Silicon Containing Hybrid Copolymers: Synthesis, Properties, and Applications 3527346643, 9783527346646

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Silicon Containing Hybrid Copolymers: Synthesis, Properties, and Applications
 3527346643, 9783527346646

Table of contents :
Cover
Silicon
Containing Hybrid Copolymers
© 2020
Contents
1 Introduction of Organosilicon Materials
2 Reactive Functionally Terminated
Polyorganosiloxanes
3 Functionalized Polyhedral Oligomeric
Silsesquioxanes (POSS) and Copolymers: Methods
and Advances
4 Nanostructured Self‐assemblies from Siliconcontaining
Hybrid Copolymers
5 Superhydrophobic Materials Derived from Hybrid
Silicon Copolymers
6 Silicone Copolymers for Healthcare and Personal
Care Applications
7 Construction of Organic Optoelectronic Materials by
Using Polyhedral Oligomeric Silsesquioxanes (POSS)
8 Hybrid POSS Nanocomposites: An Overview
of Material Toughening and Fire Retardancy
9 3D Printing Silicone Materials and Devices
Index

Citation preview

Silicon Containing Hybrid Copolymers

­Silicon Containing Hybrid Copolymers

Edited by Chaobin He and Zibiao Li

Editors Prof. Chaobin He

National University of Singapore Department of Materials Science and Engineering 9 Engineering Drive 1 117576 Singapore Singapore and Institute of Materials Research and Engineering A*STAR (Agency for Science, Technology and Research) 2 Fusionopolis Way Innovis #08‐03 138634 Singapore Singapore Dr. Zibiao Li

Institute of Materials Research and Engineering A*STAR (Agency for Science, Technology and Research) 2 Fusionopolis Way Innovis #08‐03 138634 Singapore Singapore Cover Image:

Background: © Emrah Turudu / Getty Images Front image: © molekuul_be / Shutterstock

All books published by Wiley‐VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate. Library of Congress Card No.: applied for British Library Cataloguing‐in‐Publication Data

A catalogue record for this book is available from the British Library. Bibliographic information published by the Deutsche Nationalbibliothek

The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at . © 2020 Wiley‐VCH Verlag GmbH & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Print ISBN: 978‐3‐527‐34664‐6 ePDF ISBN: 978‐3‐527‐82347‐5 ePub ISBN: 978‐3‐527‐82348‐2 oBook ISBN: 978‐3‐527‐82349‐9 Typesetting  SPi Global, Chennai, India Printing and Binding

Printed on acid‐free paper 10 9 8 7 6 5 4 3 2 1

v

Contents 1

Introduction of Organosilicon Materials  1 Huihui Shi, Jing Yang, Zibiao Li, and Chaobin He

1.1 Introduction  1 1.2 Synthesis of Polymeric Organosilicon Materials  2 1.2.1 Polysiloxanes  3 1.2.2 Polysilsesquioxanes  5 1.2.3 Other Polymeric Organosilicon Materials  7 1.3 Applications  10 1.3.1 Biomaterials  10 1.3.2 Optical and Electronic Materials  13 1.3.3 Surface Modification  15 1.4 Conclusion and Outlook  18 References  18 2

2.1 2.1.1 2.1.2 2.1.3 2.1.4 2.2 2.2.1

Reactive Functionally Terminated Polyorganosiloxanes  23 Yuanyuan Pang, Junqiang Justin Koh, Zibiao Li, and Chaobin He

Types of Functionalized Polysiloxane and Their Synthesis  23 Types of Functional Polysiloxanes  23 Polysiloxane with Monofunctional Terminals  25 Polysiloxane with Difunctional Terminals  25 Polysiloxane with Functional Side Groups  27 Functionalized Polysiloxane as Macromers  30 Modifying Degree of Polymerization of Functionalized Polysiloxanes  30 2.2.2 Cross‐Linking of Functionalized Polysiloxanes  30 2.2.3 Polysiloxane‐Containing Block and Graft Copolymers  35 2.2.3.1 Polysiloxane‐Containing Segmented and Multiblock Copolymers by Step‐Growth Polymerization  35 2.2.3.2 Polysiloxane‐Containing Graft Copolymers  41 2.2.3.3 Polysiloxane‐Containing Copolymers by Hydrosilylation and Click Chemistry  42 2.3 Functionalized Polysiloxane as Macroinitiators and Macrochain Transfer Agents  43 2.3.1 Conventional Radical Polymerization  43 2.3.2 Controlled Radical Polymerization  45

vi

Contents

2.3.2.1 Atom Transfer Radical Polymerization (ATRP)  45 2.3.2.2 Reversible Addition Fragmentation Chain Transfer (RAFT) Polymerization  47 2.3.2.3 Other Controlled Radical Polymerization Methods  50 2.3.3 Ring‐Opening Polymerization (ROP)  50 References  54 3

Functionalized Polyhedral Oligomeric Silsesquioxanes (POSS) and Copolymers: Methods and Advances  63 Huihui Shi, Jing Yang, Zibiao Li, and Chaobin He

3.1 Introduction  63 3.2 Synthetic Strategies for Functionalized POSS  64 3.2.1 Octafunctional POSS  65 3.2.1.1 Hydrolysis and Condensation from RSiX3 Monomer  65 3.2.1.2 Modification of Substituents  66 3.2.2 Monofunctional POSS  71 3.2.2.1 Corner Capping of T7R7(OH)3  71 3.2.2.2 Modification of Substituents  73 3.2.3 Bifunctional POSS  73 3.2.3.1 Some Special Cases  73 3.2.3.2 Some Developing New Strategies  74 3.3 Synthetic Protocols for Hybrid POSS‐containing Polymers  76 3.3.1 Preparation from Monomers  78 3.3.1.1 Radical Polymerization  79 3.3.1.2 Ring‐Opening Polymerization  81 3.3.1.3 Step‐Growth Polymerization  83 3.3.1.4 Other Polymerization Methods  86 3.3.2 Preparation from Polymers  87 3.3.2.1 By Conventional Organic Reactions  87 3.3.2.2 Some Advanced Methods  91 3.4 Conclusion  91 References  91 4

Nanostructured Self‐assemblies from Silicon‐containing Hybrid Copolymers  97 Hong Chi, Beng Hoon Tan, Fuke Wang, Chaobin He, and Zibiao Li

4.1 Introduction  97 4.2 Mechanism in Self‐assembly of POSS and PDMS‐Based Copolymers  99 4.2.1 Stimuli‐Responsive Micelles  100 4.2.1.1 pH‐Sensitive Micelles  100 4.2.1.2 Thermosensitive Micelles  103 4.2.1.3 Photoactive Micelles  104 4.2.2 Other Mechanisms in Different Assemblies  104 4.2.2.1 Micelles 104 4.2.2.2 Spheres 105 4.2.2.3 Sheets 106

Contents

4.3 Application  107 4.3.1 Biomedical Applications  107 4.3.2 Photodynamic Therapy  109 4.3.3 Coating  111 4.3.4 Optical Sensors  112 4.4 Conclusions and Perspectives  113 References  113 5

Superhydrophobic Materials Derived from Hybrid Silicon Copolymers  119 Lu Jiang, Xian Jun Loh, Chaobin He, and Zibiao Li

5.1 Introduction  119 5.2 Hybrid Silicon Copolymer Materials with Superhydrophobic Property  120 5.2.1 PDMS‐Incorporated Hybrid Copolymer Materials  120 5.2.2 POSS‐Incorporated Hybrid Copolymer Materials  122 5.3 Application of Superhydrophobic Silicon Copolymer Materials  128 5.3.1 Oil–Water Separation  128 5.3.1.1 PDMS‐Based Superhydrophobic Materials  131 5.3.1.2 POSS‐Based Superhydrophobic Materials  135 5.3.2 Self‐cleaning and Antifouling  136 5.3.3 Anticorrosion  137 5.3.4 Other Applications  138 5.4 Conclusion  140 References  140 6

6.1 6.1.1 6.1.1.1 6.1.1.2 6.1.1.3

Silicone Copolymers for Healthcare and Personal Care Applications  145 Weiren Cheng, Dan Kai, Xian Jun Loh, Chaobin He, and Zibiao Li

Silicone Copolymers for Biomedical and Healthcare Applications  145 Adsorption and Cell Interaction on Silicone Copolymer Surface  145 Antifouling Effect of Silicone Copolymer Surfaces  148 Antibacterial Effect of Silicone Copolymer Surfaces  148 Silicone Copolymers in Tissue Engineering and Regenerative Medicine  150 6.1.1.4 Silicone Copolymers Based Bio‐coating  150 6.1.2 Self‐assembly with Silicone Copolymers  152 6.1.2.1 Silicone Copolymers for Drug Delivery and Bioimaging  153 6.1.2.2 Silicone Copolymers in the Construction of Artificial Cells  154 6.2 Silicone for Personal Care Applications  157 6.2.1 Silicone Oil Emulsions  157 6.2.2 Silicone Copolymers as Surfactants  158 6.2.3 Silicone for Hair Care  159 6.2.4 Strategies for Depositing Silicone on Hair  160 6.2.5 Silicone for Skin Care Applications  161 6.3 Conclusions  162 References  163

vii

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Contents

7

Construction of Organic Optoelectronic Materials by Using Polyhedral Oligomeric Silsesquioxanes (POSS)  167 Fuke Wang, Xuehong Lu, Zibiao Li, and Chaobin He

7.1

Unique Properties of POSS for Building Organic Optoelectronic Materials  167 7.2 POSS‐Based Organic Electroluminescence Materials  171 7.3 POSS as a Building Block for Electrochromic Materials  181 7.4 Other Applications of POSS in Organic Optoelectronic Materials  189 7.5 Conclusions  195 References  196 8

Hybrid POSS Nanocomposites: An Overview of Material Toughening and Fire Retardancy  201 Junhua Kong, Beng H. Tan, Xuehong Lu, Zibiao Li, and Chaobin He

8.1 Introduction  201 8.2 Polypropylene/POSS Composites  202 8.3 Polycarbonate/POSS Composites  206 8.4 Polystyrene/POSS Composites  211 8.5 Polyester/POSS Composites  216 8.6 Polyepoxides/POSS Composites  220 8.7 Summary  233 References  233 9

3D Printing Silicone Materials and Devices  239 Jayven Yeo, Junqiang Justin Koh, Fuke Wang, Zibiao Li, and Chaobin He

9.1 Introduction  239 9.2 Extrusion‐Based Printing  240 9.2.1 Fused Deposition Modeling (FDM)  240 9.2.2 Direct Ink Writing (DIW)  242 9.2.2.1 Rheology‐Controlled Shape Retention  242 9.2.2.2 Coaxial Printing  245 9.2.2.3 Embedded 3D Printing  245 9.3 Jetting‐Based Printing  247 9.3.1 Inkjet 3D Printing (IJP)  247 9.3.2 Aerosol Jet Printing (AJP)  249 9.4 Vat Photopolymerization/Light‐Based/Photocurable 3D Printing  251 9.4.1 Stereolithography (SLA)  252 9.4.2 Digital Light Processing (DLP)  252 9.4.3 Photopolymerization Process  252 9.4.3.1 Photoinitiator 253 9.4.3.2 Photocurable Polymers  254 9.5 Potential Applications  260 References 261 Index  265

1

1 Introduction of Organosilicon Materials Huihui Shi1, Jing Yang 2, Zibiao Li 2, and Chaobin He1,2 1

National University of Singapore, Department of Materials Science and Engineering, 9 Engineering Drive 1, Singapore 117576, Singapore 2 Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Innovis, #08‐03, Singapore 138634, Singapore

1.1 ­Introduction The chemistry of organosilicon polymers has reached a high level of maturity during the past century, which established a fundamental basis for their application in materials science. Because of their inorganic–organic chemical compositions, the unique dual nature of organosilicon polymers makes them an important bridge between inorganic and organic polymers and contributes to an interesting combination of properties [1–3]. According to the structural differences in the backbone, organosilicon polymers can be mainly divided into polysiloxanes (Si–O), polysilsesquioxanes (Si–O), polysilanes (Si–Si), polycarbosilanes (Si–C), and polysilazanes (Si–N) [4]. Compared to carbon, the size and electronegativity of silicon significantly affect the structural properties of the bond and endow polymeric organosilicon with unique features [5]. Polysiloxane‐based materials are attractive because of their high backbone flexibility, low glass transition temperatures, good thermal and oxidative stability, high gas permeability, excellent dielectric properties, and biocompatibility [6]. Polysilsesquioxane‐based materials, mostly referred to as polyhedral oligomeric silsesquioxane (POSS)‐based materials, demonstrate improved mechanical and thermal properties, oxidation resistance, gas permeability, reduced flammability, and antibiofouling and antibacterial properties [7]. Polysilane‐based materials are characteristic of fantastic optical and electronic properties owing to delocalization of σ‐electrons and conjugation along the σ‐bonds of backbone [8]. Polycarbosilane‐based materials can exhibit excellent thermal stability at relatively low temperature and pyrrolytic properties at high temperature as well as high mechanical strength and ultralow

Silicon Containing Hybrid Copolymers, First Edition. Edited by Chaobin He and Zibiao Li. © 2020 Wiley‐VCH Verlag GmbH & Co. KGaA. Published 2020 by Wiley‐VCH Verlag GmbH & Co. KGaA.

2

1  Introduction of Organosilicon Materials

dielectric constant [9]. Polysilazane‐based materials are best known not only for pyrrolytic properties but also for possessing some similar or preferable properties relevant to polysiloxane‐based materials as a result of their isoelectronic molecular structure, such as thermal stability, low fire hazard, high mechanical resistance, and high surface energy [3]. In recent years, considerable attention has been drawn to the research of novel silicon‐containing hybrid copolymers for further expansion and improvement of materials possessing specific useful properties [10]. Thanks to the fast development of polymer science, a variety of such copolymers with well‐defined architectures as well as elements of selectivity and self‐assembly has been reported [6, 7]. Distinguishing from pure organosilicon materials, the properties of hybrid materials include not only the sum of the individual contributions of their components but also the strong synergy created by extensive hybrid interfaces [11]. For the preparation of silicon‐containing copolymers, a very popular and simple strategy is to synthesize organosilicon oligomers/polymers with reactive functionally terminated groups first by different approaches, which will be discussed in detail later. As preformed segmental components, they can go on to copolymerize with a wide range of (i) monomers via step growth [12], anionic [13], ring‐ opening [14], or living free radical polymerization [15, 16] or (ii) polymer blocks via coupling reaction such as click chemistry [17] and hydrosilylation [18], constructing well‐defined architectures such as block [19], graft [20], and star‐like [21] copolymers. The significant advantages of the silicon‐containing copolymers are their flexible chemistry, which manifests as a wide selection of substituents on the silicon atom of the backbone, controlled molecular weight of copolymers, and tailor‐designed backbone composition, implementing the facile tunability of specific material properties [6]. Organosilicon polymers with versatile properties hold great interest for a wide range of potential applications including biomaterials [22], functional coatings [23], electronic and photic devices [24], catalysts [25], ceramics [26], membranes [27], additives, and modifiers [28]. More details on this topic will be discussed later. There is no doubt that their integration with organic polymers can further enlarge the compatibility for an expansion in breadth as well as in depth of utilitarian scope. A myriad of literature has proved the emergence of quite a few new applications such as nanostructured self‐assemblies [29], shape memory materials [30], and 3D printing devices [31], which will be discussed in detail in Chapters 4, 5, and 10 respectively.

1.2 ­Synthesis of Polymeric Organosilicon Materials The synthetic strategies for polymeric organosilicon have been diverse and mature with the development over the past century [1–3]. Because of the presence of silica in nature, the monomers for polymeric organosilicon are obtained upon a synthetic route. Silanes have the general formula R4−nSiXn, where X is the reactive groups having Cl, –OR, –OOCR, and –NR2 as the most fundamental precursors for organosilicon polymerization. They can be produced by (i) direct reaction of an organic compound with silicon at elevated temperature;

1.2  Synthesis of Polymeric Organosilicon Materials

(ii)  ­chlorination of silicon and a subsequent substitution reaction by organic groups with organometallic reagents such as organolithium compounds, Grignard reagents, and organic zinc compounds; or (iii) transformation of silicon into silyl hydrides and a subsequent addition to multiple bonds in a hydrosilylation process [32]. Notably, hydrosilylation is a characteristic reaction in organosilicon chemistry and has been utilized as a prevailing approach to binding organic groups and silicon moieties [33]. Starting from functional silanes, methods for different kinds of polymeric organosilicon will be discussed in this section. In view that there are considerably more research directed toward polysiloxanes and polysilsesquioxanes (especially POSS), these two classes are emphasized, although others also receive a lot of attention from organosilicon chemists. In addition, given that this book focuses on the utilization of polymeric organosilicon in copolymers, methods bringing functionalization of chain ends will be highlighted. 1.2.1 Polysiloxanes Polysiloxanes having the general formula (–R2Si–O–)n, also termed as silicone polymers or polyorganosiloxanes, are composed of a backbone with alternate silicon and oxygen atoms while two organic substituents are linked to each silicon atom. The general route to polysiloxanes from monomers consists of two steps: (i) hydrolytic polycondensation of the bifunctional silane precursors resulting in a mixture of linear and cyclic oligomers and (ii) transformation of the oligomers into high‐molecular‐weight polymers either by polycondensation of the short‐chain linear oligomers or by ring‐opening polymerization of the cyclic oligomers. Such process is mostly based on dimethyldichlorosilane (DDS) and puts out polydimethylsiloxane (PDMS), which are the best‐known silicone ­polymers [34]. As for polycondensation routes, linear oligomers with silanol end group are reactive toward a wide range of silyl‐functional groups such as –SiH, –SiCl, – SiOR, –SiOOCR, and –SiNR2 and hence can be polymerized via either homofunctional or heterofunctional condensation when the resultant polymers are conferred with the potential of terminal functionalization. However, the application of polycondensation routes in academic and industry is limited for issues such as size dependence and side reactions [3]. In contrast, ring‐opening polymerization of cyclic oligomers permits the synthesis of high‐molecular‐weight polysiloxanes with better selectivity and precision, playing an important role in the preparation of reactive functionally terminated silicone oligomers including monofunctional silicone oligomers and α,ω‐reactive difunctionally terminated (telechelic) silicone oligomers, which are very critical starting materials for a wide range of silicone copolymers [35]. The monofunctional silicone oligomers are mostly synthesized through anionic polymerization of hexamethylcyclotrisiloxane (D3) initiated by lithium silanolate in the presence of an activator such as tetrahydrofuran (THF) or diglyme (Figure 1.1), and the functional group is introduced during the deactivation of the silanolate ion using a functional chlorosilane [10]. The preparation of telechelic silicone oligomers, which are more commonly used in

3

4

1  Introduction of Organosilicon Materials

BuLi +

Si O

O Si

Si O

Bu Si O Si O nSi O– Li

+

Cl Si R X LiCl + Bu Si O Si O nSi O Si R X

Figure 1.1  Synthetic scheme for monofunctional silicone oligomers.

Step 1 H Si O Si H

+

R

X

Pt catalyst

R2 R1 O Si R1 R2 Si O Step 2 X R Si O Si R X + O R Si O Si 2 R1 R 1 R2

X R

acid or base heat

H2C

H2C Si O Si CH2 CH2 R X

R1 X R Si O Si O R2

n

Si R X + Cyclics

Figure 1.2  Synthetic scheme for telechelic silicone oligomers.

c­ opolymerization, generally adopts acid‐ or base‐catalyzed ring‐chain equilibration or redistribution reactions, as shown in Figure 1.2. Based on the nature and the reactivity of the functional end groups, strong acids such as sulfuric acid, trichloroacetic acid, and sulfonic acid or strong bases such as sodium hydroxide, potassium hydroxide, and quaternary ammonium hydroxide can be selected as catalysts. With proper temperature ranging from 50 to 100 °C, the polymerization leads to a complex equilibria between cyclic and open‐chain populations, and the yield is dependent on the initial concentration of monomer as well as the size and polarity of organic substituents at silicon atoms [36]. The introduction of end blockers in the initiation of the ring‐opening polymerization (ROP) routes is also significant to both the precise control over polymer molecular weight and the functionalization of chain ends. The end blockers can be classified as (i) siloxane oligomers with reactive functional groups (X) such as chlorine, hydroxy, and alkoxy directly attached to the terminal silicon atoms (Si–X termination) and (ii) siloxane oligomers with a short hydrocarbon bridges (R) between them (Si–R–X termination), but actually, the latter kind turns out to be preferable for copolymerization. Si–X end blockers can simply be obtained by hydrolysis of bifunctional silanes, whereas Si–R–X end blockers need another step of hydrosilylation reaction between Si–H‐terminated silicone oligomers and vinyl or allyl terminated functional organic reagents. Although Si–X end blockers have higher reactivities for subsequent copolymerization compared to their Si–R–X counterparts, the formation of Si–O–C linkages between polysiloxanes and organic segments are so susceptible to hydrolysis under acidic or basic conditions as to become an unignorable disadvantage, while the insertion of hydrocarbon bridges (R) to be Si–R–X end blockers can effectively avoid this problem [6]. By the use of various cyclic oligomers such as octamethylcyclotetrasiloxane (D4) [37], diphenylsiloxane [38], methylphenylsiloxane [39], and methyltrifluoropropylsiloxane [40], polysiloxanes terminated with functional groups have been widely reported, which are great candidates for the preparation of silicone‐containing hybrid copolymers

1.2  Synthesis of Polymeric Organosilicon Materials

and hold unlimited potential for extensive applications. A thorough discussion about this topic is available in Chapter 2. 1.2.2 Polysilsesquioxanes Polysilsesquioxanes have a structure with empirical formula RSiO3/2, where R can be a hydrogen atom or an organic moiety and are generally prepared by precursors of formula RSiX3 via hydrolysis and subsequent polycondensation, where X is a group, such as chlorine and alkoxy, prone to hydrolysis in the presence of acid or base catalysts and R is a hydrolytically stable organic moiety [1]. A combination of multiple factors including types of substituents at silicon atoms, concentration of initial monomers and water, catalyst, temperature, and solvent will determine the structures of the resultant polymeric silsesquioxanes varying from oligomeric cages to ordered ladder structures to three‐dimensional network, as shown in Figure 1.3, and a great concentration has been cast on the area of corresponding polysilsesquioxanes [29, 41, 42]. Besides, rapid growth has also been witnessed for bridged polysilsesquioxanes, whose silicon atoms are linked by their organic moiety additionally. The main difference of bridged polysilsesquioxanes from others lies at the precursors, which possess more than one functional moiety –Si(OR)3 and are thus active for sol–gel processing to construct more attractive Si–O–Si networks [43]. Among all these structure, cage‐like POSS is the hottest compound for investigation. POSS of formula (RSiO1.5)n, where n is an even integer larger than 4 and R is an active or inert organic group, is considered as the smallest silica particle with a definite nanostructure that can

Random structure

(T8)

Ladder structure

(T10) Cage structures

Figure 1.3  Various structures of polysilsesquioxanes.

Partial cage structure

(T12)

5

6

1  Introduction of Organosilicon Materials

be incorporated into polymer matrices to fabricate a wide range of novel hybrid materials with promising properties [44]. As a result of the stability of the Si4O4 ring structure, the spontaneous formation of T8 POSS is preferential over other cubic species such as T10 and T12 and offers them a huge advantage in the synthesis to achieve functional T8 POSS molecules by flexible approaches (Figure 1.4). Thus, in the past few decades, tremendous efforts have been devoted to the development of octasilsesquioxane (T8 POSS). Quite a number of T8 POSS species have been successfully prepared from hydrolysis and condensation reactions of trifunctional chlorosilanes or alkoxysilanes. Together with the substitution of functional groups at one or more of the corner silicon atoms or the modification of functional groups, an abundant source of T8 POSS derivatives from the existing T8 core are provided. In most cases, T8 POSS molecules bear one reactive group for further modification, grafting or polymerization and seven unreactive groups for stability, solubility, and compatibility known as monofunctional POSS (T8R7R′), or eight reactive groups of the same kind named as octafunctional POSS (T8R8). Meanwhile, compounds such as T8R6R′2, T8R6R′R′′, T8R5R′3, and T8R4R′4 obtained by chemical modification have also come into notice in recent years, largely expanding the catalog of POSS molecules. However, it is noteworthy that the synthetic routes toward POSS directly from trifunctional chlorosilanes or alkoxysilanes actually produce a mixture of products and often leads to long reaction time and low yield as inherent downsides. Thus, optimization of reaction conditions is still an interesting point of research. Besides, the issue of stability of the POSS core in the process of organic modification, that is, the cleavage R

R

RSiX3 Hydrolysis and condensation

Si OH OHOH O O O Si R Si Si O Si O OR R O O Si Si O

R R Si R

Substitution at Si R' Si R'

R'

O

R

RSiX3

R

Addition of a comer

Si

O O O O O Si R Si Si O R O R O O O Si Si O Si

R

Modification of substituents R'

R'

Si

Si

O O O O O Si R' Si Si Si O OR' O O O Si Si O R'

R

O

+

R' R' R'

O

R' Si

O

O O O O Si R' Si Si O OR' O O O Si Si O Si

R'

R'

Figure 1.4  Synthetic schemes for functional POSS. Source: Cordes et al. 2010 [45]. Reproduced with permission of American Chemical Society.

1.2  Synthesis of Polymeric Organosilicon Materials

of Si─O bonds, will also cause some problems. Instead, the simplicity of corner‐ capping reactions makes them a preferred method for the functionalization of POSS, especially monofunctional POSS. In this approach, corner‐truncated cube species R7Si7O9(OH)3 (T7) are firstly synthesized by incomplete condensation of trifunctional organosilanes or hydrolytic removal of one silicon atom from a stable T8 POSS molecule, succeeded by addition of a capping agent such as chlorosilane or alkoxysilane to T7 where a variety of novel T8 POSS molecules are produced [45, 46]. Based on functionalized POSS molecules, well‐defined POSS‐containing hybrid polymers with different architectures including telechelic polymers, block copolymers, and star‐shaped polymers can be developed by advanced polymer synthetic approaches such as anionic polymerization, living radical polymerization, and click chemistry, where POSS molecules function as monomers, ROP initiators, atom transfer radical polymerization (ATRP) initiators, reversible addition fragmentation chain transfer (RAFT) agents, or are directly used to couple with the end group of polymer chains [7]. Chapter 3 will give a discussion in depth about this topic. 1.2.3  Other Polymeric Organosilicon Materials Except for polysiloxanes and polysilsesquioxanes, a relatively new class of polymeric organosilicon materials such as polysilanes, polycarbosilanes, and polysilazanes also open many interesting possibilities for hybrid materials. Polysilanes are composed of a backbone of catenating silicon atoms, which typically bear two organic side chains each. The most commonly used method to prepare polysilanes is Wurtz‐type reductive coupling of dichlorodiorganosilanes under the catalysis of alkali metals, as shown in Figure 1.5. As the reactions are carried out at a high temperature and strongly reducing environment so that the functional substituents at silicon atoms are limited to alkyl, aryl, and fluoroalkyl groups or those are well protected that can withstand such conditions, post‐polymerization functionalization is required. Meanwhile, factors such as temperature, solvent, and reductant have significant impact on the reaction as well. It is interesting that in many cases, especially when the side chains lack

(a)

(b)

Cl

R Si R

R Si R

Cl

alkali metal

R Si R R = aryl – Na

Cl

+ Na R = alkyl – NaCl

R Si R

n

R Si R R Si R

Cl

Cl

H-abstraction

R Si R

Figure 1.5  (a) Synthesis of polysilanes via Wurtz‐type reductive coupling polymerization and (b) side chain‐dependent termination.

H

7

8

1  Introduction of Organosilicon Materials

radical‐stabilizing substituents, hydrogen‐terminated chains are obtained with the hydrogen atom extracted from a solvent such as toluene or xylene, offering great possibilities of functionalizing polysilane chain ends for further copolymerization with organic polymers [47]. In addition, given the disadvantages of Wurtz‐type reductive‐coupling reaction such as polymodality and poor molecular weight distributions resulting from end‐biting and back‐biting of polysilane chains, alternative approaches including ring‐opening polymerization of cyclotetrasilanes [48], catalytic disproportionation of alkoxydisilanes [49], anionic polymerization of masked disilenes [50], and dehydrocoupling polymerization of primary silanes (RSiH3) catalyzed by transition metal [51] are also available to prepare polysilanes. Polycarbosilanes usually refer to a class of polymers characterized as difunctional organic groups in the backbone to bridge the silicon atoms, which are conventionally converted by polysilanes explicitly or implicitly upon thermolysis, namely Yajima strategy. Besides, more direct routes to prepare polycarbosilanes  without polysilane intermediates have been developed, too. One of the approaches undergoing intensive investigation is ring‐opening polymerization. Structures such as 1,3‐disilacyclobutane, 1‐silacyclopent‐3‐ene, 3,4‐benzosilacyclopentene, silacyclobutane, 2,3‐benzosilacyclobutene, and silicon‐bridged ferrocenophanes can be premodified with required organic substitutions on silicon atoms and then act as a variety of monomers for corresponding polycarbosilanes in the presence of an appropriate catalyst, as shown in Figure 1.6 [52]. Another method involving coupling reaction between halogen atoms and metal atoms is also commonly used. In early years, such polycarbosilanes are derived from chlorosilanes together with alkyl halides or solely chlorosilanes bearing alkyl chloride substituents on silicon atoms via Wurtz–Fittig coupling with sodium or potassium, while in recent years, the use of Grignard and organolithium reagents has come into mainstream gradually [3]. Moreover, various reactions including dehydropolycondensation coupling of hydrogenosilanes [53] and hydrosilylation of hydrogen silanes with alkenes and alkynes or vinylhydrogenosilanes alone [54] have been reported for the elaboration of polycarbosilanes, too. Meanwhile, cyclolinear polycarbosilanes have attracted some attention as an  emerging group of organosilicon polymers with novel beads‐on‐a‐string ­structure. They are mostly prepared from 1,3‐disilacyclobutane with alkylene or

R1 Si R2

Si

R1 R2

Si R 2 R1

Fe

Si R1 R2

Si R 2 R1

Si

Si R1 R2

R1 R2

Catalyst R1 Si CH2 R2

n

R1 Si R2

n

R1 Si R2

n

R1 Si R2

n

Figure 1.6  Various monomers for ROP of polycarbosilanes.

R1 Si R2

n

R1 Si R2

Fe n

1.2  Synthesis of Polymeric Organosilicon Materials

halogen groups, which are reactive for acyclic diene metathesis polymerization (ADMET) or Grignard coupling reactions, respectively, and polymerized into polymeric organosilicon materials with a general formula as {–[RSi(CH2)2SiR]–Y–}n, where R and Y can be different organic groups such as alkyl and aryl. Interestingly, as 1,3‐disilacyclobutane derivatives have proved to be useful monomers for conventional linear polycarbosilanes, the strained rings in the backbone of such cyclolinear polymers are naturally latent cross‐linking sites that can form network polycarbosilanes under suitable conditions [9]. Polysilazanes, as an analog to polysiloxanes, whose oxygen atoms in the main chain are replaced by nitrogen atoms, are also an unneglectable part of organosilicon polymers, although their preparative methods are less well explored. There are mainly two classes of routes to polysilazanes: one is polycondensation of functional silanes and amines and the other is ionic ring‐opening polymerization of cyclosilazanes (Figure 1.7). Ammonolysis or aminolysis reactions of dihalogenosilanes with ammonia or amine are the most commonly used methods in the former case. However, these reactions usually yield a complex mixture of linear and cyclic oligomers, and thus some chemists have turned to the latter routes for better control of the polymerization nowadays. Cyclodisilazanes, cyclotrisilazanes, and cyclotetrasilazanes synthesized from amine and dichlorosilane have exhibited potential for ionic ring‐opening polymerization in a similar mechanism to polysiloxanes. Yet, the effect of steric hindrance resulting from nitrogen substituent of cyclosilazanes is an extra factor that needs to be taken care of, which may reduce the accessibility of silicon atoms and decrease the polymerizability [55]. This section only offers a primary summary for the synthetic strategies of several representative kinds of organosilicon polymers. Beyond that, novel structures such as hyperbranched polymers [56], dendrimers [57], and conjugated organosilicon [58] are also remarkable while several literature studies specializing in this topic are already available for extensive reading.

(a)

(b)

(c)

Cl

Cl

R1 Cl Si R2

+

R1 Cl + Si R2

(Me3Si)2NH

R1 Si N R2 H

NH3

R1 Si N R2 R

R NH2

+

R1 Si N R2 H

n

n = 2,3,4

+ NH4Cl

n

n

Me3Si

+ RNH3Cl

R1 Si N R2 H

n SiMe3

Figure 1.7  Synthetic schemes for polysilazanes including polycondensation of dichlorosilanes with (a) ammonia or (b) amino, and (c) ring‐opening polymerization of cyclosilazanes.

9

10

1  Introduction of Organosilicon Materials

1.3 ­Applications The evolution of synthesis established a foundation for the extensive applications of various polymeric organosilicon materials, and the characteristic composition of main chain gives each kind of polymers their own unique properties as mentioned earlier and make them advantageous in very different fields. Owning to the excellent photophysical and electronic properties, polysilanes have been mainly investigated as optoelectronic materials such as photoresists [59], photoinitiators [60], photoconductor [61], nonlinear optical materials [62], and materials for organic electroluminescent devices [63]. Meanwhile, considerable progress has been achieved for polycarbosilanes and polysilazanes as ceramic precursors on the basis of their pyrolytic properties, which can potentially ­generate high‐performance nonoxide ceramics known as silicon carbide (SiC) and nitride (Si3N4) [64]. Moreover, the maturity and facility of the preparative ­methods for Si–O–Si‐based organosilicon materials, including polysiloxanes and polysilsesquioxanes, have significantly promoted the understanding of their structure–property relationships and further contributed to the prosperity of applications [3]. With regard to polysiloxanes, more commonly known as silicone and referring to PDMS in most cases, with good thermal and oxidative stability as well as other useful properties, they have become the earliest materials gaining widespread commercial applications such as elastomers [65], sealants [66], lubricants [67], surfactants [68], coatings [23], thermal and electrical insulation materials [69], and biomedical devices [70]. As for polysilsesquioxanes, distinguished by their nanostructure, rigidity, and thermal stability, a lot of research has been devoted to the incorporation of POSS moieties into polymers by blending or copolymerization to create polymer nanocomposite with reinforced properties, promising to be applied in nanomedicine [71], catalysts [72], electronic devices [73], military and aerospace fields [74], etc. In view that there is a myriad of applications for the latter two kinds of organosilicon hybrid materials, this section will introduce three hot fields specifically, i.e. biomaterials, optical and electronic materials, and surface modification. 1.3.1 Biomaterials Both PDMS and POSS exhibit physiological inertness, low toxicity, good thermal and oxidative stability, excellent mechanical and viscoelastic properties, and high gas permeability provided by Si─O bond in the structure, which makes them suitable for biomedical application. PDMS is a potential candidate for implantable biomedical devices including blood pumps, cardiac pacemaker leads, mammary prostheses, replacement esophagus, maxillofacial reconstruction, finger joints, contact lenses, and catheters [70]. For example, Hao et al. prepared a kind of macromonomers from octamethylcyclotetrasiloxane (D4) and 2,4,6,8‐tetramethylcyclotetrasiloxane (D4H) via ring‐opening polymerization and subsequent hydrosilylation of Si–H with allyl methacrylate, which were injectable and curable under irradiation of blue light in the presence of a photoinitiator to form a soft gel in situ within five minutes, which is promising to be used as intraocular lens in human eyes in the place of hardened natural lens for

1.3 Applications

restoring eyes’ ability [75]. Simmons et  al. developed a kind of drug‐eluting stents loaded with dexamethasone acetate (DexA) from PDMS‐based polyurethane, which consisted of hard segments of 4,4′‐methylenediphenyl diisocyanate (MDI) and 1,4‐butanediol (BDO) and mixed soft segments of PDMS and polyhexamethylene (PHMO) in the ratio of 80  :  20. The materials demonstrated excellent long‐term biostability and may be used as a delivery vehicle of therapeutic agents for drug‐controlled release in vivo [76]. Besides, soft lithography using PDMS allows one to fabricate complex microfluidic devices easily [77]. Together with optical transparent properties suitable for detection, PDMS‐based microfluidic devices hold great interest in lab‐on‐a‐chip (LOC) systems for biochemical analysis and point‐of‐care (POC) disease diagnosis. Li et al. attempted to combine the high performance in the flow control for PDMS‐based microfluidic devices and the ease of immobilizing biosensors on a chip for paper‐based microfluidic devices and reported a versatile and cost‐effective PDMS/paper hybrid microfluidic device integrated with loop‐mediated isothermal amplification (LAMP) reaction to achieve rapid, sensitive, and instrument‐free detection of meningitis‐causing bacteria [78]. Even more effort has been focused on POSS as it has been considered as a next‐generation material in several biological fields [79]. For example, Huang et  al. developed a kind of POSS‐based nanomedicine with pH and reduction dual responsiveness, as shown in Figure 1.8. Amphiphilic star‐shaped polymers were synthesized by grafting semitelechelic N‐(2‐hydroxypropyl) methacrylamide (HPMA) copolymers to a POSS rigid core through reductively degradable disulfide bond. The hydrophobic drug docetaxel (DTX) was either attached to the grafts by pH‐sensitive hydrazone bonds or encapsulated into the POSS core, ensuring high drug loading capacity. The final conjugates could self‐assemble into nanoparticles and release DTX under acidic lysosomal and reducing cytoplasmic environments, which are usually generated by tumor tissues and thus open the possibility of an efficient approach to transport hydrophobic drugs for cancer therapy [80]. Except for drug delivery, gene delivery has also become a hot topic for emerging gene therapy. Star‐shaped block polymers with a cationic poly(2‐dimethylamino) ethyl methacrylate (PDMAEMA) shell and a zwitterionic poly[N‐(3(methacryloylamino) propyl)‐N,N‐dimethyl‐N‐(3‐sulfopropyl) ammonium hydroxide] (PMPDSAH) corona grafting from a POSS‐cored initiator through atomic transfer radical copolymerization were reported by Liu et al. The copolymers could self‐assemble into core‐shell‐corona micelles in aqueous solutions, during which anticancer drug doxorubicin (DOX) and tumor‐suppressing gene p53 were encapsulated benefitting from the strong DNA condensation ability of PDMAEMA and the hydrophobicity of POSS. Meanwhile, the micelles were stabilized by antifouling PMPDSAH in serum environment, which could restrain the production of polymer‐specific antibodies and thus prolong their retention time in vivo. The micelles showed good drug and gene loading capacity, high gene transfection efficiency, reduced bovine serum albumin (BSA) protein absorption in vitro, and result in high tumor cell apoptosis in mice model, which were promising to be used for further clinical applications [81]. As we can see, the self‐assembly of POSS into ordered nanostructures plays a significant role in their applications, and this topic will be discussed with a full

11

1  Introduction of Organosilicon Materials

(a)

Cl H3N

NH

S N Si O O Si O

Cl H3N Cl H3N

O O Si

Si O

O Si

Si O

O

O

O

NH

S

N

S

N

S

O

POSS-NH2

NH

S

N

S

O

Cl H3N

S

Si S

NH3 Cl

N

O

O

NH3 Cl

O Si O Si O

O O

SPDP

NH3 Cl

S

O

O

Si NH

S

NH3 Cl

Si O

O

O

Si O

S

N

S

O

Si

Si O

HN O

O

S

O HN

Si

O Si O

POSS-PDS

O

S

HN

S S

HN

N

S

O

N

N

S

O

(b) POSS-PDS

+

Conjugated via hydrazone bonds

Disulfide bond

Self-assembly

DTX P-SH

Redox-sensitive star copolymers

DTX

Star copolymer-docetaxel conjugates

SP-DTX

(c) d En yt oc

EPR effect

is

os

Blood vessel

) se SH SH ea G rel M m D TX 10 l ( ted so la to su 0) 5. Cy cap H~ ase en (p e rele m so TX us so D cle /ly ed Nu do gat En nju co

SH

Tumor cell

SH

SH

Fibroblast

SH

SH

SH

SH

12

Tumor tissue

e ul n ub tio ot za icr ri M yme l po

Figure 1.8  Application of POSS in drug delivery. (a) Synthesis procedure of octa‐functional POSS. (b) Fabrication of drug‐loaded POSS nanoparticles. (c) Illustration of tumor accumulation and intracellular trafficking pathway of POSS nanoparticles. Source: Yang et al. 2016 [80]. Reproduced with permission of American Chemical Society.

illustration in Chapter 4. In addition, Chen et al. used amino‐modified POSS to link on the surface of upconversion nanoparticles (UCNPs) via simple addition reaction and achieve a transfer from hydrophobicity to hydrophilicity for UCNPs while their high‐size monodispersity was well preserved. Thanks to the low cytotoxicity and excellent biocompatibility of POSS, the POSS–UCNPs displayed good performance for both in vitro cell imaging and in vivo small animal imaging [82]. Besides, Lu et al. prepared a series of POSS–polyethylene glycol (PEG) hybrid hydrogels by covalently grafting POSS into tetra‐arm PEG and further cross‐linked by matrix metalloproteinase (MMP)‐degradable peptide both via Michael‐type addition reaction between thiol group and maleimide group. Such biodegradable hydrogels gained higher mechanical properties, lower equilibrium swelling ratio, and tunable degradation rate because of the incorporation of POSS, having the potential to be used as a tissue‐engineering scaffold [83].

1.3 Applications

1.3.2  Optical and Electronic Materials Modifications of side or end groups of polysiloxanes and POSS vertex groups are extensively explored in order to give access to new applications. Their derivatives with unique optical or electronic properties are just a manifestation. Polysiloxanes bearing liquid crystalline groups via hydrosilylation is mostly an important ­family of polysiloxanes. Among them, liquid crystalline elastomers (LCEs) have attracted significant attention because of their reversible anisotropic dimensional shape responses to applied stimuli, which can dramatically impact the microscopic orders or molecular structures of uniaxially aligned liquid crystal mesogens and further change the macroscopic shapes of the whole LCE materials, opening the prospect for applications such as actuators and sensors [84]. For example, Yang et al. reported multistimuli‐responsive LCE composite films prepared from a mixture of poly(methylhydrosiloxane) (PMHS), vinyl‐terminated side‐on azobenzene‐containing mesogen A44 V6, vinyl‐terminated side‐on mesogen V444, cross‐linker 11UB, Pt catalyst, and single‐walled carbon nanotubes (SWCNTs). The composites integrated the thermal‐induced liquid crystal (LC)‐ to‐isotropic phase transition effect of liquid crystalline, trans–cis tautomerization effect of azobenzene and photothermal effect of SWCNTs and could perform a fully reversible shrinking/expanding response toward heat or near‐ infrared (NIR) light stimulus, and a fully reversible bending behavior under ultraviolet (UV) irradiation, holding potential for applications of control devices, logic gate devices, etc. [85] POSS are widely explored in their applications for photoluminescence (PL) and electroluminescence (EL) devices [7]. For example, Wang et al. developed a novel kind of eight tetraphenylethene (TPE)‐modified POSS through amide condensation reaction, which was endowed with the effect of aggregation‐induced emission (AIE), as shown in Figure 1.9. Their studies indicated that POSS core had superstrong self‐assembly properties by organic modification and could form different morphologies in different solvents and concentrations through hydrogen bond interaction. Compared with TPE–NH2, POSS–TPE exhibited enhancement in aggregate emission, monomer emission, and quantum yields in the aggregate state. With appropriate selection of solvent and concentration, the monomer emission was quenched while the aggregate emission was enhanced as a result of the aggregation‐induced restriction of intramolecular rotation in combination with the increasing π–π stacking interactions of TPE units. Because of strong noncovalent interaction forces between POSS–TPE and aromatic molecules, fast detection of methamphetamine and ketamine was performed successfully by quenching aggregate emission, suggesting that POSS–TPE was hopeful in the application of a new AIE chemosensor [86]. Cheng et al. studied a POSS derivative containing eight π‐conjugated chromophores 3,6‐dipyrenylcarbazole (DPCz) obtained via hydrosilylation reaction as electroluminescence materials for nondoped blue organic light‐emitting diode (OLED) devices. POSS–DPCz displayed a sterically bulky three‐dimensional structure, which could effectively inhibit the crystallization behavior of emissive units DPCz and restrict the motion of structural perturbation, conductive to enhance the color

13

14

1  Introduction of Organosilicon Materials

NH

O

POSS-TPE

O

S O S

HN

NH

S

Si O Si O O O Si O Si O O O Si Si O O O Si Si O

S

H N

O

S

NH

O N H

S O S

NH

S O

O HN

Self-assembly 335 nm 432 nm

Monome r emission

AIE emiss ion

Quen ch by dru ed gs

335 nm 470 nm

335 nm No PL

Figure 1.9  Application of POSS for photoluminescence. Source: An et al. [86].

purity and stability, emission quantum yield, and thermal stability of the materials. As a result, the luminous efficiency and maximum brightness of the POSS– DPCz device are almost two times higher than that of the control DPCz‐based device, proving it to be a possible route to fabricate practical OLED devices with improved optoelectronic performance [87]. Moreover, POSS are also available for the application in proton exchange membranes (PEMs) of fuel cells. Gong et al. incorporated double‐decker silsesquioxane (DDSQ), a difunctional POSS unit, into the backbone of linear sulfonated polyimides (SPIs) with advantages such as enhanced oxidative and hydrolytic stabilities, low swelling ratio ­combined with high water uptake, adequate thermal stability and mechanical ­properties,

1.3 Applications

and low methanol permeability over SPI alone. Meanwhile, compared with other POSS hybrid membranes fabricated by direct blending or cross‐linking methods, POSS in the SPI main chain show more uniform dispersion and increase the confined bound water molecules within the formation of continuous proton transformation channels, thereby increasing proton conductivity [88]. Besides, POSS have been loaded in some LC media to optimize the electro‐optical (E‐O) performance of LC systems. For example, POSS is considered as one of the promising candidates for the formation of vertical alignment (VA) of LC because of some new E‐O properties such as memory effect, frequency modulation response, and low driving voltage. Jeong et  al. synthesized a cyanobiphenyl monosubstituted POSS giant molecule connected by a flexible alkyl chain, called POSS–CBP1, to improve the compatibility and interaction of POSS with LC media. POSS–CBP1 showed an excellent dispersion in nematic (N) LC media and could gradually diffuse onto the glass substrate. Owing that the interaction between POSS and substrate was much higher than that between cyanobiphenyl moieties and substrate, the molecular orientation of POSS–CBP1 was perpendicular to the surface and formed homeotropic alignment, which should lead to a clear dark state in the absence of an electric field and was proved by polarized optical microscopy (POM) images. Also, the interaction between POSS–CBP1 and substrate was strong enough for the VA of LC to remain stable under thermal fluctuations, making such materials potential for electro‐optical applications, e.g. LC displays (LCD) [89]. POSS‐based high‐performance organic optoelectronic materials have been recognized as the most promising materials in both academic research and industry applications, and Chapter 8 will delve into this topic. 1.3.3  Surface Modification The versatile properties of PDMS‐ and POSS‐based materials including excellent mechanical properties, thermal stability, oxidation resistance, reduced flammability, hydrophobicity, low surface energy, and so forth provide them broad possibilities in the application of surface modification of materials and tremendous endeavor has been devoted to achieve accomplishing various functions such as anticorrosion, antifouling, self‐cleaning, flame resistance, antireflection, anti‐ icing, and antifogging [23, 90]. Metal corrosion is a challenging problem in the twenty‐first century as most common applications in everyday human life are virtually based on metals. PDMS is an important category of anticorrosion coatings for pure and alloyed metals such as magnesium, aluminum, iron, and copper because of its hydrophobicity, chemical inertness, easy modification and processing, and environmental friendliness. For example, Guo et al. fabricated PDMS–titania (TiO2) nanocomposite coating prepared from a mixture of hydroxyl‐terminated PDMS, TiO2 nanoparticles, curing agent tetraethoxysilane (TEOS), and catalyst dibutyltin dilaurate (DBTDL) via spin‐coating method on aluminum alloys AA 2024. The good hydrophobicity and structure compactness of PDMS could delay the rate of penetration of water and corrosive ions into the substrate and obstruct the flow of the current to protect the metal, while the incorporation of TiO2 nanoparticles

15

16

1  Introduction of Organosilicon Materials

could produce a micro–nano rough structure with a larger amount of air to block the entry of water, which compensated the negative effect caused by some existing microspores and microcracks in the organic coatings and improved the durability of corrosion resistance. The results demonstrated that the coating still had a protective effect on the metal after 40 days of immersion [91]. As a class of nanoparticles, the reinforced effect of POSS was investigated by Minaee et al. in the system of polypyrrole (PPy)/POSS nanocomposite coating on a copper substrate prepared via cyclic voltammetry technique. Compared to uncoated Cu and PPy/Cu, the corrosion potential for PPy/POSS/Cu was more positive and the corrosion rate of PPy/POSS‐coated Cu was lower, indicating the effective protection against copper corrosion of PPy/POSS coatings [92]. Marine biofouling formed by settlement and accumulation of microorganisms, plants, and animals on surfaces immersed in seawater is a vexing issue. The low surface energy and elastic modulus of PDMS minimize adhesion strength for fouling organisms and make them promising to be applied as antifouling coatings. For example, Zhang et  al. reported a kind of PDMS‐based polyurea (PDMS‐PUa) synthesized from α,ω‐aminopropyl‐terminated PDMS and isophorone diisocyanate (IPDI), which could be reversibly cross‐linked because of strong hydrogen bond interaction. In addition, antifoulant 4,5‐dichloro‐2‐n‐octyl‐4‐isothiazolin‐3‐one (DCOIT) was introduced into PDMS‐PUa with controlled release behavior to improve the antifouling and fouling release performance because PDMS was not effective under static conditions. The resultant coatings exhibited persistent antifouling and fouling release ability for more than six months as well as excellent self‐repairing, enhanced mechanical properties, and good adhesion to substrates [93]. Likewise, POSS can be utilized as antifouling materials taking advantage of their superhydrophobicity and have been increasingly investigated as potential water‐ or oil‐repellent coatings. For example, Sun et  al. deposited branched poly(ethylenimine) (PEI), silver nanoparticles (AgNPs), and fluorinated decyl POSS (F‐POSS) onto cotton fabrics in turn through a simple solution‐dipping method and successfully construct a novel kind of colored cotton fabrics with multiple functions. AgNPs could act as a safe dye for cotton fabrics with antibacterial properties while F‐POSS could autonomically migrate to fabric surface because of low surface energy and confer superhydrophobicity for self‐cleaning properties. As results showed, even damaged by plasma etching and inverted to superhydrophilic surface, such fabrics could quickly restore superhydrophobicity through rearrangement of embedded F‐POSS molecules, improving the durability of cotton fabrics [94]. Flame resistance is an important feature required by materials in quite a lot of practical applications for fire safety. There have been numerous investigations focusing on the use of PDMS as a flame retardant as it can migrate to the material surface and produce a silicaceous char layer during combustion to effectively reduce the flammability of materials. For example, Dong et al. developed a novel kind of flame retardant coupling PDMS with other flame‐retardant elements such as nitrogen, sulfur, and iodine, where part of methyl groups of PDMS were substituted by iodine butyl or sulfonate amino groups. The obtained

1.3 Applications

copolymer, known as (IB‐N‐SA) PDMS, could combine to cotton fabrics with covalent bond because of the activity group and accomplish double functions as flame retardancy and water repellence [95]. Because of the good heat resistance and thermal oxidative stability of POSS, polymer‐POSS nanocomposites with improvement in fire retardance and enhancement in mechanical properties have attracted great attention. For example, Chen et al. grafted octaminopropyl POSS (oapPOSS) and 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene 10‐oxide (DOPO) to the framework of graphene oxide (GO) through reaction of the active functional groups to develop a novel flame retardant, as shown in Figure  1.10, which was then incorporated into polypropylene (PP) matrix to prepare the flame‐retarded PP nanocomposites. By the synergistic effects of three components including the barrier property of graphene sheets to prevent heat and volatile transfer, the rigid shield provided by Si–O–Si structure to improve the thermal oxidative stability, the generation of inert gas from amino group in POSS to dilute the combustible volatiles, and the catalysis of phosphorus in DOPO to promote the char formation, the flame retardancy of the PP matrix was significantly enhanced [96]. The modification and functionalization of materials with the assistance of PDMS or POSS and their derivatives to meet the requirements of applications have been a hotspot in research. A comprehensive introduction is given in Chapters 6–8.

