Investigations of the Ligand Electronic Effects on α-Diimine Nickel(II) Catalyzed Ethylene Polymerization.

Lihua Guo,Shengyu Dai,Changle Chen

doi:10.3390/polym8020037

Abstract

The synthesis and characterization of a series of dibenzhydryl-based α-diimine Ni(II) complexes bearing a range of electron-donating or -withdrawing groups are described. Polymerization with ethylene is investigated in detail, involving the activator effect, influence of polymerization conditions on catalyst activity, thermal stability, polymer molecular weight and melting point. All of these Ni(II) complexes show great activity (up to 6 × 106 g of PE (mol of Ni)−1·h−1), exceptional thermal stability (stable at up to 100 °C) and generate polyethylene with very high molecular weight (Mn up to 1.6 × 106) and very narrow molecular weight distribution. In the dibromo Ni(II) system, the electronic perturbations exhibit little variation on the ethylene polymerization. In the Ni(acac) system, dramatic ligand electronic effects are observed in terms of catalytic activity and polyethylene molecular weight.

Highlights

The initial reports by Brookhart and coworkers showed that complexes of Ni(II) and Pd(II)bearing sterically hindered α-diimine ligands could generate high-molecular-weight polymer with high catalytic activity in ethylene polymerization and incorporate polar comonomers into polyolefins for the Pd(II) catalyst [1,2]
In the Ni(acac) system, dramatic ligand electronic effects are observed in terms of catalytic activity and polyethylene molecular weight
We investigated ethylene/methyl acrylate copolymer withand highlow molecular weight and low density [33]. the

Summary

Introduction

The initial reports by Brookhart and coworkers showed that complexes of Ni(II) and Pd(II)bearing sterically hindered α-diimine ligands could generate high-molecular-weight polymer with high catalytic activity in ethylene polymerization and incorporate polar comonomers into polyolefins for the Pd(II) catalyst [1,2]. Since Brookhart’s seminal discovery, tremendous efforts have been made to explore new late-transition metal catalysts for olefin polymerization and copolymerization with polar monomers [3,4,5,6,7,8,9,10,11,12,13,14,15]. Despite these exciting features, one drawback of these catalysts is their low thermal stability, which greatly hinders their potential industrial applications (70–110 ̋ C) [16]. It has been reported that α-diimine Pd(II) and Ni(II) catalysts undergo rapid decomposition over 60 ̋ C because of increased associative chain transfer, C–H activation of the ligand, potential decomposition via in situ–generated metal hydride species and bis-ligation during the polymerization process [17,18,19,20]

Methods

Results

Conclusion