Читать книгу Poly(lactic acid) - Группа авторов - Страница 65

In azeotropic dehydration, the same principle stages as in direct melt condensation of lactic acid are present, with the exception that the last high viscosity melt‐polycondensation stage is eliminated because the polycondensation is performed in solution. The removal of the reaction water from the reaction medium thus becomes easier and a higher PLA molecular weight is achievable. The solvent, on the other hand, has to be dried from the water produced in the reaction using a drying agent (e.g., molecular sieve). Alternatively, fresh, dry organic solvent can be added during the reaction, which is undesirable from both an environmental and an economical point of view. Another disadvantage when using organic solvents in the dehydration reaction is that the prepared polymer has to be collected from the solvent, typically by using a nonsolvent for the polymer, and dried. These steps use extra labor, are time‐consuming, and usually lower the yield of the raw material used. The boiling point of the solvent also sets a restriction on the polycondensation temperature that can be used. However, the optical purity of the PLA can be retained because of the lower temperature.

Several patent applications have been filed on the azeotropic dehydration of PLA. A process was claimed wherein the organic solvent was removed from the reaction mixture and an additional solvent, that had a water content less than the water content of the solvent removed from the reaction mixture, was added to the reaction mixture [37]. The removed solvent was dried using, for example, molecular sieves, phosphorus pentaoxide, or metal hydrides and added back to the reaction mixture. In another similar application, the drying agent used was an ion exchange resin [38]. Examples of solvents that were claimed included anisole or diphenyl ether. Azeotropic dehydration of lactic‐acid‐containing impurities (e.g., chain terminators such as methanol, ethanol, acetic acid, and pyruvic acid) in a total amount of 0.3 mol% has also been reported [39]. When the lactic acid contained 0.16 mol% methanol, a molecular weight of 50,000 g/mol (viscometry, dichloromethane, 20°C) was obtained in diphenyl ether at 130°C using tin powder as a catalyst. A methanol content of 0.02 mol% yielded a PLA with M _w of 320,000 g/mol using the same polycondensation procedure.

The effect of several different catalysts on the azeotropic dehydration of lactic acid in diphenyl ether has been studied [40]. The most effective catalysts were found to be Sn compounds (Sn powder, SnO, and SnCl₂), Ni(OAc)₂, and CH₃─Ph─SO₃H. Using these catalysts, weight‐average molecular weights exceeding 100,000 g/mol according to GPC results relative to polystyrene standards (chloroform, 40°C) were obtained for the PLA. Haloiminium salts have also been utilized as polycondensation agents in azeotropic dehydration of hydroxycarboxylic acids, including lactic acid [41].

A process to further increase the M _w of the hydroxycarboxylic acid copolymerization with polyfunctional compounds was described [42]. The polyfunctional compounds were those having three or more carboxylic end groups or hydroxyl groups. In addition to this, a second compound having two or more functional end groups was present in the reaction mix. A disadvantage of the invention is that all compounds are preferably added at the same time in the beginning of the reaction, thus giving an uncontrollable reaction and therefore also reproducibility problems.