Poly(N-isopropyl acrylamide) is a thermoresponsive polymer that has been widely investigated for drug delivery. Herein, we report conditions facilitating the controlled, room-temperature RAFT polymerization of N-isopropylacrylamide (NIPAM). The key to success is the appropriate choice of both a suitable RAFT chain transfer agent (CTA) and initiating species. We show that the use of 2-dodecylsulfanylthiocarbonylsulfanyl-2-methyl propionic acid, a trithiocarbonate RAFT CTA, in conjunction with the room-temperature azo initiator 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), in DMF, at 25 degrees C, yields conditions leading to NIPAM homopolymerizations which bear all of the characteristics of a controlled/"living" polymerization. We also demonstrate facile size exclusion chromatographic analysis of PNIPAM samples in DMF at 60 degrees C, directly on aliquots withdrawn during the polymerizations, which avoids the problems previously reported in the literature.

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The reversible addition−fragmentation chain transfer (RAFT) polymerization of acrylamide (AM) was studied in order to establish reaction conditions which would provide optimal rates of monomer conversion and to determine reasons for deviation of theoretical and experimental molecular weights, the former predicted from current models. To this end, chain transfer agents (CTAs) and initiators were selected and experiments performed in water and in dimethyl sulfoxide (DMSO) at specified CTA/initiator ratios and temperatures. Higher apparent rates of polymerization were achieved utilizing CTAs with higher intermediate fragmentation rates, larger initiator concentrations, and higher temperatures. For RAFT polymerization of acrylamide under these experimental conditions, a continuing supply of radicals was required in order to achieve reasonable conversions. The deviations of experimentally measured molecular weights from those theoretically predicted are a function of the CTA utilized and parallel the extent of rate retardation. The deviations are, at least in part, consistent with significant early radical coupling of stable intermediate species during the preequilibrium period (or the recently proposed CTA “initialization” period). These effects are apparent in both aqueous buffer and DMSO. The retardation effects and eventual loss of linearity of the first-order kinetic plots at extended times are also consistent with termination processes although these experiments alone do not rule out alternative mechanisms of reversible termination or slow fragmentation of intermediate species. For RAFT polymerizations in DMSO mediated by the trithiocarbonate CTA, reaction rates are significantly faster, and near quantitative conversions can be reached with proper initiator choice.

A review of hydrogels containing dynamic bonds that are shown to provide benefits for applications including self-healing and stimuli-induced stiffness changes.

We present an account of our research into polyelectrolyte polymer brushes that are capable of acting as stimuli-responsive films. We first detail the synthesis of poly(acrylic acid) polymer brushes using ATRP in a "grafting from" strategy. Significantly, we employed a chemical-free deprotection step that should leave the anchoring ester groups intact. We have demonstrated how these polymer assemblies respond to stimuli such as pH and electrolyte concentration. We have used poly(acrylic acid) polymer brushes for the synthesis of metallic nanoparticles and review this work. We have used XPS, ATR-FTIR, and AFM spectroscopy to show the presence of silver and palladium nanoparticles within polymer brushes. Finally, we report the synthesis of AB diblock polyampholyte polymer brushes that represent an extension of polyelectrolyte polymer brushes.

We report the preparation of a poly(acrylic acid) polymer brush from a flat silica substrate using a surface-initiated atom transfer radical polymerization of tert-butyl acrylate. Significantly, we use a chemical free deprotection strategy through pyrolysis of the tert-butyl esters to the corresponding carboxylic acids. This eliminates the possibility of loss of the polymer brush from the surface via acidolysis of the ester group in the surface bound initiator. We have verified the formation of the poly(acrylic acid) brush through ATR-FTIR, ellipsometry, water contact angle analysis, and XPS. We also demonstrate the stimuli-responsive nature of these brushes with respect to pH and added electrolyte concentration.

Wang resin has been transformed into an initiator for copper(I)-mediated living radical polymerization of methacrylates at initiator loading of 0.9 and 3.5 mmol g-1. The immobilized initiator was characterized by ATR FTIR, gel phase 13C NMR, and solid-state CP/MAS 13C NMR using two different spinning frequencies as well as a TOSS pulse sequence. The immobilized initiator has been used to prepare poly(methyl methacrylate), PMMA, homopolymer, and poly(methyl methacrylate)-block-poly(benzyl methacrylate-co-methyl methacrylate), P(MMA)-block-P(BzMA-co-MMA), block copolymers. The poly(methacrylate)s have been harvested from the insoluble resin by a simple trifluoroacetic acid, TFA, wash which selectively cleaved the activated benzyl ester linkage, so as to facilitate analysis. At an initiator loading of 0.9 mmol g-1 the M n increases linearly with conversion with kinetics following first-order behavior in monomer as would be expected for living polymerization. After 3 h a 61.9% conversion of MMA is reached, with the isolated polymer chains having an average number molar mass, M n, of 8200 and a polydispersity, PDI, of 1.18. High conversions, >90%, lead to considerable increases in M n and PDI. Moreover, small amounts of “free” chains present in the supernatant, ca. 5−8% after 4 h of reaction time, were found. The morphology of the beads was monitored by SEM with the integrity being maintained throughout the transformations. Attempts to prepare true block copolymers via a two-stage process involving isolation of the Wang resins with the first block and subsequent reuse to attach the second block were not satisfactory. However, a one-shot addition of BzMA at high conversion of MMA allowed the synthesis of P(MMA)-block-P(BzMA-co-MMA) with a narrow molar mass distribution, as confirmed with SEC, DSC, and NMR. The paper demonstrates that Wang immobilized chemistry can be used to prepare excellent polymers maintaining the characteristics of analogous homogeneous living radical polymerizations while allowing for catalyst removal by simple washing procedures. The potential for automation of this chemistry for high throughput synthesis has been demonstrated.

The synthesis of AB diblock copolymer polyampholyte polymer brushes of the type Si/SiO2//poly(acrylic acid-b-vinyl pyridine) prepared using atom transfer radical polymerization is reported. Both 2- and 4-vinyl pyridine have been used. The diblock polyampholyte polymer brushes demonstrate stimuli-responsive behavior with respect to pH, showing both polyelectrolyte and polyampholyte effects. Furthermore, we have quaternized the 4-vinyl pyridine segments to form a mixed weak/strong, or annealed/quenched, polyelectrolyte system. The quaternized polymer brush exhibits different pH-responsive behavior, with decreasing film thickness being observed with increasing pH.