Spiro orthocarbonate (SOC) monomers having either an exomethylene group {3,3-dimethyl-9-methylene-1,5,7,11-tetraoxaspiro[5.5]undecane (ExoSOC)} or an allyl group {9-allyl-3,3-dimethyl-1,5,7,11-tetraoxaspiro[5.5]undecane (AllylSOC)} were radically copolymerized with vinyl monomers at several feed ratios to obtain the corresponding copolymers having SOC moieties in the side chain. The obtained copolymers were crosslinked via the double ring-opening polymerization of the SOC moieties by a treatment with boron trifluoride etherate. The volume changes during the crosslinking of the copolymers were evaluated by density measurements with a gas pycnometer. As the SOC moiety composition increased, the volume shrinkage during the crosslinking was suppressed, and that finally changed into volume expansion. The volume changes during the crosslinking of the copolymers from AllylSOC were slightly larger than those of the copolymers from ExoSOC. The higher volume expansions in the crosslinking of AllylSOC-based copolymers were ascribable to the lower steric hindrance around the SOC moieties. (c) 2006 Wiley Periodicals, Inc.