In order to develop a casting resin for precision molding, a curing method without shrinkage during the polymerization was achieved by the double ring-opening addition reaction of spiro orthoester (SOE) and acid chloride using cationic catalysts. Tetraphenylphosphonium halides (TPPXs) were found to be effective catalysts for the reaction. The catalytic reaction of SOE with acid chloride proceeded even at 25 degrees C to give the corresponding adduct. Polyaddition of a bifunctional SOE-XII with trifunctional 1,3,5-pentanetricarbonyl trichloride gave a cured material at 30 degrees C in the presence of tetraphenylphosphonium bromide (TPPB) of 1 mol%, accompanied by a negligible shrinkage of 0.21%. Gelation time was shortened with increase in the amount of catalyst and reaction temperature in the curing process. The obtained products were crosslinked copolymers that showed variable mechanical properties dependent on the curing temperature and monomer structure. This is the first demonstration of shrinkage-controlled molding in the absence of heating or cooling process. (c) 2005 Elsevier Ltd. All rights reserved.