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Synthesis of thermoresponsive mixed arm star polymers by combination of RAFT and ATRP from a multifunctional core and its self-assembly in water

TitleSynthesis of thermoresponsive mixed arm star polymers by combination of RAFT and ATRP from a multifunctional core and its self-assembly in water
Publication TypeJournal Article
Year of Publication2008
AuthorsRanganathan, K, Deng, R, Kainthan, RK, Wu, C, Brooks, DE, Kizhakkedathu, JN
JournalMacromolecules
Volume41
Pagination4226-4234
Date PublishedJun
Type of ArticleArticle
ISBN Number0024-9297
KeywordsAGGREGATION BEHAVIOR, AQUEOUS-SOLUTION PROPERTIES, DIBLOCK COPOLYMERS, FRAGMENTATION CHAIN TRANSFER, HYDROPHILIC BLOCK-COPOLYMERS, HYPERBRANCHED POLYGLYCEROL CORE, POLY(ETHYLENE OXIDE), TERTIARY AMINE METHACRYLATES, TO-GLOBULE TRANSITION, TRANSFER RADICAL POLYMERIZATION
Abstract

Mixed arm star copolymers of poly(N,N-dimethylacrylamide) (PDMA) and poly(N-isopropylacrylamide) (PNIPAm) were synthesized by a sequential reversible addition - fragmentation chain transfer (RAFT) and atom transfer radical polymerization (ATRP) from a multi-initiator-functionalized hyperbranched polyglycerol (MI-HPG) core. The MI-HPG core was synthesized from an amine-functionalized polyglycerol, modified successively with 2-chloropropionamide groups (ATRP initiator) and 4,4’-azobis(4-cyanovaleric acid) (azo initiator). N,N-Dimethylacrylamide was polymerized from MI-HPG core by the RAFT method using S,S’-bis(alpha,alpha’-dimethyla-alpha ’’-acetic acid)trithiocarbonate as a chain transfer agent (CTA) in acetic acid/sodium acetate aqueous buffer solutions. The ratio of [CTA]/[azo initiator] was critical in controlling the molecular weight of the PDMA grafts from MI-HPG core (HPG-g-PDMA). Controlled synthesis of mixed arm star copolymers was achieved by cografting PNIPAm on to the HPG-g-PDMA macroinitiator by ATRP. The temperature-induced phase transition of aqueous solutions of hybrid HPG-g-PDMA/PNIPAm star copolymers was studied by H-1 NMR, UV-vis spectroscopy, and laser light scattering. Results show that the mixed arm star copolymers exist as either single molecules or small aggregates below the phase transition temperature (LCST) of PNIPAm in aqueous solutions. All the star copolymers formed intermolecular aggregates above the LCST of PNIPAm possibly due to the hydrophobic interaction between collapsed PNIPAM chains. These aggregates have micelle-like structure with PNIPAm core and PDMA corona. The formation of intermolecular aggregates and the stabilization of aggregates depend on the molecular weight of arms and composition of the star copolymer.

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