PURPOSE: To enable a silane crosslinking agent in the solid state to be easily and conveniently incorporated into a polymer by mixing and heating a thermoplastic base polymer, a solid carrier polymer contg. a specific silane crosslinking agent, and a free-radical generator together.

CONSTITUTION: A thermoplastic base polymer (A) which is a 2-6C α-olefin homopolymer or a copolymer of two α-olefins, a solid carrier polymer (B) which is pref. a porous polymer, contains a silane crosslinking agent (a) represented by the formula (wherein R' is H or methyl; x and y are each 0 or 1 provided when x is 1, then y is 1; n is 1-12; and R is a hydrolyzable group selected from among alkoxy, aryloxy, aralkoxy, aliph. acyloxy, oximo, alkylamino, arylamino, and 1-6C alkyl groups provided not more than one R is an alkyl) and optionally a hydrolysis-condensation catalyst (b), and does not substantially contain water, and a free-radical generator (C) are mixed, and heated to the crystalline m.p. of ingredient A or higher to obtain a polymer crosslinkable by exposure to moisture in the presence of ingredient (b).