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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.

Dually responsive poly[(N,N-diethylaminoethyl methacrylate)-b-(N-isopropyl acrylamide)]s (P(DEAEMA-b-NIPAM)s) capable of "schizophrenic" aggregation in aqueous solution were synthesized via aqueous reversible addition-fragmentation chain transfer (RAFT) polymerization. The nanoassembly morphologies, dictated by the hydrophilic mass fraction, can be controlled by the polymer block lengths, solution pH, and temperature. Both P(DEAEMA 98 -b-NIPAM 209 ) (52.5 wt % NIPAM) and P(DEAEMA 98b-NIPAM 392 ) (70.8 wt % NIPAM) self-assemble into PDEAEMA-core PNIPAM-shell spherical micelles with a hydrodynamic radii (R h ) of 21 and 25 nm, respectively, at temperatures below the lower critical solution temperature of PNIPAM and at solution pH values greater than the pK a of PDEAEMA. The two block copolymers, however, display quite different temperature-responsive behaviors at pH < 7.5. At elevated temperatures (>42 °C) P(DEAEMA 98 -b-NIPAM 209 ) forms spherical micelles (R h =28 nm) with hydrophobic PNIPAM cores stabilized by a hydrophilic PDEAEMA shell. By contrast, P(DEAEMA 98 -b-NIPAM 392 ) assembles into vesicles (R h =99 nm) above 38 °C. The nanostructures were characterized by a combination of dynamic and static light scattering as well as transmission electron microscopy and are being investigated for their potential application as drug delivery vehicles. † Paper number 143 in a series on Water-Soluble Polymers.

Herein we report the synthesis and characterization of polymeric cross-linking agents and novel shell cross-linked (SCL) micelles utilizing reversible addition-fragmentation chain transfer (RAFT) polymerization. A series of pH-responsive ABC triblock copolymers consisting of R-methoxypoly(ethylene oxide)-b-poly[N-(3aminopropyl)methacrylamide]-b-poly [2-(diisopropylamino)ethyl methacrylate] (mPEO-PAPMA-PDPAEMA) have been synthesized via RAFT polymerization in aqueous media at 70 °C employing a PEO-based macrochain transfer agent (macro-CTA). These triblock copolymers molecularly dissolve in aqueous solution at low pH (<5.0) due to protonation of primary amine residues on the PAPMA block and tertiary amine residues on the PDPAEMA block. Above pH 6.0, the polymers self-assemble into micelles consisting of PDPAEMA cores, PAPMA shells, and mPEO coronas. Hydrodynamic dimensions of the triblock copolymer micelles depend on both triblock copolymer composition and solution pH. Narrowly dispersed poly(N-isopropylacrylamide) was synthesized utilizing the difunctional CTA, 2-(1-carboxy-1-methylethylsulfanylthiocarbonylsulfanyl)-2-methylpropionic acid (CMP). The chain ends of the PNIPAM were converted from carboxylic acids to Nhydroxysuccinimidyl esters (NHS) through dicyclohexylcarbodiimide (DCC) coupling, yielding an amine-reactive polymeric cross-linking agent, NHS-PNIPAM-NHS. SCL micelles were attained via reaction of PAPMA (shell) amine functionality with NHS-functionalized PNIPAM. These SCL micelles swell when the solution pH is lowered below the pK a of the PDPAEMA block. The polymeric cross-linking agent NHS-PNIPAM-NHS synthesized in this work has inherent temperature-responsive segments and a cleavable trithiocarbonate unit which have future potential in mediating drug delivery from SCL micelles.