Heterogeneous Ziegler–Natta Catalysts

These catalysts form when a soluble metal alkyl like triethylaluminum or diethyl aluminum chloride is combined with a metal salt, like titanium chloride, in a medium of an inert hydrocarbon diluent. The transition metal is reduced during the formation of the catalyst. The following chemical scheme illustrates the reactions that are believed to take place between aluminum alkyls and transition metal halides [227, 228]. Titanium chloride is used as an example:

The radicals that form in the above shown reactions probably undergo combinations or other reactions of radicals, or perhaps react with solvents and decay. The reduction of the tetravalent titanium is unlikely to be complete due to the heterogeneous nature of the catalyst. Better catalytic activity results when TiCl3 is used directly in place of TiCl4. Many catalysts, however, are prepared with TiCl4. In addition, TiCl3 exists in four different crystalline forms, referred to as a, b, g, and d.Of these, the b, g, and d forms yield highly stereospecific polymers from a-olefins. The a-form, however, yields polymers that are high in atactic material. The ratio of the transition metal compounds to those of the compounds from metals in Group I to III can affect polymerization rates. They can also affect the molecular weights, and the steric arrangement of the products. Also, additives, like Lewis bases, amines, or other electron donors

help increase the stereoregularity of the product. Thus, for instance, dialkylzinc plus titanium trichloride catalyst yields polypropylene that is 65% stereoregular. Addition of an amine, however, to this catalytic system raises stereoregularity to 93% [229]. Heterogeneous catalysts, typically, form polymers with very wide molecular weight distributions.

Many mechanisms were proposed to explain the action of heterogeneous Ziegler-Natta catalysts. All agree that the polymerizations take place at localized active sites on the catalyst surfaces. Also, it is now generally accepted that the reactions take place by coordinated anionic mechanisms. The organometallic component is generally believed to activate the site on the surface by alkylating the transition metal. Some controversy, however, still exists about the exact mechanism of catalytic action, whether it is monometallic or bimetallic. Most of the opinion leans to the former. Also, it is well accepted that the monomer insertion into the polymer chain takes place between the transition metal atom and the terminal carbon of the growing polymeric chain [230].

Only one bimetallic mechanism is presented here, as an example, the one originally proposed by Natta . He felt that chemisorptions of the organometallic compounds to transition metal halides take place during the reactions. Partially reduced forms of the di- and tri-chlorides of strongly electropositive metals with a small ionic radius (aluminum, beryllium, or magnesium) facilitate this. These chemisorptions result in formations of electron-deficient complexes between the two metals. Such complexes contain alkyl bridges similar to those present in dimeric aluminum and beryllium alkyls [231]. The polymeric growth takes place from the aluminum-carbon bond of the bimetallic electron-deficient complexes [232, 234]:

where R represents an ethyl group. The bulk of the evidence, however, indicates that chain growth occurs through repeated four-center insertion reactions of the monomer into the transition metal- carbon σ-bond . It is still not established whether the base metal alkyls serve only to produce the active centers and have no additional function. This would make the mechanism monometallic. If, however, the active centers must be stabilized by coordination with base metals then the mechanism is bimetallic [234]. The two active centers are depicted as follows:

where MT represents the transition metal, M1 a base metal, and a vacant site. Both types of active centers might conceivably be present in heterogeneous Ziegler-Natta catalysts [234]. The exact locations of the sites in the solid catalyst crystals are still debated. Some speculations center on whether they are located over the whole crystal surface or only over the edges of the crystals [235]. Most evidence points to location at the edges. An example of the monometallic mechanisms is one originally proposed by Cossee and Arlman [236, 237]. This mechanism assumes that the reaction occurs at a transition metal ion on the surface layer of the metal trichloride (or perhaps dichloride) lattice. Here the halide is replaced by an alkyl group (R). The adjacent chloride site is vacant and accommodates the incoming monomer molecule. Using titanium chloride as an illustration:

