Silicon-Oxygen Polymers Polysiloxanes Silicones

Polysiloxanes were first developed in the 1930s and 1940s and currently represent the most commercially important inorganic polymer system. These materials and their products comprise a billion dollar global industry. 

Polysiloxanes also possess a variety of other useful properties such as hydro-phobicity and exceptionally high permeability to gases. Indeed, snails can live submerged beneath the surface of low molecular weight poly(dimethylsiloxane) 

A very versatile and important methodology for controlling the properties of polysiloxanes involves hydrosilylation [eqn (10.33)]. Reaction of poly(methyl-hydrosiloxane) or poly(methylhydrosiloxane)-poly(dimethylsiloxane) copolymers with vinyl-capped species allows the introduction of side groups which give rise to a variety of interesting properties. 

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Two major classes of hydrophobic chemical substances can be applied to glass in ultrathin layers to inhibit surface wetting. Siloxanes or polysiloxanes or silicones are polymers with a backbone of alternating silicon and oxygen atoms. These macromolecules are quite chemically inert, show resistance to water, and exhibit stability at high and low temperatures. The most common siloxane polymer, polydimethylsiloxane, is composed of the monomeric (i.e., repeating) unit illustrated in Fig. 7.5.1. 

The Tg is related to chain stiffness and the geometry of the polymer chain. Flexible polymers with methylene and oxygen atoms in the chain, such as polyethylene, polyoxymethylene, and polysiloxane (silicone), have relatively low Tg values. The Tg of polyoxymethylene is somewhat higher than would be anticipated because of the dipole character of the C—O—C group, which increases the intermolecular forces and restricts segmental motion. 

The atomic composition of polymers encompasses primarily non-metallic elements such as carbon (C), hydrogen (H) and oxygen (O). In addition, recurrent elements are nitrogen (N), chlorine (Cl), fluoride (F) and sulfur (S). The so-called semi-organic polymers contain other non-metallic elements such as silicon (Si) in silicone or polysiloxane, as well as bor or beryllium (B). Although other elements can sometime be found in polymers, because of their very specific nature, we will not mention them here. The properties of the above elements lead to specific properties that are common of all polymers. These are ... 

Polysiloxanes, or silicones as they are commonly called, are polymers of silicon, not carbon. Their chains are made up of alternating silicon and oxygen bonds and are characteristically very flexible (have very low glass transition temperatures—see Chapter 7). As a result, silicones typically find use as... 

Silicone—Polysiloxane, a polymer with a backbone consisting of alternating silicon and oxygen atoms. 

As the name implies, polysilane polymers consist of chains made up exclusively of sihcon atoms. (In the ht-erature these polymers are named either as polysUanes or poly(silylenes) thus (Et2Si) can be called polydiethylsilane or poly(diethylsilylene).) Unlike the heteroatomic polysiloxanes, with alternating silicon and oxygen atoms in the polymer backbone, the poly silanes are homoatomic and therefore structurally closer to aUcene polymers. However, because the atoms in the main chain are all silicon, the polysilanes show quite special properties. ... 

A vast majority of the commercially available silicon-containing polymers today are polysiloxanes or silicones. In the same way that aldehydes and alcohols are characterized by the presence of the -CHO-H and = C-OH moieties, respectively, a siloxane is a compound containing the bond =Si-0-Si=, and silicones are materials consisting of alternating silicon and oxygen atoms in which most silicon atoms are bound to at least one monofunctional organic radical. Polymers and copolymers of R, R2, and R3 monomers are termed poly(siloxanes). Linear poly(siloxanes)... 

The most important silicon-oxygen systems of course, are the linear polysiloxane polymers. These important materials are resistant to heat or oxidation, exhibit little change in properties with temperature, are water repellent and biologically inert. The history of the discovery and development of silicones has been summarized172. 

Silicone rubbers, also known as polysiloxanes, are a series of compoimds whose polymer structure consists of silicon and oxygen atoms rather than the carbon structures of most other elastomers. The silicones are derivatives of silica, Si02 or 0=Si=0. When the atoms are combined so that the double linkages are broken and methyl groups enter the linkages, silicone rubber is produced ... 

Polysiloxane or Silicone Rubber (Q). The polymer backbones of sihcone rubbers contain no carbon atoms, but they contain alternating silicon and oxygen atoms. The predominate structural unit is generally dimethylsiloxane, as follows ... 

Polysiloxanes are inorganie polymers that eontain alternate silieon and oxygen atoms in their baekbone [1-6]. While the silicon atom has two side-groups the oxygen atom carries none. In this regard they are similar to polyphosphazenes that we have seen in Chap. 3. The most widely studied polymer of this family is poly(dimethylsiloxane) (PDMS) which contains methyl groups as the substituents on silicon. PDMS is isoelectronic with poly(isobutylene) as well as poly(dimethylphosphazene) although its properties are dramatically different (Fig. 6.1). 

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