RAFT Synthesis of Water-Soluble, Stimuli-Responsive AB Diblock Copolymers
A series of water-soluble, stimuli-responsive AB diblock copolymers were synthesized via the reversible addition-fragmentation chain transfer (RAFT) polymerization technique employing 2-(2-carboxyethylsulfanylthiocarbonylsulfanyl)propionic acid (CTA26 ) as the RAFT mediating agent. First, a series of diacid functional trithiocarbonate chain transfer agents (CTA's) were synthesized and examined for their effectiveness as mediating agents in controlling the polymerization of n -butyl acrylate (nBA). Overall, CTA26 demonstrated good control in the homopolymerization of nBA with respect to the molecular weight and the molecular weight distribution, as well as its ability to form block copolymers with high reinitiating efficiency, and thus was chosen as the CTA for the subsequent synthesis of polymers. The first series of AB diblock copolymers we synthesized were polyampholytes derived from phosphonium styrenic-based monomers (M63 and M106 ) and 4-vinylbenzoic acid (VBZ, M62 ). The homopolymerization of the trimethyl/triphenyl phosphonium styrene derivatives proceeds in a controlled fashion as evidenced from the narrow molecular weight distributions and the excellent agreement between the theoretical and experimentally determined molecular weights. We also demonstrate the controlled nature of the homopolymerization of M62 in DMSO. We subsequently prepared both statistical and block copolymers from the phosphonium/VBZ monomers to yield the first examples of polyampholytes in which the cationic functional group is a quaternary phosphonium species. We show that the kinetic characteristics of the statistical copolymerizations are different from the homopolymerizations and proceed, generally, at a significantly faster rate although there appears to be a composition dependence on the rate. Given the inherent problems in characterizing such polyampholytic copolymers via aqueous size exclusion chromatography we have qualitatively proved their successful formation via FTIR spectroscopy. Finally, we demonstrate the ability of such pH-responsive block copolymers to undergo supramolecular self-assembly characterized by 13 C NMR spectroscopy. Following this, we synthesized styrenic-based block polyelectrolytes comprised of 4-vinylbenzyltrimethylphosphonium chloride (TMP, M63 ) and N,N -dimethylbenzylvinylamine (DMBVA, M59 ) directly in aqueous media under homogeneous conditions. TMP was first homopolymerized and polyTMP was subsequently used as macro-CTA for the polymerization of the DMBVA under buffered conditions (pH 4). Copolymerizations were controlled as judged by the high blocking efficiency and the resulting narrow molecular weight distributions. The pH-dependent self-assembly properties of the AB diblock copolymers were examined using a combination of 1 H NMR spectroscopy, dynamic light scattering, and fluorescence spectroscopy. The size of the polymeric aggregates was demonstrated to be dependent upon the block copolymer composition/molar mass. Such pH-induced supramolecular self-assembly was also demonstrated to be completely reversible, as predicted given the tunable hydrophilicity/hydrophobicity of the DMBVA block. Finally, we demonstrate the ability to effectively lock the AB diblock copolymers in the self-assembled state via a straightforward core crosslinking reaction between the tertiary amine residues of DMBVA and the difunctional benzylic bromide 1,4-bis(bromomethyl)benzene. Finally, we made an AB diblock copolymer of N -isopropylacrylamide (NIPAM, M75 ) and VBZ ( M62 ) via RAFT mediated by CTA26 in DMF. NIPAM was homopolymerized first and polyNIPAM was treated as a macro-CTA in the subsequent polymerization of VBZ. By virtue of the temperature-responsive properties of the NIPAM block and the pH-responsive nature of VBZ block, this novel AB diblock copolymer was demonstrated to be able to form normal and inverse micelles in the same aqueous solution simply by controlling the temperature and solution pH. As judged by NMR spectroscopy and dynamic light scattering, raising the temperature to 40°C (above the lower critical solution temperature of the NIPAM block), while at pH 12 results in supramolecular self-assembly to yield nanosized species that, presumably, are composed of a hydrophobic NIPAM core stabilized by a hydrophilic VBZ corona. Conversely, lowering the solution pH to 2.0 at ambient temperature results in the formation of aggregates in which the VBZ block is now hydrophobic and in the core, stabilized by the hydrophilic NIPAM block.