"number, the sum of m and n being from 0 to 70, preferably 2 to 30, and more preferably 4 to 10, R.sub.3 and R.sub.4 are each hydrogen or methyl, R.sub.5 and R.sub.6 are each hydrogen or C.sub.1 to C.sub.2 alkyl, and A is a divalent linking group selected" . . . . .