A polymer according to claim 1 wherein R6 is phenyl or substituted phenyl, R3 is an alkyl group having 1 to 4 carbon atoms, R1 is selected from the group consisting of a divalent saturated alkylene having 2 to 4 carbon atoms, a divalent substituted saturated alkylene having 2 to 4 carbon atoms in the alkylene portion and including at least one alkyl, alkenyl, aryl or alkylaryl substituent bonded t