Computational study of propene polymerization promoted by postmetallocene octahedral systems: playing with steric and electronic factors

The design of stereoselective group 4 catalysts for the fine tuning of polyolefins microstructure is a challenging target thanks to the development of metallocene and postmetallocene systems. The main interactions leading to the enantioselective processes are predominantly steric in nature although several attempts to modify this general framework (e.g. by electronic factors) have been reported. Within the various strategies, the “directional site epimerization promoted electronically” published by Kol et al. immediately appeared as one of the most promising approaches, allowing for the synthesis of polypropylene samples with high isotacticities as well as high control of degree of isotacticity. It consists basically in modify the regular chain migration mechanism by placing donors of different trans influence trans to the two coordination sites in an octahedral environment. The system used is a salalen ligands, with halfsalan O,N,N donors bind in a fac mode and the half-salen O,N,N donors bind in a mer mode the metal active site (see Chart 1 A). The difference with respect to the salan ligands wrapping with a fac-fac geometry (see Chart 1 B) might allow to interplay with both steric and electronic factors. In this communication we will report about our extensive study by using DFT calculations on the propene polymerization mechanisms promoted by salan and salalen ligands. Our results offer an alternative interpretation of the experimental results.