Synthetic protocols toward polypeptide conjugates via chain end functionalization after RAFT polymerization

Abstract

We report a new protocol to synthesize conjugates of polypeptides with vinyl polymers prepared by reversible addition–fragmentation chain transfer (RAFT) polymerization. Well-defined polystyrene-b-poly(γ-benzyl L-glutamate) (PS-b-PBLG) diblock copolymers are synthesized as a typical example of such conjugates. Polystyrene is prepared by RAFT polymerization using conventional and commonly used 2-cyano-2-propyl dodecyl trithiocarbonate as a RAFT chain transfer agent bearing an inert R group. Amine functionalization is carried out by a one-pot process combining aminolysis with the in situ thiol capping reaction by acrylate or methanethiosulfonate containing a tert-butoxy carbonyl (Boc) protected amine group. After deprotection of the Boc group, the released amino group initiates ring-opening polymerization of γ-benzyl L-glutamate N-carboxyanhydride (BLG NCA) to generate the desired block copolymers. Hydrolysis of the benzyl group affords the corresponding polystyrene-b-poly(L-glutamic acid) (PS-b-PGlu) amphiphilic block copolymers which self-assemble into spherical micelles in aqueous media. Circular dichroism (CD) spectra show secondary structural changes of PGlu in different pH conditions. When methanethiosulfonate is used as the capping agent, polystyrene and PBLG segments are conjugated by a reduction-responsive disulfide linkage, which is confirmed by size-exclusion chromatography and destruction of the micelle after treating with dithiothreitol (DTT). This protocol is a simple way to synthesize diblock copolymers of amino acids with vinyl monomers without specific design of RAFT CTA, which is practical for various polymer conjugates with tunable properties.