Metallocenes stereospecificity

Catalysts which bring about a spatially regular (stereoregular) configuration in high polymers. The main types are Ziegler, Phillips, Standard Oil and Metallocene Stereospecific Polymers... 

Polymerization Ziegler-Natta, supported metal oxides such as PhiUps, Unipol and metallocene Stereospecific chain formation required weak interaction/complexation with any initiators/catalysts metal coordination complexes required... 

Sumi K, Kumobayashi H (2004) Rhodium/Ruthenium Applications. 6 63-96 Suzuki N (2005) Stereospecific Olefin Polymerization Catalyzed by Metallocene Complexes. 8 177-215... 

The molecular design of stereospecific homogeneous catalysts for polymerization and oligomerization has now reached a practical stage, which is the result of the rapid developments in early transition metal organometallic chemistry in this decade. In fact, Exxon and Dow are already producing polyethylene commercially with the help of metallocene catalysts. Compared to the polymerization of a-olefins, the polymerization of polar vinyl, alkynyl and cyclic monomers seems to be less developed. 

Also in the 1980s, the discovery of homogeneous stereospecific catalysts for the polymerization of 1-alkenes has opened up new prospects for research on stereospecific polymerization and stereoregular polyolefins. Ewen and coworkers79 achieved this discovery on the basis of earlier research on metallocenes in combination with alkyl-Al-oxanes by Sinn and Kaminsky.10... 

Section 3 will deal with catalytic systems whose stereospecificity is mainly controlled by the chirality of the environment of the transition metal, independently of the possible chirality of the growing chain (chiral site stereocontrol). In particular, in Section 3.1 the chirality and stereospecificity of homogeneous catalytic systems based on metallocenes of different symmetries and in different experimental conditions will be reviewed. In Section 3.2 the chirality of model catalytic sites, which have been supposed for isospecific first-generation TiCl3-based and high-yield MgC -supported catalysts, is described. In Section 3.3 we will present a comparison between model catalytic sites proposed for heterogeneous and homogeneous stereospecific site-controlled catalysts. 

Syndiospecific catalytic systems based on metallocenes are highly regioreg-ular. As a consequence, their stereoselectivity in possible regioirregular insertions has been experimentally investigated for propene copolymers only.78,79 However, an analysis of the stereoselectivity of possible secondary propene insertions on syndiospecific catalytic models based on -symmetric metallocenes is reported here, also due to its relevance to the rationalization of the dependence of regiospecificity on the type of stereospecificity (see Section 3.1.4.1).80... 

Corradini et al. examined in some detail by molecular mechanics15 and density functional studies100 the polymerization mechanism proposed by Cossee and the catalytic sites on TiC surfaces, including those proposed by Arl-man and Cossee13 and by Allegra.14 According to the calculations, for all these octahedral active sites a similar general mechanism of stereoselectivity occurs which is very similar to the one established several years later for stereospecific metallocenes (see previous section). The chirality of the site would determine a chiral orientation of the first C-C bond of the chain (for a A site,... 

Nonbonded energy interactions are able to rationalize not only the stereospecificities observed for different metallocene-based catalytic systems (isospecific, syndiospecific, hemi-isospecific, and with oscillating stereocontrol) but also the origin of particular stereodefects and their dependence on monomer concentration as well as stereostructures associated with regioirregular insertions. Nonbonded energy analysis also allowed to rationalize the dependence of regiospecificity on the type of stereospecificity of metallocene-based catalysts. 

Group 4 metallocenes are important as catalysts in the stereospecific polymerization of alkenes. Consequently, alkenes have been the most extensively studied substrates in reactions with metallocenes. 

Figure 12. Scheme of stereospecific 1-olefins polymerization with generic C2 and Cs symmetric metallocenes. In the framework of a regular chain migratory mechanism, the C2 and Cs symmetric catalysts lead to iso- and syndiotactic polymers, respectively. In fact, multiple insertions of the same enantioface occur with C2 symmetric metallocenes, while multiple insertions of alternating enantiofaces occur with Cs metallocenes. 

The initiation of polymerizations by metal-containing catalysts broadens the synthetic possibilities significantly. In many cases it is the only useful method to polymerize certain kinds of monomers or to polymerize them in a stereospecific way. Examples for metal-containing catalysts are chromium oxide-containing catalysts (Phillips-Catalysts) for ethylene polymerization, metal organic coordination catalysts (Ziegler-Natta catalysts) for the polymerization of ethylene, a-olefins and dienes (see Sect. 3.3.1), palladium catalysts and the metallocene catalysts (see Sect. 3.3.2) that initiate not only the polymerization of (cyclo)olefins and dienes but also of some polar monomers. 

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. 

The breakthrough in metallocene catalyst development occurred in the early 1980s when a metallocene catalyst, instead of an aluminium alkyl, was combined with methylaluminoxane (MAO) [8, 9, 10]. This catalyst system boosted the activity of metallocene-based catalyst and produced uniform polyethene with the narrow molar mass distribution typical for single-site catalysts. Efforts to polymerise propene failed, however the product was found to be fully atactic, indicating complete lack of stereospecificity of the catalyst [10]. 

With conventional polymerization processes, atactic chains are predominantly formed for the formation of isotactic and syndiotactic chains a special catalyst system is required, e.g. Ziegler-Natta catalysts. Such a process is called stereospecific polymerization. It enables the manufacture of, i.a., technically usable PP and also unbranched PE (see 4.1). The newest development is the metallocene katalyst it enables the building-up of chains-to-measure with very high degrees of chain regularity also the manufacture of syndiotactic polystyrene is technically possible in this way (see Qu. 2.47). 

Brintzinger, H. H., Fischer, D., Miilhaupt, R., Rieger, B. and Waymouth, R. M., Stereospecific Olefin Polymerisation with Chiral Metallocene Catalysts , Angew. Chem., Int. Ed. Engl., 34, 1143-1170 (1995). 

During the last decade, a variety of new catalysts have been presented for the stereospecific polymerisation of a-olefins, based on non-bridged metallocene or stereorigid ansa-metallocene as the procatalyst and a methylaluminoxane activator [29,30,37,105-107,112-114,116-135], Apart from isotactic [118,119,124, 131,132] and syndiotactic [23,118,124,133] polypropylenes and other poly(a-olefin)s [121], hemiisotactic [112,121,124], isoblock [131,132,134], syndioiso-block (stereocopolymer) [127], stereoblock isotactic [135] and stereoblock isotactic atactic [116,128,129] polypropylenes have been obtained using these new catalysts. 

Dimeric homochirotopic [rac.-Me2Si(Me3Si, t-BuCp)2YH]2 is the first singlecomponent isospecific catalyst it is suitable to study the subtle steric factors that govern the remarkably high stereospecificities exhibited in the polymerisation of a-olefins by this and related two-component class III catalysts of C2 symmetry based on group 4 metallocenes. 

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