Phosphine-Phenoxide Nickel Catalysts for Ethylene/Acrylate Copolymerization: Olefin Coordination and Complex Isomerization Studies Relevant to the Mechanism of Catalysis

Abstract

The insertion copolymerization of ethylene and acrylate remains a challenge in polymer synthesis due to decreased activities upon incorporation of the polar monomer. Toward gaining mechanistic insight, two elusive four-membered chelated intermediates generated after acrylate insertion were prepared (1-CCO and 2-CCO), and their ligand coordination and substitution behavior were studied. Specifically, an ethylene-coordinated species was characterized by NMR spectroscopy upon exposing 2-CCO to ethylene at low temperatures, a rare observation for neutral late-transition metal polymerization catalysts. Thermodynamics of chelate-opening and monomer coordination from 2-CCO were determined at −90 °C (ΔG of 0.4 kcal/mol for ethylene and 1.9 kcal/mol for 1hexene). The Gibbs energy barrier of ligand exchange from pyridine to ethylene, a prerequisite for ethylene insertion in catalysis, was determined to be 3.3 kcal/mol. Ligand-binding studies reveal that compared to NiMe and Ni(CH 2 SiMe 3 ) complexes, acrylate inserted species 1L-CCO and 2L-CCO produce compressed thermodynamic binding scales for both electronically and sterically differentiating ligands, potentially related to their more electron-deficient nickel centers as suggested by computational studies. Triethylphosphine complexes 1P, 2P, and 2P−Me were observed as both cis and trans isomers in solution. 31 P{ 1 H} EXSY NMR studies of 2P reveal conversion between a cis and trans isomers that does not involve exchange with free PEt 3 , supporting the mechanism of intramolecular isomerization. 2-CCO, a neutral Ni(II) precatalyst that does not display an auxiliary ligand, serves as a highly active catalyst for copolymerization.

Impact of Labile Ligands on Catalyst Initiation and Chain Propagation in Ni-Catalyzed Ethylene/Acrylate Copolymerization

“…The lower maximum TOF at higher tBA concentrations is consistent with more frequent acrylate insertions, generating the resting state of the catalyst with a chelating pendant ester that undergoes slow ethylene insertion. 34,56 Structural analyses of the copolymer indeed show higher tBA incorporation, which keeps the catalyst in the less reactive form.…”

“…61,62 The ester group of acrylate can also substitute L, though it is a significantly weaker ligand compared to pyridine and phosphines. 56 We thus performed ethylene uptake studies with 2-py under different acrylate concentrations (Figure 2c, Table S4), which revealed that the catalyst initiation has a similar induction period under all conditions. However, a significant difference was observed in maximum TOF.…”

“…Overall, the above results demonstrate that ligand L impacts catalyst initiation and propagation, which is notable given that the concentration of L is ca. ∼1000 times lower than that of acrylate, and the ethylene concentration is even higher than that of acrylate. , The ester group of acrylate can also substitute L, though it is a significantly weaker ligand compared to pyridine and phosphines . We thus performed ethylene uptake studies with 2-py under different acrylate concentrations (Figure c, Table S4), which revealed that the catalyst initiation has a similar induction period under all conditions.…”

“…These observations indicate that initiation is independent of acrylate concentration, likely being driven by ethylene insertion, consistent with kinetics studies on related systems, although ethylene and acrylate induced initiation could also be undistinguishable. The lower maximum TOF at higher tBA concentrations is consistent with more frequent acrylate insertions, generating the resting state of the catalyst with a chelating pendant ester that undergoes slow ethylene insertion. , Structural analyses of the copolymer indeed show higher tBA incorporation, which keeps the catalyst in the less reactive form.…”

“…Neutral Ni catalysts are attractive given their relatively high efficiency and the low cost of nickel . P,O-chelated Ni complexes represent a high-performance class of catalysts with potential for further improvement for the synthesis of polar polyolefins. ,,− ,− We have previously studied the thermodynamics of olefin and polar group coordination to the nickel center and found that both are orders of magnitude weaker than binding of typical labile ligands (e.g., PEt 3 ). , We therefore anticipated that ligand L could compete for binding to metal with vinyl and ester groups present under ethylene/acrylate copolymerization conditions even under relatively low concentrations of L as is the case for single-component catalysts (Figure b). Once coordinated, substitution of ligand L by olefins is required for subsequent monomer insertion during initiation or propagation.…”