Synthesis of PLA

Random optical copolymers Random level of meso or D-lactide in L-lactide or D-lactic acid in L-lactic acid 

Stereocomplex PLLA/PDLA LLLLLLLL mixed with DDDDDDDD 

Syndiotactic poly(meso-lactide) DLDLDLDL Al-centered R-chiral catalyst 

Polymerization through lactide formation is currently used by Cargill (NatureWorks ), which patented a continuous process [17,18]. 

ROP is carried out in solution, in the melt, in the bulk or in suspension. The involved mechanism can be ionic (anionic or cationic), coordination-insertion or free-radical polymerization [19].The cationic pol)rmerization is initiated by only two catalysts, trifluoromethane-sulphonic acid and its methyl ester [10, 15]. Initiators such as potassium methoxide, potassium benzoate, zinc stearate, n-, sec-, fer-butyl lithium or 18-crown-6-ether complexes are added for the anionic polymerization to induce a nucleophilic reaction on the carbonyl to lead to an acyl-oxygen link cleavage. According to Jedkinski et al. only the primary alkoxides, such as the first mentioned catalyst, can yield polymers with negligible racemization, transesterification and termination [10]. 

PolyQactic acid] was discovered by Carothers at DuPont in 1932. Only a low-molecular-weight PLA could be produced by heating lactic acid under vacuum while removing the condensed water [1]. 

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This chapter first presents a brief discussion on the synthesis of PLA, then, it gives an overview of various processing techniques used to fabricate PLA nanocomposites, with a special focus on the microcellular technology. Lastly, the chapter discusses various viscoelastic, thermal, mechanical, thermal degradation, electrical... 

Figure 8.2 Main routes for the synthesis of PLA. Adapted from [5].

The synthesis of PLA and its cost-driving bottlenecks are presented in Fig. 4. The first step is the fermentative synthesis of (usually L-) LA with bacteria or yeast. Elegant reviews on this anaerobic fermentation are available elsewhere [55-58]. LA is further converted to its cyclic dimeric ester lactide via a two-stage process consisting of a pre-polymerization and a backbiting reaction (second frame in Fig. 4). This L,L-lactide is the acmal mmiomer for high molecular weight PLA. 

The synthesis of PLA is a multistep process which starts from the production of lactic acid and ends with its polymerization [2-4, 6-7]. An intermediate step is often the formation of the lactide. Figure 21.2 shows that the synthesis of PLA can follow three main routes. Lactic acid is condensation polymerized to yield a low molecular weight, brittle polymer, which, for the most part, is unusable, unless external coupling agents are employed to increase its chains length. Second route is the azeotropic dehydrative condensation of lactic acid. It can yield high... 

Mu, Chen, Li, Zhang, and Jiang (2009) also smdied the effect of medium composition on the production of PLA by lactic acid bacteria. Attempting to optimize a medium for the commercial production of PLA, the authors used a response surface methodology and the results showed that addition of phenylpyruvic acid (PPA) increased the yield of PLA. The authors suggested that because the transamination step is a bottleneck in the synthesis of PLA, the addition of PPA allowed for a shortcut that bypassed the transamination of phenylalanine to PPA, which is normally the first step when phenylalanine is used as the source for PLA. 

Scheme 2.2 Synthesis of PLA multiblock stereo-copolymers by ring-opening polymerization of DL-lactide using stereoselective catalyst.

REX synthesis of PLA-based materials via ring-opening polymerization (ROP) ... 

REX Synthesis of PLA-based Materials via Ring-opening Polymerization... 

In order to achieve enhanced intumescent properties of stereo-complexed PLA, the in situ REX synthesis of PLA was similarly reported for PLA homopolymers in the presence of nanoclays and carhon nanotubes and for stereo-complexed PDLA-A-PLLA diblock copolymers in the presence of a mixture of ammonium polyphosphate, melamine and nanoclay. ... 

This section gives a brief overview of PLA matrix (for further reading, readers should refer to Chapters 1,4 and 5). As with other commodity polymers, PLA actually refers to a large family of compounds that includes copolymers with other monomers. The monomer, that is, lactic acid (2-hydroxy propanoic acid) is the simplest hydroxy acid with an asymmetric carbon atom and exists in two optically active configurations (d and l). Generally, two major routes are followed for the synthesis of PLA, such as polycondensation... 

The properties of PLA improved tremendously with the development of production using ring-opening polymerization. This route requires an intermediate substance known as lactide. Lactide is the cyclic dimers of lactic acid, and it can be in the form of L-lactide, L,D-lactide (meso-lactide) and D-lactide stereocomplex (see Figure 8.2). Nowadays, the synthesis of PLA rarely starts from chemically synthesized lactic acid. The lactic acid used is yielded from the fermentation of carbohydrates such as starch and cellulose. A large proportion is derived from the crops corn and cassava. Microorganism-based fermentation yields mainly L-lactic acid. 

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