Ene/Diene Copolymerization Catalyzed by Cationic Sc and Gd d0 Metal Complexes: Speciation, Ion Pairing, and Selectivity from a Computational Perspective

A computational mechanistic study of the butadiene homopolymerization reaction performed with scandium and gadolinium catalysts [(2-Me-Ind)2(N­(TMS)2)­M] (M = Sc, Gd) in the presence of [Ph3C]­[B­(C6F5)4] or [PhNMe2H]­[B­(C6F5)4] and Al2Et6 has been performed at the density-functional theory (DFT) level. The speciation of mono- and polymetallic complexes involving alkylaluminum and the cationizing agent has been achieved. Their dormant or active character has been revealed. This speciation shows a thermodynamically favorable transmetalation reaction between the aryl group of [B­(C6F5)4]− and the alkyl group of AlR3. The reactivity of [(2-Me-Ind)­(Et)­M]+ (M = Gd, Sc) toward monomer insertions is detailed. Computed energy profiles demonstrate the preference for cis-1,4 over trans-1,4 insertion. The influence of the counterion on the reactivity and selectivity of the catalyst has been investigated. For Gd-based system, the counterion has an impact on the reactivity and selectivity of the catalyst and cannot be omitted in the chemical model. Finally, the ability of these cationic complexes to copolymerize ethylene with butadiene is assessed and compared to neutral and efficient Nd-based catalytic systems.