(a)

COOH

OH

OH

O

COOH

O O OH

(b)

R1 Si O Si R1 O O O Si RO R1 Si 1 O O O Si R1 R1 Si O O O R1 Si O Si R1

R2

R2

OC R1 Si O Si R1 O O O Si RO R1 Si 1 O O O Si R R1 Si O 1 O O R1 Si O Si R1

R

OH

D-G

COOH

oapPOSS

R1

R1 R2

OH

R

OH

DCC

R

R R

R

R1 Si O Si R1 O O O Si RO R1 Si O O R1 Si O O Si R1 O O R1Si O Si R1 OC

R2

R

R

OH

P-G-D

R

COOH

HOOC

COOH

GO

OH

HO OH

HO

OH

HOOC

COOH

R

O=P O H

O O

OH

DOPO

R1 Si O O Si O Si O O O R1 Si O O Si O Si

Si R1 O RO 1 Si R1 O R1

R2

R1Si O Si R1 O O O Si RO R1 Si 1 O O R1 Si O O Si R1 O O R1 Si O Si R1

R=DOPO R1=CH2CH2CH2NH2 R2=oapPOSS

Figure 1.10  Application of POSS as flame retardant. Modification of (a) DOPO and (b) oapPOSS onto GO in turn. (DCC: N, N‐dicyclohexylcarbodiimide). Source: Yuan et al. [96].

17

18

1  Introduction of Organosilicon Materials

1.4 ­Conclusion and Outlook The great progress of organosilicon polymer chemistry over the past few decades provides diverse synthetic approaches to produce a variety of organosilicon polymers with controlled architectures, unique properties, and broad application prospects. In particular, because of the ease of synthesis and modification of PDMS and POSS, much effort has been devoted to the investigation of polymers based on these two classes of organosilicon. Meanwhile, although there has been relatively less literature for other classes of organosilicon such as polycarbosilanes, polysilazanes, and polysilanes so far, they also hold the interest of many researchers and are considered to be materials of the future. Nowadays, organosilicon‐based materials are used almost everywhere in our life, while the high demand also becomes a powerful driving force for the perfection and expansion of organosilicon polymers in the field of fundamental research. With the advancement of polymer chemistry, silicon‐containing hybrid copolymers based on reactive functionally terminated silicon‐containing moieties have gained more and more attention in recent years for the intriguing combination of material properties from novel polymeric organosilicon and conventional organic polymer segments, which possess some incomparable advantages over pure organosilicon materials and open up the possibility for new applications. As this book exactly aims to offer a comprehensive and systematic overview of the latest developments in functional hybrid silicon copolymers, we hope the brief introduction of the synthesis, structure, and properties for polymeric organosilicon in this chapter can help to offer a better understanding for the design and preparation of novel silicon‐containing hybrid copolymers in the following chapters.

­References 1 Ganachaud, F., Boileau, S., and Boury, B. (2008). Silicon Based Polymers: Advances

2 3

4 5 6 7 8 9

in Synthesis and Supramolecular Organization. New York: Springer Science & Business Media. Muzafarov, A.M. (2010). Silicon Polymers, vol. 235. Heidelberg: Springer Science & Business Media. Jones, R.G., Ando, W., and Chojnowski, J. (2013). Silicon‐Containing Polymers: the Science and Technology of Their Synthesis and Applications. The Netherlands: Springer Science & Business Media. Auner, N. and Weis, J. (2015). Organosilicon Chemistry‐From Molecules to Materials. New York: Wiley‐VCH. Corey, J.Y. (1989). The Chemistry of Organic Silicon Compounds, vol. 1, 1–56. New York: Wiley. Yilgör, E. and Yilgör, I. (2014). Prog. Polym. Sci. 39: 1165. Zhang, W. and Müller, A.H. (2013). Prog. Polym. Sci. 38: 1121. Miller, R.D. and Michl, J. (1989). Chem. Rev. 89: 1359. Interrante, L.V. and Rathore, J.S. (2010). Dalton Trans. 39: 9193.

­  References

10 Yilgör, İ. and McGrath, J.E. (1988). Polysiloxane Copolymers/Anionic

Polymerization, 1–86. New York: Springer.

11 Sanchez, C., Boissiere, C., Cassaignon, S. et al. (2014). Chem. Mater. 26: 221. 12 Chan, B.Q.Y., Heng, S.J.W., Liow, S.S. et al. (2017). Mater. Chem. Front. 1: 767. 13 Petr, M., Katzman, B., DiNatale, W., and Hammond, P.T. (2013). Macromolecules

46: 2823.

14 Yang, J., Zhou, Q., Shen, K. et al. (2018). RSC Adv. 8: 3705. 15 Semsarzadeh, M.A. and Amiri, S. (2013). J. Inorg. Organomet. Polym. Mater. 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

36 37 38 39 40 41 42

23: 553. Zhang, Z., Hong, L., Gao, Y., and Zhang, W. (2014). Polym. Chem. 5: 4534. Zhang, W. and Müller, A.H. (2010). Macromolecules 43: 3148. Peng, W., Xu, S., Li, L. et al. (2016). J. Phys. Chem. B 120: 12003. Liao, F., Shi, L.‐Y., Cheng, L.‐C. et al. (2019). Nanoscale 11: 285. Zhang, Z., Hou, Z., Qiao, C. et al. (2018). Colloids Surf., B 171: 647. Zhang, P., Zhang, Z., Jiang, X. et al. (2017). Polymer 118: 268. Tanaka, K. and Chujo, Y. (2013). Bull. Chem. Soc. Jpn. 86: 1231. Eduok, U., Faye, O., and Szpunar, J. (2017). Prog. Org. Coat. 111: 124. Li, Z., Kong, J., Wang, F., and He, C. (2017). J. Mater. Chem. C 5: 5283. Kung, M.C., Riofski, M.V., Missaghi, M.N., and Kung, H.H. (2014). Chem. Commun. 50: 3262. Narisawa, M. (2010). Materials 3: 3518. Chen, X. and Dumée, L.F. (2019). Adv. Eng. Mater. 21: 1800667. Misasi, J.M., Jin, Q., Knauer, K.M. et al. (2017). Polymer 117: 54. Czarnecki, S. and Bertin, A. (2018). Chem. A Eur. J. 24: 3354. Ang, J.Y., Chan, B.Q.Y., Kai, D., and Loh, X.J. (2017). Macromol. Mater. Eng. 302: 1700174. Bhattacharjee, N., Parra‐Cabrera, C., Kim, Y.T. et al. (2018). Adv. Mater. 30: 1800001. Rappoport, Z. and Apeloig, Y. (1998). The Chemistry of Organic Silicon Compounds, vol. 2. New York: Wiley. Marciniec, B. (2008). Hydrosilylation: A Comprehensive Review on Recent Advances, vol. 1. Berlin: Springer Science & Business Media. Noll, W. (2012). Chemistry and Technology of Silicones. New York: Academic Press Inc. McGrath, J.E., Riffle, J.S., Banthia, A.K. et al. (1983). Initiation of Polymerization, vol. 212 (eds. F.E. Bailey, E.J. Vandenberg, A. Blumstein, et al.), 145–172. Washington DC: American Chemical Society. Yilgor, I., Riffle, J.S., and McGrath, J.E. (1985). ACS Symposium Series, vol. 282, 161–174. Washington DC: American Chemical Society. Molenberg, A. and Möller, M. (1995). Macromol. Rapid Commun. 16: 449. Gädda, T.M. and Weber, W.P. (2006). J. Polym. Sci., Part A: Polym. Chem. 44: 3629. Ahn, H.W. and Clarson, S.J. (2011). Silicon 3: 157. Kang, D.W. and Kim, Y.M. (2002). J. Appl. Polym. Sci. 85: 2867. Unno, M., Suto, A., and Matsumoto, T. (2013). Russ. Chem. Rev. 82: 289. Hillson, S.D., Smith, E., Zeldin, M., and Parish, C.A. (2005). J. Phys. Chem. A 109: 8371.

19

20

1  Introduction of Organosilicon Materials

43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58

59

60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75

Shea, K.J. and Loy, D.A. (2001). Chem. Mater. 13: 3306. Tanaka, K. and Chujo, Y. (2012). J. Mater. Chem. 22: 1733. Cordes, D.B., Lickiss, P.D., and Rataboul, F. (2010). Chem. Rev. 110: 2081. Hartmann‐Thompson, C. (2011). Applications of Polyhedral Oligomeric Silsesquioxanes, vol. 3. London: Springer Science & Business Media. Koe, J. (2009). Polym. Int. 58: 255. Fossum, E. and Matyjaszewski, K. (1995). Macromolecules 28: 1618. Kabeta, K., Wakamatsu, S., and Imai, T. (1996). J. Polym. Sci., Part A: Polym. Chem. 34: 2991. Sakamoto, K., Yoshida, M., and Sakurai, H. (1990). Macromolecules 23: 4494. Bourg, S., Corriu, R.J., Enders, M., and Moreau, J.J. (1995). Organometallics 14: 564. Birot, M., Pillot, J.‐P., and Dunogues, J. (1995). Chem. Rev. 95: 1443. Procopio, L.J. and Berry, D.H. (1991). J. Am. Chem. Soc. 113: 4039. Tsumura, M., Iwahara, T., and Hirose, T. (1996). J. Polym. Sci., Part A: Polym. Chem. 34: 3155. Kroke, E., Li, Y.‐L., Konetschny, C. et al. (2000). Mater. Sci. Eng., R 26: 97. Jikei, M. and Kakimoto, M. (2001). Prog. Polym. Sci. 26: 1233. Dvornic, P.R. and Owen, M.J. (2009). Silicon‐Containing Dendritic Polymers, vol. 2. Berlin: Springer Science & Business Media. Ponomarenko, S.A. and Kirchmeyer, S. (2010). Conjugated organosilicon materials for organic electronics and photonics. In: Silicon Polymers (eds. S.A. Ponomarenko and S. Kirchmeyer), 33–110. Berlin: Springer. Kwark, Y.‐J., Bravo‐Vasquez, J.‐P., Ober, C.K. et al. (2003). Novel silicon‐ containing polymers as photoresist materials for EUV lithography. In: Advances in Resist Technology and Processing XX, Proceedings of SPIE, vol. 5039 (ed. T.H. Fedynyshyn), 1204–1212. Bellingham, WA: SPIE Press. Lalevee, J., El‐Roz, M., Morlet‐Savary, F. et al. (2007). Macromolecules 40: 8527. Kobayashi, T., Shimura, H., Mitani, S. et al. (2000). Angew. Chem. Int. Ed. 39: 3110. Tang, H., Luo, J., Qin, J. et al. (2000). Macromol. Rapid Commun. 21: 1125. Sharma, A., Katiyar, M., Deepak, S., and Seki, S.T. (2006). Appl. Phys. Lett. 88: 143511. Peuckert, M., Vaahs, T., and Brück, M. (1990). Adv. Mater. 2: 398. Heiner, J., Stenberg, B., and Persson, M. (2003). Polym. Test. 22: 253. De Buyl, F. (2001). Int. J. Adhes. Adhes. 21: 411. Yoo, S.‐S. and Kim, D.‐E. (2013). Int. J. Precis. Eng. Manuf. 14: 875. Hill, R.M. (2002). Curr. Opin. Colloid Interface Sci. 7: 255. Cherney, E.A. (2005). IEEE Trans. Dielectr. Electr. Insul. 12: 1108. Abbasi, F., Mirzadeh, H., and Katbab, A.‐A. (2001). Polym. Int. 50: 1279. Ghanbari, H., Cousins, B.G., and Seifalian, A.M. (2011). Macromol. Rapid Commun. 32: 1032. Leng, Y., Liu, J., Jiang, P., and Wang, J. (2014). ACS Sustainable Chem. Eng. 3: 170. Chen, H.‐L., Jiao, X.‐N., and Zhou, J.‐T. (2017). Funct. Mater. Lett. 10: 1730001. Raimondo, M., Russo, S., Guadagno, L. et al. (2015). RSC Adv. 5: 10974. Hao, X., Jeffery, J.L., Wilkie, J.S. et al. (2010). Biomaterials 31: 8153.

­  References

76 Simmons, A., Padsalgikar, A.D., Ferris, L.M., and Poole‐Warren, L.A. (2008). 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96

Biomaterials 29: 2987. Dong, R., Liu, Y., Mou, L. et al. (2018). Adv. Mater.: 1805033. Dou, M., Dominguez, D.C., Li, X. et al. (2014). Anal. Chem. 86: 7978. Wu, J.T. and Mather, P.T. (2009). Polym. Rev. 49: 25. Yang, Q., Li, L., Sun, W. et al. (2016). ACS Appl. Mater. Interfaces 8: 13251. Li, Y., Xu, B., Bai, T., and Liu, W. (2015). Biomaterials 55: 12. Ge, X., Dong, L., Sun, L. et al. (2015). Nanoscale 7: 7206. Yu, J., Liu, Z., Shen, J. et al. (2017). Macromol. Mater. Eng. 302: 1700142. Finkelmann, H., Kock, H.‐J., and Rehage, G. (1981). Makromol. Chem. Rapid Commun. 2: 317. Wang, M., Sayed, S.M., Guo, L.‐X. et al. (2016). Macromolecules 49: 663. An, Z., Chen, S., Tong, X. et al. (2019). Langmuir 35: 2649–2654. Cheng, C.‐C., Chu, Y.‐L., Chu, C.‐W., and Lee, D.‐J. (2016). J. Mater. Chem. C 4: 6461. Wu, Z., Zhang, S., Li, H. et al. (2015). J. Power Sources 290: 42. Kim, D.‐Y., Kim, S., Lee, S.‐A. et al. (2014). J. Phys. Chem. C 118: 6300. Yahyaei, H. and Mohseni, M. (2018). Polymer/POSS Nanocomposites and Hybrid Materials, 395–413. New York: Springer. Cui, X., Zhu, G., Pan, Y. et al. (2018). Polymer 138: 203. Omrani, A., Rostami, H., and Minaee, R. (2016). Prog. Org. Coat. 90: 331. Liu, C., Ma, C., Xie, Q., and Zhang, G. (2017). J. Mater. Chem. A 5: 15855. Wu, M., Ma, B., Pan, T. et al. (2016). Adv. Funct. Mater. 26: 569. Dong, C., Lu, Z., Zhang, F. et al. (2015). Mater. Lett. 152: 276. Yuan, G., Yang, B., Chen, Y., and Jia, Y. (2019). Composites Part A 117: 345.

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23

2 Reactive Functionally Terminated Polyorganosiloxanes Yuanyuan Pang1, Junqiang Justin Koh1, Zibiao Li2, and Chaobin He1,2 1

National University of Singapore, Department of Materials Science and Engineering, 9 Engineering Drive 1, Singapore 117576, Singapore 2 Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Innovis, #08‐03, Singapore 138634, Singapore

2.1 ­Types of Functionalized Polysiloxane and Their Synthesis Generally, the production of siloxanes starts from organodichlorosilane. Through hydrolysis or methanolysis, linear and cyclic siloxanes can be produced. More often, a mixture of linear and cyclic siloxanes is formed. Cyclic siloxanes are commonly used as the precursor for the synthesis of functional polysiloxanes. Hence, from the mixture, cyclic siloxanes can be separated by distillation. In addition, the linear siloxanes can also be converted into cyclic siloxanes through a heated equilibrating reaction in the presence of potassium hydroxide (KOH) and the constant removal of lower boiling point cyclic siloxanes through distillation [1]. The partial organo nature of polysiloxane allows the wealth of organic synthesis knowledge accumulated for about two centuries to be exploited for the synthesis and reactivity of functionalized polysiloxanes. The most renown of the polysiloxane family is polydimethylsiloxane (PDMS). 2.1.1  Types of Functional Polysiloxanes The different types of functional polysiloxanes are illustrated in Scheme 2.1. The functional group (X) can exist at the terminals or as side groups. The functional group can be attached directly to the terminal silicon atom (Si) (Scheme 2.1a,c,f,h) or via a short organo‐bridge (R) (Scheme 2.1b,d,g,i) between the silicon atom and the functional group. In many cases, Si–X and Si–R–X have significant differences in their reactivity because of the difference in electronegativity between the C (2.55) and Si (1.9) atoms. A classic example would be silanols, which are more acidic [2] than their carbon analogs and many of them are able to be fully

Silicon Containing Hybrid Copolymers, First Edition. Edited by Chaobin He and Zibiao Li. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2020 byWiley-VCH Verlag GmbH & Co. KGaA.

Functional terminal groups Monofunctional terminal groups

Functional group attached to Si atom

R

R R

Si

O

n

R

Si

X

X

R

R

Monosubstituted side groups

X

R O

n

Si

R

X

R

(c)

R R

Si R

(a)

Functional group transit via organobridge

Functional side groups

Difunctional terminal groups

Si

n

R

R

Si

X

R

X

n

Si

X R

R

Si

O

R

n

Si

R

X

R

R

(d)

R X

R

Si R

(e)

n

Si

X

X

X

X

R

R

R

R

X

Si

O

R

n

Si R

(i)

R

X

R

R

Si X

(j)

Scheme 2.1  Types of functionalized polysiloxanes. X and Y represent the functional groups.

O

n

Si

O

Si R

X

X

R O

n

R

X/Y

R

X Y

Si

R

R

(g)

R

n

Si

R

R O

O

R

Y

Different functional groups

X

Si

R

(h)

R

R

(b)

X O

(f)

R O

Si

Disubstituted side groups

Si

m

R

R

Y

X/Y

R

R

2.1  Types of Functionalized Polysiloxane and Their Synthesis

deprotonated in water, forming siloxide (or silanolate). The functional terminals can also exist at either one (monofunctional) (Scheme 2.1a,b) or both (difunctional) end groups (Scheme 2.1c–e) of the polysiloxane chains. 2.1.2  Polysiloxane with Monofunctional Terminals The most common way to synthesize polysiloxanes with monofunctional terminals is via the anionic ring‐opening polymerization (ROP) of cyclic siloxanes, commonly hexamethylcyclotrisiloxane (D3), initiated by strong bases such as organolithium compounds (Scheme 2.2) [3]. This living polymerization can be terminated by chlorosilane molecules. A functional terminal group can either be introduced by the organo‐segment of the initiator or by the chlorosilane molecule [4]. It is also noteworthy that such a synthetic route also allows the formation of difunctional terminals with different functional groups introduced by both the initiator and the terminator at each ends (Scheme 2.1e). Me

Me Si

– R1

Li

+

+ Me

O

O

Si

Si

Me Me

R1

O Me

Me D3

Si

O

Si

Me O

Si



O Li

+

n

Me

Me Cl

Si

Me

Me

Me

R2

Me Me R1

Si

Me O

Si

Me O

Si

Me O

Si

n

Me

Me

Me

R2

Me

Scheme 2.2  Living ROP of cyclic siloxanes, D3, initiated by organolithium compound, terminated by chlorosilane molecule.

Nonetheless, monofunctional terminated polysiloxanes can be rather useful in many areas. For example, they can be used to control the degree of cross‐linking for cross‐linked polysiloxanes. In addition, they can be employed in the synthesis of diblock copolymers containing polysiloxanes, which can be applicable for the production of surfactants. Some of the applications of these monofunctional polysiloxanes will be discussed in further detail in subsequent sections and chapters. 2.1.3  Polysiloxane with Difunctional Terminals Difunctional terminated polysiloxanes are usually synthesized via a rather ­different route from their monofunctional terminated counterparts. The t­ erminator mole-

25

26

2  Reactive Functionally Terminated Polyorganosiloxanes

cules, oligomeric siloxane or even disiloxane with hydride (Si–H) end groups, are first functionalized through a hydrosilylation reaction with a reagent containing a vinyl group. As shown in Scheme 2.3a, hydrosilylation is the reaction between the Si–H and the double bond, catalyzed either by metal‐containing compounds (e.g. chloroplatinic acid), radical precursors (e.g. peroxides), amine complexes, or aluminum chloride [5]. The functional group of interest is introduced at the terminals by the vinyl‐containing terminator molecules.

Me H

(a)

Si

Me O

Me

Si H n Me

Me X

Catalyst

X Si

R

R

+

Me

X O

Me

n>1

Si n Me

R

n>1

Me Me X R

Si

Si

Me O

Me

Si n Me

Me

X

+

R

Me Me

n>1

Si

Si

O O

Me

Si Me

Δ

Me

O

O

Me

Catalyst Me X R

(b)

Si

Me O

Me

Si m Me

X R

+

Cyclic siloxanes

m>n

Scheme 2.3  Synthesis of polysiloxanes with difunctional terminals. (a) Functionalization of the end‐capped siloxane molecules through hydrosilylation. (b) Redistribution polymerization (ring‐chain equilibration) of D4 with functionalized end‐capped siloxane molecules.

Subsequently, the functionalized terminator undergoes redistribution polymerization (ring‐chain equilibration) together with cyclic siloxanes, most commonly octamethylcyclotetrasiloxane (D4), as illustrated in Scheme  2.3b [6–8]. The average molecular weight of the difunctionalized polysiloxane can be controlled by the ratio of the functionalized terminator and the cyclic siloxane monomers. Such reactions are usually conducted at an elevated temperature between 50 and 100 °C, catalyzed by strong acids (e.g. sulfuric acid, trichloroacetic acid, and sulfonic acids) or strong bases (e.g. sodium hydroxide, potassium hydroxide, and quaternary ammonium hydroxide). The choice of a suitable catalyst depends on the reactivity of the functional group, such that the catalyst only cleaves the ionic Si∙O bonds leaving the functional group untouched. A general guideline would be to employ strong acids as a catalyst if the functional group is acidic (e.g. carboxyl), and vice versa for basic (e.g. amine) functional group. Functional groups such as epoxy and amine can be tricky because of their high reactivity

2.1  Types of Functionalized Polysiloxane and Their Synthesis

with strong acids and bases at elevated temperature. At the end of the ring‐chain equilibration, the product is a still mixture of linear and cyclic siloxanes (lesser than the initial amount), at which the lower boiling point cyclic siloxanes can be removed by vacuum distillation. The functionally terminated polysiloxanes can also undergo a second‐step functionalization, to alter or include new functional groups, through many well‐established conventional organic chemistry techniques. As such, there are many possible synthetic routes to functionalize the terminal groups with the desired functional groups. Scheme  2.4 illustrates some of the common reactions for second‐step functionalization of the frequently used hydroxyl and primary amine‐terminated polysiloxanes. For example, hydroxyl and amine‐terminated polysiloxanes can undergo esterification and amidification, respectively, with carboxylic acids (Scheme  2.4a). Besides the formation of the ester and amide linkages, the new functional group can be introduced by the carboxylic acid molecule. Reactions with isocyanates are also common for hydroxyl and primary amine‐terminated polysiloxanes (Scheme 2.4b), through the formation of urethane and urea linkages, respectively. The ability of the primary amine to undergo reaction twice (via secondary amine) also allows the formation of tetra‐functional terminals. An example is shown in Scheme 2.4b, through addition reactions [9–11]. 2.1.4  Polysiloxane with Functional Side Groups Other than functional terminals, functional side groups can also be attached on the polysiloxane main chain (Scheme  2.5a). This can be achieved through the above‐mentioned ROP of cyclotetrasiloxane. However, instead of the usual D4 or D3, which yields the renown PDMS, the methyl group(s) attached to the Si atom of the cyclic siloxanes can be substituted by various functional groups before polymerization (Scheme  2.5b) [12–14]. These functionalized cyclic oligomers can be polymerized in a similar manner to yield polysiloxanes with functional side groups. In addition, it is also possible to copolymerize cyclic siloxanes that possess different functional groups to form polysiloxane copolymer with different functional side groups (Scheme 2.1j). The side groups attached can be inert, for example, phenyl or fluropropyl, similar to the methyl side group. Regardless of its reactivity, the side group affects the physiochemical properties of the polymer. A classic example would be the poly(dimethylsiloxane‐r‐diphenylsiloxane) copolymer. Generally, the copolymer has properties between that of the homopolymers and becomes a solid when the diphenylsiloxane component exceeds a certain fraction [14, 15]. The above‐mentioned method is the most common route to acquire polysiloxanes with functional side groups. Other than such chemical synthetic processes, physical methods can also be applied to functionalize the main chain of polysiloxanes. Previously, ultraviolet radiation/ozone, as well as plasma treatment, had been applied to convert the inert methyl groups of PDMS into reactive silanol groups (Scheme 2.5c) [16, 17]. However, in contrast to the chemical method, the functionalization of these physical methods only occurs at the surface of the bulk polysiloxane.

27

Me

O Me HO

R

Si

Me O

Si

n

Me



HOOC

R







O

R

Ester

OH R´







H N

C

Si

Me O

Me Me

O

OCN

Me

C

O

R

Si

O

Me O

Me H2N

R

Si Me

OC

Si

n

Me





R

NH2

ha



el a

dd



Si





H N

Si

n

Urea

H N

R

Si

n



H N







H N



O R

O

C

Si

n

H N

C

2° Amide

Me Me O

Si

n

O R

H N

C Urea

Me

Me

Me

Me

X´ R´



R

Si Me



Scheme 2.4  Examples of further functionalizing (a) hydroxyl‐ and (b) amine‐terminated polysiloxanes.



O R



itio



Urethane

Me

C

C Ester

Me O

Me

3° Amine N

(b)

R

O

OCN Mic

Me H N

C

O

Me

2° Amide

HO

Me O





n

O R

Me Me

Urethane

(a)

Si

O

Si

n

Me

R´ R

N 3° Amine R´ X´



X R R

Si

O

Si

R

(a)

X

R

+

X

R

O

Si

Si

O O

X

Me

R (b)

Si

O

R

Si

R

R

+

Me

Me

Si

(c)

Me

X

Me

O Si Me

Me

Si

+

Me X

X

O

O

Si

Si

O O

R Me

Si Me

n

O

R

Si

n

X

R

Δ Catalyst

R

Si R

X

Me O

Si Me

O

n

R

Si

O

Si

m

R

R

Me

X

UV/ozone or plasma treatment O

R

Si

Me

Me

Si

O

R

Si

O

Si

X

O

O Me

R

Catalyst

Me Si

R

X

R Δ

X

Si X

R

X

O

Si

R

OH Si

O

n

Me

Scheme 2.5  Examples of synthesis/fabrication route to achieve polysiloxanes with functional side groups. (a) Redistribution polymerization of functionalized cyclotetrasiloxane. (b) Copolymerization of D4 and functionalized cyclotetrasiloxane. (c) Physical method to convert the inert methyl groups on the surface of PDMS into reactive silanol groups.

30

2  Reactive Functionally Terminated Polyorganosiloxanes

2.2 ­Functionalized Polysiloxane as Macromers This section discusses the utilizations of functionalized polysiloxanes as macromers to derive polysiloxanes with an increased degree of polymerization, cross‐linked polysiloxanes, as well as block and graft copolymers containing polysiloxanes. 2.2.1  Modifying Degree of Polymerization of Functionalized Polysiloxanes Polycondensation is commonly used to increase the molecular weight of hydroxyl‐terminated polysiloxanes (Scheme 2.6a). The reaction is usually conducted in the presence of acid catalysts such as polychlorophosphazenes and water produced during the reaction are constantly removed (e.g. vacuum environment) [1]. The catalyst can be deactivated by ammonia or amines once the desired chain length has been achieved [1]. Next, other than the usual manipulation of reaction time and temperature for the polycondensation reaction, there are several other techniques to control the molecular weight of the final product. An important part to note for polycondensation is that it is usually conducted at elevated temperature, at which equilibration of the polysiloxane chains occurs as well. However, polycondensation is known to occur much more rapidly than equilibration. This knowledge can be applied to manipulate the chain length of the final polysiloxane products. For example, short siloxane chains can be included into the reaction together with an extended reaction time for equilibration reaction to take place, and the final molecular weight can be controlled/reduced. Another straightforward manner to manipulate the degree of polymerization, as shown in Scheme 2.6b, is through the inclusion of monofunctional hydroxyl‐terminated polysiloxanes. Similarly, other routes that utilize difunctionally terminated polysiloxanes as macromers to increase the molecular weight of polysiloxanes are also possible based on other well‐established organic chemistry synthesis routes. The polymerization can occur between two different difunctionally terminated polysiloxanes, whose generic route is shown in Scheme 2.6c. An example would be the reaction between diisocyanate‐terminated polysiloxanes and dihydroxyl‐terminated polysiloxanes resulting in the formation of urethane linkages between them. Similarly, coupling can also be achieved via difunctional molecules, as shown in Scheme 2.6d [18–20]. However, it is noteworthy that the physiochemical properties of the final product can be greatly affected by the presence of the linkages, depending on factors such as the proportion of PDMS to linkage and the nature of the linkages. 2.2.2  Cross‐Linking of Functionalized Polysiloxanes A typical way of utilizing functionalized polysiloxanes is for the purpose of cross‐ linking or, in other words, curing. The curing of polysiloxanes, such as PDMS, allows the formation of a solid elastomer, which otherwise exist as a liquid largely

R

R HO

O

Si

(a)

n

H

+

HO

(b)

Si

n

H

+

HO

(c)

R

X

Si

(d)

R

R O

Si

O

n

+

Y

R

R

R1

m

(PNCl2)x

Si

n

R

R1

Si

Si

m

Y

R

Si

X

Si R

R

R R o = Xn, X = 0, 1, 2, 3...

Si

Si

n

R

O

R Linkage

n

Si

Linkage

Si R

Si

m

R1

+

H2O

R

Si

m

Y

R

R R

O

R O

R

R

Linkage

o

Si

R

R

m

Si

R

O

R O

H2O

Si

R O

R

R

Y

+

H

m

R O

R

R

+

Si

X

Y

O

R

R O

R

X

n

Si

R

R

R O

O

R

Si

R X

R

Si

R

Si R

R Si

HO

R

R O

R

X

m

(PNCl2)x

H

R

R R

HO

R O

Si

R O

Si

n

X

R

Scheme 2.6  Polysiloxanes as macromers to increase molecular weight. (a) Typical polycondensation reaction between two dihydroxyl‐terminated polysiloxane chains. (b) Example of using monofunctional hydroxyl‐terminated polysiloxane chains to end‐capped polycondensation reaction in order to manipulate the molecular weight of the final products. (c) Generic synthetic route to couple two different difunctional terminated polysiloxanes chains, with the formation of a linkage. (d) Generic synthetic route to couple difunctional polysiloxane chains with a difunctional coupling agent. Note: For simplicity, this figure illustrates the coupling between two polysiloxane chains but coupling need not be between two macromers.

32

2  Reactive Functionally Terminated Polyorganosiloxanes

because of their low intermolecular interactions. The cross‐linking mechanism for polysiloxanes can be classified into two types: it can occur between the functional groups of the functionalized polysiloxanes or via a cross‐linking agent that usually possesses multiple functional groups. One of the earliest used curing technique is radical generation through the decomposition of organic peroxide regent [1, 21, 22]. Polysiloxanes possessing vinyl side group are preferably included for such peroxide curing systems, where the radical generated can react with another vinyl group or with another radical. However, previous reports have shown that it is possible for methyl side groups’ hydrogens to be abstracted by certain organic peroxide regents, which lead to cross‐links of two radical combinations [22]. For example, a PDMS system with the vinyl content cannot be cured with di‐tert‐butyl peroxide, but curing is possible using benzoyl peroxide. Another important curing technique that involves two different functionalized polysiloxanes is hydrosilylation, where hydride silicone functional groups react with unsaturated bonds, under the presence of catalysts, usually platinum, to form a linkage. For example, polysiloxanes containing methylhydrosiloxane unit react with another polysiloxane chain‐containing vinyl side groups, resulting in the formation of ethylene linkages [1, 23–25]. An example is illustrated in Scheme 2.7a. Such cross‐linking methods without cross‐linking molecules usually involve at least one precursor polysiloxane component with functional side groups. On the other hand, multifunctional molecules can be used to cross‐link functionalized polysiloxanes. One classical route is condensation using tri‐ or tetra‐ functional silanes with hydrolyzable group, such as alkoxysilanes, acetoxysilanes, oxime silanes, and aminosilanes [1]. These cross‐linking molecules can undergo condensation reaction with hydroxyl groups of the polysiloxanes, releasing small volatile molecules as side products (Scheme 2.7b). For example, for the case of alkoxysilane as a cross‐linking agent, alcohol side products are produced. Such curing techniques are usually able to react even at room temperature because of the high reactivity of the cross‐linking molecules (even with ambient humidity), hence known as room temperature vulcanization. Besides such traditional techniques, other multifunctional cross‐linking molecules, for example, trimesoyl chloride, are also able to cross‐link amine‐ or hydroxyl‐terminated polysiloxanes (Scheme  2.7c) [26]. Usually, these sorts of cross‐linking techniques that utilize multifunctional (more than two) cross‐linking molecules are employed on polysiloxanes with functional terminal groups. In addition, the cross‐links between polysiloxanes can be formed by dynamic/ reversible covalent bonds. Examples of such bond formation include Diels–Alder reactions, transesterification reactions, reversible imine bonds, and disulfide bonds [27–30]. Generally, polysiloxanes cross‐linked by these reversible covalents are remoldable, hence recyclable, and at times able to induce self‐healing abilities. Scheme  2.8a shows an example of PDMS cross‐linked by reversible imine bonds, which is reported to be able to undergo autonomous self‐healing without external stimuli. Similar to dynamic covalent bonds, supramolecular interactions are reversible. Supramolecular interactions such as hydrogen bonding, metal–ligand

R

Me

Me R

Si

O

Si H

o

O

p

HO

R

Si

+

Si

Si

Me

OH H2N

R

R

Si

O

n

n

Me

Si

O

MeO

R

Si

O

o

Si

O

p

R

Polysiloxane chain

Me

Si

O

Si

n

Si Me

O

O

(b)

H N

O

Si

O

Polysiloxane chain

R

R

m

R

R

Si O

Cl

R Si

Me

Me

Si

O Cl

Sn catalysts

Me

Si

O

OMe

Me

Me

(a) Me

+

OMe

Polysiloxane chain

R

NH2

R

Me

m

Pt

R

Si

Me O

O

Me

Me

+

Me R

n

R

Me

Cl

R O

Polysiloxane chain

Polysiloxane chain

(c)

Si R

O

O R

R

NH

HN

R

Si R

Polysiloxane chain

Scheme 2.7  Examples of functionalized polysiloxane cross‐linking. (a) Hydrosilylation curing between polysiloxane chains containing methylhydrosiloxane unit and polysiloxane chains containing vinyl side groups. (b) Condensation curing of hydroxyl‐terminated polyslioxanes using methyltrimethoxysilane as a cross‐linker. (c) Cross‐linking of amine‐terminated polysiloxanes using trimesoyl chloride as the cross‐linking agent.

O Me H2N

R

Si

Me R Si n Me

O

Me

+

NH2

O

O

Me Me Polysiloxane chain

Polysiloxane chain

N

Si R

Si R

NH2

O

Me

Me

Me

Me

Me N

N

Si

O

Si

Si

Polysiloxane R chain Me

R Me

Polysiloxane chain

Polysiloxane chain

R

Me

O H2 N

NH2

Si R

Me

Polysiloxane

Me chain

(a) Polysiloxane chain

Polysiloxane chain

R1 Me R

Si Me

R1

R1

Me O

o

Si R1

O

R p CoCl2

N

N

N

Co2+ N

R1

(b)

Polysiloxane chain

Polysiloxane chain

R1

N

Polysiloxane chain

N

N Co2+ N

R1 Polysiloxane chain

R1 Polysiloxane chain

N

R1 Polysiloxane chain

Scheme 2.8  Examples of utilizing functionalized polysiloxanes to form reversible cross‐linking bonds. (a) Formation of reversible imine bonds between amine‐terminated polysiloxanes and triformyl as cross‐linking molecules. (b) Formation of reversible metal–ligand coordination bond between polysiloxanes containing pyridine side groups and cobalt ions. Red components indicate the reversible bonds.

2.2  Functionalized Polysiloxane as Macromers

­coordination, and π‐stacking interactions have been applied to cross‐link polysiloxane systems in order to induce self‐healing abilities [9, 31, 32]. In the example shown in Scheme 2.8b, polysiloxane‐containing pyridine side groups are cross‐ linked by cobalt ions through the formation of cobalt (II) pyridine coordination complex. Besides being self‐healable at room temperature, the material also exhibits solvatochromic properties [32]. On the other hand, PDMS cross‐linked by 2,6‐pyridinedicarboxamide with iron(III) centers are reported to possess the ability to self‐heal even at temperatures as low as 20 °C [32]. 2.2.3  Polysiloxane‐Containing Block and Graft Copolymers Because of the availability of a wide range of reactively functional polysiloxanes and their excellent properties, the incorporation of polysiloxanes, mainly PDMS, as “soft” segments into organic polymer materials has been of great interest over the past few decades and has been used to prepare block, graft, or segmented copolymers for special technical applications. In this section, the synthesis of polysiloxane‐containing copolymers based on condensation and addition reactions will be provided. 2.2.3.1  Polysiloxane‐Containing Segmented and Multiblock Copolymers by Step‐Growth Polymerization

Step‐growth polymerization has been widely utilized for the synthesis of ­polysiloxane‐containing segmented and multiblock copolymers. Hydroxyl‐ and amine‐terminated polysiloxanes are versatile starting materials leading to the formation of silicone–urethane, silicone–urea, silicone–ester, silicone–amide, silicone–imide, and other polysiloxane‐containing segmented/multiblock copolymers. Table 2.1 lists the typical linkage structures between silicone segments and the combined organic segments obtained from the reaction of hydroxyl/ amine‐terminated polysiloxanes and isocyanate/carboxyl/dianhydride/halogen‐ functionalized monomers. Poly(Siloxane–Urea) and Poly(Siloxane–Urethane)  The addition reactions of alcohols

and amines with isocyanates lead to the formation of urethane and urea linkages, respectively (Table  2.1). Poly(siloxane–urea) and poly(siloxane–urethane) are usually synthesized by reacting diisocyanates with telechelic PDMS. The commonly used diisocyanates have been listed in Table 2.2. Because amines are much more nucleophilic than the hydroxyl groups, urea formation reactions take  place almost instantaneously at room temperature. On the other hand, silicone–urethane copolymers are usually prepared at 60–80 °C in the presence of catalysts [33]. In general, a two‐step approach is commonly used for conducting step‐growth polymerization to prepare segmented poly(siloxane–urea)s and poly(siloxane– urethane)s [6]. Take the polymerization procedure of poly(siloxane–urethane) for instance, the first step involves reaction of excess diisocyanate with the amine/hydroxyl‐terminated siloxane oligomer to form the isocyanate‐­terminated “prepolymer” . This is followed by the “chain extension” step in which the prepolymer is reacted with an organic diol (and possibly additional diisocyanate)

35

36

2  Reactive Functionally Terminated Polyorganosiloxanes

Table 2.1  Typical linkage groups and synthetic methods for the polysiloxane–organic segmented and multiblock copolymers. Entry

Linkage

1

Urethane

2

Urea

Chemical structure O H N C O

Synthetic method

Isocyanate with hydroxyl/amine

O H N

3

Ester

O

4

Amide

O

5

Imide

O

C

C

C

H N

Esterification

O

Amidification

H N

N O

6

Ether

7

Carbonate

Nucleophilic substitution

O O O

Esterification O

Table 2.2  Isocyanate structures used for the synthesis of poly(siloxane–urea/urethane)s. Diisocyanate

Abbreviation

1,4‐/1,3‐Phenylene

PPDI/MPDI (1,4‐phenylene diisocyanate/1,3‐phenylene diisocyanate)

2,4‐Tolylene

Chemical structure NCO

NCO

TDI (2,4‐tolylene diisocyanate)

4,4′‐Methylenedipenyl MDI (4,4′‐methylenedipenyl diisocyanate) Hexamethylene

HDI (hexamethylene diisocyanate)

1,4‐Cyclohexyl

CHDI (1,4‐cyclohexyl diisocyanate)

Isophorone

IPDI (isophorone diisocyanate)

OCN

CH3

OCN

H2 C

OCN

OCN

OCN

Bis(4‐ isocyanatocyclohexyl)

HMDI (bis(4‐isocyanatocyclohexyl))

NCO / OCN

OCN

OCN

(CH2)6

NCO

NCO

NCO

NCO

H2 C

NCO

2.2  Functionalized Polysiloxane as Macromers

to form the high molecular weight poly(siloxane–urethane). The architecture of the obtained copolymer is composed of organic linkage as the hard segment and silicone part as the soft segment (Scheme 2.9) [34–38]. Similar to the synthesis of poly(siloxane–urethane)s, poly(siloxane–urea)s can also be prepared by the two‐ step route using diamine‐terminated siloxane oligomers and diisocyanate chain extenders. Copolymers produced by this two‐step route generally have a more homogeneous distribution of hard and soft segments.

Me x HO R1

Si O Me

(y-x) O C N R2

Me + yO C N

Si R1 OH

n

Me

(Prepolymer formation)

O H N C O

Me R1

Si O Me

+ m HO

R2 N C O

Me

O

Si R1 O C n Me

H N

R2

N C O x

R3 OH (Chain extension)

R2

O H N C O

Me R1

Si O Me

Me

O

Si R1 O C n Me

H N

R2

x

O O H H N C O R3 O C N m

Scheme 2.9  The two‐step synthetic route for poly(siloxane–urethane).

Another procedure, known as the “one‐shot” method, has been widely used for poly(siloxane–urea/urethane) synthesis as well. In this method, all the reagents are mixed together at the desired stoichiometric ratio. The reaction usually takes place in bulk to give a fairly random distribution of soft and hard segments along the macromolecular backbone [39, 40]. Generally, the desired mechanical properties can be achieved by controlling the contents of hard segments (urea or urethane units), while the soft silicone segments are in a rubbery state to provide the flexibility. In addition, a large number of variables, such as the molecular weight of polysiloxane, the ratio of siloxane and diisocyanate, the structure of diisocyanate, and chain extender, will influence the morphology and properties of poly(siloxane–urethane/urea)s [6, 41–48]. Both poly(siloxane–urea) and poly(siloxane–urethane) block copolymers can form hydrogen bonding interactions between the urethane or urea linkage. Urethane groups form the monodentate intermolecular hydrogen bond, while urea groups form the much stronger bidentate hydrogen bond with each other. These hydrogen bonds can lead to the formation of reinforced and cross‐linked networks [49, 50].

