Polymerization heterogeneous stereospecific

Phillips in the United States developed heterogeneous catalysts based on Mo and Cr that also produced linear polyethylene under mild conditions. 

Polymerization with Zie er-Natta catalysts is thought to occur at active sites formed by interaction of the metal alkyl with metal chloride on the surface of the metal chloride crystals. Monomer is chemisorbed at the site (thus accounting for its specific orientation when added to the chain), and propagation occurs by insertion of the chemisorbed monomer into the metal-chain bond at the active site. 

The chain thus grows out from the site like a hair from the scalp. 

Hydrogen is used as a chain-transfer agent in these reactions  

Chain transfer with the metal alkyl component in 25e er-Natta systems has been identified  

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Heterogeneous reaction, 25 728-729 Heterogeneous reactors, 27 568 Heterogeneous stereospecific polymerization, 20 410-411 Heterogeneous vapor-phase fluorination, 22 863... 

The stereospecific polymerization of alkenes is catalyzed by coordination compounds such as Ziegler-Natta catalysts, which are heterogeneous TiCl —AI alkyl complexes. Cobalt carbonyl is a catalyst for the polymerization of monoepoxides several rhodium and iridium coordination compounds... 

He was a Professor of Industrial Chemistry, School of Engineering, Polytechnic Institute of Milan, Milan, Italy since 1937. He became involved with applied research, which led to the production of synthetic rubber in Italy, at the Institute in 1938. He was also interested in the synthesis of petrochemicals such as butadiene and, later, oxo alcohols. At the same time he made important contributions to the understanding of the kinetics of some catalytic processes in both the heterogeneous (methanol synthesis) and homogeneous (oxosynthesis) phase. In 1950, as a result of his interest in petrochemistry, he initiated the research on the use of simple olefins for the synthesis of high polymers. This work led to the discovery, in 1954, of stereospecific polymerization. In this type of polymerization nonsymmetric monomers (e.g., propylene, 1-butene, etc.) produce linear high polymers with a stereoregular structure. 

Considerable interest has been shown in the new processes of stereospecific polymerization, not only so far as they concern the production of new classes of polymers, having unusual characteristics and improved properties, but also because they are representative of a peculiar new type of heterogeneous catalysis, of great interest from the practical and the theoretical points of view 1-5). 

The stereospecific polymerization of a-olefins takes place only in the presence of heterogeneous catalytic systems, including a crystalline substrate (formed by halides of transition metals, such as TiCb, TiCb, VCI3, CrCla, C0CI2, etc.) and a suitable metallorganic compound (5). 

Stereospecific Polymerization. In the early 1950s, Ziegler observed that certain heterogeneous catalysts based on transition metals polymerized ethylene to a linear, high density material at modest pressures and temperatures. N atta showed that these catalysts also could produce highly stereospecific poly-a-olefins, notably isotactic polypropylene, and polydienes. They shared the 1963 Nobel Prize in chemistry for their work. More recently, metallocene catalysts that provide even greater control of molecular structure have been introduced. 

A final example of a stereoselective heterogeneous catalytic system is the work of Laycock, Collacott, Skelton and Tchir.17 Layered double hydroxide (LDH) synthetic hydrotalcite materials were used to stereospecifically polymerize propylene oxide [PO] to crystalline isotactic and liquid atactic poly(propyleneoxide) [PPO]. These authors suggest that the LDH surface acts as other inorganic or organometallic coordination initiators or catalysts by providing specific surface orientations for propylene oxide monomer. X-ray powder diffraction showed some loss of crystallinity after calcination and X-ray photoelectron spectroscopy showed an enhancement of Mg/Al content due to restructuring of the Mg and A1 surface atoms. The surface was also rich in Cl ... 

The existence of such associated organolithium compounds has been estabhshed in various cases (19, 20, 24), In addition to isotactic polystyrene, a considerable amoimt of atactic material is always present it is formed by starting the polymerization on the nonassociated part of the organolithium compounds which probably promote a nonstereospecific anionic polymerization. The stereoregulation of the polymerization of styrene by heterogeneous alkali metal aUcyl initiators is limited by the forces on the surface of the catalyst while the dissolved organolithium initiators in their associated form cause the stereospecific polymerization. 

At the present time, the most likely concept of the mechanism of a heterogeneous polymerization catalyzed by a Ziegler-Natta catalyst involves a complex in which the organometallic component and the transition metal component—i.e., the A1 and Ti atoms—are joined by electron-deficient bonds. Natta, Corradini, and Bassi (13) have reported such a structure for the active catalyst prepared from bis (cyclopentadienyl) titanium dichloride and aluminum triethyl. Natta and Pasquon (14), Patat and Sinn (18), and Furukawa and Tsuruta (2) have proposed mechanisms for the stereospecific polymerization of a-olefins in terms of such electron-deficient complexes. 

Stereospecific Polymerization. It was found quite early in the development of Ziegler-Natta catalysts that stereospecific polymerization usually requires the presence of a crystalline catalyst surface—i.e, heterogeneous catalysis. Most recently, this has been confirmed in the careful examination of catalyst surfaces by Rodriguez, van Looy, and Gabant (35). It must be remembered, however, that this work dealt largely with a-olefins. 

Other types of complex catalysts that have received attention for stereospecific polymerization are the reduced metal oxides and the alfin catalysts (prepared from compounds of sodium). All three types are mainly used in heterogeneous polymerization, although some homogeneous processes are also commercially important. 

Unlike heterogeneous Ziegler-Natta catalysts, homogeneous matallocene catalysts often produce only low molecular weight polymers, especially in stereospecific polymerizations. The molecular weight is given by... 

Natta (194) discovered the phenomenon of stereospecific polymerization responsible for the formation of valuable isotactic polymers of a-olefins in the presence of heterogeneous catalysts and of activators and explained it by oriented adsorption. However, the latter is an obligatory condition of the multiplet theory, according to which the reacting atoms come into contact with the sui face, and the substituents must be oriented in one direction, namely, off the surface (195). According to Natta, the electronic properties also play an important part in stereo-specific polymerization. 

Hence, there can be four stereospecific polymerization mechanisms in primary polyinsertion, all of which have been documented with metallocene catalysts (Scheme 13) the two originated by the chiralities of the catalyst active sites, referred to as enantiomorphic site control (isospecific and syndio-specific site control), can be relatively strong, with differences in activation energy (AA. ) for the insertion of the two enantiofaces up to 5 kcal/mol. A value of 4.8 kcal/mol has been found by Zambelli and Bovey for a Ti-based heterogeneous catalyst. 

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