Ziegler-Natta polymerization intermediate

These results at least demonstrate that ethylene can be polymerized by an alkylidene hydride catalyst, probably by forming a metallacyclobutane hydride intermediate. The extent to which this is relevant to the more classical Ziegler-Natta polymerization systems (27) is unknown. Recent results in lutetium chemistry (28), where alkylidene hydride complexes are thought to be unlikely, provide strong evidence for the classical mechanism. 

Section 14.15 Coordination polymerization of ethylene and propene has the biggest economic impact of any organic chemical process. Ziegler-Natta polymerization is carried out using catalysts derived from transition metals such as titanium and zirconium. ir-Bonded and cr-bonded organometallic compounds are intermediates in coordination polymerization. 

An ESI mass spectrometer coupled online to a microreactor was used to intercept the catalytically active cationic intermediates of the Ziegler-Natta polymerization of ethylene with the homogeneous catalyst system [Cp2Zr(Me)Cl]-MAO (MAO = methylaluminoxane). For the first time these intermediates were studied directly in the solution and their catalytic activity proved.60... 

Over the last 50 years numerous reactions of organic compounds catalyzed by transition metal complexes have been developed (e. g., olefin oxidation -Wacker Process, hydroformylation, carbonylation, hydrogenation, metathesis, Ziegler-Natta polymerization and oligomerization of olefins) in which the reactivity of metal-carbon bonds in the active intermediate (organometallics) is crucial. 

Ziegler-Natta polymerization and hydrogenation catalyst. Also, it is used as intermediate in organic syntheses. 

Usually Ziegler-Natta polymerization has not been classified as a cationic polymerization. However, here we briefly touch on Ziegler-Natta polymerization using a microflow system because the polymerization involves cationic metal complex intermediates. 

As reported by Santos and Metzger [50], Ziegler-Natta polymerization can be carried out in a microflow system coupled directly to the ESI source of a Q-TOF mass spectrometer (Figure 14.12). In the first micromixer, catalyst (Cp2ZrCl2-MAO) and monomer solutions are mixed continuously to initiate the polymerization. The polymerization occurs in the microtube reactor. The solution thus obtained is introduced into the second micromixer M2, where the polymerization is quenched by acetonitrile. The quenched solution is fed directly into the ESI source. The transient cationic species can be characterized by mass spectrometry. This is the first case where an alkyl zirconium cation intermediate in the homogeneous Ziegler-Natta polymerization of ethylene is detected directly. 

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. 

Alkenes. —Reviews on Ziegler-Natta catalysis and the stereoregular and sequence-regular polymerization of butadiene have been published and the stereoselective oligomerizations of isoprene by lithium and palladium catalysts have been compared. Semi-empirical MO calculations suggest that Ziegler-Natta polymerization proceeds via a bis-alkene complex and a metallacyclo-pentane intermediate. ... 

For polar monomers, heterogeneity is seldom a requirement for isoselective polymerization with traditional Ziegler-Natta initiators syndiotactic polymers are obtained only with the soluble initiators. Styrene and 1,3-dienes are intermediate in behavior between the polar and nonpolar monomer. These monomers undergo isoselective polymerization with both homogeneous and heterogeneous traditional Ziegler-Natta initiators. 

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