Steric Control with Heterogeneous Catalysts

Different independent approaches were used to investigate the mechanism of polymerization of a-olefins with heterogeneous catalysts. As a result, it was shown that isotactic polymerization of racemic mixtures of α-olefins are stereoselective. Also, optically active polymers form with optically active catalysts . Stereoelectivity and stereoselectivity are due to the intrinsic asymmetry of the catalytic centers [248]. This conclusion comes in part from knowledge that propylene coordinates asymmetrically in platinum complexes [248]. A study of chemical and configurational sequences supports this . In addition, polymerization of a racemic mixture of (R)(S)-3-methyl-1-pentene with ordinary Ziegler–Natta catalysts (e.g., Al(C₂H₅)₃ + TiCl₄ or Al (C₂H₅)₂Cl + TiCl₃) yields a racemic mixture of isotactic polymers. The mixture can be separated by column chromatography into pure optically active components. These polymeric chains are exclusively either poly[(S)-3-methyl-1-pentene] or poly[(R)-3-methyl-1-pentene] [248, 250, 251]. This means that typical Ziegler–Natta catalysts have essentially two types of active sites that differ only in a chiral sense. Such sites polymerize the monomers stereoselectively, i.e., either (R)or(S) with the exclusion of the other enantiomeric form:

Fig. 4.4 Two enantiomorphs form of the catalyst. R indicates the ligands that was replaced by alkyl groups through alkylation by organometallic compounds from Groups I through III In the heterogeneous catalyst, like a-TiCl₃, the crystals are made up of elementary sheets of alternating titanium and chlorine atoms. These atoms are aligned along the principal crystal axis. The chlorines are hexagonally packed and the titanium atoms are at the octahedral interstices of the chlorine lattice. Every third titanium in the lattice is missing. There is a vacancy between pairs of titanium atoms. Many ligand vacancies are present in the crystals in order to accommodate electrical neutrality and the titanium atoms at the surface are bonded to only five chlorines instead of six. Neighboring transition metal atoms that are bridged by chlorines have opposite chiralities. This means that two enantiomorphic forms exist (. The monomers coordinate at either one of the two faces (at the vacant sites). Coordination results in formation of one of two diastereoisomeric intermediate transition states. Both result in isotactic placements, but the products are either meso or racemic [240]. In addition, enantioselectivity between the two faces of the crystals requires a minimum amount of steric bulk at the active site. This is enhanced by larger-sized monomers . A schematic representation of a catalytic center, showing chirality by an asterisk, is as follows [252]:

The above means that the active sites act as templates or molds for successive orientations of the monomers. The monomers are forced to approach these sites with the same face. This sort of monomer placement is called enantiomorphic site control or catalyst site control. So far, the evidence for an initial complex formation between the catalyst and the olefin is not strong. The working hypothesis, commonly accepted today, is as follows. The position in the p-complex of Dia stereoisomeric and rotameric equilibria and/or activation energy for the insertion reaction cause large regioselectivity. That can also account for the enantioface discrimination necessary for the synthesis of stereoregular poly-a-olefins. 13C NMR analyses of isotactic polypropylenes formed in d-TiCl₃–Al(₁₃CH₃)2I catalyzed reactions show that the enriched 13Cis located only in the isopropyl end group. Also, it is located predominantly at the threo position, relative to the methyl group on the penultimate unit . This supports the concept that steric control must come from chirality of the catalytic centers . It might be interesting to note that the proponents of the carbene mechanism (mentioned earlier). point out that this is also consistent with their mechanism . The reaction can consist of (a) an insertion of a metal into an a-CH bond of a metal alkyl to form a metal–carbene hydride complex. This is followed by (b) reaction of the metal-carbene unit with an alkene to form a metal-cyclobutane hydride intermediate. The final step (c), is a reductive elimination of hydride and alkyl groups to produce a chain-lengthened metal alkyls. This assures that a chiral metal environment is maintained . It is generally believed , however, that stereospecific propagation comes from concerted, multicentered reactions, as was shown in the Cossee–Arlman mechanism. The initiator is coordinated with both the propagating chain end and the incoming monomer. Coordination holds the monomer in place during the process of addition to the chain. This coordination is broken simultaneously with formation of a new coordination with the new monomer [258]:

where, MT means metal. This capability of the initiator to control the placement of the monomer overrides the common tendency for some syndiotactic placement. While syndiotactic polypropylene has been prepared with heterogeneous catalysts, the yield of syndiotactic placement is low. Soluble Ziegler–Natta catalysts on the other hand can yield high amounts of syndiotactic placement. This is discussed in the next section. When 1,2-disubstituted olefins are polymerized with Ziegler–Natta catalysts, the di tacticity of the products depends on the mode of addition. It also depends on the structure of the monomer, whether it is cis or trans. A threo diisotactic structure results from a syn addition of a trans monomer. A syn addition of a cis monomer results in the formation of an erythro diisotactic polymer. For instance, cis and trans-1-d-propylenes give erythro and threo diisotactic polymers, respectively . To avoid 1,2-interactions in the fully eclipsed conformation, the carbon bonds in the monomer units rotate after the addition of the monomer to the polymeric chain . Also, a systematic investigation of chain propagation by ethylene insertion into a metal–C6H5 bond showed that the backside insertion barriers depend little on the identity of the metal, because backside insertion requires little deformation of the metal–ligand framework . For systems that are sterically unencumbered the insertion reaction proceeds through frontside and backside channels in equal parts. This is due to the fact that both transition states are close in energy. The influence of the ligand upon insertion is such that good p donor ligands lower the front-side insertion barrier, because they favor trigonal planar geometry over trigonal pyramidal coordination. The activity of various metal centers can be strongly influenced, however, by sterically bulky ligands. Insertion barriers are generally lowered by steric bulk, because compression of the active site favors the transition state geometry over the p-complex geometry.

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7 مشروبات طبيعية لتقليل التوتر والقلق

دراسة: "خطر مبكر" يهدد صحة الأطفال

الجوز أو الفول السوداني.. أيهما يتفوّق في حماية القلب؟

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مواضيع ذات صلة

Polymerization of Aldehydes

Isomerization Polymerizations with Coordination Catalysts

Reduced Transition Metal Catalysts on Support

Post Ziegler and Natta Coordination Polymerization of Olefins

Steric Control in Polymerization of Conjugated Dienes

Steric Control with Homogeneous Catalysts

Homogeneous Ziegler–Natta Catalysts

Heterogeneous Ziegler–Natta Catalysts

Coordination Polymerization of Olefins

Termination in Anionic Polymerization

Hydrogen Transfer Polymerization

Steric Control in Anionic Polymerization