PURPOSE: To obtain the titled substrate, consisting essentially of a specific polytriorganovinylsilane, having excellent light transmittance and heat resistance, low moisture absorption property without changing shape and deteriorating quality even in preserving under various environments for a long period and used for a high-density and large-capacity recording method.

CONSTITUTION: The aimed substrate consisting essentially of a polytriorgano vinylsilane expressed by formula I (R1WR3 are 1W6C monofunctional hy drocarbon; n≥1,000). The polymer expressed by formula I is produced by anionic polymerization of a triorganovinylsilane expressed by formula II as a raw material monomer in the presence of n-butyllithium as a catalyst at 50W70°C in an inert gas atmosphere for 20W50hr.