Compositions comprising an oligomer which (a) on average comprises at least one in-chain residue of the general formula ##STR1## wherein Ar.sup.1 is an aromatic group, R.sup.1 is hydrogen or a hydrocarbyl group, X is a group which activates Ar.sup.1 to electrophilic attack, and Y.sup.1 is an organic residue bearing a carboxyl substituent; and (b) has one or more pendant and/or terminal acyloxymethyl groups are described. They may be used in dental, electronic and electrical applications.