These central portions may react with end unit compounds containing active methylene groups such as malononitrile, NC.CH2 COOR', where R' is an alkyl group containing from 1 to 6 carbon atoms, e.g. methyl, ethyl, propyl, butyl and hexyl groups, R'SO2 CH2 CN and R'SO2 CH2 COR' in which R' is as defined above, ##STR5## in which R is H or OH, ##STR6## in which each R' independently represent H or a