The Cossee-Arlman Mechanism

The mechanism proposed for the solid titanium chloride catalysts is essentially the same for all catalysts and it is usually referred to as the Cossee-Arlman mechanism [33]. Titanium is hexacoordinated in the TiCl3 or supported catalysts 

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Figure 9.21. The Cossee-Arlman mechanism of chain growth in ethylene polymerization involves the insertion of ethylene in the...

The Cossee-Arlman mechanism as originally proposed has a weakness—the back-flip is required to explain isoselective placement since the two active (coordination) sites are assumed to be enantiotopic. However, the structure of the traditional Ziegler-Natta heterogeneous initiators is not sufficently understood to either support or reject the assumption of enantiotopic sites. Further, even if the sites are enantiotopic, there is no overwhelming reason why the polymer chain is more stable at one site than the other—which is the rationale for the back-flip. The mechanism of isoselectivity with various metallocene initiators is much better understood since these are initiators whose molecular structures are well-established [Busico and Cipullo, 2001 Busico et al., 1997, 1999 Cavallo et al., 1998 Ewen, 1999 Rappe et al., 2000 Resconi et al., 2000], Considerable advancements in understanding heterogeneous Ziegler-Natta initiators occur if one assumes that the active sites in these initiators mimic those in metallocene initiators. Two types of metallocene initiators offer possible models... 

Natta postulated that for the stereospecific polymerization of propylene with Ziegler-Natta catalysts, chiral active sites are necessary he was not able to verify this hypothesis. However, the metallocene catalysts now provide evidence that chiral centers are the key to isotacticity. On the basis of the Cossee-Arlman mechanism, Pino et al. (164,165) proposed a model to explain the origin of stereoselectivity The metallocene forces the polymer chain into a particular arrangement, which in turn determines the stereochemistry of the approaching monomer. This model is supported by experimental observations of metallocene-catalyzed oligomerization. 

Two major mechanisms have been proposed for alkene polymerization. These are the Cossee-Arlman mechanism and the Green-Rooney mechanism. A modified version of the latter has also been considered to explain the behavior of homogeneous, metallocene catalysts. The original Cossee-Arlman mechanism was proposed for the TiCl3 based heterogeneous catalyst. In the following sections we discuss these different mechanisms in some detail. In the following discussion in accordance with the results obtained from the metallocene systems, the oxidation states of the active surface sites are assumed to be 4+. 

The Cossee-Arlman mechanism proposes direct insertion of alkene into the metal-alkyl bond (see Section 2.3.2) without the formation of any intermediate. In the solid catalyst anion vacancies at the crystal edges are formed by simple... 

In the Cossee-Arlman mechanism insertion is considered to be direct. The transition state of 6.5 to 6.6 by the Cossee-Arlman mechanism is therefore as designated by 6.8. In 6.8, for clarity the Cl ligands are not shown and represents the growing polymer chain. 

Polymerization takes place at the edges or corners of crystallites where metal atoms are necessarily coordinatively unsaturated. The reaction steps are those expected for a migratory alkyl transfer mechanism (Section 21-6) and has become known as the Cossee-Arlman mechanism ... 

One such process is the Cossee-Arlman mechanism,proposed for the Ziegler-Natta polymerization of alkenes (also discussed in Section 14-4-1). According to this mechanism, a polymer chain can grow as a consequence of repeated 1,2 insertions into a vacant coordination site, as follows ... 

The mechanism proposed for the solid titanium chloride catalysts is essentially the same for all catalysts and it is usually referred to as the Cossee-Arlman mechanism [46]. Titanium is hexacoordinated in the TiCls or supported catalysts by four bridging chlorides and one terminal chloride that is replaced by an alkyl (P, for polymer chain) from the alkylating agent (Et2AlCl or EtsAl), and a vacancy that is available for propene coordination (see Fig. 6.12). The front and back of the complex shown are not equiveilent the "blocked" chlorine at the front causes more steric hindrance than the "exposed" one at the back [46]. 