37

38

2  Reactive Functionally Terminated Polyorganosiloxanes

Poly(Siloxane–Ester)  This part will be focused on the synthesis of siloxane–ester multiblock or segmented copolymers by step‐growth polymerization, while the preparation of block copolymers (polycaprolactone–PDMS and polylactide– PDMS) by ROP using hydroxyl‐terminated PDMS as macroinitiators will be provided in Section 2.3.3. Several condensation methods have been reported for the preparation of ester linkages between polysiloxane and organic segments, as illustrated in Scheme 2.10. The most commonly used method is direct polycondensation of diacids and diols in the presence of catalysts (Scheme 2.10a, b) [51–56]. In addition, transesterification of diols and diesters catalyzed by tetra‐n‐butyl titanate or lipase has also been used to synthesize silicone–ester segmented/block copolymers [57–59]. Another method is to react the polysiloxane diols with acid chloride, using triethylamine or pyridine as a hydrochloric acid acceptor [60–62]. Me m HO R1

Me

Si O

n

Me

(a)

m HOOC R1

Me

Me

Me

Si O

Si R1 COOH + m HO R2 OH

(c)

m HO R1

Si O

Me n

Me

Me

Me

Me

O

O

Si R1 OH + m Cl C R2 C Cl

Me

Si O

Catalyst -H2O

R1

Catalyst -H2O

R1

Me

Me m HO R1

(d)

n

Me

(b)

Me

Si R1 OH + m HOOC R2 COOH

TEA/pyridine -HCl

O O Catalyst Si R1 OH + m Me O C R2 C O Me -MeOH n Me

R1

Me

Si O

O

Si R1 O C R2

n

Me

Me

Me

Me

Si O

Si R1 C O R2

n

O

Me

Me

Me

Me

Si O

Si R1 O C R2

Me

n

Si O Me

m

O

Me

Me R1

m

Me

m

O

Si R1 O C R2 n Me

m

Scheme 2.10  Synthetic methods of poly(siloxane–ester). (a) and (b) Direct polycondensation reaction of diacids and diols. (c) Esterification reaction of diols and acid chloride. (d) Transesterification reaction of diols and diesters.

Transition metal catalysts such as tin or titanium complexes can be used to perform the esterification reaction between alcohols and acids at high temperature. For instance, dicarboxylic acid‐terminated poly(ethylene glycol) (PEG) was copolymerized with dihydroxy propyl‐terminated PDMS using dibutyltin oxide as a catalyst at 180 °C under N2 purge to give the PDMS–PEG block copolymers [52]. However, if the poly(siloxane–ester) materials are to be employed in biomedical or agrichemical applications, or for other environmental considerations, the utilization of toxic metallic catalyst will be undesirable. Therefore, enzymatic polymerization has been receiving increased attention because it is a “green” pathway to the new materials. The immobilized lipase B from Candida antarctica (CALB), sold under the name of Novozym 435, has been used for synthesizing silicone‐containing polyesters in solvent or solvent‐free system under mild condition (50–80 °C) [54]. The molecular weights (Mn) of the copolymer products would depend on the reaction temperature, pressure, enzyme activity, and

2.2  Functionalized Polysiloxane as Macromers

enzyme concentration. In general, there is a positive correlation between Mn with these reaction conditions [54]. Similar to the direct esterification of diols and diacids, transesterification of diols and diesters has been used for the synthesis of poly(siloxane–ester). A series of poly(siloxane–ester)s with different mass ratios of hard poly(butylene terephthalate) (PBT) to soft PDMS segments have been synthesized by transesterification of dimethyl terephthalate and methylester of carboxypropyl‐terminated PDMS with 1,4‐butanediol as a chain extender in melt [59]. In addition, if the acid groups are first transferred to acyl chloride groups for the activation of the polycondensation process, esterification reactions between chloride groups and diol‐functionalized PDMS will occur in the presence of triethylamine/pyridine as the acid acceptor. This reaction has been proven to be effective and fast for the preparation of poly(siloxane–ester)s [60–62]. Poly(Siloxane–Amide) and Poly(Siloxane–Imide)  As compared to siloxane–urethane,

siloxane–urea, and siloxane–ester segmented or multiblock copolymers, studies on poly(siloxane–amide) are fairly limited. The most common synthetic route for poly(siloxane–amide) is a one‐pot, two‐step process as shown in Scheme  2.11. The first step involves the preparation of α,ω‐dichloroformyl‐terminated amide oligomers. In the second step, the terminal‐functionalized amide oligomers react with aminopropyl‐terminated PDMS oligomers to form PDMS–amide segmented copolymers [63–65]. Reactions are conducted in solution at low temperature in the presence of triethylamine as a hydrogen chloride acceptor. Moreover, similar to the synthesis of ester linkage, poly(siloxane–amide) can also be made by the one‐shot reaction between acyl chloride and diamine groups [66–68], or transesterification reaction of diamine‐terminated PDMS with dimethyl terephthalate (DMT) under vacuum. CALB is usually used as an enzymatical catalyst in the transesterification reaction. However, the molecular weight of resulted copolymers in this method turns out to be fairly low [56]. In contrast to poly(siloxane–amide), silicone–imide copolymers have been extensively investigated. This is because they combine the excellent thermal and O

(1)

O

H2N R1 NH2 + Cl C R2 C Cl

Me (2) mH2N (CH2)3 Si O Me

TEA -HCl

Me

TEA -HCl

O

O

Cl C R2 C

O

O

Si (CH2)3 NH2 + m Cl C R2 C n

O O H H N R1 N C R2 C

O O H H N R1 N C R2 C

Me

Me H N (CH2)3 Si O Me

Me n

O

O

Si (CH2)3 NH C R2 C

O O H H N R1 N C R2 C

Me

Scheme 2.11  One‐pot, two‐step synthesis method of poly(siloxane–amide). TEA; triethylamine.

x

Cl

x

Cl

xm

39

40

2  Reactive Functionally Terminated Polyorganosiloxanes

thermooxidative stability, chemical resistance, mechanical and electrical properties of high‐performance polyimides together with the extremely high flexibility, good solubility, high gas permeability, hydrophobicity, fire retardancy, and interesting surface properties of silicones. Most studies conducted so far on the poly(siloxane–imide)s have focused on the relationship between the composition, content, and molecular weight of the segments and properties thereby affected, while the approach used in poly(siloxane–imide) synthesis is relatively simple [69– 74]. Generally, methods for preparing conventional polyimide were performed for the poly(siloxane–imide) synthesis. Typically, the copolymers can be prepared by condensation of diamine‐terminated PDMS, aromatic dianhydride, and sometimes an additional aromatic diamine as a comonomer, using a one‐pot solution method [75–80]. High‐boiling nonpolar solvent, such as ortho‐dichlorobenzene (ODCB), is preferred in this method. This is because the polymerization temperature has to be raised to ~180 °C for imidization to take place and to remove the generated water (Scheme 2.12a). In addition, a two‐step method is also popular for the synthesis of poly(siloxane–imide). First, diamines and dianhydride react in a dipolar aprotic solvent at subambient temperature to form high‐molecular weight polyamic acid solution. Then, the polyamic acid can be imidized by a thermal process at elevated temperature [72, 73, 81–85], or a solution process in the presence of acetic anhydride and triethylamine (Scheme 2.12b) [71, 86]. O

O (x + y) O

Ar

N

n

Si R NH2 + yH2N Ar´ NH2 Me

O

N

Ar

Me

N R

Si O

O

O

n

Me

O

O

Me Si R

x m

Me

Me

Me

N N Ar´ N N R Si O Si R Ar Ar H y H x n H H HOOC COOH m Me Me HOOC COOH O

O Imdizing

y

O

O

(b)

N Ar´

Me

O

O

O

NMP 25 °C

Me

O

O

(a)

Si O

O

O

ODCB 180 °C

Me

O + xH2N R

Ar

N O

Ar

N Ar´ O

O

O y

N O

Ar

N R O

Me Si O Me

Me n

Si R Me

x m

Scheme 2.12  (a) One‐step method and (b) two‐step method for synthesis of poly(siloxane‐imide).

Other Types of  Polysiloxane‐Containing Segmented and  Multiblock Copolymers  In

addition to the above‐mentioned polysiloxane‐based segmented and multiblock copolymers, poly(siloxane–sulfone), poly(siloxane–carbonate), and a variety of

2.2  Functionalized Polysiloxane as Macromers

combinatorial copolymers obtained from polycondensation have been extensively studied. For instance, poly(siloxane–sulfone) can be obtained in solution by the condensation reaction of chloro‐terminated polysulfone (PSU) oligomers and α,ω‐ dihydrogensilyl‐PDMS [87], or by the reaction of chloro‐terminated PDMS and hydroxyl‐terminated polysulfone [88]. Besides, the transesterification reaction has also been used for the preparation of PDMS–polycarbonate (PDMS–PC) multiblock copolymers. Reaction between α,ω‐bis(bisphenol A)‐terminated PDMS and α,ω‐bis(ortho‐nitrophenylcarbonate) end‐capped PC yields multiblock copolymers and ortho‐nitrophenol as the by‐product [89, 90]. In addition, intensive efforts have been spent on investigating the preparation and characterization of silicone‐containing copolymers with mixed PDMS, ­polyurea/urethane, polyamide/imide, polyether, polyester, and polycarbonate segments or blocks [91–98]. This shows the versatility and diversity of utilizing polysiloxanes as macromers to achieve a variety of segmented/multiblock copolymer structures, in order for them to meet the performance requirements in various applications. 2.2.3.2  Polysiloxane‐Containing Graft Copolymers

Generally, polysiloxane‐containing graft copolymers can be divided into two groups, in which polysiloxane constitutes the backbone or as the side chains. Synthetic methods for each type will be discussed separately in the following part. It should be noted that polysiloxanes may not exactly act as macromers in some of the methods, in particular, graft copolymers with polysiloxane as backbones. However, they are also mentioned in this section to describe the preparation of polysiloxane‐containing graft copolymers using macromolecular siloxanes. Graft Copolymers with  Polysiloxane as  Side Chains  When the polysiloxane is

functionalized with a polymerizable group, such as an ethylenic double bond, at one chain end, it is able to polymerize or copolymerize with common vinyl monomers to produce graft copolymers via a “grafting through” method. ω‐Methacryloyl‐PDMS and ω‐acryloyl‐PDMS are two of the most commonly used macromers for the formation of silicone‐containing graft copolymers via radical copolymerization [99–101]. On the other hand, controlled radical polymerization methods, including atom transfer radical polymerization (ATRP) and reversible addition fragmentation chain transfer (RAFT) polymerization, have been extensively investigated for the synthesis of well‐defined PDMS‐based graft copolymers [102–108]. In addition, norbornenes with oligosiloxane substituents have been synthesized and polymerized via ring‐opening metathesis polymerization to form a variety of graft copolymers with interesting self‐assembly properties [109–111]. Other than the above‐mentioned “graft through” method with the polysiloxane‐based macromers, the “graft to” route for preparing copolymers with polysiloxane side chains has also been investigated [112, 113]. For example, the hydrosilylation reaction between polybutadiene (PB) and ω‐silane‐PDMS will give rise to the PB‐g‐PDMS copolymers [112]. When the main chain is a polycondensation polymer, polysiloxanes may also be attached as the side chains via “graft through” or “graft to” method. Examples

41

42

2  Reactive Functionally Terminated Polyorganosiloxanes

of this type of copolymers include polyimide‐g‐PDMS [114], PSU‐g‐PDMS [115], and PC‐g‐PDMS [89, 116]. Graft Copolymers with  Polysiloxane as  Backbones  Poly(hydromethylsiloxane)

(PHMS) or its copolymers with dimethylsiloxane may be used for the formation of graft copolymers with polysiloxane backbones. It has been reported that polysiloxane‐g‐PEG can be synthesized by hydrosilylation of ω‐allyl‐PEG [117, 118]. In addition, specific functional groups can be introduced into the polysiloxane backbone first, and these functional groups can be used to graft side chains in the next step. Furthermore, polysiloxanes with specially functionalized side groups can act as the macroinitiators in ring‐opening or radical polymerizations and have been developed for the formation of graft copolymers with polysiloxane backbones [119]. Polysiloxanes as macroinitiators will be discussed in detail in Section 2.3.

2.2.3.3  Polysiloxane‐Containing Copolymers by Hydrosilylation and Click Chemistry Hydrosilylation  Hydrosilylation is a well‐established reaction for the preparation

of organo‐silicon compounds, in which vinyl groups react with silanes (Si–H) catalyzed by transition metal complexes, typically Pt(II) complexes. Hydrosilylation polymerization has proven to be an extremely effective method for the synthesis of many types of organic–inorganic hybrid silicones. The large variety of research works on hydrosilylation have been reviewed by Marciniec et al. [120] Polysiloxane‐containing segmented or multiblock copolymers can be synthesized by the hydrosilylation‐type step‐growth polyaddition between difunctionally terminated polysiloxanes with Si∙H terminal bonds and organic monomers/ oligomers with α,ω‐allyl groups. Similar to other step‐growth polymerization reactions, stoichiometric ratio of the reactants is extremely important to the molecular weights. Because polysiloxanes with Si∙H terminal bonds are commercially available and can be easily prepared, polyhydrosilylation becomes a convenient route for the synthesis of new functional materials and has been extensively studied in the past few decades [121]. However, the isomerization of the allyl group leads to an inhibition of the addition reaction and prevents the formation of high molecular weights [122]. In addition to the segmented or multiblock copolymers, di‐ and triblock copolymers obtained by hydrosilylation reaction have been developed. For example, amphiphilic PDMS–PEG diblock copolymers can be synthesized from ­hydrosilane‐terminated PDMS and allyl‐terminated PEG homopolymers via the equimolar hydrosilylation reaction in the presence of platinum catalyst [123, 124]. Similarly, PSU–poly(alkylene oxide)–PDMS (PSU–PAO–PDMS) triblock copolymers can be obtained by addition of the preformed α,ω‐bis(hydrogensilyl) poly(dimethylsiloxane) oligomers to allyl end‐capped PSU–PAO [125, 126]. Poly(3‐hexylthiophene)–PDMS (P3HT–PDMS) can also be synthesized as a donor material for organic photovoltaic devices by hydrosilylation between allyl‐ terminated P3HT and PDMS–SiH [127]. Hydrosilylation is one of the most powerful reactions with high efficiency in silicone chemistry. It has been widely used to functionalize polysiloxanes and to

2.3  Functionalized Polysiloxane as Macroinitiators and Macrochain Transfer Agents

prepare linear polyorganosiloxanes. Moreover, hydrosilylation can also be used to construct cross‐linked organic–inorganic hybrid silicones, as well as the graft copolymers. Click Chemistry  Click reaction is a kind of highly efficient organic reactions that

are stereospecific, quickly proceeding, and high yielding and produces negligible by‐products. Examples include the Cu‐catalyzed azide/alkyne cycloaddition (CuAAC), thiol‐ene, and Diels–Alder reaction. The CuAAC and thiol‐ene reactions have been demonstrated to be applicable for the fabrication of diverse silicone‐containing copolymer architectures [128–141]. They are simple methods to conjugate a variety of components together. So far, the CuAAC reaction has been used for the synthesis of graft and block copolymers. For instance, the coupling between azide‐terminated PDMS and alkyne‐ terminated organic oligomers leads to the formation of diblock copolymers of PDMS–PMMA [137], PDMS–PS [137], and PDMS–PEG [139], as well as a triblock copolymer of poly(oxazoline)–PDMS–poly(oxazoline) [134]. Moreover, the click reactions, especially the thiol‐ene reactions, have been widely used to cross‐link poly(organo‐siloxane)s [129–132]. Indeed, click chemistry has been extensively used to functionalize the end groups or pendant groups of polysiloxanes. Thus, the combination of click chemistry with controlled radical polymerization techniques and other organic reactions has become more powerful in the construction of novel well‐defined silicone‐containing copolymers [142].

2.3 ­Functionalized Polysiloxane as Macroinitiators and Macrochain Transfer Agents This section will discuss the utilization of functionalized polysiloxanes as macroinitiators or macrochain transfer agents (macro‐CTA) to synthesize block copolymers. We will focus on the preparation of polysiloxane macroinitiators/ macro‐CTAs and polymerization methods used for the formation of p ­ olysiloxane‐ based copolymers. 2.3.1  Conventional Radical Polymerization Polysiloxane macroinitiators with thermally decomposable groups are generally used for the synthesis of siloxane‐vinyl block copolymers. Thermolysis of this kind of macroinitiators generates macroradicals, which are able to initiate the radical polymerization in the presence of vinyl monomers. The most commonly used polysiloxane macroinitiators for conventional radical polymerization contain the azo groups or peroxy groups [143]. Silicone macro‐azo‐initiator (MAI) can be synthesized by condensation (­esterification/amidification) or hydrosilylation reactions between functionalized azo precursor and PDMS‐containing reactive groups [144]. For instance, polycondensation of 4,4'‐azobis‐4‐cyanovaleryl chloride and α,ω‐bis(3‐hydroxypropyl)polydimethylsiloxane yields the poly(azo‐containing siloxane ester)s

43

44

2  Reactive Functionally Terminated Polyorganosiloxanes

(Scheme 2.13a) [145]. The silicone‐containing MAI has been used to initiate the radical polymerization of various vinyl monomers, including styrene, acrylates and methacrylates, vinyl esters, and acrylamides, and resulting in the formation of a variety of poly(siloxane‐vinyl) copolymers [144, 146–150]. The molecular weight of these block copolymers are determined by the monomer/azo ratio, reaction temperature, macroinitiator molecular weight, and the composition of the initial mixture. Me m H2N (CH2)3 Si O Me

(a)

O

Si (CH2)3 NH2 + m Cl

C (CH2)2 C

n Me

Me

Me

O

N N C (CH2)2 C

CN

Cl

CN

Me

Si O Me

n

Si R1 OH + 2 O C N R2 N C O Me

O H O C N R2 N C O

Me R1

Me

Si O Me

O

O H Si R1 O C N n

R2

N C O + 2 tBu O OH

Me Me

O

H H tBu O O C N R2 N C O (b)

Me

Me Me Me Me O O H N (CH2)3 Si O Si (CH2)3 NH C (CH2)2 C N N C (CH2)2 C n m Me Me CN CN

TEA -HCl

HO R1

Me

R1

Si O Me

Me

O H Si R1 O C N

n Me

R2

O H N C O O tBu

Scheme 2.13  Synthesis of (a) azo macroinitiators and (b) peroxycarbamate macroinitiators.

Other types of widely used polysiloxane macroinitiators for conventional radical polymerization are those with peroxycarbamate and peroxyester groups. Hydroxyl‐terminated PDMS are first reacted with excessive amount of diisocyanate; subsequently, the isocyanate chain ends can further react with tert‐butyl hydroperoxide to produce peroxycarbamate‐based polysiloxane macroinitiator (Scheme  2.13b). The peroxide‐based macroinitiator has been reported to initiate the polymerization of styrene, methyl methacrylate, itaconate diesters, and vinyl pyrrolidone; however, the presence of free tert‐butoxy radicals may lead to the undesirable homopolymer contamination in the copolymer products [151]. Conventional radical polymerization has been widely used at an industrial level because it does not require reagents of high purity like living anionic polymerization. In addition, radical polymerization is applicable to a wide range of vinyl monomers. Moreover, such polymerization can take place in dispersed aqueous media instead of organic solvent media. Unfortunately, most of the conventional radical polymerization techniques do not allow very good control of  the molecular weight distribution and may lead to extensive homopolymer contamination.

2.3  Functionalized Polysiloxane as Macroinitiators and Macrochain Transfer Agents

2.3.2  Controlled Radical Polymerization With the development of controlled radical polymerization (CRP, or called “living” radical polymerization) techniques in polymer science, a great interest has grown in associating polysiloxanes with other organic monomers to produce new functional materials. CRP techniques have allowed the controlled synthesis of a large number of polysiloxane‐containing block and graft copolymers with predefined structures and molecular weights. Because of the variable functionalization of polysiloxane, macroinitiators for almost all the controlled radical polymerization methods, including ATRP, RAFT, nitroxide‐mediated radical polymerization (NMP), and iodine‐transfer polymerization (ITP), can be utilized to synthesize polysiloxane‐containing copolymers. 2.3.2.1  Atom Transfer Radical Polymerization (ATRP)

ATRP was first reported by Matyjaszewski et  al. It has been widely used in the construction of well‐defined polymers with complex architectures and functionalities [152–154]. The typical ATRP process involves the generation of radicals from the homolytic cleavage of the alkyl halide bond by a transition metal catalyst in lower oxidation state. The radicals then initiate the polymerization or become a dormant species by reacting with the halide atom (typically Br or Cl) of the transition metal complex to regenerate the alkyl halide [155, 156]. This is a fast and efficient dynamic activation–deactivation equilibrium and is essential in ATRP for synthesis of polymers with predefined molecular weight and narrow dispersion. ATRP is an efficient approach to combine polysiloxanes, mainly PDMS, as a backbone into block copolymers. This is because hydroxyl‐ or amine‐terminated PDMS can be easily converted to bromoalkyl‐ or chloroalkyl‐terminated PDMS by the esterification or hydrosilylation reaction into ATRP macroinitiators [6, 143]. The most commonly used PDMS‐based macroinitiators bear chlorobenzyl or bromo‐isobutyrate end groups. Structures of the reported PDMS macroinitiators and the monomers that can be initiated by them are listed in Table 2.3. Most importantly, ATRP should be conducted in a medium of a matched transition metal complex as a catalyst (typically is a CuI/N‐containing ligand system), an appropriate solvent together with suitable reaction temperature and time. Matyjaszewski et al. were the first to report the preparation of polystyrene– PDMS–polystyrene (PS–PDMS–PS) triblock copolymers by using benzyl chloride‐terminated PDMS macroinitiators [157]. Then, the preparation of ­ PDMS–poly(methyl methacrylate) (PMMA) and PDMS–poly(n‐butyl acrylate) (PnBA) di‐ and triblock copolymers was developed by ATRP using 2‐bromoisobutyrate‐ or benzyl chloride‐terminated PDMS macroinitiators [158–160, 163]. Following these pioneer works of Matyjaszewski et  al., ATRP has been applied to a large variety of monomers for the preparation of polysiloxane‐containing block copolymers (Table 2.3) [161, 162, 164–167, 172–179]. ATRP brings optimal results for the controlled synthesis of complex architectures from PDMS oligomers and gives the desired copolymer structures with well‐defined molecular weight and acceptable monomer conversions. Furthermore, the halogen end groups of polymers prepared by ATRP can undergo nucleophilic substitution reactions to transform into other functional groups [168].

45

2.3  Functionalized Polysiloxane as Macroinitiators and Macrochain Transfer Agents

2.3.2.2  Reversible Addition Fragmentation Chain Transfer (RAFT) Polymerization

RAFT polymerization has been widely used in the construction of well‐defined polysiloxane‐containing copolymers with complex architectures. The mechanism of RAFT has been proposed and illustrated in detail in several review papers [180–185]. It is generally based on the principle of degenerative chain transfer, with the dithiocarbonyl group as a transfer agent. Initiation of the polymerization reaction is accomplished by conventional thermal, photochemical, or redox methods. Chemical initiators, such as α,α'‐azoisobutyronitrile (AIBN), are widely used for initiating the RAFT polymerization. More importantly, the success of RAFT polymerization is highly dependent on the suitability of the RAFT transfer agent for specific monomers as well as the reaction medium. A polysiloxane‐based macromolecular chain transfer agent (macro‐CTA) is required for the preparation of silicone‐containing block copolymers via RAFT polymerization. Typically, two conventional esterification pathways can be used to prepare polysiloxane macro‐CTAs (representative synthetic routes are shown in Scheme 2.14). One is the direct one‐pot esterification of commercially available hydroxyl‐terminated PDMS with carboxylic acid functional trithiocarbonate RAFT agents (Scheme  2.14a). The other is through nucleophilic substitution reactions between hydroxyl‐terminated PDMS and acyl chloride‐functionalized RAFT agents (Scheme 2.14b). Besides these two conventional methods, ­xanthate‐ capped PDMS as macro‐CTA can be prepared by a two‐step route of esterification and substitution reaction in sequence (Scheme 2.14c) [186, 187].

(a)

Me

Me

Si O

Si R1 OH + HO C C S C S R2

n

Me

O Me

Me

Me

O Me

S

S

HO C C S C S R2

Oxalyl chloride

DCC/DMAP CH2Cl2

O Me

(c)

Me

Si O

Si R1 OH + Cl C C S C S R2

O Me

S

Me

Me

Me

Me

Me

Me

Si O

Si R1 OH + Br C

C Br

n

O

Me

Me

Si O

Si R1 O C C Br + KS C Me

TEA

Me

Me

n

n

O Me

S

Me

Me

Me

Me

Me

O Me

Si O

Si R1 O C C S C S R2

S

S

O CH2 Me

n

Me

Si O

Si R1 O C C Br

n

O Me

Me

Me Me

Me

Si O

Si R1 O C C S C

Me

S

Me

Me

Me

Me

O Me

Me

TEA

Me

Me

Me

Si R1 O C C S C S R2

Me

Me n

Me

Si O

Cl C C S C S R2

Me

(b)

Me

n

Me

O Me

Me

S

O CH2 Me

Scheme 2.14  Representative synthetic route of the PDMS‐based macro‐CTAs. (a) One‐pot esterification reaction. (b) Nucleophilic substitution reaction. (c) Two‐step route with esterification and substitution reactions. DCC; dicyclohexylcarbodiimide, DMAP; 4‐dimethylaminopyridine.

47

50

2  Reactive Functionally Terminated Polyorganosiloxanes

A variety of PDMS‐based macro‐CTAs can be produced by the above‐mentioned methods (Table 2.4), and RAFT process can be used for controlled polymerization of a wide range of monomers, including styrene [191], methacrylates [186, 188–190, 197, 198], acrylates [192, 195], acrylamides [193, 199], vinyl amide [187], and vinyl pyridines [196]. By controlling the molar ratio of initiator, macro‐CTA and monomer, the reaction temperature and time, silicone‐containing block copolymers with predefined architectures can be obtained with desired molecular weight and low polydispersity. The primary advantage of RAFT method is that the polymerization can be rather well controlled. Moreover, the polymer products will not be contaminated by the metal salts, unlike ATRP. Nonetheless, the drawback of RAFT is the decomposition of polymer chain ends, which may produce harmful by‐products, odor, and cause discoloration. Fortunately, some methods to eliminate the RAFT end group from the polymer have been established [200]. 2.3.2.3  Other Controlled Radical Polymerization Methods

In addition to the widely used ATRP and RAFT methods, NMP and ITP have also been reported as possible pathways to prepare the polysiloxane‐containing block copolymers. Polysiloxane precursors used in NMP techniques can be synthesized by the anionic polymerization of D3 from a (2,2,6,6‐tetramethylpiperidin‐1‐yl)oxyl (TEMPO)‐based lithium salt [201, 202]. Styrene has been the most widely studied monomer for NMP with PDMS precursors [201–206]. NMP produces polymers with low polydispersity indexes (PDIs), but high reaction temperature is required [207]. In addition, TEMPO has a low efficiency in controlling the polymerization of other monomers besides styrene and its derivatives. These drawbacks limit the application of NMP in the synthesis of PDMS‐based block copolymers. ITP relies on the use of alkyl iodides as transfer agents. PDMS‐based macro‐ CTAs can be synthesized by esterification reaction between hydroxyl‐terminated PDMS and 2‐bromopropionic acid. Subsequently, the end‐capped bromine can be substituted by iodine to form a macro‐CTA involved in ITP [208–211]. ITP is a low‐cost method, and it is applicable in dispersed aqueous medium. PDMS–PS [208] and PDMS–PVAc [209–211] di‐ or triblock copolymers have been successfully synthesized by ITP. However, the main drawback of ITP is the less‐controlled efficiency of block length and molecular weight distribution in comparison with other CRP methods, which will lead to ill‐defined architecture of the polymer products. 2.3.3  Ring‐Opening Polymerization (ROP) Preparation of polysiloxane‐containing block copolymers using ROP has been  widely demonstrated. The type of block copolymer obtained by ROP is

2.3  Functionalized Polysiloxane as Macroinitiators and Macrochain Transfer Agents

­ etermined by the nature of the starting siloxane oligomer. α,ω‐Reactive funcd tionally terminated macroinitiators will lead to the ABA‐type triblock or branched copolymers, while monofunctionalized macroinitiators will lead to the AB‐ or AxB‐type block copolymers. The molecular weight of the obtained copolymers can be controlled by the ratio of the cyclic monomers in the reaction mixture to the polysiloxane macroinitiators. The most commonly reported method is anionic ring‐opening polymerization (AROP) with organometallic compounds such as tin octoate or dibutyltin dilaurate, as catalyst. Commercially available polysiloxanes with hydroxyl or amine end groups can initiate the polymerization of cyclic esters, such as ε‐caprolactone (ε‐CL) or lactide, to give the silicone‐containing block copolymers (Table 2.5). For instance, AROP reaction is first initiated by the attack of the highly nucleophilic alkoxide end group of PDMS to the electropositive ester carbonyl of ε‐CL monomer, and then the oligomeric active species propagate by continuously attacking the ε‐CL monomers to form a triblock copolymer of PCL (polycaprolactone)–PDMS–PCL (Scheme 2.15) [213]. This kind of triblock copolymers are terminated by hydroxyl groups on both ends. As a result, they can be used as reactive oligomers for the preparation of novel segmented polyurethanes, polyesters, or other copolymers as described in Section 2.2.3.1. Similar to PCL–PDMS–PCL, AROP of d, l‐lactide, l‐lactide, or γ‐benzyl‐ l‐glutamate–N‐carboxyanhydride (BLG–NCA) with α,ω‐hydroxyl/amine‐ terminated PDMS as microinitiators lead to the formation of PLA (poly(d,l‐lactide))–PDMS–PLA, PLLA (poly(l‐lactide))–PDMS–PLLA, or PBLG (poly(γ‐benzyl‐l‐glutamate))–PDMS–PBLG triblock copolymers [212, 215]. If the monohydroxyl/amine‐terminated polysiloxanes were used for initiating AROP reaction, diblock copolymers such as PDMS–PCL or PDMS–PLA will be obtained [217–226]. In addition, by employing polysiloxane with two or more functionalities at each or both chain ends will produce block copolymers with branched architectures, e.g. Y‐ or H‐shaped block copolymers [216, 227]. These branched copolymers have attracted significant research attention because of their interesting bulk mechanical and morphological behavior compared to their linear counterparts. In addition to the above‐mentioned AROP method, cationic ring‐opening polymerization (CROP) has also been reported for preparation of silicone‐based block copolymers. The CROP approach begins with the conversion of a commercially available α,ω‐bishydroxypoly(dimethylsiloxane) into a macroinitator, e.g. converting the hydroxyl group into a trifluoromethanesulfonic acid ester or a tosylate. Then, CROP initiates by the forming of oxazolinium cations from 2‐ methyl‐2‐oxazoline. Propagation occurs via nucleophilic attack of the monomer on the oxazolinium species, resulting in the formation of amide by isomerization [228, 229].

51

54

2  Reactive Functionally Terminated Polyorganosiloxanes O

Me O + H2N (CH2)3 Si O Me

O H

O (CH2)5 C

m

H N

O

Me

O

Si (CH2)3 NH2 + n Me

Me (CH2)3 Si O Me

n

Catalyst

Me

O

Si (CH2)3 NH

C (CH2)5 O

H m

Me

Scheme 2.15  Synthesis of PCL–PDMS–PCL triblock copolymers via ROP of ε‐CL using an amine‐terminated PDMS macroinitiator.

­References 1 Moretto, H.H., Schulze, M., and Wagner, G. (2000). Ullmann’s Encyclopedia of

Industrial Chemistry. Weinheim: Wiley‐VCH.

2 Lickiss, P.D. (1995). Adv. Inorg. Chem. 42: 147–262. Chojnowski, J., Cypryk, M., Fortuniak, W. et al. (2003). Controlled synthesis of 3

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

all siloxane‐functionalized architectures by ring‐opening polymerization. In: Synthesis and Properties of Silicones and Silicone‐Modified Materials, vol. 838, 12–25. Washington, DC: American Chemical Society. Elkins, C.L. and Long, T.E. (2004). Macromolecules 37: 6657–6659. Marciniec, B. (2008). Hydrosilylation: A Comprehensive Review on Recent Advances. The Netherlands: Springer Science & Business Media. Yilgör, E. and Yilgör, I. (2014). Prog. Polym. Sci. 39: 1165–1195. Yilgör, İ. and McGrath, J.E. (1988). Polysiloxane copolymers/anionic polymerization, 1–86. Berlin, Heidelberg: Springer Berlin Heidelberg. Jacobson, H. and Stockmayer, W.H. (1950). J. Chem. Phys. 18: 1600–1606. Zhang, D.‐D., Ruan, Y.‐B., Zhang, B.‐Q. et al. (2017). Polymer 120: 189–196. Genest, A., Portinha, D., Fleury, E., and Ganachaud, F. (2017). Prog. Polym. Sci. 72: 61–110. Chruściel, J.J. and Leśniak, E. (2015). Prog. Polym. Sci. 41: 67–121. Yur’evich, K.A., Ivanov, V.S., Yegorov, A.S., and Men’shikov, V.V. (2018). Orient. J. Chem.: 34. Evmenenko, G., Yu, C.J., Kmetko, J., and Dutta, P. (2002). Langmuir 18: 5468–5472. Morariu, S., Brunchi, C.E., Cazacu, M., and Bercea, M. (2011). J. Chem. Eng. Data 56: 1468–1475. Evmenenko, G., Mo, H., Kewalramani, S., and Dutta, P. (2006). Langmuir 22: 6245–6248. Xiao, D., Zhang, H., and Wirth, M. (2002). Langmuir 18: 9971–9976. Sprung, M.M. and Guenther, F.O. (1961). J. Organomet. Chem. 26: 552–557. Yılgör, E. and Yılgör, İ. (2001). Polymer 42: 7953–7959. Sirrine, J.M., Schexnayder, S.A., Dennis, J.M., and Long, T.E. (2018). Polymer 154: 225–232. Su, X. and Shi, B. (2016). React. Funct. Polym. 98: 1–8.

­  References

21 Barrall Ii, E.M., Hawkins, R., Fukushima, A.A., and Johnson, J.F. (1984). J. Polym.

Sci. Polym. Symp. 71: 189–202.

22 Dunham, M.L., Bailey, D.L., and Mixer, R.Y. (1957). Industr. Eng. Chem. 49:

1373–1376.

23 Sturgess, C., Tuck, C.J., Ashcroft, I.A., and Wildman, R.D. (2017). J.Mater. Chem. 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

C 5: 9733–9743. González‐Rivera, J., Iglio, R., Barillaro, G. et al. (2018). Polymers 10. Zhan, X., Cai, X., and Zhang, J. (2018). RSC Adv. 8: 12517–12525. Fu, Q., Kim, J., Gurr, P.A. et al. (2016). Energy Environ. Sci. 9: 434–440. Zhang, H., Cai, C., Liu, W. et al. (2017). Sci. Rep. 7: 11833. Zhang, B., Zhang, P., Zhang, H. et al. (2017). Macromol. Rapid Commun. 38: 1700110. Sun, J., Pu, X., Liu, M. et al. (2018). ACS Nano 12: 6147–6155. Zhao, J., Xu, R., Luo, G. et al. (2016). J. Mater. Chem. B 4: 982–989. Burattini, S., Colquhoun, H.M., Greenland, B.W., and Hayes, W. (2009). Faraday Discuss. 143: 251–264. Li, C.‐H., Wang, C., Keplinger, C. et al. (2016). Nat. Chem. 8: 618. Yilgör, I., Yilgör, E., and Wilkes, G.L. (2015). Polymer 58: A1–A36. Hernandez, R., Weksler, J., Padsalgikar, A., and Runt, J. (2007). Macromolecules 40: 5441–5449. Hernandez, R., Weksler, J., Padsalgikar, A. et al. (2008). Macromolecules 41: 9767–9776. Choi, T., Weksler, J., Padsalgikar, A., and Runt, J. (2009). Polymer 50: 2320–2327. Choi, T., Weksler, J., Padsalgikar, A., and Runt, J. (2010). Polymer 51: 4375–4382. Riehle, N., Thude, S., Götz, T. et al. (2018). Eur. Polym. J. 101: 190–201. Liu, Y., Zhu, D., Sun, J. et al. (2018). Polym. Adv. Technol. 29: 2064–2071. Grübel, M., Meister, S., Schulze, U. et al. (2016). Macromol. Chem. Phys. 217: 59–71. Sheth, J.P., Yilgor, E., Erenturk, B. et al. (2005). Polymer 46: 8185–8193. Yilgor, I., Eynur, T., Yilgor, E., and Wilkes, G.L. (2009). Polymer 50: 4432–4437. Yilgor, I., Eynur, T., Bilgin, S. et al. (2011). Polymer 52: 266–274. Yildirim, E., Yurtsever, M., Yurtsever, E. et al. (2012). J. Inorg. Organomet. Polym. Mater. 22: 604–616. Yilgor, E., Atilla, G.E., Ekin, A. et al. (2003). Polymer 44: 7787–7793. Sheth, J.P., Aneja, A., Wilkes, G.L. et al. (2004). Polymer 45: 6919–6932. Adhikari, R., Gunatillake, P.A., and Bown, M. (2003). J. Appl. Polym. Sci. 90: 1565–1573. Adhikari, R., Gunatillake, P.A., McCarthy, S.J. et al. (2003). J. Appl. Polym. Sci. 87: 1092–1100. Yilgor, E., Yilgor, I., and Yurtsever, E. (2002). Polymer 43: 6551–6559. Sheth, J.P., Klinedinst, D.B., Wilkes, G.L. et al. (2005). Polymer 46: 7317–7322. Antic, V.V., Balaban, M.R., and Djonlagic, J. (2001). Polym. Int. 50: 1201–1208. Rutnakornpituk, M., Ngamdee, P., and Phinyocheep, P. (2005). Polymer 46: 9742–9752. Guo, L., Zhang, Z., Zhu, Y. et al. (2008). J. Appl. Polym. Sci. 108: 1901–1907. Poojari, Y., Palsule, A.S., Cai, M. et al. (2008). Eur. Polym. J. 44: 4139–4145. Frampton, M.B., Subczynska, I., and Zelisko, P.M. (2010). Biomacromolecules 11: 1818–1825.

55

56

2  Reactive Functionally Terminated Polyorganosiloxanes

56 Poojari, Y. and Clarson, S.J. (2010). Macromolecules 43: 4616–4622. 57 Ardal, D., Yilgor, E., and Yilgor, I. (1999). Polym. Bull. 43: 207–214. 58 Antic, V.V., Govedarical, M.N., and Djonlagic, J. (2004). Polym. Int. 53:

1786–1794.

59 Antic, V.V., Govedarica, M.N., and Djonlagic, J. (2003). Polym. Int. 52:

1188–1197.

60 Cazacu, M., Munteanu, G., Racles, C. et al. (2006). J. Organomet. Chem. 691:

3700–3707.

61 Cazacu, M., Vlad, A., Marcu, M. et al. (2006). Macromolecules 39: 3786–3793. 62 Tokoro, Y., Miyoshi, M., and Oyama, T. (2017). J. Photopolym. Sci. Technol. 30:

177–180.

63 Abduallah, A.B.E. and Mallon, P.E. (2010). J. Appl. Polym. Sci. 115: 1518–1533. 64 Kishida, A., Kanda, T., Furuzono, T. et al. (2000). J. Appl. Polym. Sci. 78:

2198–2205.

65 Senshu, K., Furuzono, T., Koshizaki, N. et al. (1997). Macromolecules 30:

4421–4428.

66 Khan, M.S.U., Akhter, Z., Naz, T. et al. (2013). Polym. Int. 62: 319–334. 67 Gill, R., Mazhar, M., and Siddiq, M. (2010). Polym. Int. 59: 1598–1605. 68 Henkensmeier, D., Abele, B.C., Candussio, A., and Thiem, J. (2004). Polymer 45: 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89

7053–7059. Sysel, P., Patrova, A., Lanc, M., and Friess, K. (2018). e‐Polym. 18: 105–110. Pei, X., Chen, G., Liu, J., and Fang, X. (2015). Polymer 56: 229–236. Jiang, X., Gu, J., Shen, Y. et al. (2011). J. Appl. Polym. Sci. 119: 3329–3337. Othman, M.B.H., Ramli, M.R., Tyng, L.Y. et al. (2011). Mater. Des. 32: 3173–3182. Shoji, Y., Ishige, R., Higashihara, T. et al. (2013). Macromolecules 46: 747–755. Othman, M.B.H., Ahmad, Z., Akil, H.M. et al. (2015). Mater. Des. 82: 98–105. Ghosh, A. and Banerjee, S. (2008). J. Appl. Polym. Sci. 107: 1831–1841. Pareek, K., Ghosh, A., Sen, S.K., and Banerjee, S. (2010). Des. Monomers Polym. 13: 221–236. Ghosh, A., Banerjee, S., Haeussler, L., and Voit, B. (2010). J. Macromol. Sci. Part A: Pure Appl.Chem. 47: 671–680. Ghosh, A., Banerjee, S., and Voit, B. (2010). High Perform. Polym. 22: 28–41. Ghosh, A., Sen, S.K., Dasgupta, B. et al. (2010). J. Membr. Sci. 364: 211–218. Cazacu, M., Vlad, A., Airinei, A. et al. (2011). Dyes Pigm. 90: 106–113. Hamciuc, C., Hamciuc, E., Cazacu, M., and Okrasa, L. (2008). Polym. Bull. 59: 825–832. Shoji, Y., Ishige, R., Higashihara, T. et al. (2010). Macromolecules 43: 805–810. Shoji, Y., Ishige, R., Higashihara, T. et al. (2010). Macromolecules 43: 8950–8956. Pei, X., Chen, G., and Fang, X. (2011). High Perform. Polym. 23: 625–632. Hamciuc, C., Hamciuc, E., Serbezeanu, D. et al. (2011). Polym. Int. 60: 312–321. Jiang, X., Gu, J., Shen, Y. et al. (2011). Desalination 265: 74–80. Bercea, M., Airinei, A., Hamciuc, V., and Ioanid, A. (2004). Polym. Int. 53: 1860–1865. Hamciuc, V., Giurgiu, D., Marcu, M. et al. (1998). J. Macromol. Sci., Pure Appl. Chem. 35: 563–575. Islam, M.M., Seo, D.‐W., Jang, H.‐H. et al. (2011). Macromol. Res. 19: 1278–1286.

­  References

90 van Aert, H.A.M., Nelissen, L., Lemstra, P.J., and Brunelle, D.J. (2001). Polymer

42: 1781–1788.

91 Rutnakornpituk, M. and Ngamdee, P. (2006). Polymer 47: 7909–7917. 92 Choi, T., Masser, K.A., Moore, E. et al. (2011). J. Polym. Sci., Part B: Polym.

Phys. 49: 865–872.

93 Chakrabarty, S., Nisenholt, M., and Wynne, K.J. (2012). Macromolecules 45:

7900–7913.

94 Chavan, J.G., Rath, S.K., Praveen, S. et al. (2016). Prog. Org. Coat. 90: 350–358. 95 Kozakiewicz, J., Przybylski, J., and Sylwestrzak, K. (2016). Polym. Adv. Technol.

27: 258–265.

96 Yi, T., Ma, G., Hou, C. et al. (2017). J. Appl. Polym. Sci. 134: 44927. 97 Qiu, H., Holken, I., Gapeeva, A. et al. (2018). Materials 11: 2413. 98 Dandeniyage, L.S., Adhikari, R., Bown, M. et al. (2019). J. Biomed. Mater. Res. B

Appl. Biomater. 107: 112–121.

99 Hou, Y.X., Tulevski, G.S., Valint, P.L., and Gardella, J.A. (2002). Macromolecules

35: 5953–5962.

100 Martinelli, E., Guazzelli, E., Glisenti, A., and Galli, G. (2019). Coatings 9: 153. 101 Huber, R.O., Beebe, J.M., Smith, P.B. et al. (2018). Macromolecules 51:

4259–4268.

102 Shinoda, H. and Matyjaszewski, K. (2001). Macromol. Rapid Commun. 22:

1176–1181.

103 Hong, S.C., Neugebauer, D., Inoue, Y. et al. (2003). Macromolecules 36: 27–35. 104 Neugebauer, D., Zhang, Y., Pakula, T., and Matyjaszewski, K. (2005).

Macromolecules 38: 8687–8693.

105 Li, J., Yi, L., Lin, H., and Hou, R. (2011). J. Polym. Sci. Part A: Polym. Chem. 49:

1483–1493.

106 Halim, A., Gurr, P.A., Blencowe, A. et al. (2013). Polymer 54: 520–529. 107 Lejars, M., Margaillan, A., and Bressy, C. (2013). Polym. Chem. 4: 3282. 108 Pavlović, D., Lafond, S., Margaillan, A., and Bressy, C. (2016). Polym. Chem. 7:

2652–2664.

109 Cheng, L.C., Gadelrab, K.R., Kawamoto, K. et al. (2018). Nano Lett. 18:

4360–4369.

110 Rokhlenko, Y., Kawamoto, K., Johnson, J.A., and Osuji, C.O. (2018).

Macromolecules 51: 3680–3690.

111 Pesek, S.L., Lin, Y.H., Mah, H.Z. et al. (2016). Polymer 98: 495–504. 112 Ciolino, A.E., Pieroni, O.I., Vuano, B.M. et al. (2004). J. Polym. Sci. Part A:

Polym. Chem. 42: 2920–2930.

113 Jin, Z., Fan, H., Li, B.‐G., and Zhu, S. (2016). Polymer 83: 20–26. 114 Ghosh, A., Banerjee, S., Wang, D.‐Y. et al. (2012). J. Appl. Polym. Sci. 123:

2959–2967.

115 Chen, Z., Yang, J., Guan, S. et al. (2010). J. Appl. Polym. Sci. 118: 2434–2441. 116 Mollah, M.S.I., Seo, D.W., Islam, M.M. et al. (2011). J. Macromol. Sci. A 48:

400–408.

117 Pricop, L., Hamciuc, V., Marcu, M. et al. (2005). High Perform. Polym. 17:

303–312.

118 Yao, M. and Fang, J. (2012). J. Micromech. Microeng. 22: 025012. 119 Wang, G., Xiong, Y., and Tang, H. (2015). J. Organomet. Chem. 775: 50–54.

