1.1 Introduction
In the late 1990s, the bio-based nature of PLA was highlighted and its production as a bio-based polymer started. In this case, the newly developed polymers ought to have high-performances and long-life utilities that can compete with those of the ordinary engineering plastics. Various types of bio-based polymers are now under development, and several PLA types are also developed as promising alternatives to commercial commodities. In particular, PLLA polymers comprising high l-contents and stereo-complex PLA polymers showing high melting temperatures are now expected to be candidates for high-performance materials. The above historical view reveals the three specific features of PLA in terms of application, i.e. bio-absorbable, bio-degradable and bio-based.
Now, the synthesis of PLA polymers can be performed by direct polycondensation of lactic acid as well as by ring-opening polymerization of lactide (LA), a cyclic dimer of lactic acid. While the former method needs severe conditions to obtain a high-molecular-weight polymer (high temperature of 180–200 °C, low pressure as low as 5 mmHg and long reaction times),the latter method can afford a high-molecular-weight PLA with narrow molecular weight distribution at relatively mild reaction conditions (low temperature of 130 °C and short reaction times).[15] Consequently, ROP of l-lactide is adopted in the ordinary industrial production of PLLA. On the other hand, since Ikada discovered the formation of stereo-complexes of PLLA and its enantiomer poly(d-lactic acid) (PDLA) in 1987, many trials have been done for its industrial production.[16]
Figure 1. Processing route of PLA+
1.2 Synthesis of Lactic Acids
1.2.1 Stereoisomers of Lactic Acid
Lactic acid (2-hydroxypropanoic acid) is the simplest 2-hydroxycarboxylic acid with a chiral carbon atom and exists in two optically active stereoisomers, namely l and d enantiomers (S and R in absolute configuration, respectively), as shown in Scheme 1.1. These l- and d-lactic acids are generally synthesized by fermentation using suitable micro-organisms. Racemic dl-lactic acid (RS configuration) consisting of the equimolar mixture of d- and l-lactic acids shows characteristics different from those of the optically active ones. dl-lactic acid is conveniently synthesized by chemical method rather than fermentation.
Scheme 1.1 Structures of three kinds of acids.
1.2.2 Fermentation with Lactic Acid Bacteria
Lactic acid fermentation is one of the bacterial reactions long utilized by mankind along with alcoholic fermentation.[17] The lactic acid bacteria are generally divided into several classes in terms of cell morphology, i.e. Lactobacillus, Streptococcus, Pediococcus, Aerococcus, Leuconostoc and Coryne species. They are also divided into various genera. Most of them produce l-lactic acid while some produce d- or dl-lactic acids. Table 1.1 compares which of d- or l-lactic acid is produced by different bacteria.[18] The species belonging to the same Lactobacillus genus produce either l- or d-lactic acid preferentially. Lactobacillus helvetics and Sporolactobacillus produce dl- and d-lactic acids, respectively. In the lactic acid formation, therefore, stereoselectivity is much lower than in the amino acid formation where the absolute l-selectivity is shown. Table 1.2 shows the mono- and di-saccharides assimilated by the lactic acid bacteria. Each bacterium assimilates most mono-saccharides, but shows its own assimilation ability for di-saccharides. This difference in assimilation ability is important in the selection of bacteria. Since the breakdown of cellulose and starch often produces di-saccharides, the species that can assimilate these di-saccharides must be used in the fermentation. In the ordinary lactic acid fermentation, the yields of l- and d-lactic acids reach 85–90% and 70–80% based on carbon usage, respectively.
Table 1.1 Formation of d- and l-lactic acids with different lactic acid-producing bacteria.
Table 1.2 Saccharides assimilated by the representative lactic acid bacteria.
1.2.3 Isolation and Purification of Lactic Acids
The fermenting liquor finally obtained in the above fermentation contains lactic acid together with various impurities such as un-reacted raw materials, cells and culture media-derived saccharides, amino acids, carboxylic acids, proteins and inorganic salts. Therefore, the isolation and purification steps are needed for obtaining a highly pure product needed in the polymer’s synthesis. In the usual fermentation process, the generated lactic acid is neutralized in situ with calcium oxide or ammonia. When calcium oxide is used for the neutralization, calcium lactate is precipitated out. This salt is isolated by filtration in the final step, washed with water and acidified with sulfuric acid to liberate free lactic acid with formation of calcium sulfate as solids. When ammonia is used for the neutralization, the ammonium lactate is formed and directly converted into butyl lactate by esterification with n-butanol, as shown in Scheme 1.2.[19] Here, the ammonia is recovered and recycled. The following distillation and hydrolysis of butyl lactate gives an aqueous lactic acid with high efficiency. The lactic acid obtained by this method has higher purity than that obtained by the calcium salt method. The technologies for the above lactic acid fermentation and purification have well been established, and the production of both d- and l-lactic acids is conducted industrially in a plant scale of 100000 ton year−1.
Scheme 1.2 Formation of butyl lactate from ammonium lactate
1.2.4 Chemical Synthesis of Lactic Acids
Racemic dl-lactic acid can be synthesized by fermentation using appropriate bacteria (Lactobacillus helvetics in Table 1.1), but it is more easily synthesized by following the chemical process shown in Scheme 1.3.[20] Here, the dl-lactic acid is produced by hydrolysis of lactonitrile that is generally formed by the addition reaction of acetaldehyde and hydrogen cyanide. Industrially, the lactonitrile is obtained as a by-product of acrylonitrile production (Sohio process). [21] The lactic acid thus prepared is purified by distillation of its ester as described above.
