Provided are polymer compositions made by a process comprising: (a) providing a first reactive composition containing: (i) a polymerization initiator that is capable, upon a first activation, of forming two or more free radical groups, at least one of which is further activatable by subsequent activation; (ii) one or more ethylenically unsaturated compounds; and (iii) a crosslinker; (b) subjecting the first reactive composition to a first activation step such that the first reactive composition polymerizes therein to form a crosslinked substrate network containing a covalently bound activatable free radical initiator, (c) combining the crosslinked substrate network with a second reactive composition containing one or more ethylenically unsaturated compounds; and (d) activating the covalently bound activatable free radical initiator of the crosslinked substrate network such that the second reactive composition polymerizes therein with the crosslinked substrate network to form a grafted polymeric network and a byproduct polymer. Also provided are precursors to the polymer compositions, processes for preparation of the polymer compositions, and methods of using the polymer compositions, for instance in medical devices.