57

58

2  Reactive Functionally Terminated Polyorganosiloxanes

120 Marciniec, B., Maciejewski, H., Pietraszuk, C., and Pawluc, P. (2009).

121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150

Hydrosilylation: A Comprehensive Review on Recent Advances. The Netherlands: Springer. Putzien, S., Nuyken, O., and Kühn, F.E. (2010). Prog. Polym. Sci. 35: 687–713. Sargent, J.R. and Weber, W.P. (1999). Macromolecules 32: 2826–2829. Li, D., Li, C., Wan, G., and Hou, W. (2010). Colloids Surf. A 372: 1–8. Kickelbick, G., Bauer, J., Hüsing, N. et al. (2003). Langmuir 19: 3198–3201. Rusu, G.G., Airinei, A., Hamciuc, V. et al. (2014). Superlattices Microstruct. 65: 91–105. Bercea, M., Airinei, A., and Hamciuc, V. (2017). Polym. Eng. Sci. 57: 114–118. Tsuchiya, K., Ando, K., Shimomura, T., and Ogino, K. (2016). Polymer 92: 125–132. Schmidt, U., Zehetmaier, P.C., and Rieger, B. (2010). Macromol. Rapid Commun. 31: 545–548. Lobez, J.M. and Swager, T.M. (2010). Macromolecules 43: 10422–10426. Rambarran, T., Gonzaga, F., and Brook, M.A. (2012). Macromolecules 45: 2276–2285. Cao, J., Wang, D., An, P. et al. (2018). J. Mater. Chem. A 6: 18025–18030. Peng, J., Deng, J., Quan, Y. et al. (2018). ACS Omega 3: 5222–5228. Cohen, C., Damiron, D., Ben Dkhil, S. et al. (2012). J. Polym. Sci. Part A: Polym. Chem. 50: 1827–1833. Isaacman, M.J., Barron, K.A., and Theogarajan, L.S. (2012). J. Polym. Sci. Part A: Polym. Chem. 50: 2319–2329. Rambarran, T., Gonzaga, F., and Brook, M.A. (2013). J. Polym. Sci. Part A: Polym. Chem. 51: 855–864. Bretzler, V., Grübel, M., Meister, S., and Rieger, B. (2014). Macromol. Chem. Phys. 215: 1396–1406. Luo, Y., Montarnal, D., Kim, S. et al. (2015). Macromolecules 48: 3422–3430. Ozmen, M.M., Fu, Q., Kim, J., and Qiao, G.G. (2015). Chem. Commun. 51: 17479–17482. Luo, Y.D., Kim, B., Montarnal, D. et al. (2016). J. Polym. Sci. Part A: Polym. Chem. 54: 2200–2208. Wang, M., Guo, L.‐X., Lin, B.‐P. et al. (2016). Liq. Cryst. 43: 1626–1635. Shen, K., Hu, H., Wang, J., and Liu, G. (2017). Polymer 132: 198–205. Yang, Z., Liu, X., Xu, X. et al. (2016). Mater. Chem. Phys. 179: 65–71. Pouget, E., Tonnar, J., Lucas, P. et al. (2010). Chem. Rev. 110: 1233–1277. Feng, L., Fang, H., Zhou, S., and Wu, L. (2006). Macromol. Chem. Phys. 207: 1575–1583. Hamurcu, E.E., Hazer, B., Misirli, Z., and Baysal, B.M. (1996). J. Appl. Polym. Sci. 62: 1415–1426. Park, H.‐W., Park, J.‐W., Lee, J.H. et al. (2019). Eur. Polym. J. 112: 320–327. Li, H., Li, X., Luo, C. et al. (2014). Thin Solid Films 573: 67–73. Yu, D.M., Zhao, Y.H., Li, H. et al. (2013). Prog. Org. Coat. 76: 1435–1444. Simionescu, C.I., Harabagiu, V., Comanita, E. et al. (1990). Eur. Polym. J. 26: 565–569. Nakamura, K., Fujimoto, K., and Kawaguchi, H. (1999). Colloids Surf. A 153: 195–201.

­  References

151 Baysal, B.M., Uyanik, N., Hamurcu, E.E., and Cvetkovska, M. (1996). J. Appl.

Polym. Sci. 60: 1369–1378.

152 Matyjaszewski, K. (2018). Adv. Mater. 30: 1706441. 153 Matyjaszewski, K. (2012). Macromolecules 45: 4015–4039. 154 Matyjaszewski, K. and Tsarevsky, N.V. (2014). J. Am. Chem. Soc. 136:

6513–6533.

155 Matyjaszewski, K. and Xia, J.H. (2001). Chem. Rev. 101: 2921–2990. 156 Krys, P. and Matyjaszewski, K. (2017). Eur. Polym. J. 89: 482–523. 157 Nakagawa, Y., Miller, P.J., and Matyjaszewski, K. (1998). Polymer 39:

5163–5170.

158 Miller, P.J. and Matyjaszewski, K. (1999). Macromolecules 32: 8760–8767. 159 Brown, D.A. and Price, G.J. (2001). Polymer 42: 4767–4771. 160 Strissel, C., Matyjaszewski, K., and Nuyken, O. (2003). Macromol. Chem. Phys.

204: 1169–1177.

161 Liang, J., He, L., Zhao, X. et al. (2011). J. Mater. Chem. 21: 6934–6943. 162 Muppalla, R. and Jewrajka, S.K. (2012). Polymer 53: 1453–1464. 163 Huan, K., Bes, L., Haddleton, D.M., and Khoshdel, E. (2001). J. Polym. Sci. Part

A: Polym. Chem. 39: 1833–1842.

164 Duquesne, E., Habimana, J., Degee, P., and Dubois, P. (2006). Macromol. Chem.

Phys. 207: 1116–1125.

165 Semsarzadeh, M.A. and Amiri, S. (2012). J. Inorg. Organomet. Polym. Mater. 23:

553–559.

166 Robert‐Nicoud, G., Evans, R., Vo, C.‐D. et al. (2013). Polym. Chem. 4:

3458–3470.

167 Semsarzadeh, M.A. and Ghahramani, M. (2017). Polym. Plast. Technol. Eng. 56:

1923–1936.

168 Xu, J.F., Qiu, M., Ma, B.M., and He, C.J. (2014). ACS Appl. Mater. Interfaces 6:

15283–15290.

169 Semsarzadeh, M.A. and Abdollahi, M. (2012). J. Appl. Polym. Sci. 123:

2423–2430.

170 Pan, A. and He, L. (2014). J. Colloid Interface Sci. 414: 1–8. 171 Shankar, R., Jangir, B., and Sharma, A. (2017). RSC Adv. 7: 344–351. 172 Luo, Z.‐H., Yu, H.‐J., and Zhang, W. (2009). J. Appl. Polym. Sci. 113:

4032–4041.

173 Martinelli, E., Galli, G., Krishnan, S. et al. (2011). J. Mater. Chem. 21:

15357–15368.

174 Shi, L.‐Y., Shen, Z., and Fan, X.‐H. (2011). Macromolecules 44: 2900–2907. 175 Zhang, C., Wang, D., He, J. et al. (2014). Polym. Chem. 5: 2513–2520. 176 Wenning, B.M., Martinelli, E., Mieszkin, S. et al. (2017). ACS Appl. Mater.

Interfaces 9: 16505–16516.

177 Zhou, X., Kong, J., Sun, J. et al. (2017). ACS Appl. Mater. Interfaces 9:

30056–30063.

178 Liang, J., He, L., Zuo, Y. et al. (2018). Soft Matter 14: 5235–5245. 179 Zhou, X., Lee, Y.‐Y., Chong, K.S.L., and He, C. (2018). J. Mater. Chem. B 6:

440–448.

180 Moad, G., Rizzardo, E., and Thang, S.H. (2006). Aust. J. Chem. 59: 669–692. 181 Moad, G., Rizzardo, E., and Thang, S.H. (2008). Polymer 49: 1079–1131.

59

60

2  Reactive Functionally Terminated Polyorganosiloxanes

182 Moad, G., Rizzardo, E., and Thang, S.H. (2009). Aust. J. Chem. 62: 1402–1472. 183 Keddie, D.J., Moad, G., Rizzardo, E., and Thang, S.H. (2012). Macromolecules

45: 5321–5342.

184 Moad, G., Rizzardo, E., and Thang, S.H. (2012). Aust. J. Chem. 65: 985–1076. 185 Moad, G., Rizzardo, E., and Thang, S.H. (2013). Chem. Asian J. 8: 1634–1644. 186 Guan, C.M., Luo, Z.H., Qiu, J.J., and Tang, P.P. (2010). Eur. Polym. J. 46:

1582–1593.

187 Pavlović, D., Lou, Q., Linhardt, J.G. et al. (2017). J. Polym. Sci. Part A: Polym.

Chem. 55: 3387–3394.

188 Duong, T.H., Bressy, C., and Margaillan, A. (2014). Polymer 55: 39–47. 189 Duong, T.H., Briand, J.F., Margaillan, A., and Bressy, C. (2015). ACS Appl.

Mater. Interfaces 7: 15578–15586.

190 Pramanik, N.B., Mondal, P., Mukherjee, R., and Singha, N.K. (2017). Polymer

119: 195–205.

191 Wadley, M.L. and Cavicchi, K.A. (2010). J. Appl. Polym. Sci. 115: 635–640. 192 Li, B., Li, X., Zhang, K. et al. (2015). Prog. Org. Coat. 78: 188–199. 193 Pai, T.S.C., Barner‐Kowollik, C., Davis, T.P., and Stenzel, M.H. (2004). Polymer

45: 4383–4389.

194 Pavlović, D.E., Linhardt, J.G., Künzler, J.F., and Shipp, D.A. (2008). J. Polym. Sci.

Part A: Polym. Chem. 46: 7033–7048.

195 Zhao, W., Fonsny, P., FitzGerald, P. et al. (2013). Polym. Chem. 4: 2140–2150. 196 Hur, Y.H., Song, S.W., Kim, J.M. et al. (2018). Adv. Funct. Mater. 28: 1800765. 197 Lopez‐Oliva, A.P., Warren, N.J., Rajkumar, A. et al. (2015). Macromolecules 48:

3547–3555.

198 Zhou, Y.‐N., Guan, C.‐M., and Luo, Z.‐H. (2010). Eur. Polym. J. 46: 2164–2173. 199 Wang, B., Xu, Q., Ye, Z. et al. (2016). ACS Appl. Mater. Interfaces 8:

27207–27217.

200 Moad, G., Rizzardo, E., and Thang, S.H. (2011). Polym. Int. 60: 9–25. 201 Miura, Y., Hirota, K., Moto, H., and Yamada, B. (1999). Macromolecules 32:

8356–8362.

202 Miura, Y., Sakai, Y., and Taniguchi, I. (2003). Polymer 44: 603–611. 203 Morgan, A.M., Pollack, S.K., and Beshah, K. (2002). Macromolecules 35:

4238–4246.

204 Ryan, J., Aldabbagh, F., Zetterlund, P.B., and Okubo, M. (2005). Polymer 46:

9769–9777.

205 McHale, R., Aldabbagh, F., Zetterlund, P.B. et al. (2006). Macromolecules 39:

6853–6860.

206 McHale, R., Aldabbagh, F., Zetterlund, P.B., and Okubo, M. (2006). Macromol.

Rapid Commun. 27: 1465–1471.

207 Nicolas, J., Guillaneuf, Y., Lefay, C. et al. (2013). Prog. Polym. Sci. 38: 63–235. 208 Pouget, E., Tonnar, J., Eloy, C. et al. (2006). Macromolecules 39: 6009–6016. 209 Tonnar, J., Pouget, E., Lacroix‐Desmazes, P., and Boutevin, B. (2008). Eur.

Polym. J. 44: 318–328.

210 Tonnar, J., Pouget, E., Lacroix‐Desmazes, P., and Boutevin, B. (2009).

Macromol. Symp. 281: 20–30.

211 Farrokhi, M. and Abdollahi, M. (2016). J. Polym. Res. 23: 122.

­  References

212 Rodwogin, M.D., Spanjers, C.S., Leighton, C., and Hillmyer, M.A. (2010). Acs

Nano 4: 725–732.

213 Kricheldorf, H.R. and Langanke, D. (2001). Macromol. Biosci. 1: 364–369. 214 Poojari, Y. and Clarson, S.J. (2009). Silicon 1: 165–172. 215 Ho, C.‐H., Wang, C.‐H., Lin, C.‐I., and Lee, Y.‐D. (2009). Eur. Polym. J. 45:

2455–2466.

216 Ekin, A. and Webster, D.C. (2006). Macromolecules 39: 8659–8668. 217 Schoener, C.A., Weyand, C.B., Murthy, R., and Grunlan, M.A. (2010). J. Mater.

Chem. 20: 1787–1793.

218 Zhang, D., Giese, M.L., Prukop, S.L., and Grunlan, M.A. (2011). J. Polym. Sci.

Part A: Polym. Chem. 49: 754–761.

219 Yilgör, E., Isik, M., Söz, C.K., and Yilgör, I. (2016). Polymer 83: 138–153. 220 Ibarboure, E., Rodriguez‐Hernandez, J., and Papon, E. (2006). J. Polym. Sci.

Part A: Polym. Chem. 44: 4668–4679.

221 Papadopoulos, P., Floudas, G., Schnell, I. et al. (2006). Biomacromolecules 7:

618–626.

222 Kotharangannagari, V.K., Sanchez‐Ferrer, A., Ruokolainen, J., and Mezzenga, R.

(2012). Macromolecules 45: 1982–1990.

223 Ragheb, R.T. and Riffle, J.S. (2008). Polymer 49: 5397–5404. 224 Satti, A.J., Molinari, E.C., de Freitas, A.G.O. et al. (2017). Mater. Chem. Phys.

188: 100–108.

225 Pitet, L.M., Wuister, S.F., Peeters, E. et al. (2013). Macromolecules 46: 226 227 228 229

8289–8295. Fan, W., Wang, L., and Zheng, S. (2009). Macromolecules 42: 327–336. Minehara, H., Pitet, L.M., Kim, S. et al. (2016). Macromolecules 49: 2318–2326. Glassner, M., D’hooge, D.R., Park, J.Y. et al. (2015). Eur. Polym. J. 65: 298–304. Nardin, C., Hirt, T., Leukel, J., and Meier, W. (2000). Langmuir 16: 1035–1041.

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3 Functionalized Polyhedral Oligomeric Silsesquioxanes (POSS) and Copolymers: Methods and Advances Huihui Shi1, Jing Yang2, Zibiao Li2, and Chaobin He1,2 1

National University of Singapore, Department of Materials Science and Engineering, 9 Engineering Drive 1, Singapore, 117576, Singapore 2 Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Innovis, #08‐03, Singapore, 138634, Singapore

3.1 ­Introduction The chemistry of organosilsesquioxanes could be traced to 1946 when Scott first isolated (CH3SiO3/2)n compounds through thermal rearrangement of a cohydrolysis product of Me2SiCl2 and MeSiCl3 [1]. After several decades of inactivity, the field of silsesquioxane chemistry was revitalized in the 1990s and has grown dramatically within the past 20–25 years [2]. Especially, quite much attention has been paid to polyhedral oligomeric silsesquioxanes (POSS) with the increasing demand for organic–inorganic hybrid polymer materials in various applications [3]. As a kind of highly ordered discrete molecular species with general formula (RSiO3/2)n (n ≥ 4, even integer), POSS molecules typically consist of a chemically inert and thermally stable inorganic silicon and oxygen framework externally covered by potentially reactive and readily modified organic substituents on vertex, among which octasilsesquioxanes (T8) are the most extensively investigated because of ease of synthesis and versatility (Figure 3.1) [4]. With definite, three‐ dimensional nanostructures (1–3 nm in diameter), POSS species may be thought of as the smallest particles of silica, which can be well dispersed in hybrid polymers even on a molecular level. Meanwhile, the diversity of organic corner groups provide POSS compounds with tailor‐made reactivity and solubility, making them compatible with almost any conventional polymer systems [5]. Thus, POSS derivatives are considered as ideal building blocks for the construction of novel hybrid materials and represent the new frontier in material chemistry, science, and engineering.

Silicon Containing Hybrid Copolymers, First Edition. Edited by Chaobin He and Zibiao Li. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2020 byWiley-VCH Verlag GmbH & Co. KGaA.

3  Functionalized Polyhedral Oligomeric Silsesquioxanes (POSS) and Copolymers: Methods and Advances Organic groups or polymers Inert and hard core structure with Si–O–Si bonds

O

R O R

Si

Si O

Si O O

R

O

O

Si

O R

Si R

O O

O Si O

Si

Si

R

X

Functional or reactive groups

64

R

Hybrid inorganic-organic nanostructure of POSS

Figure 3.1  The structure of functional POSS.

Nowadays, numerous functional POSS‐based reagents have been developed and incorporated into the polymeric matrices via copolymerization, grafting, or blending, affording a tremendous potential for modification of polymer properties, which include, but are not limited to improved mechanical and thermal properties, oxidation resistance, gas permeability, reduced flammability, and viscosity during processing, and holding great interest for a wide range of potential applications such as biomaterials, electronic and photic devices, functional coatings, catalysts, additives, and modifiers [6]. This chapter will focus on the synthesis of T8 cages and their functionalization first and then the chemistry of polymers and copolymers containing covalently bonded POSS moieties. The properties and applications of POSS species within polymeric materials are beyond the scope of this chapter, however, and will be elaborated in the following chapters.

3.2 ­Synthetic Strategies for Functionalized POSS As there is a myriad of synthetic routes for the formation POSS compounds, they can be cataloged by two major classes depending on whether the reactions involve the new generation of Si∙O∙Si bond to give the completely condensed POSS. The first class is based on the hydrolysis and condensation of chloro‐ or alkoxysilanes, which can be subdivided into direct synthesis of T8 from monomers and corner capping reaction of partially condensed silsesquioxanes T7. The second class of synthetic strategies is the modification of functional groups on existing T8 core by miscellaneous reactions either at silicon atoms or at organic substituents. According to the number of reactive groups that are potential for further modification, grafting, or polymerization, the synthesis of POSS derivatives will be mainly discussed from three sections, namely octafunctional, monofunctional, and bifunctional POSS.

3.2  Synthetic Strategies for Functionalized POSS

3.2.1  Octafunctional POSS 3.2.1.1  Hydrolysis and Condensation from RSiX3 Monomer

Hydrolytic condensation of trifunctional RSiX3, where R is a chemically stable organic substituent (such as alkyl, aryl, and vinyl) and X is a highly reactive substituent (such as chlorine, alkoxy and acetoxyl), in the presence of a stoichiometric amount of water is the most fundamental and common scheme for producing silsesquioxanes [7]. The organosilanes are first hydrolyzed to afford trisilanol monomers that undergo self‐condensation subsequently, giving different silsesquioxanes (Figure 3.2) depending on the processing parameters. To optimize the yield of a specific structure, there are several factors taking into account, including the concentration of the initial monomers, the identity of substituents R and X, the nature of the solvent, the type of the catalyst, the reaction temperature, and the rate and quantity of water addition [8]. For example, inter‐ and intramolecular condensation of the oligosiloxane intermediates during the reaction can lead to the formation of random or ladder structures and cage structures, respectively, and the latter is favorable when the systems are dilute. Also, temperature will make a difference and low temperature is preferable to obtain final products with POSS in majority [9]. In the field of POSS chemistry, the stability of the Si4O4 ring structure makes the formation of T8 cages preferential over other cubic species, and several simple T8R8 compounds have been prepared in this way with the general equation shown in Scheme 3.1. Details for the synthesis of some representative T8R8 derivatives are listed in Table 3.1.

Random structure

(T8)

Ladder structure

(T10) Cage structures

Figure 3.2  Various types of silsesquioxanes.

Partial cage structure

(T12)

65

66

3  Functionalized Polyhedral Oligomeric Silsesquioxanes (POSS) and Copolymers: Methods and Advances R

R Si O O R O O Si Si O R Si O R Si OO R O O Si Si O R R Si

8 RSiX3 + 12 H2O

Acid or base

O

+

24 HX

Scheme 3.1  Synthesis of T8R8 by hydrolytic condensation.

Table 3.1  T8R8 compounds prepared by hydrolytic condensation. R or T8R8

Starting materials

Yield (%)

References

‐H

HSiCl3 + H2O, FeCl3·nH2O

19

[10]

‐Me

MeSi(OEt)3 + NEt3, HCl, AcOH

88

[11]

‐i‐Bu

i‐BuSi(OMe)3 + KOH

96

[12]

‐Cy

CySi(OEt)3 + NBu4F

95

[13]

‐Ph

PhSiCl3 + PhCH2NMe3OH

98

[14]

‐CH=CH2

CH2=CHSi(OEt)3 + NMeOH

80

[15]

‐(CH2)3NH2

H2N(CH2)3Si(OEt)3 + NEt4OH

72

[16]

{T8[(CH2)3NH3]8}Cl8

H2N(CH2)3Si(OEt)3 + HCl

30

[17]

‐(CH2)3Cl

Cl(CH2)3Si(OEt)3 + HCl

37

[18]

‐(CH2)3SH

HS(CH2)3Si(OEt)3 + HCl

17

[19]

[NMe4]8[T8O8]

Si(OEt)4 + NMe4OH

99

[20]

It is noteworthy that the yields of some T8R8 compounds directly synthesized from hydrolytic condensation are very poor and uneconomic, in spite of many attempts for improvements by researchers. Besides, introduction of some functional groups such as hydroxy is infeasible because of the nature of the reaction. Thus, octafunctional POSS prepared by this means only account for a minor portion, while some of them are quite useful precursors to a wide range of other T8R8 compounds. 3.2.1.2  Modification of Substituents By Hydrosilylation  Hydrosilylation is another simple and characteristic reaction in

organosilicon chemistry, involving the addition of Si–H to unsaturated alkene or alkyne groups in the presence of platinum catalyst. In most cases, the functionalization of T8R8 cages via hydrosilylation are carried out between a T8R8 compound bearing Si–H (often T8H8 or T8(OSiMe2H)8) and an unsaturated organic compound, as shown in Scheme  3.2, while in some cases, T8R8 compounds terminated with unsaturated hydrocarbon bond (often T8(CH=CH2)8 or T8[OSiMe2CH=CH2]8) are

(a)

T8H8

Pt catalyst

+ 8

T8(OSiMe2H)8

R

T8[(CH2)2R]8

+

T8[CH(CH3)R]8

1 Pt catalyst

+ 8

R

2

+

T8[OSiMe2(CH2)2R]8

T8[OSiMe2CH(CH3)R]8

1

2

R

(b)

T8(CHCH2)8

+ 8

Si

R′

Pt catalyst

+

T8[(CH2)2SiRR′R″]8

H

T8[CH(CH3)SiRR′R″]8

1

R″

2

R T8[OSiMe2CHCH2]8

+ 8

Si H R″

R′

Pt catalyst

T8[OSiMe2(CH2)2SiRR′R″]8 1

+

T8[OSiMe2CH(CH3)SiRR′R″]8 2

Scheme 3.2  Hydrosilylation of T8R8 bearing (a) silicon–hydrogen bond and (b) unsaturated hydrocarbon bond. 1: α-product and 2: β-product.

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3  Functionalized Polyhedral Oligomeric Silsesquioxanes (POSS) and Copolymers: Methods and Advances

utilized as precursors to react with Si–H containing compounds. Given that there are two types of addition (i.e. Markovnikov and anti‐Markovnikov), the products can be either α‐ or β‐isomers in theory. However, α‐product is usually in the majority as a result of the strong directing effect of silicon and in only a few reactions have the products reported to be a mixture of isomers of which the β‐product can be removed as needed. Another potential problem of hydrosilylation may occur when there is an alcohol or carbonyl functionality in the alkene or alkyne, as the reaction probably takes place at oxygen to afford Si∙O bond. Yet, such a side reaction is also possible to be reduced through careful choice of catalysts, solvents, and reaction conditions. With simplicity and high yield of reaction as well as abundant selection of reactants, hydrosilylation has been widely applied in recent years to expand the T8 compound library [21]. Details for the modification of some representative T8R8 derivatives are listed in Table 3.2. Apart from hydrosilylation, some other reactions at silicon centers, for example, substitution reactions such as alkoxide–chloride, alkoxide–alkoxide, and metal–halogen exchange, are also frequently adopted to produce T8R8 compounds. Octa‐anion [T8O8]8− holds great potential for introducing functional groups by reacting with chlorosilanes such as RMe2SiCl. Some representative examples of them have been listed in Table 3.2. By Conventional Organic Reactions  The prosperity of conventional organic chemistry makes it fairly appealing for the modification of substituents at the periphery of POSS core, which allows introducing more complicated functionalities. Substitution reactions ranging from nucleophilic and electrophilic substitution to Heck, Suzuki, Sonogashira, and Kumada coupling of either aliphatic or aromatic substituents make up one of the largest categories of T8R8 derivatives. Meanwhile, reactions involved with unsaturated derivatives of POSS such as cross‐metathesis (or silylative coupling) and addition reactions provide a great number of interesting cases for creating novel octafunctional POSS as well, mainly starting from precursors T8(CH=CH2)8 or T8[(CH2)2NH2]8. Table  3.3 offers some representative examples of the reactions mentioned above. It should be pointed out that quite a few conventional organic reactions use strong bases as catalysts, which are possible to break the stability of POSS core by cleaving Si∙O bonds. Besides, the products of these reactions are commonly a mixture of isomers, especially for aromatic T8R8 derivatives, and some T8R8 compounds with bulky substituents may not react completely because of the steric effect, increasing the difficulty of post‐processing and reducing the yield. Thus, optimization for the reaction routes and conditions are necessary because there are plenty of mature organic reactions for choice [4]. By using a combination of the synthetic techniques mentioned above, T8R8 cages can be modified with virtually any functionality as required and incorporated into polymeric systems subsequently, where POSS molecules act as cross‐ linkers for polymer networks or cores for star‐shaped polymers and even hyperbranched polymers.

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3  Functionalized Polyhedral Oligomeric Silsesquioxanes (POSS) and Copolymers: Methods and Advances

Table 3.3  T8R8 compounds prepared by conventional organic reactions. R

Starting materials

Yield (%)

References

‐(CH2)3OH

T8[(CH2)3Cl]8 + Ag2O

91

[41]

‐(CH2)3N3

T8(CH2)3Cl + NaN3

79

[42]

‐(CH2)3NCO

T8[(CH2)3NH2]8 + COCl2

47

[43]

Substitution reaction

‐(CH2)3SCN

T8[(CH2)3Cl]8 + NaSCN

90

[19]

‐(CH2)3NHC(O) (CH2)2CO2H

T8[(CH2)3NH2]8 + succinic anhydride, NEt3

79

[17]

‐C6H4NO2

T8Ph8 + HNO3

92a

[44]

‐C6H4‐3‐SO3H

T8Ph8 + ClSO2OH

74

[45]

‐C6H4Br

T8Ph8 + Br2, FeCl3

60a

[46]

‐C6H4‐4‐I

T8Ph8 + Icl

90

[47]

‐C6H4NH2

T8(C6H4NO2)8 + FeCl3, C, N2H4·H2O

93a

[48]

‐C6H4N3

T8(C6H4NH2)8 + NaN3, H2SO4, NaNO2

—a

[49]

‐C6H4‐4‐CºCCO2Me

T8(C6H4‐4‐I)8 + HCºCCO2Me, CuI, Pd(PPh3)4, NEt3

67

[50]

76

[51]

Cross‐metathesis or silylative coupling ‐CH=CH(CH2)3Br

T8(CH=CH2)8 + CH2=CHC6H4‐4‐CH2Cl, [Mo]b

‐CH=CHCH2Si(OMe)3 T8(CH=CH2)8 + CH2=CHCH2Si(OMe)3, [Mo]

99

[51]

‐CH=CH(CH2)8CO2Et T8(CH=CH2)8 + CH2=CH(CH2)8CO2Et, [Mo]

99

[51]

‐CH=CHC6H4‐4‐ CH2Cl

T8(CH=CH2)8 + CH2=CHC6H4‐4‐CH2Cl, [Ru]c 65

[52]

‐CH=CHC6H4‐4‐ C6H3‐3,5‐(CHO)2

T8(CH=CH2)8 + CH2=CHC6H4‐4‐C6H3‐3,5‐ (CHO)2, [Ru]

78

[53]

‐(CH2)2CHO

T8(CH=CH2)8 + CO/H2, PtCl2(sixantphos), SnCl2



[12]

‐(CH2)2Br

T8(CH=CH2)8 + HBr(g), (BzO)2

65

[54]

‐(CH2)2CO2Me

T8(CH=CH2)8 + CO + MeOH, MeSO2OH, Pd2dba3, C6H4‐1,2‐[CH2P(t‐Bu2)]2

43

[54]

Addition reaction

‐(CH2)2S‐2‐C5H5N

T8(CH=CH2)8 + HS‐2‐C5H5N, AIBN



[55]

‐CH2CH(O)CH2

T8(CH2CH=CH2)8 + MCPBA

75

[56]

‐(CH2)3N[(CH2)2CO 2Me]2

T8[(CH2)2NH2]8 + CH2=CHCO2Me

73

[57]

‐(CH2)3NHC(O) NHCH2CH=CH2

T8[(CH2)2NH2]8 + CH2=CHCH2NCO

90

[43]

‐C6H4NHC(O) C6H4‐4‐CF=CF2

T8[(CH2)2NH2]8 + ClC(O)C6H4‐4‐CF=CF2, NEt3

73a

[58]

a) Product contains a mixture of isomers. b) Shrock’s catalyst. c) The first‐generation Grubbs’ catalyst.

3.2  Synthetic Strategies for Functionalized POSS

3.2.2  Monofunctional POSS 3.2.2.1  Corner Capping of T7R7(OH)3

As for T8 POSS compounds bearing more than one kind of substituents such as T8R7R′, T8R6R′2, T8R6R′R=, and T8R4R′4, they can also be synthesized by cohydrolysis and condensation of corresponding silane monomers in controlled feed ratios. For example, Hendan et al. prepared a series of T8R(8−n)R′n, where R=‐n‐ Pr and R′=‐(CH2)3Cl, R′=‐(CH2)3I, R′=‐(CH2)3SH, or R′=‐CH2CH=CH2, and the products with different values of n (=0, 1, 2) were successfully separated by HPLC [59]. However, as most of the reported synthetic results of POSS compounds obtained by this route illustrate, the products are often a mixture of hetero‐substituted POSS with different isomers, limiting the use of this route because they are too complicated to be identified and separated. Therefore, another strategy developed by Feher et al., namely corner capping of incompletely condensed silsesquioxane, T7R7(OH)3, or its sodium salt Na3[T7R7(O)3], with a trifunctional chloro‐ or alkoxysilane in the presence of base catalyst, has been very popular nowadays to give cT8R7R′ derivatives because of high selectivity and relatively good yields (Scheme 3.3). The source of T7 species can be hydrolytic condensation of organosilanes, or alternatively, acid‐ or base‐catalyzed cleavage of T8R8 [8]. As the feasibility of both approaches are significantly affected by the nature of the substituent, R groups for such precursors are mostly ‐Ph, ‐i‐Bu, ‐i‐Oct, ‐c‐C5H9, ‐Cy, ‐(CH2)2CF3, etc. Based on these precursors, a wide range of T8R7R′ compounds have been synthesized by varying the R′ groups in organosilanes. Preparation for some T8R7R′ derivatives with representative functional groups are listed in Table 3.4. Particularly, because of the existence of sufficiently bulky R groups, T8R7R′ derivatives bearing highly reactively R′ group such as ‐Cl, ‐SiCl3, ‐OsiCl3, and ‐(CH2)3NH2 present much better stability compared to their volatile T8R8 analogs, which are more readily handled for subsequent usage. R

R

R

Si

O Si

R

O

O

O R Si O Si O

OH OH OH R + Si OO R O Si

R′ Si O O R O O Si Si O R + 3 HX Si O R Si OO R O O Si Si O R R

Si

Si

R′ SiX3

Base

R

O

Scheme 3.3  Synthesis of T8R7R′ by corner capping.

By the way, T7R7(OH)3 derivatives can also be treated with an appropriate transition metal complexes in the form of alkoxides or chlorides to generate T8‐like metallasilsesquioxane with formulation as T7R7MLn, which may be monofunctional at the metal site as well, according to the nature of the ligand [70].

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3.2  Synthetic Strategies for Functionalized POSS

3.2.2.2  Modification of Substituents

In general, the same reactions mentioned in Section 3.2.1.2 apply equally to the modification of T8R7R′ compounds. The products of corner capping reactions that possess various substituents such as hydride, halide, alkoxide, olefin, and amine provide an abundance of precursors for the elaboration of T8R7R′ derivatives. Besides, by controlling the ratio of reactants appropriately, T8R8 compounds are also able to be modified into T8R7R′ derivatives. For example, the successful monosubstitution of T8H8 and T8(OSiMe2H)8 via hydrosilylation with unsaturated alkene or alkyne has been reported in some early work, and details for the reaction are listed in Table 3.5. Although the yield here is not as high as many other hydrosilylation reactions, it is still significant as not only the mono‐substituted group can be further modified by conventional organic reactions but also the seven unreacted ‐H or ‐OSiMe2H hold the potential for functionalization, which are useful to increase the diversity of R group in T8R7R′ derivatives. With flexible selection of reactive R′ group and inert R group, monofunctional POSS can be well compatible with almost all polymeric systems and are of great interest in the development of various hybrid polymer such as telechelic polymers and block copolymers. 3.2.3  Bifunctional POSS 3.2.3.1  Some Special Cases

When the chemistry of octafunctional and monofunctional POSS is prosperous in recent years, there is little evolution in the synthesis of T8 cage with two or more substituents different from others. In terms of bifunctional POSS, it remains a big challenge to seek for universal methods for the preparation of T8R6R′2 or T8R6R′R″ compounds with controlled manner because the eight corners of POSS cage share the same reactivity and the products tend to be a mixture of three kinds of isomers (Figure 3.3) that are mostly inseparable. Only in a limited number of cases, the synthesis of T8R6R′2 derivatives has been reported by aforementioned approaches including cohydrolysis and condensation of two different organosilanes and modification of T8R8 compounds Table 3.5  Monosubstitution of T8R8 by hydrosilylation. Yield (%)

References

T8H8 + CH2=CH(CH2)3CH3, H2PtCl6

20

[71]

‐(CH2)2Ph

T8H8 + CH2=CHPh, H2PtCl6

15

[71]

‐CH2=CHPh

T8H8 + CH2=CPh, H2PtCl6



[72]

‐(CH2)8CHBrCH2Br T8H8 + CH2=CH(CH2)6CHBrCH2Br, H2PtCl6

43

[10]

T8H8 + CH2=CHCH2CO2‐n‐C16H33, H2PtCl6

30

[73]

T8(OSiMe2H)8 + CH2=CHFc, Pt(dvs)

18

[74]

R

R′

Starting materials

‐H

‐n‐C6H13

‐H ‐H ‐H ‐H

‐(CH2)3CO2‐n‐ C16H33

‐OSiMe2H ‐OSiMe2(CH2)2Fc

73

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3  Functionalized Polyhedral Oligomeric Silsesquioxanes (POSS) and Copolymers: Methods and Advances R Si

O

R′ or R″

Si O O R O O Si Si O R O R Si O O Si R′ O O Si Si O R R Ortho-

R

R R′ or R″ R′ or R″ Si O Si Si O O O O R O R O O Si O Si O Si Si O R R O R Si O O Si O R Si O O Si R R O O O O Si Si Si O Si O R R′ R′ R Si

O

Meta-

Para-

Figure 3.3  Three types of isomers in T8R6R′2 or T8R6R′R″.

by hydrosilylation, substitution, or addition reactions under a controlled reactant ratio. Some examples for these reactions are listed in Table 3.6. Two unusual cases where disubstitution of POSS compound are achieved via enzymatic reaction and Co2(CO)8‐catalyzed hydrosilylation, respectively, are also included. Besides, Wen et al. reported the synthesis of a bifunctional POSS according to Scheme 3.4 [83]. R is epoxy ethyl in the literature but can be replaced by other functional group as required. Such a structure of POSS is quite interesting because all the three kinds of substituents on the vertex hold great potential for further functionalization. 3.2.3.2  Some Developing New Strategies

In recent years, several kinds of novel strategies have been developed for the preparation of bifunctional POSS with various structures (Figure 3.4), which is driven by the active research on incompletely condensed silsesquioxane chemistry. For example, side‐opening disilanol POSS, T8R8(OH)2, derived from acid‐ or base‐catalyzed net hydrolysis of one Si–O–Si linkage in close‐caged T8R8, which has been commercial already, can be incorporated into polymers directly or after chemical modification [84, 85]. Meanwhile, inspired by the corner capping reaction in the synthesis of T8R7R′, some T8R6R′R″(or T8R6R′2 if R″ is the same as R′) compounds are able to be prepared by making use of incompletely condensed silsesquioxane as the intermediate [86–89]. Scheme  3.5a demonstrates the two‐step process of such strategy. First, monofunctional POSS, mostly T8(i‐Bu)7[(CH2)3NH2], is treated with stoichiometric base to make the T8 cage undergo partial cleavage and afford T7R6R′(OH)3. This reaction is not fully selective because the corner bearing R′ may be cleaved as well, but according to results, the ratio of desired T8(i‐ Bu)7[(CH2)3NH2](OH)3 in the mixture can reach about 88%. Subsequently, corner‐capping reaction of the fresh trisilanol compound with organosilane R″SiX3 is conducted to give T8R6R′R″ POSS derivatives. With good reaction selectivity and yield, this approach is potential for the preparation of para‐bifunctional‐type POSS. Also, T8‐like metallasilsesquioxane, which has been mentioned in Section 3.2.2.1 which mainly refers to titanium‐based silsesquioxane (Ti‐POSS) here, can be derived from the incompletely condensed silsesquioxane in a similar route with metal complexes such as Ti(i‐PrO)4 added in, and some work on this to create novel bifunctional POSS species has been reported [90].

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3  Functionalized Polyhedral Oligomeric Silsesquioxanes (POSS) and Copolymers: Methods and Advances + O K O

SiO2

+

+

KOH

3

O Si

O O

Si

O

O

O O Si O Si HO

O

O Si OO

Si

O

O

OH

Si

Base

O O O

OR

OH

R-Cl

O

O O Si

O Si

HO

OR O

(Me4N+)6

Si(OEt)4 NMe4OH

O

O Si O

O

OH

Scheme 3.4  Synthesis of bifunctional POSS from tetra‐coordination siloxane.

Beyond the scope of T8 compounds, double‐decker silsesquioxane (DDSQ), with 10 silicon atoms in each molecule and structure similar to POSS, is also a good candidate for the controlled synthesis of bifunctional POSS. The synthetic route for DDSQ is shown in Scheme 3.5b, where R group is mainly phenyl and R′ group is mostly hydrogen or vinyl, and thus, the product can be further modified via hydrosilylation or other conventional organic reactions [91, 92]. As the R group of the aforementioned three structure is limited and thus restrict the applications of POSS in polymer systems, Imoto et al. newly designed a kind of bridged silsesquioxane where the R groups are more diverse [93]. As shown in Scheme 3.5c, the synthesis of such a corner opening bifunctional compound begins with the modification of an open‐caged T8R7(OH)3, followed by hydrosilylation with a second POSS derivate T8R7H. Because of strong steric hindrance, only one functional group in the incompletely condensed silsesquioxane will react in the second step, while the other two are still effective for other reactions. When the POSS moiety in most of the traditional POSS‐based polymers serves as a side group or an end cap, bifunctional POSS can take part in the construction of the polymer backbones and afford a bead‐like chain with novel properties, which are gaining more and more attention nowadays. However, the synthesis of bifunctional POSS is still preliminarily developed and is expected to be promoted by the investigation of polymer structure–property relationships in turn.

3.3 ­Synthetic Protocols for Hybrid POSS‐containing Polymers The convenience for the synthesis of POSS and the diversity of POSS substituents establish a good foundation for its versatility in polymer science. With tremendous research efforts dedicated to this field, POSS have been incorporated

R

R′ Si

R

R′ Si O R O O Si Si O R Si Si R O OO R O O Si Si O R″ R

R′

Si R O O R O O Si Si O R Si Si R O OO R O O Si Si O R R

O

Si

R′

O Si

Si

O

R

R O O Si O R Si R′ O O Si Si O R Si O R

R1

Si O O Si

O Si R O R O

Double-decker type

R

R′ Si O R O O Si Si O R O R Si O O Si R O O Ti Si O R″ R O

O

Si

Para-bifunctional type

Side-opening type

R

O

Si

O

Si

O

Si

R′ O Si Si Si O Si R1 O R1 O O Si O Si O R′ R1 Si R1 O

R1

R1 O O

R2 R2 O Si O Si R2 O O O Si R2 Si O O Si Si O R2 O O O Si R2 Si O

Corner-opening type

Figure 3.4  Bifunctional POSS with various structures.

R2

78

3  Functionalized Polyhedral Oligomeric Silsesquioxanes (POSS) and Copolymers: Methods and Advances R′ Si O O R O O Si Si O R Si Si R O OO R O O Si Si O R R

(a)

R′ Si O O R O O Si Si O R Si Si R HO OO R HO O Si HO

O

Si

Si

Base

NaO

NaOH

Si

O

R1

R1

O Si

O

O

OR1 Si O Si O

OH OH R1 Si OO R1 O Si

Si R1

Et3N

R1

R1

R1 Si

(c)

Cl

Si

R1

R

R O O Si ONa R ONa O Si Si O R Si O R

O O Si

R′MeCl 2

Si

R1 O O O

Si

Si

O

Si O Si O Si R1 O R1 O O Si O Si O R Si R1

1

Si

O Si

O

R′

Si

Si

O

O

O

OR1 Si O Si O

R

R O O Si O R Si R′ O O Si Si O R Si O R

Si

O O Si

O Si R O R O

R1

OH

O

base

R

(b) Si

R″ SiX3

O

Si

O

Si

NaO Si R O R O

R1

R′ Si O R O O Si Si O R Si Si R O OO R O O Si Si O R″ R

O

R

R

RSi(OMe)

R

R

R

Si O

Si O

Si

R1 Si

OO

O

T8(R2)7H O

Si

Pd(dvs)

R1

Si R1

R2 R2 O Si O Si R2 O O O Si R2 Si O O Si Si O R2 O O O Si R2 Si O R2

Scheme 3.5  Synthetic routes for (a) para‐bifunctional POSS, (b) double‐decker silsesquioxane, and (c) open‐cage silsesquioxane.

into virtually all polymeric systems, such as polyester, polyurethane, polyamide, polyimide, polyether, and polyolefin successfully, creating a wide range of POSS‐ containing hybrid polymer materials. In this section, synthetic protocols of these hybrid polymers with various architectures based on mono‐, bi‐, or octafunctional POSS (Figure  3.5) will be summed up as two categories, dependent on whether involved with monomer polymerization. Considering the fact that most of the protocols share the same techniques as the copolymerization of polysiloxanes and other organic polymers, which have been discussed in Chapter  2, rationales for them will not be repeated here and more attention will be paid to the specific cases. 3.3.1  Preparation from Monomers Profiting from the maturity of polymer chemistry, a variety of synthetic methods, such as free or living radical polymerization, ring‐opening polymerization,

3.3  Synthetic Protocols for Hybrid POSS‐containing Polymers

End-capped

Pendent

Hemi-telechelic

Eso-telechelic

Di-telechelic

Random

(b)

Bead-like

Star-shaped

(a) Multi-telechelic

Block

(c)

Network

Figure 3.5  Different architectures of polymers based on (a) monofunctional, (b) bifunctional, and (c) octafunctional POSS.

step‐growth polymerization, and so on, are available for the construction of POSS‐containing hybrid polymers from monomers. In the process, POSS molecules can serve as initiators or monomers up to the functional group on vertex. 3.3.1.1  Radical Polymerization POSS Functions as Monomers  As early as 1990s, POSS compounds mono‐substituted

with polymerizable groups styryl or (meth)acrylate have been reported to be copolymerized with various monomers via free radical polymerization initiated by azobisisobutyronitrile (AIBN) or benzoyl peroxide (BPO) to give random linear copolymers, including styrene, 4‐methylstyrene, acetoxy styrene, vinyl pyrrolidone, methyl methacrylate, octafluoropentyl acrylate, N,N‐dimethylacrylamide, and 4‐vinyl‐4′‐methyl‐2,2′‐bipyridine [65, 94–97]. Meanwhile, synthesis of star‐shaped polymers are reported by making use of octafunctional POSS terminated with vinyl or methacrylate group, along with a small amount of low cross‐linking polymers [98–102]. As the reactivity of groups on POSS is mostly lower than its comonomer, for example, methyl methacrylate (MMA), it will be initiated first to form propagating poly(methyl methacrylate) (PMMA)‐free radicals and termination reaction occurs when they contact with POSS‐free radicals, more likely to generate star‐shaped polymers rather than network [100]. However, by using monomer including N‐isopropylacrylamide, itaconic acid, and octavinyl POSS with a well‐designed feed ratio, Eftekhari‐Sis et  al. successfully obtained copolymer network where POSS serves as cross‐links via one‐pot free radical copolymerization catalyzed by AIBN [103]. According to the work of Zucchi et al.,

79

80

3  Functionalized Polyhedral Oligomeric Silsesquioxanes (POSS) and Copolymers: Methods and Advances

cross‐linked network can also be developed from monofunctional propylmethacryl POSS, which is copolymerized with isobornyl methacrylate and diethylene glycol dimethacrylate in the presence of BPO [104]. Besides, Chen et al. reported a free‐ radical graft polymerization of heptacyclopentyl propylmethacrylate POSS onto poly(amic acid) (PAA), which was pretreated to produce side peroxide groups as the initiation site [105]. With the emergence of advanced living radical polymerization, atom transfer radical polymerization (ATRP) and radical addition fragmentation chain transfer (RAFT) polymerization are widely utilized to prepare copolymers with predefined molecular weights and structures, especially block copolymers. The controlled radical polymerization can be started from small‐molecule initiators for ATRP or chain transfer agents (CTAs) for RAFT polymerization with monomers added successively [106, 107], or from existing polymers that are modified into macroinitiators or macro‐CTAs in advance [108, 109]. Although the former route may limit the comonomers to olefin species, which are mostly (meth) acrylate and (meth)acrylamide derivatives including methyl methacrylate, n‐butyl acrylate, glycidyl methacrylate, N‐isopropylacrylamide, and 2‐dimethylaminoethyl methacrylate [106, 107, 110–112], the latter one allows the introduction of other types of polymers such as poly(ethylene glycol) (PEG), poly(lactide acid) (PLA), poly(ε‐caprolactone) (PCL), and polydimethylsiloxane (PDMS) [108, 109, 113, 114]. Apart from linear copolymers, star‐shaped copolymers with POSS block can be prepared from living radical polymerization using multifunctional initiators or CTAs, for example, 1,1,1‐tris‐(4‐(2‐bromoisobutyryloxy)‐phenyl)‐ethane [115]. POSS Functions as  Initiators  ATRP techniques have been widely used to develop

hemi‐telechelic and star‐shaped POSS‐containing polymers in recent years because of the ease of modification on POSS compounds to be effective ATRP initiators. Monofunctional POSS molecules are mostly functionalized with bromine or chlorine at the terminus to prepare a variety of POSS end‐capped polymers and copolymers varying from polystyrene (PS) to polyacrylate and polyacrylamide derivatives [116, 117], while their octafunctional POSS analogs serve as multifunctional initiators for POSS‐core star‐shaped polymer and copolymers [118, 119]. Moreover, Wang et al. reported an eso‐telechelic POSS‐containing polymer by modifying mono‐substituted POSS with diol group, which reacted with 2‐bromoisobutyryl bromide and then to afford a difunctional initiator for ATRP of styrene [120]. In another interesting example, Kim et al. utilized heptacyclopentyl propylmethacrylate POSS and poly(ethylene glycol) methyl ether methacrylate as comonomers and octafunctional POSS as initiators, obtaining a kind of star‐shaped random copolymer with arms containing pendent POSS groups [121]. Similarly, mono‐ or octa‐substituted POSS molecules can act as CTAs in RAFT polymerization after properly functionalized with dithioester or linear trithiocarbonate moieties, and quite a few investigations of their usage in the preparation of either hemi‐telechelic or star‐shaped POSS‐containing polymers have been reported [122, 123]. Besides, di‐telechelic POSS‐containing polymers can be constructed with the assistance of a RAFT agent premodified with

3.3  Synthetic Protocols for Hybrid POSS‐containing Polymers

monofunctional POSS on both ends by esterification and click reaction successively, as shown in Figure 3.6 [124]. Nitroxide‐mediated polymerization (NMRP) is also available for the synthesis of hemi‐telechelic and star‐shaped POSS‐containing polymers and copolymers despite much less reports than the other two techniques [125, 126]. In the work of Miyamoto et al., POSS‐containing NMRP initiator was created by addition reaction between a monofunctional POSS with an isocyanate group and a 2,2,6,6‐ tetramethylpiperidine‐1‐oxy (TEMPO)‐based alkoxyamine with a hydroxy group, and hemi‐telechelic PS was then polymerized from styrene (Figure 3.7) [125]. Lu et al. synthesized octa‐substituted POSS initiator with similar functionalities by hydrosilylation and obtained star‐shaped diblock copolymers of poly(styrene‐ block‐4‐vinylpyridine)‐POSS ((PS‐b‐P4VP)8‐POSS) and poly‐(styrene‐block‐acetoxystyrene) ((PS‐b‐PAS)8‐POSS) via NMRP [126]. 3.3.1.2  Ring‐Opening Polymerization POSS Functions as  Monomers  POSS molecules possessing functionalities of

norbornene are known as monomers for ring‐opening metathesis polymerization (ROMP). As a synthetic technique peculiar to cycloalkene, possible comonomers for such POSS species are thus limited. POSS mono‐substituted with norbornene ethyl has been reported to copolymerize with other norbornene derivatives or cyclooctene catalyzed by Grubb’s catalyst into linear random or block copolymers [127, 128]. Besides, both mono‐ or octafunctional POSS are tried to form network through reaction with dicyclopentadiene (DCPD) where POSS is pendent moieties [129, 130]. Other than ROMP, ring‐opening polymerization (ROP) of POSS‐based monomer is reported by Lee et  al. In their work, POSS is octa‐functionalized with benzoxazine via hydrosilylation and subsequently undergoes copolymerization with mono‐ and difunctional benzoxazine monomers, achieving the construction of polybenzoxazine (PBZ) network with POSS as cross‐links. POSS Functions as  Initiators  POSS species modified with hydroxyl or amine end groups are commonly used as effective initiators for the anionic ROP (AROP) of cyclic esters, especially lactide and caprolactone, in the presence of tin(II) 2‐ethylhexanoate catalyst. Preparation of hemi‐telechelic or star‐shaped POSS‐ containing polymers or copolymers from mono‐ or octafunctional POSS, respectively, have been investigated a lot [131, 132]. Also, initiated by POSS bearing a diol group, eso‐telechelic polymers such as PCL and poly(lactide‐co‐glycolide) (PLGA) are reported [133, 134]. Based on bifunctional POSS of DDSQ‐type, PCL with a POSS moiety in backbone can be synthesized [135]. Besides, Kim et  al. reported three types of octafunctional POSS initiators, which were 3‐iodopropyl, 3‐tosylpropy, and 2‐(p‐iodobenzyl)ethyl, respectively, for the ROP of 2‐methyl‐2‐ oxazolin into star‐shaped polyoxazolines (POZ) [136]. Monticelli et al. put forward the usage of heptaisobutyl (isopropoxide)titanium‐POSS (Ti–POSS), a kind of monofunctional T8‐like metal silsesquioxane, as a novel initiator for the ROP of l‐lactide, where the polymerization probably proceeds via coordination‐insertion mechanism (Figure 3.8) [137].

