Addition-fragmentation oligomers of the general formula Z—By-A-(B-A)x-B-A-By—Z, where the A monomers units are derived from a diester or diacid, the B monomer units are derived from a difunctional monomer having functional groups co-reactive with the acid or ester groups of the A monomer, x+y is zero to 60, Z comprises an ethylenically unsaturated, polymerizable group, A monomers comprise a 1-methylene-3,3-dimethylpropyl group, and at least one B monomer unit comprises a high refractive index group.
