For example, chlorobutyl rubber having tertiary carbon-chlorine bonds can be reacted with allyltrimethylsilane to give a polyisobutylene having an unsaturated group then reacted under addition conditions with platinum catalyst using a hydrosilane compound of the general formula wherein R2 is a hydrogen atom, an alkyl group containing 1 to 20 carbon atoms, an aryl group containing 6 to 20 carbon at