molecular weight in the range of about 2,000 to about 30,000 and being essentially unreactive under copolymerization conditions, Z having the general formula: ##STR6## wherein R2 is a hydrogen atom or a lower alkyl group, R3 is a lower alkyl group, n is an integer from about 20 to about 500, and R4 is a monovalent radical selected from the group consisting of: ##STR7## and --CO2 R6 wherein R5 is a