Cossee-Arlman theory

The basis of the Cossee-Arlman theory is the concept of a chloride vacancy on the catalyst surface. This idea was first suggested by Boor in a private communication (Arlman, 1964). At the vacancy, monomer is adsorbed on the surface. According to the theory, not all titanium ions are active because most of them are fully coordinated in the octahedral interstices of the crystal. The active sites are found only on certain surfaces of a titanium trichloride crystal where vacancies exist. These arise because in order to have electrical neutrality some titanium ions must be coordinated to fewer than six chloride ions. The above interpretation is consistent with microscopic observations that poisoner growth occurs at boundaries defined by edges or steps of platelets (Hargitay et al., 1959 Rodriguez and Gabant, 1963). 

The Cossee-Arlman theory was presented in three parts. The first dealt with the reasons for activity (Cossee, 1964), the second... 

Natta and a great many others have described the addition step as anionic coordinated. The term anionic is used because, for example, the reaction of tritiated alcohol with the system gives radioactive polymer. Mechanistically, the term anionic seems plausible because the polarization of the carbon-titanium bond should favor a partial negative charge on carbon. The Cossee-Arlman theory suggests, however, that the electronic rearrange-... 

Many workers have attempted to account for the role of the organometallic component by postulating a bimetallic complex. As outlined above, this is not necessary. Furthermore, aluminum-27 NMR studies by DiCarlo and Swift (1964) are more consistent with the Cossee-Arlman theory than with the bimetallic complex hypothesis. 

The Cossee-Arlman theory requires attack from the same side of the molecule as well as cis opening of the double bond. Miya-zawa and Ideguchi (1963) have showed this experimentally. From cis-l-deuteriopropylene they obtained erytfero-diisotactic poly (propylene-1 di) and from tra s-l-deuteriopropylene, threo-diisotactic poly (propylene-ldi). 

The Cossee-Arlman mechanism for the polymerization of olefins is the most widely accepted theory but as yet it is not complete. Cossee developed his early ideas of polyethylene growth at a titanium-carbon bond and supported the theory by molecular orbital calculations. The role of the alkyl aluminium co-catalyst was in the generation of the active species, via the alkylation of the titanium chloride bonds, and to remove impurities in both the gas stream and catalyst preparative procedure. There was also the suggestion that it might be involved in the insertion of each monomer molecule, and also in the regeneration of dormant sites or the formation of new active sites. 

A guide to the manner in which structural theory may be applied to a detailed consideration of the mechanism of a surface-catalyzed reaction is found in papers by Cossee (113), Arlman (114), and Arlman and Cossee (115) concerning the mechanism of the stereoregular heterogeneous catalyzed polymerization of propylene. Particular crystallographic sites are shown to be the active centers at which the reactants combine and ligand field theory is used to demonstrate a plausible relationship between the activation energy for the conversion of adsorbed reactants to the product and the properties of the transition metal complex which constitutes the reaction center. 

Many attempts have been made to explain the stereoregular polymerization of a-olefins. An excellent discussion of the various hypotheses advanced prior to 1962 has been published by Bawn and Ledwith (1962). No effort will be made here to review different mechanistic schemes which in this writer s opinion are superseded by the theory advanced by Cossee (1964) and Arlman (1964). 

It is generally accepted that the d-orbitals in the transition element are the main sonrce of catalytic activity and that it is the Ti-alkyl bond that acts as the polymerization center where chain growth occurs. The fimction of the aliuninum alkyl is thus only to alkylate TiQs. The monometallic mechanism described below is the one based mainly on the ideas of Cossee and Arlman (1964). The quantum theory developed by Cossee (1%7) provided a sound theoretical basis for this mechanism. 

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