Polylactic acid is a widely used family of bioplastics that emerged in commercial utility with over six decades of research. Carothers et al, were the first to document the polymerization and depolymerization of oligomeric polylactic acid in the year 1932. However, until the 1960s, researchers struggled to get high molecular weight polylactic acid, and later found methods to produce highmolecullar weight polymer which was still relatively expensive to make.
With extensive research on polymerization catalysts, and polymerization mechanisms and comonomers, in the early 1990s, Cargill Corporation successfully started to produce lactide and polylactide (polylactic acid) in a continuous process. The polylactic acid production is now owned by Nature Works ( a subsidiary of Cargill), and produces and sells the resin as well as its products including packaging materials, fibers, films and clothing.
Polylactic acid synthesis
Commercial production of polylactic acid is done using the corn as the main feed stock. The process involves producing unrefined glucose by milling the corn, followed by fermentation with microorganisms to make lactic acid.
Lactic acid is also a natural product, and is the smallest molecule with an asymmetric carbon. As shown in following figure, lactic acid exists with two optical isomers – L (+) lactic acid which is the naturally occurring isomer and D(-) Lactic acid.
Thus, when the lactic acid is converted in to its dimer, lactide, 3 different stereoisomers of lactide is obtained – L- lactide, D- Lactide and Meso Lactide–. This is usually done through reactive distillation process. The two step process involves converting lactic acid to polylactic acid oiligomers, and cyclizing the oligomers to get the cyclic ester of two molecules of lactic acid.
Lactide Biomonomer manufacturers:
The world’s major lactide manufacturers include Cargill, Dupont, Boehringer, and Purac Biochem.
Polylactic acid can be synthesized by two different routes – polycondensation of lactic acid, or by ring opening polymerization of lactide–. The former method is not widespread industrially due to the difficulty in getting the high molecular weight polymer.
Polylactide Synthesis: Ring Opening Polymerization of Lactide
The ring opening polymerization of lactide is the preferred method industrially to produce high molecular weight polylactide in high yield.
Ring opening polymerization (ROP) of lactides yields polylactide acid polymer which exhibit distinctive properties depending on the molecular weight, chain architecture (linear vs. branched), and the degree of crystallinity. Unlike the polycondensation process of lactic acid, the ROP process yields high molecular weight polymer. The branching depends mainly depends on the catalyst and the purity of the monomer.
Catalysis of Ring Opening Polymerization (ROP) of Lactide
The ring opening polymerization of lactide can be catalyzed by various compounds including metal alkoxides, metal oxides and enzymes. One of the most widely used catalysts is stannous octoate.
It’s proven to yield high molecular weight polylactic acid with narrow molecular weight distribution.