The thermal degradation of polysiloxanes—I. Poly(dimethylsiloxane)

Evolution of volatile materials from pure poly(dimethylsiloxane) heated under temperature programmed conditions (10° min−1 under vacuum) is detectable at 343° and reaches a maximum at 443°. The volatile products comprise a continuous mixture of cyclic oligomers from trimer upwards. Replacement of the terminal hydroxyl by trimethyl silyl structures (end-blocking) results in a considerable increase in stability but no change in the distribution of products. The reaction is strongly accelerated by KOH and methane then appears as a significant product. Thermal gravimetry and molecular weight measurements confirm that the hydroxyl terminated polymer degrades in a stepwise fashion from chain ends. These observations are discussed from a mechanistic point of view. The reaction is accelerated by oxygen and cross-linking occurs.