Cossee—Arlman mechanism

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 

Coordinatively unsaturated 6.1 represents the surface Ti4+ responsible for catalyzing the polymerization reaction. By reaction with the Et3Al (or Et2AlCl), a Ti-Et bond, as shown for 6.2, is formed. Ethylene coordination at the vacant 

As far as the fate of 6.6 is concerned, there is a third possibility. The polymer chain may remain attached to the metal atom that is, the metal-alkyl bond remains intact. The product of the overall reaction in this case is 6.6. Polymerization of this type is often called living polymerization. Note that in this case there is no polymer chain termination step, and the catalytic cycle is not completed. In other words, although a single Ti4+ ion may be responsible for the polymerization of thousands of ethylene molecules, in the strictest sense of the term it is not a true catalytic reaction. In the event 6.6 is converted to 6.7 either by reaction with H2 or by /8-elimination, 6.7 further reacts with ethylene. Insertion of ethylene into the Ti-H bond regenerates 6.2 and completes the catalytic cycle. 

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

Cossee-Arlman mechanism, is based on the observed stereoselectivity and molecular modeling studies [Arlman and Cossee, 1964 Ewen, 1999 Rappe et al., 2000 Resconi et al., 2000]. A variation on this mechanism involves a lowering of the transition state barrier to insertion by an a-agostic interaction, specifically, an attractive interaction between titanium and a hydrogen on the first carbon attached to Ti. 

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. 

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. 

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... 

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. 

Scheme 4.2 Cossee-Arlman mechanism for (a) primary insertion and (b) secondary insertion in Ziegler-Natta polymerization of propene P = polymer chain.

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