The efficient copolymerization of acrylates with ethylene using Ni catalysts remains a challenge. Herein, we report two neutral Ni­(II) catalysts (POP-Ni-py (1) and PONap-Ni-py (2)) that exhibit high thermal stability and significantly higher incorporation of polar monomer (for 1) or improved resistance to tert-butylacrylate (tBA)-induced chain transfer (for 2), in comparison to previously reported catalysts. Nickel alkyl complexes generated after tBA insertion, POP-Ni-CCO­(py) (3) and PONap-Ni-CCO­(py) (4), were isolated and, for the first time, characterized by crystallography. Weakened lutidine vs pyridine coordination in 2-lut facilitated the isolation of a N-donor-free adduct after acrylate insertion PONap-Ni-CCO (5) which represents a novel example of a four-membered chelate relevant to acrylate polymerization catalysis. Experimental kinetic studies of six cases of monomer insertion with aforementioned nickel complexes indicate that pyridine dissociation and monomer coordination are fast relative to monomer migratory insertion and that monomer enchainment after tBA insertion is the rate limiting step of copolymerization. Further evaluation of monomer insertion using density functional theory studies identified a cis–trans isomerization via Berry-pseudorotation involving one of the pendant ether groups as the rate-limiting step for propagation, in the absence of a polar group at the chain end. The energy profiles for ethylene and tBA enchainments are in qualitative agreement with experimental measurements.

A series of palladium and nickel catalysts bearing heteroaryl-aryl moieties were prepared and applied to ethylene polymerization and copolymerization reactions.

Abstract:A series of alkyl-and aryl-substituted iminopyridine Fe(II) complexes 1a-7a and Co(II) complexes 2b, 3b, 5b, and 6b were synthesized. The activator effect, influence of temperature, and, particularly, the alkyl and aryl substituents' effect on catalytic activity, polymer molecular weight, and regio-/stereoselectivity were investigated when these complexes were applied in isoprene polymerization. All of the Fe(II) complexes afforded polyisoprene with high molecular weight and moderate cis-1,4 selectivity. In contrast, the Co(II) complexes produced polymers with low molecular weight and relatively high cis-1,4 selectivity. In the iminopyridine Fe(II) system, the alkyl and aryl substituents' effect exhibits significant variation on the isoprene polymerization. In the iminopyridine Co(II) system, there is little influence observed on isoprene polymerization by alkyl and aryl substituents.

Ring-opening polymerization is a powerful method for the synthesis of biodegradable and biorenewable polyesters. In this contribution, we report that the combination of alkali alkoxides and commercially available cyclic amides catalyzes fast and controlled ring-opening polymerization of l-lactide. The constrained cis CN bond in the imidate catalyst is critical for achieving high catalytic activity. By optimizing the basicity of the catalyst, a good balance between activity and control (M w/M n < 1.1) is realized. A high amide/initiator ratio is essential for producing narrow dispersities and inhibiting transesterification.

The insertion copolymerization of polar olefins and ethylene remains a significant challenge in part due to catalysts' low activity and poor thermal stability. Herein we demonstrate a strategy toward addressing these obstacles through ligand design. Neutral nickel phosphine enolate catalysts with large phosphine substituents reaching the axial positions of Ni achieve activity of up to 7.7 × 10 3 kg mol À 1 h À 1 (efficiency > 35 × 10 3 g copolymer/g Ni) at 110 °C, notable for ethylene/acrylate copolymerization. NMR analysis of resulting copolymers reveals highly linear microstructures with main-chain ester functionality. Structureperformance studies indicate a strong correlation between axial steric hindrance and catalyst performance.

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.

The binuclear organoscandium half-sandwich complexes (Me3SiCH2)2(THF)­Sc­[C5Me4–Si­(CH3)2–(CH2) n –Si­(CH3)2-C5Me4]­Sc­(CH2SiMe3)2(THF) (n = 0, Sc-C 0 -Sc; n = 2, Sc-C 2 -Sc) and monometallic C5Me4SiMe3Sc­(CH2SiMe3)2(THF) (Sc1) were prepared and fully characterized by conventional spectroscopic, analytical, and diffraction techniques. These complexes are active catalysts for isoprene polymerization and ethylene/isoprene copolymerization upon activation by the co-catalysts trityl perfluoroarylborate (Ph3C+)­B­(C6F5)4 – (B 1 ) and trityl bisperfluoroarylborate (Ph3C+)2[1,4-(C6F5)3BC6F4B­(C6F5)3]2– (B 2 ). Marked catalyst and co-catalyst nuclearity effects on product polymer microstructure are achieved in isoprene polymerization. Thus, the percentage of cis-1,4- units in the polyisoprene products increases from 24% (Sc1) to 32% (Sc-C 2 -Sc) to 48% (Sc-C 0 -Sc) as the catalyst nuclearity increases and the Sc···Sc distance contracts. The binuclear catalysts regulate the isometric unit distributions and favor 3,4–3,4–3,4 blocks. Furthermore, the percentage of polyisoprene trans-1,4- units increases ∼5 times when binuclear co-catalyst (B 2 ) is used, in comparison to B 1 . In ethylene/isoprene copolymerizations, the binuclear catalysts produce polymers with higher molecular weights (Mn = (3.4–6.9) × 104; polydispersity of Đ = 1.4–2.0) and with comparable isoprene enchainment selectivity versus Sc1 under identical reaction conditions. However, isoprene incorporation is curiously reduced by ∼50% when B 2 is used versus B 1 . These results highlight the importance of both ion pairing and imposed nuclearity in these polymerizations, and these results indicate that both catalyst and co-catalyst nuclearities can be used to access specific polyisoprene polymer/copolymer microstructures.

Metal-catalyzed ethylene homopolymerization and ethylene-polar monomer copolymerization to produce new kinds of polyolefins with novel microstructures are of great interest. So far, there are some disadvantages for traditional transition metal catalyst systems. Therefore, it is critical to develop new catalysts or alternative strategies. In recent years, some cationic [P, O] palladium complexes have been demonstrated with the abilities to obtain oligomers and the high molecular weight polymers. Most importantly, these complexes showed high activity and generated polymers with specific microstructures when used for copolymerization of ethylene with industrially relevant polar monomers. This review summarizes several types of high performance cationic [P, O] palladium catalysts in ethylene oligomerization, ethylene homopolymerization and the copolymerization of ethylene with polar monomers. Specially, the regulation of steric and electronic effects at specific sites of the metal complexes was focused.