Flexible Axial Shielding Strategy for Improving Ethylene (Co)polymerization with 8-Cycloalkylnaphthyl α-Diimine Catalysts

8-aryl or alkyl-naphthyl substituents are widely used as an effective axial shielding strategy for the suppression of chain transfer in late-transition metal-catalyzed ethylene (co)polymerization to yield high molecular weight polyethylene and copolymers. In this study, two 8-cycloalkylnaphthyl acenaphthene-based α-diimine ligands and the corresponding four nickel and palladium complexes were designed and synthesized to explore the effect of axial flexible shielding on ethylene (co)polymerization. In ethylene polymerization, the nickel complexes displayed high activities (up to 1.99 × 106 g mol-1 h-1) and generated lightly branched (34-54/1000 C) polyethylenes with high molecular weights (up to Mn = 1075 kg/mol), whereas the corresponding palladium complexes exhibited moderate activities (level of 104 g mol-1 h-1), producing highly branched (111-125/1000 C) polyethylenes with high molecular weights (up to Mn = 37.6 kg/mol). Highly branched (110-123/1000 C) E-MA copolymers with moderate insertion ratios (1.97-5.56 mol %) were produced by these palladium complexes in ethylene/methyl acrylate (MA) copolymerization. In addition, the size of the 8-cycloalkyl ring in these α-diimine catalysts strongly influences the ethylene (co)polymerization. Compared to cyclopentyl groups, cyclohexyl groups are more effective in suppressing chain transfer reactions in the polymerization of ethylene and the copolymerization of ethylene and MA, leading to higher molecular weight polyethylene and E-MA copolymers. Most interestingly, compared to the reported rigid planar 8-arylnaphthyl catalysts, the flexible 8-cyclohexylnaphthyl catalysts exhibited higher activity and produced higher molecular weight polyethylene in ethylene polymerization. Moreover, in nickel-catalyzed ethylene polymerization, the cyclohexyl catalyst produced significantly reduced branched polyethylene, while in palladium-catalyzed ethylene (co)polymerization, the cyclohexyl catalyst produced more highly branched polyethylene and copolymers. In contrast to the previously reported flexible 8-butylnaphthyl nickel catalysts, the 8-cycloalkylnaphthyl catalysts reported in this work yielded polyethylene with narrow unimodal molecular weight distributions.

Effect of N-Aryl Para-Benzhydryl Substituent on the Thermal Stability of α-Diimine Nickel Catalyst

The thermal stability of α-diimine nickel catalysts has always been the focus of research. The introduction of large groups in the backbone or N-aryl ortho-position is a relatively mature solution. However, the question of whether the N-aryl bond rotation is a factor affecting the thermal stability of nickel catalysts is still open. In this work, the effects of N-aryl para-benzhydryl substitutes on catalyst thermal stability are investigated, and the results of ethylene polymerization and the factors affecting thermal stability (steric effect, electronic effect, five-membered coordination ring stability, N-aryl bond rotation, etc.) are systematically analyzed. It is believed that the introduction of large steric hindrance groups at the N-aryl para-position hinders the rotation of the N-aryl bond. This obstacle effect is beneficial to improving catalyst thermal stability, and the obstacle capacity is weakened with the increase of ortho-substituent size.

N,N-Bis(2,4-Dibenzhydryl-6-cycloalkylphenyl)butane-2,3-diimine-Nickel Complexes as Tunable and Effective Catalysts for High-Molecular-Weight PE Elastomers

