Polysiloxanes and Related Polymers

At the present time, polysiloxanes are unique among inorganic and semi-inorganic polymers. They have been the most studied by far, and are the most important with regard to commercial applications. Thus, it is not surprising that a large number of review articles exist describing the synthesis, properties, and applications of these materials.1 55 

The major categories of homopolymers and copolymers to be discussed are31 (i) linear siloxane polymers [-SiRR O-] (with various alkyl and aryl R,R side groups), 

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J.E. Mark, H.R. Allcock and R. West (2005) Inorganic Polymers, 2nd edn, Oxford University Press, Oxford -Chapter 4 Polysiloxanes and related polymers . 

Radiation-induced Degradation.—There have been several reports on radiation effects in polymers,288 including single crystals,287 fluoropolymers,288 polyamides,289 polysiloxanes,270 polyethylene and its copolymers,271 polypropylene,272 polyolefins,273 polystyrene and its copolymers,274 poly(vinyl chloride) and related polymers,275 rubbers,278 polysulphones and other sulphur-containing polymers,277 polycarbonate,278 nylon,279 poly(vinylpyridines),280 and wool.281... 

Of the semi-inorganic polymers, the siloxane or "silicone polymers have been studied the most, and are also of the greatest commercial importance (i-i5). The present review provides an overview of some of these polysiloxanes and related materials, emphasizing their structures, most important and interesting physical properties, and a variety of their applications. 

Ihe literature " describes a number of dendrimers and the closely related star-like 120-123 polysiloxanes. Ihe hyperbranched polysiloxanes are the primary example of more random structures. Although the emphasis has been on synthesis and characterization,i i modeling on hyperbranched polymers has also been carried out. Some of the most interesting species involve polysiloxane chains. Star polymers, some with nanosized silica cores, have also been synthesized.i °-i i Hyperbranched polysiloxanes have been prepared with controllable molecular weights and polydispersities,i -1 - with epoxy terminal groups some are UV-curable ° and some serve as a source of molecular silica. Hyperbranched polysiloxanes have also been used in the sol-gel preparation of polypropylene/silica nanocomposites. ... 

In some cases, cyclic and linear PDMS have been combined to form conetworks, and unsaturated cyclic side-chain fragments have been placed into polysiloxanes to make them thermoreactive. Interactions between ring polymers have also been analyzed and related to loops in chromatin. Also, some cyclic oligosiloxanes having polar end groups show liquid-crystal-line behavior, specifically smectic A and E phases. Finally, several PDMS cyclics have been exposed to vacuum pyrolysis and the products analyzed by matrix-isolation spectroscopy. The pyrolysis products obtained under a variety of conditions identified the radical reactions that were involved. 

Polysiloxanes are among the most flexible polymers known and this is reflected by the very low glass-transition temperatures of this polymer family. Data for some representative examples of polysiloxanes along with some related polymers are given in Table 6.2. 

Table 6.2. Glass-transition temperatures of polysiloxanes and some related polymers...

The copolymer under investigation in the current work was a cyano-biphenyl and methylated benzoic ester side-chain substructural polysiloxane of high positive dielectric anisotropy (A8). The structure and transition details are given in Fig. 2 and the polymer is called PG 296 to relate to other work being published. 

Cyclophosphazenes are a fascinating group of inorganic heterocyclic compounds whose chemistry is multi-faceted, well developed and reasonably well understood. They are closely related to the linear poly-phosphazenes this relationship is unlike any other existing between ring-polymer systems. Although cyclic siloxanes and polysiloxanes have a close interrelationship, the number and types of cyclophospha-zene derivatives that are known, together with their exact counterparts in polyphosphazenes, underscore the utility of cyclophosphazenes as models for the more complex polyphosphazenes. The literature on cyclophosphazenes has appeared earlier in the form of books (1,2), chapters of books (3-5), authoritative compilations of data (6,7), and several reviews (8-21). The current literature on this subject is reported annually in the Specialist Periodic Reports published by the Royal Society of chemistry (22). This review deals mostly with chlorocyclo-... 

An important source of experimental and theoretical studies of equilibria in ring formation is represented by the field of so-called macrocyclisation equilibria (Flory, 1969). Interest in this field appears to have been restricted so far to chemists conventionally labelled as polymer chemists. Experimental evidence of cyclic oligomer populations of ring-chain equilibrates such as those obtained in polysiloxanes (Brown and Slusarczuk, 1965) may be delated to the statistical conformation of the corresponding open-chain molecules (Jacobson and Stockmayer, 1950 Flory, 1969). In these studies experimental results are expressed in terms of molar cyclisation equilibrium constants Kx (14) related to the x-meric cyclic species Mx in equilibrium with the... 

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