Activation Energy Of Propagation Reaction Research Articles

Kinetic evidences for reversible transformations of active centers in ethylene polymerization by titanium–magnesium catalyst: Effect of the polymerization temperature

The kinetics of ethylene polymerization at varied polymerization temperature over two modifications of titanium–magnesium catalysts that differ in their compositions and the molecular weight of the resulting polymers were studied by polymerization quenching by radioactive carbon monoxide. An increase in polymerization temperature in the range of 40–80 °C was found to significantly increase the number of active centers (CP), which contributes to the polymerization rate more significantly than an increase in the propagation rate constant (kP) does. The rise in the number of active centers with increasing polymerization temperature is shown to result in effective activation energy (Eeff) determined from the temperature dependence of polymerization rate being significantly higher than the activation energy of propagation reaction (EP) determined from the temperature dependence of the kP value. It was shown too the changes of the number of active centers and polymerization rate at variation of temperature in the single experiment have the reversible nature. The data on the number of active centers and the propagation rate constants at ethylene polymerization in the presence of hydrogen as a chain transfer agent are obtained. Introduction of hydrogen reduces the number of active centers at elevated polymerization temperatures (60–80 °C) and has virtually no effect on the CP value at lower temperatures; it also reduces the kP value at 40–80 °C. The findings on the effect of polymerization temperature on the number of active centers were used to propose a kinetic scheme including the reaction of reversible transformation of the precursors of active centers (alkyl derivatives of titanium) into an inactive state due to adsorption of a co-catalyst (triethylaluminum). The transformation of these complexes into active centers depends on polymerization temperature, presence of hydrogen, concentrations of monomer and organoaluminum activator.

Kinetic study of ethylene polymerization over 2,6-bis(imino)pyridine cobalt complexes with bulky substituents in ligand

The number of active centers (СP) and propagation rate constants (kP) for ethylene polymerization with single-site homogeneous catalysts composed of cobalt (II) bis(imino)pyridine complexes: 2-tBuLCoCl2 and 2,5-tBu2LCoCl2 (L: 2,6-(C6H3NCMe)2C5H3N) and methylaluminoxane (MAO) as an activator have been determined by polymerization quenching with radioactive carbon monoxide. The above cobalt complexes provided formation of catalysts, characterized with an increased activity due to the increase in the СP value. The values of propagation rate constant, determined for the studied catalysts, were close (3.6–2.4×103Lmol−1s−1). The values of the constant of the chain transfer reaction with ethylene decreases sharply in the order 2,6-Me2LCoCl2>2-tBuLCoCl2>2,5-tBu2LCoCl2, being the reason for the increase in the molecular weight of PE, produced with corresponding complexes. For the 2,5-tBu2LCoCl2+MAO catalyst, the effect of polymerization temperature on the CP and kP values have been studied and the activation energy of propagation reaction was calculated (4.5kcalmol−1), that was noticeably lower as compared with the effective activation energy (12.5kcalmol−1), owing to the increase in the CP value with polymerization temperature growth.

Ethylene polymerization over homogeneous Bis(imino)pyridine vanadium catalysts: data on the number and reactivity of active sites

Method of polymerization quenching with radioactive carbon monoxide (14СО) has been used to determine the number of active centers (СP) and propagation rate constants (kP) for ethylene polymerization over the homogeneous catalysts 2,6-R2LVCl3+MAO (L: 2,6-(C6H3N=CMe)2C5H3N, R=Me, Et). For both catalysts the maximum number of active centers at 60 °C was found to be 0.42–0.50 mol molV−1 and the kP values—(16.4–20.0) × 103 L mol−1 s−1. It has been shown that noticeable decrease in the catalysts activity with polymerization time results from the decrease in the active centers number and simultaneous transformation of some part of the initial centers into a new ones with lower reactivity. Analysis of the СP and kP values, measured at different polymerization times, together with the values of the polyethylene molecular-weight distribution, allowed to calculate the content of the active centers of both types and the kP values of these centers. For the 2,6-Et2LVCl3+МАО catalyst, the effect of polymerization temperature on the СP and kP values has been studied. It has been found that with polymerization temperature growth from 25 to 60 °С, the СP value substantially increases. As a result, the value of the effective activation energy (Eeff = 22.6 kcal mol−1) noticeably exceeded that of the activation energy of propagation reaction (EP = 14.2 kcal mol−1). It is proposed that the reason for the unusually high EP value is formation of “dormant” sites with vanadium-polymer bond at lower polymerization temperature.

Effect of temperature on the number of active sites and propagation rate constant at ethylene polymerization over supported bis(imino)pyridine iron catalysts

AbstractThe inhibition of ethylene polymerization with radioactive carbon monoxide (14CO) was used to obtain data on the number of active sites (CP) and propagation rate constant (kP) at ethylene polymerization in the temperature range of 35–70 °C over supported catalysts LFeCl2/Al2O3, LFeCl2/SiO2, and LFeCl2/MgCl2 (L: 2,6‐(2,6‐(Me)2C6H3N = CMe)2C5H3N) with activator Al(i‐Bu)3. The values of effective activation energy (Eeff), activation energy of propagation reaction (EP), and temperature coefficients of variation of the number of active sites (ECp = Eeff − EP) were determined. The activation energies of propagation reaction for catalysts LFeCl2/Al2O3, LFeCl2/SiO2, and LFeCl2/MgCl2 were found to be quite similar (5.2–5.7 kcal/mol). The number of active sites diminished considerably as the polymerization temperature decreased, the ECp value being 5.2–6.2 kcal/mol for these catalysts at polymerization in the presence of hydrogen. The reactions of reversible transformations of active centers to the surface hydride species at polymerization in the presence and absence of hydrogen are proposed as the derivation of ECp. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6621–6629, 2008

Determination of propagation-rate constant of vinylidene chloride by emulsion polymerization

The propagation-rate constant of vinylidene chloride (VDC) was determined at 40 and 50 °C, respectively, by applying the so-called Ugelstad plot to the polymerization-rate data of the seeded and unseeded emulsion polymerizations of VDC. The values of the propagation-rate constant kp thus determined are kp = 64 dm3/mol · s at 50 °C and kp = 52 dm3/mol · s at 40 °C, respectively. From these kp values, the activation energy for propagation reaction was determined to be Ep = 4.2 kcal/mol, which is close to that of vinyl chloride (3.7 kcal/mol). © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1005–1015, 2001

Polymerization of butadiene with tii4‐Al(i‐Bu)3 Catalyst: Part II: Kinetic model development

AbstractExpressions for conversions, Mn and Mw have been derived for the proposed kinetic scheme for the polymerization of butadiene using Til4 — Al(i‐Bu)3 as catalyst. The rate constants kp, kta and kd and the active sites were estimated from experimental data of conversion and molecular weight averages. The logarithm of both kp and kta are linearly related to Al/Ti ratio. On the other hand, kd and Co appear to be uncorrelated with the reaction variables studied in this investigation. The activation energy for propagation reaction was found to be 12.05 ± 1.31 which agrees well with those reported previously. The small difference in the activation energies for kp and kta accounts for the independence of the molecular weight of the polymer on the reaction temperature.