(461f) Quantum Chemical Analysis of Mechanisms for Rotep By Aluminum Catalysts

Conversion of cyclic esters by ring-opening transesterification polymeration (ROTEP) allows for transformation into aliphatic polyesters. A major class of catalysts utilized are the aluminum-salen catalysts. Two Al-salen catalysts (either with two- or three-carbon length backbones) are investigated to understand the operative mechanism. Additionally, the impact of steric (through inclusion of tert-butyl and methyl groups) and electronic (addition of OMe, Br or NO₂ groups on the aromatic ring) features of the catalyst are used to gain insight into the thermodynamics of the reaction mechanism. Inclusion of the steric features in the two-carbon catalyst structure lead to an overall increase of 1.8 kcal/mol (mean unsigned difference) for the various stationary points along the pathways considered. Further modification to the electronic characteristics of the catalyst by switching from (OMe to Br to NO_­2), led to a decrease in the activation energy relative to OMe of -1.2 and -2.8 kcal/mol (Br and NO­₂ respectively). According to experimental results, the three-carbon catalyst shows a 100-fold rate enhancement over the original two-carbon catalyst. Investigation into the structural difference between the two catalyst structures has shown the two-carbon catalyst to have a more square pyramidal structure (t = 0.389) while the three-carbon catalyst is more trigonal bipyramidal (t = 0.799). Further work will aid in understanding the origin of the enhanced rate of the three-carbon catalyst.