0000000000068717
AUTHOR
Tom Ziegler
DFT Study of Ethylene and Propylene Copolymerization over a Heterogeneous Catalyst with a Coordinating Lewis Base
The copolymerization of ethylene and propylene over a heterogeneous Ti(III) catalyst containing tetrahydrofuran (THF) as a Lewis base and MgCl2 as a support has been studied by means of DFT. Two feasible models of active sites have been examined thoroughly, and one of them turned out to be favorable in terms of both catalytic activity and the microstructure of the resulting polymer. The external barriers of olefin insertion for this model range from 3.1 to 16.0 kcal/mol and are influenced by a variety of factors, such as the structure of the growing polymer chain and the nature of the incoming olefin as well as the orientation of the ligands around the titanium atom. Stochastic simulations …
Synthesis of low density poly(ethylene) using nickel iminophosphonamide complexes
Ethylene polymerization using a catalyst derived from the reaction of the phosphorane (Me3Si)2NP(NSiMe3)2 (1) with either Ni(COD)2 or bis(π-allyl)Ni complexes affords branched poly(ethylene) (PE) of variable MW (103−106) depending on conditions. The branched PE of high MW is semicrystalline with Tm < 100 °C. High field 13C NMR spectra reveal the presence of methyl branches (ca. 10−15 per 1000 C atoms), branches longer than six C atoms (15−20 per 1000 C atoms) and trace levels of ethyl, propyl, n-butyl, and sec-butyl branches (total <2 per 1000 C atoms). The branching distribution changes modestly in response to changes in ethylene pressure in a manner consistent with a chain-walking mechani…
"Dormant" secondary metal-alkyl complexes are not omnipresent.
This theoretical study was inspired by the perpetual debate over the so-called “dormancy” of the active sites in propylene polymerization, i.e., a drop in their activity after a regioerror (2,1-insertion), which was reported to occur in many (although not all) catalytic systems. To explore the range of possible situations, we have selected two homogeneous systems of fundamentally different structure: an octahedral system of C 2 symmetry with a tetradentate O N N O ligand and a bridged indenyl catalyst. This choice was not accidental; it is in these two systems where the experimentalists cannot reach a consensus about dormancy. Our density-functional theory calculations explain why in certa…