Scheme 1.3 Chemical synthesis of lactic acid via lactonitrile.
1.3 Synthesis of Lactide Monomers
1.3.1 Stereoisomers of Lactides
Scheme 1.4 shows three lactides consisting of different stereoisomeric lactic acid units. l- and d-lactides consist of two l- and d-lactic acids, respectively, while meso-lactide consists of both d- and l-lactic acids. Racemic lactide (rac-lactide) is an equimolar mixture of d- and l-lactides. The melting points (Tm) of these lactides are compared in Table 1.3. Note that the Tm is higher in rac-lactide and is lower in meso-lactide.
Scheme 1.4 Structures of various lactides 
Table 1.3 Thermal properties of lactides.
1.3.2 Synthesis and Purification of Lactides
Each of the aforementioned lactides is usually synthesized by depolymerization of the corresponding oligo(lactic acid) (OLLA) obtained by polycondensation of relevant lactic acid, as shown in Scheme 1.5.Because of the ring-chain equilibrium between lactide and OLLA, unzipping depolymerization generates lactide through the back-biting mechanism involving the -OH terminals of OLLA as the active site as shown in Scheme 1.6. [22] This reaction is well catalyzed by metal compounds involving Sn, Zn, Al and Sb ions, etc. The crude lactide can be purified by melt crystallization or ordinary recrystallization from solution.
Scheme 1.5 Synthetic route to lactide from lactic acid via oilgolactide.
Scheme 1.6 Expected formation mechanism of lactide (back-biting mechanism).
1.4 Polymerization of Lactide Monomers
Structural Diversities of the Polylactides
As shown in Scheme 1.7, there are two major synthetic routes to PLA polymers: direct polycondensation of lactic acid and ring-opening polymerization (ROP) of lactide. Industrial production of PLA mostly depends on the latter route.
The polymerization of optically pure l- and d-lactides gives isotactic homopolymers of PLLA and PDLA, respectively. Both PLLA and PDLA are crystalline, showing a Tm around 180 °C. Their crystallinity and Tm usually decrease with decreasing optical purity (OP) of the lactate units. [23] Optically inactive poly(dl-lactide) (PDLLA), prepared from rac- and meso-lactides, is an amorphous polymer, having an atactic sequence of d and l units. However, crystalline polymers can be obtained when the sequence of both d and l units are stereo-regularly controlled.[24]
The most interesting issue comes from the fact that mixing of isotactic PLLA and PDLA in 1:1 ratio affords stereo-complex crystals (sc-PLA) whose Tm is 50 °C higher than that of PLLA or PDLA. This sc-PLA is formed by co-crystallization of the helical macromolecular chains having opposite senses. Stereo-block copolymers (sb-PLA) consisting of isotactic PLLA and PDLA sequences are also synthesized by stereo-regular polymerization techniques involving block copolymerization. These structural diversities of PLA polymers provide a broad range of physicochemical properties for PLA materials when processed.
Scheme 1.7 A variety of microstructures of lactides and PLAs.
Reference
[15] Moon, S., Lee, C., Miyamoto, M. and Kimura, Y., 2000. Melt polycondensation ofL-lactic acid with Sn(II) catalysts activated by various proton acids: A direct manufacturing route to high molecular weight Poly(L-lactic acid). Journal of Polymer Science Part A: Polymer Chemistry, 38(9), pp.1673-1679.
[16] Hyon, S., Jamshidi, K. and Ikada, Y., 1997. Synthesis of polylactides with different molecular weights. Biomaterials, 18(22), pp.1503-1508.
[17] Kowalski, A., Libiszowski, J., Duda, A. and Penczek, S., 2000. Polymerization ofl,l-Dilactide Initiated by Tin(II) Butoxide. Macromolecules, 33(6), pp.1964-1971.
[18] Ikada, Y., Jamshidi, K., Tsuji, H. and Hyon, S., 1987. Stereocomplex formation between enantiomeric poly(lactides). Macromolecules, 20(4), pp.904-906.
[19] Vaidya, A., Pandey, R., Mudliar, S., Kumar, M., Chakrabarti, T. and Devotta, S., 2005. Production and Recovery of Lactic Acid for Polylactide—An Overview. Critical Reviews in Environmental Science and Technology, 35(5), pp.429-467.
[20] Moses, A. and Miller, M., 1969. Assessment of pituitary reserve in subjects pretreated with dexamethasone. Metabolism, 18(5), pp.376-386.
[21] Filachione, E. and Costello, E., 1952. Lactic Esters by Reaction of Ammonium Lactate with Alcohols. Industrial & Engineering Chemistry, 44(9), pp.2189-2191.
[22] The ACS Office of Public Outreach, The SOHIO Acrylonitrile, BP Chemicals Inc., Warrensville Heights, Ohio, American Chemical Society, 1996.
[23] Nishida, H., Mori, T., Hoshihara, S., Fan, Y., Shirai, Y. and Endo, T., 2003. Effect of tin on poly(l-lactic acid) pyrolysis. Polymer Degradation and Stability, 81(3), pp.515-523.
[24] Urayama, H., Moon, S. and Kimura, Y., 2003. Microstructure and Thermal Properties of Polylactides with Different L- and D-Unit Sequences: Importance of the Helical Nature of the L-Sequenced Segments. Macromolecular Materials and Engineering, 288(2), pp.137-143.