Racemic lactide polymerization

Similarly, a turn-over frequency (TON) of 227 of the polymerization process was distinctly low for 77d with [M]/[I] = 350, at 110 °C for 6 h, using in the melt polymerization conditions. Biocompatible calcium complex 77a used as catalyst at 110 °C produced in 30 min PLAs with high molecular weight (65,000-110,600) and narrow polydispersities (1.02-1.05) using [M]/[I] = 350-700. It is worthy of note that complex 77a displayed a notable heteroselectivity (probability of racemic linkages between monomers, = 0.73, see Sect. 4.2) in polymerization of rac-lactide in THF at 33 °C. Data on the aforementioned calcium initiators and their lactide polymerization are listed in Table 4. 

ROP of lactones and lactides using lanthanide alkoxide-based initiators is a relatively recent discovery. The first example of lactone polymerization by lanthanide alkoxide complexes was reported in a DuPont patent written by McLain and Drysdale in 1991 [89]. In general, the activity of these catalysts is much higher than that determined for aluminum alkoxides, especially in lactide polymerization [90-92]. Polymers of relatively high molecular weight and narrow MWD are formed. The negative side-reactions such as macrocycle formation, transesterification, and racemization are absent. 

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." ... 

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. 

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. 

Ag(carbene)2][AgCl2] (carbene = l,3-dimesitylimidazol-2-ylidene) Was also reported for use as a pre-catalyst for the polymerization of L-lactide with low PDI and in the absence of monomer racemization.69... 

Spassky and coworkers discovered a remarkable stereocontrol of an enantiomerically pure A1 complex (7 )-161a for the ROP of rac-lactide resulting in a tapered stereoblock PLA microstructure with high melting point =187 °C) (Fig. 26) [160]. Structurally analogous, racemic salen-Al complex 162 resulted in highly isotactic PLA [161]. Feijen s enantiopure chiral complex (RJ )-163 (Fig. 26) exhibited an excellent reverse stereocontrol by preferential polymerization of L-lactide over D-lactide monomer (Kss/Krr = 14) that resulted in PLA with... 

Three cyclic initiators from dibutyltin oxide and 1,2-ethanediol, 2-mer-captoethanol, and 1,2-dimercaptoethane as indicated in Fig. 43 were prepared. Polymerizations of L-lactide and racemic D,L-lactide were conducted... 

Coates and co-workers reported polymerizations using both the enantiomerically pure (7) and the racemic (8) Ao-propoxide analogues [21], Complex 8 polymerized rac-lactide iso-selectively producing stereoblock PLA (P = 0.96). The isoselectivity was achieved due to the high selectivity of 7 for RR-lactide and corresponding S -enantiomer for S S -lactide. Further studies of the aluminium-salen system have led to the proposal that several factors influence the polymerization... 

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