1
Introduction of Organosilicon Materials

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

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 s-electrons and conjugation along the s-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 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-n SiX n , 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; (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 a,?-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 copolymerization, 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,...