Free acidity, for example, lactic acid or lactoyl lactic acid, slows down the rate of polymerization and limits the achievable degree of polymerization. According to the patent literature, free acidity of polymer grade lactide should be <10 meq/kg, and preferably no more than 5 meq/kg.
Water causes hydrolysis of lactide and also limits the attainable degree of polymerization of PLA.
Metal ions need to be specified in low quantities, because Sn, Zn, Fe, and Al cations accelerate polymerization, but may also affect hydrolysis, oxidation, or other degradation mechanisms. Sodium in particular causes racemization even in ppm amounts.
Stereochemical purity expresses the sum of meso‐lactide and D‐lactide in L‐lactide and vice versa. The higher the stereochemical purity of the lactide monomer, the higher the stereochemical purity of the obtained PLA, which controls material properties such as melting point, crystallinity, and mechanical strength.
REFERENCES
1 1. R. Narayan, Drivers & rationale for use of biobased materials based on life cycle assessment (LCA), GPC 2004 Paper Abstract #18, Michigan State University, 2004.
2 2. D. Garlotta, J. Polym. Environ. 2001, 9(2), 63–84.
3 3. A. P. Gupta, V. Kumar, Eur. Polym. J. 2007, 43, 4053–4074.
4 4. M. H. Hartmann, High molecular weight polylactic acid polymer, in: D. L. Kaplan (Ed.), Biopolymers from Renewable Resources, Springer, Berlin, 1998, Chapter 15, pp. 367–411.
5 5. J. Nieuwenhuis, Clin. Mater. 1992, 10, 59–67.
6 6. D. R. Witzke, Introduction to properties, engineering, and prospects of polylactide polymers, Ph.D. thesis, Department of Chemical Engineering, Michigan State University, East Lansing, MI, 1997.
7 7. D. E. Henton, P. Gruber, J. Lunt, J. Randall, Polylactic acid technology, in: A. K. Mohanty, M. Misra, L. T. Drzal (Eds.), Natural Fibers, Biopolymers, and Biocomposites, CRC Press, Boca Raton, FL, 2005, Chapter 16, pp. 527–577.
8 8. D. W. Grijpma, A. J. Pennings, Macromol. Chem. Phys. 1994, 196, 1649–1663.
9 9. J. J. Kolstad, J. Appl. Polym. Sci. 1996, 62, 1079–1091.
10 10. R. G. Sinclair, E. S. Lipinsky, U.S. Patent PATN 5,502,158, 1996 (to Ecopol LLC).
11 11. Cargill/NatureWorks LLC Press Release, July 1, 2009, available at http://www.natureworksllc.com (accessed date: July 24, 2009).
12 12. C. W. Scheele, Kgl. Vetenskaps‐Academiens nya Handlingar (Stockholm) 1780, 1, 116–124.
13 13. L. Dobbin, The Collected Papers of Carl Wilhelm Scheele, G. Bell & Sons Ltd, London, 1931.
14 14. C. H. Holten, A. Müller, D. Rehbinder, Lactic Acid, Verlag Chemie, Weinheim, 1971.
15 15. H. Benninga, A History of Lactic Acid Making, Kluwer Academic Publishers, Dordrecht, 1990.
16 16. C. Avery, U.S. Patent 243,827, 1881 (to Avery Lactate Company).
17 17. F. Kraft, W. A. Dyes, Ber. Dtsch. Chem. Ges. 1895, 28, 2589–2597.
18 18. H. Borsook, H. M. Huffman, Y. P. Liu, J. Biol. Chem. 1993, 102, 449–460.
19 19. A. Schouten, J. A. Kanters, J. van Krieken, J. Mol. Struct. 1994, 323, 165–168.
20 20. PURAC internal data.
21 21. G. Saville, H. A. Gundry, Trans. Faraday Soc. 1959, 55, 2036–2038.
22 22. A. Šepitka, Průmysl Potravin 1961, 13, 661–665.
23 23. R. A. Troupe, W. L. Aspy, P. R. Schrodt, Ind. Eng. Chem. 1951, 43, 1143–1146.
24 24. W. Ostwald, Z. Phys. Chem. 1889, 3, 170–197, 241–288, 369–322.
25 25. H. M. Huffman, E. L. Ellis, H. Borsook, J. Am. Chem. Soc. 1940, 62, 297–299.
26 26. G. S. Parks, S. B. Thomas, D. W. Light, J. Chem. Phys. 1936, 4, 64–69.
27 27. J. van Breugel, J. van Krieken, A. Cerda Baro, J. M. Vidal Lancis, M. Camprubi Vila, WO 00/56693, 2000 (to PURAC).
