Polyisobutylene-based helical block copolymers with ph-responsive cationic side-chain amino acid moieties by tandem living polymerizations.

Abstract

Syntheses of diblock copolymers of isobutylene (IB) with amino acid-based monomers were carried out by a combination of living carbocationic and reversible addition–fragmentation chain transfer (RAFT) polymerizations. Cationic polymerization of IB followed by hydroboration oxidation produced hydroxyl end-capped polyisobutylene, which was subsequently converted into macro-chain-transfer agent (macro-CTA) for the RAFT polymerization of tert-butyloxycarbonyl (Boc)-protected amino acid-based monomers such as Boc-l-alanine methacryloyloxyethyl ester (Boc-l-Ala-HEMA) and Boc-l-leucine methacryloyloxyethyl ester (Boc-l-Leu-HEMA). The block length and molecular weight could be controlled while maintaining a narrow molecular weight distribution. These block copolymers form core–shell type micellar structure in methanol—a good solvent for side-chain amino acid block but bad solvent for polyisobutylene segment. Specific rotation and circular dichroism (CD) studies indicated the chiral nature of the block copolymers and revealed that they adopt a stable helical conformation with predominantly one-handed screw sense in acetonitrile. Removal of the Boc groups under acidic conditions gave pendant side chain primary amino groups, and all the block copolymers exhibited “smart” pH responsiveness and amphiphilic character in aqueous media as confirmed by 1H NMR, fluorescence spectroscopy, dynamic light scattering (DLS), atomic force microscopy (AFM), and scanning electron microscopy (SEM). Block copolymers carrying primary amine groups as pendants can be useful for postpolymerization modification to prepare other PIB containing functional materials.