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Emergence, Chemical Nature, Classification, Environmental Impact, and Analytical Challenges of Various Plastics
Published in Hyunjung Kim, Microplastics, 2023
More recent developments have centered on single-site catalysts such as metallocene systems. Such metallocenes typically consist of a metal atom, usually titanium or zirconium, linked with two aromatic five-carbon rings and two other groups (primarily carbon of –CH2 groups). The reactivity of the catalyst can be controlled by varying the nature of substituents on the five-carbon rings, by modifying the symmetry of the structure after changing the position of substituents, and by the use of co-catalysts such as methyl aluminoxanes (Edmondson and Gilbert, 2017).
Monomers, Polymers, and Plastics
Published in James G. Speight, Handbook of Petrochemical Processes, 2019
A Ziegler–Natta catalyst is a catalyst used in the synthesis of polymers of α-olefins (alpha-olefins, 1-alkenes). Two general classes of Ziegler–Natta catalysts are employed and are distinguished by their solubility: (i) heterogeneous supported catalysts such as those based on titanium compounds which are used in polymerization reactions in combination with co-catalysts—organo-aluminum compounds such as triethylaluminum (Al(C2H5)3) and (ii) homogeneous catalysts usually based on complexes of titanium, zirconium, hafnium which are usually used in combination with a different organo-aluminum co-catalyst, methyl aluminoxane (or methylalumoxane)—these catalysts traditionally contain not only metallocene derivatives but also feature multidentate oxygen- and nitrogen-based ligands.
Olefin Copolymer Viscosity Modifiers
Published in Leslie R. Rudnick, Lubricant Additives, 2017
The desire to achieve higher levels of control over stereoregularity, composition, and molecular weight distribution led to the development of activated metallocene catalysts. Although known to Ziegler and Natta, the technology was rediscovered by Kaminsky and Sinn in 1980 and further developed by workers such as Brintzinger, Chien, Jordan, and others [16–20]. Metallocene catalysts consist of compounds of transition metals (usually Group IVB: Ti, Zr, Hf) with one or two cyclopentadienyl rings attached to the metal. The most common activator is methylaluminoxane (MAO). A large number of variants have been reported, but the highest levels of stereospecificity have been achieved with bridged, substituted bis-cyclopentadienyl metallocenes (Figure 11.3). One of the major advantages of metallocenes over Ziegler–Natta catalysts is the ability to incorporate higher α-olefins and other monomers into the ethylene chain.
Transition metal(II) complexes featuring push-pull dianionic Schiff base ligands: synthesis, crystal structure, electrochemical, and NLO studies
Published in Journal of Coordination Chemistry, 2020
Salvador Celedon, Thierry Roisnel, David Carrillo, Isabelle Ledoux-Rak, Jean-Rene Hamon, Carolina Manzur
Crystals of the neutral bimetallic 3 suitable for X-ray crystallography were grown by slow diffusion of diethyl ether into a solution of 3 in DMF/MeOH mixture. Perspective view of the asymmetric Schiff base 3 is shown in Figure 2. Bond distances and angles for the first copper(II) coordination sphere are provided in Table 1, whereas other relevant bond distances and angles are gathered in Table S1 (Supplementary material). Complex 3 crystallizes in the triclinic centrosymmetric space group P-1 with two crystallographically nonequivalent molecules (3A, 3B) found in the asymmetric unit. The crystal structure of 3 is formed with no unusual intermolecular contacts, all interactions between neighboring molecules being on the basis of the van der Waals’ weak attractions. Compound 3 consists of a ferrocenyl unit linked to a copper(II)-centered N2O2 unsymmetrical macroacyclic Schiff base framework doubly substituted with a phenol and a nitro group. The ferrocenyl moiety is unexceptional, featuring in both 3A and 3B a typical linear η5-Fe-η5 metallocene structure in accordance with a Fe(II) oxidation state; Table S2 (Supplementary material) summarizes selected metrical parameters. The cyclopentadienyl rings are parallel and staggered by 23–30°.