Propagation constants olefins

Hie reaction of a monomer as described in scheme (1) is a composed quantity of several donentaiy stqw. 6.e., diffiiaion of the monomer to the active centre, adsorption of the monomer active centre and finally insertion into the Ti-C-bond) hoioe, the propagation constants, kij. in the above scheme are likewise cwnposed quantities as a result of the diffusion, adsorption and insertion processes. We siqqwse that the adsorption behaviour of the relatively small ethylene molecule in comparision to the long a-olefin molecule may be different and therefore the value of the term [MI2 / [M], i.e., the monomer... 

For olefins, cyclic, or better hi- or tricyclic ring structures with large ring strain (norborn-2-enes or norbornadienes for instance) are required. Alternatively, 1-alkynes can be used. In this case, the term 1-alkyne polymerization applies. This reaction proceeds via a- or j6-insertion of the alkyne into the metal-carbon double bond (Scheme 1). Both insertion mechanisms lead to a conjugated polymer. With a few exceptions [1-3], polymerizations based on a-insertion are the preferred ones, since they offer better control over molecular weights due to favorable values of kj/kp (ki, kp = rate constants of initiation and propagation, respectively). 

For N-vinylcarbazole in methylene chloride solutions cycloheptatrienyl ion has been shown to be a very efficient initiator, reacting by a rapid and direct addition to the olefin (82). A mechanistic scheme involving virtually instantaneous and quantitative initiation, rapid propagation (and transfer) and no true termination appears to operate, enabling rate constants for propagation kp, to be determined very simply from initial slopes of conversion/time curves. Under the experimental conditions used the initiators were almost totally dissociated and there seems every reason to suppose that the propagating cations are similarly dissociated (Section II.C.2). The derived rate constants therefore refer to the reactivity of free poly-(N-vinylcarbazole) cation, kp, and relevant data are summarised in Table 7. 

Although the isospecific polymerisation of styrene monomers has much less steric demands for the Ziegler-Natta catalysts than that of x-oldins, it proceeds with much lower propagation rate constants by comparison with the polymerisation of x-olefins for example, on a molar basis, styrene is ca 100 times less reactive than propylene in the polymerisation [30,33], Also, compare the relatively slow polymerisation of styrene and other vinylaromatic monomers with the relatively fast polymerisation of vinylcyclohexane [20,31,34-36]. 

Three distinct propagating species, each having different equilibrium olefin binding constants and insertion rate constants, are present. They are Cp2ZrH(olefin)+, Cp2ZrR (olefin)+ and Cp2ZrR(olefin)+. 

We now propose that the diradical intermediates in the cycloaddition reactions of captodative olefins are also the initiators in the observed spontaneous polymerizations of these olefins. Tail-to-tail combination of captodative olefins are expected to provide a low but constant concentration of diradicals, which are capable of initiation. Whether or not polymerization ensues must depend on the experimental conditions, propagation equilibria and rates. 

Much remains to be done in this new field. Absolute propagation rate constants need to be determined for those captodative olefins which yield high polymers. No data on ceiling temperatures of captodative olefins have yet been reported. Such data for olefins with substituents with systematically varying steric requirements would be of great interest. 

Secondly, the modified S-F model(II) was constructed. The chain growth of readsorbed olefins was assumed to take place independently from the S-F propagation path. The numerical simulation was made, assuming that the reaction rate constants Are not... 

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. 

Readsorption steps (J3r,n) decrease the total termination probability (/3t, ) by reversing the olefin termination step. Readsorption can be enhanced by increasing the olefin reactivity (i.e., /3r, the ratio of readsorption to propagation rate constants), or by increasing olefin concentrations within pellets and reactors. 

Number of C fCp) and their reactivity in a propagation reaction (propagation rate constant kp). Cp data can be used to determine the mole fraction of active metal centers, the localization of active centers on the surface of the solid catalyst, and the role of individual components of the catalytic system in the formation of active centers. Systematic data on the influence of the catalyst composition and polymerization conditions on Cp and the rate constants of individual steps are important for the determination of the composition of the active centers and the elucidation of the mechanism of these steps. Various methods for determining Cp and kp in catalytic polymerizations of olefins have been reported A direct method for the determination of Cp is the radioactive tracer technique. In this method radioactive compounds react with the AC thus introducing radioactivity into the growing polymer chain. The use of radioactive alcohols is the classical example of this technique... 

Zakharov, V.A. Bukatov, G.D. Barabanov, A.A. Recent data on the number of active centers and propagation rate constants in olefin polymerization with supported ZN catalysts. Macromol. Symp. 2004, 213, 19-28. 

Since little anti-form is present under equihbrium conditions (without irradiation) in Mo(NAr )(CHR)(OCMe(CF3)2)2, and syn- into anti-conversion is slow (ca. 10 s ), cis-polymers are proposed to form from the syn-species of a catalyst via olefin attack on the CNO-face of the initiator [94]. In a t-butoxide system, where interconversion is relatively fast (ca. 1 s ), it was proposed that the anti-form was the only propagating alkylidene species. This proposal was further supported by studies carried out by Feast and co-workers [100]. Using sterically hindered and therefore unreactive monomers such as 1,7,7-trimethylnorbornene, only the reaction of the anti-rotamer at a very slow, monomer concentration-independent rate was observed. Additionally, the calculated rate constant was essentially identical with the one for syn-anti[Pg.165]

Several reviewers have attempted to summarize existing data on the determination of propagation rate constants (k ) and the number of active centers (C ) in olefin polymerization (40-43). Although... 

The Propagation Rate-Constants in Cationic Polymerisations 151 Table 1. Some values for olefinic monomers (M sec )... 

Kinetic data on olefin polymerization by polymer-immobilized zirconocene are scarce. It is generally accepted that homogeneous metallocene catalysts contain uniform active sites however, if they are immobilized on a polymer support, the MWD polymer production becomes broader compared with a homogeneous catalyst [103]. Kinetic analysis of gas-phase ethylene polymerization catalyzed by (CH3)2[Ind]2ZrCl2 bound at a hydroxylated copolymer of styrene with divinylbenzene and previously activated with MAO (0.17 wt.% Zr) has been carried out [104]. The influence of temperature (333 to 353 K), ethylene partial pressure (2 to 6 atm) and MAO level (molar ratio of MAO to zirconium from 2600 to 10,700) were studied. The activity of the catalyst in the gas-phase process changed from 5 to 32 kg PE (g of Zr atm h)It is possible that there are two types of active site. They are stable to temperature and deactivated by the same mechanism. A first-order reaction takes place. The propagation rate constants of two active sites show a similar dependence on temperature. 

---