where, represents a vacant site in the d-orbital. The newly formed transition metal–alkyl bond becomes the active center and a new vacant site forms in place of the previous transition metal–alkyl bond. The driving force for the reactions [236, 237] depends on p-type olefin complexes. In these complexes, the p-electrons of the olefins overlap with the vacant d-orbitals of the transition metals. This results in the p-bonds being transitory. Also, the d-orbitals of the metals can simultaneously overlap with the vacant anti-bonding orbitals of the olefins. This decreases the distances between the highest filled bonding orbitals and the empty (or nearly empty) d-orbitals. In such situations, the carbon–metal bonds of the transition metals weaken and the alkyl groups migrate to one end of the incoming olefin . The insertion process results in a cis-opening of the olefinic double bond . The above scheme of propagation might also be pictured for bimetallic active centers. Complexations precede monomer insertions at the vacant octahedral sites and are followed by insertion reactions at the metal–carbon bonds . When the transition metals are immobilized in crystal lattices, the active centers and the ligands are expected to interchange at each propagation step. The above model for monometallic mechanism, though now widely accepted, is still occasionally questioned. Some evidence, for instance, has been presented over the years to support a bimetallic mechanism [242]. It was shown that elimination of the organometallic portion of the complex catalyst during polymerization of propylene results in deactivation of the catalyst. By contrast, replacement of the initial organometallic compound with another one results in a change in the polymerization rate, but not in deactivation of the catalyst. In addition, some monometallic mechanisms based on a different mode of monomer insertion were also proposed. An example is a reaction mechanism that was proposed by Ivin et al. [241]. This mechanism is based on an insertion mechanism involving an a-hydrogen reversible shift, carbene, and a metallocyclobutane intermediate:

where, MT means metal. The stereospecificity is dependent upon the relative configuration of the substituted carbons of the metallocyclobutane ring. Hydrogen transfer from the metal to the more substituted carbon exclude branching [241]. The following evidence supports the above mechanism.

1. There are no unambiguous examples where a characterized metaloalkyl-olefin compound may be induced to react.

2. There is a close identity between the catalysts that cause the Ziegler–Natta type polymerizations and those that cause metathesis type polymerization via a carbene mechanism . This mechanism was argued against, however, as an over simplification, because it ignores the experimentally observed regiospecificity of may propylene polymerizations . On the other hand, it is argued that, correct regiospecificity of the monomer is accounted for by isotactic or syndiotactic propagation. Different energies of steric control can be qualitatively explained by the Ivin mechanism. This can be done through simple considerations of different distances between the substituted carbons of a four-membered ring . In addition, Cavallo et al.  reported a study of ethylene polymerization using a model for the heterogeneous Ziegler–Natta catalyst (Mg2CI6Ti). Propagation as well as the termination reactions were considered. From this study they concluded that in the absence of a coordinated olefin, the Ti–C (chain) s-bond does not occupy an octahedral coordination position but is oriented along an axis that is intermediate between the two octahedral coordination positions that are available. The propagation reactions occur in stepwise fashions, and the most favored mechanism requires rearrangements of the growing chains out of the four-center transition-state planes. The insertion reactions are facilitated by a-agostic interactions. The most favored termination reactions are b-hydrogen transfers to the monomers. This type of reaction is favored, relative to the C–H s-bond activation of a coordinated monomer, as well as to the b-hydrogen transfer to the metal.

اخر الاخبار

اخبار العتبة العباسية المقدسة

العتبة العباسية المقدسة تكرم المشاركين في مشروع مقرأة العميد الرقمية

تقديم محاضرة عن عالم الأرقام في القرآن الكريم ضمن حفل إطلاق مشروع مقرأة العميد

ضمن فعاليات إطلاق مقرأة العميد الرقمية.. عرض فيديو توثيقي عن فكرة المشروع

جامعة تكنولوجيا المعلومات: مشروع مقرأة العميد منصة رقمية متكاملة لتعليم القرآن الكريم وتلاوته وحفظه

المزيد

الآخبار الصحية

عرض شائع على الأصابع.. قد يكشف سرطان الرئة مبكرا

7 مشروبات طبيعية لتقليل التوتر والقلق

دراسة: "خطر مبكر" يهدد صحة الأطفال

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

المزيد

الآخبار التكنلوجية

مفاجأة علمية عن الإنجاب.. كم طفلا يطيل عمر المرأة؟

استطلاع يكشف "الأكثر قلقا" من تأثر الوظائف بالذكاء الاصطناعي

هل يمنحك تناول الجزر يوميا سمرة طبيعية؟

إنجاز طبي تاريخي.. جراحة قلب بلا شق في الصدر

المزيد

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

Steric Control with Heterogeneous Catalysts

Coordination Polymerization of Olefins

Termination in Anionic Polymerization

Hydrogen Transfer Polymerization

Steric Control in Anionic Polymerization