Olefin polymerization propagation

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... 

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 ... 

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]. 

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]. 

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. 

Chien JCW, Llinas GH, Rausch MD, Lin YG, Winter HH, Atwood JL, Bott SG (1992) Metallocene catalysts for olefin polymerizations. XXIV. Stereoblock propylene polymerization catalyzed by rac-anri -ethylidene(l-T 5-tetramethylcyclopentadienyl)(l-r 5-indenyl) dimethyltitanium A two-state propagation. J Polym Sci A 30 2601-2617... 

For example, the decomposition of a hydroperoxide to generate an alkoxy free radical can result in the reaction of the alkoxy radical with an olefin. A carbon radical then forms. Olefin chain propagation and polymerization can follow to yield high-molecular-weight deposits. 

If we consider as an example the addition of HC1 to ethylene, we find that whereas the propagation step for polymerization will be exothermic by about 30 kcal mole-1,146 abstraction of H from HC1 by the R—CH2- radical will be endothermic by 5 kcal mole-1. Activation energies for typical polymerization propagation steps are in the range of 6-10 kcal mole-1,147 and that for abstraction from HC1 will have to be greater than the 5 kcal mole-1 endothermicity. These data are at least indicative that radical addition of HC1 will not be favorable experimentally, it is indeed rare, but can be made to occur with excess HC1.148 With HBr the situation is different. Now the hydrogen abstraction is exothermic by about 10 kcal mole-1 and occurs to the exclusion of telomeriza-tion.149 Hydrogen iodide does not add successfully to olefins because now the initial addition of the iodine atom to the double bond is endothermic. 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then react with excess monomer to start propagation. The kinetics and mechanisms of cationic polymerization and polycondensation have been studied extensively.925-928 Kennedy and Marechal926 have pointed out that only cations of moderate reactivity are useful initiators, since stable ions such as arenium ions were found to be unreactive for olefin polymerization. On the other hand, energetic alkyl cations such as CH3CH2+ were too reactive and gave side products. 

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. 

The Catalyst System Eleven years ago, Kaminsky invented a novel olefin polymerization catalyst derived from Cp2ZrCl2 (Cp = 7 -5-C5H5) and methylaluminoxane (1), a result that has stimulated intense interest in synthesis and reactions of metallocenium ions. Important questions still remain, however, regarding the nature of the Kaminsky catalyst. These include (1) what is methylaluminoxane and how does it interact with Cp2ZrMe2 to initiate polymerization and (2.) what are the mechanisms of chain initiation, propagation, transfer and termination A collateral question is how these steps may be controlled. 

Based on these kinetic and microscopic observations, olefin polymerization by supported catalysts can be described by a shell by shell fragmentation, which progresses concentrically from the outside to the centre of the support particles, each of which can thus be considered as a discrete microreactor. A comprehensive mathematical model for this complex polymerization process, which includes rate constants for all relevant activation, propagation, transfer and termination steps, serves as the basis for an adequate control of large-scale industrial polymerizations with Si02-supported metallocene catalysts [A. Alex-iadis, C. Andes, D. Ferrari, F. Korber, K. Hauschild, M. Bochmann, G. Fink, Macromol. Mater. Eng. 2004, 289, 457]. 

Determination of propagation rate constants in cationic (and in anionic) systems is complicated by the simultaneous occurrence of different types of propagating sites. In olefin polymerizations, some portion of the active centers may exist as free ions and others as ion pairs of varying degrees of solvation. In the solvents in which cationic polymerizations are normally carried out, the polymerization is mainly due to free ions. In low dielectric constant media like benzene or hydrocarbon monomers, however, ion pairs will dominate the reaction. 

The relative rates of chain transfer to monomer (briefly transfer), termination and propagation, determine molecular weights and ultimate conversions in most cationic olefin polymerizations. The corresponding model reactions are proton elimination, alkylation by the counteranion and formation of Cfg and higher fractions. Thus, a quantitative analysis of [Cfa]. [C13 or C14] and [Cj ] could give clues as to the relative rates of these competing reactions. The following equations further illustrate the concept ... 

The two-component catalytic systems used for olefin polymerization (Ziegler-Natta catalysts) are combinations of a compound of a IV-VIII group transition metal (catalyst) and an organometallic compound of a I-III group non-transition element (cocatalyst) An active center (AC) of polymerization in these systems is a compound (at the surface in the case of solid catalysts) which contains a transition metal-alkyl bond into which monomer insertion occurs during the propagation reaction. In the case of two-component catalysts an AC is formed by alkylation of a transition metal compound with the cocatalyst, for example ... 

The kinetic parameters of the propagation of olefin polymerization on different active centers are compiled in Table 10. Apparently, for both the transition and non-transition metal compounds the insertion of the olefin into Mt—C bonds proceeds with the participation of coordinatively unsaturated metalalkyl compounds via intermediate n-complexes. The higher reactivity of transition metal compounds compared with organoaluminium compounds is primarily due to the lower activation energy of the propagation step when Mt is a transition metal. Many facts indicate that polarization of the Mt—C bond does not determine the reactivity of metalalkyl compounds in olefin addition, e.g. due to the decrease of reactivity in the order... 

Apparently, the reactivity of organometallic compounds in the addition of olefins to Mt—C bonds is determined by the capability of these compounds to coordinate olefins. The formation of intermediate n-complexes ensures further insertion of olefin by a concerted mechanism with a low activation energy. Thus, a high reactivity of active centers, containing a transition metal, comparable to the reactivity of the radical active centers, is achieved. The activation energy of the propagation in olefin polymerization on catalysts containing transition metals (2-6 kcal/mol) does not exceed its value for the radical polymerization (Table 10). 

Table 10. Kinetic parameters for the propagation reaction of olefin polymerization on various active centers...

In this section a brief review of quantum-chemical studies on the electron structure of the active center and the nature of the elementary steps of the chain propagation and transfer reactions for olefin polymerization is given. 

On the Mechanism of Olefin Polymerization by Ziegler-Natta Catalysts propagation reaction ... 

C5Mes)2Lu(R) complexes undergo all of the key reactions involved in olefin polymerization including olefin insertion (chain propagation), )7-H and ) -alkyl elimination, and Lu—R bond hydrogenolysis (chain transfer/ termination) (23). 

In Section 7.1 the effects of chain-transfer on polymerization rate were not considered. There is, however, evidence from the dependence of rate on monomer concentration to show that initiation reactions in olefin polymerization are slower than propagation. If the rate equation... 

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