Ziegler-Natta polymerization systems

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. 

Certain Ziegler-Natta polymerization systems showing stable kinetic ratetime profiles of the type (a) in Fig. 9.6 agree well with Eq. (9.39) in rate behavior. Describe how kinetic experiments would be planned and the results analyzed in order to derive the kinetic parameters of this equation. 

Keaton, R. J. Jayaratne, K. C. Fettinger, J. C. Sita, L. R. Structural characterization of zirconium cations derived from a living Ziegler-Natta polymerization system New insights regarding propagation and termination pathways for homogeneous catalysts. J. Am. Chem. Soc. 2000, 122, 12909-12910. 

Coordination polymerization of isoprene using Ziegler-Natta catalyst systems (Section 6 21) gives a material similar in properties to natural rubber as does polymerization of 1 3 butadiene Poly(1 3 buta diene) is produced in about two thirds the quantity of SBR each year It too finds its principal use in tires... 

On the basis of these observations, criticize or defend the following proposition Regardless of the monomer used, zero-order Markov (Bernoulli) statistics apply to all free radical, anionic, and cationic polymerizations, but not to Ziegler-Natta catalyzed systems. 

Polyacetylenes. The first report of the synthesis of a strong, flexible, free-standing film of the simplest conjugated polymer, polyacetylene [26571-64-2] (CH), was made in 1974 (16). The process, known as the Shirakawa technique, involves polymerization of acetylene on a thin-film coating of a heterogeneous Ziegler-Natta initiator system in a glass reactor, as shown in equation 1. 

Before the development of the Ziegler-Natta catalyst systems (Section 6.21), polymerization of propene was not a reaction of much value. The reason for this has a stereochemical basis. Consider a section of polypropylene ... 

By performing excellent model reactions [144], Grubbs and his co-workers demonstrated direct olefin insertion into an M-C bond. Thus, complex 115 was treated with AlEtCl2 to give complex 116, whose decomposition afforded methylcyclopentane. Under the same conditions, the polymerization of ethylene took place. In this way, the insertion of a-olefins into a Ti-C single bond in a model Ziegler-Natta catalyst system was directly observed (Eq. 9). 

Various mechanisms have been proposed to explain the stereoselectivity of Ziegler-Natta initiators [Boor, 1979 Carrick, 1973 Corradini et al., 1989 Cossee, 1967 Ketley, 1967a,b Tait and Watkins, 1989 Zambelli and Tosi, 1974]. Most mechanisms contain considerable details that distinguish them from each other but usually cannot be verified. In this section the mechanistic features of Ziegler-Natta polymerizations are considered with emphasis on those features that hold for most initiator systems. The major interest will be on the titanium-aluminum systems for isoselective polymerization, more specifically, TiCl3 with A1(C2H5)2C1 and TiCLt with A1(C2H5)3—probably the most widely studied systems, and certainly the most important systems for industrial polymerizations. 

Zambelli, A. and Tosi, C. Stereochemistry of Propylene Polymerization. Vol. 15, pp. 31-60. Zucchini, U. and Cecchin, G. Control of Molecular-Weight Distribution in Polyolefins Synthesized with Ziegler-Natta Catalytic Systems. Vol. 51, pp. 101-154. 

Catalysts. Iodine and its compounds are very active catalysts for many reactions (133). The principal use is in the production of synthetic rubber via Ziegler-Natta catalysts systems. Also, iodine and certain iodides, eg, titanium tetraiodide [7720-834], are employed for producing stereospecific polymers, such as polybutadiene rubber (134) about 75% of the iodine consumed in catalysts is assumed to be used for polybutadiene and polyisoprene polymerization (66) (see Rubber CHEMICALS). Hydrogen iodide is used as a catalyst in the manufacture of acetic acid from methanol (66). A 99% yield as acetic acid has been reported. In the heat stabilization of nylon suitable for tire cordage, iodine is used in a system involving copper acetate or borate, and potassium iodide (66) (see Tire cords). 

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... 

The reactions between TiCl4 and AlMe3 are of particular interest in studies of olefin-polymerization catalysts as a Ziegler-Natta catalyst system.14 The reaction between a surface TiClx group and AlMe3 gas is known to give an active olefin polymerization catalyst.1... 

In discussions about the nature of the active species in the polymerization of dienes by Ziegler/Natta catalyst systems allyl species have already been suggested in the 1960s [273-278]. This discussion has continued through the past decades [139,279-283]. Today, it is widely accepted that Nd-allyl-groups are the key element in the insertion of dienes into the Nd carbon bond. 

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