Analysis of polymeryl chain transfer between group 10 metals and main group alkyls during ethylene polymerization

The ability of various group 10 α-diimine and salicylaldimine polymerization catalysts to undergo chain transfer with main group metal alkyls during ethylene polymerization has been investigated in depth. The catalyst systems with the most efficient chain transfer were found to be cationic (α-diimine)Ni catalysts paired with dialkyl zinc chain-transfer reagents, in which all growing polymeryl chains were transferred to Zn on the basis of ¹³C NMR analysis. In these systems, chain transfer was found to be dependent on the sterics of both the catalyst and the chain-transfer reagent (CTR). When less sterically encumbered catalysts or CTRs were utilized, the relative rate of bimetallic chain transfer to chain propagation was increased; however, in cases where chain termination via β-H elimination was extremely rapid, chain transfer to Zn was kinetically not viable. Importantly, chain transfer from (α-diimine)Ni catalysts to Zn alkyls is also very sensitive to the strength of the Zn-C bond: ZnMe₂ (186 kJ/mol) is a significantly poorer chain-transfer reagent than ZnEt₂ (157 kJ/mol), despite being less sterically encumbered. Finally, the nature of the catalyst counteranion (MAO or B(ArF)₄^- ArF = 3,5-(CF₃)₂C₆H₃) does not have a significant impact on the rate of chain transfer to ZnR₂ relative to propagation, indicating that the same factors that determine propagation rates also determine bimetallic chain-transfer rates.