The synthesis of polyesters with pendant oxetane groups by the chemoselective alternating copolymerization of 3-ethyl-3-(glycidyloxymethyl)oxetane (EGMO) with carboxylic anhydride and the photochemical reaction of the resulting polymer was examined. The alternating copolymerization of EGMO with phthalic anhydride proceeded chemoselectively with quaternary onium salts under appropriate reaction conditions, and the corresponding soluble polymers with pendant oxetane groups with number-average molecular weights of 4700-7200 were obtained in 72-87% yields. Furthermore, the photochemical reaction of the resulting polymers was examined with certain photoacid generators in the film state upon UV irradiation, and it was found that the photocrosslinking reaction of the pendant oxetane groups proceeded smoothly to give the insoluble polymers. (C) 2003 Wiley Periodicals, Inc.