Post Ziegler and Natta Coordination Polymerization of Olefins

In the current industrial practice, coordinated anionic catalysts differ considerably from the original ones, developments by Ziegler, Natta and others. Using the same basic chemistry, new compounds were developed over the years that yield large quantities of polyolefins from small amounts of catalysts. In addition, catalysts can now be designed to yield products that are either wide or narrow in molecular weight distribution, as needed . The multinuclear olefin polymerization catalysts were reviewed recently . The new catalysts for ethylene polymerization can be divided into three groups :

1. Products from reactions of trivalent alkoxy chlorides of transition metals with certain halogen-free organoaluminum compounds, e.g., triisobutylaluminum .Such catalysts are used without any support.

2. Products from reactions of magnesium compounds with titanium compounds. In these catalysts the transition metals are attached chemically to the surfaces of solid magnesium compounds. The reactions take place between the halogen atoms from titanium compounds and the hydroxyl groups at the surfaces of magnesium hydroxide:

Titanium compounds bonded to the surface OH groups of Mg(OH)₂ are mainly inactive. The active sites are the ones associated with the coordinated and unsaturated negative oxygen ions . Reactions with aluminum alkyls activate the catalysts . For high efficiency, special carriers must be used together with a correct balance of the reactants, and proper reaction conditions. Some choice combinations are : Mg(OH)Cl with TiCl₄; MgCl_{2 3}Mg(OH)₂ with Ti(OR)xCly; MgSO₄ • 3Mg(OH)₂ with Ti(OR)xCly; and Mg(OH)₂ with TiCl₄. Highly active, unsupported catalysts form from reactions of magnesium alkoxides with tetra valent titanium chlorides. The same is true of reaction products of MgCl₂ or MgCl₂-electron donor adducts, like MgCl₂ 6C₂H₅OH, with tetravalent titanium compounds. 3. Products from reactions of organisilanols with chromium trioxide are also very active catalysts. The silylchromate which forms is deposited on a silica support and activated with alkyl aluminum compounds :

The activity and performance of coordination catalysts for the polymerization of propylene has been also very much improved. Titanium compounds are also supported on some carriers and then activated by reactions with aluminum alkyls . The patent literature describes a variety of inorganic supports . The most common ones, however, are based on MgCl₂ treated with various Lewis bases, like ethyl benzoate. One such catalytic system is described as being prepared by treating a complex, TiCl₃ •3C₅H₅N, with diethyl aluminum chloride in the presence of highly dispersed MgCl₂. The product of that reaction is then combined with triethylaluminum . The reaction between an electron donor, a Lewis base, and MgCl₂ is a two step exothermic reaction . The first one is a rapid adsorption to the inorganic surface and the second one, a slower one, is formation of the complex. The most commonly used Lewis bases are ethyl benzoate, di-n-butyl phthalate, and methyl p-toluenate. Amines, like 2,2,6,6-tetramethylpiperidine, and some phenols are also used. A common practice is to ball mill the Lewis base with the support material first. The transition metal component is then added and the mixture is milled some more or thoroughly mixed. This Lewis base is called the internal Lewis base. This is followed by addition of Group I–III metal-alkyl component with an additional Lewis base. The base that is added the second time may be the same or a different one from one used in the original milling. In either case it is called the external Lewis base. It is not uncommon to use an ester as the internal base and an organosilane compound, like phenyltriethoxysilane, as the external one.

Initially, a series of soluble, highly isospecific catalysts were developed for propylene polymerization . These materials are zirconium, titanium, or hafnium based metallocenes, such as racemic 1,1-ethylene-di-Z5-indenylzirconium dichloride. The term metallocene applies to complexes of transition metals sandwiched between two aromatic rings, usually two cyclopentadienyl. They are rigid structures, due to ethylene bridges between the two five-membered rings. Syntheses of these compounds yield racemic mixtures of two enantiomers. Both produce isotactic polypropylene:

Methylaluminoxane co catalyst is required by these metallocenes to become highly isospecific catalysts capable of very high isotactic placement. They are also very active, yielding very large quantities of polypropylene per each gram of zirconia. Catalysts that yield highly syndiotactic polypropylene (86% racemic pentads) were also developed. One of them is i-propylene(Z5-cyclopentadienyl-Z3fluorenyl) zirconium dichloride. Initial disclosures of metallocene catalysts were followed by numerous publications in the literature that described similar materials for the polymerizations of either ethylene or propylene, or both, and for formation of various copolymers. Thus, for instance, Kaminsky et al. reported preparation of a zirconium dichloride-type catalyst for copolymerization of cyclic olefins with ethylene. These cyclic olefins are cyclopentene, norbornene, and the hindered cyclopentadiene adducts of norbornene. The catalytic system consists of a bridged indene derivative that is combined with methylaluminoxane:

It is interesting that the copolymers of some of these cycloolefins with ethylene were reported to be transparent, amorphous, and oxidatively stable materials. In addition, it is claimed that they have glass transition temperatures in excess of 200C, are melt processable, solvent stable, and possess high mechanical strength . Another example is work by Han et al. who synthesized ansa-dichloro[o-phenylenedi-methylenebis(Z5-1-indenyl)]zirconium. Both racemic and meso forms were obtained and separated:

