Polymerization Cossee-Arlman mechanism

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

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+. 

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 ... 

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. 

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

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]. 

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... 

Figure 3.3 Cossee-Arlman mechanism for the coordination polymerization of ethylene mediated by a heterogeneous...

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... 

The copolymerization of a polar comonomer with nonpolar olefins by coordination polymerization is thought to be possible if the insertion of the polar comonomer takes place on the same active catalyst center as the nonpolar olefin according to the Cossee-Arlman mechanism [131, 132]. The prerequisite for this is that the polar comonomer coordinates to the metal center by its C=C double bond rather than by its polar group [133]. 

The chain migratory insertion mechanism is universally accepted and the original mechanistic description originated with Cossee and Arlman ° for heterogeneous ZN catalyst polymerizations. Figure 10 shows the Cossee-Arlman mechanism... 

Figure 10 The Cossee-Arlman mechanism for 1-alkene polymerization requires a chain migratory insertion step. M=transition metai atom and = vacant coordination site.

Dimerization, oligomerization, and polymerization all rely on the Cossee-Arlman mechanism that consists of repeated alkene 1,2-insertion into the M-C bond of the growing polymer chain (Fig. 

The Cossee-Arlman mechanism for the polymerization of olefins is the most widely accepted theory but as yet it is not complete. Cossee developed his early ideas of polyethylene growth at a titanium-carbon bond and supported the theory by molecular orbital calculations. The role of the alkyl aluminium co-catalyst was in the generation of the active species, via the alkylation of the titanium chloride bonds, and to remove impurities in both the gas stream and catalyst preparative procedure. There was also the suggestion that it might be involved in the insertion of each monomer molecule, and also in the regeneration of dormant sites or the formation of new active sites. 

In Natta s catalyst, the steric constraints near the active sites due to the presence of the chloride anions make the coordination of propylene stereospecific. In other words, coordination of propylene through one particular enantioface is energetically favored. This mechanism for the stereospecific propylene polymerization is referred to as the Cossee-Arlman mechanism. 

Figure 4. Schematic representation of Cossee-Arlman type reaction mechanism for homogeneous polymerization with metallocene-based catalysts.(Reprinted with permission from ref 51. Copyright 1994.)...

The debate on the mechanism of polymerization, whether an insertion mechanism (Cossee-Arlman) [6], or a metathesis-type mechanism initiated by a-H elimination from the alkyl complex to give a hydrido-carbene intermediate (Green-Rooney) [108], was solved in favor of the former on the basis of the absence of isotope effect on the rates of insertion, and on the stereochemistry of alkene intramolecular insertion, when a-D alkyls were used in the cyclizafion reaction shown in Eq. 6.21 [109]. 

Following that year, many work groups studied the theoretical concepts concerning this type of catalytic system. Griffiths et al. [27] presented the first theoretical studies on this catalyst system, having assumed the generally accepted Cossee-Arlman polymerization mechanism [25-27] (Scheme 3.13). 

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