39] wherein m and n are each a positive number, the sum of m and n being from 5 to 70, R1 and R2 are each hydrogen, methyl or ethyl, preferably hydrogen or methyl and A is a divalent linking group selected from the group consisting of straight or branched chain alkylene (usually containing from 1 to 8 carbon atoms), phenylene, C1-C9 alkyl substituted phenylene and a group represented by the follow