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Thermo‐responsive block copolymers based on poly(N‐vinylcaprolactam) (PNVCL) have been prepared by cobalt‐mediated radical polymerization (CMRP) for the first time. The homopolymerization of NVCL was controlled by bis(acetylacetonato)cobalt(II) and a molecular weight as high as 46,000 g/mol could be reached with a low polydispersity. The polymerization of NVCL was also initiated from a poly(vinyl acetate)‐Co(acac)2 (PVAc‐Co(acac)2) macroinitiator to yield well‐defined PVAc‐b‐PNVCL block copolymers with a low polydispersity (Mw/Mn = 1.1) up to high molecular weights (Mn = 87,000 g/mol), which constitutes a significant improvement over other techniques. The amphiphilic PVAc‐b‐PNVCL copolymers were hydrolyzed into unprecedented double hydrophilic poly(vinyl alcohol)‐b‐PNVCL (PVOH‐b‐PNVCL) copolymers and their temperature‐dependent solution behavior was studied by turbidimetry and dynamic light scattering. Finally, the so‐called cobalt‐mediated radical coupling (CMRC) reaction was implemented to PVAc‐b‐PNVCL‐Co(acac)2 precursors to yield novel PVAc‐b‐PNVCL‐b‐PVAc symmetrical triblock copolymers. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Recent developments in cobalt-mediated radical polymerization (CMRP) and progress in the mechanistic understanding enabled to optimize the copolymerization of n-butyl acrylate (nBA) with vinyl acetate (VAc), as well as to control the homopolymerization of nBA by means of bis(acetylacetonato)cobalt-(II) (Co(acac) 2 ). Critical experimental parameters such as the initiating system, the temperature, and the presence of additives were varied and discussed. Under optimized conditions, an alkylcobalt(III) adduct R 0 -(CH 2 -CHOAc) <4 -Co(acac) 2 (R 0 = primary radical from the V-70 decomposition) allowed a better control of the nBA/VAc copolymerization than the previously studied V-70/Co(acac) 2 pair regarding the molecular weight control and the polydispersities. Importantly, the homopolymerization of nBA was controlled by Co(acac) 2 for the first time using the alkylcobalt(III) adduct or the lauroyl peroxide (LPO)/ Co(acac) 2 redox pair as initiating system. Typically, poly(n-butyl acrylate) with polydispersity around 1.2 and molar mass as high as 200 000 g/mol was achieved with this cobalt complex.

The cobalt-mediated radical polymerization (CMRP) of 1-vinyl-3-ethylimidazolium bromide (VEtImBr) is described. Polymerizations were performed at 30 °C in solution either in dimethylformamide (DMF) or in methanol (MeOH) or in a mixture of both solvents, using a preformed alkyl–cobalt(III) adduct, CH3OC(CH3)2CH2–C(CH3)(CN)–(CH2–CHOAc)<4–Co(acac)2, as the mediating agent. Excellent control over molecular weights and dispersities (M w/M n ∼ 1.05–1.06) was achieved in MeOH, with a linear increase of experimental molecular weights with the monomer conversion. Substituting methanol for DMF induced much faster polymerization process, even under quite high diluted conditions: for instance, about 80% monomer conversion was reached in 30 min in DMF, compared to 10 h in MeOH. However, size exclusion chromatography (SEC) traces of PVEtImBr samples synthesized in DMF revealed a side population in the high molecular weight region, presumably due to the occurrence of irreversible coupling reactions of a small proportion of growing chains. Well-defined diblock copolymers featuring both a poly(vinyl acetate) (PVAc) block and a PVEtImBr-based poly(ionic liquid) block, PVAc-b-PVEtImBr, were next obtained by sequential CMRP of VAc and VEtImBr. To this end, a PVAc-Co(acac)2 was first prepared by CMRP and employed as a macroinitiator for the polymerization of VEtImBr either in methanol or in a mixture of DMF and MeOH (2/1: v/v) at 30 °C. Finally, cobalt-mediated radical coupling (CMRC) of the aforementioned PVAc-b-PVEtImBr diblock copolymers, using isoprene as a simple coupling agent, led to unprecedented and structurally well-defined PVAc-b-PVEtImBr-b-PVAc triblock copolymers.

International audienceReversibly crosslinked poly(vinyl alcohol)-b-poly(N-vinylcaprolactam) PVOH-b-PNVCL nanogels were prepared by using a redox-responsive crosslinking agent, 3,3′-dithiodipropionic acid (DPA), to crosslink the PVOH corona, above the lower critical solution temperature (LCST) of the PNVCL block. The stability of the as-prepared nanogels against heating and diluting with water was studied by dynamic light scattering (DLS) to follow the evolution of the hydrodynamic diameter and size distribution. Stability under reductive conditions was also studied by DLS and transmission electron microscopy (TEM) after exposure to dithiothreitol (DTT) buffer solutions at different pH. The reversibility of the crosslinking was evaluated by treating the de-crosslinked nanogels with hydrogen peroxide (H2O2) above the LCST. As a hydrophobic drug model, Nile red (NR) was loaded into the nanogels, and triggered release behaviours were studied after exposure to the same DTT buffer solutions. Moreover, two PVOH-b-PNVCL copolymers with different compositions and LCST were used to evaluate the effect of the LCST on the release behaviours of the nanogels. The cytotoxicity of the nanogels against a mouse fibroblast-like L929 cell line was assessed via the MTS assay, and preliminary studies on cellular uptake of the nanogels within human melanoma MEL-5 cells were also carried out by fluorescence microscopy and fluorescence-activated cell sorting