28 28. J. van Krieken, WO 02/022546, 2002 (to PURAC).
29 29. W. P. Hammes, C. Hertel, The genera Lactobaccilus and Carnobacterium, in: M. Dworkin, S. Falkow, E. Rosenberg, K.‐H. Schleifer, E. Stackebrandt (Eds.), The Prokaryotes, 3rd edition, Springer, New York, 2006, pp. IV/320 ff.
30 30. M. T. Madigan, J. M. Martinko, J. Parker, Brock Biology of Microorganisms, 9th edition, Prentice Hall, Englewood Cliffs, NJ, 2000, pp. 118–121.
31 31. O. Kandler, Antonie van Leeuwenhoek 1983, 49, 209–224.
32 32. C. Plumed‐Ferrer, et al., Appl. Environ. Microbiol. 2008, 74, 5349–5358.
33 33. M. H. Saier, et al., J. Bacteriol. 1996, 178, 314–316.
34 34. A. M. Rodas, et al., Int. J. Syst. Evol. Microbiol. 2006, 56, 513–517.
35 35. M. Cocaign‐Bousquet, C. Garrigues, P. Loubiere, N. D. Lindley, Antonie van Leeuwenhoek 1996, 70, 253–267.
36 36. A. P. Oliveira, J. Nielsen, J. Förster, BMC Microbiol. 2005, 5, 39.
37 37. B. Teusink, et al., J. Biol. Chem. 2006, 281, 40041–40048.
38 38. N. V. Narendranath, et al., Appl. Environ. Microbiol. 1997, 63, 4158–4163.
39 39. W. F. Kemper, et al., Proc. Natl. Acad. Sci. USA 2001, 98, 723–728.
40 40. Z. Ying Zhang, B. Jin, J. M. Kelly, Biochem. Eng. J. 2007, 35, 251–263.
41 41. A. Vaidya, et al., Crit. Rev. Environ. Sci. Technol. 2005, 35, 429–467.
42 42. O. Michio, K. Kimitoshi, Jpn. Patent 61293388, 1986 (to Daicel Chemical Industries).
43 43. J. P. de Boer, et al., Appl. Environ. Microbiol. 1993, 59, 2474–2478.
44 44. V. Rajgarhia, et al., WO 03/102201, 2003 (to Cargill Dow LCC).
45 45. M. Wada, et al., WO 05/033324, 2005 (to Mitsui Chemicals).
46 46. G. Bustos, A. B. Moldes, J. M. Cruz, J. M. Dominquez, J. Agric. Food Chem. 2004, 52, 801–808.
47 47. M.‐T. Gao, et al., Bioresour. Technol. 2008, 99, 3659–3664.
48 48. M.‐T. Gao, M. Hirata, E. Toorisaka, T. Hano, Bioresour. Technol. 2007, 97, 2414–2420.
49 49. T. J. Carlson, E. M. Peters, U.S. Patent 6,475,759, 2002 (to Cargill Inc.).
50 50. S. Saitoh, et al., Appl. Environ. Microbiol. 2005, 71, 2789–2792.
51 51. A. J. A. van Maris, et al., Appl. Environ. Microbiol. 2004, 70, 2898–2905.
52 52. W. N. Konings, et al., Antonie van Leeuwenhoek Int. J. Gen. Mol. Microbiol. 1997, 71(1–2), 117–128.
53 53. D. Visser, J. van Breugel, J. M. de Bruijn, P. A'Campo, WO 08/000699, 2008 (to PURAC Biochem BV).
54 54. R. L. Whistler, J. N. BeMiller, Carbohydrate Chemistry for Food Scientists, 1st edition, American Association of Cereal Chemist Inc., St. Paul, MN, 1997, 117 pp.
55 55. R. Anuradha, A. K. Suresh, K. V. Venkatesh, Process Biochem. 1999, 35, 367–375.
56 56. E. Y. Park, P. Ngoc Anh, N. Okuda, Bioresour. Technol. 2004, 93, 77–83.
57 57. S. I. Abe, M. Tagaki, Biotechnol. Bioeng. 1991, 37, 93–96.
58 58. S. Ding, T. Tan, Process Biochem. 2006, 41, 1451–1454.
59 59. E. Ohleyer, H. W. Blanch, C. R. Wilke, Appl. Biochem. Biotechnol. 1985, 11, 317–332.
60 60. A. A. Dietz, E. F. Degering, H. H. Shopmeyer, Ind. Eng. Chem. 1947, 39, 82–85.
61 61. B. I. Veldhuis‐Stribos, et al., WO 0127064 A1, 2000 (to PURAC).
62 62.