Synthesis and aqueous solution behavior of a pH-responsive schizophrenic diblock copolymer

A novel zwitterionic poly[4-vinylbenzoic acid-block-2-N-(morphohno)ethyl methacrylate] (VBA(63)-b-MEMA(123)) diblock copolymer was synthesized via atom transfer radical polymerization using protecting group chemistry for the acidic residues. The acidic VBA block has a pK(a) of 7.1, which is higher than that of the conjugate acid of the MEMA block (pK(a) = 4.9). This has important consequences for the aqueous solution properties of this zwitterionic diblock copolymer. For example, unlike other zwitterionic diblock copolymers such as poly[methacrylic acid-block-2-(dimethylamino)ethyl methacrylate], precipitation does not occur at the isoelectric point (IEP) of around pH 6.2. This is most likely due to the relatively low degree of charge density on both the VBA and MEMA blocks at this pH. The VBA(63)-MEMA(123) copolymer exhibits interesting schizophrenic micellization behavior. Below pH 6, VBA-core micelles are formed, while above pH 6, the diblock copolymer can be dissolved as unimers. In the presence of sufficient Na2SO4 or at elevated temperature, well-defined MEMA-core micelles are formed in alkaline media. Thus, if dissolved in the presence of 0.80 M Na2SO4, the zwitterionic diblock copolymer can be switched from VBA-core micelles to MEMA-core micelles (and vice versa) simply by manipulating the solution pH. At intermediate pH around the IEP precipitation occurs, in contrast to the solution behavior in the absence of salt. The most likely explanation is that the weakly hydrophilic copolymer is simply salted out under these conditions. Alternatively, the added electrolyte leads to a higher charge density on the copolymer chains and precipitation occurs due to polyelectrolyte complexation. Both the VBA-core micelles and the inverted'' MEMA-core micelles have been characterized at different solution pHs, ionic strengths, and temperatures using potentiometric titration, aqueous electrophoresis, dynamic and static light scattering, and H-1 NMR spectroscopy measurements, respectively.