The present invention provides a process for producing a comb polymer comprising the steps of: a) providing: (i) (w+z) molar equivalents of a monomer (ii) one molar equivalent of an initiator compound of formula (IX), wherein B3 represents a halogen, B2 represents H or a halogen, Y1 represents a group capable of attaching the residue of an antibody or fragment thereof or capable of being converted into such a group, L represents a linker group, y is 1, 2 or 3, w is at least 1 and z is 0 or greater (iii) a catalyst capable of catalysing the polymerization of a plurality of the monomers to produce the comb polymer and b) causing the catalyst to catalyse, in combination with the initiator, the polymerization of a plurality of the monomers (i) to produce the comb polymer.
