Insight into the Mechanism of Thermal Stability of α-Diimine Nickel Complex in Catalyzing Ethylene Polymerization

Abstract

The union of experimental and computational methods can accelerate the development of polymerization catalysts for industrial applications. Herein, we report complementary experimental and computational studies of the thermal stability of α-diimine nickel complexes by using thermally stable Cat. 1 and a typical Brookhart catalyst (B-Cat) as models. Experimentally, we found that many more nickel atoms could be activated for Cat. 1 at elevated temperature during the ethylene polymerization process compared to those for B-Cat. Computationally, first-principle calculations showed that the decomposition energies of Cat. 1 were found to be higher than those of B-Cat, contributing to the activation observed for Cat. 1. We found that the presence of ethydene evidently affected the conformation of C1−N1−Ni− N2−C2 five-membered ring (where the nickel center is located) of Cat. 1, turning the envelope conformation (B-Cat) into a half-chair conformation (Cat. 1). According to calculations, the decomposition energy of the latter was 17.4 kJ/mol higher than that of the former. These results provide information to elucidate the mechanism of thermal stability of α-diimine nickel catalyst and significantly advance the development of thermally stable αdiimine nickel catalysts used in industry.

A continuing legend: the Brookhart-type α-diimine nickel and palladium catalysts

“…16) for ethylene polymerization, which demonstrated higher thermal stability and higher catalytic activity than classic Brookhart-type α-diimine catalyst C30f at elevated temperatures. 104 Gao and He groups studied a family of three-dimensional geometry Ni-and Pd-based α-diimine precatalysts C32 (Fig. 17) bearing a dibenzobarrelene backbone.…”

“…In the process of exploring various Brookhart catalysts, various olefinic monomers have been polymerized and copolymerized. 27,31,[125][126][127] The applicable monomers have also been rapidly growing to include linear 128,129 and branched 130,131 α-olefins, cycloalkyl substituted α-olefins, 132-136 cycloolefins, 137 dienes, 138 trienes, 139,140 internal olefins, [141][142][143][144] norbonene type monomers, [101][102][103][104] and different polar comonomers. 3,27,28 It should be noted that the continuous interest in Brookhart-type catalysts largely originates from their capabilities in copolymerizations of olefins with polar functionalized comonomers.…”

Synthesis of multiblock ethylene/long‐chain α‐olefin copolymer via chain walking polymerization using thermostable α‐diimine nickel catalyst

“…α‐Diimine nickel complex (ArN=C(C 12 H 8 )C=NAr)NiBr 2 ( Cat.1 , Ar = 2,6‐diisopropylphenyl, C 12 H 8 = acenaphthene‐1,8‐diyl) was prepared in our laboratory . The FI catalyst bis[N‐(3‐tert‐butylsalicylidene)cyclopentylaminato] zirconium(IV) dichloride ( Cat.FI ) was prepared according to literature .…”

“…Diethyl aluminum chloride (DEAC) was purchased from Akzo Noble. Dried methylaluminoxane (dMAO) was prepared and used as mentioned in our previous work . Ethylene (polymerization grade, supplied by Minxing Gas, Hangzhou, China) was purified by passing through columns of molecular sieve and manganese‐based deoxygen agent in a gas purification system made by Dalian Samat Chemicals, China.…”

“…Also, since such reactions generally require low to moderate reaction temperature, only α‐olefins with short chain lengths (usually with carbon number < 20) are applied because of their good solubility at medium reaction temperature. Considering the poor solubility of long‐chain α‐olefins (LCO) (with carbon number > 30) in common solvents at low to medium temperature, we introduced a novel thermostable Brookhart‐type α‐diimine nickel complex (ArN=C(C 12 H 8 )C=NAr)NiBr 2 ( Cat.1 , Ar = 2,6‐diisopropylphenyl, C 12 H 8 = acenaphthene‐1,8‐diyl) to catalyze ethylene/LCO copolymerization at elevated reaction temperatures . The chemical structure of the α‐diimine nickel complex used is shown in Scheme .…”