Four examples of N,N-bis(aryl)butane-2,3-diimine-nickel(II) bromide complexes, [ArN=C(Me)-C(Me)=NAr]NiBr₂ (where Ar = 2-(C₅H₉)-4,6-(CHPh₂)₂C₆H₂ (Ni1), Ar = 2-(C₆H₁₁)-4,6-(CHPh₂)₂C₆H₂ (Ni2), 2-(C₈H₁₅)-4,6-(CHPh₂)₂C₆H₂ (Ni3) and 2-(C₁₂H₂₃)-4,6-(CHPh₂)₂C₆H₂ (Ni4)), disparate in the ring size of the ortho-cycloalkyl substituents, were prepared using a straightforward one-pot synthetic method. The molecular structures of Ni2 and Ni4 highlight the variation in the steric hindrance of the ortho-cyclohexyl and -cyclododecyl rings exerted on the nickel center, respectively. By employing EtAlCl₂, Et₂AlCl or MAO as activators, Ni1-Ni4 displayed moderate to high activity as catalysts for ethylene polymerization, with levels falling in the order Ni2 (cyclohexyl) > Ni1 (cyclopentyl) > Ni4 (cyclododecyl) > Ni3 (cyclooctyl). Notably, cyclohexyl-containing Ni2/MAO reached a peak level of 13.2 × 10⁶ g(PE) of (mol of Ni)^-1 h^-1 at 40 °C, yielding high-molecular-weight (ca. 1 million g mol^-1) and highly branched polyethylene elastomers with generally narrow dispersity. The analysis of polyethylenes with ¹³C NMR spectroscopy revealed branching density between 73 and 104 per 1000 carbon atoms, with the run temperature and the nature of the aluminum activator being influential; selectivity for short-chain methyl branches (81.8% (EtAlCl₂); 81.1% (Et₂AlCl); 82.9% (MAO)) was a notable feature. The mechanical properties of these polyethylene samples measured at either 30 °C or 60 °C were also evaluated and confirmed that crystallinity (X_c) and molecular weight (M_w) were the main factors affecting tensile strength and strain at break (ε_b = 353-861%). In addition, the stress-strain recovery tests indicated that these polyethylenes possessed good elastic recovery (47.4-71.2%), properties that align with thermoplastic elastomers (TPEs).

Recent Advances in Transition Metal-Based Catalysts for Ethylene Copolymerization with Polar Comonomer

The synthesis of polar functionalized polyolefin (PFP) offers improvement in mixing properties, polymer surface, and rheological properties with the potential of upgraded polyolefins for modern and ingenious applications. The synthesis of PFP from metal-based catalyzed olefin (non-polar in nature) copolymerization with polar comonomers embodies energy-efficient, atom-efficient, and apparently an upfront methodology. Despite their outstanding success during conventional polymerization of olefin, 3^rd and 4^th group (early transition metal)-based catalysts, owing to their electrophilic nature, face challenges mainly due to Lewis basic sites of the polar monomers. On the contrary, late transition metal-based catalysts have also made progress, in recent years, for PFP synthesis. The recent past has also witnessed several advancements in the development of dominating palladium-based catalysts while their lower resistance towards ligand functional groups has limited the practical application of abundant and cheaper nickel-based catalysts. However, the relentless efforts of the scientific community, during the past half-decade, have indicated rigorous progress in the development of nickel-based catalysts for PFP synthesis. In this review, we have abridged the recent research trends in both early as well as late transition metal-based catalyst development. Furthermore, we have highlighted the role of transition metal-based catalysts in influencing the polymer properties.

Direct Synthesis of Polyethylene Thermoplastic Elastomers Using Hybrid Bulky Acenaphthene-Based α-Diimine Ni(II) Catalysts

Recently, polyolefin thermoplastic elastomers can be obtained directly using ethylene as a single feedstock via α-diimine nickel-catalyzed ethylene chain walking polymerization. Here, a new range of bulky acenaphthene-based α-diimine nickel complexes with hybrid o-phenyl and -diarylmethyl anilines were constructed and applied to ethylene polymerization. All the nickel complexes under the activation of excess Et₂AlCl exhibited good activity (level of 10⁶ g mol^-1 h^-1) and produced polyethylene with high molecular weight (75.6-352.4 kg/mol) as well as proper branching densities (55-77/1000C). All the branched polyethylenes obtained exhibited high strain (704-1097%) and moderate to high stress (7-25 MPa) at break values. Most interestingly, the polyethylene produced by the methoxy-substituted nickel complex exhibited significantly lower molecular weights and branching densities, as well as significantly poorer strain recovery values (48% vs. 78-80%) than those by the other two complexes under the same conditions.

Direct Synthesis of Partially Chain-Straightened Propylene Oligomers and P-MA Co-Oligomers Using Axially Flexible Shielded Iminopyridyl Palladium Complexes

