Olefin polymerization stereospecificity

Ziegler-Natta polymerization Stereospecific polymerization of olefines using a Ziegler catalyst. See titanium(IIl) chloride. 

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 course of stereospecific olefin polymerization was studied by using the molecular mechanics programs, MM-2 and Biograph, based on the optimized geometries of the ethylene complex and the transition state [13,203]. Interestingly, the steric interaction at the transition state mainly controls the stereochemistry in polymerization, which proceeds specifically isotactic or syndiotactic depending on the kind of catalyst. 

Moreover, the molecular catalysts have provided systematic opportunities to study the mechanisms of the initiation, propagation, and termination steps of coordination polymerization and the mechanisms of stereospecific polymerization. This has significantly contributed to advances in the rational design of catalysts for the controlled (co)polymerization of olefinic monomers. Altogether, the development of high performance molecular catalysts has made a dramatic impact on polymer synthesis and catalysis chemistry. There is thus great interest in the development of new molecular catalysts for olefin polymerization with a view to achieving unique catalysis and distinctive polymer synthesis. 

In Section 2 a brief description of the generally assumed polymerization mechanism and the elements of chirality for the stereospecific olefin polymerization is presented. 

A necessary (but not sufficient) prerequisite for models of catalysts for the stereospecific polymerization of 1-olefins polymerization, is the stereoselectivity of each monomer insertion step. The possible origin of stereoselectivity in this class of systems was investigated through simple molecular mechanics calculations [11, 14, 24, 32, 52, 78-80, 82-86]. 

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. 

As already stated in the first chapter, several catalytic systems show a certain stereospecificity in the a-olefin polymerization. 

The behavior of the different catalytic systems (containing transition metal crystalline compounds) in the a-olefin polymerization, except for the different degree of stereospecificity, may be connected with a definite kinetic scheme. This was shown by experimental work performed at the Institute of Industrial Chemistry of the Milan Polytechnic. 

A very important field of polymerization, stereospecific polymerization, was opened in 1955. In this year, Natta and his coworkers (1—3) polymerized a-olefins to crystalline isotactic poly-a-olefins with the Ziegler catalyst, and Pruitt and Baggett (4,5) polymerized dl-propylene oxide to crystalline polypropylene oxide, which was later identified as an isotactic polymer by Price and his coworkers (6,7). Since then, a large number of compounds including both unsaturated and cyclic compounds were polymerized stereospecifically and asymmetrically. Development of the stereospecific polymerization stimulated... 

Catalytic reactivity and stereospecificity of organometallic compounds in olefin polymerization. J. Polymer Sci. 36, 275—286 (1959). 

Brintzinger HH, Fischer D, Miilhaupt R, Rieger B, Waymouth RM, Stereospecific Olefin Polymerization with Chiral Metallocene Catalysts, Angew Chem Int Ed Engl 34, 1143 (1995), and refs Therein... 

Danusso s conclusion that the monomer is only weakly polarized in the complex must certainly be true for the stereospecific coordinated anionic catalysts, such as AlR3/TiCl3, which are used for a-olefin polymerization. If it were not so, then rapid termination would take place through hydride abstraction by the strongly polarized monomer. This appears to be the case when strongly acidic alkyl metal chlorides are used in catalysts. For example, low molecular weight polypropylene oils are obtained with RAlCl2/TiCla catalyst (308) and polyethylene oils are obtained with RjA Cla/TiC catalysts in reactive alkyl halide solvents (309). 

The different behavior of the catalysts apparently arises from the nature of the transition metal of the catalyst. It seems reasonable to treat the mechanism of stereospecific olefin polymerization in terms of coordination ionic catalysts, regarding the valence state, coordination number, and nature of ligands of the transition metal as a matter of primary importance. In such an approach the polymerization mechanism is based on the character of metal—carbon bond by which a growing polymer chain is linked to the transition metal. 

Doubly bridged ansa-titanium complexes are considered as a new class of stereorigid catalysts for stereospecific a-olefin polymerization. Doubly silyl bridged -complexes are considerably more strained than their singly bridged analogs. 

Rioka, M. Tsutsui, T. Ueda, T. Rashiwa, N. Stereospecific Polymerization of a-Olefin with an Ethylene Bis(l-indenyl)hafnium Dichloride and Methyl-aluminoxane Catalyst System. In Catalytic Olefin Polymerization, Studies in Surface Science and Catalysis-, Reii, T., Soga, R., Eds. Elsevier New York, 1990, p 483. 

Brintzinger, H.H. Fischer, D. Mulhaupt, R. Rieger, B. Waymouth, R.M. Stereospecific olefin polymerization with chiral metallocene catalysts. Angew. Chem. Int. Ed. Engl. 1995, 34, 1143. 

Cationic metallocene-alkyl complexes are attracting much interest because of their catalytic activity for a-olefin polymerization [47,48]. The most convenient method for generating the ion pairs is the reaction of metallocene dialkyl complexes of zirconium and hafnium with B(C6F5)3, forming cationic complexes [Cp2MR] [RB(C6F5)3]" (M = Zr, Hf) [49,50]. ansa-TypQ cationic compounds have been applied in the stereospecific polymerization of a-olefins [51]. 

Some other contributions of organometallic compounds to fundamental research are (a) the detection of free alkyl radicals by the pyrolysis of lead alkyls (b) the classification of hydrocarbon acidity via organoalkali compounds (c) the study of Lewis acid-base interactions with Group III alkyls (d) the development of the concept of electron-deficient compounds by the study of metal alkyls (e) the discovery of stereospecific olefin polymerization and (f) the investigation of nucleophilic additions to unsaturated organic compounds via reactive metal alkyls. 

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