Mechanistic Studies on Nickel-Catalyzed Ethylene Polymerization: Ligand Effects and Quantitative Structure–Activity Relationship Model

The
polymerization mechanisms of ethylene catalyzed by two cationic
nickel catalysts, viz., diimine-type Ni complex A [(ArNC­(Me)-C­(Me)NAr)­NiMe]+ (Ar = 2,6-diisopropylphenyl) and α-imino-ketone Ni
complex B [(ArNC­(Me)-C­(Me)O)­NiMe]+ (Ar = 2,6-diisopropylphenyl), have been comparatively studied
through density functional theory (DFT) calculations. It has been
found that the calculated activity trend toward ethylene polymerization
is consistent with experimental results. In comparison with species A, the higher activity of complex B is originated
from the stronger interaction between the monomer and the catalyst
in the chain growth process. The stronger interaction can be explained
by the more positive NBO charge of the metal center and the smaller
energy gap between the HOMO of ethylene and the LUMO of active intermediate BP1. We also report a combination
of multivariate linear regression (MLR) and DFT calculations to analyze
and verify the factors affecting the ethylene polymerization activity
of catalysts A and B. The trained MLR model
features an R2 value of 0.85 and a leave-one-out Q2 value of 0.78. Strong correlations were found
between the energy barrier of ethylene polymerization (ΔG⧧) and the distance of the nickel–carbon
bond (dNi–C3) as well as the NBO
charge (QN) on the coordinating N atom
in the transition state of the chain growth.