Synthesis of Zwitterionic Poly(sulfobetaine methacrylate)s with Asymmetric N-Substituents by Post-polymerization Betainization Reactions

Zwitterionic polymers, with one cationic and one anionic group in each monomer unit, have shown tremendous potential in numerous applications, ranging from antifouling to drug delivery, lubrication, and water-oil separation. With a net charge of zero, these polyelectrolytes are commonly synthesized from zwitterionic monomers. It is known that the poly(sulfobetaine methacrylate) (PSBMA) with two N-methyl substituents can be readily synthesized from poly(2-(N,N-dimethylamino)ethyl methacrylate) by reaction with 1,3-propanesultone. However, no betainization was observed for the PSBMA with two N-isopropyl substituents as reported in the literature, which is attributed to the steric hindrance. This thesis work shows that PSBMAs with one N-methyl and one N-n-alkyl substituent can undergo complete betainization with 1,3-propanesultone. A series of poly(2-(N-methyl-N-n-alkylamino)ethyl methacrylate) homopolymers with controlled molecular weights and narrow dispersities were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization, including poly(2-(N-methyl-N-ethylamino)ethyl methacrylate), poly(2-(N-methyl-N-n-propylamino)ethyl methacrylate), poly(2-(N-methyl-N-n-pentylamino)ethyl methacrylate), and poly(2-(N-methyl-N-n-hexylamino)ethyl methacrylate). The betainization reactions of these polymers with 1,3-propanesultone were conducted in tetrahydrofuran at 70 °C for 72 h and found to be quantitative by ¹H NMR analysis; the reaction mixtures all turned white gel-like. This finding provides an alternative route for the synthesis of PSBMAs with asymmetric N-substituents, given the difficulty commonly encountered in the characterization of zwitterionic polymers by size exclusion chromatography. It opens up new opportunities for the synthesis of more complex wholly zwitterionic polymer architectures, such as block copolymers, star copolymers, and molecular bottlebrushes, by post-polymerization modification. As a demonstration, a wholly zwitterionic diblock copolymer was successfully prepared by the reaction of its precursor diblock copolymer with 1,3-propanesultone under our optimized betainization conditions. In addition, a new monomer, 2-(N-methyl-N-benzylamino)ethyl methacrylate and the corresponding homopolymer with a controlled molecular weight and a narrow dispersity were synthesized by RAFT polymerization.