The synthesis of low molecular weight (M n (NMR) < 7000 g/mol) lactic acid prepolymers by condensation polymerization of l-lactic acid was investigated. Besides the l-lactic acid polymer, hydroxyl- and carboxyl-terminated telechelic prepolymers were also prepared by the addition of small amounts of 1,4-butanediol and adipic acid, respectively. All polymerizations were carried out in a melt with tin octoate as the catalyst. The products were characterized by differential scanning calorimetry, gel permeation chromatography (GPC), IR, 1H-NMR, and 13C-NMR. According to NMR, the resulting prepolymers contained less than 1 mol % of lactic acid monomer and less than 4.1 mol % of lactide. End group analysis of the polymers was carried out by comparing the NMR spectra of different polymers. According to NMR, the lactic acid can be copolymerized so that the resulting prepolymer chains have only one kind of end group, hydroxyl or carbonyl. The integrated area of the identified end group peak (hydroxyl or acid) was then used in molecular weight calculations. In 13C-NMR studies, the molecular weights were calculated by using the peaks in the methine area. The molecular weights were also calculated by using the peak integrals of 1H-NMR spectra of different polymers. The calculated molecular weights were systematically smaller than the molecular weights determined by GPC, and on about the same order as the molecular weights determined by titrimetric methods. The number-average molecular weights of prepared prepolymers determined by GPC varied from 2800 to 18 000 g/mol, depending on the amount of difunctional substance added. The glass transition temperatures varied from 16.7 to 46 °C.

The synthesis of low molecular weight (M̄ n(NMR) < 27 000 g/mol) lactic acid polymers by condensation polymerization of l-lactic acid was investigated. All polymerizations were carried out in the melt, using different catalysts and polymerization temperatures. The products were characterized by DSC, GPC, titrimetric methods, and 13C-NMR. According to NMR, the resulting polymers contained less than 1 mol % of lactic acid monomer and less than 6.6 mol % of lactide. In 13C-NMR studies, the molecular weights were calculated by using the previously identified end group peaks in the methine area. The calculated molecular weights were systematically smaller than the weight-average molecular weights determined by GPC and on the same order as the molecular weights determined by titrimetric methods. The weight-average molecular weights of prepared prepolymers determined by GPC varied from 3600 to 32 600 g/mol, depending on the catalyst and polycondensation conditions. In DSC studies the glass transition temperatures of the resulting polymers varied from 24 to 51 °C, and crystallinity varied from 0% to 52%. The annealing of the polymer samples had only a small effect on glass transition temperatures and crystallinity. According to our results, the best polycondensation catalyst was sulfuric acid, which produced the highest molecular weights and over 50% crystallinity. Sn(II) octoate produced quite a high molecular weight polymer which was totally amorphous (the proportion of d-lactic acid structures was about 48 mol %).

: We studied a two step process for lactic acid polymerization: in the first step, the lactic acid is condensation polymerized to a low molecular weight hydroxyl terminated prepolymer and then the molecular weight is raised by joining prepolymer chains together using diisocyanate as the chain extender. The resulting polymer is a thermoplastic poly(ester-urethane). The polymer samples were carefully characterized with 13 C-NMR, GPC, DSC, and IR. The results indicate that high conversions of lactic acid can be achieved, as well as independent control of the stereostructure, long chain branches, molecular weight average, and molecular weight distribution. Lactic acid is converted into a poly(ester-urethane) with a weight average molecular weight as high as 390,000 g/mol and a glass transition temperature of 53.7ЊC. The analyzed content of the monomer in the prepolymer is less than 1 mol % and the lactide content 2.4 mol %, while the final poly(ester-urethane) is essentially monomer and lactide free. The mechanical properties of the poly(ester-urethane) are comparable to those of polylactides.

We studied a two step process for lactic acid polymerization: in the first step, the lactic acid is condensation polymerized to a low molecular weight hydroxyl terminated prepolymer and then the molecular weight is raised by joining prepolymer chains together using diisocyanate as the chain extender. The resulting polymer is a thermoplastic poly(ester-urethane). The polymer samples were carefully characterized with 13 C-NMR, GPC, DSC, and IR. The results indicate that high conversions of lactic acid can be achieved, as well as independent control of the stereostructure, long chain branches, molecular weight average, and molecular weight distribution. Lactic acid is converted into a poly(ester-urethane) with a weight average molecular weight as high as 390,000 g/mol and a glass transition temperature of 53.7ЊC. The analyzed content of the monomer in the prepolymer is less than 1 mol % and the lactide content 2.4 mol %, while the final poly(ester-urethane) is essentially monomer and lactide free. The mechanical properties of the poly(ester-urethane) are comparable to those of polylactides.

Five different iron monocarboxylates were used as catalysts in the two-step preparation route of lactic acid based poly(ester-urethane)s (PEU). In the first step, a hydroxyl-terminated poly(lactic acid) prepolymer was prepared, which in the second step was linked with 1,6-hexamethylene diisocyanate. The resulting polymers were characterized by titration, size exclusion chromatography, differential scanning calorimetry, and nuclear magnetic resonance spectroscopy, and the mechanical properties were tested as well. Iron monocarboxylates proved to be efficient catalysts in the preparation of a hydroxyl-terminated prepolymer (lowest acid number obtained: 0.08). The same catalyst systems proved also to be highly efficient in the linking step yielding a high molar mass PEU. Semicrystalline PEUs could be prepared at 160 and 180 degrees C by using the iron acetate of different oxidation state. PEU prepared at 200 degrees C was amorphous, which could be related to racemization during the polycondensation. By using the fluorinated iron acetate amorphous PEUs was prepared at all reaction temperatures. The molar mass of the prepolymers and the PEUs increased as a function of the polycondensation temperature for all catalysts used. The highest weight-average molar masses (M(w)) were obtained by using the fluorinated iron acetate.