Light-induced crosslinking polymerization of both bis-methacrylates and bis-epoxides yields highly transparent glassy products, suitable for optical applications. Rheological changes of vitrifying liquids during photopolymerization strongly influence the shape accuracy of the final product. Comparison of a free-radical-initiated methacrylate polymerization and a cationically initiated ring-opening polymerization of an epoxide showed that different reaction mechanisms led to very different chemorheological responses and, hence, to pronounced differences in the shape accuracy of the products obtained with either of these materials. An ethoxylated bisphenol-A dimethacrylate (HEBDM) gels below 1-2% conversion and vitrifies; at 15%. At higher conversion, large stresses develop through polymerization shrinkage. Relaxation occurs upon release of a product from its mold, leading to large shape deviations. Ring-opening polymerization of the diglycidyl ether of bisphenol-A (DGEBA) has an intrinsically lower polymerization shrinkage. Moreover, gelation of DGEBA polymer networks occurs at 25-30% conversion, leading to much lower stresses since most of the volume change occurs in the liquid state in which replenishment of a monomer can still occur. Upon release from a mold, there is hardly any driving force for relaxation, so a much better copy of the mold is obtained. (C) 2003 Wiley Periodicals, Inc.