The advantages of halo-isocyanates as 'transformation' reagents in the synthesis of block and graft copolymers are elaborated. These compounds permit polymeric blocks of vinyl units to be attached to a wide range of starting materials, natural or synthetic. Two stages are involved-isocyanation and grafting-and both may be carried out homogeneously or heterogeneously. Attention is here focused on poly(ether-urethanes), e.g. Biomer, dextran and poly(ethylene glycol) to which chains of several water-soluble polymers have been attached.