The most common tooth-filling material presently alternative to Amalgam is dental composites. These materials consist mainly of a polymer and a filler part. The polymer can cure by a photoinitiator for visible light. Dental composites used at present are accompanied with a significant polymerization shrinkage. This phenomenon is associated with the curing contraction of the polymer and will produce internal stress of the material when placed in a tooth-cavity and result in insufficient seal.- The main objective of this R & D project is to identify practically useful monomers which will not contract or even expand during polymerization. This objective reached will be highly innovative and may have a great impact on the entire polymer industry. From prior experience it is known that monomers, that polymerize by ring-opening reactions show less contraction or none.