"wherein R1 is an alkyl group having 1 to 20 carbon atoms, with the proviso that the total number of carbon atoms of 4 alkyl groups is at least 13, and 4 alkyl groups may be the same or different from one another; benzyltrialkylphosphonium salts represented by a formula (7): wherein R1 is an alkyl group having 1 to 20 carbon atoms, with the proviso that the total number of carbon atoms of 3 alkyl g" . . . .