Читать книгу Poly(lactic acid) - Группа авторов - Страница 82

The simplest type of reactor system is a reaction vessel with an agitator. The number of vessels can vary depending on the desired polymerization conditions [110]. A combination of this type of reactor and a static mixer has also been developed for a continuous polymerization process for preparing polyesters from glycolide, lactide, or CL. The column type of plug‐flow reactor is preferably equipped with agitation blades in order to ensure appropriate mixing [111]. A similar concept is described in another patent, but the static mixer here can optionally be linked to an extruder as the final process step [112]. A static mixer for continuous ROP of lactide is described in another US patent. The mixer is equipped with mixing elements designed to enable mixing in both axial and crosswise directions [113]. ROP can also be performed by reactive extrusion, provided that the residence time and catalyst efficiency match [114]. A more recent approach for ROP of lactide is described where the lactide is fed into a first polymerization step (e.g., a stirred vessel or loop‐type bubble column) where pre‐polymerization takes place. The pre‐polymerized product is transferred to a second polymerization reactor where the polymerization step is conducted in a tubular reactor equipped with non‐mixing baffles [115]. Another continuous ROP process for cyclic ester monomers has been developed to operate at temperatures between 100 and 240°C by continuously providing cyclic ester monomer and polymerization catalyst to a mixing reactor to form a pre‐polymerized reaction mixture, which is transferred to a plug flow reactor where the reaction mixture is polymerized to a degree of polymerization of at least 90% whereafter the polymer is continuously removed from the plug flow reactor [116].