Structure of the monomeric unit of crosslinked polymer and its relation to the thermal stability of polymethylsiloxanes

A study has been made of the products of thermal polymerization of oligomethylsiloxanes prepared by cohydrolysis of methylchlorosiloxanes with silicone tetrachloride, the initial mixtures having functionalities in the range from 2·5 to 3·25. The resulting three dimensional polymethylsiloxanes (PMS) differ as to their structures and the length of chain segments between crosslinks (the distance between network points or the segmental molecular weight Mc). The thermal and thermo-oxidative degradation of PMS have been investigated. It is shown that the thermal and thermo-oxidative stability of PMS increase regularly as the value ofMc decerases, the structure of the monomeric unit of the PMS network influencing the thermal more than the thermo-oxidative stability.