81

OH

S

S

OH

HO S

S

O

SOCl2

O

O

O S

S

O

O

N3

R Si O Si O OR O Si O Si O OR Si OO Si R O O Si O Si R R

R

R R

O Si

Si O O

OR Si O Si O

CuBr, PMDETA

OO Si O Si

R

S

N

Si OR Si O

O

N

O

S

N

R

N O

S

N

N

R

O

O

O

Si O R O Si O O

Si O Si

OR Si OO Si O O O Si

R

R

Si R

R

R

R

NIPAAm

R

AIBN

O Si

O

OR Si O Si R

O

Si O R Si O

O

Si O

OO

Si O

N N

N

O

n

O R

O

NH

N

n S

HN

R

N

S

S

O

O

N

R

O

Si O R O Si O O

Si O Si

O

OR Si OO Si O O Si O

Si

Si R

R

Figure 3.6  Preparation of di‐telechelic POSS‐containing PNIPAAm using POSS‐containing RAFT agent. PMDETA, N,N,N′,N′′,N′′‐ pentamethyldiethylenetriamine; NIPAAM, N‐isopropylacrylamide. Source: Wanget al. 2011 [124]. Reproduced with permission of American Chemical Society.

R

R

3.3  Synthetic Protocols for Hybrid POSS‐containing Polymers O

N

O R

O Si Si O O R O O Si Si O R O R Si O O Si R O O Si Si O R: ∗ R R Si

N H

O

O

O

+

125 °C O

N

O R O

O

Si Si O R O O Si Si O R O R Si O O Si R O O Si Si O R: ∗ R R O

Si

N H

O

n

O

Figure 3.7  Preparation of hemi‐telechelic POSS‐containing PS using POSS‐containing NMRP initiators. Source: Wang et al. 2011 [124]. Reproduced with permission of American Chemical Society.

3.3.1.3  Step‐Growth Polymerization

Step‐growth polymerization is a technique commonly used for the preparation of POSS‐containing polyurethanes on the basis of isocyanate/hydroxyl reaction. In such a process, polymer segments such as polyether and polyester bearing hydroxy group at both ends are synthesized in advance to be macromonomers in the copolyaddition with difunctional POSS monomers, two of which are linked by diisocyanate such as lysine methyl‐ester diisocyanate (LDI), 4,4′‐methylenebis(phenyl isocyanate) (MDI), and hexamethylene diisocyanate (HDI) either by a two‐step approach to give alternative copolymers or by one‐shot method to give random copolymers. Thus, for mono‐substituted POSS, the functionalities need to be diol group, mostly employing structure shown in Figure  3.9, while the polymer segments are PEG, poly(tetramethylene ether)glycol (PTMG), PCL, PLA, etc. [138–141] In the meanwhile, bifunctional POSS derivatives of side‐opening type

83

POSS Ti

OPri

O n O

O PriO

O O

POSS Ti

O

O

POSS Ti

O

O POSS Ti

OPri

O

O

O O

O

O

O

O R R POSS Ti

OPri

R

Si

O Si

O

OPri

2n

O

O

O R Si O Si O

O Ti O O Si R Si OO R O Si O

∗ R:

R

Figure 3.8  Scheme for Ti‐POSS initiated ROP of l‐lactide. Source: Monticelli et al. 2011 [137]. Reproduced with permission of John Wiley & Sons.

3.3  Synthetic Protocols for Hybrid POSS‐containing Polymers OH HO HO O HO O Si

R Si

O

O

Si O R O O Si Si O R O R Si O O Si R O O Si Si O R R O

R O

Si

Si O R O O Si Si O R O R Si O O Si R O O Si Si O R R O

OH

HO HO HO

R Si

O

Si O R O O Si Si O R O R Si O O Si R O O Si Si O R R O

Si

R

O Si O R O O Si Si O R O R Si O O Si R O O Si Si O R R O

Si

O

R: isobutyl, phenyl, cyclopentyl, cyclohexyl, trifluoroprolyl, etc.

Figure 3.9  Structures of POSS‐diol monomers for step‐growth polymerization.

and DDSQ type have also been reported to be as a monomer in the preparation of bead‐like polyurethanes with PTMG and polypropylene glycol (PPG) segments, respectively [84, 142]. As for octafunctional POSS, Gu et al. prepared a octahydroxy‐terminated POSS with PLA arm generated from ROP and then mixed the POSS macromonomers with linear diol‐terminated polytetramethylene ether glycol (PTMEG), PLA, and HDI, finally producing a polyurethane‐based network cross‐linked at POSS moieties [143]. Given that the epoxy group is susceptive to amine and hydroxy group when heated, POSS compounds substituted with epoxy groups become another type

85

86

3  Functionalized Polyhedral Oligomeric Silsesquioxanes (POSS) and Copolymers: Methods and Advances

of commonly used monomers in step‐growth polymerization, especially applied for the construction of polyepoxide networks, namely epoxy resin. Among them, octa(propylglycidyl ether) POSS (OpePOSS) has been widely investigated and incorporated into a variety of polymer matrices for the cross‐linking of linear polymers at hydroxy side groups, such as polyphenolic [144]. In the work of Liu et al., PPG are pretreated with toluene‐2,4‐diisocyanate to afford polyurethane as macromonomers and then mixed with OpePOSS, where the secondary amine urethane linkages serve as the reaction site and lead to the formation of network [145]. Sometimes, mono‐substituted POSS can also be useful to give networks. Lee et al. reported a hemi‐telechelic POSS‐containing polymer from the mixture of monofunctional POSS‐epoxide, 1,4‐butanediol diglycidyl ether, and PPG bearing amine group at both the ends by step‐growth polymerization, which can be regarded as a macromonomer with several reaction sites at both end and side groups. With further reaction with poly(diglycidyl ether of bisphenol A), polyepoxide networks can thus be easily prepared (Figure 3.10) [146]. 3.3.1.4  Other Polymerization Methods

Living anionic polymerization has been conducted by Hirai et al. to prepare POSS‐containing block copolymer based on monofunctional POSS with

O

O

R O Si O O R O O Si O Si R R Si O Si O O O R O Si O Si R R R = c-C6H11 (cyclohexyl), c-C5H9 (cyclopentyl)

O

O

O

Si

H N

R O Si O O R O O Si O Si R R Si O Si R O O O O Si O Si R R Si

OH

BDGE

H

+

H2N

O

C CH3

C H2

CH CH3

NH2 x

JEFFAMINE D-230 60 °C 3h H

H O C CH NH C CH C O H2 H2 H2 x OH CH3 CH3 C

(CH2)4

O

C CH C N H2 H2 H OH

C

O

CH3

C H2

CH CH3

NH2 x

POSS-epoxy prepolymer

+ OH

O O

O

O

O

DGEBPA n~ 0 - 0.5

O

n

100 °C 24 h

POSS-reinforced epoxy resin

Figure 3.10  Synthesis of POSS-epoxy system via step-growth polymerization. Source: Lee and Lichtenhan 1998 [146]. Reproduced with permission of American Chemical Society.

3.3  Synthetic Protocols for Hybrid POSS‐containing Polymers

methacrylate group (methacrylisobutyl polyhedral oligomeric silsesquioxane [MAPOSS]) [147]. The polymerization was initiated by sec‐BuLi while excess amount of 1,1‐diphenyletylene (DPE) was added to inhibit the attack of highly reactive reagents toward carbonyl groups in methacrylate group, resulting in a nice homopolymer of PMAPOSS as well as a block copolymer of PMMA‐b‐ PMAPOSS and PS‐b‐PMAPOSS. Compared to living radical polymerization such as ATRP, the degree of polymerization (DP) of the POSS monomer exhibited to be much better. Hydrosilylation polymerization is reported by Seino et al. to prepare bead‐ like copolymers from DDSQ‐type POSS monomers with diphenylacetylene (DPA), diethynylbenzene (DEB), 1,4‐bis(phenylethynyl)benzene (BPEB), and 9,10‐bis(phenylethynyl)‐anthracene (BPEA) in the presence of Karstedt’s catalyst [148]. Because of the strong steric hindrance of POSS cage, the resultant double bond is almost impossible to be consumed in further hydrosilylation and the products possess single structure. Metallocene‐catalyzed polymerization is used to introduce polyolefins into POSS‐containing polymer systems. As reported by Seurer et  al., POSS mono‐ functionalized with norbornene group is copolymerized with ethylene and propylene in the presence of ethyl(bis‐indenyl)hafnium dichloride and methaluminoxane (MAO) [67]. 3.3.2  Preparation from Polymers Except for polymerization, POSS‐containing polymers can also be obtained from coupling of POSS compounds with existing polymers through various reactions such as conventional organic reaction, click reaction, and hydrosilylation. When monofunctional POSS moieties can be grafted onto polymers as end groups to afford hemi‐, di‐, and multi‐telechelic polymers, respectively, or as side groups to give block or random copolymers, octafunctional POSS moieties can be grafted with polymers and result in star‐shaped polymers. 3.3.2.1  By Conventional Organic Reactions

Amidation is one of the most effective methods for the coupling of POSS and polymers, which has been applied in the preparation of POSS‐containing polymers with almost all architectures. For example, di‐telechelic POSS‐containing PAA was prepared from the reaction of POSS mono‐substituted with phthalimide and PAA terminated with anhydride, where the former reactant is synthesized from the very beginning of corner capping reaction of T7(Cy)7(OH)3, followed by a series of conventional organic reactions and the latter reactant is synthesized via step‐growth polymerization of pyromellitic dianhydride and 4,4′‐oxydianiline (Figure 3.11) [149]. Similarly, cyclooctene and 5‐norbornene‐ exo,exo‐2,3‐dicarboxylic anhydride are reported to be copolymerized via ROMP and further modified by POSS mono‐substituted with aminopropyl, obtaining random copolymers with pendent POSS moieties [150]. Besides, star‐shaped copolymers are reported to be gained from synthesis of copolymer arm poly(benzyl l‐aspartate)‐b‐PEG terminated with amino via ROP first, followed by grafting onto octacarboxyl POSS [151].

87

Cl

Cy Si Cy

OH

OH O OH O Si O Si Si O Si Cy Cy O O O Cy O Si O Si

Cy

Si

Cl Si Cl Cl Cy

TEA

Cy

Cy

Cl O

Si

O O O O Si O Si Si O Si Cy Cy O O O Cy O O Si Si

Cy HO

NO2

Cs2CO3, DMF, Nal, THF, 80 °C

Cy

Cy- cyclohexyl

O

Si Cy

Cy

(a)

O

Si

Cy

NH2-POSS

Si

Cy

NO2-POSS O

NH2

O

O N O

Cy

O

O O O O Si O Si Si O Si Cy Cy O O O Cy O Si O Si

O

Cy

O Cy

MeOH, THF

Si

Cy

O O O O Si O Si Si O Si Cy Cy O O O Cy O Si O Si

Cl-POSS

Zn / NH4Cl

NO2

O Cy

Si Cy

DMAc/THF, heat

Cy

O

Si

O O O O Si O Si Si O Si Cy Cy O O O Cy O Si O Si Cy

phthalimide-POSS (model compound)

Figure 3.11  Synthesis of functional (a) POSS and (b) copolymers for coupling. PMDA, pyromellitic dianhydride; ODA, 4,4′‐oxydianiline. Source: Leu et al. 2003 [149]. Reproduced with permission of American Chemical Society.

O

O

O O

O PMDA

+ H2N

O

O

NH2

ODA DMAc RT,N2

O

CONH

O

HOOC

CONH

HNOC

COOH

O O

COOH

Cy Si O Si O O O O Si O Si Cy Si O Si Cy O O O O Cy Si O Si Cy Cy

(b)

HNOC

O O

HOOC

O

HNOC

CONH

HOOC

COOH

O

NH2-POSS

HOOC

CONH

- HNOC

COOH

O

HNOC n

CONH

HOOC

COOH

O Cy Si

PAA/POSS

Cy

Figure 3.11  (Continued)

n

poly(amic acid) (PAA) DMAc,THF RT,N2

O

O

Imidization Polyimide chain end linked POSS

O Si O O O Si O Si Cy Cy Si OO Si CyO O Cy O Si O Si Cy Cy O

Br

+

Br NaN3

n

CuBr, PMDETA Heating to 110 °C under Argon gas

N3

n

DMF/ THF = 9:1 At room temperature

Initiator

PPh3

NH2

n

THF/ Water = 10:1 heating to 60 °C

PS-Br

PS-N3

PS-NH2 N3 O

O

O O

R

O O

HN O

n

DMF

O

PS-PPLG

NH2

N H

m

O

R

O

O

Si

O

Si O O R Si O Si O R

Si O R O Si Si O R O O Si

NH2 m

N H

n

O

O

N

N N

R

CuBr, PMDETA

R

DMF R

O Si Si O R O O O Si Si O Si O R O R Si O O O Si Si O R R

PS-PPLG-POSS

Figure 3.12  Synthesis of POSS‐pendent copolymers via click reaction. PMDETA, N,N,N′,N′′,N′′‐pentamethyldiethylenetriamine; DMF, N,N‐dimethylformamide; THF, tetrahydrofuran; PPLG, poly(γ‐propargyl‐l‐glutamate. Source: Lin and Kuo 2012 [154]. Reproduced with permission of Royal Society of Chemistry.

­  References

Urethane reaction is also practical for coupling, as reported by Cardoen et al., and POSS is mono‐substituted with isocyanate, and then tethered to hydroxyl‐ terminated PS synthesized by living anionic polymerization to give hemi‐telechelic architecture [152]. In another case, Leu et al. reported the modification of side groups on aromatic polyimide with POSS, involving the reaction between phenol and benzyl chloride [153]. Considering the fact that coupling of POSS and polymer by conventional organic may need multistep preprocessing and complicated reaction conditions many times, which compromise the stability of POSS cage and the synthetic yield, more facile coupling reaction between POSS and polymers is in demand. 3.3.2.2  Some Advanced Methods

Click chemistry has attracted a lot of attention in recent years because of high yield, mild reaction conditions, and good tolerance for functional groups and is thus suitable for the coupling of POSS and polymers, among which copper‐catalyzed azide/alkyne cycloaddition are most widely. For example, Lin et al. developed a kind of polypeptide with POSS as pendent groups, where the block copolymer is synthesized via ATRP and ROP in order and modified by azide‐terminated monofunctional POSS with alkyne on side group via click reaction (Figure 3.12) [154]. Besides, the preparation of hemi‐, di‐, multi‐, eso‐telechelic polymers and star‐shaped polymers with the help of click reaction are also reported [155–159]. Also, hydrosilylation can be useful in some cases between vinyl‐substituted POSS and PDMS species bearing Si–H or between vinyl‐terminated polymers and POSS bearing Si–H. For example, star‐shaped polymer is developed by Maitra et al. from octahydridodimethylsiloxy POSS and allyl‐PEG functionalized by substitution [160].

3.4 ­Conclusion Benefiting from the growth of silsesquioxane chemistry and polymer chemistry, a variety of synthetic methods have been available for the design and preparation of functional POSS and their polymers with desired composition and architecture and establish a good foundation for structure–property investigation and application.

­References 1 Scott, D.W. (1946). J. Am. Chem. Soc. 68: 356. Wu, J. and Mather, P.T. (2009). Polym. Rev. 49: 25. 2 Pielichowski, K., Njuguna, J., Janowski, B., and Pielichowski, J. (2006). Polyhedral 3

oligomeric silsesquioxanes (POSS)‐containing nanohybrid polymers. In: Supramolecular Polymers Polymeric Betains Oligomers, 225–296. Berlin, Heidelberg: Springer.

91

92

3  Functionalized Polyhedral Oligomeric Silsesquioxanes (POSS) and Copolymers: Methods and Advances

4 Cordes, D.B., Lickiss, P.D., and Rataboul, F. (2010). Chem. Rev. 110: 2081. Zhang, W. and Müller, A.H. (2013). Prog. Polym. Sci. 38: 1121. 5 6 Li, G., Wang, L., Ni, H., and Pittman, C.U. (2001). J. Inorg. Organomet. Polym.

11: 123.

7 Harrison, P.G. (1997). J. Organomet. Chem. 542: 141. Hartmann‐Thompson, C. (2011). Applications of Polyhedral Oligomeric 8

Silsesquioxanes, vol. 3. The Netherlands: Springer Science & Business Media.

9 Dasgupta, D., Srividhya, M., Sarkar, A. et al. (2017). Progress in Rubber

10 11

12

13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Nanocomposites, Woodhead Publishing Series in Composites Science and Engineering (eds. S. Thomas and H.J. Maria), 231–247. Woodhead Publishing. Tsuchida, A., Bolln, C., Sernetz, F.G. et al. (1997). Macromolecules 30: 2818. Gu, Y., Liang, G., Zhang, Z., et al. (2006). Synthesis method for substituting sesquialter siloxane by non‐functional alkyl, CN100344636C, filed 13 December 2005 and issued 19 July 2006. Kuehnle, A., Jost, C., Abbenhuis, H. C. L., and Gerritsen, G. (2006). Polyedric oligomeric silicon/oxygen cluster with at least one aldehyde group and method for production thereof, WO2006027074A1, filed 09 August 2005 and issued 16 March 2006. Bassindale, A.R., Liu, Z., MacKinnon, I.A. et al. (2003). Dalton Trans.: 2945. Ni, Y., Zheng, S., and Nie, K. (2004). Polymer 45: 5557. Gao, J., Wang, S., Zhang, X., and Run, M. (2005). Youjigui Cailiao 19: 5. Zhang, Z., Liang, G., Ren, P., and Wang, J. (2008). Polym. Compos. 29: 77. Tanaka, K., Inafuku, K., Adachi, S., and Chujo, Y. (2009). Macromolecules 42: 3489. Ibrahim, G.M., Ahmad, M.I., and El‐Gammal, B. (2009). J. Appl. Polym. Sci. 113: 3038. Dittmar, U., Hendan, B.J., Flörke, U., and Marsmann, H.C. (1995). J. Organomet. Chem. 489: 185. Liu, H., Kondo, S., Takeda, N., and Unno, M. (2008). J. Am. Chem. Soc. 130: 10074. Lickiss, P.D. and Rataboul, F. (2008). Adv. Organomet. Chem. 57: 1. Mya, K.Y., Wang, Y., Shen, L. et al. (2009). J. Polym. Sci., Part A: Polym. Chem. 47: 4602. Hartmann‐Thompson, C., Keeley, D.L., Dvornic, P.R. et al. (2007). J. Appl. Polym. Sci. 104: 3171. Liu, L.K. and Dare, E.O. (2004). J. Chin. Chem. Soc. 51: 175. Chen, K., Chen, H., Yang, S., and Hsu, C. (2006). J. Polym. Res. 13: 237. Létant, S.E., Maiti, A., Jones, T.V. et al. (2009). J. Phys. Chem. C 113: 19424. Chen, W.Y., Wang, Y.Z., Kuo, S.W. et al. (2004). Polymer 45: 6897. Crivello, J.V. and Malik, R. (1997). J. Polym. Sci. Part A: Polym. Chem. 35: 407. do Carmo, D.R., Paim, L.L., Dias Filho, N.L., and Stradiotto, N.R. (2007). Appl. Surf. Sci. 253: 3683. Jutzi, P., Batz, C., and Mutluay, A. (1994). Z. Naturforsch. B 49: 1689. Constantopoulos, K., Clarke, D.J., Markovic, E. et al. (2004). Abstr. Pap. Am. Chem. Soc. 227: U441. Vautravers, N.R., André, P., and Cole‐Hamilton, D.J. (2009). Dalton Trans.: 3413.

­  References

33 Faustini, M. and Tanaka, H. (2003). Octa‐siloxane polymers and their

34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

56 57 58 59 60 61 62 63 64 65

preparation, JP2003268107A, filed 14 March 2002 and issued 25 September 2003. Soh, M.S., Yap, A.U., and Sellinger, A. (2007). Eur. Polym. J. 43: 315. Zhang, X., Tay, S.W., Hong, L., and Liu, Z. (2008). J. Membr. Sci. 320: 310. Hagiwara, Y., Shimojima, A., and Kuroda, K. (2007). Chem. Mater. 20: 1147. Hasegawa, I., Niwa, T., and Takayama, T. (2005). Inorg. Chem. Commun. 8: 159. Yim, J.H., Lyu, Y.Y., Jeong, H.D. et al. (2003). J. Appl. Polym. Sci. 90: 626. Ropartz, L., Morris, R.E., Foster, D.F., and Cole‐Hamilton, D.J. (2002). J. Mol. Catal. A: Chem. 182: 99. Coupar, P.I., Jaffrès, P.A., and Morris, R.E. (1999). J. Chem. Soc., Dalton Trans.: 2183. Liu, Y., Yang, X., Zhang, W., and Zheng, S. (2006). Polymer 47: 6814. Ervithayasuporn, V., Wang, X., and Kawakami, Y. (2009). Chem. Commun.: 5130. Feher, F.J., Wyndham, K.D., Soulivong, D., and Nguyen, F. (1999). J. Chem. Soc., Dalton Trans.: 1491. Iyer, P. and Coleman, M.R. (2008). J. Appl. Polym. Sci. 108: 2691. Hartmann‐Thompson, C., Merrington, A., Carver, P.I. et al. (2008). J. Appl. Polym. Sci. 110: 958. He, C., Xiao, Y., Huang, J. et al. (2004). J. Am. Chem. Soc. 126: 7792. Roll, M.F., Asuncion, M.Z., Kampf, J., and Laine, R.M. (2008). ACS Nano 2: 320. Zhang, J., Xu, R., and Yu, D. (2007). J. Appl. Polym. Sci. 103: 1004. Ak, M., Gacal, B., Kiskan, B. et al. (2008). Polymer 49: 2202. Asuncion, M.Z., Roll, M.F., and Laine, R.M. (2008). Macromolecules 41: 8047. Feher, F.J., Soulivong, D., Eklund, A.G., and Wyndham, K.D. (1997). Chem. Commun.: 1185. Cheng, G., Vautravers, N.R., Morris, R.E., and Cole‐Hamilton, D.J. (2008). Org. Biomol. Chem. 6: 4662. Vautravers, N.R., André, P., and Cole‐Hamilton, D.J. (2009). J. Mater. Chem. 19: 4545. Drylie, E.A., Andrews, C.D., Hearshaw, M.A. et al. (2006). Polyhedron 25: 853. Knig, H.J., Marsmann, H.C., and Letzel, M.C. (2003). Organosilicon Chemistry V (eds. N. Auner and J. Weis), 425–428. Weinheim, Germany: Wiley‐VCH Verlag GmbH. Lee, L.H. and Chen, W.C. (2005). Polymer 46: 2163. Feher, F.J. and Wyndham, K.D. (1998). Chem. Commun.: 323. Suresh, S., Zhou, W., Spraul, B. et al. (2004). J. Nanosci. Nanotechnol. 4: 250. Hendan, B.J. and Marsmann, H.C. (1994). J. Organomet. Chem. 483: 33. Saito, H. and Ikeda, M. (2004). Method for producing silicon compound, JP2004051848A, filed 22 July 2002 and issued 19 February 2004. Zeng, K., Liu, Y., and Zheng, S. (2008). Eur. Polym. J. 44: 3946. Zeng, K., Wang, L., Zheng, S., and Qian, X. (2009). Polymer 50: 685. Gießmann, S., Fischer, A., and Edelmann, F.T. (2004). Z. Anorg. Allg. Chem. 630: 1982. Iacono, S.T., Budy, S.M., Mabry, J.M., and Smith, D.W. (2007). Polymer 48: 4637. Kim, K.M. and Chujo, Y. (2003). J. Mater. Chem. 13: 1384.

93

94

3  Functionalized Polyhedral Oligomeric Silsesquioxanes (POSS) and Copolymers: Methods and Advances

66 Y. Morimoto, H. Oikawa, M. Yamahiro, et al. (2004). Production process for

67 68 69 70 71 72

73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97

silsesquioxane derivative and silsesquioxane derivative, US20040030084A1, filed 06 August 2003 and issued 12 February 2004. Seurer, B. and Coughlin, E.B. (2008). Macromol. Chem. Phys. 209: 1198. Ohno, K., Tsujii, Y., and Fukuda, T.( 2004). Silicon compound, WO2004014924A1, filed 07 August 2003 and issued 19 February 2004. Drazkowski, D.B., Lee, A., Haddad, T.S., and Cookson, D.J. (2006). Macromolecules 39: 1854. Hanssen, R.W.J.M., van Santen, R.A., and Abbenhuis, H.C.L. (2004). Eur. J. Inorg. Chem. 2004: 675. Calzaferri, G., Herren, D., and Imhof, R. (1991). Helv. Chim. Acta 74: 1278. Marcolli, C., Imhof, R., and Calzaferri, G. (1997). Progress in Fourier Transform Spectroscopy (eds. J. Mink, G. Keresztury and R. Kellner), 493–496. Vienna: Springer. Goto, R., Shimojima, A., Kuge, H., and Kuroda, K. (2008). Chem. Commun.: 6152. Moran, M., Casado, C.M., Cuadrado, I., and Losada, J. (1993). Organometallics 12: 4327. Jerman, I., Vuk, A.Š., Koželj, M. et al. (2008). Langmuir 24: 5029. Rościszewski, P., Kazimierczuk, R., and Sołtysiak, J. (2006). Polimery 51: 3. Kobayashi, T., Hayashi, T., and Tanaka, M. (1998). Chem. Lett. 27: 763. Matějka, L., Strachota, A., Pleštil, J. et al. (2004). Macromolecules 37: 9449. Xu, H., Yang, B., Gao, X. et al. (2006). J. Appl. Polym. Sci. 101: 3730. Han, S.‐Y., Wang, X.‐M., Shao, Y. et al. (2016). Chem. Eur. J. 22: 6397. Ihara, N., Kurisawa, M., Chung, J.E. et al. (2005). Appl. Microbiol. Biotechnol. 66: 430. Said, M.A., Roesky, H.W., Rennekamp, C. et al. (1999). Angew. Chem., Int. Ed. 38: 661. Wen, Y. and Liu, A. (2005). Youji Huaxue 25: 470. Raftopoulos, K.N., Jancia, M., Aravopoulou, D. et al. (2013). Macromolecules 46: 7378. Wang, P., Tang, Y., Yu, Z. et al. (2015). ACS Appl. Mater. Interfaces 7: 20144. Olivero, F., Renò, F., Carniato, F. et al. (2012). Dalton Trans. 41: 7467. Maegawa, T., Irie, Y., Fueno, H. et al. (2014). Chem. Lett. 43: 1532–1534. Maegawa, T., Irie, Y., Imoto, H. et al. (2015). Polym. Chem. 6: 7500. Maegawa, T., Miyashita, O., Irie, Y. et al. (2016). RSC Adv. 6: 31751. Carniato, F., Boccaleri, E., and Marchese, L. (2008). Dalton Trans.: 36. Liao, Y.‐T., Lin, Y.‐C., and Kuo, S.‐W. (2017). Macromolecules 50: 5739. Żak, P., Delaude, L., Dudziec, B., and Marciniec, B. (2018). Chem. Commun. 54: 4306. Imoto, H., Katoh, R., and Naka, K. (2018). Polym. Chem. 9: 4108. Haddad, T.S. and Lichtenhan, J.D. (1996). Macromolecules 29: 7302. Haddad, T.S., Viers, B.D., and Phillips, S.H. (2001). J. Inorg. Organomet. Polym. 11: 155. Xu, H., Kuo, S.‐W., Huang, C.‐F., and Chang, F.‐C. (2004). J. Appl. Polym. Sci. 91: 2208. Seurer, B. and Coughlin, E.B. (2008). Macromol. Chem. Phys. 209: 2040.

­  References

98 Zhao, C., Yang, X., Wu, X. et al. (2008). Polym. Bull. 60: 495. 99 Yang, B., Xu, H., Wang, J. et al. (2007). J. Appl. Polym. Sci. 106: 320. 00 Xu, H., Yang, B., Wang, J. et al. (2007). J. Polym. Sci. Part A: Polym. Chem. 45: 1

5308.

101 Markovic, E., Clarke, S., Matisons, J., and Simon, G.P. (2008). Macromolecules

41: 1685.

102 Xu, H., Yang, B., Wang, J. et al. (2005). Macromolecules 38: 10455. 103 Eftekhari‐Sis, B., Rahimkhoei, V., Akbari, A., and Araghi, H.Y. (2018). React. 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133

Funct. Polym. 128: 47. Zucchi, I.A., Galante, M.J., and Williams, R.J. (2009). Eur. Polym. J. 45: 325. Chen, Y., Chen, L., Nie, H., and Kang, E.T. (2006). J. Appl. Polym. Sci. 99: 2226. Pyun, J., Matyjaszewski, K., Wu, J. et al. (2003). Polymer 44: 2739. Mya, K.Y., Lin, E.M., Gudipati, C.S. et al. (2010). J. Phys. Chem. B 114: 9119. Tan, B.H., Hussain, H., and He, C.B. (2011). Macromolecules 44: 622. Li, B., Li, X., Zhang, K. et al. (2015). Prog. Org. Coat. 78: 188. Zheng, Y., Wang, L., Yu, R., and Zheng, S. (2012). Macromol. Chem. Phys. 213: 458. Matějka, L., Janata, M., Pleštil, J. et al. (2014). Polymer 55: 126. Zhang, Z., Xue, Y., Zhang, P. et al. (2016). Macromolecules 49: 8440. Wang, L., Li, J., Li, L., and Zheng, S. (2013). J. Polym. Sci. Part A: Polym. Chem. 51: 2079. Tan, B.H., Hussain, H., Leong, Y.W. et al. (2013). Polym. Chem. 4: 1250. Pyun, J. and Matyjaszewski, K. (2000). Macromolecules 33: 217. Ohno, K., Sugiyama, S., Koh, K. et al. (2004). Macromolecules 37: 8517. Fei, M., Jin, B., Wang, W., and Liu, L. (2010). J. Polym. Res. 17: 19. Yang, Y.‐Y., Wang, X., Hu, Y. et al. (2013). ACS Appl. Mater. Interfaces 6: 1044. Li, Y., Xu, B., Bai, T., and Liu, W. (2015). Biomaterials 55: 12. Wang, W., Ding, W., Yu, J. et al. (2012). J. Polym. Res. 19: 9948. Kim, D.‐G., Sohn, H.‐S., Kim, S.‐K. et al. (2012). J. Polym. Sci., Part A: Polym. Chem. 50: 3618. Yu, Z.‐W., Gao, S.‐X., Xu, K. et al. (2016). Chin. Chem. Lett. 27: 1696. Zhang, W., Fang, B., Walther, A., and Müller, A.H. (2009). Macromolecules 42: 2563. Wang, L., Zeng, K., and Zheng, S. (2011). ACS Appl. Mater. Interfaces 3: 898. Miyamoto, K., Hosaka, N., Otsuka, H., and Takahara, A. (2006). Chem. Lett. 35: 1098. Lu, C.‐H., Wang, J.‐H., Chang, F.‐C., and Kuo, S.‐W. (2010). Macromol. Chem. Phys. 211: 1339. Xu, W., Chung, C., and Kwon, Y. (2007). Polymer 48: 6286. Zheng, L., Hong, S., Cardoen, G. et al. (2004). Macromolecules 37: 8606. Pittman, C.U. Jr., Li, G.‐Z., and Ni, H. (2003). Macromol. Symp. 196: 301. Constable, G.S., Lesser, A.J., and Coughlin, E.B. (2004). Macromolecules 37: 1276. Goffin, A.‐L., Duquesne, E., Moins, S. et al. (2007). Eur. Polym. J. 43: 4103. Li, L., Lu, B., Wu, J. et al. (2016). New J. Chem. 40: 4761. Knight, P.T., Lee, K.M., Chung, T., and Mather, P.T. (2009). Macromolecules 42: 6596.

95

96

3  Functionalized Polyhedral Oligomeric Silsesquioxanes (POSS) and Copolymers: Methods and Advances

134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160

Mirmohammadi, S.A., Imani, M., Uyama, H. et al. (2014). Polym. Int. 63: 479. Song, X., Zhang, X., Li, T. et al. (2019). Polymers 11: 373. Kim, K.‐M., Ouchi, Y., and Chujo, Y. (2003). Polym. Bull. 49: 341. Monticelli, O., Cavallo, D., Bocchini, S. et al. (2011). J. Polym. Sci. Part A: Polym. Chem. 49: 4794. Fu, B.X., Hsiao, B.S., Pagola, S. et al. (2001). Polymer 42: 599. Wang, W., Guo, Y., and Otaigbe, J.U. (2009). Polymer 50: 5749. Wu, J., Ge, Q., and Mather, P.T. (2010). Macromolecules 43: 7637. Guo, Q., Knight, P.T., Wu, J., and Mather, P.T. (2010). Macromolecules 43: 4991. Wei, K., Wang, L., and Zheng, S. (2013). Polym. Chem. 4: 1491. Gu, S.Y. and Gao, X.F. (2015). RSC Adv. 5: 90209. Liu, Y., Zeng, K., and Zheng, S. (2007). React. Funct. Polym. 67: 627. Liu, Y., Ni, Y., and Zheng, S. (2006). Macromol. Chem. Phys. 207: 1842. Lee, A. and Lichtenhan, J.D. (1998). Macromolecules 31: 4970. Hirai, T., Leolukman, M., Hayakawa, T. et al. (2008). Macromolecules 41: 4558. Seino, M., Hayakawa, T., Ishida, Y. et al. (2006). Macromolecules 39: 3473. Leu, C.‐M., Reddy, G.M., Wei, K.‐H., and Shu, C.‐F. (2003). Chem. Mater. 15: 2261. Monticelli, O., Fina, A., Ullah, A., and Waghmare, P. (2009). Macromolecules 42: 6614. Pu, Y., Zhang, L., Zheng, H. et al. (2014). Macromol. Biosci. 14: 289. Cardoen, G. and Coughlin, E.B. (2004). Macromolecules 37: 5123. Leu, C.‐M., Chang, Y.‐T., and Wei, K.‐H. (2003). Chem. Mater. 15: 3721. Lin, Y.‐C. and Kuo, S.‐W. (2012). Polym. Chem. 3: 882. Zhang, W. and Müller, A.H.E. (2010). Polymer 51: 2133. Gungor, E., Bilir, C., Hizal, G., and Tunca, U. (2010). J. Polym. Sci. Part A: Polym. Chem. 48: 4835. Gungor, E., Bilir, C., Durmaz, H. et al. (2009). J. Polym. Sci. Part A: Polym. Chem. 47: 5947. Doganci, E., Tasdelen, M.A., and Yilmaz, F. (2015). Macromol. Chem. Phys. 216: 1823. Zhang, W., Yuan, J., Weiss, S. et al. (2011). Macromolecules 44: 6891. Maitra, P. and Wunder, S.L. (2002). Chem. Mater. 14: 4494.

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4 Nanostructured Self‐assemblies from Silicon‐ containing Hybrid Copolymers Hong Chi1, Beng H. Tan 2, Fuke Wang 2, Chaobin He 2,3, and Zibiao Li 2 1

Qilu University of Technology (Shandong Academy of Sciences), Shandong Provincial Key Laboratory of Molecular Engineering, School of Chemistry of Pharmaceutical Engineering, Jinan, 250353, P. R. China 2 Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Innovis, #08‐03, Singapore, 138634, Singapore 3 National University of Singapore, Department of Materials Science and Engineering, 9 Engineering Drive 1, Singapore, 117576, Singapore

4.1 ­Introduction Recent advances in nanosciences have created a new surge of interest in the development of silicon‐containing hybrid copolymers because many intelligent functions are directly determined by their functionalities and dimensions [1–3]. Self‐assembly of copolymers from a single molecule to functional architecture copolymers is an efficient strategy to create the required product with well‐ organized structures. Silicon‐based hybrid organic/inorganic copolymers have gained great attention because of their versatility and possible use in the form of films, coatings, monoliths, or nanomaterials in various fields [4]. The most com­ mon hybrid copolymer structures of silica are polyhedral oligomeric silsesquiox­ anes (POSSs) and polydimethylsiloxane (PDMS). As shown in Figure  4.1a–c, POSSs are the smallest hybrid nanosilica with the formula (RSiO1.5)n (n = 6, 8, 12, etc.) and the dimension ranging from 1 to 3 nm [5]. The size of POSSs depends on the surrounding R groups, where R could be hydrogen atom or organic functional groups, which could be precisely func­ tionalized via the living/controlled polymerization techniques. POSSs have been reported to construct hybrid polymers with well‐defined structures, including telechelic‐shaped [6–8], star‐shaped [9–11], dendrimers [12–14], block copoly­ mers (BCP) [15–17], and alternative copolymers [18, 19]. These interesting structures and properties of POSSs make them widely used in hybrid materials [20, 21] , drug deliveries [22, 23], biomedical applications [24, 25], catalytic sup­ ports [26, 27], and so on [28–30], which have been reviewed extensively [28, 31–33]. In general, the superhydrophobic POSSs could provide strong aggregation ten­ dency for controllable assembly and confined motion of polymer chains to the Silicon Containing Hybrid Copolymers, First Edition. Edited by Chaobin He and Zibiao Li. © 2020 Wiley‐VCH Verlag GmbH & Co. KGaA. Published 2020 by Wiley‐VCH Verlag GmbH & Co. KGaA.

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4  Nanostructured Self‐assemblies from Silicon‐containing Hybrid Copolymers R R

Si

R

Si O O

O

Si Si O R O

O

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R O

Si

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O Si

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O

Si

Si O R O

Si

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O

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Si R

O

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O

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(a)

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O Si

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O

Si

Si O

Si

O

O

R

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HO O

O O Si

Si

O

Si

O

Si

OH

n

Si O (d)

Figure 4.1  Schematic illustration of single (a), double (b), and multiple (c) armed POSS and PDMS (d).

required nanometer size and tailored properties [34–37]. Self‐assembly of POSS hybrids is normally driven by a combination of attractive and repulsive forces between the POSS segments and the organic segments. The biocompatibility and nontoxicity of POSSs have indicated their potential applications such as cytocompatibility [38, 39], construction of capillary bed [40], antithrombogenic­ ity [41, 42], etc. Moreover, because of its special composition and cage‐like nano­ structure, not only it has oxidation resistance and thermal stability [43] but it could also strengthen the mechanical properties [44], enhance the stability of the micelles, and influence the viscoelastic and homogeneous properties of the materials [45–50]. For example, Sarwat Butool Rizvi et  al. prepared poly(carbonate–urea) urethane (POSS–PCU) amphiphilic block copolymer micelles to encapsulate and stabilize quantum dots (QDs) in water. The coated QDs showed both colloidal stability and highly photostability [51]. PDMS is also known as a common linear polyorganosiloxane copolymer (Figure 4.1d). It is readily prepared by R(OSiMe2)mOR reacting with molecular water to form some new simpler PDMS‐type silicones [52]. Because of the tor­ sional motion along the backbone, PDMS exhibits a low glass transition tem­ perature of −123 °C. It has been demonstrated that the very low intermolecular force in PDMS could result in a large molar volume (75.5 cm3/mol) and a low cohesion energy density. The low surface tension, surface energy, solubility parameter, and dielectric constant observed for PDMS can also be ascribed to the low intermolecular forces between the PDMS chains. The presence of a polar methyl groups around the Si–O–Si polymer backbone explains their high lipo­ philic and hydrophobic characters. The value of the Si─O bond dissociation

4.2  Mechanism in Self‐assembly of POSS and PDMS‐Based Copolymers

energy (BDE) (110 kcal/mol) is high, which explains the excellent thermal stabil­ ity of polyorganosiloxanes. In addition, PDMS has a good gas permeability and is transparent to visible and UV light (the methyl groups do not absorb radiation above 300 nm) [53]. Polysiloxanes are normally synthesized using three principal routes: ring‐opening polymerization (ROP), polycondensation, and redistribu­ tion reaction. The ROP of cyclosiloxanes enables one to synthesize high‐molecu­ lar‐weight siloxanes with better precision than the polycondensation and redistribution methods. The most common cyclic siloxane monomers are octa­ methylcyclotetrasiloxane (Me2SiO)4 and hexamethylcyclotrisiloxane (Me2SiO)3 [53]. With the aim to illustrate how to take advantage of the specific structures and physical properties of POSS or PDMS in building functional materials, we sum­ marized recent progress on the silicon‐based amphiphilic copolymer on the mechanism of self‐assembly and some applications, particularly in the area of biomedical, photodynamic therapy, coating, and sensing.

4.2 ­Mechanism in Self‐assembly of POSS and PDMS‐ Based Copolymers Self‐assembly is a process whose components form ordered arrangements or structures spontaneously [54]. The structures are varied and dependent on the type of the substance used and the environment located [55]. Recently, there has been tremendous interest on hybridized polymers with POSS or PDMS [56–59]. Thus, a variety of interesting studies have been conducted to develop new syn­ thetic protocols and explore their self‐assembled nanostructure. Fundamentally, the formation of these assemblies depends on the force balance of both hydro­ philic and hydrophobic parts arising from the shapes, sizes, sequences, and rela­ tive properties. For example, the phase behavior of amphiphiles is defined by the molecular shape that can be quantitatively expressed via critical packing param­ eter p = v/al, where v stands for the effective volume of the hydrophobic parts, a represents the effective hydrophilic part surface area, and l is the maximum effective length [60]. Stephen Z. D. Cheng et  al. have reported a series of famous and important works on POSS‐induced assembled nanostructure  [6, 61–63]. Recently, they found that different phase‐separated nanostructures are determined by precisely controlling the sequence and the number of incorporated POSS cages (composi­ tion) because the sequence of the symmetry could affect the cross‐sectional areas of the hydrophobic/hydrophilic POSS domains [64]. In another work, they also found that both the compositional variation and specific sequences could induce unconventional phase formation from specifically designed chain‐like giant molecules. To study the sequence‐phase relationships and the sequences effect, they synthesized a variety of amphiphilic giant molecules by interconnect­ ing both hydrophobic (7 isobutyl groups functionalized hydrophobic POSS cage [BPOSS] (~1 kDa with 7 sec‐butyl groups) and hydrophilic 14 hydroxyl groups functionalized hydrophilic POSS cage (DPOSS) (~1.5 kDa with 14 hydroxyl groups) nanoparticles in a precisely defined sequence. Driven by a strong

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c­ ollective hydrogen bonding and nanophase separation, the assemblies are con­ structed by a core of DPOSS nanoparticles (NPs) together and covered with a thick shell formed by BPOSS NPs. The distinct locations of DPOSS directed the giant molecules into different conformations, leading to different supramolecu­ lar lattices [64]. The self‐assembly of POSS‐based amphiphilic copolymers are normally done by dispersing copolymers in a good solvent and then adding to water with sonication or stirring, and the assemblies are obtained when the solu­ tions are dialyzed followed by filtration. Just like POSS, the PDMS copolymers can self‐assemble into many interesting structures such as films. Kee‐Hong Lee et al. used PS‐b‐PDMS (labeled SD44) to form nanoparticles with a PDMS volume fraction ƒPDMS = 0.67 and the number‐ average molecular weight of the PDMS block of 28.0 kg/mol and of the final BCP of 44.0 kg/mol (the polydispersity of BCP was 1.06). The bulk morphology of SD44 consisted of lamellae with a period of 50 nm corresponding to PS and PDMS layer thicknesses of approximately 13 and 37 nm, respectively. With dif­ ferent UV irradiation times, the copolymer would be assembled into different morphologies as shown in Figure 4.2 [65]. 4.2.1  Stimuli‐Responsive Micelles Micelle is the most common self‐assembled structure of POSS copolymers with varieties of morphologies, such as spherical micelles, necklace‐like micelles, and rod‐like micelles, among which the spherical micelles are the most frequently used and it could be divided into pH‐responsive, reduction‐responsive, tempera­ ture‐responsive, and photoactive micelles. Among various stimuli‐sensitive block copolymers, the pH‐sensitive micelles mainly consist of acrylic acid (AA) [66, 67], 2‐diisopropylaminoethyl meth­ acrylate (DPA) [68], and 2‐(dimethylamino) ethyl methacrylate (DMAEMA) [69, 70] blocks. 4.2.1.1  pH‐Sensitive Micelles

A series of POSS‐containing pH‐sensitive block copolymers (HBCP) of poly(methacrylisobutyl‐POSS)‐b‐poly(4‐vinylpyridine) (PMAiBuPOSS‐b‐P4VP) and poly(methacrylisobutyl‐POSS)‐b‐polystyrene‐b‐poly(4‐vinylpyridine) (PMAi­ BuPOSS‐b‐PS‐b‐P4VP) were synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization by Yiting Xu et  al. [71]. The self‐assembly behavior of HBCP is attributed to a balance of three kinds of driving forces: the hydrophobic interaction of POSS, the electrostatic interaction of P4VP blocks, and the π–π stacking interaction of the pyridine group. The low pH value enables the P4VP block highly protonated, resulting in a larger hydrodynamic size of the aggre­ gates because of the strong electrostatic repulsion‐induced molecular stretch. The increase of pH value resulted in more curled molecular chains of P4VP because of the decrease of electrostatic repulsion of P4VP. When the pH value further increases close to the pKa of P4VP, the protonation degree of the P4VP block is lower enough to change P4VP to hydrophobic. Consequently, a stronger π–π stacking interaction leads to larger HBCP aggregates.

