Dental resins with reduced shrinkage during hardening.

Unsaturated spiro-orthocarbonates are monomers that expand on polymerization and will co-polymerize with conventional BIS-GMA based resins. This combination has good physical properties and bonding to enamel with some polymerization expansion evident at 50 C. Dispersion of the carbonate in microcrystalline form in liquid monomer appears to be a method to allow its reaction in the initial polymerization.