Synthesis and Solution Self-assembly of pH-responsive ABC Triblock Copolymers

Two polylysine-containing ABC-type triblock hybrid copolymers,poly(ethylene glycol)-b-poly-(TFA-L-lysine)-b-poly(styrene) (PEG-b-PTLL-b-PS) and poly(ethylene glycol)-b-poly(TFA-L-lysine)-b-poly(N-isopropylacrylamide) (PEG-b-PTLL-b-PNIPAM),were successfully synthesized via sequential ring-opening polymerization (ROP) and atom transfer radical polymerization (ATRP). The diblock precursor,PEG-b-PTLL-NH2 was prepared via the ROP of e-trifluoroacetyl-L-lysine N-carboxyanhydride (Lys(TFA)-NCA) with amino-terminated PEG as the initiator. Then the diblock was converted to a macromolecular initiator (PEG-b-PTLL-Br) through the amidation with 2-bromoisobutyryl bromide in the presence of triethylamine (TEA) and 4-dimethylaminopyridine (DMAP). Two series of triblock hybrid copolymers, PEG-b-PTLL-b-PS and PEG-b-PTLL-b-PNIPAM,were obtained via ATRP under appropriate conditions. The GPC and NMR characterizations verified the successful preparation of triblock copolymers. The above results showed that the DP of each block can be adjusted by molecular design and variation of reaction time. Considering the rich amido groups in both backbone and side chain of PTLL block,we applied a relatively high amount of Cu(I) salts and ligand for effective activation and performed the ATRP in high-polarity organic solvent to overcome the effects of amide groups. Moreover,the self-assembly of amphiphilic PEG-b-PLL-b-PS was explored in mixture solvents at different pH values. It was found that the morphologies of resultant micelles depended on pH values in the water/DMF mixed solvent. When pHK(a) (ca. 10.5),the PEG(45)-b-PLL106-b-PS20 triblock formed spherical micelles. When pH>pK(a),the PEG(45)-b-PLL106-b-PS20 triblock formed disk-like micelles.