(b) 2-methoxy-2- 4-(4-phenylphenoxy)phenyl!but-1-yl diphenylphosphate, the compound in which n is 1, R1 is a methyl group, R3 is an ethyl group, the group Ar attached to the same carbon atom as the group R3 is a 4-(4-phenylphenoxy)phenyl group, and the other two groups Ar are phenyl groups; and