Cossee-Arlman chain propagation

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... 

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]. 

Scheme 18 (a) Chain propagation through classical Cossee-Arlman mechanism, (b) Chain transfer by P-H elimination to monomer... 

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... 

Propagation occurs when olefins insert into the metal-carbon bond, extending the chain. In the following the Cossee-Arlman transition state is shown ... 

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. 

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