A process for the preparation of a polymethane graft copolymer for the encapsulation and immunoisolation of living cells, comprising the steps of reacting an isocyanate with a solution of a polyol in the presence of a catalyst followed by adding a chain extender thereto to form a mixture and curing the mixture to obtain the polyurethane, subjecting the polyuretnane to a step of swelling in a solution of a vinyl monomer,initiator and crosslinker to obtain the swollen polymer followed by irra- diation to form the polyurethane graft copolymer.