Mechanism of Organoscandium-Catalyzed Ethylene Copolymerization with Amino-Olefins: A Quantum Chemical Analysis

The direct, efficient copolymerization of ethylene with polar monomers represents a "holy grail" for the synthesis of polar polyethylenes; however, developing effective catalysts for such copolymerizations remains a largely unsolved challenge. Very recently, organoscandium catalysts were shown to be very active for ethylene + polar monomer [H2C=CH(CH2)(n)CH(2)FG, FG = polar functional group] copolymerization. Interestingly, comonomer enchainment selectivity decreases with increasing linker length (n), while overall polymerization activity is largely unaffected, and the intriguing mechanistic origins are not yet understood. In this study, density functional theory (DFT) methods are employed to investigate the mechanism of organoscandium-catalyzed ethylene + amino olefin (AO) copolymerization, using (C5Me4SiMe3)Sc(CH2CH2CH3)B+(C6F5)(4)(-) (Sc-1) as the model active species and N-(1-butenyl)"Pr-2 and N-(1-octenyl)"Pr-2 as model comonomers. Among conceivable scenarios in monomer coordination, activation, and insertion, it is found that copolymerization activity is largely governed by intermolecular amino olefin N-coordination. Amino olefin n-dependent enchainment patterns arise from chain-length regulation of the energy barrier for an amino olefin chelating "self-assisted" enchainment pathway. Short-chain N-(1-butenyl)"Pr-2 enchains via a self-assisted insertion pathway (6.0 kcal/mol energy barrier), while long-chain N-(1-octenyl)"Pr-2 enchains via unassisted 1,2-insertion with exogenous amine coordination (7.2 kcal/mol energy barrier). These findings explain the experimental results, showcase the characteristic reactivity of Sc catalysts in polar monomer copolymerization, and highlight the potential and challenges in developing catalysts for polar monomer copolymerization.