Reaction of vinyl chloride with group 4 metal olefin polymerization catalysts

Ancillary ligand effects on α-olefin polymerization catalyzed by zirconium metallocene: a computational study

The polymerization of α-olefins catalyzed by zirconium metallocene catalyst was systematically studied through experiments and density functional theory (DFT) calculations. Having achieved an agreement between theory and experiment, it was found that the effect of the catalyst ligand on the C[double bond, length as m-dash]C insertion reaction was significantly greater than that on the β-H elimination reaction. Therefore, the molecular weight of polymers can be increased by improving the activity of the C[double bond, length as m-dash]C insertion. In addition, in comparison with propylene, the chain length of α-olefins can directly affect the stereotacticity of polymerization products, owing to steric hindrance between the polymer chain and monomer.

Active Sites in a Heterogeneous Organometallic Catalyst for the Polymerization of Ethylene

Heterogeneous derivatives of catalysts discovered by Ziegler and Natta are important for the industrial production of polyolefin plastics. However, the interaction between precatalysts, alkylaluminum activators, and oxide supports to form catalytically active materials is poorly understood. This is in contrast to homogeneous or model heterogeneous catalysts that contain resolved molecular structures that relate to activity and selectivity in polymerization reactions. This study describes the reactivity of triisobutylaluminum with high surface area aluminum oxide and a zirconocene precatalyst. Triisobutylaluminum reacts with the zirconocene precatalyst to form hydrides and passivates -OH sites on the alumina surface. The combination of passivated alumina and zirconium hydrides formed in this mixture generates ion pairs that polymerize ethylene.

Compatibilization of iPP/HDPE Blends with PE-g-iPP Graft Copolymers

The compatibilization of polyethylene (PE) and isotactic polypropylene (iPP) blends is of particular interest due to the challenges associated with recycling these plastics from mixed waste streams. Polyethylene-graft-iPP copolymers (PE-g-iPP) were prepared using a grafting-through strategy by copolymerization of ethylene with allyl-terminated iPP macromonomers in the presence of a hafnium pyridylamido catalyst. Graft copolymers with a variety of graft lengths (M_n = 6-28 kg/mol), graft numbers, and graft spacings were prepared. These graft copolymers were melt-blended with high-density polyethylene (HDPE) and iPP (iPP/HDPE = 30/70 w/w), and the blend properties were evaluated by tensile testing. The blends showed enhanced tensile strength at 5 and 1 wt % loading, with higher tensile strength observed for larger block numbers and graft lengths. These results indicate that graft copolymers are efficient compatibilizers for blends of HDPE and iPP.

Copolymerization of Ethylene and Vinyl Fluoride by Self-Assembled Multinuclear Palladium Catalysts

The self-assembled multinuclear Pd^II complexes {(Li-OPO^OMe2)PdMe(4-5-nonyl-pyridine)}₄Li₂Cl₂ (C, Li-OPO^OMe2 = PPh(2-SO₃Li-4,5-(OMe)₂-Ph)(2-SO₃⁻-4,5-(OMe)₂-Me-Ph)), {(Zn-OP-P-SO)PdMe(L)}₄ (D, L = pyridine or 4-^tBu-pyridine, [OP-P-SO]³⁻ = P(4-^tBu-Ph)(2-PO₃²⁻-5-Me-Ph)(2-SO₃⁻-5-Me-Ph)), and {(Zn-OP-P-SO)PdMe(pyridine)}₃ (E) copolymerize ethylene and vinyl fluoride (VF) to linear copolymers. VF is incorporated at levels of 0.1–2.5 mol% primarily as in-chain -CH₂CHFCH₂- units. The molecular weight distributions of the copolymers produced by D and E are generally narrower than for catalyst C, which suggests that the Zn-phosphonate cores of D and E are more stable than the Li-sulfonate-chloride core of C under copolymerization conditions. The ethylene/VF copolymerization activities of C–E are over 100 times lower and the copolymer molecular weights (MWs) are reduced compared to the results for ethylene homopolymerization by these catalysts.

An efficient and mild route to highly fluorinated polyolefins via copolymerization of ethylene and 5-perfluoroalkylnorbornenes

Highly efficient copolymerization of ethylene/5-(perfluoro-n-alkyl)norbornenes has been realized for the synthesis of fluorinated COC for the first time.

