Reactions siloxane interchange

Silicone-based materials have been known for their ability to restructure under certain conditions [68]. Recently, tetramethylammonium silanolate-initiated ringopening copolymerization of octamethylcyclotetrasiloxane (D4) and bis (heptamethylcyclotetrasiloxanyl) ethane (bis-D4) showed that these crosslinked polymers with ethylene bridges and active silanolate end groups are able to be restructured [69]. Figure 15d depicts siloxane interchange reactions, which maintain their activity under ambient conditions, thus providing ready-to-respond active groups. 

FIGURE 19. The siloxane bond interchange reaction for cyclic PDMS. The intermolecular process (a) results in a much higher molecular weight cyclic polymer than that of the starting material. The intramolecular process (b) gives a mixture of cyclic oligomers. Reproduced from Bannister, D. J. and Semiyen, J. A., Polymer 1981, 22, 377, by permission of the publishers, Butterworth and Co. (Publishers) Ltd. 

Figure 10.7 (8) illustrates the stress relaxation of a poly(dimethyl siloxane) network, silicone rubber, in the presence of dry nitrogen. The reduced stress, o(t)/(T(0),is plotted,so that under the initial conditions its value is always unity. Since the theory of rubber elasticity holds (Chapter 9), what is really measured is the fractional decrease in effective network chain segments. The bond interchange reaction of equation (10.2) provides the chemical basis of the process. While the rate of the relaxation increases with temperature, the lines remain straight, suggesting that equation (10.2) can be treated as the sole reaction of importance. 

Thomas and Kendrick observed the establishment of the ring-chain equilibria in systems of linear poly(dimethylsilox-ane) at high temperatures without catalysts and interpreted their results as the effea of a siloxane-bond interchange reaction involving a four-center transition state. This reaction is in fact a ring-insertion/exclusion reaction (Scheme 3) (or ring-expansion/contraction reaction if only tydic compoimds are involved, cf. Scheme 4) ... 

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