Racemic lactide

Chiral catalyst 171 was used to effect kinetic resolution of the racemic lactide in the polymerization of the racemic lactide [216]. At low conversion high enantiomeric enrichment in the polymer was observed (Scheme 6.169). The stereochemistry of the catalyst overrides the tendency for syndiotactic placements that are typically favored by chain-end control. At higher conversions, the ee in the polymer decreases. 

Ligand design in the enantioselective polymerization of racemic lactide... 

As discussed above, A1 catalysts supported by salen ligands are some of the most isoselective catalysts for the polymerization of racemic lactide. However, slow polymerization rates and high water/air sensitivity preclude their use in an industrial setting. " We were interested in exploiting chiral indium salen complexes to achieve high activity, selectivity and control over the polymerization process. Although achiral indium salen complexes had been reported previously, " prior to our work there was only one example of a chiral indium salen complex, which was not used as a catalyst for polymerization." ... 

Figure 21.3 shows the different stereoforms of lactide. The cyclic dimer of lactic acid combines two of its molecules and gives rise to L-lactide or LL-lactide, D-lactide or DD-lactide, and meso-lactide or LD-lactide (a molecule of L-lactic acid associated with another one of D-lactic acid). A mixture of L- and D-lactides is a racemic lactide (rac-... 

Schwach G., Coudane J., Engel R., Vert M., Stannous octoate-versus zinc-initiated polymerization of racemic lactide, Polym. Bull., 32, 1994, 617-623. 

DD and LL pair linkages during the ring-opening polymerization of racemic lactide. /. Polym. ScL, Part A Polym. Chem., 35 (9), 1651-1658. 

Scheme 1.4 shows three lactides consisting of different stereoisomeric lactic acid units, l- and D-lactides consist of two l- and o-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 is higher in rac-lactide and is lower in meso-lactide. 

Radano, C. P. Baker, G L. Smith, M. R. III. Stereoselective polymerization of a racemic monomer with a racemic catalyst. Direct preparation of the poly(lactic acid) stereocomplex from racemic lactide. J.Am. Chem. Soc. 2000,122, 1552-1553. 

Nomura, N. Ishii, R. Akakura, M. Aoi, K. Stereoselective ring-opening polymerization of racemic lactide using aluminum-achiral ligand complexes Exploration of a chain-end control mechanism. J. Am. Chem. Soc. 2002,124, 5938-5939. 

Hormnirun, P Marshall, E. L. Gibson, V. C. White, A. J. R Withams, D. J. Remarkahle stereocontrol in the polymerization of racemic lactide nsing aluminum initiators supported hy telradentate aminophenoxide ligands. J. Am. Chem. Soc. 2004,126,2688-2689. 

Cai, C.-X. Amgoune, A. Lehmann, C. W. Carpentier, J.-F. Stereoselective ring-opening polymerization of racemic lactide using alkoxy-amino-bis(phenolate) group 3 metal complexes. Chem. Common. 2004, 330-331. 

Pure poly lactic acid (PLA) is a semi cry.stalline polymer whereas polymers prepared from meso and racemic lactides are amorphous. The melting temperature (T ,), degree of crystallinity and solubility are dependent on the MW, thermal history, optical purity and the copolymer ratio. 

Because of the presence of an asymmetric carbon atom in the lactic acid molecule, there are two forms of lactic acid with an L (S) and D R) configuration leading to different cyclic dimers, namely L- and D-lactides composed of two lactate units respectively with the configuration L,L and D,D, meso-lactide composed of one L-and one D-unit, and racemic lactide (rac-lactide) which is a 50/50 mixture of L- and D-lactides (Figure 9.1). 

Each lactide dimer can have three stereoisomeric possibilities dd, ll, and DL. Mixtures of dimers can form dd/ll cocrystallized pairs (racemic lactide, m. p. 128 °C) and DL-lactide crystals (meso-lactide m. p. 43 ""C). Poly(m-lactide) contains more syndiotactic dyads with poly(r-lactide) contains more isotactic dyads. The most atactic poly(lactic acid) is derived from d,L-lactic acid (DLLA) [77]. 

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