Herein we report the synthesis of vinyl sulfone end-functionalized PEGylated polymers by reversible addition-fragmentation chain transfer (RAFT) polymerization for conjugation to proteins. Poly(ethylene glycol) methyl ether acrylate (PEGA) was polymerized in the presence of I-phenylethyl dithiobenzoate with 2,2'-azobis(2-methylpropionitrile) as the initiator to generate well-defined polyPEGAs with number-average molecular weights (M-n) by gel permeation chromatography (GPC) of 6.7, 11.8, and 16.1 kDa. Postpolymerization, the majority of polymer chains contained the dithioester functional group at the omega chain end, and the polydispersity indexes (PDI) of the polymers ranged from 1.08 to 1.24. The dithioesters were subsequently reduced via aminolysis, and the resulting thiols were trapped with divinyl sulfone in situ to produce semitelechelie, vinyl sulfone polyPEGAs with efficiencies ranging between 85% and 99%. It wits determined that the retention or vinyl sulfone was directly related to reaction time, with the maximum dithioester being transformed into a vinyl sulfone within 30 min. Longer reaction times resulted in slow decomposition of the vinyl sulfone end group. The resulting semitelechelie vinyl sulfone polymers were then Conjugated to a protein containing a free cysteine, bovine serum albumin (BSA). Get electrophoresis demonstrated that the reaction was highly efficient and that conjugates of increasing size were readily prepared. After polymer attachment, the activity of the BSA was 92% of the unmodified biomolecular.