Understanding the Insertion Pathways and Chain Walking Mechanisms of α-Diimine Nickel Catalysts for α-Olefin Polymerization: A ¹³C NMR Spectroscopic Investigation

Nickel α-diimine catalysts have been previously shown to perform the chain straightening polymerization of α-olefins to produce materials with melting temperatures (T_m) similar to linear low density polyethylene (T_m = 100-113 °C). Branching defects due to mechanistic errors during the polymerization currently hinder access to high density polyethylene (T_m = 135 °C) from α-olefins. Understanding the intricacies of nickel α-diimine catalyzed α-olefin polymerization can lead to improved ligand designs that should allow production of chain-straightened polymers. We report a ¹³C NMR study of poly(α-olefins) produced from monomers with ¹³C-labeled carbons - specifically 1-decene with a ¹³C-label in the 2-position and 1-dodecene with a ¹³C-label in the ω-position - using a series of α-diimine nickel catalysts. Furthermore, we developed a mathematical model capable of quantifying the resulting ¹³C NMR data into eight unique insertion pathways: 2,1- or 1,2- insertion from the primary chain end position (1°), the penultimate chain end position (2p^°), secondary positions on the polymer backbone (2°), and previously installed methyl groups (1m^°). With this model, we accurately determined overall regiochemistry of insertion and overall preference for primary versus secondary insertion pathways using nickel catalysts under various conditions. Beyond this, our model provides the tools necessary for determining how ligand structure and polymerization conditions affect catalyst behavior for α-olefin polymerizations.