Novel stimuli-responsive block copolymers via reversible addition-fragmentation chain transfer polymerization: Synthesis, characterization, and surface immobilization in aqueous media

Brent Sterling Sumerlin

Abstract

The extension of reversible addition-fragmentation chain transfer (RAFT) polymerization to the synthesis of well-defined, stimuli-responsive (co)polymers directly in water has been investigated. (Co)polymer structures that were once unattainable using the traditional methods of controlled polymerization have been prepared, and their aqueous solution properties examined. The first study involved the controlled polymerization of two functional, acrylamido monomers in homogeneous aqueous media. Well-defined homopolymers of sodium 2-acrylamdio-2-methylpropane sulfonate (AMPS) (59 ) and sodium 3-acrylamido 3-methylbutanoate (AMBA) 60 were obtained by employing 4-cyanopentanoic acid dithiobenzoate (54 ) as the RAFT chain transfer agent (CTA). Analysis of the degree of molecular weight control and the polymerization kinetics confirmed the controlled nature of the polymerizations. Successful reactivation of the dithioester end groups was achieved by successful block copolymerizations employing either PAMPS ( 65 ) or PAMBA (66 ) as a macro-chain transfer agent. A series of well-defined AB diblock copolymers were investigated for their ability to self-assemble in response to a change in solution pH. The second study involved the synthesis and solution characterization of a novel series of AB diblock copolymers with neutral, water-soluble N,N -dimethylacrylamide (DMA) (61 ) A blocks and pH-responsive N,N -dimethylvinylbenzylamine (DMVBA) (57 ) B blocks. This represents the first example of an acrylamido-styrenic block copolymer being prepared in homogeneous aqueous solution. In order to yield well-defined block copolymers with minimal homopolymer impurity, the DMA block must be prepared first, followed by polymerization of DMVBA. The DMA-DMVBA block copolymers ( 72 ) reversibly self-assembled to form well-defined micelles with DMA coronas and dehydrated DMVBA cores. By employing a hydrophobic, difunctional crosslinking agent, core-crosslinked micelles were also prepared. The aqueous solution properties of these block copolymers were investigated using 1 H NMR spectroscopy and DLS. The third study involved the immobilization of (co)polymers prepared by RAFT to transition metal surfaces. The in situ reduction of dithioester end groups, which resulted by virtue of the RAFT process, in the presence of a suitable transition metal complex facilitated long-term nanoparticle stabilization in aqueous solutions as confirmed by transmission electron microscopy and UV-Vis spectroscopy. (Abstract shortened by UMI.)