where A, A???, R and q are defined as above; Y is an optionally substituted alkyl or aryl group; Z and Z???, which can be the same or different, are each an optionally substituted alkyl, aryl or aralkyl group; X is hydrogen or an aliphatic group, preferably an alkyl group having 1 to 30, particularly 6 to 20, carbon atoms, or a triorganosilyl group such as trimethylsilyl; m is for example 41000; a