In this study, a series of partially chain-straightened propylene oligomers and functional propylene−methyl acrylate (P-MA) co-oligomers were synthesized with 8-alkyl-iminopyridyl Pd(II) catalysts. The molecular weight and polar monomer incorporation ratio could be tuned by using Pd(II) catalysts with various 8-alkyl-naphthyl substituents (8-alkyl: H, Me, and n-Bu). In propylene oligomerization, all the 8-alkyl-iminopyridyl Pd(II) catalysts convert propylene to partially chain-straightened (119−136/1000 C) oligomers with low molecular weights (0.3−1.5 kg/mol). Among the catalysts, Pd1 with non-substituent (H) on the ligand showed the highest activity of 5.4 × 104 g/((mol of Pd) h), generating oligomers with the lowest molecular weight (Mn: 0.3 kg/mol). Moreover, polar-functionalized propylene-MA co-oligomers with very high incorporation ratios (22.8−36.5 mol %) could be obtained in the copolymerization using these 8-alkyl-iminopyridyl Pd(II) catalysts. Additionally, Pd1 exhibited the best performance in propylene-MA copolymerization as it displayed the highest MA incorporation ratio of up to 36.5 mol%. All the three catalysts are capable of generating partially chain-straightened P-MA co-oligomers and the activities decrease gradually while the molecular weight increases with the increasing steric hindrance of the alkyl substituent (H < Me < n-Bu). Compared to Pd4 with the rigid 8-aryl substituent, the flexible 8-alkyl-iminopyridyl Pd(II) catalysts (Pd1-3) not only showed much higher activities in the propylene oligomerization, but also yielded P-MA co-oligomers with significantly higher incorporation ratios in the propylene co-oligomerization.

Exploring the Relationship between the Polyethylene Microstructure and Spatial Structure of α-Diimine Pd(II) Catalysts via a Hybrid Steric Strategy

The branching density of polyethylene generated in the α-diimine Pd(II) system is usually very high, largely independent of simple ligand modifications with steric or electronic perturbations, or the polymerization conditions. In this study, we designed and synthesized a class of bulky hybrid α-diimine Pd(II) catalysts combining ortho-diarylmethyl and ortho-phenyl moieties to explore the relationship between the polyethylene microstructure and the spatial structure of catalysts. In ethylene polymerization, the hybrid α-diimine Pd(II) catalysts exhibited high activities (well above 10⁵ g·mol^-1·h^-1) and yielded highly branched (90-110/1000C) polyethylenes with high molecular weights (up to 278.3 kg/mol). Compared with the two corresponding symmetrical ortho-diarylmethyl-based or ortho-phenyl-based Pd(II) catalysts, the hybrid catalysts generated polyethylene of significantly higher branching densities (92 vs 28-34/1000C) in marked higher activities. Similar phenomena are also observed in the copolymerization of ethylene with polar monomers. Moreover, the hybrid Pd(II) catalysts can more efficiently promote the copolymerization of ethylene with various polar monomers in comparison to the corresponding symmetrical catalysts. The more open spatial environment around the metal center by using a hybrid steric strategy was proposed to be responsible for above advantages.

Efficient suppression of the chain transfer reaction in ethylene coordination polymerization with dibenzosuberyl substituents

Catalysts with dibenzosuberyl substituents possess a superior ability to suppress chain transfer in ethylene (co)polymerization, producing high molecular weight polyethylenes and functionalized polyethylenes (ethylene-MA copolymers).

A rigid-flexible double-layer steric strategy for ethylene (co)oligomerization with pyridine-imine Ni(ii) and Pd(ii) complexes

A rigid-flexible double-layer steric strategy enhances the molecular weight of the resulting ethylene oligomers and promotes the co-oligomerization of ethylene and methyl acrylate.

Facile Synthesis of Hyperbranched Ethylene Oligomers and Ethylene/Methyl Acrylate Co-oligomers with Different Microscopic Chain Architectures

Low-molecular-weight (MW) ethylene oligomers with hyperbranched microstructures are often difficult to be synthesized by traditional catalytic processes. In this study, a series of N-terphenyl iminopyridyl ligands and the corresponding Pd(II) and Ni(II) complexes bearing remote conjugated substituents with different electronic effects (H, Me, F, Cl, and tBu) were synthesized in a simple and efficient way. These Pd(II) and Ni(II) complexes were highly effective in the ethylene oligomerization and co-oligomerization with methyl acrylate (MA). Low-MW ethylene oligomers with hyperbranched microstructures were generated using the iminopyridyl Pd(II) and Ni(II) complexes in ethylene oligomerization. More importantly, polar functionalized ethylene-MA co-oligomers with low MWs and varying incorporation ratios were generated via ethylene and MA co-oligomerization using the Pd(II) complexes. Most notably, these ethylene oligomers obtained by different metal species showed a significant difference in microscopic chain architectures. The remote conjugated electron effect showed little effect on the polymerization parameters of the iminopyridyl system, which is very different from those of the salicylaldiminato system.