For isospecific polymerization by the Cossee-Arlman mechanism, migration of the vacant site back to its original position is necessary, as otherwise an alternating position is offered to the incoming monomer and a syndiotactic polymer would result. This implies that the tacticity of the polymer formed depends essentially on the rates of both the alkyl shift and the migration. Since both these processes slow down at lower temperatures, syndiotactic polymer would be formed when the temperature is decreased. In fact, syndiotactic polypropylene can be obtained at —IQPC. 

The polymer may be isotactic, syndiotactic, or atactic according to the nature of the catalyst/cocatalyst system. The Cossee-Arlman mechanism for the ZNP of propene is depicted in Scheme 4.2. 

Figure 5.9 outlines the steps for the chain polyaddition mechanism involved in the coordination polymerizations for any kind of active species initiated through different cocatalysts. The counteranion species was suppressed for practical representation of the active site. Once the cationic species is created, it starts the growth of the polymeric chain through continuous addition of monomer. The propagation step is forward described in Figure 5.9 according to the most accepted reaction cycle proposed by Cossee and Arlman, which is known as the Cossee-Arlman mechanism [51]. 

Figure 5.9 The Cossee-Arlman mechanism a) Olefin coordination, b) Olefin concerted insertion, c) Insertion step, d) Chain migration.

Two examples of 1,2 insertions are in Figure 14.11. An important application of 1,2 insertions of alkenes into metal-alkyl bonds is in the formation of polymers. One such process is the Cossee-Arlman mechanism, proposed for the Ziegler-Natta polymerization of... 

For the Cossee-Arlman mechanism to operate, migration of the vacant site back to its original position is necessary otherwise, an alternating position is offered to the chemisorbed monomer and a syndiotactic polymer would result. This implies... 

Alkene pol5anerization is catalysed at a surface Ti(III) centre in which there is a terminal Cl atom and a vacant coordination site. The Cossee-Arlman mechanism is the accepted pathway of the catalytic process and a simplified representation of the mechanism is shown in Figure 27.16. Coordinatively unsaturated TiCls units are the catalytically... 

The extensive structural characterization of iPP produced by ZN catalysts has established unequivocally that the polymer backbone arises from a predominant head-to-tail propagation of monomer units, with occasional head-to-head dyad regioerrors being incorporated. In terms of the Cossee-Arlman mechanism, these structural data can be accounted for by assuming a preferred primary 1,2-regioseIective mode of migratory insertion, with secondary 2,1-regioisomeric... 

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 [254, 255], 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 [254]. It is generally believed [258], however, that stereospecific propagation comes from concerted, multicentered reactions, as was shown in the Cossee-Arlman mechanism. The initiator is coordinated... 

The Cossee-Arlman mechanism proposed for the Ziegler-Natta polymerization of alkenes. 

The approach and insertion of an olefin molecule may or may not pass through a local minimum or coordination complex (first in brackets in eq. 16) recent theoretical work (128) indicates that the well, if it indeed exists, is very shallow. The insertion of the new molecule into the growing chain is represented in equation 13 as a structure intermediate between reactants and products. The mechanism for this apparently concerted reaction does not involve the participation of metal-based electrons, and can be considered to be a Lewis acid-assisted anionic attack of the zirconium alkyl (ie, the polymer chain) upon one end of a carbon-carbon double bond. The concept of this reaction pre-dates metallocene study, and is merely a variant of the Cossee-Arlman mechanism (129) routinely invoked in Ziegler-Natta polymerization. Computational studies indicate (130) that an a-agostic interaction (131) provides much needed stabilization during the process of insertion. 

Polymer Chain Growth. The essential characteristic of Ziegler-Natta catalysis is the polymerization of an olefin or diene using a combination of a transition-metal compound and a base-metal alkyl cocatalyst, normally an aluminum alkyl. The function of the cocatalyst is to alkylate the transition metal, generating a transition-metal-carbon bond. It is also essential that the active center contains a coordination vacancy. Chain propagation takes place via the Cossee-Arlman mechanism (23), in which coordination of the olefin at the vacant coordination site is followed by chain migratory insertion into the metal-carbon bond, as illustrated in Figure 1. 

Three mechanisms have been proposed to explain metallocene-based homogeneous and Ziegler-Natta polymerization schemes. The Cossee-Arlman mechanism... 

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