Synthesis and properties of diarylsiloxane and (aryl/methyl)siloxane polymers: 1. Thermal properties

Abstract

Poly(diarylsiloxane)s are highly crystalline, high-melting polymers with excellent high-temperature properties. As a consequence of the rigid polymer backbone, they show crystal-liquid-crystal transitions, and both nematic and smectic states have been observed by optical microscopy. The polymers of greatest interest in this investigation have been those containing phenyl and p-tolyl substituents on the siloxane backbone. We have prepared a series of polymers with various combinations of these substituents, as well as the end members of the series, poly(diphenylsiloxane) and poly(di( p-tolyl)siloxane). The polymers are prepared with controlled molecular weights and distribution by ring-opening anionic polymerization of the cyclic trimers in solution. The crystal-liquid-crystal transition temperatures, T 1c, for poly(diphenylsiloxane) and poly(di( p-tolyl)siloxane) are very high (265 and 300°C, respectively) and the polymers are only soluble in a few solvents at temperatures above 150°C. However, the T 1c values are reduced for the mixed poly((phenyl p- tolyl)siloxane)s and the polymers become soluble at lower temperatures. The symmetric poly((phenyl p- tolyl)siloxane) is of particular interest since T 1c is reduced to 150–160°C, and the polymer is soluble at room temperature in common solvents such as toluene, tetrahydrofuran and chloroform. In contrast to the retention of the crystalline and liquid-crystalline character in the mixed diarylsiloxanes, the replacement of a single phenyl group by a methyl group in the repeat hexaphenyl triad sequence of poly(diphenylsiloxane) is sufficient to destroy both the crystalline and liquid crystalline character of the polymer.