These catalysts also polymerize ethylene and propylene in the presence of methyl-aluminoxane . The meso-1/MAO analog was found to be more active than the rac-1/MAO in ethylene polymerization. In propylene polymerization, however, the racemic one is active and the meso is inactive. Ethylene polymerization activity with rac-1/MAO increases as the temperature increases. An inverse temperature effect is observed for propylene polymerization with the rac-1/MAO catalyst system. The stereoregularity of polypropylene formed with this racemic catalyst is low [293]. This led Han et al. to conclude  that the a-olefin polymerization is influenced by changes in the structure of ansa chiral metallocene catalysts such as variation of the transition metal, the steric and electronic properties of p- and s-ligands, and the length and rigidity of the bridging groups. The activity and the stereoselectivity of a-olefin polymerization reactions can be significantly affected by slight structural variations of the bridging group in metallocene catalysts. The length of the bridging chain affects the angle between the cyclopentadiene's and the metal atom. The chiral metallocenes having p-ligands bridged with one atom (carbon or silicon) or two carbon atoms have either C₂ (racemic) or C₈ (meso) symmetry, if the ansa-ligand is enantiotropic . The mechanism of monomer insertion and steric control in polymerizations of a-olefins by the metallocene catalysts received considerable attention. There is no consensus on the mechanism of polymerization. Many studies of chain propagation tend to support the Cossee-Arleman mechanism . An example is work by Miyake et al. [294] who synthesized unsymmetrical ansa-metallocenes and separated them into threo and erythro isomers. Both isomers coupled with methylaluminoxane polymerize propylene in toluene to highly isotactic polymers of Mw ¼ 105,000. The isotactic placement is greater that 99.6% and the polymer melting point is 161C.

Based on experimental evidence obtained with the above catalysts, it was concluded by Miyake et al., that the isotactic propylene polymerization with zirconium catalysts takes place by a regioselective 1,2-insertion of the propylene monomer into the metal-polymer bond . Monomer insertion is believed to take place at two active sites on the metal center in an alternating manner. In addition, it was shown  that the substituents on the cyclopentadiene rings determine the conformation of the polymer chain end, and the fixed polymer chain end conformation in turn determines the stereochemistry of olefin insertion in the transition state as a form of indirect steric control. With the above catalysts, however, because the stereochemistry of the two sites is different, Miyake et al. suggest that monomer insertion takes place at the same active site on the metal center:

Lohrenz et al. reported [296] quantum mechanical calculations on model systems support the Cossee-Arleman mechanism. Insertion of ethylene into Cp2Zr*-Me is preceded by an initial olefin complexation with the vacant coordination site and formation of л-complex 1 as shown below:

After the formation of complex 1, the insertion follows a low activation barrier (Eact = 3 kL/mol), yielding the y-agostic product 3a. These calculations also led to the conclusion about the importance of agostic interactions present in the product [296]. The most stable conformation is the ẞ-agostic structure (3b) at 22 kJ/mole that is more stable than the primary insertion

product (3a). It is assumed that this conformation serves as a model for the resting state of the growing chain attached to the cationic group-4 metallocenes between insertions .

To shed additional light on complex 3b, shown above, calculations were carried out  on the reaction of ethylene with a model complex, Cl1⁄2Zr + Et. Two reaction paths (A) and (B) are possible as illustrated below:

Front-side attack leads to the formation of a p-complex that in turn can undergo two possible reactions. Insertion of the olefin into the Zr–Ca bond may take place after rotation around Zr–Ca that moves Cb out of plane. The b-hydrogen can be transferred from the polymer chain to the olefin. This leads to chain termination and formation of a vinyl-terminated polymer and an ethyl-zirconocene that can start a new polymer chain [296]. The backside attack on the other hand allows insertion of the olefin without rotation around Zr–Ca bond. The front-side insertion is accompanied by chain migration from one side to the other whereas backside attack does not involve inversion at the metal center. Lohrenz et al. [296] concluded that insertion into the metal-polymer bond takes place exclusively from the backside. That means that no inversion takes place. They also feel that in propylene polymerization two orientations occur. In the first step the polymer chain points to the larger ligand side and the propylene methyl group points away from the large ligand and the polymer chain. The next step is governed by a stronger interaction of propylene with the polymer chain than with the ligand [296]. An analysis of molecular mechanics using model metallocene complexes ,as possible intermediates for propylene polymerization was also reported by Guerra et al. [298]. The two coordination positions available for the monomer and the growing chain are diastereotopic. The conclusion was that the energy difference between the corresponding diastereoisomeric pre-insertion intermediates appear to be relevant for the model complexes . It was also concluded  that energy differences can be related to an increased probability of a back-skip of the growing chain toward the outward coordination position after the monomer insertion and prior to the coordination of a new olefin molecule. Busico et al., on the other hand, came to a conclusion  that the stereoregularity of poly propylene produced with C2-symmetric group 4 ansa-metallocene catalysts is a result of the interplay of two competing reactions. These are: isotactic monomer polyinsertion and a side process of epimerization of the polymer chain at its active end. That makes this class of homogenous catalysts different from the typical Ziegler–Natta catalyst, because with these catalysts enantioselectivity and stereoselectivity are not necessarily coincidental . Zang et al. reported that they achieved highly efficient, rapid, and reversible chain transfer reactions between active transition-metal based propagating centers with catalysts derived from {Cp*Hf (Me)[N(Et)C(Me)N(Et)]} [B(C6F5)4] (Cp* ¼ Z5-C₅Me₅) or {Cp*Hf (Me)[N(Et)C(Me)N (Et)]} [B(C₆F₅)₃Me]

