Redox Potential as a Predictor of Polyethylene Branching Using Nickel α-Diimine Catalysts

Catalyst Editing via Post-Synthetic Functionalization by Phosphonium Generation and Anion Exchange for Nickel-Catalyzed Ethylene/Acrylate Copolymerization

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.

Impact of o-aryl halogen effects on ethylene polymerization: steric vs. electronic effects

Ligand steric hindrance and electronic effects play a crucial role in late-transition metal-catalyzed olefin polymerization. In this research, a series of o-aryl halogenated α-diimine ligands bearing bulky dibenzhydryl substituents, along with their corresponding nickel catalysts, have been synthesized and thoroughly characterized. The nickel catalysts demonstrated very high activity in ethylene polymerization, achieving a high rate of up to 107 g mol-1 h-1. The produced polyethylenes displayed a broad spectrum of molecular weights (12.2-871.7 kg mol-1) but maintained consistent branching densities (50-82/1000 C) when polymerized at a fixed temperature with different nickel catalysts. Notably, the polymerization temperature has a significant influence on both the molecular weight and branching density of the resulting polyethylene. Higher temperatures led to the formation of polyethylenes with lower molecular weights and increased branching densities. Interestingly, the o-aryl halogens significantly impact the molecular weight of the polyethylene. The size of the halogen substituents primarily determines the molecular weight of the polyethylene. However, in terms of branching density, the steric and electronic effects of these substituents appear to counteract each other. In addition, the branched high molecular weight polyethylenes from the bromine and chlorine substituted nickel catalysts are excellent polyethylene thermoplastic elastomers with high strain at break values (688-2478%) and high strain recovery values (42-62%).

Significantly Tunable Foaming Behavior of Blowing Agent for the Polyethylene Foam Resin with a Unique Designed Blowing Agent System

The chemical blowing agent plays a crucial role in enhancing the performance of the polyethylene (PE) foaming resin during the rotational foaming process. Previously, the conventional blowing agent of the PE resin commonly used pure azodicarbonamide (AZ). It had the unavoidable drawbacks of releasing NH3 and exhibiting strong reactions during the rotational foaming process. Meantime, pure AZ had a relatively high decomposition temperature, resulting in a sharp foaming process. To address the above issues, this work developed a uniquely designed blowing agent system. In this study, a novel blowing agent for the PE resin was successfully synthesized by a one-pot method. This blowing agent consisted of an activator and AZ, which exhibited a lower decomposition temperature and a milder decomposition rate than AZ. The activator was constituted of small-sized ammonium dihydrogen phosphate on the AZ surface, which could be decomposed properly and deliver phosphoric acid and H2O during the foaming process. Then, AZ reacted with H2O under phosphoric acid catalysis. Also, this reaction generated CO2 emission while reducing the emission of NH3 through recombination with phosphoric acid. Moreover, phosphoric acid catalysis caused a decrease in the AZ decomposition temperature. Meantime, the thermal coupling appeared during the foaming process, which could further reduce the decomposition rate. Consequently, the small activator played a key role in regulating cell formation and diffusion. Compared to AZ, the novel blowing agent system significantly reduced the cell diameter of the PE foam resin and enhanced its flexural modulus by 50%. Furthermore, the novel blowing agent facilitated better demolding performance and improved the surface morphology of the PE foam product. This research provides significant foaming behavior regulation for the PE resin during industrial applications.

A co-anchoring strategy for the synthesis of polar bimodal polyethylene

Since polar groups can poison the metal centers in catalysts, the incorporation of polar comonomers usually comes at the expense of catalytic activity and polymer molecular weight. In this contribution, we demonstrate polar bimodal polyethylene as a potential solution to this trade-off. The more-polar/more-branched low-molecular-weight fraction provides polarity and processability, while the less-polar/less-branched high-molecular-weight fraction provides mechanical and melt properties. To achieve high miscibility between these two fractions, three synthetic routes are investigated: mixtures of homogeneous catalysts, separately supported heterogeneous catalysts, and a co-anchoring strategy (CAS) to heterogenize different homogeneous catalysts on one solid support. The CAS route is the only viable strategy for the synthesis of polar bimodal polyethylene with good molecular level entanglement and minimal phase separation. This produces polyolefin materials with excellent mechanical properties, surface/dyeing properties, gas barrier properties, as well as extrudability and 3D-printability.

New Ni(II)-Ni(II) Dinuclear Complex, a Resting State of the (α-diimine)NiBr2/AlMe3 Catalyst System for Ethylene Polymerization

A novel room-temperature stable diamagnetic nickel complex 2 was detected upon activation of Brookhart-type ethylene polymerization pre-catalyst LNiBr2 (1, L = 1,4-bis-2,4,6-trimethylphenyl-2,3-dimethyl-1,4-diazabuta-1,3-diene) with AlMe3. Using in situ 1H, 2H, and 13C NMR spectroscopy, as well as DFT calculations, this species has been identified as an antiferromagnetically coupled homodinuclear complex [LNiII(μ-Me)(μ-CH2)NiIIL]+Br−. Its behavior in the reaction solution is characteristic of the resting state of nickel catalyzed ethylene polymerization.

Ethylene polymerization using heterogeneous multinuclear nickel catalysts supported by a crosslinked alpha diimine ligand network

A crosslinked alpha diimine ligand supporting a nickel metal center polymerizes ethylene to produce polyethylene with controlled microstructures, high activities, and can be removed from the product.

Identifying the Real Chemistry of the Synthesis and Reversible Transformation of AuCd Bimetallic Clusters

The capability of precisely constructing bimetallic clusters with atomic accuracy provides exciting opportunities for establishing their structure-property correlations. However, the chemistry (the charge state of precursors, the property of ligands, the amount of dopant, and so forth) dictating the fabrication of clusters with atomic-level control has been a long-standing challenge. Herein, based on the well-defined Au₂₅(SR)₁₈ cluster (SR = thiolates), we have systematically investigated the factors of steric hindrance and electronic effect of ligands, the charge state of Au₂₅(SR)₁₈, and the amount of dopant that may determine the structure of AuCd clusters. It is revealed that [Au₁₉Cd₃(SR)₁₈]^- can be obtained when a ligand of smaller steric hindrance is used, while Au₂₄Cd(SR)₁₈ is attained when a larger steric hindrance ligand is used. In addition, negatively charged [Au₂₅(SR)₁₈]^- is apt to form [Au₁₉Cd₃(SR)₁₈]^- during Cd doping, while Au₂₄Cd(SR)₁₈ is produced when neutral Au₂₅(SR)₁₈ is used as a precursor. Intriguingly, the reversible transformation between [Au₁₉Cd₃(SR)₁₈]^- and Au₂₄Cd(SR)₁₈ is feasible by subtly manipulating ligands with different steric hindrances. Most importantly, by introducing the excess amount of dopant, a novel bimetallic cluster, Au₄Cd₄(SR)₁₂ is successfully fabricated and its total structure is fully determined. The electronic structures and the chirality of Au₄Cd₄(SR)₁₂ have been elucidated by density functional theory (DFT) calculations. Au₄Cd₄(SR)₁₂ reported herein represents the smallest AuCd bimetallic cluster with chirality.