Understanding the Insertion Pathways and Chain Walking
Mechanisms of α‑Diimine Nickel Catalysts for α‑Olefin
Polymerization: A <sup>13</sup>C 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 (<i>T</i><sub>m</sub>) similar to linear low density polyethylene (<i>T</i><sub>m</sub> = 100–113 °C). Branching defects
due to mechanistic errors during the polymerization currently hinder
access to high density polyethylene (<i>T</i><sub>m</sub> = 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 <sup>13</sup>C NMR study
of poly(α-olefins) produced from monomers with <sup>13</sup>C-labeled carbonsspecifically 1-decene with a <sup>13</sup>C-label in the 2-position and 1-dodecene with a <sup>13</sup>C-label
in the ω-positionusing a series of α-diimine nickel
catalysts. Furthermore, we developed a mathematical model capable
of quantifying the resulting <sup>13</sup>C 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 (2<sub>p</sub><sup>°</sup>), secondary
positions on the polymer backbone (2°), and previously installed
methyl groups (1<sub>m</sub><sup>°</sup>). 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