Manganese, copper and zinc catalysts in rac-lactide polymerization

Diiminopyrrolide ligands bearing two chiral N-methylbenzyl substituents were prepared by a condensation reaction of the 1H-pyrrole-2,5-dicarbaldehyde and S-methylbenzylamine. Reaction of the ligand with Cu(OMe)2 in the presence of 2 equiv of pyridylmethanol or dimethylaminoethanol yielded the dimeric Cu(II) catalysts L2Cu2(μ-OR)2. Application of these complexes in rac-lactide polymerization gave isotactic (Pm = 0.73) and atactic (Pm = 0.50) PLA, respectively. Kinetic studies conducted on these two complexes indicated the presence of two different active species. GPC results obtained for the copper catalyst containing two pyridylmethoxide bridges indicate the growth of only one chain per dimer (thus one alkoxide remains as a spectator ligand), while in the complex bearing two dimethylaminoethoxides both alkoxides inserted lactide. A ligand mediated chain-end control mechanism, which is accomplished by the epimerization of the catalytic site based on the chirality of the last inserted unit, is proposed. The presence of a coordinated and an uncoordinated imine “arms” facilitates epimerization since it requires only dissociation/re-coordination. The effects of the ligand framework (steric bulk) on activity and stereocontrol of the catalyst were investigated by variation of the imine substituents to benzyl, bromobenzyl, xylyl, diphenylmethyl and cyclohexyl. Benzyl, bromobenzyl and cyclohexyl were the only imine substituents providing the desired dimeric copper catalyst with pyridylmethoxide. Benzyl and cyclohexyl substituted complexes produced isotactic PLA. Substituent chirality was thus not required for stereocontrol. The bromobenzyl-substituted complex was the only one providing an atactic catalytic site with both imines coordinated and produced atactic PLA. Monoiminopyrrolide copper(II) complexes with pyridylmethoxide ligands were prepared with naphtyl, diphenylmethyl, xylyl and 2,6-diisopropylphenyl N-substituents. They showed reduced stereocontrol which is assumed to be due to slower epimerization (rotation around the Cu-pyrrole bond is now required). All complexes provided isotactic PLA, but the stereocontrol did not surpass that of the diiminopyrrolide complexes. Substitution of the 5-position of the pyrrole by chloride led to loss of activity while a methyl substituent provided atactic PLA. Phenoxy-imine ligands were prepared by a condensation reaction of the salicylaldehyde derivative and the desired amine (benzyl, cyclohexyl, xylyl and diphenylmethyl). Their complexes bearing either dimethylaminoethoxide or pyridylmethoxide ligands were structurally similar to the iminopyrrolide complexes. All complexes were active in rac-lactide polymerization, but although GPC results indicated the growth of only one chain per dimer for the pyridylmethoxide complexes and thus indicated a similar active species, only atactic PLA was produced. A zinc analog of the isotactic copper complex with the N,N’-bis(methylbenzyldiiminopyrrolide ligand was prepared and structurally characterized, but produced heterotactic PLA (Pr = 0.75). Zinc complexes with 2,4-di-tert-butyl-6-aminomethyl-phenol ligands with amino = N,N,N’,N’-tetramethyldiethylenetriamine or di-(2-picolyl)amine substituents were prepared and structurally characterized. They showed a chiral tetrahedral zinc center with one coordinated and one uncoordinated for the ethylenediamine substituents and a five-coordinated zinc center with both amino groups coordinated for picolylamine. NMR investigations indicated fast epimerization of the metal center on the NMR time scale. Both zinc complexes are highly active in lactide polymerization and reach full conversion in only a few minutes, placing them among the most active zinc catalysts known. Slightly isotactic PLA (Pm up to 0.6) was obtained for both complexes, showing in proof-of-principle the advantage of introducing catalytic site epimerization. The picolylamine-substituted complex showed a suppression of stereocontrol at high catalyst concentrations, which is not fully understood. Bulk polymerization of lactide was conducted with manganese diamino-diphenolate complexes following a coordination-insertion mechanism. Their activity was low and only heterotactic PLA was obtained. Tri/tetradentate phenoxy-imine copper complexes were likewise used in bulk polymerization, following an activated monomer mechanism with benzyl alcohol as co-initiator. Polymerizations were stable in air and in the presence of water or acetic acid, but polymer molecular weight control was low with evidence for facile intramolecular transesterification reactions. Surprisingly high heterotacticities were obtained in molten monomer (Pr up to 0.85), but there was no evidence that the additional basic site on the ligand participates in stereocontrol.

Keyphrases: Catalysis, Copper complexes, Manganese complexes, Polylactic acid, Lactide polymerization, Mechanism, Isotactic stereocontrol