roup having 2 to 16 carbon atoms, thiuram compounds, e.g. thiuram disulfide, having the following general formula [VI]: ##STR7## wherein R1, R2, R3 and R4 are an alkyl or aryl group having 2 to 16 carbon atoms, and organo tin mercaptides having the following general formula [VII]: ##STR8## wherein R1 and R2 are the same or different and each is an alkyl or aryl group having 2 to 16 carbon atoms, a