Patent ID: 8354477

Claim:
A method for preparing a conjugate of a purified branched water-soluble polymer, comprising: a. providing an impure polymer composition comprising (i) a branched water-soluble polymer having the structure: where Z is a moiety comprising a site suitable for interacting with ion exchange chromatography media, and mPEG a and mPEG b are each independently a methoxy polyethylene glycol, and (ii) one or more polymeric impurities selected from the group consisting of PEG diol, mPEG-OH, and activated mPEG, b. purifying the impure polymer composition by ion exchange chromatography under conditions effective to essentially remove the polymeric impurities to thereby provide a purified branched water-soluble polymer that is in essentially pure form, c. optionally reacting the purified branched water-soluble polymer with a reagent effective to transform Z to a Z-activated moiety suitable for reaction with a nucleophilic group of a biologically active molecule, and d. reacting the purified branched water soluble polymer of step b. or step c. with one or more of the nucleophilic groups of the biologically active molecule under conditions effective to form a conjugate of the purified branched water soluble polymer and the biologically active molecule.