Search

Alkene polymerization reactions were studied with three catalyst systems containing complexes of vanadium and titanium with the acetylacetonate ligand (V(acac)3, VO(acac)2, Ti(acac)2Cl2) and a binary activator, a combination of Al(C2H5)2Cl and Mg(C4H9)2 at an [Al]:[Mg] molar ratio of ca 3. All three catalysts are highly active in ethylene homopolymerization reactions. They copolymerize ethylene and 1‐hexene to random, compositionally uniform products with 1‐hexene content of up to 20–25%. The Ti(acac)2Cl2–Al(C2H5)2Cl/Mg(C4H9)2 combination is also an effective catalyst for propylene polymerization to an essentially atactic product. Gel permeation chromatography analysis showed that all the studied M(acac)n–Al(C2H5)2Cl/Mg(C4H9)2 catalysts contain several types of active centers which differ in the average molecular weight of polymer material they produce. © 2021 Society of Industrial Chemistry. [ABSTRACT FROM AUTHOR]

Reaction of (TBBP)AlMe ⋅THF with [Cp*2Zr(Me)OH] gave [(TBBP)Al(THF)−O−Zr(Me)Cp*2] (TBBP=3,3',5,5'-tetra- tBu-2,2'-biphenolato). Reaction of [DIPPnacnacAl(Me)−O−Zr(Me)Cp2] with [PhMe2NH]+[B(C6F5)4]− gave a cationic Al/Zr complex that could be structurally characterized as its THF adduct [(DIPPnacnac)Al(Me) −O−Zr(THF)Cp2]+[B(C6F5)4]− (DIPPnacnac=HC[(Me)C=N(2,6- iPr2−C6H3)]2). The first complex polymerizes ethene in the presence of an alkylaluminum scavenger but in the absence of methylalumoxane (MAO). The adduct cation is inactive under these conditions. Theoretical calculations show very high energy barriers (Δ G=40-47 kcal mol−1) for ethene insertion with a bridged AlOZr catalyst. This is due to an unfavorable six-membered-ring transition state, in which the methyl group bridges the metal and ethene with an obtuse metal-Me-C angle that prevents synchronized bond-breaking and making. A more-likely pathway is dissociation of the Al-O-Zr complex into an aluminate and the active polymerization catalyst [Cp*2ZrMe]+. [ABSTRACT FROM AUTHOR]

Full quantum chemical calculations with density functional theory (DFT) show that a principal role of donors in Ziegler-Natta (ZN) olefin polymerization catalysts is to coordinate to the metal center at the active sites on the MgCl2 surface. Thereby, the behavior of the catalyst is modulated to favor insertion over termination and, thus, polymerization occurs. This is shown to be true for a range of different donors. The calculations indicate that active sites that feature anionic chloride ligands at the titanium center (the conventional model for the active site) would lead to lower-molecular-weight polymers. If an -OC2H5 group were present instead of a chloride ligand, the active site would much more effectively produce long chain polymers. Therefore, the current work provides important new insights into the nature of the ZN polymerization process. [ABSTRACT FROM AUTHOR]