The optimal conditions of rosin modification of unsaturated polyesters were found and the reaction mechanism which consists of both Diels-Alder addition reaction and esterification were investigated by means of IR

UV

NMR

and MS and by comparison with some related model compounds. It has been found that this chemical modification is mainly due to the reaction of the terminal hydroxyl groups of the polyesters-and the branching effect of Diels-Alder products

and hence

the water resistance

chemical resistance

chemical resistance and the compatibility with styrene of the polyesters are improved.