Ring-opening polymerization of D,L-lactide was carried out in the presence of monohydroxylated poly(ethylene glycol) (PEG) with Mn of 2000 and 5000, using zinc powder as catalyst. The resulting PEG-b-polylactide (PEG-PLA) diblocks with various ethylene oxide/lactyl (EO/LA) ratios were coupled with adipoyl chloride to yield PEG-PLA-PEG triblock copolymers. N-Dimethylaminopyridine (DMAP) was used as catalyst. The obtained PEG-PLA-PEG triblock copolymers were characterized by various analytical techniques such as IR, 1H NMR, size exclusion chromatography, X-ray diffraction, and differential scanning calorimetry. Data showed that all the copolymers were semicrystalline with the PEG-type crystalline structure, the crystallinity decreasing with increasing PLA block length. Bioresorbable hydrogels were prepared from the water-soluble triblock copolymers. Rheological measurements showed a gel-sol transition with increasing temperature and gelation was found to be thermoreversible. The copolymer solution behaves like a viscoelastic liquid above the gel point and like a viscoelastic solid below the gel point. The critical gelation concentration, the gel-sol transition temperature at a given concentration, and corresponding moduli depend on both the EO/LA ratio and the molecular weight of the copolymers. It is assumed that gelation results from interactions between PEG blocks at low temperatures and that these interactions are disrupted as the temperature is elevated. The shrinking of PEG blocks with increasing temperature seems to be in agreement with the variation of the gel-sol transition temperatures.
As part of the search of novel degradable polymers, amphiphilic and cationic poly(epsilon-caprolactone)-g-poly(l-lysine) (PCL-g-PlL) copolymers have been synthesized following a grafting "onto" or a grafting "from" method both applied to a macropolycarbanionic PCL derivative. The first approach led to PCL-g-PZlL containing 36% of epsilon-caprolactone and 64% of N-epsilon-Z-l-lysine units, by reaction of activated poly(N-epsilon-Z-l-lysine) on the macropolycarbanion derived from PCL. The second route was based on the anionic ring opening polymerization of N-carboxyanhydride of N-epsilon-benzyloxycarbonyl-l-lysine initiated by the macropolycarbanion derived from PCL and led to a similar copolymer containing 45% of of epsilon-caprolactone and 55% of N-epsilon-Z-l-lysine units. After deprotection of the lysine units, PCL-g-PlL copolymers were obtained. These copolymers are water-soluble and form nanometric micelle-like objects with mean diameters between 60 and 500 nm in distilled water depending on the synthesis route.
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