in which Ar is an aromatic hydrocarbon group such as phenyl or naphthyl (which can optionally be ring-substituted as described above, particularly with C1-C20 alkyl or C7-C30 aralkyl groups); X is a direct bond, O, or ???CO???NR1??? or ???SO2???NR1??? in which R1 is hydrogen or alkyl (i.e., an amide) or is attached to the aromatic group through a ???CO??? group to form an imide (e.g., a phthalimid