Coordination polymerization Cossee-Arlman

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 ... 

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

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. 

From this evidence and the reaction mechanisms of Natta [27], Cossee-Arlman [28] and Rodriguez-van Looy [29], the mechanism of the polymerization with R3Al-TiCL, (w = 3,4) is considered as follows Ti has a six-coordination structure. 

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]. 

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

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. 

If the rate of anti-syn isomerization is relatively low, then the cis-trans selectivity can be determined by the formation of the anti- or the 5y/i-butenyl structure, for example from the t] -cis or the if-trans coordinated butadiene, in the catalyst complex. This is the mechanism of stereoregulation which was suggested in the mid-1960s by Cossee and Arlman [34, 35] for titanium-catalyzed butadiene polymerization, and which was reconsidered more recently for the allylne-odymium complex catalysts to explain their cis-trans selectivity [39], But it is also possible that the difference in reactivity between the anti and the syn structure of the catalytically active butenyl complex can determine the cis-trans selec-... 

Once an active catalytic system is generated and exposed to olefin, polymerization proceeds through a chain reaction. As proposed by Cossee and Arlman in 1964, the chain propagation cycle starts with a vacant coordination site and the coordination of an olefin to this vacant coordination site at the metal, eq 1. Olefin coordination is followed by the insertion... 

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