Stereospecific polymerizations heterogeneous catalysts

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. Natta 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. 

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... 

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. 

It should be noted that the monomer coordination step shown in Eq. (2.82) may not be a distinct step as discussed previously. An important feature of this mechanism which affects the stereospecificity of olefin polymerizations using these types of soluble catalysts is the fact that the insertion of the monomer into the transition metal-carbon bond involves a secondary insertion reaction, i.e., the more substituted carbon of the double bond in the monomer becomes bonded to the transition metal (Corradini et al., 1985). In contrast, a primary insertion mechanism to form a transition metal bond to the less substituted carbon on the double bond of the monomer Ti-CH2CHR-P is involved in polymerizations using typical heterogeneous catalysts, e.g., from titanium halides and alkylaluminum compounds (Boor, 1979). 

A different mechanism, however, was offered by Nakano and co-workers. They felt that there must be a relationship between the crystal structures of the heterogeneous catalysts and the resultant stereoregularity of the polymers. If the crystal structures of the catalysts are tetrahedral and the crystals have active edges, stereoregular polymers should form even at room temperature. In addition, shorter active edges make the catalysts more suitable for stereospecific polymerization. The following mechanism was therefore proposed. ... 

Conclusions on the stereospecificity of the catalyst cannot be drawn in a simple way from the aggregate state, but heterogeneous catalyst systems seem to be necessary for the polymerization of isotactic poly(a-oleflns). Conversely, syndiotactic poly(propylene) has so far only been produced with a homogeneous catalyst system. Other syndiotactic poly (a-olefins) are unknown. 

Corradini, P Busico, V. Cavallo, L. Guerra, G Vacatello, M. Venditto, V. Structural analogies between homogeneous and heterogeneous catalysts for the stereospecific polymerization of 1 -aUcenes. J. Mol. Catal. 1992, 74, 433-442 and references therein. 

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