Ziegler-Natta olefin polymerization reactions involved

The coordination polymerization of ethylene and a-olefins with Ziegler-Natta catalysts involves, in general, many elementary reactions, such as initiation (formation of active centers), chain propagation, chain transfers and chain terminations. The length of growing polyolefin chains is limited by the chain-terminating processes, as schematically represented (for ethylene) by 21,49 51)... 

The involvement of the a-elimination reaction in this cycle has been in question following experiments on cyclopentadienyl cobalt complexes, where evidence for olefin insertion for Ziegler-Natta polymerization catalysis has been obtained by labelling experiments, using C2H4 with a deuterated cobalt complex (70) ... 

Ziegler-Natta polymerization is well known to involve a two-stage process [148, 149]. In the first stage, an aluminum alkyl (such as trialkyl aluminum) is reacted with TiCU in order to produce active jS-TiCls. The alkyl radicals, which are also produced in this reaction, are terminated by coupling and create inert products. Subsequent alkylation of -TiCb then occurs to generate the titanium species that is capable of initiating the polymerization of olefins such as ethylene (Scheme 11.37). 

Figure 1.13 One possible mechanism for the polymerization of olefins using the Ziegler-Natta catalyst A 1( 2115)3.71014 which involves both metal atoms in the catalytic complex. The coordination of the monomer with the Ti atom in the first stage of the reaction leads to this type of polymerization being described as coordination polymerization.

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. 

If reaction had proceeded in the same manner as Ziegler-Natta polymerization of ethylene and substituted ethylenes (Section 29.6B), a 1,2-addition polymer would have been formed. What is formed, however, is an unsaturated polymer in which the number of double bonds in the polymer is the same as that in the monomers polymerized. This process is called ring-opening metathesis polymerization, or ROMP, after the olefin metathesis involving reaction of acyclic alkenes and nucleophilic carbene catalysts described in Section 24.6. 

Transition metal-carbon a-bond formation by this method most commonly involves the insertion of some carbon-containing species into a metal-hydrogen bond. The name insertion reaction for this process has no mechanistic significance but merely describes the outcome of the reaction. This method of bond formation, while not the most widely applied for the synthesis of metal alkyls, is nevertheless significant because it is often a key step in transition metal-catalyzed reactions of olefins such as Ziegler-Natta polymerization, hydrogenation, carbonylation, dimerization, and isomerization. 

The polymerization of cycloolefins in the presence of Ziegler-Natta catalysts generally involves the main steps known for this type of reaction from work with acyclic olefins [198] (e.g., cycloolefin coordination to the metal center, monomer insertion into the metal-carbon bond, chain termination, and reaction transfer) ... 

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