A reaction mixture for producing a film-forming polycarbosilazane polymer comprises a first compound containing at least two unsaturated groups and a second compound containing at least two SiH.sub.2 hydrosilyl functional groups, wherein the reaction mixture is capable of forming the film-forming polycarbosilazane polymer utilizing hydrosilylation of the first compound by the second compound. Also disclosed are methods of forming a silicon and carbon containing film on a substrate comprising the steps of producing a film-forming polycarbosilazane polymer by a polymerization of a reaction mixture of a first compound containing at least two unsaturated groups and a second compound containing at least two hydrosilyl functional groups, forming a solution containing the film-forming polycarbosilazane polymer, and contacting the solution with the substrate via a spin-on coating, spray coating, dip coating, or slit coating technique to form the silicon and carbon containing film on the substrate.

Provided are a thermoplastic resin excellent in terms of properties for optical uses, such as refractive index and chromatic aberration, and a molded article including such thermoplastic resin. This thermoplastic resin has: a silane constituent unit (S) which is a constituent unit represented by general formula (1) and is the OSi(R.sup.1R.sup.2)O moiety; and a diol constituent unit (A) derived from a dihydroxy compound and is represented by M. (In general formula (1), R.sup.1 and/or R.sup.2 represents an optionally substituted C10-C30 polycyclic aryl group.)

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