Controlled Radical Polymerization of Methacrylic Monomers in the Presence of a Bis(ortho-chelated) Arylnickel(II) Complex and Different Activated Alkyl HalidesA novel class of homogeneous nickel(II) catalysts, i.e [Ni{o,o‘(CH2NMe2)2C6H3}Br], denoted as Ni(NCN‘)Br, is reported to mediate in the presence of activated alkyl halides, e.g., CCl4 or α-halocarbonyl compounds, a well-controlled radical polymerization of methacrylic monomers [methyl and n-butyl methacrylate), (MMA, n-BuMA)] at rather low temperatures (<100 °C). The number-average molecular weight of the polymer gradually increased with the monomer conversion and was inversely proportional to the initiator concentration of alkyl halides. The molecular weight distribution (MWD) remained very narrow during the whole course of the polymerization (MWD < 1.3). All the experimental data including a successful block copolymerization (n-BuMA-b-MMA) experiment were in agreement with a living polymerization process, and remarkably enough, poly(methyl methacrylate) (PMMA) with molecular weight up to at least 105 g/mol was synthesized in a controlled fashion. Increased thermal stability of the PMMA is a further indication of the high regioselectivity and the virtually absence of termination reactions. Owing to the compatibility of the Ni(II) complexes toward water, extension to aqueous suspension polymerization was attempted successfully as attested by the promising preliminary results. Indications on the mechanism let us suggest that the reactive alkyl halide or the corresponding growing chain end is reversibly activated/deactivated by single electron transfer together with the halogen transfer.
Polylactide macroporous biodegradable implants for cell transplantation. II. Preparation of polylactide foams by liquid-liquid phase separationC Schugens, Véronique Maquet, Ch. Grandfils et al.|Journal of Biomedical Materials Research|1996 Potential of thermally induced phase separation as a porogen technique has been studied in an effort to produce a surgical implant suitable for cell transplantation. Emphasis has been placed on the liquid-liquid phase separation of solutions of amorphous poly DL-lactide and semicrystalline poly L-lactide in an 87/13 dioxane/water mixture. The related temperature/composition phase diagrams have been set up by turbidimetry, and the possible occurrence of a gel has been discussed. Freeze-drying of some phase-separated polylactide solutions can produce flexible and tough foams with an isotropic morphology. Interconnected pores of 1-10 microns in diameter are expected to result from the spinodal decomposition of the polylactide solutions with formation of co-continuous phases. Thermodynamics of the polymer/solvent pair has a decisive effect on the final macroporous foams, as shown by the dependence of their porosity, density, porous morphology, and mechanical behavior on molecular weight and crystallinity of polylactide and concentration of the original solutions. On the basis of the foam characteristics, potential of the liquid-liquid phase separation (spinodal decomposition) has been compared with the solid/liquid phase separation (solvent crystallization) as a porogen technique.