to 20 carbon atoms and an aralkyl group having 7 to 20 carbon atoms, and a mercaptan compound represented by the formula (5): ##STR43## wherein R1 and R2 are the same or different and each is a member selected from the group consisting of an alkyl group having 1 to 20 carbon atoms, an aryl group having 6 to 20 carbon atoms, an aralkyl group having 7 to 20 carbon atoms, and a triorganosiloxy group