A thermoplastic composition comprising:(a) a vinyl chloride homopolymer or copolymer, (b) a silane of structure R'RSiY2 where R' is a monovalent unsaturated hydrocarbon or hydrocarbonoxy radical, Y is a hydrolyzable organic group and R is a R' or a Y radical, said silane being present in an amount to produce a graft vinyl chloride homopolymer or copolymer which is moisture cross-linkable to prod