alkyl group thereof containing 1 to 18 carbon atoms); R2 =a hydrogen atom, an alkyl group (containing 1 to 18 carbon atoms), or a di(substituted phenyl) amino group (the substituent including a hydrogen atom and an alkyl group containing 1 to 18 carbon atoms), provided that when n=1, two R2 groups may be combined to form any one of the following groups: ##STR181## (wherein R3, R4, R5 and R6 =hydr