Figure 4.2  Representative SEM images of PDMS microdomains with solvent vapor annealing from toluene: heptane 5 : 1 volumetric mixture. (a) As‐cast morphology; (b–d) annealed without UV irradiation; (e) Five minutes UV irradiation before annealing; (f ) UV irradiation during first five minutes of annealing; and (g, h) UV irradiation during last five minutes of annealing. The film thickness was (a) 36 nm; (b) 25 nm; (c) 36 nm; (d) 78 nm; (e) 36 nm; (f ) 36 nm; (g) 36 nm; and (h) 80 nm.

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4  Nanostructured Self‐assemblies from Silicon‐containing Hybrid Copolymers Controlled thiol-disulfide exchange reaction O

O

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Si O Si OO OO Si O Si Si O Si OO OO

S S

pH 12 S S

Si O Si O

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Core/shell separation

pH 12

Activation

S

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Si O Si OO OO Si O SiO Si Si OO OO Si O Si

S S O O

O

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OO

S S

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Si Si O OO OO Si O SiO Si Si O O OO Si O Si

Catalytic RSH

S

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pH 7

pH 12

Termination

Reactivation

pH-triggered self-assembled evolution

pH = 12

pH = 7

pH = 12

pH = 7

pH = 12

Figure 4.3  Controllable morphology evolution of the copolymers with a pH‐switched on/off function. Source: Wang et al. [72]. https://www.nature.com/articles/s41467‐018‐05142‐3. Licensed under CC BY 4.0.

Another example reported by the same group is poly(methacrylate isobutyl POSS)‐b‐poly(3‐dimethyl(methacryloyloxyethyl) ammonium propane sul­ fonaten‐co‐2‐(diethylamino) ethyl methacrylate‐co‐styrene) (PMAiBuPOSS‐ b‐P(DMAPS‐co‐DMAEMA‐co‐St)) prepared by the ring‐opening reaction [49]. As shown in Figure 4.3, the PMAPOSS‐b‐P(DMAEMA‐co‐St) can be assembled into relatively uniform micelles in water. The size of micelles is determined by the length of the DMAEMA block, pH value, and the content of DMAPS; with longer DMAEMA block, the size is larger than that of the shorter DMAEMA because of better hydrophilicity of chains. Besides, the micelle size also exhibited a great dependence on pH, which decreased with the increase of the pH value. Furthermore, higher content of DMAPS showed strong electrostatic interac­ tions, which weaken the interactions of molecular chains. Thus, the micelles become more compact because molecular chains are shrunk. A special pH‐ and reduction‐sensitive micelle is reported by Xing Wang et al. [72]. By activation and/or termination exchange reaction of the POSS–(succin­ imidyl succinate PEG [SSPEG])8 copolymer via the thiol‐disulfide bonds, they can modulate the constructive linkage of POSS‐embedded segments into micelles and return through the poly(ethylene glycol) (PEG) segments (Figure 4.2). The activated POSS–(SSPEG)8 acted as the preassemblies and can be further grown to the micellar connection, axial growth, bending, and cyclization pro­ cesses driven by the highly active connection points. Then, the preassemblies could provide a platform for the formation of all the nanoscale, microscale, and macroscale morphologies. In this system, the rigid POSS‐embedded backbone

4.2  Mechanism in Self‐assembly of POSS and PDMS‐Based Copolymers

with strong aggregation tendency can promote the hybrid polymers with unique self‐assembly behaviors, thereby various hierarchical nanostructures can be fur­ ther generated. 4.2.1.2  Thermosensitive Micelles

In general, amphiphilic block copolymers can self‐assemble in selective solvents to form micelles. However, thermoresponsive polymers are completely soluble in the solvent in all proportions at temperature below the lower critical solution temperature (LCST) and become insoluble above the LCST. The thermosensi­ tive micelles mainly consist of N‐isopropylacryl amide (NIPAM) [69, 73–75], propylene glycol [47, 76], ε‐caprolactone [77–79], and oligo(ethylene glycol) methacrylate (OEGMA) [80–82]. Poly(methacrylate isobutyl POSS)‐b‐poly(N‐isopropylacrylamide‐co‐oligo (ethylene glycol) methyl ether methacrylate) (PMAPOSS‐b‐P(NIPAM‐co‐ OEGMA)) was synthesized via RAFT polymerization. With hydrophobic nature of PMAPOSS segment and hydrophilic nature of P(NIPAM‐co‐OEGMA) seg­ ment, it can self‐assemble into spherical micelles. By adjusting the content of NIPAM or OEGMA domain, the essentially predetermined sharp and intensive LCST can be modulated. In addition, these hybrid micelles could be reversibly associated or disassociated by a heating and cooling solution with several cycles and the degree of reversibility is greatly concentration dependent [83]. Poly(propylene glycol) (PPG) is another thermally responsive polymer with tunable hydrophilic‐hydrophobic properties triggered by external tempera­ ture. The phase transition can be tuned with temperature ranging from 14 to 100 °C, depending on the architecture and molecular weight, which makes it more attractive in temperature‐responsive self‐assemblies. Hybrid copoly­ mers prepared from poly(ethylene glycol) methacrylate (PEGMA) meth­ acrylate POSS (POSSMA) together with poly(propylene glycol) methacrylate (PPGMA) was reported by Zibiao Li et al. [47]. The synthesized poly(PEGMA– PPGMA–POSSMA) (PEPS) exhibited LCST ranging from 31 to 33 °C. Static light scattering and dynamic light scattering (SLS and DLS) studies showed core–shell micellar morphologies. Compared to samples without POSS, PEPS copolymers with only 3.1 wt% POSS could effectively lower the critical micelle concentration (CMC) of the micelles at room temperature by 1 order of mag­ nitude. In addition, PEPS with 6.7 wt% POSS exhibited a constant hydrody­ namic radius, with Rh equal to 65 nm, and an aggregation number, with Nagg equal to 350, when the temperature varies from 20 to 70 °C. They have found that such interesting findings of PEPS hybrid copolymers could open up new opportunities to protect unfolded proteins from aggregation under high temperatures. Star‐shaped polymeric micelles with poly(ε‐caprolactone)‐poly(2‐(2‐methox­ yethoxy)ethyl methacrylate)‐co‐poly(ethylene glycol) methacrylate) (POSS‐ (PCLP(MEO2MA‐co‐PEGMA))16 [PCLP]) were synthesized via atom transfer radical polymerization (ATRP), ROP, and click reaction [77]. Owing to the hydrophobic property of POSS and PCL cores and the hydrophilic P(MEO2MA‐co‐ PEGMA) segments, the amphiphiles were found to self‐assemble into ellipsoidal structures with a moderately uniform size. In addition, the thermoresponsive

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4  Nanostructured Self‐assemblies from Silicon‐containing Hybrid Copolymers

properties could be finely tuned by changing the feed ratios of MEO2MA and PEGMA. Shasha Li et  al. found that the solution behaviors of POSS‐P(MEO2MA‐co‐ OEGMA) are mainly determined by the balance between hydrophilic and hydro­ phobic moieties. At low temperature, the amphiphilic P(MEO2MA‐co‐OEGMA) formed hydrogen bonds with water, whereas the POSS showed competitive hydrophobic effect. However, this balance was disrupted when temperature is higher than the LCST. The dehydration interaction induced that the formation of large aggregates would self‐assemble into a core–shell nanostructure with hydro­ phobic POSS as the core of the micelles and the hydrophilic P(MEO2MA‐co‐ OEGMA) as the corona of the micelles [84]. 4.2.1.3  Photoactive Micelles

The optical‐sensitive micelles are normally triggered by photoresponsive chro­ mospheres. A smart multi‐stimuli‐responsive copolymer (PEG‐b‐PDMAEMA‐ azo, PPA) of mono‐cyclodextrin‐substituted isobutyl POSS (mCPOSS) and azobenzene end‐capped poly‐(ethylene glycol)‐b‐poly(2‐(dimethylamino) ethyl methacrylate) was prepared [85]. The mechanism of its micelle formation in aqueous solution started from the nanosphere formation of mCPOSS self‐ assembled because of its amphipathic property. Then, the trans‐azo end groups on PPA interacted with cyclodextrin followed by the supramolecular assembly between PPA and mCPOSS. The morphology, formation/dissociation, and size could be adjusted by a ratio of PPA and mCPOSS, visible and ultraviolet light, and pH, respectively. 4.2.2  Other Mechanisms in Different Assemblies 4.2.2.1 Micelles

It has been found that POSS is a powerful hydrophobic unit in the assembly of  poly(acrylic acid)‐co‐poly(acrylate‐POSS) [86]. Aminopropylisobutyl POSS (ap‐POSS‐Br) initiated polymerization of methylmethacrylate (MMA), and methacrylisobutyl POSS (MA‐POSS) was reported to self‐assemble into ­ 150–300 nm core–shell micelles with ap‐POSS/MA‐POSS as the core and poly(methylmethacrylate) (PMMA) as the shell, or core–shell‐crown micelles with P(MA‐POSS) as the core, PMMA as the shell, and ap‐POSS as the crown when P(MA‐POSS) content is increased [87]. With low POSS ratio in the copoly­ mers, 40–110 acrylic acid repeat units in one POSS, the copolymers could self‐ assemble to form nanoaggregates. Increasing the volume fraction of insoluble blocks could result in morphological transformations [88]. The solvent effect mentioned showed that the copolymer POSS‐b‐PDMAEMA‐b‐PMMA self‐ assembled into polymeric micelles with different shapes such as spherical, rod, and necklace morphology in various solutions [89]. In specific solvents, P(MMA‐co‐GMA)‐b‐PMAPOSS could form ordered micellar‐like structures such as spherical, cylindrical, or vesicle‐like morphologies by tuning the copoly­ mer and mixed solvent compositions [88]. Controlling the length of the particu­ lar blocks also makes it possible to tune the micellar structures. Copolymers with

4.2  Mechanism in Self‐assembly of POSS and PDMS‐Based Copolymers

short‐range ordering lead to spherical micelles or cylindrical and lamellar aggre­ gates generally. Weian Zhang et al. studied that the chain length effect of PHEMPOSS‐b‐poly (methyl methacrylate) (PMAA) possessed different lengths of hydro­ philic chains [9]. The polymethacrylate monomer POSS (PHEMAPOSS45)‐b‐ PMAA523 formed typical core–shell spherical micelles with the hydrophobic PHEMAPOSS blocks as the core and hydrophilic PMAA blocks as the shell. The micelles are not conventional core–shell micelles because the dispersion of POSS moieties in the aggregates. Longer PMAA chain of PHEMAPOSS45‐b‐ PMAA1173 resulted in irregular aggregates after self‐assembly, whereas shorter hydrophilic PMAA chain of PHEMAPOSS45‐b‐PMAA308 lead to a dendritic cyl­ inder assembles. Thus, the assembled morphologies of PHEMAPOSS‐b‐PMAA block copolymers can be mediated. In addition, the electrostatic interaction between the two kinds of micelles was reported in the fabrication of mixed micelles as well [90]. Qiu et al. reported a similar process to assemble PDMS‐based block copoly­ mer to micelles with a wide variety of shapes and gel‐like phases [91]. They dem­ onstrated a process that amphiphilic P‐H‐P and H‐P‐H cylindrical triblock comicelles with hydrophobic (H) or polar (P) segments able to self‐assemble by side or end to end way in nonsolvents. Block copolymers with a crystallizable poly(ferrocenyldimethylsilane) (PFS) core‐forming block were used as precur­ sors and possessed either a nonpolar, corona‐forming H block of [poly(dimethylsiloxane) (PDMS) or poly(methylvinylsiloxane) (PMVS) or a com­ plementary P block of poly(2‐vinylpyridine) (P2VP) to form the micelle periph­ ery. Noncentrosymmetric H‐H‐P [92] and centrosymmetric P‐H‐P [93] amphiphilic triblock comicelles self‐assemble in polar media to form spherical supermicelles of size 1–5 mm with various aggregation numbers. Various chain lengths of poly(2‐ethyloxazoline‐block‐dimethylsiloxane‐block‐ 2‐ethyloxazoline) PEOXA‐b‐PDMS‐b‐PEOXA were synthesized. The individual tubular vesicle formed from PEOXA11‐b‐PDMS72‐b‐PEOXA11 exhibited a uni­ form external diameter of 29 nm and length in the range 50–250 nm. A tubular vesicle formed from the PEOXA14‐b‐PDMS72‐b‐PEOXA14 had a diameter of 36 nm and length in the range 50–450 nm. This communication provides a viable route toward artificial membrane systems that exploit the properties of mimick­ ing cellular functions. Worm‐shaped tubular vesicles are formed from amphi­ philic block copolymers with varying hydrophilic fractions by film rehydration techniques [94]. 4.2.2.2 Spheres

The amount of the microdomains increased with the content of poly (ε‐caprolactone)‐block‐poly(butadiene‐g‐POSS)‐block‐poly(ε‐caprolactone) denoted as PCL‐b‐P(B‐g‐POSS)‐b‐PCL increase. PCL‐b‐P(B‐g‐POSS)‐b‐PCL was found to self‐assemble into spherical microdomains with sizes of 20−50 nm in the epoxy matrix [95]. Because of the different viscoelastic properties of the poly(B‐g‐POSS) segments of the organic polymers, the P(B‐g‐POSS) blocks sep­ arated from the epoxy network to form dispersed microdomains.

105

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4  Nanostructured Self‐assemblies from Silicon‐containing Hybrid Copolymers

To study the chain length effect, Weian Zhang et  al. prepared various PHEMAPOSS‐b‐PDMAEMA copolymers with different lengths of PDMAEMA via RAFT reaction [15]. By varying the length of hydrophilic PDMAEMA block, the morphologies of self‐assembly can be tuned from irregular aggregates to spherical core–shell micelles and further from pearl‐necklace‐liked structure to capsules. The incorporation of POSS clusters as side groups was found to affect the size of assemblies because of the stretching effect of the polymer chains. Chui‐Song Meng et al. prepared multiple cluster‐wrapped polymers and block copolymers via the ROP polymerization [96]. Garnier et al. reported a new approach to obtain spherical nanodomains using polystyrene‐block‐polydimethylsiloxane (PS‐b‐PDMS). Adding PS homopoly­ mer into a low molar mass cylindrical morphology, the PS‐b‐PDMS system drives it toward a spherical morphology. Besides, by controlling the as‐spun state, spherical PDMS nanodomains could be kept and thermally arranged. Excellent long‐range‐order spherical microdomains were therefore produced on flat surfaces and inside graphoepitaxy trenches with a period of 21 nm and in‐plane sphere diameter of 8 nm with a 15 minutes thermal annealing [97]. Javier Arias‐Zapata et  al. prepared polystyrene–polydimethylsiloxane (PS–PDMS) blended with dioctylsebacate (DOC) and diisooctyladipate (DOA). It has been found that the composites formed highly ordered self‐assembled body‐centered cubic spherical PS–PDMS after spin coating without any annealing. During spin coating, plasticizer molecules remain in the PS chains because of their negligible vapor pressure of less than 1 mPa. After spin‐coating on functionalized Si sub­ strates, a CF4 followed by an O2 plasma etching step was performed to remove the PDMS top layer and the PS matrix, respectively. This two‐step plasma etch­ ing reveals an oxidized‐PDMS cubic‐centered sphere network having long‐range order without any post‐coating treatment. A morphology transition from bcc spheres to horizontal cylinders is obtained [98]. 4.2.2.3 Sheets

Generally, block copolymers are able to form a variety of ordered nanostructures via self‐assembly. Specially, POSS‐b‐PEO (poly(ethylene oxide)s) block copoly­ mers were reported to crystallize in a selected solvent that drives the block copolymers to be organized into large nanothick sheets by Cheng‐Bin Yu et al. [99]. The sheet size can be modulated ranging from micrometers to tens of micrometers with an increasing ratio of POSS in the samples. The sheet forma­ tion was attributed to a balance between the PEO block crystallization and the solubility of POSS block in the mixed solvent. The solubility of the whole nPOSS‐ b‐5.0kPEO in the mixed solvent was found to increase with the increasing con­ tent of POSS along with the slowing down of sheet growth and the decrease of the dimension. Zhiguang Li et  al. proposed and verified the packing model of POSS‐ (PMMA‐b‐poly(trifluoroethyl methacrylate) [PTFEMA])8, POSS‐(PTFEMA‐b‐ PMMA)8, and POSS‐(PTFEMA‐b‐poly[poly(ethylene glycol)methyl ether methacrylate] [PMPEGMA])8) at the air–water interface by Langmuir–Blodgett method [100]. With compression, all the molecules occupied a large surface area at the beginning and the POSS adopted a condensed globular state. The copoly­

4.3 Application

mer chains exhibited almost 2D conformation and the elasticity increases with the surface concentration. Georgopanos et  al. synthesized a series of linear (n  =  1, 2) and star‐block copolymers (n = 3, 4) of (PS‐b‐PDMS)n consisting of PS‐b‐PDMS diblock copol­ ymer via chlorosilane chemistry and studied their self‐assembly in bulk. The diblock copolymer precursors exhibit expected morphologies, and a chevron texture was identified during the self‐assembly of a (PS‐b‐PDMS)3‐type star block copolymer. For the linear diblock copolymer (SD‐1), alternating bright and dark layers can be clearly observed in the transmission electron microscopy (TEM) images, suggesting the formation of a lamellar phase. By increasing the arm number to two (sample (SD‐1)2), the TEM image and the small‐angle X‐ray scattering (SAXS) profile exhibit similar morphologies. When the arm number is further increased (n = 3, sample [SD‐1]3), an interesting zigzag pattern can be clearly identified in the TEM image, and there are no cracks or indication of stretching at the grain boundaries of the lamella regions. By further increasing the arm number to 4 (n = 4, [SD‐3]4), there is no formation of such a chevron structure [101]. Yoon Hyung Hur et al. reported a system containing a random‐ copolymerized block (poly(2‐vinylpyridine‐co‐4‐vinylpyridne)‐b‐poly(dimethyl­ siloxane) (P(2VP‐co‐4VP)‐b‐PDMS)). The self‐assembly behavior of a P2VP‐ b‐PDMS (7.34 kg/mol) thin film was studied by grazing incident small‐angle X‐ray scattering (GISAXS) measurements and scanning electron microscopy (SEM). For GISAXS measurement, the incident angle (αi) was set to 0.120°, which is below the critical angle (0.140°). They controlled composition of 2VP and 4VP to study the effect on morphology. Calculations were done based on small‐angle X‐ray scattering (SAXS) measurement with the addition of 4VP in the P(2VP‐co‐4VP) block. They suggested that a 4VP fraction of 33% provided the best pattern quality while ensuring a kinetic pathway for the self‐assembled pattern formation via thermal or microwave annealing [102].

4.3 ­Application 4.3.1  Biomedical Applications Drug delivery with self‐assembled amphiphilic copolymers have been exten­ sively studied for in the past two decades because of the ease of synthesis and structure modulation [103–105]. Among various drugs, drugs for cancer treat­ ment and diagnosis have attracted increasing attention [106, 107]. As a drug car­ rier, stability is quite important because once the micelle disassemble into free polymer chains, it will result in a burst release of encapsulated drugs, which is not so desirable for clinical usage [108, 109]. To endow carriers with controlled delivery, Zibiao Li et al. have well reviewed the development of various hybrid nanocarriers in remotely triggered drug release [110]. In terms of POSSs, Qingqing Yang et al. reported a star‐like organic–inorganic conjugate of POSS‐based nanomedicine [111]. The hybrids were synthesized by grafting semitelechelic N‐(2‐hydroxypropyl) methacrylamide (HPMA) copoly­ mers to POSS through reductively degradable disulfide bonds. As shown in

107

4  Nanostructured Self‐assemblies from Silicon‐containing Hybrid Copolymers

Cl H3N

NH

S N Si O O Si O

Cl H3N Cl H3N

O O Si

Si O

O Si

Si O

O

S

O

N

S

O

Cl H3N

NH

S

N

S

N

S

O

POSS–NH2

NH

O Si

S

NH3 Cl

S

N

O

O

NH3 Cl

O Si O Si O

O O

SPDP

NH3 Cl

S

O

O

Si NH

S

NH3 Cl

Si O

O

O

Si O

S

N

S

O

Si

Si O

HN O

O

S

O

HN Si

O Si O

POSS–PDS

O

S

HN

S S

HN

N

S

O

N

N

S

O

(a)

POSS-PDS

+

Conjugated via hydrazone bonds

Disulfide bond

Self-assembly

DTX P-SH

Redox-sensitive star copolymers

DTX

Star copolymer-docetaxel conjugates

SP-DTX

(b) do

En EPR effect

sis

to

cy

Blood vessel

SH

SH

e ul n ub tio ot za icr eri M lym ) e po SH as G le M re m TX 10 D l( d so late 0) 5. se yto su H~ a C ap (p ele enc r e m TX so D us so ed cle /ly at Nu do jug En con

Tumor cell

SH

SH

SH

SH

SH

SH

108

Tumor tissue

Fibroblast

(c)

Figure 4.4  (a) Synthesis procedure of pyridyldisulfanyl‐functionalized POSS (POSS‐PDS). (b) Self‐assembly of the star‐shaped POSS‐based conjugates. (c) Tumor accumulation and intracellular trafficking pathway of SP‐DTX nanoparticles. EPR, enhanced permeability and retention; SPDP, N‐[O‐succinimdy]‐3‐(2‐pyridyldithio) propionate. Source: Reprinted with permission from Yang et al. 2016 [111]. Copyright 2016, American Chemical Society.

Figure 4.4, the anticancer drug docetaxel (DTX) was attached to the grafts via pH‐sensitive hydrazone bonds and also encapsulated into the POSS. Such star‐ shaped conjugates could self‐assemble into nanoparticles (spherical particle [SP]‐DTX) and exhibited conspicuous drug‐loading capacity (20.1 wt%). The stimuli‐responsive DTX release under acidic lysosomal and reducing cytoplas­ mic environments was verified as well. SP‐DTX also displayed uniform tumor distribution and tumor growth inhibition of 78.9% compared with non‐redox‐ sensitive SP‐DTX‐A (67.4%), in contrast to SP‐DTX‐C which contained DTX only in the core exhibiting only 65.5% and linear P‐DTX showed 60.7% suppres­ sion through enhanced depletion of cancer‐associated fibroblasts and induction of apoptosis. The star‐shaped POSSs show advantages in increased drug‐loading capacity, uniform particle distribution, and enhanced drug stability. Another pH‐sensitive drug delivery micelle is prepared with poly (ε‐­caprolactone)‐poly(2‐(dimethylamino)ethyl methacrylate)‐co‐poly(ethylene

4.3 Application

glycol) methacrylate) (POSS‐PCL‐P(DMAEMA‐co‐PEGMA))16 using PCL as cores and star‐shaped P(DMAEMA‐co‐PEGMA) as coronas during dialysis pro­ cess [112]. Different concentrations of micelles could be obtained by diluting. The triggered self‐assembly behavior of these triblock copolymers could be modulated by pH values from 5.0 to 7.4 for controlled doxorubicin release. The drug release efficiency was up to 82% (w/w) with identified location of doxoru­ bicin (DOX) in HeLa cells and no associated cytotoxicity. The PDMS‐type silicone precursors possess established antibiofouling (AF) functionalities; thus, it is widely utilized in a variety of AF coatings. The presence of PDMS moiety offers the needed enhancements in surface stickiness, mechani­ cal strength, and hardness. Most of these PDMS coating materials are also func­ tionalized with amino [113, 114], fluorinated [115–117], zwitterionic [118–120], and quaternary ammonium [121–123], biocidal derivatives of relatively long alkyl groups. The presence of these chemical groups enhances the resistance to enzymatic activities, discourage bacterial biofilm formation, and also reduce bacterial settlement/growth [52]. Lei et al. utilized the hydrophobic interaction to anchor zwitterionic polysilox­ anes grafted with cysteine onto surfaces of hydrophobic block copolymer, referred to as PDMS‐b‐(PDMS‐g‐Cys). This kind of zwitterionic polymer has been used to cope with nonspecific protein adsorption and biofouling problems for a wide range of materials, including biomedical devices. The hydrophobic interaction moieties of the additional PDMS blocks improved the hydrophilic cysteine‐ grafted blocks. These findings suggest that the addition of hydrophobic moieties provides an effective approach to construct antifouling interfaces with zwitteri­ onic polymers in aqueous solution. The adsorption kinetics and protein resistance of the PDMS‐b‐(PDMS‐g‐Cys) adsorption layer have been examined. The con­ centration of each sample solution is set at 3 mg/ml, which is far higher than their CMCs, to form a sufficiently assembled layer of amphiphilic copolymers [124]. A similar research also performed by Chen et al. presented a superhydropho­ bic underwater self‐repairing and anti‐biofouling coating through the self‐ assembly of hydrophilic polymeric chain‐modified hierarchical microgel spheres. The obtained surface material not only has excellent underwater superoleopho­ bicity but also owns very good subaqueous anti‐biofouling property because of PDMS. More importantly, this surface material can recover the oil‐ and biofoul­ ing‐resistant properties once its surface is mechanically damaged, which is simi­ lar to the skins of some marine organisms such as sharks or whales [125]. Zhang et al. synthesized gelatin‐mono epoxy‐terminated polydimethylsiloxane polymer (PDMS‐E grafted gelatin [PGG]) and found that the excellent self‐cleaning func­ tion of PGG indicates potential application in biomaterials. The delicate supra­ molecular architectures present excellent antiwater, anticontamination, and antiradiation properties on the surface of skin [126]. 4.3.2  Photodynamic Therapy A novel amphiphilic diblock copolymer, PHEMAPOSS‐b‐P(DMAEMA‐co‐ coumarin methacrylate [CMA]), was prepared for photodynamic therapy (PDT) application via RAFT polymerization [127]. As shown in Figure  4.5,

109

110

4  Nanostructured Self‐assemblies from Silicon‐containing Hybrid Copolymers

O

CDB / AIBN O

65°C, 48 h

O

O

S S O

n O O

O O

NH

S

AIBN 65°C, 10 h

S

O

DMAEMA CMA

O

Si

n

O

O O

S

O

O O Si O Si O Si O O O O OSi O Si Si O Si O

O O Si O Si O Si O O O O OSi O Si Si O Si O Si

O N

S

O

NH

m

O O

O

O

O

NH

O O Si O Si O Si O O O O O Si O Si Si O Si

O

Si

O

HEMAPOSS

PHEMAPOSS O O

P(DMAEMA-co-CMA)-b-PHEMAPOSS

Etching UV = 365 nm

HF SO2H

Self-assembly aqueous phase

HO2S

Release

NH N N NH

SO2H xH2O

SO2H

Degradation P(DMAEMA-co-CMA)-b-PHEMAPOSS GSH

Figure 4.5  Schematic illustration of the synthesis of PHEMAPOSS‐b‐P(DMAEMA‐co‐CMA) block copolymer and structural changes in the process of loading and release of drugs. Source: Reprinted with permission from Zhang et al. [127]. Copyright 2016, American Chemical Society.

­ hotodimerization of coumarin leads to the micelles assembled with POSS core p and stimuli‐responsive shell, and then hollow polymeric capsules could be finally obtained via etching the POSS core. The hollow polymeric capsules are multi‐ responsive to redox potential and pH and could be utilized in the encapsulation and release of tetraphenylporphyrin tetrasulfonic acid hydrate (TPPS). The cap­ sule showed a relatively low TPPS release at pH = 7.4. However, a burst release of TPPS was found in the presence of 10 mM glutathione (GSH) at pH = 5.5. TPPS‐ loaded polymeric capsules also demonstrated low dark toxicity toward MCF‐7 cells. Because of the size effect of POSS, it could be an ideal sacrificial template for the encapsulation of drug molecules. Later, the same group reported stable unimolecular micelles self‐assembled from POSS‐(PCL‐b‐PDMAEMA)8‐biotin with an inorganic POSS nucleus, a

4.3 Application

hydrophobic poly(ɛ‐caprolactone) (PCL) middle layer, and a hydrophilic poly (2‐(dimethylamino)ethyl methacrylate) (PDMAEMA) outer corona [128]. The micelles were utilized in encapsulation and release of hydrophobic pheophor­ bide A (PPa) photosensitizers for photodynamic therapy (PDT). PPa‐loaded tumor‐targeted micelles could boost the internalization rate in HeLa cells ­effectively. In addition, the micelles also showed low dark toxicity and high PDT efficacy toward HeLa cells according to the 3‐(4,5‐dimethylthiazol‐2‐yl)‐2, 5‐diphenyltetrazolium bromide (methyl thiazolyl tetrazolium [MTT]) assay. 4.3.3 Coating Because of the intrinsic properties of POSS molecules, the motion of the chains can be effectively controlled and therefore induces molecules to self‐assemble, which could provide the copolymer with promising properties used as coat­ ings. Furthermore, POSS groups were reported to show low surface energy and thus could migrate and aggregate on the coating surfaces, endowing them with ­hydrophobicity for antifouling, anti‐icing, or protective coatings [129–131]. Poly(dimethylsiloxane) (PDMS) can also be used for superhydrophobic coat­ ings. It should also be mentioned that superhydrophobicity can also be achieved with silicon‐based polymers such as commercial poly(dimethylsiloxane) (PDMS). Chuan Li et  al. developed highly transparent antifogging/anti‐icing coatings with POSS‐poly[2‐(dimethylamino)ethyl methacrylate]‐block‐poly(sulfobetaine methacrylate) (POSS‐PDMAEMA‐b‐PSBMA) with a small amount of ethylene glycol dimethacrylate (EGDMA) [132]. The POSS clusters aggregated and well dispersed within the polymer matrix in the size of 10–80 nm. With the hygrosco­ picity of both PDMAEMA and PSBMA blocks with polymerization of EGDMA and the hydrophobicity of POSS, the copolymers exhibited excellent antifogging properties. In addition, the hygroscopic coatings could manipulate water mole­ cules well dispersed into the hydrophilic matrix via hydrogen‐bonded interac­ tions. Interestingly, the amphiphilic coatings exhibited the anti‐icing ability with a freezing delay time of more than two minutes at −15 °C, owing to the aggrega­ tion tendency of hydrophobic POSS and the self‐lubricating aqueous layer gen­ erated by nonfreezable bond water on the surface. Another promising application of hybrid inorganic/organic block copolymer is the functionalization of glass surfaces (at the nano‐ to macroscale), paper, or textiles (cotton) with water‐repellent ((super) hydrophobic) or both oil‐ and water‐repellent ((super) amphiphobic) coatings. Superhydrophobicity indicates that the generated surfaces have a contact angle greater than 150°, and in the case of superamphiphobic surfaces, these surfaces are both superhydrophobic and superoleophobic, that is, contact angles greater than 150°along with low contact angle hysteresis not only toward water but also for low‐surface‐tension oils. Indeed, hybrid block copolymers with an organic polymer segment such as poly(meth)acrylates with perfluorinated aliphatic side chains or a second inor­ ganic block segment composed of polydimethylsiloxane (PDMS) can be used to achieve this goal. The polymer block containing the silane can be easily grafted via sol–gel chemistry to these various surfaces to the silanols of glass surfaces or

111

112

4  Nanostructured Self‐assemblies from Silicon‐containing Hybrid Copolymers

silica nanoparticles or to the hydroxy groups of the d‐glucose units of cellulose fibers (paper and cotton) [4]. Developing self‐powered flexible electronic systems is of great importance for wearable and implanted biomedical devices [133–135]. In particular, mechanical energy‐harvesting systems using nanogenerators have attracted a considerable amount of research attention because of the technological possibility of realizing self‐powered electronic systems [136]. Several relevant studies have been per­ formed based on piezoelectric devices [137], electromagnetic induction [138], and triboelectric effects [139]. Yim et al. found a reliable and universal doping route for few‐layer transition metal dichalcogenides (TMDs) by employing sur­ face‐shielding nanostructures during a plasma‐doping process. It is shown that the surface‐protection‐oxidized polydimethylsiloxane nanostructures obtained from the sub‐20 nm self‐assembly of Si‐containing block copolymers can pre­ serve the integrity of 2D TMDs and maintain high mobility while affording exten­ sive control over the doping level. The oxidized polydimethylsiloxane (ox‐PDMS) surface‐masking nanostructures can easily and controllably be formed using the self‐assembly of Si‐containing block copolymers. The assembled copolymers provide a periodically arranged plasma‐blocking region and plasma‐accepting region, which protect the lattice from plasma attack and allow incident plasma irradiation, respectively. Using this protected plasma doping approach, n‐doping and p‐doping of few‐layer TMDs were, respectively, achieved by sulfur vacancy creation (n‐doping) and oxygen incorporation (p‐doping) [140]. 4.3.4  Optical Sensors The electrical detection of target species such as gas and biological molecules is one of the emerging research topics in the field of nanotechnology [141]. Electric field‐effect transistor (FET) types of sensors provide an attractive platform to sense the contents of biological or gas species because of the direct conversion of target samples to electronic signals, thus allowing more convenient and rapid detection [142, 143]. Nanostructured channels in electric FET‐type sensors have been intensively investigated for electrical sensing, as the dimensional features of the nanostructured channel are comparable to the scale of gas and biological species. The fabrication route for nanostructured channels can be categorized into two representative types: top‐down and bottom‐up [144, 145]. Organic–inorganic hybrid 3‐(trimethoxysily)propylmethacrylate‐coplatinum porphyrin‐co‐methacrylolsobutyl‐polyhedral oligomeric silsesquioxane (3‐(tri­ methoxysily)propylmethacrylate [TPMA]‐PtTPP‐POSS) copolymer films were prepared and applied as high‐performance oxygen sensors [146]. The strongly repulsive interactions between the organic (TPMA) and inorganic POSS blocks of the copolymers allow the formation of ordered nanostructures with smaller feature sizes. The introduction of POSS could enhance surface roughness and surface area for sensing. The oxygen sensor showed high sensitivity with a KSV of 1.833 per kPa, swift response capability (0.6 seconds). Such high sensitivity was ascribed to the worm‐like structures of the copolymers. The structure can ensure the homogeneous dispersion of PtTPP, which can be perturbed by a trace oxygen environment. Jung et  al. [147] reported an electrical gas sensor that

­  References

employed highly ordered conductive polymer nanowires enabled by bottom‐up self‐­assembly. To fabricate polymer nanowires, poly(3, 4‐ethylenedioxythio­ phene) : polystyrene sulfonate (PEDOT : PSS), SiO2, and poly(dimethylsiloxane) (PDMS) monopolymer brush were formed in sequence on a silica‐coated silicon substrate with 1.3 μm trenches. Afterward, flattened PDMS cylinders were self‐ assembled in a PS matrix within the trenches, utilizing PS‐b‐PDMS. Using the array of PDMS cylinders as an etching mask, PEDOT:PSS nanowires were pro­ duced through a reactive ion etching processes. Reported by Lingnan Chen et al., a metal‐sensitive organic–inorganic hybrid amphiphile consists of a POSS hydrophobic head and a hydrophilic PEG tail functionalized along with a bidendate ligand were synthesized [148]. The metal– ligand coordination was strong enough to stabilize the micelles in Zn2+/POSS‐ MA‐PEG‐DPA system, although the hydrogen bond interactions were damaged. Accordingly, their results suggest that the micelle morphology of POSS‐MA‐ PEG‐DPA in solution was sensitive to Zn2+ because of the coordination reaction. Thus, this new type of amphiphile indicates potential application in the fabrica­ tion of metal‐sensitive sensors.

4.4 ­Conclusions and Perspectives In this chapter, we have demonstrated that POSS and PDMS are two of the most interesting and important building blocks to construct and drive the assembly of amphiphilic hybrid copolymers. The key driving force of POSS and PDMS in their assembly comes from its unique hybrid‐cage structures together with its interesting physical properties such as hydrophobic and stable Si–O core with functional periphery organic groups. Moreover, interactions of the amphiphilic copolymers also include simple amphiphilic assembly, and it can be increasingly effective when specific van der Waals forces are introduced. However, challenges in this area still remains to be dissolved is the incorporation of bioactive segment onto these copolymers, which would allow more specificity and target for trig­ gered release. In addition, it would be more meaningful if the assemblies can be prepared uniformly in batches for preclinical and clinical test for developing practical applications. Moreover, great opportunities still remain for the devel­ opment of diversiform self‐assembly systems because various functional silox­ ane precursors are now commercially available, and also the precisely control of the substitution is possible because of the synthetic flexibility.

­References 1 Hendricks, M.P., Sato, K., Palmer, L.C., and Stupp, S.I. (2017). Acc. Chem. Res. 50:

2440.

2 Krieg, E., Bastings, M.M., Besenius, P., and Rybtchinski, B. (2016). Chem. Rev.

116: 2414.

3 Kim, Y., Li, W., Shin, S., and Lee, A.M. (2013). Acc. Chem. Res. https://doi.

org/10.1021/ar400027c.

113

114

4  Nanostructured Self‐assemblies from Silicon‐containing Hybrid Copolymers

4 Czarnecki, S. and Bertin, A. (2018). Chem. Tech. 24: 3354. 5 Ullah, A., Ullah, S., Khan, G.S. et al. (2016). Eur. Polym. J. 75: 67. 6 Yu, X., Zhong, S., Li, X. et al. (2010). J. Am. Chem. Soc. 132: 16741. 7 Ni, B., Huang, M., Chen, Z. et al. (2015). J. Am. Chem. Soc. 137: 1392. 8 Wang, X., Yang, Y., Gao, P. et al. (2015). ACS Macro Lett. 4: 1321. 9 Pu, Y., Zhang, L., Zheng, H. et al. (2014). Polym. Chem. 5: 463. 10 Yang, Y.‐Y., Wang, X., Hu, Y. et al. (2013). ACS Appl. Mater. Interfaces 6: 1044. 11 Fan, X., Cao, M., Zhang, X., and Li, Z. (2017). Mater. Sci. Eng., C 76: 211. 12 Weng, J.‐T., Yeh, T.‐F., Samuel, A.Z. et al. (2018). Nanoscale 10: 3509. 13 He, H., Chen, S., Tong, X. et al. (2017). Langmuir 33: 13332. 14 Tanaka, K., Inafuku, K., Naka, K., and Chujo, Y. (2008). Org. Biomol. Chem.

6: 3899.

15 Hong, L., Zhang, Z., Zhang, Y., and Zhang, W. (2014). J. Polym. Sci., Part A: 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

Polym. Chem. 52: 2669. Hong, L., Zhang, Z., and Zhang, W. (2014). Ind. Eng. Chem. Res. 53: 10673. Chi, H., Lim, S.L., Wang, F. et al. (2014). Macromol. Rapid Commun. 35: 801. Zhang, Z., Hong, L., Li, J. et al. (2015). RSC Adv. 5: 21580. Mohamed, M.G., Hsu, K.‐C., Hong, J.‐L., and Kuo, S.‐W. (2016). Polym. Chem. 7: 135. Han, Y., Wang, F., Lim, C.Y. et al. (2017). ACS Appl. Mater. Interfaces 9: 32418. Zhang, X., Chi, H., Li, T. et al. (2018). J. Polym. Sci., Part A: Polym. Chem. 56: 1225. Almutary, A. and Sanderson, B. (2017). J. Appl. Pharm. Sci. 7: 101. Yang, D.P., Oo, M.N.N.L., Deen, G.R. et al. (2017). Macromol. Rapid Commun. 38: 1700410. Pramudya, I., Rico, C.G., Lee, C., and Chung, H. (2016). Biomacromolecules 17: 3853. Yahyaei, H., Mohseni, M., Ghanbari, H., and Messori, M. (2016). Mater. Sci. Eng., C 61: 293. Falk, A., Dreimann, J.M., and Vogt, D. (2018). ACS Sustainable Chem. Eng. 6: 7221. Zhou, Y., Yang, G., Lu, C. et al. (2016). Catal. Commun. 75: 23. Zhang, W., Camino, G., and Yang, R. (2017). Prog. Polym. Sci. 67: 77. Lin, H.K. and Liu, Y.L. (2017). Macromol. Rapid Commun. 38: 1700051. Shang, D., Fu, J., Lu, Q. et al. (2018). Solid State Ionics 319: 247. Li, Z., Kong, J., Wang, F., and He, C. (2017). J. Mater. Chem. C 5: 5283. Wang, F., Lu, X., and He, C. (2011). J. Mater. Chem. 21: 2775. Ye, Q., Zhou, H., and Xu, J. (2016). Chem. Asian J. 11: 1322. Zhang, W. and Müller, A.H.E. (2013). Prog. Polym. Sci. 38: 1121. Hirai, T., Leolukman, M., Liu, C.C. et al. (2009). Adv. Mater. 21: 4334. Zheng, L., Hong, S., Cardoen, G. et al. (2004). Macromolecules 37: 8606. Liu, Y., Yu, C., Jin, H. et al. (2013). J. Am. Chem. Soc. 135: 4765. Raghunath, J., Zhang, H., Edirisinghe, M.J. et al. (2009). Biotechnol. Appl. Biochem. 52: 1. Wang, B., Lin, Q., Shen, C. et al. (2014). RSC Adv. 4: 52959. Kannan, R.Y., Salacinski, H.J., Edirisinghe, M.J. et al. (2006). Biomaterials 27: 4618.

­  References

41 Kannan, R.Y., Salacinski, H.J., De Groot, J. et al. (2006). Biomacromolecules 7: 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81

215. Ata, S., Banerjee, S.L., and Singha, N.K. (2016). Polymer 103: 46. Suenaga, K., Tanaka, K., and Chujo, Y. (2017). Chem. Eur. J. 23: 1409. Tanaka, K., Kozuka, H., Ueda, K. et al. (2017). Mater. Lett. 203: 62. Liu, L., Tian, M., Zhang, W. et al. (2007). Polymer 48: 3201. Liu, H. and Zheng, S. (2005). Macromol. Rapid Commun. 26: 196. Li, Z., Tan, B.H., Jin, G. et al. (2014). Polym. Chem. 5: 6740. Li, Y., Xu, B., Bai, T., and Liu, W. (2015). Biomaterials 55: 12. Xu, Y., Huang, J., Li, Y. et al. (2017). Macromol. Res. 25: 817. Ueda, K., Tanaka, K., and Chujo, Y. (2016). Polym. J. (Tokyo, Jpn.) 48: 1133. Rizvi, S.B., Yang, S.Y., Green, M. et al. (2015). Bioconjugate Chem. 26: 2384. Eduok, U., Faye, O., and Szpunar, J. (2017). Prog. Org. Coat. 111: 124. Pouget, E., Tonnar, J., Lucas, P. et al. (2010). Chem. Rev. 110: 1233. Whitesides, G.M. and Grzybowski, B. (2002). Science 295: 2418. Ding, C. and Li, Z. (2017). Mater. Sci. Eng., C 76: 1440. Li, C., Bai, S., Li, X. et al. (2018). ACS Omega 3: 7371. Ishizaki, Y., Yamamoto, S., Miyashita, T., and Mitsuishi, M. (2018). Langmuir 34: 8007. Zhang, W., Chu, Y., Mu, G. et al. (2017). Macromolecules 50: 5042. Zhu, H., Akkus, B., Gao, Y. et al. (2017). ACS Appl. Mater. Interfaces 9: 28144. Israelachvili, J.N., Mitchell, D.J., and Ninham, B.W. (1976). J. Chem. Soc., Faraday Trans. 2, 72: 1525. Li, Y., Zhang, W.‐B., Hsieh, I.F. et al. (2011). J. Am. Chem. Soc. 133: 10712. Yu, X., Zhang, W.‐B., Yue, K. et al. (2012). J. Am. Chem. Soc. 134: 7780. Huang, M., Yue, K., Huang, J. et al. (2018). ACS Nano 12: 1868. Zhang, W., Zhang, S., and Guo, Q. (2018). ACS Macro Lett. 7: 635–640. Lee, K., Kreider, M., Bai, W. et al. (2016). Nanotechnology 27: 465301. Niu, H., Zhang, L., Gao, M., and Chen, Y. (2005). Langmuir 21: 4205. Schilli, C.M., Zhang, M., Rizzardo, E. et al. (2004). Macromolecules 37: 7861. Wu, W., Wang, W., Li, S. et al. (2014). J. Polym. Res. 21: 494. Zhang, B.‐Y., He, W.‐D., Li, W.‐T. et al. (2010). Polymer 51: 3039. Pietsch, C., Mansfeld, U., Guerrero‐Sanchez, C. et al. (2012). Macromolecules 45: 9292. Xu, Y., Chen, M., Xie, J. et al. (2013). React. Funct. Polym. 73: 1646. Wang, X., Gao, P., Yang, Y. et al. (2018). Nat. Commun. 9: 2772. Feng, Z., Lin, L., Yan, Z., and Yu, Y. (2010). Macromol. Rapid Commun. 31: 640. Fan, X., Wang, X., Cao, M. et al. (2017). Polym. Chem. 8: 5611. Thakur, N., Sargur Ranganath, A., Sopiha, K., and Baji, A. (2017). ACS Appl. Mater. Interfaces 9: 29224. Miladinovic, Z.R., Micic, M., Mrakovic, A., and Suljovrujic, E. (2018). J. Polym. Res. 25: 1. Li, L., Lu, B., Wu, J. et al. (2016). New J. Chem. 40: 4761. Washington, K.E., Kularatne, R.N., Du, J. et al. (2017). J. Mater. Chem. B 5: 5632. Truong, T.T., Thai, S.H., Nguyen, H.T. et al. (2018). J. Mater. Sci. 53: 2236. Fang, Q., Chen, T., Zhong, Q., and Wang, J. (2017). Macromol. Res. 25: 206. Alejo, T., Prieto, M., García‐Juan, H. et al. (2018). Polym. J. (Tokyo, Jpn.) 50: 203.

115

116

4  Nanostructured Self‐assemblies from Silicon‐containing Hybrid Copolymers

82 Topuzogullari, M., Elalmis, Y.B., and Isoglu, S.D. (2017). Macromol. Biosci. 17:

1600263.

83 Xu, Y., Xie, J., Chen, L. et al. (2014). Polym. Adv. Technol. 25: 613. 84 Li, S., Liu, Y., Ji, S. et al. (2014). Colloid. Polym. Sci. 292: 2993. 85 Li, J., Zhou, Z., Ma, L. et al. (2014). Macromolecules 47: 5739. 86 Wang, Z., Tan, B.H., Hussain, H., and He, C. (2013). Colloid. Polym. Sci. 291:

1803.

87 Shao, Y., Aizhao, P., and Ling, H. (2014). J. Colloid Interface Sci. 425: 5. 88 Matějka, L., Janata, M., Pleštil, J. et al. (2014). Polymer 55: 126. 89 Li, L., Lu, B., Fan, Q. et al. (2016). RSC Adv. 6: 61630. 90 Xu, Y., Cao, Y., Xie, J. et al. (2016). J. Mater. Res. 31: 2046. 91 Qiu, H., Hudson, Z.M., Winnik, M.A., and Manners, I. (2015). Science: 347. 92 Rupar, P.A., Chabanne, L., Winnik, M.A., and Manners, I. (2012). Science

337: 559.

