Olefin polymerization centers

The propagation centers of the catalysts of olefin polymerization contain the active transition metal-carbon olefin polymerization may be divided into two vast classes according to the method of formation of the propagation center two-component and one-component.1... 

In the propagation centers of chromium oxide catalysts as well as in other catalysts of olefin polymerization the growth of a polymer chain proceeds as olefin insertion into the transition metal-carbon tr-bond. Krauss (70) stated that he succeeded in isolating, in methanol solution from the... 

In essence the active centers for catalytic polymerization of olefins are organometallic complexes of transition metals. For this reason a search for individual organometallic compounds that would possess catalytic activity in olefin polymerization is of great interest. The first attempts to use organometallic compounds of transition metals as catalysts for olefin polymerization were made long ago [e.g. CH3TiCl3 as a catalyst for polymerization of ethylene 116). However, only in recent years as a result of the application of relatively stable organometallic compounds of transition... 

To determine the number of propagation centers in one-component catalysts, in principle the same methods used to study two-component catalysts of olefin polymerization may be applied Qsee (18, 160, 160a) ]. The most widely used methods for the determination of the number of propagation centers in polymerization catalysts are ... 

The conclusion may be drawn that the data obtained of comparative studies of olefin polymerization by the one-component catalyst (TiCl2) and two-component systems (TiCl2 + AlEtxCl ) confirm the concept of monometallic active centers on the surface of titanium chlorides developed by Cossee and Arlman (170-173). 

Despite the difference in composition of various olefin polymerization catalysts the problems of the mechanism of their action have much in common. The difference between one-component and traditional Ziegler-Natta two-component catalysts seems to exist only at the stage of genesis of the propagation centers, while the mechanism of the formation of a polymer chain on the propagation center formed has many common basic features for all the catalytic systems based on transition metal compounds. 

Two possible reasons may be noted by which just the coordinatively insufficient ions of the low oxidation state are necessary to provide the catalytic activity in olefin polymerization. First, the formation of the transition metal-carbon bond in the case of one-component catalysts seems to be realized through the oxidative addition of olefin to the transition metal ion that should possess the ability for a concurrent increase of degree of oxidation and coordination number (177). Second, a strong enough interaction of the monomer with the propagation center resulting in monomer activation is possible by 7r-back-donation of electrons into the antibonding orbitals of olefin that may take place only with the participation of low-valency ions of the transition metal in the formation of intermediate 71-complexes. 

Unfortunately, at present the information characterizing the properties of the active bond in polymerization catalysts is very scant. The analogy between the features of the active bonds in the propagation centers and those of the transition metal-carbon bond in individual organometallic compounds is sure to exist, but as in the initial form the latter do not show catalytic activity in olefin polymerization this analogy is restricted to its limits. 

The Phillips Cr/silica catalyst is prepared by impregnating a chromium compound (commonly chromic acid) onto a support material, most commonly a wide-pore silica, and then calcining in oxygen at 923 K. In the industrial process, the formation of the propagation centers takes place by reductive interaction of Cr(VI) with the monomer (ethylene) at about 423 K [4]. This feature makes the Phillips catalyst unique among all the olefin polymerization catalysts, but also the most controversial one [17]. 

When combined with the isolation and reactivity studies of the patterned aminosilica (7), the increased activity of the patterned catalysts provide further evidence that the patterning technique developed allows for the synthesis of aminosilicas which behave like isolated, single-site materials (although a true single site nature has not been proven). As the olefin polymerization catalysts supported by the patterned materials show a marked improvement over those materials supported on traditional aminosilicas, these patterned materials should be able to improve supported small molecular catalysis as well. Future improvements in catalysis with immobilized molecular active sites could be realized if this methodology is adopted to prepare new catalysts with isolated, well-defined, single-site active centers. 

Since group 3 metallocene alkyls are isoelectronic with the cationic alkyls of group 4 catalysts they may be used as olefin polymerization initiators without the need for cocatalysts. The neutral metal center typically results in much lower activities, and detailed mechanistic studies on the insertion process have therefore proved possible.216-220 Among the first group 3 catalysts reported to show moderate activities (42 gmmol-1 h-1bar-1) was the yttrocene complex (77).221... 

As a typical case, olefin-metal complexation is described first. Alkene complexes of d° transition metals or ions have no d-electron available for the 7i-back donation, and thus their metal-alkene bonding is too weak for them to be isolated and characterized. One exception is CpfYCH2CH2C(CH3)2CH=CH2 (1), in which an intramolecular bonding interaction between a terminal olefinic moiety and a metal center is observed. However, this complex is thermally unstable above — 50 °C [11]. The MO calculation proves the presence of the weak metal-alkene bonding during the propagation step of the olefin polymerization [12,13]. 

With these features in mind, we envisioned a new family of macrocyclic ligands for olefin polymerization catalysis (Fig. 9) [131, 132], We utilized macrocycles as the ligand framework and installed the catalytic metal center in the core of the macrocycles. Appropriate intra-annular binding sites are introduced into cyclophane framework that not only match the coordination geometry of a chosen metal but also provide the appropriate electronic donation to metal center. The cyclophane framework would provide a microenvironment to shield the catalytic center from all angles, but leaving two cis coordination sites open in the front one for monomer coordination and the other for the growing polymer chain. This could potentially protect the catalytic center and prevent it from decomposition or vulnerable side reactions. 

Mejzlik, J., P. Vozka, J. Kratochvila, and M. Lesna, Recent Developments in the Determination of Active Centers in Olefin Polymerization, pp. 79—90 in Transition Metals and Organometallics as Catalysts for Olefin Polymerization, W. Kaminsky and H. Sinn, eds., Springer-Verlag, Berlin, 1988. 

A further modification of the active-center model was based on the consideration that the vacancy in the active center is strongly shielded by the polymer chain, mainly by the CH2 and CH3 groups of the second monomer unit. As a result, the vacant site is blocked and inaccessible for olefin coordination. Kissin et al. suggest a polymerization center with two vacancies one shielded and the other open for complexation.344,345 After each insertion step the end of the growing polymer chain flips from side to side and the two vacant sites are alternately available for alkene coordination. 

The use of late transition metals as olefin polymerization catalyst requires the suppression of chain transfer while at the same time a high chain growth rate should be maintained. These new catalysts have an electron-deficient, in most cases, 14-electron and cationic metal center with a vacant coordination site. The most... 

A number of structures were hypothesized for the catalytic active centers in the organometallic catalysts for the a-olefins polymerization. 

It is now clear that, when propagation centers are formed, olefin polymerization by all solid catalysts (including the Phillips Petroleum catalyst from chromium deposited on oxides, and the Standard Oil catalyst of molybdenum oxide on aluminum oxide) essentially follows the same mechanism chain growth through monomer insertion into the transition-metal-carbon bond, with precoordination of the monomer. Interestingly,... 

Yu. I. Yermakov and V. A. Zakharov, The Number of Propagation Centers in Solid Catalysts for Olefin Polymerization and Some Aspects of Mechanism of Their Action, in Ref. 9, p. 91. 

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