Synthesis and properties of diarylsiloxane and (aryl/methyl)siloxane polymers: 3. New aryl substituents

Abstract

In this paper we describe the synthesis and characterization of a new series of diarylsiloxane polymers, in which the aryl substituents are either m -tolyl, 4-methoxyphenyl or 4-propylphenyl. The polymers were prepared by the ring-opening anionic polymerization of cyclic trimers in solution at high temperatures. The structure and compositions of the cyclic trimers were confirmed by 1 H and 29 Si n.m.r. The new diaryl polymers are all highly crystalline, and each melts to a liquid crystalline state. The crystal-liquid crystal transition temperatures depend on the nature of substituents and follow the order: di(4-methoxyphenyl)≅di( p -tolyl)>di(phenyl)>di(4-propylphenyl)>di( m -tolyl). The transition temperatures of polydi(4-methoxyphenyl)siloxane and polydi( p -tolyl)siloxane are both very high (300°C). The solubility of the diarylsiloxane polymers is strongly affected by the nature of the aryl substituents. Polymers having di(4-methoxyphenyl), di( p -tolyl) or di(phenyl) substituents are only soluble at temperatures above 150°C in solvents such as dimethylsulfoxide and diphenyl ether. Polydi( m -tolyl)siloxane and the mixed poly (p- tolyl phenyl)siloxanes are soluble in toluene and chloroform at room temperature. Polydi(4-propylphenyl)siloxane crystallizes slowly, and if recovered and tested before becoming fully crystalline it will dissolve in common solvents such as toluene and chloroform at room temperature. However, after annealing it becomes soluble only at high temperatures. The cyclic trimer of di( o -tolyl)siloxane could only be prepared in trace quantities, and we were unsuccessful in preparing polydi( o -tolyl)siloxane, probably because of the steric interactions of the o -tolyl groups.