Mechanism of the Living Lactide Polymerization Mediated by Robust Zinc Guanidine Complexes

Abstract

Zinc bis(chelate) guanidine complexes promote living lactide polymerization at elevated temperatures. By means of kinetic and spectroscopic analyses the mechanism has been elucidated for these special initiators that make use of neutral N-donor ligands. The neutral guanidine function initiates the polymerization by a nucleophilic ring-opening attack on the lactide molecule. DFT calculations on the first ring-opening step show that the guanidine is able to act as a nucleophile. Three transition states were located for ligand rearrangement, nucleophilic attack, and ring-opening. The second ring-opening step was modeled as a representation for the chain growth because here, the lactate alcoholate opens the second lactide molecule via two transition states (nucleophilic attack and ring-opening). Additionally, the resulting reaction profile proceeds overall exothermically, which is the driving force for the reaction. The experimental and calculated data are in good agreement and the presented mechanism explains why the polymerization proceeds without co-initiators.

New Racemic and Single Enantiopure Hybrid Scorpionate/Cyclopentadienyl Magnesium and Zinc Initiators for the Stereoselective ROP of Lactides

“…As a comparison, the new [NNCp]-hybrid scorpionate/ cyclopentadienyl Mg and Zn-alkyl/aryloxide complexes proved to be more active for the ROP of rac-LA than the previously described related alkyls [Mg(R)(κ 2 -η 5 -bpzcp)] 11a and [Zn(R)-{κ 2 -η 1 (π)-bpzcp}] 11a (Table 3, entries 9 and 17; only traces of product were obtained for both initiators at 50°C), the recently reported robust zinc guanidine complexes of the type [Zn(guanidine) 2 OTf]OTf, 28 Poly(rac-lactide) Microstructure Analysis. Microstructural analysis by homonuclear decoupled 1 H NMR spectroscopy of the poly(rac-lactide)s revealed that both families of racemic mixtures of magnesium and zinc initiators 1−3, 5−9, and 11−14 lead to a moderate level of heterotacticity.…”

Coordination Chemistry and Reactivity of Zinc Complexes Supported by a Phosphido Pincer Ligand

“…[61][62][63][64][65][66][67][68] However, the potential-energy surfaces for ROP of caprolactone and lactide were not completely mapped, and different reaction pathways cannot be ruled out. Our DFT calculations support the hypothesis that the hemilability of the coordinated ligand is an important prerequisite for coordination of the monomer and for opening straightforward reaction pathways at the metal center.…”

Guanidine Metal Complexes for Bioinorganic Chemistry and Polymerisation Catalysis

“…As a result of the high initiation barrier, only a few catalysts are active in the ROP at lower temperature. This leads to higher experimental molecular weights than expected from the theoretical molecular weights [170].…”

“…Furthermore, in the UV-vis spectra, the absorption of the guanidinequinoline ligands was observed in the corresponding polylactide samples. The reason is the π À π* transition of the aromatic quinoline system in the polymer [170].…”