Unusual Effect of α-olefins as Chain Transfer Agents in Ethylene Polymerization over the Catalyst with Nonsymmetrical Bis(imino)pyridine Complex of Fe(II) and Modified Methylalumoxane (MMAO) Cocatalyst.

Ethylene polymerization with bis(imino)pyridlyiron precatalysts generally produces linear polyethylene (PE) even with the presence of α-olefins because α-olefins are not incorporated into polymeric products. Interestingly, α-olefins, such as hexene-1 or butene-1, have been found to act as effective chain transfer agents in the ethylene polymerization promoted by nonsymmetrical bis(imino)pyridyliron complexes with modified methylalumoxane (MMAO), resulting in higher catalytic activities with higher amounts of polymers with lower molecular weights, and, more importantly, narrower molecular weight distributions of the resultant polyethylenes (PE). This phenomenon confirms the assistance of α-olefins in the chain-termination reaction of iron-initiated polymerization and regeneration of the active species for further polymerization. Besides higher activities of the catalytic system, the formation of linear PE with trans-vinylene terminal groups and lower molecular weights are explained. The observation will provide a new pathway for enhancing catalytic activity and improving the quality of polyethylenes obtained by regulation of molecular weights and molecular weight distribution.

Effect of Mono- and Multichlorinated Organic Compounds-Chlorocyclohexane and Hexachloro-p-xylene-On the Catalytic Properties of Titanium-Magnesium Catalysts in the Homo- and Copolymerization of Ethylene with 1-Hexene.

Ethylene polymerization and ethylene/1-hexene copolymerization over the titanium-magnesium catalytic system in the presence of chlorocyclohexane (CHC) and hexachloro-p-xylene (HCPX) has been studied. Modification of TMC with chlorocyclohexane and hexachloro-p-xylene increased catalyst activity severalfold for both ethylene polymerization and ethylene/1-hexene copolymerization. The key kinetic regularities of ethylene homopolymerization and ethylene/1-hexene copolymerization in the presence of CHC and HCPX were determined, and the copolymerization constants were measured. Molecular characteristics and the copolymer composition were determined for the synthesized samples of ethylene homopolymers and ethylene/hexene copolymers. Modification of the titanium-magnesium catalyst with chlorinated organic compounds reduced 1-hexene content in the copolymer; polymerization was sensitive to 1-hexene as a regulator of polymer molecular weight. The potential mode of action of chlorinated organic modifiers on catalytic properties of titanium-magnesium catalyst is discussed.

NMR spectroscopic identification of Ni(ii) species formed upon activation of (α-diimine)NiBr2 polymerization catalysts with MAO and MMAO.

The nature of Ni(ii) species formed upon the activation of the Brookhart's α-diimine polymerization pre-catalyst LNiBr2 with MAO and MMAO (L = 1,4-bis-2,4,6-dimethylphenyl-2,3-dimethyl-1,4-diazabuta-1,3-diene) has been established using 1H and 13C NMR spectroscopy. The heterobinuclear ion pair [LNiII(µ-Me)2AlMe2]+[MeMAO]- is observed at the initial stage of the reaction of LNiBr2 with MAO at -40 °C, whereas the ion pair [LNiII-tBu]+[MeMMAO]- predominates at the initial stage of the reaction of LNiBr2 with MMAO under the same conditions. At higher temperatures, both ion pairs transform into a Ni(i) species displaying an axially anisotropic EPR spectrum (g‖ = 2.21, g⊥ = 2.06, A⊥ = 1.06 mT).

Formation and evolution of chain-propagating species upon ethylene polymerization with neutral salicylaldiminato nickel(II) catalysts.

Formation of Ni-polymeryl propagating species upon the interaction of three salicylaldiminato nickel(II) complexes of the type [(N,O)Ni(CH3 )(Py)] (where (N,O)=salicylaldimine ligands, Py=pyridine) with ethylene (C2 H4 /Ni=10:30) has been studied by (1) H and (13) C NMR spectroscopy. Typically, the ethylene/catalyst mixtures in [D8 ]toluene were stored for short periods of time at +60 °C to generate the [(N,O)Ni(polymeryl)] species, then quickly cooled, and the NMR measurements were conducted at -20 °C. At that temperature, the [(N,O)Ni(polymeryl)] species are stable for days; diffusion (1) H NMR measurements provide an estimate of the average length of polymeryl chain (polymeryl=(C2 H4 )n H, n=6-18). At high ethylene consumptions, the [(N,O)Ni(polymeryl)] intermediates decline, releasing free polymer chains and yielding [(N,O)Ni(Et)(Py)] species, which also further decompose to form the ultimate catalyst degradation product, a paramagnetic [(N,O)2 Ni(Py)] complex. In [(N,O)2 Ni(Py)], the pyridine ligand is labile (with activation energy for its dissociation of (12.3±0.5) kcal mol(-1) , ΔH(≠) 298 =(11.7±0.5) kcal mol(-1) , ΔS(≠) 298 =(-7±1) cal K(-1) mol(-1) ). Upon the addition of nonpolar solvent (pentane), the pyridine ligand is lost completely to yield the crystals of diamagnetic [(N,O)2 Ni] complex. NMR spectroscopic analysis of the polyethylenes formed suggests that the evolution of chain-propagating species ends up with formation of polyethylene with predominately internal and terminal vinylene groups rather than vinyl groups.

Formation of trivalent zirconocene complexes from ansa-zirconocene-based olefin-polymerization precatalysts: an EPR- and NMR-spectroscopic study.

Reduction of Zr(IV) metallocenium cations with sodium amalgam (NaHg) produces EPR signals assignable to Zr(III) metallocene complexes. The chloro-bridged heterodinuclear ansa-zirconocenium cation [(SBI)Zr(µ-Cl)2AlMe2](+) (SBI = rac-dimethylsilylbis(1-indenyl)), present in toluene solution as its B(C6F5)4(-) salt, thus gives rise to an EPR signal assignable to the complex (SBI)Zr(III)(µ-Cl)2AlMe2, while (SBI)Zr(III)-Me and (SBI)Zr(III)(µ-H)2Al(i)Bu2 are formed by reduction of [(SBI)Zr(µ-Me)2AlMe2](+) B(C6F5)4(-) and [(SBI)Zr(µ-H)3(Al(i)Bu2)2](+) B(C6F5)4(-), respectively. These products can also be accessed, along with (SBI)Zr(III)-(i)Bu and [(SBI)Zr(III)](+) AlR4(-), when (SBI)ZrMe2 is allowed to react with HAl(i)Bu2, eliminating isobutane en route to the Zr(III) complex. Further studies concern interconversion reactions between these and other (SBI)Zr(III) complexes and reaction mechanisms involved in their formation.