Straightforward Design for Phenoxy-Imine Catalytic Activity in Ethylene Polymerization: Theoretical Prediction

DFT insight into metals and ligands substitution effects on reactivity of phenoxy-imine catalysts for ethylene polymerization

The reaction mechanism of ethylene (ET) polymerization catalyzed by the phenoxy-imine (FI) ligands using DFT calculations was studied. Among five possible isomers, isomer A which has an octahedral geometry and a (cis-N/trans-O/cis-Cl) arrangement is the most stable pre-reaction Ti-FI dichloride complex. The isomer A can be activated by MAO to form the active catalyst and the active form was used for the study of the mechanism for Ti-FI. The second ethylene insertion was found to be the rate-determining step of the catalyzed ethylene polymerization. To examine the effect of group IVB transition metals (M = Ti, Zr, Hf) substitutions, calculated activation energies at the rate-determining step (EaRDS) were compared, where values of EaRDS of Zr < Hf < Ti agree with experiments. Moreover, we examined the effect of substitution on (O, X) ligands of the Ti-phenoxy-imine (Ti-1) based catalyst. The results revealed that EaRDS of (O, N) > (O, O) > (O, P) > O, S). Hence, the (O, S) ligand has the highest potential to improve the catalytic activity of the Ti-FI catalyst. We also found the activation energy to be related to the Ti-X distance. In addition, a novel Ni-based FI catalyst was investigated. The results indicated that the nickel (II) complex based on the phenoxy-imine (O, N) ligand in the square-planar geometry is more active than in the octahedral geometry. This work provides fundamental insights into the reaction mechanism of M - FI catalysts which can be used for the design and development of M - FI catalysts for ET polymerization.

Highly Active Homoleptic Zinc and Magnesium Complexes Supported by Constrained Reduced Schiff Base Ligands for the Ring-Opening Polymerization of Lactide

New homoleptic zinc and magnesium complexes containing constrained reduced Schiff base ligands based on substituted 7-hydroxy-1-indanone were successfully synthesized and used as a catalyst for the polymerization of lactide. The ligands contain a side arm having different basicity because dimethylamino, pyridyl, and furfuryl groups are shown to greatly affect the polymerization rates. The homoleptic zinc complex containing constrained reduced Schiff base ligands and a dimethylamino side arm was highly active, giving a 92% conversion of l-lactide in 3 min using [LA]:[Zn]:[BnOH] = 500:1:2 at room temperature. The polymerization is pseudo-first-order dependent on the LA concentration. Well-controlled and living behavior of the zinc complex was observed and demonstrated in the preparation of stereodiblock and triblock copolymers of l-, d-, and rac-lactide in a one-pot sequential synthesis with a predictable block length, block sequence, and narrow dispersity rapidly in 10 min. Stereocomplex formation was observed for PLA made sequentially from 100 l-LA, 100 rac-LA, and 100 d-LA having a high T_m of up to 220 °C.