POLYMERIC MIXTURES AND PROCESS THEREFOR

A tough polymeric mixture comprises the polymerized product of admixture I or II, wherein admixture I comprises 5 to 45 weight percent of a first monomer and a photoinitiator therefor, said first monomer being at least one free-radically polymerizable monomer, and 95 to 55 weight percent of a second monomer and a photoinitiator therefor, said second monomer being at least one set of polyurethane precursors, and admixture II comprises 5 to 45 weight percent of a first monomer and a photoinitiator therefor, said first monomer being at least one set of polyurethane precursors, and 95 to 55 weight percent of a second monomer and a photoinitiator therefor, said second monomer being at least one free-radically polymerizable monomer, said polymeric mixture having been prepared by a process involving two sequential actinic radiations of wavelengths ranges centered around lambda 1 and lambda 2, respectively, wherein lambda 1 and lambda 2 differ from each other by at least 30 nm, and wherein the actinic radiation centered lambda 1 does not substantialy polymerize the second monomer, and wherein the area under a stress-strain curve as determined by ASTM D638-89 for said polymeric mixture is at least 10 percent greater, preferably 20 percent greater, and more preferably at least 50 percent greater when said precursor monomers are cured by said sequential photoinitiation process compared to when the same mixture is cured by a single photostage cure.