As the polymerization initiator, organic peroxides such as 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 1,1-bis(t-butylperoxy)cyclohexane, t-butylperoxy-3,3,5-trimethylhexanoate, t-butylperoxyisopropyl carbonate, 2,5-dimethyl-2,5-di(benzoylperoxy)hexane, t-butylperoxy benzoate, dicumylperoxide, 2,5-dimethyl-2,5-di(t-butyl peroxy)hexane, t-butylperoxy(2-ethylhexanoate), di-t-butyl hydroperoxide, t-butylperoctoate, benzoylperoxide, etc. are used. In order to obtain a superior surface quality in a usual molding time, t-butyl-peroxybenzoate alone or in an admixture with another compound is preferably used.
