Other comonomers include the esters of olefinically unsaturated carboxylic acids, preferably the lower alkyl esters of alpha, beta-olefinically unsaturated carboxylic acids and more preferred the esters having the structure ##STR19## wherein R1 is hydrogen, an alkyl group having from 1 to 4 carbon atoms, or a halogen and R2 is an alkyl group having from 1 to 2 carbon atoms.