The Kol group at the School of Chemistry of Tel Aviv University has been developing catalysts and technologies for the ring opening polymerization of lactones, and in-particular lactide to produce PLLA-based polymers, block-copolymers and tailor-made copolymers.
Recently, a new family of catalysts was developed by the Kol group.

These catalysts combine several attractive properties:
1. They are highly active.
2. They are based on non-toxic metals.
3. They may be employed both in solution and under melt conditions at 180 °C.
4. They may be employed in polymerization of unpurified lactide.
5. They may be employed at very low loadings giving very high turnover numbers.
6. They produce PLLA which is stable under melt-processing conditions.

In addition, these catalysts exhibit high stereoselectivity:
1. They are able to polymerize racemic-lactide to almost perfectly heterotactic-PLA.
2. They are able to polymerize meso-lactide to syndiotactic-PLA which exhibits record tacticities and melting temperatures. Highly syndiotactic PLA was previously available only in milligram quantities and low molecular weights having been produced by sluggish catalysts. The new technology developed by the Kol group enables the production of the syndiotactic PLA on much higher scales.

Most importantly, these catalysts exhibit a tendency to polymerize meso-lactide faster than L-lactide, as well as racemic-lactide faster than L-lactide. This tendency is expressed even when polymerizations are done in the melt. Consequently, in polymerizations of mixtures of L-lactide and meso-lactide, or L-lactide and racemic-lactide, a PLLA is formed which has a microstructure of stereo-gradient copolymer, rather than stereo-random copolymer. The industrial two-step process for L-lactide production from L-lactic acid leads to substantial formation of meso-lactide as a byproduct. Polymerization of the stereo-impure L-lactide (namely, L-lactide contaminated with meso-lactide, or L-lactide contaminated with D-lactide) by the industrial catalyst tin octanoate which is non-stereoselective leads to formation of a stereo-random copolymer of reduced crystallinity. The melting temperature of the PLLA decreases and its crystallization time increases as the degree of stereo-impurities increases. At ca. 10% of meso-lactide impurity in the L-lactide stream the polymer becomes amorphous. Therefore, current technology requires the removal of meso-lactide from the mixture of L-lactide / meso-lactide prior to polymerization. The technology developed by the Kol group enables the production of stereo-gradient copolymers from mixtures of L-lactide / meso-lactide. These novel stereo-gradient copolymers exhibit enhanced crystallinities.

The Kol catalysts can be employed as drop-in replacements for the industrially-employed tin-octanoate and produce crystalline PLA from L-lactide streams contaminated with meso-lactide. This is expected to reduce the cost of PLLA production as well as enable the introduction of novel types of PLA-based materials.

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