PURPOSE: To obtain a curable polymer compsn. which is of a one-component type, has an excellent long-term stability, and exhibits a satisfactory curability when used by compounding a specific vinyl polymer and a specific compd. into the compsn.

CONSTITUTION: This polymer composition contains a vinyl polymer having a hydrolyzable silyl group of the formula (wherein R1 is alkyl, aryl, or aralkyl; X is halogen, alkoxy, acyloxy, or hydroxyl; and (k) is 1-3) (e.g. a polymer obtd. by polymerizing t-butyl methacrylate, styrene, γ-methacryloyloxypropyl trimethoxysilane, methyl methacrylate, and ethyl acrylate) and at least one compd. selected from the group consisting of an aryldiazonium salt, a diaryliodinium salt, a triarylsulfonium salt, a triarylselenonium salt, a phenacylsulfonium salt, a 4-hydroxyphenylsulfonium salt, and a sulfonic ester (e.g. triphenylsulfonium hexafluoroantimonate).