Single molecules of a monomer are held together by Van Der Waal's forces at Van Der Waal's distances of 4 angstroms.  Application of heat, acid base reactions or free radical formation are catalysts that join monomers to form polymers where molecules are at atomic bond distances of 1.9 angstroms.  Volumetric shrinkage can be dramatic, for example, acrylic materials shrink as more than 20%.

Filler materials reduce shrinkage by creating less resin.  They further increase the physical properties of composite.  Filler affects on composite behavior depend on filler material, surface, size, load, shape, surface modifiers, optical index, filler load and size distribution.  It is critical for filler to be wet by resin.  This limits the amount of filler that can be incorporated into resin.  A filler to resin interface is a high stress area when resin cures and for this reason, surface treatment of fillers is common to overcome interface breakdown.

Resin cure is inhibited by oxygen.  Oxygen inhibition is useful during the restorative process to allow layering techniques that minimize negative affects of shrinkage.  Layering techniques that maintain an oxygen inhibited layer result in a uniform structure as if the composite had been placed in bulk.  Layering techniques during restoration minimize negative affects of shrinkage by creating small incremental shrinkage.  The final outer layer must be covered to eliminate contact with air and complete composite polymerization.  Coating composite with glycerin and light curing is a popular technique.