The present disclosure relates to a new polymerization process in which ethylenically unsaturated monomers are polymerised by a living radical polymerization process in the presence of an initiator and a catalyst. Polymers produced by this new process are also thought to be novel and may be used to derivatise biological molecules to improve their efficacy as therapeutic treatments. A preferred polymer is of formula The polymers are particularly suitable for derivatising proteins, such as interferon-α.