Compositions comprising a matrix polymer and a mixture of monomers are used for making polymer mixtures containing the matrix polymer and a second polymer formed from the monomer mixture. Preferably, the matrix polymer comprises a polyester, polystyrene, polyacrylate, thiol-cured epoxy polymer, thiol-cured isocyanate polymer, or mixtures thereof. Preferably, the monomer mixture comprises a thiol monomer and at least one second monomer selected from the group consisting of ene monomer and yne monomer. The compositions may be used to fabricate optical elements such as lenses.