with multiple equivalents of dialkylzinc (ZnR2) acting as “surrogate” chain-growth sites. This was done to achieve living coordinative chain-transfer polymerization of ethylene with a-olefins, and α,w non conjugated dienes. It is claimed by these investigators that these living coordinated chain transfer processes provide a work-around solution to the “one chain per metal” cap on product yield. In addition, they are claimed to provide access to practical volumes of a variety of unique new classes of precision polyolefins of tunable molecular weights and very narrow polydispersity (Mw/Mn 1.1). A Japanese patent issued to Watanabe and Okamoto  describes preparation and illustrates an iron containing catalyst for polyethylene preparation. It is shown here as an illustration:

This catalyst was reported to yield 2.66 kg of polyethylene per one minimol of nickel. Subsequent research efforts have been focused on discovering more efficient catalytic processes benefiting from cooperative effects between active centers in multinuclear complexes. The idea is to ultimately mimic the advantageous enzyme characteristics. Thus, for instance, Li, Stern, and Marks  reported that in their earlier work they demonstrated that the constrained geometry of binuclear catalyst + binuclear co catalyst combination

affords, via the modification of chain transfer pathway kinetics, significantly enhanced branching in ethylene homopolymerizations and enhanced comonomer incorporation, as well as ethylene plus 1-hexene copolymerization. Such catalysts, however typically produce unacceptably low molecular weight polyolefins. Earlier, they reported preliminary results that showed that metal–metal proximity and cocatalyst structure have significant impact in terms of product molecular weight and comonomer enchainment selectivity. Following that they reported in the subject publication the synthesis of single methylene-bridged (m-CH_2-3,3) {(n⁵-indenyl) [1-Me₂Si(tBuN)] (ZrMe₂)}₂Zr based catalyst:

to examine achievable metal-metal proximity effects. They also reported preparing sterically encumbered naphthyl-derivatized catalyst {1-Me₂Si [3-(1)-naphthyl indenyl]-(tBuN)}ZrMe₂ (N-Zn) to additionally probe catalytic center steric effects. Placing the two metals into closer proximity was shown by them to significantly increase polyethylene molecular weight in ethylene homopolymerizations and a-olefin enchainment in ethylene copolymerizations with 1-hexene. Additionally, they found that when methyl alumoxane is used as the co catalyst, the catalyst shown above, yields even greater enhancement in weight average molecular weight. Also, when they used a polar solvent, C₆H₅Cl in the reaction, it caused a weakening in the catalyst-cocatalyst ion pairing, a significant alteration in catalyst response and polymer product properties was observed. Their results showed that Zr…Zr spatial proximity, as modulated by ion pairing, significantly influences chain transfer rate and selectivity for comonomer enhancement. These proximity effects are highly cocatalyst and solvent sensitive. In a subsequent publication [304] they reported the synthesis and activity of binuclear catalysts 2,7-di[(2,6-isopropylphenyl) imino]-1,8-naphthalenediolato group 4 metal complexes and copolymerization of ethylene with various mono olefins. The bimolecular catalysts exhibited enhanced activity as compared to their mononuclear analogs. The polymerizations were illustrated as follows:

At the present time, in spite of all the research effort, there is still a need for catalyst systems that will enable preparation of low-density polyethylene with controlled levels of both short and long branches. Homura and coworkers [305] investigated a series of half-titanocenes containing pyrazole ligands that have been employed as catalyst precursors for ethylene polymerization, syndiospecific styrene polymerization, and copolymerization of ethylene with 1-hexene, styrene, and norbornene in the presence of methyl aluminoxane cocatalyst. The catalyst was TiCl2 (3,5R₂C₃HN₂), where R was hydrogen, methyl, propyl, and phenyl. The ethylene/styrene copolymerization proceeds in a living manner, irrespective of the styrene concentration in toluene at 25C and the same system exhibits relatively high catalytic activity for the ethylene/norbornene copolymerization with highly efficient norbornene incorporation [305]. The catalyst system can be illustrated as follows:

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العتبة العباسية المقدسة تكرم المشاركين في مشروع مقرأة العميد الرقمية

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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

Steric Control in Polymerization of Conjugated Dienes

Steric Control with Homogeneous Catalysts

Homogeneous Ziegler–Natta Catalysts

Steric Control with Heterogeneous Catalysts

Heterogeneous Ziegler–Natta Catalysts

Coordination Polymerization of Olefins

Termination in Anionic Polymerization

Hydrogen Transfer Polymerization

Steric Control in Anionic Polymerization