Ziegler polymerizations mechanisms

Figure 2 shows the profile of the 27-29 ppm spectral region of three polymers which served as models (1J ) for ethylene propylene rubber. The better agreement between the observed spectrum and the five-parameter model strongly suggests the three-parameter model is less realistic as an explanation for the polymerization mechanism. Table VII compares the observed profiles of EPDM rubbers made with a Ziegler catalyst system. The ratio of... 

High magnetic fields and in particular C-NMR spectroscopy allow the analysis of even longer configurational sequences (tetrads up to nonads). This proved to be important in particular for the analysis of polyolefins like polypropylene or cycloolefin copolymers (COC). These polymers are available via transition-metal mediated (Ziegler-Natta, metallocene) insertion polymerizations, and the configurational analysis provides deep insight into the respective polymerization mechanisms as well as into the structure-property relationships. 

The polymerization mechanism with Ziegler-Natta catalysts can be explained as follows ... 

The Ziegler polymerizations of olefins (92, 5) and the aluminum (108), gallium, beryllium, and indium (ill) alkyl growth reactions also seem to be examples of olefin insertion reactions of metal-carbon compounds. Despite great effort concerning the mechanism of the polymerization reactions, relatively little has been learned about the actual catalytic species involved. 

Ziegler-Natta catalysts are not active at all in polymerization of disubstituted acetylenes.415 Mo- and W-based systems (for alkynes with small substituents) and Nb- and Ta-based catalysts (for alkynes with bulky groups), in turn, are very effective catalysts used to convert disubstituted acetylenes into polymers with very high molecular weight.414 415 A polymerization mechanism similar to that of metathesis polymerization of cycloalkenes are supported by most experimental observations.414 423 424... 

Styrene is a very versatile monomer. It can be polymerized by most types of polymerization mechanisms, e.g. free radical (FR), Ziegler-Natta (ZN), anionic, and cationic. Classical ZN polymerization of styrene yields isotactic polystyrene. However, if methylalumoxane (MAO) is added as a co-catalyst, syndiotactic polystyrene is formed. The resulting polymers formed using the various mechanisms of polymerization are summarized in Scheme 8.1. 

The subsequent polymerization mechanism should be analogous to that accepted for Ziegler-Natta reactions shown at the top of the following page. 

Cp2TiMe2 is activated by various borate salts to give catalytic systems for the polymerization of ethylene, propylene, and styrene. A conventional Ziegler-Natta coordination polymerization mechanism is proposed for ethylene and propylene polymerization, while a carbocationic polymerization mechanism has been suggested for styrene.1497... 

Staudinger, Hermann (1881-1965). Fundamental research on high-polymer structure, catalytic synthesis, polymerization mechanisms, resulting eventually in development of stereospecific catalysts by Ziegler and Natta (stereoregular polymers). Nobel Prize 1963. 

Ziegler-Natta Various types of Ziegler-Natta catalysts Polymerization mechanism and applications of molecular mechanics to the study of enantioselectivity of some stereospecific catalytic systems having chiral site stereocontrol Mechanism of enantioselectivity and its relevance to catalytic systems 56... 

The difference in polymerization mechanism between one-component metal oxide catalysts and traditional Ziegler-Natta two-component catalysts seems to exist only in the initiation stage, while the mechanism of continued propagation of polymer chain has many common features for all the catalyst systems based on transition metal compounds. Thus most studies of the chromium oxide catalyst system, for example, deal either with the nature... 

The mechanism by which Ziegler polymerizations proceed has been the subject of investigation and speculation ever since Ziegler s discovery. The numerous theories that have been proposed fall in general into two classes ... 

Fr . 1. Mechanisms of Ziegler polymerization (a) Bimetallic mechanism, (b) Monometallic mechanism [after Cossee]. (c) Monometallic Cossec mechanism (5). Crystal structure of a-TiC. la with active center in the surface. A propylene rnolocule is inserted in the Cl v acancy forming a 7r-bond with Ti ion A. Figures lb and Ic wore reprinted with the permission of the Faraday Society. 

The alkyl-lithium initiated, living anionic polymerization of elastomers was described in 1928 by Ziegler. To polymerize styrene-isoprene block copolymers Szwarc et al., [1956] used sodium naphthalene as an anion-radical di-initiator, while Shell used an organolithium initiator. The polymerization mechanism was described by By water [1965]. 

It was first observed by Woon (1974) and Farona (1974) that acetylenes could be polymerized by catalysts of the type Mo(CO)3(toluene). This was followed by the discovery that conventional metathesis catalysts such as M0CI5 (Masuda 1974) and WCls (Navarro 1976 Masuda 1976), with or without a cocatalyst, could also bring about polymerization of acetylenes. At first there was some doubt as to whether these polymerizations were being propagated by the metathesis mechanism (Scheme 10.2) or whether a Ziegler-Natta mechanism was operating. However, the observation that metal carbene complexes could react with acetylenic molecules to form simple adducts as in reaction (20) (Fischer, H. 1980), and the fact that such complexes could initiate the polymerization of acetylenes, albeit somewhat slowly, but cleanly and in fair yield, soon allayed these doubts. 

Solutions of complex compound cyclopentadienyl-dititana in hexane and sodium cyclopentadienyl complex have high catalytic activity. Films with a metallic luster can be obtained by slowly removing the solvent from the formed gel polyacetylene in vacuum. It is assumed that the active complex has a tetrahedral structure. Polymerization mechanism is similar to the mechanism of olefin polymerization on catalyst Ziegler-Natta. Obtained at -80°C cis-polyacetylene films after doping had a conductivity of 240 (Ohm-cm" ). 

Since the discovery of Ziegler-Natta catalyst, isotactic polypropylene has been widely used as a commodity material due to its low cost and excellent physical properties. As the modulus of the resin is closely related to isotacticity, an understanding in polymerization mechanism is important in the chemistry of propylene polymerization as well as in the industry. 

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