No abstract

The ring-opening transesterification polymerization (ROTEP) of rac-lactide (rac-LA) using LZn catalysts (L = ligand having phenolate, amine, and pyridine donors with variable para substituents X on the bound phenolate donor; X = NO, Br, t-Bu, OMe) was evaluated through kinetics experiments and density functional theory, with the aim of determining how electronic modulation of the ligand framework influences polymerization rate, selectivity, and control. After determination that zinc-ethyl precatalysts required 24 h of reaction with benzyl alcohol to convert to active alkoxide complexes, the subsequently formed species proved to be active and fairly selective, polymerizing up to 300 equiv of rac-LA in 6-10 min while yielding isotactic (P = 0.72-0.78) polylactide (PLA) with low dispersities: Đ = 1.06-1.17. In contrast to previous work with aluminum catalysts for which electronic effects of ligand substituents were significant (Hammett ρ = +1.2-1.4), the LZn systems exhibited much less of an effect (ρ = +0.3). Density functional calculations revealed details of the initiation and propagation steps, enabling insights into the high isotacticity and the insensitivity of the rate on the identity of X.

Computational screening of a series of aluminum complexes for their activity in the ring-opening transesterification polymerization (ROTEP) of εcaprolactone (CL) was performed using a "framework distortion energy" (FDE) hypothesis. An {N,N,N,N}-aluminum complex with a bis-indolide Schiff-base ligand was predicted on the basis of that screening to be an efficient catalyst, and this prediction was tested and verified experimentally through the synthesis and characterization of the complex and evaluation of its ROTEP reactivity.

Aluminum alkoxide complexes supported by salen ligands [salen = N, N'-bis(salicylaldimine)-2-methylpropane-1,2-diamine or N, N'-bis(salicylaldimine)-2,2-dimethylpropane-1,3-diamine] with o-adamantyl substituents have been synthesized and investigated for the polymerization of ε-caprolactone. Geometric analysis of the catalysts used for the reaction reveals the metal coordination geometries to be intermediate between square-pyramidal and trigonal-bipyramidal. A detailed kinetic study accompanied by density functional theory modeling of key mechanistic steps of the reaction suggest that, in addition to the length of the backbone linker, the o-aryl substituents have a significant impact on the catalyst's reactivity. Bulky ortho substituents favorably distort the precatalyst geometry and thereby foster the achievement of the rate-limiting transition-state geometry at low energetic cost, thus accelerating the reaction.