A series of alkyl- and aryl-substituted iminopyridine Fe(II) complexes
⁻
and Co(II) complexes
,
,
, and
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
-1,4 selectivity. In contrast, the Co(II) complexes produced polymers with low molecular weight and relatively high
-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.
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.
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 CN 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.
Linear α-olefins are accessed efficiently from ethylene in the Shell higher olefin process (SHOP), an industrial process practiced on a million-ton scale that generates a statistical mixture of ...C4-C30 products. An analogous general process for the preparation of a range of α,ω-dienes is lacking. We herein report a two-step method that generates industrially relevant α,ω-dienes. In the first step, ethylene and butadiene are copolymerized to generate copolymers with a variable butadiene content. A titanium bisphenoxide bisthiolate catalyst activated with methylaluminoxane (MAO) was selected for this step due to the high selectivity for 1,4-butadiene incorporation. Ethenolysis of the copolymers results in a distribution of α,ω-dienes in the C10-C20 range. Depending on the conditions, linear trienes are also generated. The reported protocol provides a strategy for copolymer upcycling to value-added olefin products.
A class of in situ-generated Ni-based multimetallic complexes based on a bisphosphine phenoxide ligand (POP) was studied for ethylene-acrylate copolymerization. Starting from a previously prepared ...Ni-POP complex, addition of several metal precursors results in coordination involving the second phosphine not bound to Ni. These isolated complexes show lower copolymerization activity compared to Ni-POP. Structural characterization of relevant bimetallic complexes shows ancillary ligand binding in a PO-bidentate fashion to each metal with phenoxide bridging. Additionally, halide bridges are observed in several complexes, highlighting a potential inhibition mechanism by binding to Ni. In contrast, in situ addition of Al(O i Pr)3 as a secondary metal results in an increase in activity for ethylene/acrylate copolymerization while maintaining high levels of acrylate incorporation. Overall, these results demonstrate various effects in bimetallic systems for polymerization catalysis relevant to future catalyst design.
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 1-hexene). 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(CH2SiMe3) 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. 31P{1H} EXSY NMR studies of 2P reveal conversion between a cis and trans isomers that does not involve exchange with free PEt3, 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 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.
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 7.7×103 kg mol−1 h−1 (efficiency >35×103 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. Structure‐performance studies indicate a strong correlation between axial steric hindrance and catalyst performance.
Incorporation of polar functionalities into the polyethylene backbone via coordination copolymerization can provide value‐added polyolefins with improved properties. Nickel enolate catalysts with steric constraints in the axial positions were developed and applied to polyolefin synthesis. They are highly active (up to 7.7×103 kg mol−1 h−1) and thermally stable in ethylene/acrylate copolymerization, and their behavior correlates with the level of axial shielding.
Polar polyolefin synthesis by coordination polymerization is of high interest, but the catalysts’ low activity limits industrial implementation. Herein, we demonstrate how the nature of the labile ...ligand, L, impacts the performance of neutral nickel catalysts supported by bidentate phosphine-phenoxide and phosphine-enolate ligands in ethylene/acrylate copolymerization. By tuning L, the copolymerization activity reaches ∼24000 kg/(mol h). In situ studies indicate that a weaker L leads to faster chain propagation and more efficient catalyst initiation. Overall, this work highlights the potential of a strategy to improve catalyst activity in polar polyolefin synthesis complementary to design optimization for the bidentate ligand.
Rapid, efficient development of homogeneous catalysts featuring desired performance is critical to numerous catalytic transformations but remains a key challenge. Typically, this task relies heavily ...on ligand design that is often based on trial and error. Herein, we demonstrate a “catalyst editing” strategy in Ni-catalyzed ethylene/acrylate copolymerization. Specifically, alkylation of a pendant phosphine followed by anion exchange provides a high yield strategy for a large number of cationic Ni phosphonium catalysts with varying electronic and steric profiles. These catalysts are highly active in ethylene/acrylate copolymerization, and their behaviors are correlated with the electrophile and the anion used in late-stage functionalization.
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