of 1 to 3 carbon atoms, ##STR2## wherein the plural R1 may be the same or different and are as defined above, and the plural R2 may be the same or different and are each an alkyl group of 1 to 10 carbon atoms having an amino group, ##STR3## wherein the R1 groups may be the same or different and are as defined above, and n is an integer of 1 or 2, ##STR4## wherein the plural R1 may be the same or d