Unsymmetrical Strategy Makes Significant Differences in α‐Diimine Nickel and Palladium Catalyzed Ethylene (Co)Polymerizations

Abstract

AbstractLigand steric bulk is one of the most important parameters on determining activity, polymer molecular weight, and branching density in α‐diimine Ni(II) and Pd(II) catalyzed ethylene polymerization. In this contribution, we delineated an unsymmetrical strategy to shed light on the effect of steric bulk in α‐diimine species via the unsymmetrically pentiptycenyl/dibenzhydryl α‐diimine Ni(II) and Pd(II) catalysts Ipty/Ph‐Ni and Ipty/Ph‐Pd versus symmetrical pentiptycenyl analogues Ipty‐Ni and Ipty‐Pd and symmetrical dibenzhydryl analogues Ph‐Ni and Ph‐Pd. In the Ni(II) catalyzed ethylene polymerization, new features have been revealed: (1) with the increase of steric bulk (Ph‐Ni > Ipty/Ph‐Ni > Ipty‐Ni), in a relatively long 30 min polymer molecular weights increase, yet Ipty/Ph‐Ni produces the highest molecular weight (1230 kDa) in a short 5 min; (2) with increasing steric bulk, branching density first rises and then falls, liking a downward parabola. In the Pd(II) catalyzed ethylene polymerization, increasing steric bulk enhanced activity and molecular weight or not, dependent on temperature, but usually decreased branching density. Consequently, Ipty/Ph‐Pd gave the highest activity and the highest molecular weight (412 kDa) at challenging high temperature of 70 °C. Plausible insights have been given to address these differences from previous results. Notably, unsymmetrical Ni(II) and Pd(II) catalysts also enabled copolymerizations of ethylene with various polar comonomers.