Polymerization-induced thermal self-assembly (PITSA)

Polymerization-induced self-assembly (PISA) is a versatile technique to achieve a wide range of polymeric nanoparticle morphologies. Most previous examples of self-assembled soft nanoparticle synthesis by PISA rely on a growing solvophobic polymer block that leads to changes in nanoparticle architecture during polymerization in a selective solvent. However, synthesis of block copolymers with a growing stimuli-responsive block to form various nanoparticle shapes has yet to be reported. This new concept using thermoresponsive polymers is termed polymerization-induced thermal self-assembly (PITSA). A reversible addition-fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide from a hydrophilic chain transfer agent composed of N,N-dimethylacrylamide and acrylic acid was carried out in water above the known lower critical solution temperature (LCST) of poly(N-isopropylacrylamide) (PNIPAm). After reaching a certain chain length, the growing PNIPAm self-assembled, as induced by the LCST, into block copolymer aggregates within which dispersion polymerization continued. To characterize the nanoparticles at ambient temperatures without their dissolution, the particles were crosslinked immediately following polymerization at elevated temperatures via the reaction of the acid groups with a diamine in the presence of a carbodiimide. Size exclusion chromatography was used to evaluate the unimer molecular weight distributions and reaction kinetics. Dynamic light scattering and transmission electron microscopy provided insight into the size and morphologies of the nanoparticles. The resulting block copolymers formed polymeric nanoparticles with a range of morphologies (e.g., micelles, worms, and vesicles), which were a function of the PNIPAm block length

Aqueous Solution Properties of pH-Responsive AB Diblock Acrylamido Copolymers Synthesized via Aqueous RAFT

AB diblock and AB statistical copolymers of sodium 2-acrylamido-2-methylpropanesulfonate (AMPS) and sodium 3-acrylamido-3-methylbutanoate (AMBA) have been prepared via aqueous reversible addition-fragmentation chain transfer (RAFT) polymerization. Such AB diblock copolymers were tailored to undergo reversible pH-induced self-assembly given the tunable hydrophilic/hydrophobic nature of the AMBA species. Using a combination of H-1 NMR spectroscopy, dynamic light scattering, and fluorescence spectroscopy, it has been shown that these AB diblock copolymers exhibit reversible micellization below pH 5.5 with the aggregate sizes dependent on the block copolymer composition

Synthesis of Block Copolymers of 2- and 4-Vinylpyridine by RAFT Polymerization

The synthesis of block copolymers of 2- and 4-vinylpyridine (VP) by reversible addition-fragmentation chain transfer (RAFT) polymerization was discussed. All polymerization were conducted at 60 °C under a nitrogen atmosphere in septa-sealed vials. Absolute molecular weights were determined by online light scattering. Good end-group control was demonstrated by utilizing the resulting homopolymers as macro-chain-transfer agents

RAFT Polymerization in Homogeneous Aqueous Media

Controlled radical polymerization has been the focus of intense research during the last decade. However, to date, research has focused primarily on polymerizations conducted in homogeneous organic media or bulk with common monomers such as styrene, methyl methacrylate or butyl acrylate. The ability to conduct polymerizations in homogeneous aqueous solution is advantageous from both environmental and commercial viewpoints. In this chapter we will present a summary of our research efforts in the area of aqueous Reversible Addition-Fragmentation chain Transfer (RAFT) polymerization. We will demonstrate the broad versatility of RAFT, showing its applicability to neutral, anionic, cationic, and zwitterionic monomers from a range of monomer classes, including styrenics, acrylamides and (meth)acrylates

Aqueous Solution Properties of pH-Responsive AB Diblock Acrylamido-Styrenic Copolymers Synthesized Via Aqueous Reversible Addition-Fragmentation Chain Transfer

Here we report the synthesis and solution characterization of a novel series of AB diblock copolymers with neutral, water-soluble A blocks consisting of N,N-dimethylacrylamide and pH-responsive B blocks of N,N-dimethylvinylbenzylamine. To our knowledge, this represents the first example of an acrylamido-styrenic block copolymer prepared directly in a homogeneous aqueous solution. The best blocking order [with poly(N,N-dimethylacrylamide) as a macro-chain-transfer agent] yielded well-defined block copolymers with minimal homopolymer impurities. The reversible aggregation of these block copolymers in aqueous media was studied with H-1 NMR spectroscopy and dynamic light scattering. Finally, an example of core-crosslinked micelles was demonstrated by the addition of a difunctional crosslinking agent to a micellar solution of the parent block copolymer. (C) 2004 Wiley Periodicals, Inc