This article describes cationic ring-opening copolymerization of a bicyclic orthoester having hydroxy group (BOE-OH) and glycidyl phenyl ether (GPE), and the volume shrinkage behavior during the copolymerization. THF soluble polyethers [poly(BOE-OH-co-GPE)] were obtained by the copolymerizations at 80-180 degrees C, while crosslinked poly(BOE-OH-co-GPE) was obtained by the copolymerizations at 220-250 degrees C. This crosslinking reaction may originate from the dehydration of methylol groups in the side chain of poly(BOE-OH-co-GPE). The volume shrinkage during the cationic copolymerization reduced as the increase of the BOE-OH feed ratio. By contrast, the volume shrinkage on the crosslinking polymerization was almost independent on the BOE-OH feed ratio. Poly(BOE-OH-coGPE)s with higher BOE-OH composition showed lower thermal weight loss temperature owing to the release of H2O by dehydration of methylol groups. The BOE-OH component in the THF soluble poly(BOE-OH-co-GPE)s lowered the glass transition temperature (T-g), while that in the crosslinked poly(BOE-OH-co-GPE) increased the T-g probably because of the higher crosslinking density. (c) 2006 Wiley Periodicals, Inc.