Solid-state 45Sc NMR studies of Cp*2Sc–X and Cp*2ScX(THF)

A systematic study showing how the Sc–X bond affects solid-state ⁴⁵Sc NMR quadrupolar coupling constants in Cp*₂Sc–X.

Olefin Insertion into a Pd-F Bond: Catalyst Reactivation Following β-F Elimination in Ethylene/Vinyl Fluoride Copolymerization

The discrete (phosphinoarenesulfonate)Pd fluoride complex (PO^Bp,OMe )PdF(lutidine), where PO^Bp,OMe =(2-MeOC₆ H₄ )(2-{2,6-(MeO)₂ C₆ H₃ }C₆ H₄ )(2-SO₃ -5-MeC₆ H₃ )P, inserts vinyl fluoride (VF) to form (PO^Bp,OMe )PdCH₂ CHF₂ (lutidine) and inserts multiple ethylene (E) units to generate polyethylene that contains -CH₂ F chain ends. These results provide strong evidence that the -CHF₂ and -CH₂ F chain ends in E/VF copolymer generated by (phosphinoarenesulfonate)PdR catalysts form by β-F elimination of Pd(β-F-alkyl) species, VF or E insertion of the resulting (PO)PdF species, and subsequent chain growth. These results also imply that β-F elimination is not an important catalyst deactivation reaction in this system.

Catalytic, Stereoselective Dihalogenation of Alkenes: Challenges and Opportunities

Although recent years have witnessed significant advances in the development of catalytic, enantioselective halofunctionalizations of alkenes, the related dihalogenation of olefins to afford enantioenriched vicinal dihalide products remains comparatively underdeveloped. However, the growing number of complex natural products bearing halogen atoms at stereogenic centers has underscored this critical gap in the synthetic chemist's arsenal. This Review highlights the selectivity challenges inherent in the design of enantioselective dihalogenation processes, and formulates a mechanism-based classification of alkene dihalogenations, including those that may circumvent the "classical" haliranium (or alkene-dihalogen π-complex) intermediates. A variety of metal and main group halide reagents that have been used for the dichlorination or dibromination of alkenes are discussed, and the proposed mechanisms of these transformations are critically evaluated.

Synthesis of Allyl-Terminated Polar Macromonomers by Metallocene-Catalyzed Polymerizations of 10-Undecene-1-ol

Poly(10-undecene-1-ol) macromonomers were synthesized by metallocene-catalyzed polymerization using vinyl chloride as a chain transfer agent. Using this technique, predominantly allyl terminated polymers could be obtained, which was verified by NMR spectroscopy and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS). By means of the mass spectra, a detailed interpretation of the nature of the head and end groups of these polymers was possible.

Carbon−Oxygen Bond Cleavage with η9,η5-Bis(indenyl)zirconium Sandwich Complexes

Treatment of the eta9,eta5-bis(indenyl)zirconium sandwich complex, (eta9-C9H5-1,3-(SiMe3)2)(eta5-C9H5-1,3-(SiMe3)2)Zr, with dialkyl ethers such as diethyl ether, CH3OR (R=Et, nBu, tBu), nBu2O, or iPr2O resulted in facile C-O bond scission furnishing an eta5,eta5-bis(indenyl)zirconium alkoxy hydride complex and free olefin. In cases where ethylene is formed, trapping by the zirconocene sandwich yields a rare example of a crystallographically characterized, base-free eta5,eta5-bis(indenyl)zirconium ethylene complex. Observation of normal, primary kinetic isotope effects in combination with rate studies and the stability of various model compounds support a mechanism involving rate-determining C-H activation to yield an eta5,eta5-bis(indenyl)zirconium alkyl hydride intermediate followed by rapid beta-alkoxide elimination. For isolable eta6,eta5-bis(indenyl)zirconium THF compounds, thermolysis at 85 degrees C also resulted in C-O bond cleavage to yield the corresponding zirconacycle. Both mechanistic and computational studies again support a pathway involving haptotropic rearrangement to eta5,eta5-bis(indenyl)zirconium intermediates that promote rate-determining C-H activation and ultimately C-O bond scission.