α-Diimine Ni-Catalyzed Ethylene Polymerizations: On the Role of Nickel(I) Intermediates

Nickel(II) complexes with bidentate N,N-α-diimine ligands constitute a broad class of promising catalysts for the synthesis of branched polyethylenes via ethylene homopolymerization. Despite extensive studies devoted to the rational design of new Ni(II) α-diimines with desired catalytic properties, the polymerization mechanism has not been fully rationalized. In contrast to the well-characterized cationic Ni(II) active sites of ethylene polymerization and their precursors, the structure and role of Ni(I) species in the polymerization process continues to be a “black box”. This perspective discusses recent advances in the understanding of the nature and role of monovalent nickel complexes formed in Ni(II) α-diimine-based ethylene polymerization catalyst systems.

The crystal structure of {N 1,N 2-bis[2,4-dimethyl-6-(4-(tert-butyl)phenyl)(phenyl)methyl]acenaphthylene-1,2-diimino-κ2 N, N′}-dibromido-nickel(II) – dichloromethane(1/2), C64H64Br2Cl4N2Ni

C64H64Br2Cl4N2Ni, monoclinic, C2/c (no. 15), a = 31.9723(18) Å, b = 11.9826(6) Å, c = 18.4722(11) Å, β = 125.038(2)°, V = 5794.4(6) Å3, Z = 4, R gt (F) = 0.0536, wR ref (F 2) = 0.1640, T = 273.15 K.

Flexible "Sandwich" (8-Alkylnaphthyl α-Diimine) Catalysts in Insertion Polymerization

8-Arylnaphthyl substituents are privileged motifs frequently integrated into late-transition-metal catalysts, endowing them with an ability to retard chain transfer in ethylene polymerization. In this contribution, we disclose a sort of novel α-diiminenickel and -palladium complexes containing flexible 8-alkylnaphthyl in lieu of rigid 8-arylnaphthyl and their catalytic performance in ethylene polymerization. An interesting feature of these 8-alkylnaphthyl-substituted α-(diimine)PdMeCl complexes is that they present as a mixture of syn and anti isomers (syn:anti = ca. 1:1 ratio, determined by ¹H and ¹³C NMR spectroscopy). In ethylene polymerization, these nickel complexes displayed high activity (up to 3.37 × 10⁶ g mol^-1 h^-1) and generated branched polyethylenes with broad or bimodal molecular weight distributions (4.6-29.3), while the corresponding palladium complexes exhibited moderate activity, producing highly branched polyethylenes with unimodal and narrow molecular weight distributions (<1.8). In ethylene (E)/methyl acrylate (MA) copolymerization, highly branched E-MA copolymers with considerable MA incorporations were achieved by these palladium complexes. Most interestingly, compared to rigid 8-arylnaphthyl-substituted α-diiminenickel and -palladium complexes, the flexible 8-alkylnaphthyl ones showed significantly improved activity and generated lower or comparable molecular weight polyethylenes or E-MA copolymers.

Efficient incorporation of a polar comonomer for direct synthesis of hyperbranched polar functional ethylene oligomers

Hyperbranched polar functional ethylene oligomers with very high incorporation can be accessible via ethylene–MA co-oligomerization with iminopyridyl Pd(ii) catalysts.

Suppression of chain transfer via a restricted rotation effect of dibenzosuberyl substituents in polymerization catalysis

Catalysts with dibenzosuberyl substituents possess superior thermostability and outstanding ability to suppress chain transfer in ethylene (co)polymerization, producing high molecular weight polyethylene and functionalized polyethylene.

Influence of intramolecular π–π and H-bonding interactions on pyrazolylimine nickel-catalyzed ethylene polymerization and co-polymerization

Pyrazolylimine-based nickel catalysts bearing intramolecular π–π and H-bonding interactions show high activity, thermal stability, and Mn of polyethylene.

Tunable branching and living character in ethylene polymerization using “polyethylene glycol sandwich” α-diimine nickel catalysts

The key effects of polyethylene glycol units as a unique secondary coordination sphere in α-diimine Ni(ii)-mediated ethylene polymerization are comprehensively disclosed.

The synergistic effect of rigid and flexible substituents on insertion polymerization with α-diimine nickel and palladium catalysts

The introduction of flexible cycloalkyl groups greatly enhanced the chain growth in the Ni(ii) catalytic system and facilitated the insertion of polar monomers in the Pd(ii) catalytic system.

Direct synthesis of various polar functionalized polypropylene materials with tunable molecular weights and high incorporation ratios

As compared with the classical α-diimine catalyst, iminopyridyl catalysts were observed to be highly efficient for the direct synthesis of polar functionalized polypropylene with tunable molecular weights and high incorporation ratios.