93 Sosna, M., Stoica, L., Wright, E. et al. (2012). Phys. Chem. Chem. Phys. 14:

11882.

94 Dhir, S., Salahub, S., Mathews, A.S. et al. (2018). Chem. Commun. (Camb.)

54: 5346.

95 Peng, W., Xu, S., Li, L. et al. (2016). J. Phys. Chem. B 120: 12003. 96 Meng, C.‐S., Yan, Y.‐K., and Wang, W. (2017). Polym. Chem. 8: 6824. 97 Garnier, J., Arias‐Zapata, J., Marconot, O. et al. (2016). ACS Appl. Mater.

Interfaces 8: 9954.

98 Arias‐Zapata, J., Böhme, S., Garnier, J. et al. (2016). Adv. Funct. Mater. 26: 5690. 99 Yu, C.‐B., Ren, L.‐J., and Wang, W. (2017). Macromolecules 50: 3273. 100 Li, Z., Ma, X., Guan, X. et al. (2016). Colloid. Polym. Sci. 295: 157. 101 Georgopanos, P., Lo, T.‐Y., Ho, R.‐M., and Avgeropoulos, A. (2017). Polym.

Chem. 8: 843.

102 Hur, Y.H., Song, S.W., Kim, J.M. et al. (2018). Adv. Funct. Mater.: 28. 103 Rösler, A., Vandermeulen, G.W.M., and Klok, H.‐A. (2012). Adv. Drug Delivery

Rev. 64: 270.

104 Gaucher, G., Dufresne, M.‐H., Sant, V.P. et al. (2005). J. Controlled Release

109: 169.

105 Biswas, S., Kumari, P., Lakhani, P.M., and Ghosh, B. (2016). Eur. J. Pharm. Sci. 106 107 108 109 110 111 112 113 114 115 116 117

83: 184. Chen, W., Ouyang, J., Liu, H. et al. (2017). Adv. Mater. 29: 1603864. Xu, X., Saw, P.E., Tao, W. et al. (2017). Adv. Mater. 29: 1700141. Zhang, X., Wang, H., and Dai, Y. (2017). Nanotechnology 28: 205601. Jaskula‐Sztul, R., Xu, W., Chen, G. et al. (2016). Biomaterials 91: 1. Li, Z., Ye, E., Lakshminarayanan, R., and Loh, X.J. (2016). Small 12: 4782. Yang, Q., Li, L., Sun, W. et al. (2016). ACS Appl. Mater. Interfaces 8: 13251. Zhang, J., Liu, X., Neri, G., and Pinna, N. (2016). Adv. Mater. 28: 795. Zhao, Q., Yan, M., and Lin, D. (2016). J. Appl. Polym. Sci.: 133. Hu, L., Cheng, J., Li, Y. et al. (2017). Appl. Surf. Sci. 413: 27. Owen, M.J. (2010). J. Appl. Polym. Sci. 35: 895. Owen, M.J. (2004). Surf. Coat. Int., Part B 87: 71. Xiao, K.Z., Poojari, Y., Drechsler, L.E. et al. (2008). J. Inorg. Organomet. Polym. Mater. 18: 246.

­  References

118 Sundaram, H.S., Han, X., Nowinski, A.K. et al. (2014). ACS Appl. Mater.

Interfaces 6: 6664.

119 Yesudass, S.A., Mohanty, S., Nayak, S.K., and Rath, C.C. (2017). Eur. Polym. J. 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148

96: 304. Vaterrodt, A., Thallinger, B., Daumann, K. et al. (2016). Langmuir 32: 1347. Qin, X., Li, Y., Fang, Z. et al. (2015). Appl. Surf. Sci. 328: 183. Ye, S., Majumdar, P., Chisholm, B. et al. (2010). Langmuir 26: 16455. Liu, Y., Leng, C., Chisholm, B. et al. (2013). Langmuir 29: 2897. Lei, Y., Lin, Y., and Zhang, A. (2017). Appl. Surf. Sci. 419: 393. Chen, K., Zhou, S., and Wu, L. (2016). ACS Nano 10: 1386. Zhang, Z., Hou, Z., Qiao, C. et al. (2018). Colloids Surf., B 171: 647. Zhang, Z., Xue, Y., Zhang, P. et al. (2016). Macromolecules 49: 8440. Zhang, P., Zhang, Z., Jiang, X. et al. (2017). Polymer 118: 268. Li, X., Zhang, K., Zhao, Y. et al. (2015). Prog. Org. Coat. 89: 150. Zhang, K., Li, X., Zhao, Y. et al. (2016). Prog. Org. Coat. 93: 87. Liu, B., Zhang, K., Tao, C. et al. (2016). RSC Adv. 6: 70251. Li, C., Li, X., Tao, C. et al. (2017). ACS Appl. Mater. Interfaces 9: 22959. Wang, Z.L. (2013). ACS Nano 7: 9533. Lin, Z.H., Xie, Y., Yang, Y. et al. (2013). ACS Nano 7: 4554. Hwang, G.‐T., Byun, M., Jeong, C.K., and Lee, K.J. (2015). Adv. Healthcare Mater. 4: 646. Park, K.I., Son, J.H., Hwang, G.T. et al. (2014). Adv. Mater. 26: 2514. Jeong, C.K., Kim, I., Park, K.I. et al. (2013). ACS Nano 7: 11016. Beeby, S.P., Torah, R.N., Tudor, M.J. et al. (2007). J. Micromech. Microeng. 17: 1257. Yoo, H.G., Byun, M., Jeong, C.K., and Lee, K.J. (2015). Adv. Mater. 27: 3982. Yim, S., Sim, D.M., Park, W.I. et al. (2016). Adv. Funct. Mater. 26: 5631. Hwang, I.S., Kim, Y.S., Kim, S.J. et al. (2009). Sens. Actuators, B 136: 224. Hsu, S.C., Hsin, C.L., Huang, C.W. et al. (2012). CrystEngComm 14: 4570. Stern, E., Wagner, R., Sigworth, F.J. et al. (2007). Nano Lett. 7: 3405. Choi, S.J., Ahn, J.H., Han, J.W. et al. (2011). Nano Lett. 11: 854. Ahn, J.H., Choi, S.J., Han, J.W. et al. (2011). J. Mater. Chem. C 10: 1405. Mao, Y., Zhao, Q., Wu, J. et al. (2017). J. Mater. Chem. C 5: 11395. Jung, Y.S., Jung, W., Tuller, H.L., and Ross, C.A. (2008). Nano Lett. 8: 3776. Chen, L., Zeng, B., Xie, J. et al. (2013). React. Funct. Polym. 73: 1022.

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5 Superhydrophobic Materials Derived from Hybrid Silicon Copolymers Lu Jiang1, Xian Jun Loh1, Chaobin He1,2, and Zibiao Li1 1

Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Innovis, #08‐03, Singapore 138634, Singapore 2 National University of Singapore, Department of Materials Science and Engineering, 9 Engineering Drive 1, Singapore 117576, Singapore

5.1 ­Introduction Superhydrophobic materials and surfaces have attracted increasing attention in both research and industry areas over the past three decades since 1990s [1]. The unique water‐repellent and self‐cleaning [2–4] properties of superhydrophobic materials greatly promote their study and applications, for instance, nonwetting [5, 6], oil–water separation [7–9], antifouling [10–12], anti‐icing [13, 14], and anticorrosion [15, 16] applications. The study of superhydrophobic phenomena was initially inspired by naturally occurring plants and insects surfaces [17–19], and the most notable ones are lotus leaf [20–22] and gecko foot. Even now, “lotus effect” is still synonymous with superhydrophobic behavior [23, 24]. To better understand and investigate the superhydrophobic behavior, a series of equations and parameters have been developed to define and measure the surface wetting properties and hydrophobicity, including the most common Young’s static and dynamic contact angle equations [25], dynamic contact angles (advancing and receding), and roll‐off angle [26–28]. Meanwhile, the effects of surface roughness or topology on contact angle have also been described using Wenzel and Cassie–Baxter regimes [23, 29, 30]. According to the above basics, a superhydrophobic surface can trap pockets of air in their porous texture and therefore display a high water contact angle (WCA) (>150°), a very low roll‐off angle (160°. Evaporation dynamics of a water droplet placed on these nano/microstructured substrates was investigated in this study [71]. In 2013, Mabry and Ramirez also synthesized a range of F‐POSS/MMA copol­ ymers using F‐POSS macromer and MMA via reversible addition–fragmenta­ tion chain transfer (RAFT) polymerization. These copolymers showed excellent wetting‐resistant behavior, and they were also the first example of covalently bound F‐POSS nanocomposites for improved surface robustness. The obtained F‐POSS/MMA copolymers coated cotton fabrics were proved to show both superhydrophobic and oleophobic behavior [72]. In 2010 and 2011, Qing group reported the synthesis of two series of POSS‐based hybrid terpolymers P(POSS–MMA–(HFPO)3MA) and P(POSS–MMA–VBFC) by radical polymerization (Scheme 5.2b,c). It has been found that the introduction of POSS cage in these terpolymers can improve their thermal properties; for P(POSS–MMA–VBFC), the Td > 250 °C can be achieved. Meanwhile, the POSS content in terpolymers would influence the hydrophobicity of coated cotton

123

H2 C O

CH3 C

x

CH3

H2 C

C

O

y

H2C

O

O

O

O

O

O Si

O Si

Si O

Si

Si O

O R Si Si O O O R Si Si O RO

H2 C

H2 C

CH3 C C O

O

m

H2 C

Si

C O O

CH3

CH2

CF3

O

O SiO

Si

Si O O

O Si

CH3

H2 C

H2 C

H2C

n

Rf

O Si O

O

CH3 C

O

O Si

C C O

Si O

H2 C

m

O

CH2

O Si O

O

n

CH3

O Si

Si

H C

O

O C O

CF3

Rf

Rf = CF3CF2CF2OCFCF2OCF-

R

O R Si O O Si O R Si O R

(a)

Si O

CF3

CF3

Rf = CF3CF2CF2OCFCF2OCF-

POSS-PMMA copolymer

P(POSS–MMA–(HFPO)3MA) copolymer

(b)

P(POSS–MMA–(VBFC) copolymer

(c)

Br m

O

O

O O

O

x

O

O

O R Si O R Si

y

O

m

O

O Si

O Si O Si O R

R

(f)

C H

Si

R Si O RO

OSi O R Si O O Si O R Si O R

R

(e)

R

P(MMA–co–BA–co–HEMA–co–FMA)–b–PMAPOSS copolymer

O

O

OH

R = H3C

O

Si O O SiO

n

O

R H2 C

O O

OH

O O

O

O

O

O FC

(CH2)3

CH3

n

O O

R Si O RO

O

O

O O

m

O O

Rf

P(MMA–co–BA–co–HEMA)–b–PMAPOSS copolymer

x

O

(CH2)2OH Si O O R Si O

O

z

O

O

(CH2)2CH3

(d)

y

O

O

O

O

CH3

C2H5

n

Si O O Si O

O Si

O Si O R Si O O Si O R Si O R

(H3CO)3Si

CF2

F3C

POSS–acrylic copolymer (PAC) copolymer

CF F

R

O

O

O

O

(CF2)5 F3 C

R

6

R = –C(CH3)3

(g)

O

O

OH

R Si O RO

O

O

CF3

O SiO Si O

O Si

R Si O R O

OSi O R Si O O Si O R Si O R

R

PFPEM–POSS copolymer

Scheme 5.2  Chemical structures of (a) POSS–PMMA, (b) P(POSS–MMA–(HFPO)3MA), (c) P(POSS–MMA–VBFC), (d) P(MMA‐co‐BA‐co‐HEMA)‐b‐PMAPOSS, (e) P(MMA–BA–HEMA–FMA)‐b‐PMAPOSS, (f ) POSS‐acrylic (PAC), and (g) PFPEM–POSS copolymers.

5.2  Hybrid Silicon Copolymer Materials with Superhydrophobic Property

f­abrics. The coated fabrics can achieve superhydrophobicity and high oleo­ phobicity when the incorporated POSS content in terpolymers increased to 13.4 wt% for P(POSS–MMA–(HFPO)3MA) and 7.1 wt% for P(POSS–MMA– VBFC). The corresponding water and salad oil CAs were 152° and 144° for P(POSS–MMA–(HFPO)3MA), and 159° and 141° for P(POSS–MMA–VBFC), respectively [73, 74]. In 2012, Xu and Nari et al. presented the fabrication procedure of fluorinated polyhedral oligomeric silsesquioxanes–poly(vinylidene fluoride‐co‐hexafluoro propylene) (fluoroPOSS–PVDF–HFP) nanocomposite mixtures. The fluoro­ POSS–PVDF–HFP nanocomposite was produced by mixing two fluorinated POSS materials (FP8 and FPSi8) with PVDF–HFP solution individually. Meanwhile, the transparent superhydrophobic coatings on glass substrates have also been fabricated by electrospinning. The prepared surface showed uniform, nonbeaded, and continuous nanofibers, and its superhydrophobicity was dem­ onstrated by the high WCA (157.3°) and the low SA 1000 S/m), high WCA (>150°) and low CAH 50°), which can be used to transport water droplets [91]. In 2017, Men and Zhang et al. developed a one‐pot and environmentally friendly method to prepare robust porous polydivinylbenzene (PDVB)–PDMS‐decorated superhydrophobic melamine sponge and filter membrane. By this method, in the presence of PDMS adhesive, hierarchically nanoporous coating can be formed with the growth of PDVB cross‐linking networks in situ on the substrates via polymerization. The prepared melamine sponge had high repeatable oil‐sorption capacity, and it can be utilized as oil‐collecting devices to immediately collect oils in situ on a large scale from water surfaces. Furthermore, the decorated sponge with chemical and mechanical stability was able to separate complex oil–water systems, including cold or hot water system, corrosive aqueous solutions, and extremely turbulent water system. The modified membrane also exhibited high separation efficiency for various surfactant‐stabilized water‐in‐oil emulsions [92]. In 2018, Liu and Zhao proposed a novel strategy of using reversible dynamic bonds to prepare the self‐healing coating material. In their work, a self‐healing coordinated silicon elastomer coating@fabrics, Co‐PDMS@PET fabrics, was

5.3  Application of Superhydrophobic Silicon Copolymer Materials

developed by cross‐linking pyridine‐functionalized PDMS via cobalt (Co)‐based coordination and then coating on polyethylene terephthalate (PET) fabrics. The obtained fabric proved its hydrophobic properties with a CA of >140°, and it has been applied for oil–water separation with a high separation factor of around 99% [93]. In 2019, Li and Tan presented a smart and recyclable oil absorber, hybrid nano­ particles Fe3O4@SiO2@P4VP–PDMS–P4VP, which was based on pH‐responsive block copolymer‐modified magnetic nanoparticles. The poly(4‐vinylpyridine‐b‐ dimethyl siloxane‐b‐4‐vinylpyridine) (P4VP–PDMS–P4VP) triblock copolymer was used for surface modification of superparamagnetic iron(III) oxide (Fe3O4) nanoparticle cores to achieve switchable superoleophilic and superoleophobic properties. The efficient and highly controlled oil–water separation performance of the modified NP was demonstrated by its great absorption of octadecene from oil‐in‐water emulsion (up to 78.2 times of its own weight) through simply switch­ ing the pH and applying a magnetic field (Scheme 5.4). The different oil miscibil­ ity and wettability of the nanoparticles are attributed to different dipoles of P4VP–PDMS–P4VP and protonated P4VP–PDMS–P4VP, as supported by den­ sity functional theory (DFT) calculations. In addition, the modified NPs can be easily recycled by acidic water washing [94]. P4VP–PDMS–P4VP TEOS Fe3O4

Fe3O4@SiO2

OCH3 I

Si OCH3 OCH3

Fe3O4@SiO2@P4VP–PDMS–P4VP At high pH

Magnet

Nanoemulsion At low pH

Magnet

Seperation

Bulk oil

Scheme 5.4  Schematic illustration showing the fabrication of pH‐sensitive and magnetic hybrid Fe3O4@SiO2@P4VP–PDMS–P4VP nanoparticle and working mechanism for remotely controlled oil‐in‐water nanoemulsion separation.

133

134

5  Superhydrophobic Materials Derived from Hybrid Silicon Copolymers High acid, alkali or salt oil–water mixture

Water-in-oil emulsion

Oil

Water

Hydrophilic segments Water attraction Crosslinking water

Heat treated

O

N

S S m

O S

Si O

HO

Si

O

Si n

O OH

O O

Si

O

Si O n

O

O

Adsorbed water droplet

S

O

O

O

Modified textile

oil

S

S O

F FF FF FF F O F Si F O OF FF F F F F

N

m

Oil

Gravity-directed separation

O

Figure 5.2  Schematic illustration of the modification process of PDMS–FA–PVP coating on textile (left) and separation process of the water‐in‐oil emulsions using the modified textile (right).

In 2019, Zhang et al. developed a superhydrophpbic, highly durable, and robust coating on cotton textiles by a facile sol–gel approach for effective oil–water separation (Figure  5.2). In their work, cotton textiles have been modified by PDMS–FA–PVP coating through sol–gel method using functional copolymers PVP–PDMS–PVP and PDMS. The PVP–PDMS–PVP copolymer was prepared by RAFT polymerization with the PDMS macromolecular chain transfer agent. Solely driven by gravity, the coated cotton textiles showed effective separation for both oil–water mixture (flux ∼ 7500 l/m2/h) and surfactant‐stabilized water‐ in‐oil emulsion. Moreover, this material still remained stable and showed great separation performance under harsh conditions (such as acidic, alkaline, or salty conditions) [95]. In 2019, Wang and Ahmed et al. developed a robust and simple dip–rinse pro­ cess to fabricate smart surfaces on various substrates, such as melamine sponge and nonwoven textile. In detail, the process included oxidative self‐polymeriza­ tion of dopamine on the material surface, followed by functionalizing with a block copolymer, poly(2‐vinylpyridine‐b‐dimethylsiloxane) (P2VP‐b‐PDMS) block copolymer. The fabricated surfaces process stable and continuous switch­ able surface oil wettability when responding to aqueous pH value change at room temperature. Furthermore, the materials were applied in oil–water separation with high performance for different kinds of oils and exhibited durability in strong alkaline and corrosive conditions [96]. In 2019, Li et al. prepared a series of scalable superhydrophobic/superoleophilic (SHBOI) porous materials by depositing SHBOI PDMS‐co‐PMHS coating onto various porous materials, including sponge (SP), fiber paper (FP), and glass fiber (GF) membrane [97]. The PDMS‐co‐PMHS with network structure was synthe­ sized by hydrosilylation reaction between terminal vinyl poly(dimethylsiloxane) (VPDMS) and trimethylsilyl‐terminated‐poly (dimethylsiloxane)‐co‐poly‐meth­ ylhydrosiloxane (TPDMS‐co‐PMHS). It was a novel SHBOI coating to replace the use of nanoparticles and fluorinated compounds. The oil–water separation per­ formance of these fabricated porous materials was investigated (Figure 5.3). The results revealed that PDMS‐co‐PMHS@SP processed SHBOI characteristics, high oil‐adsorption capacity, and recyclability, and PDMS‐co‐PMHS@FP also

5.3  Application of Superhydrophobic Silicon Copolymer Materials CH3

CH3

C Si O Si C H nH CH3 CH3

H2C

CH2

V-PDMS CH3 H3C

Si

O

CH3

H Si

CH3 O

CH3

Si O n CH3

CH3 Si

O m

CH3

H H

CH3 Si CH3

CH3

+

H H

TPDMS-co-PMHS

H2PtCl6

PDMS-co-PMHS with network structure Water droplet THF solvent

PDMS-co-PMHS coating

Porous substrate

Surface coating

Heating

PDMS-co-PMHS@Porous Substrate

Figure 5.3  The hydrosilylation reaction between VPDMS and TPDMS‐co‐PMHS and the fabrication process of PDMS‐co‐PMHS@porous substrate.

showed SHBOI characteristics and high immiscible water–oil separation effect with high oil/organic solvents fluxes (~2000–3000 l/m2/h), while PDMS‐co‐ PMHS@GF membrane exhibited great water rejection (∼99.90%) while separat­ ing water‐in‐oil emulsions under gravity‐driven condition. 5.3.1.2  POSS‐Based Superhydrophobic Materials

In 2015, Tang et  al. reported the preparation of superhydrophobic graphene oxide (GO)–epoxy‐functionalized polyhedral oligomeric silsesquioxane (ePOSS) composite by direct esterification of GO with ePOSS. The superhydrophobicity of GO–ePOSS composite was proved by a WCA of 145°. The efficient and robust oil–water separation was achieved by the GO–ePOSS membrane for oils with density >1 g/ml [98]. In 2016, Wen et al. prepared a new superhydrophobic/superoleophilic cotton fabric by a simple dip‐coating method (WCA > 150° and oil CA ~ 0°), in which a  cross‐linkable fluorinated copolymer poly(methyl methacrylate‐co‐butyl acrylate‐co‐hydroxyethyl methacrylate‐co‐perfluoroalkylethyl methacrylate‐co‐ stearyl methacrylate‐co‐methacrylisobutyl polyhedral oligomeric silsesquioxane) (P(MMA–BA–HEMA–FMA–SMA–MAPOSS)) was used as the coating mate­ rial. The coated cotton fabric can maintain its superhydrophobic property toward various harsh conditions, such as ultrasonic treatment in ethanol, thermal treat­ ment, and acidic or alkaline conditions. The cotton fabric can separate various oil–water mixtures with separation efficiency all above 96%; in particular, for n‐ hexane/water mixture, it can keep high separation efficiency (>98%) after 50 separation cycles [99]. Later, the same group fabricated another kind of superhydrophobic–­ superoleophilic stainless steel meshes with hierarchical structures by spraying a POSS hybrid acrylic copolymer P(MMA–SMAMAPOSS) on substrate in 2018 (Figure 5.4). The coated mesh had a static WCA of 153° and a SA of 4.5°. It has been tested for various oil–water mixture separation, and all separation efficien­ cies are nearly 99%. The coated mesh was proved to maintain the high separation

135

136

5  Superhydrophobic Materials Derived from Hybrid Silicon Copolymers

O

O O

O H3C

(CH2)17

R R O Si Si O O R Si O O Si O BA/XL AIBN O R R Si Si O O 80 °C 10 h R O O Si Si O R (CH2)3 O R=

SMA

CH2

O

C

x

OCH3

O

C y

C

CH3 H2C

MAPOSS

H2C

CH

CH3

* z O

O

(H2C)3

(CH2)17 H3C

C

O

R

CH3

O

MMA

CH3

CH3 CH2 C

*

R

R O O Si Si O Si Si O O R O OR O Si Si O Si O O Si O R R

Copolymer: P(MMA–SMA–MAPOSS)

(a)

Spraying

Water

Copolymer solution (b)

Figure 5.4  (a) Synthesis of the POSS‐containing acrylic copolymer and (b) formation of a superhydrophobic–superoleophilic mesh.

efficiency (~99%) after 25 cycles for n‐hexane/water mixture separation. Meanwhile, the good mechanical stability of coated mesh has also been demon­ strated by high WCA (~145°) and n‐hexane/water mixture separation efficiency (~99%) after 20 abrasion cycles [100]. Wang and Chen newly developed a kind of superhydrophobic smart fabrics with durable UV‐cured coatings from pH‐responsive polyurethane (pH‐PU) and fluorated octavinyl polyhedral oligomeric silsesquioxane (F‐OV‐POSS). The prepared fabric coatings processed durable superhydrophobicity, self‐cleaning, and pH‐controllable oil–water separating ability even under harsh conditions, like mechanical damage, seawater, and UV irradiation conditions. The wettabil­ ity of the fabric coating would change between the superhydrophobicity and underwater superoleophobicity with response to pH value change and therefore can separate oil or water from oil–water mixtures. Meanwhile, the smart fabrics showed self‐healing ability under heating to recover their special wettability, which is probably because of the migration of preserved fluorocarbon chains in the coatings [101]. 5.3.2  Self‐cleaning and Antifouling Beside oil–water separation, several reports have illustrated the potential of hybrid silicon copolymer‐based superhydrophobic materials in self‐cleaning and antifouling applications. For instance, Kang and Lee synthesized a novel amphiphilic organic/inorganic hybrid star‐shaped copolymer (SPP) using poly(ethylene glycol) methyl ether

5.3  Application of Superhydrophobic Silicon Copolymer Materials

methacrylate (PEGMA) and MAPOSS as monomers and octakis(2‐bromo‐2‐ methylpropionoxypropyldimethylsiloxy)‐octasilsesquioxane (OBPS) as an initia­ tor by ATRP in 2012. The polysulfone (PSf ) ultrafiltration membranes coated with SPP were fabricated and the coated membranes showed good bio‐ and oil‐ fouling resistance and flux recovery ability for oil–water emulsion separation. The dual effective antifouling properties of the coated membranes was attrib­ uted to the simultaneous enhancement of hydrophilic PEG and hydrophobic MAPOSS moieties on the membrane surfaces to decrease the interactions with proteins and increase the repellence to oils [102]. In 2018, a triblock copolymer based on PDMS–poly(tert‐butyl acrylate)– poly(methacrylol‐sobutyl POSS) has been reported by He et al., which was syn­ thesized via combination of single‐electrontransfer living radical polymerization (SET‐LRP) and ATRP from a PDMS macroinitiator. The synthesized copolymers can form hierarchical structures through self‐assembly followed by the nonsol­ vent vapor‐induced phase separation. The superhydrophobic property of opti­ mized surfaces was demonstrated by a high WCA (156.7° ± 0.5°), a low roll‐off angle (200 °C and viscosities ranging from 5 cSt to about 1 million cSt. Amino‐functional silicones are also widely used in hair products. Hair made of proteins is negative charged. Traditional conditioner attaches on the hair surface through van der Waals attractions, which is weak and can be easily removed. Amino silicones bind to hair surface through strong electrostatic force. The amino group with positive charge is responsible for the binding properties, while the silicone segment functions as a conditioning agent in reducing hair damage or providing a shiny and glamorous appearance. Based on the theory, Aeby et al. developed a new hair care composition containing a certain amount of one terminally functionalized diquaternary silicone compound and one quaternized amine compound [74]. The product provided a hair care composition having improved care properties, especially the feel and combability of the treated hair in the wet and dry state as well as a reduced load on the hair. 6.2.4  Strategies for Depositing Silicone on Hair The deposition of silicone influences the improvement of properties of hair ­substrates. To improve the silicone deposition on hair, various strategies were developed. Cationic polymers have been widely used as an agent for improving silicone deposition on hair. Many properties of cationic polymers, such as molecular weights, surface charges and charge densities, importantly influence on the ­silicone deposition. Nanavati et  al. found that adding quaternary ammonium compounds, such as benzalkonium chloride, benzethonium chloride and dimethyldioctadecyl ammonium chloride polymers, into silicone emulsions were able to improve deposition of silicone on hair [75]. The cationic compounds exhibited strong bonding properties with silicone oils and functionalized as linking agents to enhance the silicone deposition on hair. The researchers also found that the

6.2  Silicone for Personal Care Applications

interaction and linkage between silicone and hair increased with the increasing molecular weights of quaternary ammonium compounds. La Torre et al. demonstrated that virgin hair (without treatment) showed much less silicone deposition, whereas the deposition was greatly increased for hair samples treated with emulsions with positive surface charge [76]. The result indicated that hair surface treated with positive charge were favourable for higher silicone deposition on hair. It is also found that polymers with higher charge density would help the silicone deposition on hair. Besides cationic polymers, other additives or formulation strategies can also help to deposit silicone onto hair. Gamez‐Garcia found that cystine proteins in shampoo could strongly interact with silicones due to their multiple ionic and polar sites, leading to a higher viscosity of shampoo and the improvement of silicone deposition on hair [77]. The higher deposition was beneficial for repairing damaged cuticle scales of hair. It is reported that a shampoo with liquid crystal colloidal structures could affect the electrostatic and hydrophobic properties on hair, and hence improved the silicone deposition [78]. 6.2.5  Silicone for Skin Care Applications Skin is the largest organ of the integumentary system covering our body. Due to the interaction with outside environment, skin plays an important immunity role and protective barrier in protecting our body against extra water loss and pathogens. Skin also has some other important functions, including temperature regulation, sensation, UV absorption, synthesis of vitamin D and sebum secretion. There are four basic types of healthy skin: normal, dry, oily and combination skin, according to the sebum production and moisture degree of the skin. Different type of skin requires skin care treatment. Silicone‐based formulations appears in various skin care products, including serum, lotion, cream, antiperspirant formulations and sunscreen products. Silicone oil emulsions in the formulations provide several special properties of  silicone, such as excellent spreading and film‐forming properties, gloss, dry non‐sticky feel. Silicone polymers can be used as a film forming agent for the protection of skin barriers, which is considered as an evolutional technology for peristomal skin care. It is found that dimethicone in water cream can treat skin with denuded stratum corneum with better barrier properties compared to those creams without silicone. More importantly, silicone‐based barrier creams resulted in a negligible pain on application compared to the severe pain from the application of alcohol‐based peristomal skin products [79]. The Protectant film barriers provide a long lasting physical barrier that protects the skin, especially peristomal skin, from irritants such as stomal effluent and urine. Hoggarth et  al. studied the efficacy of six skin protectants against a skin irritant introduced by sodium lauryl sulphate. The result indicated that W/O products using petrolatum are more efficacious than O/W products with dimethicone in protecting the skin. However, dimethicone‐containing products exhibited higher hydration properties [80]. Silicone can also be used in adhesive remover products. Hexamethyldisiloxane in the products is able to penetrate rapidly the adhesive‐skin bond by spreading effectively and rapidly on the peristomal skin

161

162

6  Silicone Copolymers for Healthcare and Personal Care Applications

surface and forming an interposing layer under the interface. Such adhesive remover can be a product of choice for the removal of stoma appliances, and also be effective for the applications of removing urinary sheaths and dressings where trauma to delicate skin may occur [81]. Silicone copolymers possess good film‐forming properties, and hence they can be used as delivery matrixes for active ingredients, such as oil‐soluble vitamins and sunscreens. Organic UV filters, such as octyl methoxycinnamate, butyl‐ methoxydibenzoylmethane and ethylhexyl‐methoxycinnamate, can be more efficiently delivered from a sunscreen formulation together with silicone polymers, generating a higher sun protection factor (SPF). It is reported that a sunscreen formulation containing 4% silicone elastomer could increase the SPF value from ~5 to 18. The study indicated that the silicone elastomer is capable to maximize the effectiveness of sunscreen agents in a formulation. By adjusting the silicone percentage, the amount of organic UV filters can be reduced. Therefore, the cost of the formulation can be well‐controlled and the side effect caused by the organic UV filters (irritation or cytotoxicity) can be minimized. Silicone also enable more flexibility of the sunscreen formulation, in order to increase the wash‐off resistance, translating to longer‐lasting protection from water and sweat for sports or beach applications. Moreover, the unique properties of silicone polymers are able to reduce the oily and sticky feel induced by oily organic UV filters. Recently, new advancement in silicon technology focuses on grafting organic UV filters onto silicone backbones. Pattanaargson et al. grafted p‐methoxycinnamate (OMC) moieties, a widely used UV‐B filter, onto the amino functionalized silicone polymer through amide linkages [81]. They found that the new copolymer showed the similar UV absorption profiles compared to that of neat OMC with less sensitivity to solvent difference. Silicone‐based polymers can also be used in facial or body cleansing formulations. Some water‐soluble polydimethylsiloxane polyether can be used as a secondary surfactant, which helps for stabilizing foaming, improving skin feel properties and reducing eye and skin irritation. Volatile (poly)dimethylcyclosiloxane and their deviates are very commonly used in non‐rinsable makeup removers and cleansers. Such polymers are good solvents for organic‐based oils, and they are able to offer a smooth, silky and non‐greasy feeling on human skin. Some low molecular weight silicones, such as cyclomethicone, are often utilized in cleanser formulations for makeup removal, due to their low surface tension, good wetting properties and the capacity to eliminate dirt or colour cosmetic residues.

6.3 ­Conclusions In conclusion, silicone copolymers especially PDMS‐based have fully demonstrated their versatility and feasibility as biomaterials in the area of biomedical and healthcare. Due to the low surface energy of the PDMS block, films prepared with PDMS copolymers have surfaces with unique microdomains which have profound effect on protein adsorption and cell adhesion. Furthermore, via silicone specific reactions like hydrosilylation and silane coupling, and interactions,

­  References

PDMS substrate can be easily and reliably coated with silicone copolymers to  enhance surface biocompatibility and properties. Through these surface enhancements, silicone copolymers were explored in various applications like antifouling, antibacterial and tissue engineering and regenerative medicine. Silicone copolymers in aqueous solution have unique self‐assembly behaviors. Among many different structures, micelles and polymersomes are the most commonly reported self‐assemblies from silicone‐based diblock or triblock copolymers and these self‐assemblies are found to be useful in the area of drug delivery and artificial cell study. Although silicone‐based polymers are widely used in almost all types of personal care products, critics around the world attempt to stop the use of silicone due to its safety concerns and environmental issues. Similar to plastic wrap, silicones easily form a film barrier on the surface of skin. Such barrier can keep moisture, but also trap dirt, sweat, bacteria, sebum, dead skin cells and other debris along with it. Therefore, silicone would irritate the skin, cause allergic reactions, clog the pores and induce acne. More importantly, silicones, unlike natural sebum, is difficult to be removed by washing with a normal shampoo with anionic surfactant. It is found that silicones deposited by typical 2‐in‐1 shampoos can only be removed gradually after a number of washes with sodium lauryl ether sulphate solution [82]. Silicone is not a natural ingredient, and its side effects are bad for hair. It gives the hair the illusion of shine, but it is a fake plastic shine instead of natural hair shine. Silicone prevents hair from moisture, making it limp, lifeless, and dull. With time it will dry the hair out. Due to lack of moisture and nutrients, the hair will become very brittle and could lead to frizz and breakage. A study in Europe showed that cyclomethicone (D4) could be an endocrine disruptor. Animal studies suggested that D4 exhibits toxicity profiles that may have indirect or direct impact on human health [83]. On the other hand, silicones could cause detrimental effects on the environment. Silicones are not biodegradable. Although some types of silicones can be recycled, but it is unlikely to happen to the silicones that are used on the skin and washed down the drain. In 2008, an Environment Canada Review found that certain cyclic silicones (D4 and D5) could harm the environment and have potential risks in aquatic organisms. Personal care industries should take responsibility to address the safety and environmental concerns, and material scientists and formulators should be working out more human‐friendly and eco‐friendly formulations for benefiting our lives.

­References 1 Abbasi, F., Mirzadeh, H., and Katbab, A.‐A. (2001). Polym. Int. 50: 1279. Whitford, M.J. (1984). Biomaterials 5: 298. 2 Chumbimuni‐Torres, K.Y., Coronado, R.E., Mfuh, A.M. et al. (2011). RSC Adv. 3

1: 706.

4 Childs, M.A., Matlock, D.D., Dorgan, J.R., and Ohno, T.R. (2001). Biomacromolecules

2: 526.

5 Bates, F.S. and Fredrickson, G.H. (1999). Phys. Today 52: 32.

163

164

6  Silicone Copolymers for Healthcare and Personal Care Applications

6 Deppisch, R., Gohl, H., and Smeby, L. (1998). Nephrol. Dial. Transplant. 13:

1354.

7 Zdrahala, R.J. and Zdrahala, I.J. (1999). J. Biomater. Appl. 14: 67. 8 Pergal, M.V., Nestorov, J., Tovilovic, G. et al. (2014). J. Biomed. Mater. Res. Part A

102: 3951.

9 Stefanovic, I.S., Djonlagic, J., Tovilovic, G. et al. (2015). J. Biomed. Mater. Res. 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

Part A 103: 1459. Okano, T., Aoyagi, T., Kataoka, K. et al. (1986). J. Biomed. Mater. Res. 20: 919. Seo, J.H., Matsuno, R., Takai, M., and Ishihara, K. (2009). Biomaterials 30: 5330. Banerjee, I., Pangule, R.C., and Kane, R.S. (2011). Adv. Mater. 23: 690. Wong, I. and Ho, C.M. (2009). Microfluid. Nanofluid. 7: 291. Zhang, H. and Chiao, M. (2015). J. Med. Biol. Eng. 35: 143. Ajit Walter, P., Muthukumar, T., and Reddy, B.S. (2015). Lett. Appl. Microbiol. 61: 274. Shi, M., Zhu, J., and He, C. (2016). RSC Adv. 6: 114024. Liu, C., Xie, Q., Ma, C., and Zhang, G. (2016). Ind. Eng. Chem. Res. 55: 6671. Sommer, S., Ekin, A., Webster, D.C. et al. (2010). Biofouling 26: 961. Zhang, X.M., Li, L., and Zhang, Y. (2013). Physics Procedia 50: 328. Ren, W., Cheng, W., Wang, G., and Liu, Y. (2017). J. Polym. Sci., Part A Polym. Chem. 55: 632. Kugel, A., Chisholm, B., Ebert, S. et al. (2010). Polym. Chem. 1: 442. Novi, C., Mourran, A., Keul, H., and Möller, M. (2006). Macromol. Chem. Phys. 207: 273. Zhang, Q., Liu, H., Chen, X. et al. (2015). J. Appl. Polym. Sci. 132: 41944. Jonca, J., Tukaj, C., Werel, W. et al. (2016). J. Mater. Sci. ‐ Mater. Med. 27: 55. Hussey, G.S., Dziki, J.L., and Badylak, S.F. (2018). Nat. Rev. Mater. 3: 159. Kai, D., Prabhakaran, M.P., Chan, B.Q. et al. (2016). Biomed. Mater. 11: 015007. Ferreira, L., Squier, T., Park, H. et al. (2008). Adv. Mater. 20: 2285. Bratt‐Leal, A.M., Carpenedo, R.L., Ungrin, M.D. et al. (2011). Biomaterials. 32: 48. Carpenedo, R.L., Seaman, S.A., and McDevitt, T.C. (2010). J. Biomed. Mater. Res., Part A 94: 466. Song, L., Ahmed, M.F., Li, Y. et al. (2017). Tissue Eng. Part C 23: 627. Iwasaki, Y., Takamiya, M., Iwata, R. et al. (2007). Colloids Surf. B. 57: 226. Miller, P.J. and Matyjaszewski, K. (1999). Macromolecules 32: 8760. Seo, J.H., Matsuno, R., Konno, T. et al. (2008). Biomaterials. 29: 1367. Nagahashi, K., Teramura, Y., and Takai, M. (2015). Colloids Surf. B. 134: 384. Halim, A., Reid, T.D., Ren, J.M. et al. (2014). J. Polym. Sci., Part A Polym. Chem. 52: 1251. Zhang, Y., Zhang, Z., Wang, Q. et al. (2006). Macromol. Rapid Commun. 27: 1476. Pavlović, D., Linhardt, J.G., Künzler, J.F., and Shipp, D.A. (2008). J. Polym. Sci., Part A Polym. Chem. 46: 7033. Li, X., Gao, Y., Boott, C.E. et al. (2016). J. Am. Chem. Soc. 138: 4087. Qiu, M., Zhao, X.‐Z., Liu, D.‐P., and He, C.‐J. (2015). RSC Adv. 5: 17851. Car, A., Baumann, P., Duskey, J.T. et al. (2014). Biomacromolecules 15: 3235.

­  References

41 Pavlović, D., Lou, Q., Linhardt, J.G. et al. (2017). J. Polym. Sci., Part A Polym.

Chem. 55: 3387.

42 Raez, J., Tomba, J.P., Manners, I., and Winnik, M.A. (2003). J. Am. Chem. Soc.

125: 9546.

43 Qian, J., Lu, Y., Chia, A. et al. (2013). ACS Nano. 7: 3754. 44 Raez, J., Manners, I., and Winnik, M.A. (2002). J. Am. Chem. Soc. 124: 10381. 45 Ghamrawi, S., Bouchara, J.P., Tarasyuk, O. et al. (2017). Mater. Sci. Eng., C

75: 969.

46 Kihara, Y., Ichikawa, T., Abe, S. et al. (2013). Polym. J. 46: 175. 47 Kwon, G.S. and Kataoka, K. (1995). Adv. Drug Delivery Rev. 16: 295. 48 Kataoka, K., Harada, A., and Nagasaki, Y. (2001). Adv. Drug Delivery Rev. 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75

47: 113. Protat, M., Bodin, N., Gobeaux, F. et al. (2016). Langmuir. 32: 10912. Cabral, H., Miyata, K., Osada, K., and Kataoka, K. (2018). Chem. Rev. 118: 6844. Kiene, K., Schenk, S.H., Porta, F. et al. (2017). Eur. J. Pharm. Biopharm. 119: 322. Dou, Q.Q., Liow, S.S., Ye, E. et al. (2014). Adv. Healthcare Mater. 3: 977. Erdodi, G. and Kennedy, J.P. (2006). Prog. Polym. Sci. 31: 1. Xu, C., Hu, S., and Chen, X. (2016). Mater. Today 19: 516. Buddingh, B.C. and van Hest, J.C.M. (2017). Acc. Chem. Res. 50: 769. Martin, L., Gurnani, P., Zhang, J. et al. (2019). Biomacromolecules 1297: 20. Carlsen, A., Glaser, N., Le Meins, J.F., and Lecommandoux, S. (2011). Langmuir 27: 4884. Salva, R., Le Meins, J.F., Sandre, O. et al. (2013). ACS Nano. 7: 9298. Dao, T.P.T., Fernandes, F., Fauquignon, M. et al. (2018). Soft Matter 14: 6476. Kowal, J., Wu, D., Mikhalevich, V. et al. (2015). Langmuir. 31: 4868. Klermund, L., Poschenrieder, S.T., and Castiglione, K. (2016). J. Nanobiotechnol. 14: 48. Dinu, M.V., Spulber, M., Renggli, K. et al. (2015). Macromol. Rapid Commun. 36: 507. Kawaguchi, M. and Kubota, K. (2004). Langmuir. 20: 1126. Mehta, S.C., Somasundaran, P., and Kulkarni, R. (2009). J. Colloid Interface Sci. 333: 635. Kawaguchi, M., Kimura, Y., Tanahashi, T. et al. (1995). Langmuir 11: 563. Horozov, T.S., Binks, B.P., and Gottschalk‐Gaudig, T. (2007). Phys. Chem. Chem. Phys. 9: 6398. Michaut, F., Hébraud, P., Lafuma, F., and Perrin, P. (2003). Langmuir 19: 10086. Sakai, K., Ikeda, R., Sharma, S.C. et al. (2010). Langmuir 26: 5349. Lentsch, S. E., Man, V. F., Ihns, D. A., et al. (2005). US6956019B2, filed 19 July 2004 and issued 18 October 2005. Torgerson, P. and Uehara, N. (2007). AU2007254194B2, filed 17 May 2007 and issued 28 November 2007. Bolish Jr, R. E., Cobb, D. S., Kwasniewski, V. J. et al. (1990). US4902499A, filed 27 March 1987 and issued 20 February 1990. Wood, J. L. and Mariahazy, A. (1993). US5362484A. Crudele, J., Bhatt, D., Kamis, K., and Milczarek, P. (2000). US6056946A. Aeby, J., Irrgang, B., Mager, H. et al. (2004). US6696052B2. Nanavati, S. and Hami, A. (1994). J. Soc. Cosmet. Chem. 45: 135.

165

166

6  Silicone Copolymers for Healthcare and Personal Care Applications

76 La Torre, C. and Bhushan, B. (2006). J. Cosmet. Sci. 57: 37. 77 Gamez‐Garcia, M. (1998). J. Cosmet. Sci. 49: 213. 78 Brown, M.A., Hutchins, T.A., Gamsky, C.J. et al. (2010). Int. J. Cosmet. Sci.

32: 193.

79 Berry, J., Black, P., RorySmith, D., and Stuchfield, B. (2007). Br. J. Nurs. 16: 778. 80 Hoggarth, A., Waring, M., Alexander, J. et al. (2005). Ostomy. Wound Manage.

51: 30.

81 Pattanaargson, S., Hongchinnagorn, N., Hirunsupachot, P., and Sritana‐Anant, Y.

(2007). UV absorption and photoisomerixation of p‐methoxycinnamate grafted silicone. Photochem. Photobiol. 80 (2): 322–325. 82 Haake, H.‐M., Lagrené, H., Brands, A. et al. (2007). J. Cosmet. Sci. 58: 443. 3 Dudzina, T., von Goetz, N., Bogdal, C. et al. (2014). Environ. Int. 62: 86. 8

167

7 Construction of Organic Optoelectronic Materials by Using Polyhedral Oligomeric Silsesquioxanes (POSS) Fuke Wang1, Xuehong Lu2, Zibiao Li1, and Chaobin He1,3 1

Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Innovis, #08‐03, Singapore, 138634, Singapore 2 Nanyang Technological University, School of Materials Science and Engineering, 50 Nanyang Avenue, 639798, Singapore 3 National University of Singapore, Department of Materials Science and Engineering, 9 Engineering Drive 1, Singapore, 117576, Singapore

7.1 ­Unique Properties of POSS for Building Organic Optoelectronic Materials Organic optoelectronic materials can be generally divided into small molecular weight semiconductors and conjugated polymers with high opto/electronic performance. This interesting electroluminescence and conducting properties were first discovered in the 1970s, but the real surge of the interest occurred in the past 20 years with significant improvements in material performance through creative material design and high material purification [1, 2]. Small molecular weight semiconductors are interesting because of their high purity, ordered structures, and ideal model for fundamental mechanism studies of excitons and charge carriers. Conjugated polymers are mainly used for device fabrication for their advantages of large‐area device fabrication, high performance, and multiple functions. Currently, organic materials receive considerable attention because of their successful applications in organic light‐emitting diodes (OLEDs), solar cell, liquid crystal display (LCD), thin film transistors, sensors, electrochromic devices, and many others [1, 3–6]. Thanks to the extensive academic and industrial efforts, the mechanism and the relationship between the molecular structure and optoelectronic properties have been illustrated, and various device structures have been developed. Although organic optoelectronic materials have experienced tremendous progress over the past 10 years in fundamental physics and real applications, more work is necessary such as theoretical descriptions in charge generation and transport, distinction in the contributions of various interactions, thermal stability improvement, and commercialization of existing applications [7, 8]. Various new phenomena are still being discovered every day and device performance is continuing to rise [2, 9–13]. Among these efforts, the incorporation of polyhedral oligomeric silsesquioxanes (POSSs) into organic Silicon Containing Hybrid Copolymers, First Edition. Edited by Chaobin He and Zibiao Li. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2020 byWiley-VCH Verlag GmbH & Co. KGaA.

