Ziegler-Natta polymerization chain termination reactions

Ziegler-Natta polymerization is characterized by a series of elementary reactions which can be represented by suitable models. A scheme of such reactions, as proposed by Grieveson and including Natta s original hypotheses as well, shows (Table 2) besides the initiation and propagation steps, the various possible types of chain transfer and termination processes, both in the presence and in the absence of... 

Ziegler-Natta polymerizations have the characteristics of living polymerization with regard to catalyst active sites but not individual propagating chains. Thus the propagating chains have lifetimes of seconds or minutes at most, while active sites have lifetimes of the order of hours or days. Each active site produces many polymer molecules. The termination of a polymer chain growing at an active center may occur by various reactions, as shown below with propylene as an example. 

On the basis of the nature of the initiation step, polymerization reactions of unsaturated hydrocarbons can be classified as cationic, anionic, and free-radical polymerization. Ziegler-Natta or coordination polymerization, though, which may be considered as an anionic polymerization, usually is treated separately. The further steps of the polymerization process (propagation, chain transfer, termination) similarly are characteristic of each type of polymerization. Since most unsaturated hydrocarbons capable of polymerization are of the structure of CH2=CHR, vinyl polymerization as a general term is often used. 

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)... 

Molecular hydrogen has been known for a long time as an effective chain-transfer agent in the coordination polymerization of ethylene and a-oiefms with Ziegler-Natta catalysts 99-101,50). The mechanism for the reaction of a growing polymer chain with H2 has not been established, The living coordination polymerization system is well suited for an elucidation of the mechanism, since the reaction with H2 can be studied independently of any interference from other chain-terminating processes. 

Additionally, if the initiation reaction is more rapid an the chain propagation, a very narrow molecular weight distribution, MJM = 1 (Poisson distribution), is obtained. Typically living character is shown by the anionic polymerization of butadiene and isoprene with the lithium alkyls [77, 78], but it has been found also in butadiene polymerization with allylneodymium compounds [49] and Ziegler-Natta catalysts containing titanium iodide [77]. On the other hand, the chain growth can be terminated by a chain transfer reaction with the monomer via /0-hydride elimination, as has already been mentioned above for the allylcobalt complex-catalyzed 1,2-polymerization of butadiene. 

Much effort has been devoted during the last 30 years toward understanding the mechanisms operative in the coordination catalysis of ethylene and a-olefin polymerization using Ziegler-Natta systems (metal halide and aluminum alkyl, sometimes with Lewis base modifiers). Aspects of the complex heterogeneous reactions have been elucidated (jL- ) but the intimate mechanistic detail - for example the role of inhibitors and promoters, kinetics and thermodynamics of chain growth, modes of chain transfer and termination - comes primarily from studies of homogeneous catalysts ... 

Metallocene d° complexes of the early metals, which are very important catalysts for olefin polymerization, are not known to form H2 complexes, but a Tire metallocene H2 complex is stable calculationally from Car-Parrinello ab initio molecular dynamics.154 This methodology155 can reveal novel reaction pathways that traditional static computations may overlook. An allyl H2complex computed to be stable in the gas phase at 300 is located as a secondary minimum in -elimination, the most common assumed unimolecular chain termination process in Ziegler-Natta ethylene polymerization. Static calculations show that the H2... 

High-temperature SEC finds wide application in polymerization studies, as the molecular mass distribution is an artefact of the various reactions involved in polymerization, initiation, termination, and transfer. It is diagnostic of living systems and random polymerization reactions, such as condensation and radical initiated polymerizations, for which the distributions are Poisson and normal respectively. In the polymerization of ethene and propene by Ziegler-Natta catalysts, the determination of the concentration of active centres as a function of conversion defines catalyst type. Similar studies have been made in the study of chain scission by thermal degradation or by irradiation, in defining the number of molecules produced from the inverse of the number average molecular mass and random chain scission eventually leads to a normal molecular mass distribution, with polydispersities close to 2.0. This has, of course, been widely used to produce narrow from broad molecular mass distribution samples prior to fractionation. 

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) ... 

In metallocene-catalyzed olefin polymerization, the propagation reaction is terminated usually by chain transfer. It is generally believed that three major chain-transfer reactions exist in homogeneous Ziegler-Natta catalysts (Scheme 3) [8] ... 

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