Synthesis of polyampholytic diblock copolymers via RAFT aqueous solution polymerization†
We report the synthesis of two new classes of polyampholytic diblock copolymers by RAFT aqueous solution polymerization. In each case, poly(methacrylic acid) (PMAA) is the anionic block while the cationic block comprises either poly(2-N-(morpholino)ethyl methacrylate) (PMEMA) or poly(2-(methacryloyloxy)ethyl trimethylammonium chloride) (PMETAC). Empirically, we found that polymerizing methacrylic acid as the second block afforded more well-defined diblock copolymers. Using this protocol, a series of copolymers of varying diblock composition is prepared for both classes. Robust derivatization protocols are developed to aid the characterization of such diblock copolymers via gel permeation chromatography (GPC). Thus the carboxylic acid groups within the PMAA block of the PMEMA–PMAA diblock copolymers are selectively methylated without quaternization of the tertiary amine groups on the PMEMA chains. In contrast, PMETAC–PMAA diblock copolymers are subjected to forced hydrolysis of the PMETAC ester groups to produce a PMAA homopolymer, which is then methylated to produce poly(methyl methacrylate) samples for GPC analysis. The aqueous solution properties of such polyampholytic diblock copolymers are explored using dynamic light scattering (DLS) and aqueous electrophoresis. These techniques enable identification of the isoelectric point. Unlike most other polyampholytic diblock copolymers reported in the literature, the PMEMA–PMAA diblock copolymers exhibit minimal variation in their isoelectric point when adjusting the diblock copolymer composition. This is because the pKa of the acidic PMAA block is close to the pKa of the conjugate acid form of the basic PMEMA block. For the PMETAC–PMAA system, no IEP is observed for PMETAC-rich copolymers because there is insufficient anionic charge to compensate for the cationic charge even if the PMAA chains are fully ionized.