7  Construction of Organic Optoelectronic Materials

optoelectronic materials to achieve improved performances has been attracting attention particularly because of the unique and interesting hybrid structures of POSS. Polyhedral oligomeric silsesquioxanes are inorganic–organic hybrid particles with the hard inorganic ceramic core surrounded by organic silicone polymers or functional groups. The general formula of POSS is Rn(SiO1.5)n, where n is commonly 6, 8, 10, or 12; the ratio of Si to O is 1.5; and the value of n = 8 is studied most extensively [14, 15]. A typical chemical structure of POSS is shown in Figure 7.1, which includes the siloxane cage with Si∙O∙Si bonds, which provides high thermal stability and chemical robust framework [16, 17]. The peripheral organic arms can be small molecules or polymers, which provide excellent solubility or miscibility to the organic matrix. The regular element of the Si∙O∙Si framework in the core with silica architecture makes POSS a perfect nanofiller for polymer nanocomposites [13, 18]. The well‐defined sizes of POSS and various functional group modifications allow their use in composition with polymer matrix with unique properties. The resulting organic–inorganic hybrid materials often display improved thermal, mechanical, optical, or chemical properties [19–24]. Because of these aspects, functional silsesquioxanes possess great application potential in organic optoelectronic material construction to provide deep insight into the charge carriers’ dynamics and to improve device stability or performance. In general, the following physicochemical features of POSS could be considered to build a new organic optoelectronic material [25]. Firstly, POSS can be used to reinforce the thermal stability of the resulting materials. The stiff and inert SiO core structure in POSS will not only reinforce the mechanical properties of the polymer materials but also function as a thermal barrier to isolate heat from circumstance because of its low thermal conductivity. Therefore, POSS has been widely used as a building block to enhance the mechanical and thermal properties of organic optoelectronic materials. Pramudya et al. recently reported a significant enhancement in both mechanical strength (sixfold higher) and optical transmittance by copolymerization of POSS Organic groups or polymers

Inert and hard core structure with Si—O—Si bonds

R O R

O O

Si O O

R

Si R

O

O X

O

Si O

R

O

O Si

R Si O Si

Si

Si

Functional or reactive groups

168

R

Hybrid inorganic–organic nanostructure of POSS

Figure 7.1  The hybrid nanostructure of POSS.

7.1  Unique Properties of POSS for Building Organic Optoelectronic Materials

into the active conjugated polymers [26, 27]. The enhancement was attributed to the small size of the silica‐like cage that acts as nanoparticle fillers to isolate heat from surroundings. We also showed that coupling POSS with small functional molecules can significantly enhance their thermal stability and thus broaden their application temperatures. For instance, by coupling POSS with a light‐ active azobenzene dye, a 177 °C increase in the decomposition temperature of the POSS‐coupled dye over the unmodified one was observed [28]. Secondly, POSS can function as a 3D scaffold to build a star‐shaped structure to change the molecular aggregation behavior and to achieve new properties for organic optoelectronic materials. Monosubstituted and higher phenyl‐substituted octahydrosilsesquioxanes have been used as models to simulate the electronic structures of POSS. The results showed that both the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) were localized on the phenyl functional groups [29]. These results suggest that POSS can help to suppress the self‐quenching of organic luminescent dyes, luminescent oligomers, or polymers in their aggregate but improve their quantum efficiencies in photoluminescence and electroluminescence. This interesting property has been successfully used in the design of OLEDs and sensing organic optoelectronic materials to improve their performance. As we know that the strong π–π stacking in most organic luminescent materials normally causes a significant drop of quantum yield in the solid state. By using the 3D scaffold POSS structures, the self‐assembly behaviors of these conjugated luminescent materials can be suppressed together with improved processability [30]. The same strategy was also employed in electrochromic materials to disrupt conjugated polymer packing and provide a loose and porous film, allowing free ion movement and more accessible sites for the redox reaction. It was also demonstrated that the star‐shaped structures of POSS could help to change the crystallinity of materials and thus improve the device performance. An interesting example is the POSS‐based ionic liquid developed by Wang et  al. [31–33] Imidazolium iodides are one the most popular additives as liquid electrolytes in dye‐sensitized solar cell (DSSC) application, but their crystallinity inhibits pore filling and interfacial contact. However, coupling of the imidazolium salt with POSS gave an amorphous nature of the solid electrolytes (Figure  7.2), with improved solar cell conversion efficiency without adding a crystal growth inhibitor. A similar conducting ionic liquid based on POSS such as POSS octacarboxy anions (POSS‐COO−)8 and (POSS–PEG)8 was also reported [34, 35]. In addition, the glass transition temperatures (or application temperatures of the devices) can be easily tuned by changing the length of alkyl chains that are linked to the imidazolium iodides. Last but not least, the sizes of POSS core (1–3 nm) are close to a large molecule, together with their uniform size, and POSS is an ideal building unit for organic optoelectronic model compounds to gain insight into the kinetic or dynamic mechanism of carrier and energy transfer. Very recently, we successfully incorporated POSS into the donor–acceptor conjugated system to study the intra‐ and interchain energy transfer in the luminescent system [36]. As can be seen in Figure  7.3, POSS provides a circumference shielding of the polymer ­backbone to prevent close packing of the polymer chains, indicating that the

169

170

7  Construction of Organic Optoelectronic Materials R Si

R

O Si O Si

R

R

O

O Si

O R O

Si O

O Si R O O O O

POSS-8lm-M

R=

N + N l-

POSS-8lm-E

R=

N + N l-

POSS-8lm-B

R=

N + N l-

POSS-8lm-O

R=

N + N l-

Si R

Si R

Figure 7.2  Structures of POSS‐based imidazolium iodides with various alkyl groups of different lengths. Source: Wang et al. 2013 [33]. Reproduced with permission of American Chemical Society.

(a)

(b)

Figure 7.3  The comparison of the molecular design of side chains (a) and POSS (b) shielding polymers. Source: Zhang et al. 2018 [36]. Reproduced with permission of John Wiley & Sons.

intrachain energy transfer dominates in large concentration range. Our results showed that POSS can effectively shield interchain energy transfer of the polymers, suggesting that it is an effective model for energy transfer study with less interchain effects [37]. Similarly, POSS has been used to control supramolecular assemblies into crystalline lattice for the applications in the aggregation‐induced emission (AIE)‐based sensors and biodevices [38, 39] to build the self‐assembly macromolecules and polymers for the liquid crystal applications [14, 40, 41]. In this chapter, we will summarize POSS‐based organic optoelectronic materials for various applications, particularly in the area of electroluminescent materials,

7.2  POSS‐Based Organic Electroluminescence Materials

electrochromic devices, and other important applications such as liquid crystal display and sensor.

7.2 ­POSS‐Based Organic Electroluminescence Materials Organic electroluminescent materials have been attracting considerable attention in recent years because of their advantages of facile device fabrication and low power consumption; particularly, the development of the organic white light‐emitting materials hold great promises for the production of highly efficient large area light sources. There has been a constant endeavor to enhance the brightness and efficiency of OLED by means of using novel small‐molecule and polymeric materials to accelerate this technology for practical applications [42–44]. Hybridation of silsesquioxane architecture with either small‐molecule or organic polymer fragments has been applied to OLEDs to improve the device efficiency and light purity. Sellinger et al. introduced this composition concept first in 2003 to build the compounds containing a spherical “silica” core with a hole‐transporting chromophore [45]. The incorporation of POSS into OLED materials offers many advantages for OLEDs that include introducing amorphous properties with enhanced thermal resistance, light stability at higher temperatures, high solubility and easy processing, and high purity. POSS helps to reduce the aggregation of chromophores, which result in quenching the fluorescence [22, 46]. The most popular highly efficient conjugated chromophores such as carbazole, fluorene, terfluorene, and pyrene have been hybridized with POSS with the aim to further improve properties that includes having a greater efficiencies, ­brightness, thermal stability, and stable color emission to name a few quantum [47–49]. As we discussed in Section  7.1, employing POSS as a spacer to reduce the intermolecular interaction is one of the most widely explored approaches by our and other research groups in the design of electroluminescent materials [50–54]. The special considerations for using POSS here include its larger size, low dielectric constant, and its high band gap (intensive absorption at 6 eV and emission at 4.2 eV), which is much higher than those of conventional organic compounds or oligomers. These properties make POSS an ideal candidate for isolating organic arms while limiting the influence of its electronic properties on organic moieties. For example, in one recent study, we reported a series of nanohybrid light‐emitting dots with a diameter ranging from 2 to 4 nm by directly implanting organic conjugated chains to POSS [50]. Because of the unique architecture, it was noted that POSS could prevent the overlapping of organic chains, whereas the intermolecular interaction was significantly reduced by the outer layer of alkyl/alkoxy chains attached to the top of the organic chains and thus achieved a drastic reduction in aggregation. The nanohybrids’ distinctive structure gave them outstanding photoluminescence properties that differ from conjugated polymers and most organic molecules. Moreover, there was a tremendous increase in the PL quantum efficiency (PLQE), regardless in solution or in condensed states.

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7  Construction of Organic Optoelectronic Materials

400

400

200

200

0 200 (a)

(b)

250 300 350 Wavelength (nm)

PL intensity (a.u.)

In comparison to the photoluminescence (PL) spectrum of the nano‐hybrid in most organic solvents, blue shift was observed when it was in solid film [50]. Using the similar strategy, POSS‐assisted quantum confinement effect was proposed for organic photoluminescent materials and synthesized organic quantum dots (QD) cluster materials where POSS were used to spatially isolate the small organic molecules [51, 52]. As shown in Figure 7.4, the clusters were made up of a cubic core with short organic arms extending from all eight corners. Quantum confinement effects were achieved because of the core acting as a stable spacer, thus separating the organic arms. In addition to acting as luminescent centers, the organic arms hindered intermolecular aggregation as the ends were capped with alkyl chains. The nano‐sized and rigid cage‐like POSS enhanced the material properties in mechanical and thermal stabilities and further reduced the electronic effect faced by the organic moieties. The torsion energy barrier was increased by the incorporation of methyl groups to the R position of the neighboring phenyl ring, resulting in a reduction of phenyl ring rotation, preventing PL line broadening (Figure 7.4) [51]. Similarly, Chang et al. developed a blue‐light electroluminescent nanoparticle containing eight carbazole chromophore arms and octakis[N‐(9‐ethyl‐9H‐­ carbazol‐3‐yl)undecanamide‐11‐dimethylsiloxy]silsesquioxane (POSS‐C11‐Cz), which can form well‐ordered dispersive structure in bulk state [53]. In their design, the carbazole units were linked to the POSS cage through alkyl chain spacers, creating a bulky POSS core, resulting in the prohibition of interchain interactions, which ultimately caused a reduction in aggregation and excimer formation. This was confirmed through the photoluminescence and optical spectra of POSS‐C11‐Cz in both solution and solid state. Particularly, the photoluminescence measurement of a blend of polyfluorene (97 wt%) and POSS‐C11‐ Cz (3 wt%) showed stable color luminescence even after the sample was heated for five hours at 200 °C. Here, we can see that both merits of POSS that include the unique star‐shaped 3D structure and high heat resistance of the silica core were used in their design [53].

PLE intensity (a.u.)

172

0 400

Figure 7.4  (a) Organic clusters made up of a cubical core with eight short organic arms stretching from the corners were synthesized. (b) The photoluminescence excitation (PLE) and PL results proved that these organic clusters possessed properties similar to quantum dots (QD). Source: He et al. 2004 [51]. Reproduced with permission of American Chemical Society.

7.2  POSS‐Based Organic Electroluminescence Materials

From the above works, we can see that a careful computer simulation is important for these material designs. Generally, molecular dimension, packing, energy band gap, and optical properties should be considered to achieve the best performance. For instance, single molecular POSS‐based white‐light‐emitting hybrid materials were recently designed based on the computer simulation [55]. Density functional theory (DFT) method was used to simulate the absorption and emission spectra of blue (B) and yellow (Y) emitting units and chromophores B and Y with similar absorption spectra and a different Stokes shift in their emission spectra were selected. Based on theoretical simulations, three POSS‐based luminous single‐molecule nanohybrids containing a yellow monochromatic emitter (Y: propylene nitrile) with a large Stokes shift and a blue monochromatic emitter (B : 9‐ethynyl anthracene) were designed and prepared as shown in Figure  7.5 [55]. Both emitters possess similar absorption spectra to avoid the strong self‐ absorption between different emitters. The partial energy transfer from B to Y is allowed for effectively adjusting the balance of the white light emission. The POSS‐based white‐light‐emitting single‐molecule nanohybrid, which was precisely and controllably prepared via click chemistry by simply controlling the feed ratio of blue and yellow‐light‐emitting units (Figure 7.5). The results show that the incorporation of nano‐sized inorganic POSS effectively restrained intramolecular rotation and a significant decoupling effect of the emitter and an AIE effect, which provided an important contribution to the high emission efficiency of hybrid molecules. The resultant nanometer organic–inorganic hybrid exhibited a significantly enhanced emission in the solid film (Φfilm = 95%) because of the significant AIE effect, which is attributed to the incorporation of nano‐sized POSS moieties. POSS‐assisted reduction of aggregation is the most often employed strategy when applied in organic electroluminescent material design. Hyperbranched organic–inorganic hybrid conjugated polymers were synthesized from NC

+

m

CN

+

n

O Si CH2N3

1

O

8

ODZMS NC KF, Cu(PPh3)3Br DMF

H2 O Si C N

CN

B/Y N N 8

m : n = 7 : 1 W71 6 : 2 W62 5 : 3 W53

Y=

O

B=

Si O Si O Si O SiO O O O O OSi O OSi Si O Si

Figure 7.5  The synthetic route of white‐light hybrids, where the blue monochromatic emitter (B) is 9‐ethynyl anthracene and propylene nitrile is the yellow monochromatic emitter (Y). Source: Gang et al. 2018 [55]. Reproduced with permission of Royal Society of Chemistry.

173

174

7  Construction of Organic Optoelectronic Materials

4,7‐bis(3‐ethylhexyl‐2‐thienyl)‐2,1,3‐benzothiadiazole and POSS at varying concentrations through FeCl3‐oxidative polymerization. Compared with its linear counterpart, the POSS‐coupled polymers showed 80–400% increase in PL quantum efficiencies because of the reduction of the aggregation and thus lower down the intermolecular quenching at solid state. Its electroluminescent (EL) spectra showed an emission λmax at 660 nm, which was within the range for pure red. Furthermore, its EL spectrum was much narrower than its linear counterpart. Under similar voltage and film thickness, the hyperbranched polymer displayed ~500% greater EL brightness, showing its high potential as an effective red electroluminescent material for light‐emitting diode (LED) applications [54]. Naka et  al. reported a bifunctional terminal polyhedral octasilicate (OS)‐core dendrimer containing carbazole and 1,8‐naphthalimide on its peripheries (OS‐NC). The photoluminescence spectrum of OS‐NC coating film showed exciplex emission and displayed greater red shift emission in comparison to binary blends [56]. In addition to the reduction of aggregation, POSS‐based luminescence materials also displayed higher efficiencies, brightness, and thermal stability [7, 18, 30, 57–63]. For instance, using the Gilch polymerization method, POSS05‐poly(p‐ phenylenevinylene)s (PPV) was synthesized to carry 5 mol% POSS‐attached PPV units, whereas POSS25‐PPV and POSS100‐PPV carried 25 mol% and 100 mol%, respectively. EL devices containing the POSS–PPV possessed better thermal stability with the glass transition temperature in the range of 64–77 °C compared to 58 °C of MEH‐PPV. MEH‐PPV had a maximum brightness of 3880 cd/m2 at 14.8 V and luminance efficiency of 0.075 cd/A at 3880 cd/m2, whereas POSS25‐ PPV showed higher maximum brightness and luminance efficiency of 6340 cm/m2 at 13.2 V and 0.26 cd/A at 6190 cd/m2, respectively, indicating that the presence of the POSS moieties caused an improvement in the EL properties (Figure 7.6). However, a binary bland made up of 95 wt% of MEH‐PPV and 5 wt% of POSS25‐PPV showed a luminance efficiency of 0.48 cd/A at 10 540 cd/m2, which was 6.4 times better than that of POSS25‐PPV. It also displayed a maximum brightness of 11 010 cd/m2 at 14.3 V. This surprising enhancement was due to insulation domains of the POSS moieties being formed within the polymer matrices. Therefore, in addition to the PL efficiencies of the emitting polymers, the EL efficiencies of the devices are dependent on the charge mobility and there should be a balance of charge carrier of electrons and holes [57, 58]. In another report, the addition of POSS into polyfluorene derivatives showed a decrease in interchain interaction along with the formation of keto, which led to a remarkable increase in fluorescence quantum yields. The POSS units also resulted in a reduction in fluorescence quenching and also giving the POSS‐tethered polymer better thermal stability [59].

Figure 7.6  (a) The electroluminescence spectra of devices with an ITO/PEDOT:PSS/polymer blend (POSS25‐PPV in MEH‐PPV)/Ca/Al configuration. (b) Relationship of voltage and luminance of POSS25‐PPV in MEH‐PPV. (c) Relationship of voltage and luminance efficiencies of POSS25‐PPV in MEH‐PPV. Source: Kang et al. 2006 [57]. Reproduced with permission of American Chemical Society.

7.2  POSS‐Based Organic Electroluminescence Materials 1.2

POSS25-PPV, 5% in MEH-PPV POSS25-PPV, 25% in MEH-PPV

Normalized intensity

1.0 0.8 0.6 0.4 0.2 0.1

(a)

400

500

600 Wavelength (nm)

700

Luminance (cd/m2)

10 000

1000

100

10

1 POSS25-PPV, 5% in MEH-PPV POSS25-PPV, 25% in MEH-PPV

0.1 0

2

4

6

Luminance efficiency (cd/A)

(b)

10

12

14

0.1

0.01

1E-3

POSS25-PPV, 5% in MEH-PPV POSS25-PPV, 25% in MEH-PPV 0

(c)

8 Voltage (V)

2

4

6

8 Voltage (V)

10

12

14

175

176

7  Construction of Organic Optoelectronic Materials

The ratio of the POSS core with the chromophores also affects the device performance. Yang et al. synthesized POSS macromolecules with platinum complex and carbazole moieties [60]. It showed that the monomer/excimer emission balance was directly affected by the ratio of carbazole to platinum complex moiety combined with the POSS. In comparison to analogous devices that made use of a mix of platinum complexes and polymer matrix, light‐emitting devices that used the POSS macromolecule showed remarkably greater efficiencies with a peak external quantum efficiency of c. 8%. Moreover, there is an increase in the device efficiency along with the ratio of monomer and excimer/aggregate ­emission intensity in the electroluminescent spectra when there is a decrease in the amount of platinum complex in the POSS macromolecule. This is due to the declined concentration quenching and reduction of interaction between the platinum complexes [60]. In a similar manner, POSS nanoparticles were introduced to the choromophore matrix to synthesize a novel organic–inorganic amorphous 3,6‐dipyrenylcarbazole‐POSS hybrid material (POSS‐DPCz) (Figure  7.7). The introduction of the POSS nanoparticles was to facilitate the formation of a three‐dimensional nanostructure. OLED device fabrication was able to take place because of the presence of a suitable HOMO–LUMO energy level. When the POSS derivative was used in a three‐layer OLED device, a maximum brightness of 8900 cd/m2 was observed where a stable blue light at 450 nm was observed. Results showed that when compared to the control DPCz‐based devices, its performances were nearly two times higher [61]. With these enhanced properties, most electroluminescent materials showed stable and pure color emission, in particular blue light [64–68]. Almost in most of the applications of POSS in organic optoelectronic material development, increased thermal stability was observed. Therefore, the use of POSS to increase device application temperature and enhance thermal stability

R′ O H O Si Si Si O O Si O Si O RO O R′ O O Si O Si R′ Si Si O O R′ R′

R′

R′ =

O

Si

H

Q8M8

+

Pt-dvs Toluene, 80 °C

N

DPCz

Figure 7.7  Synthetic procedure for POSS‐DPCz and its high electroluminescence property in solution. Source: Cheng et al. 2016 [61]. Reproduced with permission of Royal Society of Chemistry.

7.2  POSS‐Based Organic Electroluminescence Materials

l l

Si O Si O O Si O Si O O O Si OO Si O Si O Si O

l

Reaction sequence: 1 and 2 or 2 and 1

l

l 1. Heck coupling: 2. Heck coupling: 3. Sonogashira coupling:

l

1 and 3 l

Si O Si O O Si O Si O O O Si OO Si O Si O Si O

l

n Pd2(dba)3 / (Pd(PtBu3)2), dioxane, NCy2Me, 60 °C, 48 h Pd2(dba)3 / [Pd(PtBu3)2], Cul dioxane, NCy2Me, 60 °C, 48 h

=

R

R = H, p-OMe, NMe2 =

;

Figure 7.8  The synthesis of high thermally stable hybrid luminescent materials by using Sonogashira reaction. Source: Gon et al. 2016 [69]. Reproduced with permission of Royal Society of Chemistry.

has become a popular choice. In 2016, Chujo et  al. applied Sonogashira reaction  to couple octa(iodophenyl)POSS with π‐conjugated phenylacetylenes (Figure 7.8). All of these systems were verified in terms of their thermal resistance, which was proved to be incredible high (Td  = 496–526 °C). Their photophysical properties were disclosed via UV–vis and photoluminescence analysis and compared with their organic, trimthylsilyl analogs. The absorption spectra revealed the presence of two maxima wavelengths in the area λab = 323–327 and 347–350 nm and did not present any significant changes regarding the structural differences of substituents. The emission spectra were recorded in solution (CHCl3) and in a solid state. The relatively broader and red‐shifted emission was recorded for Ph3POSS along with higher quantum efficiency in comparison to the organic analog. This was explained by the inorganic core presence as well as the mobility and π‐conjugation of phenyl rings (undisturbed for alkyl derivatives). On the other hand, a strong, red‐shifted λem maxima at c. 405–427 nm was recorded, resulting from unfavorable intermolecular interactions. The authors reported high thermal resistance for the luminescence properties (after UV irradiation λ = 365 nm) that were maintained even at 250 °C [69, 70]. Through the nickel‐catalyzed Yamamoto coupling reaction, Lee et al. prepared the highly efficient blue polymer POSS‐substituted polyfluorene (PFPOSS) [64]. With restricted interchain interactions, it showed a decrease in undesired green emission (>500 nm) of poly(dialkylfluorene). The POSSs group contributed to the high thermal stability of the polymer, which further led to a better color stability of PFPOSSs blue emission despite undergoing thermal annealing at 150 °C. An exceptionally steady blue light emission with great performance was emitted by (Indium tin oxide) ITO/Ca/Al/OEDOT‐PSS/polymer LED devices that made use of PFPOSS [64]. Through the interaction of 4‐uracilbutyl‐1‐methylpyrene ether (U‐PY) and octakis[dimethyl(N‐[6‐acetamidopyridin‐2‐yl])siloxy] silsesquioxane (ODAP‐POSS), a star‐like POSS core star‐like supramolecule was developed as an efficient blue‐light electroluminescent material [65].

177

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7  Construction of Organic Optoelectronic Materials

Hydrosilation reaction of POSS is one the efficient ways to build POSS‐based luminescent materials with POSS core structures in the main backbone to produce materials that exhibited high efficiencies and luminance. Cho et al. synthesized POSS‐FL3, a POSS‐based blue light electroluminescent nanoparticle containing a terfluorene chromophore on each arm [66]. Poly(dihexylfluorene) had a maximum absorption wavelength (391 nm in tetrahydrofuran (THF) solution) that overlaps well with the maximum emission wavelength (394 nm in THF solution) of POSS‐FL3. This indicated that the quantum efficiencies of blue light‐emissive conjugated polymers such as polyfluorenes can be increased by energy transfer and isolation of the chromophores through the use of POSS‐FL3 as a dopant [66]. Using the similar approach, our group recently designed a series of pearl‐necklace‐like polymers through the hydrosilylation condensation between hybrid polymers inorganic bifunctional POSS and organic light‐emitting oligofluorene segments (Figure  7.9). Optoelectronic analyses showed that the presence of the interconnecting POSS can effectively reduce the red shift in photoluminescence and electroluminescence during film formation. The obtained hybrid poly(oligofluorenes) display stable blue emission with high color purity. Thermal analyses recorded a high glass transition temperature of 125 °C, suggesting the thermal plastic behavior of the materials [67]. In addition to the peral‐necklace‐like structure, a hybrid POSS material composed of carbazole (POSS‐8Cz) was developed using the hydrosilylation reaction [68]. It was found that POSS‐8Cz possessed high thermal stability and showed blue emission in both solid film and solution in the photoluminescence spectroscopic analysis. Hydrosilylation approach was also used to anchor the chromophore part onto the POSS that has been disclosed by Lee, Mochizuki, and Jabbour group [69]. They used a hydrosilylation process for the synthesis of a series of macromolecular materials composed of the POSS core and organic fluorescent emitters covalently attached either monochromatically or in a combination of mixed emitters (POSS with different chromophore systems) (Figure 7.10). Hydrosilylation was performed using Karstedt’s catalyst with fixed and controlled reagents’ stoichiometry. All compounds were proved to have higher thermal stability than their corresponding organic emitters. Interestingly, it was found monochromatic products (1–3) have similar absorbance and emission spectra to their free emitter counterparts both in solution and in a solid state. Therefore, using POSS core as a scaffold does not influence the color of the emitters applied. Remarkably, it was found that lower energy emissions (orange or yellow) dominated in the photoluminescence spectrum for materials 4–7 because of a strong intramolecular energy transference from the neighboring higher energy blue emitter on the POSS. This high degree of energy transference derives from two major factors: the overlapping of the blue emitter’s emission band with its lower‐energy absorption band and the blue and orange emitters’ short intramolecular distance. Many attempts have been made to integrate POSS‐based luminescent materials with excellent properties into various apparatuses including dyes [71] and light‐ emitting devices [49, 72, 73]. For example, Singh et al. recently developed iridium‐ based stellate POSS macromolecular dyes along with an inkjet printing process, which can deposit the dyes to study the POSS‐based light‐emitting diodes [71].

7.2  POSS‐Based Organic Electroluminescence Materials

Br

n R

Br

b

n

R

R

n=1 1 n=2 2 n=3 3

O Si Ph NaO O Si NaO Ph

O Si O O Si Ph

R

n=1 4 n=2 5 n=3 6

R=C8H17

Ph

OCH3 c Si OCH3 OCH3

a

SiMe3

Ph Si O O

O Si Ph O O Si

Ph Si

O

Ph Si O

Ph

d

ONa

Me

Ph

O

H Si

O

O

Ph Si

O

Si

H

Si O ONa Ph Si O Ph

O

Si O O Si Ph

Si O O Ph Si O Ph

Me

B-POSS

Ph

O

Si

B-POSS +

e

4/5/6

Ph O O Si

Me

Ph Si O O O

Si O

O Ph

Si O O Si Ph

Ph Si

O

Me Si

Si O O Ph Si Ph

O

R

n R N

n = 1 P1 n = 2 P2 n = 3 P3

(a)

R R

n

POSS R R

n

POSS R R

n

(b)

Figure 7.9  (a) Synthetic procedure and (b) schematic illustration of blue‐light emissive poly(oligofluorenes) with bifunctional POSS in the main chain. Source: Chi et al. 2014 [67]. Reproduced with permission of John Wiley & Sons.

179

Si O O Si Si O Si O Si O Si O O Si O O O Si O Si O O O Si O Si O Si O Si O Si O Si Si

O

1

Si O O Si Si O Si O O O O O Si Si Si Si O O O Si O Si O Si O Si O O O Si O Si O Si O Si

5

Si O O Si Si O Si O Si O Si O O Si O O O Si O Si O Si O O O O Si Si O Si O Si O Si

O Si Si O Si O Si O O O Si O Si Si O O O Si O Si O O O Si O Si O Si Si O O Si O Si Si

O

Si

2

Blue emitting dye

N

4

7

NC CN

O

Yellow emitting dye

O

Si O O Si Si O Si O O O O O Si Si Si Si O O O Si O Si O Si O Si O O O Si Si Si O O O Si

6

=

Si

3

Si O O Si Si O Si O O O O O Si Si Si Si O O O Si O Si O Si O Si O O O Si O Si Si O O Si

NC =

Si O O Si Si O Si O Si O Si O O Si O O O Si O Si O Si O Si O O O Si O Si O Si O Si

O

=

N

CN

O

Orange emitting dye

Figure 7.10  Synthetic hydrosilylation route to POSS‐based systems with mixed organic fluorescent emitters. Source: Gon et al. 2016 [69]. Reproduced with permission of Royal Society of Chemistry.

7.3  POSS as a Building Block for Electrochromic Materials

It showed that the peak luminance was at ~10 000 cd/m2, and with printed layers, it was possible to obtain peak quantum efficiencies of ~2.5%. Efficiencies of the printed devices were plausibly reduced because of the thickness of the printed layer along with the effect of the emissive region where the surface roughness strengthens the metal cathode quenching [71]. As for the POSS‐based light‐emitting devices, Lo et al. recently synthesized novel amorphous POSS core materials using Heck coupling. In this study, octavinylsilsesquioxane (OVS) was reacted with different amounts of 1‐bromopyrene resulting in polysilsesquioxanes (SSQ)‐ based materials that contained a spherical “silica” core and pyrene‐functionalized emissive periphery (Figure 7.11) [72]. The advantage of using SSQ‐based materials within organic light‐emitting diodes (OLEDs) includes high solubility, high Tg, high yield reactions, low polydispersities, and high purity. The materials showed an external quantum efficiency of 2.63% and current efficiency of 8.28 cd/A, making them as efficient emitters. In addition, iridium complexes attached to POSS showed high performance in monochromatic light‐emitting devices, where the peak external quantum efficiency is in 5–9% range [73]. This can be driven at a voltage that is less than 10 V to achieve a luminance of 1000 cm/m2. Moreover, white‐light‐emitting devices with R, G, and B POSS emitters exhibited an external quantum efficiency of 8% with a power efficiency of 8.1 lm/W at 1000 cd/m2. Encouraging efficiency was achieved in the devices based on hole‐transporting and Ir‐complex moieties of dual‐functionalized POSS materials without using host materials, demonstrating that triplet dye and carrier‐transporting moieties of functionalized POSS material is a viable approach for the development of solution‐processable electrophosphorescent devices [73].

7.3 ­POSS as a Building Block for Electrochromic Materials An electrochromic material is one where a reversible color change takes place upon reduction (gain of electrons) or oxidation (loss of electrons), on passage of

8 equiv

C144H88O12Si8 Mol wt: 2234.9 8 substitutions most abundant

O O SiO O Si Si Si O O O Si OSi OSi O Si O

O

24 equiv

C240H136O12Si8 Mol wt: 3436.3 14 substitutions most abundant

Figure 7.11  Synthetic route of pyrene‐substituted (P8‐OVS and P14‐OVS) using Heck coupling. Source: Lo et al. 2016 [72]. Reproduced with permission of American Chemical Society.

181

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7  Construction of Organic Optoelectronic Materials

electrical current after the application of an appropriate electrode potential [74]. Nowadays, electrochromic materials and their associated device technologies have been widely used in reflective‐type displays [75], smart windows and smart mirrors [76, 77], and e‐papers [78, 79]. Various materials, including inorganics (WO3, ZnO, etc.) [80–85], organic small molecules [86], macromolecules and polymers (deoxyribonucleic acid (DNA), polyaniline, polypyrrole, etc.) [87–89], as well as their hybrids (ZnO‐PEDOT, MoO3‐WO3, WO3‐PPy, WO3‐rGO, etc.) [90–94], are electrochromically active and have been extensively researched until now. Generally, electrochromic materials are classified into four distinct categories including metal oxides, coordination complexes, small organic molecule, side‐chain‐substituted polymers or main‐chain polymers [95, 96]. Compared with the others, organic conjugated polymers, such as polycarbazole, polypyrrole, polyaniline, and polythiophene, have wide application in electrochromic device because of their better stability, faster switching speed, and high color contrast rations [97]. Electrochromic polymers normally feature in high conjugation with band gap ranging from 0.5 to 3.0 V. The band gap of conjugated polymers is tunable through altering the chemical structure of the polymers or corresponding monomers, which allows further improvement of the optical contrast, one of the most critical and demanded parameters in the area of electrochromism. Some typical examples include introducing additional electrochromic groups onto the polymer chains as side pendants or copolymerized segments [98, 99], incorporating rigid or bulky substituents [100], and increasing the size of the functional groups on the polymer chains [101]. On the other hand, porous polymers or nano‐sized domains do fasten up the switching response compared with bulky solid polymers because of the facilitated ion movement and shortened ion pathway [102, 103]. However, the porosity and small size limited influence on the optical contrast. Based on the above discussion, we can conclude that the manipulation of the chemical structures of the conjugated polymers turns out to be the most useful strategy to modulate the optical contrast, while the morphological optimization reduces the switching time significantly. This enhances the overall electrochromic performance of the corresponding devices. Being beneficial from the cage‐ like geometry of the POSS as well as the convenience and diversity in attaching/ grafting functional groups or long organic chains onto the corners of the cages, the incorporation of POSS into electrochromic polymers in a chemical way brings possible alteration to the morphology and structure of the polymers. Typically, such “chemical way” involves tethering electrochromic chains onto the nanocages of POSS through covalent bonding, disrupting the molecular packing of the chains. The incorporation of POSS will help to enlarge the interspaces between the electrochromic chains because of the separation effect from the POSS cages. This allows relatively free ion movement during the redox switching. Also, POSS‐induced morphology change leads to a loose, nonparallel packing of the polymer chains, offering more accessible sites for the redox reaction, inducing more distinct color change, i.e. higher optical contrast. Lu’s group has done massive and valuable work on POSS‐modified electrochromic polymers. In 2007, for the first time, they grafted polyaniline (PANI)

7.3  POSS as a Building Block for Electrochromic Materials

onto the corners of POSS cages (POSS–PANI) through oxidative emulsion copolymerization of aniline with octa(aminophenyl) silsesquioxane (OAPS), as shown in Figure 7.10 [104]. After the copolymerization, the dense packing of the rigid aniline chains was prohibited, which is verified by the poorer crystallinity as well as the reduction of the crystal size [105]. Furthermore, nanoporous structure was created because of the loose packing structure of the star‐like geometry (Figure 7.12), offering free pathways for ion injection and extraction. This is evidenced by the increased ionic conductivity and decreased electrical conductivity of the POSS–PANI over that of pure PANI. Because of the above adjustments, the optical contrast of POSS–PANI with OAPS concentration of 0.5 mol% increases by about 40% over that of pure PANI, and the switching time that required to achieve 90% of the total absorbance change decreases from 3.2 seconds for pure PANI to 2.8 seconds for POSS–PANI (0.5 mol% OAPS), as shown in Figure 7.12. It is noted that increasing the OAPS concentration reduces the chromophore concentration that induced by the excess amount of nanometer‐ sized POSS cages. This reduces the switching time further to 1.5 seconds, however, showing no more positive effect on the enhancement of the contrast. Therefore, the optimization of POSS content for the grafting must be taken into careful consideration to achieve best electrochromic performance. Besides the improvement of optical contrast and switching time, the electrochemical stability of POSS–PANI is also significantly higher than that of pristine PANI [105]. This may be because of the stronger interactions between dopant anions and PANI chains brought by the larger conformation freedom of the anions in POSS–PANI. It is worth noting that following the same strategy, Toppare’s group attached polypyrrole (PPy) chains onto thiophene‐processed OAPS nanocages through click chemistry [106]. The lower band gap of π–π* transitions as well as the nanoscale porous structure from the loose packing of the star‐like geometry give rise to higher optical contrast, fast switching response, and color properties compared with pristine PPy. By combining POSS–PANI with other electrochromically active materials through proper assembling techniques, the optical contrast and switching response can be further enhanced from the synergistic effect of both phases. The typical techniques include electrostatic layer‐by‐layer (LBL) assembling and complementary dual‐layer device fabrication. LBL is a unique strategy to produce low‐roughness electroactive polymer films with excellent homogeneity and high optical contrast [107]. Utilizing this strategy, Lu’s group fabricated alternative multilayer thin films of POSS–PANI with poly(2‐acrylamido‐methane‐2‐ propanesulfonic acid) (PAMPS) [108] and sulfonated polyaniline (SPANI) [109], respectively, as shown in Figure  7.13. It was verified that the redox process of POSS–PANI/PAMPS is close to nondiffusion‐controlled process. Being attributed to the unique morphology brought by the synergistic combination of star‐ like geometry and the LBL assembling technique, the POSS–PANI/PAMPS possesses optical contrast of 30% higher as well as much faster switching kinetics than its counterpart, PANI/PAMPS, under dynamic switching condition. Besides the nondiffusion‐controlled redox mechanism in the case of POSS–PANI/ SPANI, strong interaction between POSS–PANI and SPANI was formed. The interaction increases the amount of electroactive units and conjugation length in

183

7  Construction of Organic Optoelectronic Materials PANI H2N H 2N H2N

NH2 Si O Si O O O Si OO Si Si O Si O O O O Si Si O

H2N

NH2 NH2 + NH2

(NH4)2S2O8

~0.5 nm

DBSA

NH2

OAPS

(a)

POSS–PANI

Homopolyaniline

Crystal region

POSS-polyaniline

(b) 0.8

POSS–PANI-a

0.7

PANI

POSS–PANI-b PANI/OAPS blend

0.6 ABS

184

0.5 0.4 0.3 0.2 0.1 0

(c)

20 Time (s)

40

Figure 7.12  (a) Schematic demonstration of POSS–PANI from emulsion polymerization. (b) A possible loose packing structure of POSS–PANI (c) Optical absorbance at λmax for the devices with pure PANI (dotted line), POSS–PANI with 0.5 mol% OAPS (POSS–PANI‐a, thick solid line), and the one with 1.0 mol% OAPS (POSS–PANI‐b, thin solid line) as the electrochromic layers under square‐wave potentials oscillating between −3.0 and +3.0 V. Source: Xiong et al. 2007 [104, 105].

POSS–PANI/SPANI, leading to lower band gap energies and higher electrical conductivity. Furthermore, a more favorable redox reaction was achieved as verified by the lower oxidation and reduction potential values. The above alteration

7.3  POSS as a Building Block for Electrochromic Materials

PAMPS Substrate H+–O3S

POSS–PANI

(a)

SO3–

SPANI

H N

SO3H H N

SO3–

H N

n N H

O

SO3H H N

Substrate

POSS–PANI PANI

(b)

Spin-coated SPANI (POSS–PANI/PAMPS)50

ABS

(POSS–PANI/SPANI)50

(c)

Time (s)

Figure 7.13  LBL assembling of (a) POSS–PANI with PAMPS and (b) POSS–PANI with SPANI and (c) the potential step absorptiometry of the LBL electrochromic devices compared with that of reference. Source: Jia et al. 2010 [109, 108].

results in 35% electrochromic contrast improvement for the POSS–PANI/SPANI over POSS–PANI/PAMPS, and the switching kinetics of POSS–PANI/SPANI is much faster than that of spin‐coated SPANI (Figure 7.13). This demonstrates the great usefulness of LBL technique in effectively enhancing the electrochromic performance of polymer thin films through the intimate assembling with synergistic polyions. A complementary electrochromic device refers to the one with two electrochromic layers with coincident coloration and bleaching potentials, including anodically colored layer and cathodically colored layer. The integrated function of the dual‐layer configuration could bring more than the sum of two single layers. POSS–PANI has been used to combine with WO3 [110] and poly(4‐styrenesulfonicacid) polystyrene sulfonate (PSS)‐doped poly(3,4‐ethylenedioxythiophene

185

7  Construction of Organic Optoelectronic Materials

(PEDOT) (PEDOT : PSS) [111] to fabricate dual‐layer complementary electrochromic devices. Under such configuration, more favorable accessible doping sites are formed because of the loosely packed structure of POSS–PANI, releasing more protons or Li+ during the oxidation process. The ionic concentration gradient creates additional internal driving force to diffuse more ions toward the other electrochromic layer, i.e. WO3 or PEDOT  :  PSS. Therefore, the layer of WO3 or PEDOT : PSS adds significant contribution to the overall electrochromic performance of the corresponding dual‐layer device, leading to much higher optical contrast than the counterparts and even the sum of the two corresponding single layers (Figure 7.14). Moreover, the switching speed and the coloration efficiency are also greatly improved, thanks to the loosely packed structure and the formed internal driving force. 1.2 1.1 Absorbance

1.0

POSS–PANI/WO3 PANI POSS–PANI PANI/WO3 WO3 (2V)

0.9 0.8 0.7 0.6 0.5 0.4 0.3

0

40

(a) 0.9

80 120 Time (s)

160

200

1 2

0.8 0.7 Absorbance

186

0.6

3

0.5 4

0.4 0.3 0.2 0.1

(b)

5 0

20

40 Time (s)

60

80

Figure 7.14  Optical absorbance for the devices with (a) PANI(I) (dash dot line), POSS–PANI (II) (dot line), WO3 (III) (short dot line), PANI/WO3 (IV) (dash line), and POSS–PANI/WO3 (V) (solid line) as the electrochromic layers, and (b) POSS–PANI/PEDOT:PSS (1), PANI/PEDOT:PSS (2), POSS–PANI (3), PANI (4), and PEDOT:PSS (5) as the electrochromic layers. Source: Zhang et al. 2009 [110, 111].

7.3  POSS as a Building Block for Electrochromic Materials

In addition to the above‐stated function of POSS, i.e. attaching organic electrochromic chains onto the corners, POSS has also been used to prepare new structures or modify existing morphologies regarding electrochromic properties. Bekaroglu’s group designed a novel sandwich‐like bis(phthalocyaninato) lanthanide(III) complexes LuPc2, which contain a mercaptopropylisobutyl‐POSS functional group on each Pc moiety [112]. The further reaction with lithium followed by Lu(OAc)3·3H2O in refluxing 1‐pentanol leads to formation of a special compound, which exhibits electrochromic green–blue transition. Differently, Orel’s group used POSS as a pigment surface modifier [113]. In a typical work, Thin TiO2 (anatase) pigment coating was prepared through milling metatitanic acid (mTiA) agglomerates in the presence of trisilanol heptaisobutyl silsesquioxane (trisilanol POSS)/butanol/hexane. The coating was constructed with mTiA and rutile (mTiR) nanoparticles in the size of less than 100 nm, on the surface of which forms a thin layer of POSS dispersant through establishing hydrogen bonding. Being used as an electrochromic layer, the trisilanol POSS/mTiA pigment coating exhibits dark blue color at coloration stage. It possesses higher transmittance contrast than the coating without trisilanol POSS because of the small and uniform nanoparticle size that facilitated by the trisilanol POSS as well as the interaction between the mTiA pigment coating and the trisilanol POSS dispersant layer on the surface. In an electrochromic device, electrolyte is another critical component that determines the conduct of the ions during redox switching. Because of the large specific surface area as well as the functionality number, POSS can also be used to modify electrolytes to improve the electrochromic performance of the corresponding devices. Orel’s group used POSS as the nano‐sized building blocks to construct functionalized 1,3‐alkylimidazolium iodide solid and room temperature ionic liquids, as shown in Figure 7.15 [114]. Using MePrIm+Ix−·IB7·T8·POSS as the electrolyte of WO3/Pt‐based electrochromic device, adequate coloring‐ bleaching change was established for up to 1000 repetitive cycles. Following this, Lu’s group incorporated octa(3‐hydroxy‐3‐methylbutyldimethylsiloxy) polyhedral oligomeric silsesquioxane (POSS‐OH) into electrospun poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF‐co‐HFP) nanofibers with good dispersion and used as host of the ionic liquid (IL), LiClO4/1‐butyl‐3‐methylimidazolium tetrafluoroborate (BMIM+BF4−) [115]. The ionic conductivity is enhanced in the presence of POSS‐OH because of the greater extents of dissociation of the IL and LiCLO4 that is induced by the Lewis acid–base interactions of POSS‐OH with BF4− and ClO4−. As the electrolyte of an electrochromic configuration of ITO glass/electrochromic layer (PANI–TiO2)/electrolyte/ITO glass, the PVDF‐co‐ HFP (5 wt% POSS‐OH)/LiClO4/BMIM+BF4− helps to achieve higher optical contrast (71.9%) and shorter coloration/bleaching time (2.0 seconds/2.3 seconds) compared with PVDF‐co‐HFP/BMIM+BF4− (53.9%, 5.9 seconds/7.9 seconds, respectively). The improvement can be ascribed to the small size of Li+ as well as the increased number of free Li+. The above presented work suggests that POSS with adjustable functional groups on the corners can be a potential modifier to the electrolyte for electrochromic performance enhancement. Except the covalently binding POSS in the main polymer backbone to tune the polymer structures and porosities, people also proposed to graft POSS to the

187

H3C

H3C CH3

H3C O

Si

H3C O H3C

Si

O

H3C H3C

Si

O HO

+

O

O

H3C

OH

HO

Si

O

H3C

Si

H3C

l O

CH3

O

H3C O H3H CC 3

Si

H3C

Phosphazene base P1-t-Bu

CH3

Si H3C

Si O

CH3

CH3

Si

O

H3C

CH3 O

O

Si

Si

O

Si

O

H3C

O

l

Si

CH3

H3C

CH3

H3C H 3C

H3C

1

2 H3C

O H3C H3C H3C H3C H3C

N

CH3

H3C Si Si

O O

O

O

+

O

H3C CH3

H3C Si O H3C O

O

H3C H3C

O O

CH3

Si H3C CH3 H3C

O N

N

O

CH3

H3C H3C

O O

Si

l– Si

Si

O Si O

Si O

Si

N+

O

N

R

O O

O

CH3

Si H3C O

O H3C

2

CH3

a

l

Si

Si O

Si

N

H3C

Si

O Si

CH3

O Si H3C

O

O

H3C

O O

Si O

Si

O

b

CH3

H3C H3C

3a R = Me 3b R = DEGME

Figure 7.15  Schematic demonstration of the synthesis of 1‐methyl‐3‐propyl hepta(i‐butyl) octasilsesquioxane imidazolium iodide (MePrIm+Ix− IB7 T8 POSS, 3a), 1‐(2‐(2‐[2‐methoxyethoxy]ethoxy)ethyl) 3‐propyl hepta(i‐butyl) octasilsesquioxane imidazolium iodide (3b). Source: Colovic et al. 2011 [114]. Reproduced with permission of Elsevier.

7.4  Other Applications of POSS in Organic Optoelectronic Materials

side chain of polymers in PANI‐based electrochromic materials development [116]. New electrochromic polyamic acid containing oligoaniline segments in the main chain via oxidative coupling polymerization. Then various additional functional groups, including tetraaniline, POSS, and fluorene, were grafted to the polymeric architecture through the post‐polymerization functionalization. With the introduction of tetraaniline pendants, the resultant polymer exhibits improved electrochromic performance with high optical contrast value and rapid switching rate because of the high content of electrochromic units in the polymeric structure. The polyamic acid functionalized with polyhedral oligomeric silsesquioxane demonstrates a great enhancement of switching rate in the electrochromism because of the rapid electrolyte migration through polymer film under electrochemical potentials.

7.4 ­Other Applications of POSS in Organic Optoelectronic Materials Except the successful applications of POSS in electroluminescent and electrochromic materials, it was also widely used in other interesting areas by using its unique structure and physical properties. Because of the uniform‐sized nanostructure and good dispersion in most organic matrix, POSSs have been widely used in the liquid crystal (LC) alignment with improved contrast ratio and time response [117, 118]. It has been reported that addition of POSSs into liquid crystal can induce a spontaneous vertical alignment, which eliminated the alignment layers required in a conventional LC device and avoided the drawbacks of the traditional high‐temperature process [119, 120]. In the LCD industry, changing the pretilt angle θp by using the synthetic polyimide to produce the homogeneous (θp ~ 0°) and homeotropic (θp ~ 90°) surface alignments has become a mature technology. However, only a small tuning range of pretilt angle can be achieved by using the commercial polyimides. By using the POSS (nanoparticles)‐induced vertical alignment technique, controlling of the liquid crystal pretilt angles over the range of 0°