Читать книгу Poly(lactic acid) - Группа авторов - Страница 75

Many technical processes involve esterification reactions, and these have traditionally been of main importance in the preparation of polyesters. Previous chapters have dealt with the preparation of mainly high‐molecular‐weight LA polymers involving polycondensation. This chapter focuses on processes where the polycondensation is a process step in the ROP polylactide manufacturing chain and the PLA prepared is generally of low molecular weight.

The lactide manufacturing is done by depolymerization of PLA that preferably is in the M _w range of 400–2500 g/mol [83]. Both catalyzed and uncatalyzed polycondensation reactions of LA as such have been known as an industrial process since the 1940s and can be considered to be common knowledge [84]. This explains the fact that not more than a few relevant patents can be found for the polycondensation of LA into low‐molecular‐weight polymer. The patent literature found is mainly related to other inventions in connection with the polycondensation process, for instance, the use of different catalyst systems, such as solid inorganic catalysts containing alumina silicate [85] or alkali metal compounds [26]. The former reference also provides the conditions for the polycondensation processes in terms of temperature increase (from 105 to 150°C) along with pressure reduction (350–30 mmHg). Further references are related to technical solutions for the polycondensation process. One invention describes the production of an LA polycondensate with a degree of polymerization of 1.59–2.63 and immediately separation of the lactide contained in the polycondensate [86]. Another uses an adiabatic reactor at 120–180°C and has a recycling loop, and it eliminates the water vapor [87]. Two additional references for polycondensation processes focus on improving water removal [88, 89].