An (ethylene, vinyl acetal) copolymer is disclosed which comprises (i) a plurality of ethylenic moieties A having a structure according to the following formula: wherein R 2 and R 3 independently represent hydrogen, a halogen or an optionally substituted linear, branched or cyclic alk(en)yl group, or an optionally substituted aromatic or heteroaromatic group; and (ii) a plurality of acetal moieties B having a structure according to the following formula: wherein L 1 represents a divalent linking group; x = 0 or 1; and R 1 represents an optionally substituted aromatic or heteroaromatic group including at least one hydroxyl group; and (iii) a plurality of acetal moieties C and/or moieties D which are different from the acetal moieties B, and which include at least one nitrogen atom.