Polysilsesquioxanes,-[RSi(O)1.5 ]x-, exhibit many properties that are potentially quite useful for industrial applications. These properties include high temperature stability (−600°C in O2); good adhesion and, liquid crystal-like behavior for some derivatives. Moreover, [MeSi(O)l.5]x, polymethylsilsesquioxane has been used successfully as a precursor for the fabrication of carbon fiber/“black glass” (SiO2/SiC/C) composites and “black glass” fibers.
Current methods of preparation depend on hydrolysis of RSiCl3 or RSi(OR)3. Unfortunately, this approach leads to several products that are difficult to purify because polysilsesquioxanes exhibit a great propensity for forming gels. We describe here a simple catalytic approach to the synthesis of polymethylsilsesquioxane copolymers of the type -[MeRSiO].3[MeSi(O)1.5].7- where R - H, OMe, OEt, OnPr and OnBu. The R - H copolymer is produced by catalytic redistribution of -[MeHSiO]xoligomers using dimethyltitanocene, Cp2TiMe2 as the catalyst precursor.
Following catalytic redistribution, the resulting copolymer, -[MeHSiO].3[MeSi(O)1.5].7−
, is reacted in situ with alcohols to produce -[Me(R'O)SiO].3[MeSi(0)1.5].7− (where R' - Me, Et, nPr and nBu) which serve as masked forms of the polymethylsilsesquioxane. These new copolymers have been characterized by 1H, 13C and 29Si NMR TGA and DTA. The NMR studies allow us to assign structures for the copolymer.
These new copolymers exhibit improved tractability. Their high temperature properties are all quite similar; although, the MeO-, EtO- and especially the nPrO- derivatives give much higher ceramic yields than expected.