Poly bond interchange

Fig. 24. A typical time-temperature cure cycle illustrating bond interchange and phase behavior of castor oil/urethanes and poly(ethylene terephthalate) semi-IPNs. Axes not drawn to scale.

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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