We describe alkoxo‐aluminum catalysts of chiral bipyrrolidine‐based salan ligands that follow the dual‐stereocontrol mechanism wherein a given combination of stereogeneities at the metal site and the proximal center of the last inserted lactidyl (“match”) is active towards lactide having a proximal stereogenic center of the opposite configuration, while the diastereomeric combination of stereogeneities (“mismatch”) is inactive towards any lactide. Polymerization of rac‐LA by the enantiomerically pure catalysts was sluggish and gave stereoirregular poly(lactic acid) (PLA) because selective insertion to a match diastereomer gives a mismatch diastereomer. The racemic catalysts showed higher activity and led to highly heterotactic PLA following polymeryl exchange between two mismatched catalyst enantiomers. A succession of match diastereomers in selective meso‐LA insertions led to syndiotactic PLAs reaching a syndiotacticity degree of α=0.96. This polymer featured a Tm of 153 °C matching the highest reported value, and the highest crystallinity (ΔHm=56 J g−1) ever reported for syndiotactic PLA.
Zirconium
complexes of amine tris(phenolate) ligands wherein the
substituents ortho to the phenolate oxygens are aryl groups are introduced,
and their application in the catalysis of ring-opening polymerization
of l-lactide at 180 °C is described. Spectroscopic and
crystallographic characterization revealed that for ortho-phenyl substituents, bridging dinuclear complexes were obtained,
whereas for ortho-mesityl-substituents, fluxional
mononuclear complexes were obtained. Melt polymerizations employing
catalyst ratios reaching as low as 5 ppm for unpurified l-lactide and 1 ppm for purified l-lactide revealed ultra-high
activities for the zirconium complex featuring the mesityl-phenolate
substituents, yielding high-molecular-weight stereoerror-free poly(l-lactic acid) that exhibited enhanced stability toward thermal
degradation. Slightly lower activities were found for the corresponding
hafnium complex and for the zirconium complex featuring the phenyl-phenolate
substituents. These catalysts could thus serve as drop-in replacement
for the industrially employed stannous octanoate.
Aspects of the proposed pathway combining chain-end and enantiomorphic site control for the stereospecific polymerization of lactide (LA) were investigated through studies of aluminum complexes supported by enantiopure and racemic bipyrrolidine-based salan ligands, Lig 1 AlOBn and Lig 2 AlOBn. Spectroscopic analysis of stoichiometric initiation reactions and the definition of the stereochemistry of the selective formation of the "match" single-insertion products by X-ray crystallography led to key conclusions about the observed stereocontrol. Notably, it was determined to rely heavily on the preference for the trio of stereocenters around the metal to have a "match" formation (RR-ligand + S-polymer), which works synergistically with the enantiomorphic site preference of the catalyst to ring-open next to a stereocenter of a monomer of the same chirality as that of the ligand, resulting in highly heterotactic or syndiotactic PLA from rac-or meso-LA, respectively.
The production of L-lactide from L-lactic acid involves a substantial formation of meso-lactide as an impurity, and, upon polymerization with the industrial catalyst tin octanoate, results in poly(L-lactic acid) of reduced crystallinity due to stereoerrors randomly distributed along the polymer chains. We describe a new approach wherein, instead of avoiding stereoerrors by removing the meso-lactide prior to polymerization, the stereoerrors in the polymer are tolerated, by crowding them in a stereogradient copolymer. A zirconium complex of an amine tris(phenolate) ligand is found to exhibit very high syndioselectivity in the ring opening polymerization catalysis of meso-lactide at room temperature, and gives rise to stereogradient copolymers in the polymerization of mixtures of meso-lactide/L-lactide in the melt at 180 °C. Relative to the stereo-random copolymers obtained with tin octanoate, the stereogradient copolymers exhibit enhanced crystallinities manifested in lower solubilities and higher melting temperatures and enthalpies.
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