in the particular embodiment described in the specification , a contact lens hydrogel is prepared by copolymerizing 75 parts of n - vinyl pyrrolidone and 23 parts of phenyl ethyl methacrylate together with allyl methacrylate as a crosslinker , t - butyl peroctoate as a thermal initiator and 2 , 2 - dimethoxy - 2 - phenylacetophenone or benzoin methyl ether as a photoinitiator . the mixture is maintained at a temperature of 5 ° c . and protected from light prior to photopolymerization which is carried out under ultraviolet radiation . the copolymer is then post - cured thermally . cloudy or hazy hydrogels are avoided by preventing prepolymerization of the mixture .