Ethylene polymerization kinetic curves

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...

The shape of the kinetic curves depends on the catalyst type and polymerization conditions (ethylene pressure, temperature, concentration of inhibitors in reaction medium) (89, 97, 98). The types of the kinetic curves obtained. at ethylene polymerization under various conditions are presented in Fig. 1. 

Fig. 1. Examples of the kinetic curves during ethylene polymerization by chromium oxide catalysts. Support—SiOs temperature—80°C polymerization at constant ethylene pressure in perfect mixing reactor. Curve 1—catalyst reduced by CO at 300°C. Curve 2— catalyst activated in vacuum (400°C) polymerization in the case of (1) and (2) in solvent (heptane) ethylene pressure 10 kg/cm2 02 content in ethylene 1 ppm, HsO 3 ppm. Curves 3, 4, 5, 6—catalyst activated in vacuum (400°C) polymerization without solvent ethylene pressure 19 (curve 3), 13 (curve 4), 4 (curve 5), and 2 (curve 6) kg/cm2 02 content in ethylene 1 ppm, HsO = 12 ppm.

The change of shape of the kinetic curves with monomer and inhibitor concentration at ethylene polymerization by chromium oxide catalysts may be satisfactory described 115) by the kinetic model based on reactions (8)-(14). 

In ethylene polymerization with the catalytic system TicyMgClj—TIBA a progressive change of the kinetic curve from stationary-type of decay-type upon addition... 

The existence of a further type of active centers was demonstrated by Pino and Rotzinger93> by polymerizing ethylene with a MgQ2-supported catalyst in the presence of an electron donor. A comparison of the ethylene and propylene kinetic curves shows that, while propylene polymerization is characterized by the well known rapid decrease in rate, the ethylene polymerization rate increases reaching a constant value after about 30 min. This has been attributed to the existence of active... 

Another kinetic method has been proposed [17]. This method allows one to determine the kp value and the number of active sites using the dependence on monomer concentration of the stationary polymerization rate and of the polymerization rate during the acceleration period of the kinetic curve, according to Eqs. (2) and (6). The method has been applied for determination of these kinetic parameters in the polymerization of propylene and ethylene with VCl3/Al(i-Bu)3 catalyst. 

Fig. 10 Two types of kinetic curve for ethylene polymerization over Phillips-type catalysts, (a) Hybrid of two typical types of kinetic curve fast activation followed by fast decay (A) and slow activation followed by slow decay (B). (b) Single-type curve with slow activation followed by...

Fig. 11 Kinetic curves of the TEA-modified Phillips catalyst (PC600/TEA) at Al/Cr molar ratios of 2.08 white symbols), 3.12 (grey symbols), and 4.16 (black symbols), before ethylene slurry polymerization. Polymerization conditions catalyst amount, 100 mg polymerizatitui temperature, 60°C ethylene pressure, 0.15 MPa solvent heptane, 20 niL...

Fig. 12 Kinetic curves of ethylene polymerization of S-2 catalysts modified with various cocatalysts before gas phase polymerization. Polymerization conditions catalyst amount, 200 mg polymerization temperature, 92°C ethylene pressure,...

Fig. 13 Kinetic curves of ethylene polymerization using Phillips catalyst PC600 activated by TEA during slurry polymerization with Al/Cr molar ratio (a) 7.5 ...

Fig. 15 (a-c) Kinetic curves of ethylene homopolymerization using Phillips catalyst PC600/CO activated by TEA during slurry polymerization with Al/Cr mole ratio of (a) 7.5, ib) 15.0,and (c) 22.5. d-g) Kinetic curves of ethylene/1-hexene copolymerization under (d) Al/Cr ratio of 7.5 with 10 vol% of 1-hexene,... 

Figure 3.2 Two types of kinetic curves for ethylene polymerization over Phillips chromium catalysts (A) a typical hybrid kinetic curve and (B) a typical single kinetic curve.

After activation, the catalyst is intrcxiuced into the polymerization reactor as slurry in a saturated hydrocarbon such as isobutane. The precise mechanism of initiation is not known, but is believed to involve oxidation-reduction reactions between ethylene and chromium, resulting in formation of chromium (II) which is the precursor for the active center. Polymerization is initially slow, possibly because oxidation products coordinate with (and block) active centers. Consequently, standard Phillips catalysts typically exhibit an induction period. The typical kinetic profile for a Phillips catalyst is shown in curve C of Figure 3.1. If the catalyst is pre-reduced by carbon monoxide, the induction period is not observed. Unlike Ziegler-Natta and most single site catalysts, no cocatalyst is required for standard Phillips catalysts. Molecular weight distribution of the polymer is broad because of the variety of active centers. 

The kinetics of ADMET with complex 6 were compared to those of complex 2 by measuring the volume of ethylene liberated from ADMET reactions over time [35], Obtaining an approximate second order rate constant from the DP versus time curves, it was found that molybdenum complex 2 polymerizes 1,9-decadiene 24 times faster than ruthenium complex 6 (Tab. 6.1). 

For example, both the rapid development in rate and the subsequent decay were found to be dependent on the ethylene concentration with Cr/AIPO4 catalysts. This dependence is shown in Figure 171, in which the kinetics of three polymerization runs are shown as the ethylene concentration was varied. It is perhaps not surprising that the reduction and/or initiation slowed substantially with decreased ethylene concentration. When these curves were analyzed according to the model in Scheme 35, the polymerization rate itself was found to be first-order in ethylene concentration. However, the development in activity was found to be more sensitive to ethylene concentration. In contrast, the decay in activity was found to be unaffected by ethylene concentration. 

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