Coating composition and hydroxy acrylic copolymer precursor therefor

A thermosetting coating composition with enhanced flexibility and impact strength and excellent weatherabilty adapted for use as an automotive topcoat which upon curing forms a hard, glossy, durable coating. The coating composiition comprises:- (A) a modified hydroxy acrylic copolymer which is the a stepwise reaction product of:- (i) a copolymer bearing pendant hydroxyl groups and having a number average molecular (Mn) of between about 1000-5000, (ii) a dicarboxylic acid anhydride comprising at least about 50 percent by weight of an alkyl hexahydrophthalic anhydride, and (iii) a monoepoxide functional monomer comprising at least about 90 percent by weight of a C2-C10 monoepoxide functional monomer; (B) an amine-aldehyde or polyisocyanate crosslinking agent; (C) optionally a hydroxy functional additive, and (D) solvent. The anhydride is reacted in an amount sufficient to esterify at least about 30 percent of the pendant hydroxyl groups of copolymer (i) thereby generating carboxyl functionality, substantially all of which is thereafter reacted with the monoepoxide functional monomer. The crosslinking agent is included in the composition in an amount sufficient to provide at least about 0.5 equivalent of crosslinking functionality per equivalent of hydroxy functionality included in the composition either as a hydroxyl group of the modified hydroxy acrylic copolymer or as a hydroxyl group of the optional hydroxy-functional additive.