Synthesis and High Temperature Chemistry of Methylsilsesquioxane Polymers Produced by Titanium Catalyzed Redistribution of Methylhydridooligo- and Polysiloxanes.

摘要

The synthetic utility of homogeneous titanium catalyzed redistribution of the cyclomers-(MeHSiO) (x)-(where x=4 or 5) and the linear oligomer -MeHSiO(x) - MeHSiO(x)-(M(n) approx. 2,200 D) was explored as a route to MeSiH3 and a methylsilsesquioxane-methylhydridopolysiloxane copolymer of approximate composition- MeHSiO 0.35 MeSi(O)1.5)0.65-. The high temperature behavior of the titantium derived methylsilsesquioxane copolymer follows closely that of similar polymers prepared by sol-gel processing. Heating under nitrogen to 800-1000 C at a heating rate of 5 C/min gives a black glass with an apparent composition of: SiO2 (70%); SiC (19%) and C (10%). The apparent composition bellies the true nature of the material which is probably a metastable glass, best described by the various Si-X bonding arrangements (X= O, C, H). Thus, MAS NMR and DRIFT spectroscopy show the existence of species containing 4,3 and 2 Si-O bonds with the remaining bonds either Si-C or SiH. At 600 C and below (under N2), the Si-C bonds are almost exclusively Si-H3, which is an indication of the extraordinary thermal stability of silsesquioxane polymers. Above 600 C, the CH3 groups react with Si-O bonds to generate Si-OH bonds, new Si-C bonds and Si-H bonds. This type of reactivity is illustrative of the basic chemistry involved in the carbothermal reduction of SiO2 to Si and/or SiC as well as the degradation mechanisms of the polymer. Keywords: Ceramic materials; Silica glass. (aw)