Cocatalysts polymerization kinetics

FIGURE 175 Polymerization kinetics on Cr/AlP04 catalyst (P/Al atomic ratio of 0.8) activated at three temperatures, and tested at 95 °C without cocatalyst. 

FIGURE 201 Polymerization kinetics on Cr/silica-titania catalyst activated at 540 °C and tested with and without BEt3 cocatalyst. 

Ionic-polymerization Kinetics. The kinetics of ionic polymerization share some common principles with that of the free-radical reaction. Both are based on the basic steps of initiation, propagation, termination, and chain transfer, and in both the ultimate average molecular weight depends on the ratio of the reaction rates of propagation and termination. There are, however, important differences. In ionic polymerization the termination step appears to be unimolecular, while it is bimolecular in free-radical type polymerization. The dependence of the kinetic scheme of the reaction on the various parameters is therefore different in the two reactions. Likewise, the fact that a cocatalyst has to be brought into the ionic reaction scheme has to be taken into account. 

Olefin polymerization kinetics are considered and discussed in many reviews [ 1-6]. In this section, the influence of the main parameters such as the concentrations of catalysts and cocatalysts and time of polymerization on polymerization rate, and the main reactions in the olefin polymerization process will be briefly reviewed. We also consider the problems of deviation from the linear law of polymerization rate with changing monomer concentration, the effect of hydrogen in the ethene and propene polymerizations, as well as the nature of the comonomer effect, which are under discussion in the literature and the natures of which are not yet completely clear. 

Ethylene homopolymerization using Phillips catalyst PC600 calcined at 600°C followed by activation with DEAE cocatalyst during the slurry polymerization process was carried out with Al/Cr molar ratios of 7.5, 15.0, and 22.5 [84]. As shown in Fig. 14, a typical single-type polymerization kinetics corresponding to type b in Fig. 10b was observed, which was completely different from the kinetics with the same catalyst activated by TEA at the same conditions (as shown in Fig. 13). This t3 pe of polymerization kinetics could be ascribed to one type of active site (Site-B) formed in two ways. One was similar with the PC600 activated by TEA some chromate Cr(VI) species were reduced to Cr(II) species by ethylene monomer and coordinated with formaldehyde, then formaldehyde-coordinated Cr(ll) sites were transformed to DEAE-coordinated Cr(II) sites by substitution, as shown in Scheme 8. On the other hand, some chromate Cr(VI) species were reduced by DEAE, and then the Al-alkoxy product coordinated with the Cr(Il) sites. Site-B had relatively low activity and high stability. Based on the microstructure analysis, the relative amount of SCBs of polymers obtained from the DEAE systems was even more than that from TEA catalyst systems. This can be explained as follows. Firstly, the reduction ability of DEAE was weaker than that of TEA. More Cr(VI) species... 

Fig. 27 Ethylene polymerization kinetic curves of catalysts activated by TEA cocatalyst during slurry polymerizatimi (a) Phillips catalyst al) and Cat-A/1.5 catalyst a2) (Al/Cr molar ratio = 20.0) (b) Cat-A/1.5 catalyst (W) and S-2 catalyst b2) (Al/Cr molar ratio = 15.0). Polymerization conditions catalyst amount, 160 mg polymerization temperature, 90°C ethylene pressure, 0.15 MPa solvent, heptane, 70 mL...

F. 28 Polymerization kinetic curves of catalysts activated by TEA cocatalyst during the slurry polymerizatirai process ... 

Even though the discussion has been mainly on homopolymerization, the same polymerization mechanism steps are valid for copolymerization with coordination catalysts. In this case, for a given catalyst/cocatalyst system, propagation and transfer rates depend not only on the type of coordinating monomer, but also on the type of the last monomer attached to the living polymer chain. It is easy to understand why the last monomer in the chain will affect the behavior of the incoming monomer as the reacting monomer coordinates with the active site, it has to be inserted into the carbon-metal bond and will interact with the last (and, less likely, next-to-last or penultimate) monomer unit inserted into the chain. This is called the terminal model for copolymerization and is also commonly used to describe free-radical copolymerization. In the next section it will be seen that, with a proper transformation, not only the same mechanism, but also the same polymerization kinetic equations for homopolymerization can be used directly to describe copolymerization. 

Therefore, at steady-state, the complete CLD or MWD of polyolefins can be predicted using Equation 2.84 and the value of x calculated for the concentrations of monomer, cocatalyst, and chain transfer agent in the reactor, as well as the several required polymerization kinetic constants calculated at a given polymerization temperature. This is a rather straightforward procedure. 

Syndiotactic polystyrene (SPS) can be readily polymerized using homogeneous or heterogeneous metallocene catalysts, based on group 4 metal compounds, especially titanium compounds like T1CI4, CpTiClj, and Cp Ti(OCH3)3 with methyl aluminoxane (MAO) as cocatalyst [1-3]. The recent developments of transition metal catalysts and reaction mechanisms are discussed in earlier chapters. This chapter will be focused on the quantitative aspects of SPS polymerization kinetics and related physical and chemical phenomena. 

Xia W, Liu B, Fang Y, Hasebe K, Terano M Unique polymerization kinetics obtained from simultaneous interaction of Phillips Cr(VI)Ox/Si02 catalyst with Al-alkyl cocatalyst and ethylene monomer, J Mol Catal A Chem 256(1—2) 301—308, 2006. 

Many other compounds have been shown to act as co-catalysts in various systems, and their activity is interpreted by analogous reactions [30-33]. However, the confidence with which one previously generalised this simple picture has been shaken by some extremely important papers from Eastham s group [34], These authors have studied the isomerization of cis- and Zraws-but-2-ene and of but-l-ene and the polymerization of propene and of the butenes by boron fluoride with either methanol or acetic acid as cocatalyst. Their complicated kinetic results indicate that more than one complex may be involved in the reaction mechanism, and the authors have discussed the implications of their findings in some detail. 

Olefins can only be polymerized by metal halides if a third substance, the co-catalyst, is present. The function of this is to provide the cation which starts the carbonium ion chain reaction. In most systems the catalyst is not used up, but at any rate part of the cocatalyst molecule is necessarily incorporated in the polymer. Whereas the initiation and propagation of cationic polymerizations are now fairly well understood, termination and transfer reactions are still obscure. A distinction is made between true kinetic termination reactions in which the propagating ion is destroyed, and transfer reactions in which only the molecular chain is broken off. It is shown that the kinetic termination may take place by several different types of reaction, and that in some systems there is no termination at all. Since the molecular weight is generally quite low, transfer must be dominant. According to the circumstances many different types of transfer are possible, including proton transfer, hydride ion transfer, and transfer reactions involving monomer, catalyst, or solvent. 

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