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l-Lactide was ring-opening polymerized in the melt by using different organic monocarboxylic iron complexes. The complexes were those of iron and acetic acid, butyric acid, isobutyric acid, dichloroacetic acid, and trifluoroacetic acid. The polymerization temperature was in the range 170−210 °C, polymerization time between 0.5 and 25 h, and amount of catalyst added varied between 0.12 and 1.20 wt %. Iron butyrate and iron dichloroacetate complexes were low efficient catalysts in the ring-opening polymerization due to hydrolysis during the preparation. Iron acetate, iron trifluoroacetate, and iron isobutyrate complexes were efficient catalysts yielding a high molar mass poly(l-lactide) with a high monomer conversion. Under optimum conditions a poly(l-lactide) with a molar mass (M w) of ca. 150 000 g/mol could be prepared. Monomer conversions over 85% were obtained in many experiments. High polymerization temperatures are required though with these kinds of iron catalysts, and some racemization of the polymerization products is evident. The polymerization experiments indicate that the oxidation state of the iron has an influence on the efficiency of the catalysts and that the iron is chemically bound to the polymer.

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Poly(lactic acid) (PLA) stereocomplexes have high potential as renewable materials for advanced polymer applications, mainly due to their high melting temperature (Tm, typically 230–240°C). The properties of PLA stereocomplexes consisting of linear high molar mass homopolymers have been studied extensively in the past, but the available information about the possibilities to affect the thermal properties of the stereocomplex by varying the structure of the blend components has not been sufficient. Novel stereocomplexes containing linear or star‐shaped D‐lactic acid (D‐LA) oligomers and high molar mass poly(L‐lactide) (L‐PLA) were thus prepared. The Tm and melting enthalpy (ΔHm) of the racemic crystallites were found to depend strongly on both the blending ratio and the arm‐length of the D‐lactic acid oligomer. The preparation method of the oligomers, i.e. step‐growth polymerization or ring‐opening polymerization (ROP), did not affect the Tm or ΔHm of the blends significantly. Slightly higher ΔHm values were, however, obtained, when linear oligomers were used. The results thus indicated that the Tm and ΔHm of PLA stereocomplexes could be optimized, simply by selecting a D‐LA oligomer having a suitable arm‐length and structure as the other blend component. The possibility to adjust the melting behavior of the stereocomplex blend is a significant advantage and could make PLA suitable for a wider range of products used at elevated temperatures. Copyright © 2010 John Wiley & Sons, Ltd.

ABSTRACT:The condensation reaction product of poly-(lactic acid) (PLA) and a hydroxyl-terminated four-armed poly(-caprolactone) (PCL) was studied by size-exclusion chromatography, DSC, and NMR. The use of both l-lactic acid (LLA) and rac-lactic acid (rac-LA) was studied and the use of two different catalysts, stannous 2-ethylhexanoate [Sn(Oct) 2 ] and ferrous acetate [Fe(OAc) 2 ], was also investigated. The thermal stability and adhesive properties were also measured for the different formulations. The characterization results suggested the formation of a blend of PLA and a block-copolyester of PLA and PCL. The results further indicated partial miscibility in the amorphous phase of the blend showing only one glass-transition temperature in most cases, although no randomized structures could be detected in the block-copolymers. The polymerization in the Fe(OAc) 2 -catalyzed experiments proceeded slower than in the Sn(Oct) 2 -catalyzed experiments. The discoloring of the polymer was minor when Fe(OAc) 2 was used as catalyst, but significant when Sn(Oct) 2 was used. The ferrous catalyst also caused a slower thermal degradation. Differences in the morphology and in the adhesive properties could be related to the stereochemistry of the poly(lactic acid).