osphorous ester thereof represented by any one of the formulae III--VI below ##STR2## wherein R1 and R3 represent an alkyl group containing from 1 to 18 carbon atoms respectively, R2 and R5 represents an alkyl group containing from 1 to 8 carbon atoms respectively, R4 represents an alkyl group containing from 1 to 30 carbon atoms, l is an integer from 0 to 30, m is an integer from 1 to 40, and n i