Ethylene polymerization active site concentration

In the polymerization of ethylene by (Tr-CjHsljTiClj/AlMejCl [111] and of butadiene by Co(acac)3/AlEt2Cl/H2 0 [87] there is evidence for bimolecular termination. The conclusions on ethylene polymerization have been questioned, however, and it has been proposed that intramolecular decomposition of the catalyst complex occurs via ionic intermediates [91], Smith and Zelmer [275] have examined several catalyst systems for ethylene polymerization and with the assumption that the rate at any time is proportional to the active site concentration ([C ]), second order catalyst decay was deduced, since 1 — [Cf] /[Cf] was linear with time. This evidence, of course, does not distinguish between chemical deactivation and physical occlusion of sites. In conjugated diene polymerization by Group VIII metal catalysts -the unsaturated polymer chain stabilizes the active centre and the copolymerization of a monoolefin which converts the growing chain from a tt to a a bonded structure is followed by a catalyst decomposition, with a reduction in rate and polymer molecular weight [88]. 

The kinetics of polymerization catalyzed by Cr/silica can be manipulated over a wide range, depending on temperature and ethylene concentration. These observations suggest that the active-site concentration rises over time and (rarely for Cr/silica) sometimes also declines. At low temperatures (e.g., 60 °C), there is little polymerization activity in comparison with that characterizing the commercial process. At high temperatures (e.g., 135 °C) used in the solution process, there is no induction time at all. Full and constant reaction is immediately observed. This variable induction time means that attempts to measure the activation energy of polymerization are often dominated by the initiation rather than the polymerization itself. Thus, one must be especially cautious when extrapolating from vacuum-line experiments. 

Chien and Wang [23] determined the initial active site concentration of this homogeneous catalyst system using a radiolabeling technique based on CHjOT and found that 80% of all the Zr atoms are catalytically active at 70°C and an Al/Zr molar ratio of 1,000, and that 100% of the Zr atoms are active at an Al/Zr ratio of 10,000. Lowering the polymerization temperature to 50°C and the Al/Zr ratio to 550 reduces the Zr active site content to 20%. The ethylene propagation rate constant at 70 C and an Al/Zr ratio of 11,000 was calculated to be 1.6 x 10 M/sec with the maximum ethylene polymerization rate obtained in about one minute at 1.06 x 10 M/sec and the rate decayed to about one-half the maximum rate of polymerization in about 40 minutes. 

Cr(VI)/Silica develops polymerization activity only gradually when exposed to ethylene at 100°C in a slurry autoclave. An example is shown in Fig. 5, which depicts an experiment in which the catalyst was not immediately active upon introduction into the reactor, but first underwent a dormant period or induction time. The rate of polymerization then increased during the remainder of the experiment. This is thought to be due to the slow reduction of Cr(VI) by ethylene to the Cr(II) active site, or perhaps to the desorption of by-products such as formaldehyde (32). Thus, the concentration of active sites is probably not constant but increases with time. Below 100°C the induction time becomes longer until at about 60°C there is almost no activity. Conversely, increasing the temperature shortens the induction time. At 150°C the catalyst exhibits an immediate and constant activity in solution phase polymerization. 

It is interesting to note the effect of chromium content on reaction rate at high pressures (,—500 p.s.i.g.). Experiments (5) were carried out with normal air-activated catalysts (Figure 4). Catalysts were used with chromium contents ranging from 0.7 to 0.0005 wt. % of the total catalyst. Results of one-hour ethylene polymerization tests at 132°C. and 450 p.s.i.g. with these catalysts, activated at 500°C., are given. As the concentration of chromium was decreased, catalyst charge was increased to compensate for poisoning of catalyst sites by trace impurities and to keep total rate of production about constant. 

The second relevant phenomenon is the decrease of the overall rate or the increase of the decay rate which is often observed when the alkyl concentration increases beyond certain limits. According to some authors 88 11this may be due to the adsorption of the Al-alkyl on catalytic sites in competition with the monomer. Still others 92,95 107) attribute it to an overreduction of titanium. This seems plausible when considering the results obtained by Kashiwa78) who showed that Ti2+ is less active than Ti3+ or Ti4+ in ethylene polymerization and completely inactive in the polymerization of propylene. Keii98), in turn, based on the results of Fig. 32, hypothesizes that the decay rate is due to a bimolecular disproportionation of the Ti—R bonds, favored by Al-alkyl reversibly adsorbed on the catalyst surface. 

The nature of the catalyst sites was given particular attention. Reduction of the catalyst, from the amount of carbon dioxide produced, indicated an average valence state of the chromium of 4.5 0.2 comparable to that observed in reduction by ethylene. The conclusion was that the active sites contained chromium in a valence state above Cr — possibly Cr. However, in view of a relationship between polymerization activity and Cr concentration [161] for extensively reduced catalysts, other catalytic species may exist. 

Catalysts containing only chromate were quite active for ethylene polymerization, which again demonstrates that chromate can be a precursor of an active site. However, some of the most active catalysts also contained some dichromate. Therefore, one cannot dismiss its potential role as a possible active-site precursor as well. Speculating a little, one might even attribute some of the observed changes in polymer properties to these changes in dichromate concentration. 

CrA sites, the most numerous of the three in this preparation, were identified as the most reactive species and comprise the sites that are active in ethylene polymerization, whereas Crc sites were found to be inactive. CrB sites were also thought to exhibit polymerization activity, but distinctions were made. The concentration of these surface species could be varied with the chromium content and with the conditions of the thermal pretreatments [279]. Heating the catalyst under vacuum at 700 °C, which caused a deactivation for ethylene polymerization, was found to convert CrA into Crc whereas CrB remained unaffected. 

The carbonium ions are known to be the important intermediates in the reactions involving formation and breaking of the carbon-carbon bonds. In the case of a solid-acid catalyst, the Bronsted acid sites are considered to be the active sites for initiation of carbonium ion formation, which in turn lead to various reactions such as polymerization, alkylation and aromatization[5]. However, a comparision of the data in Fig.2 and Table 1 shows that the conversion of ethylene over ZSM-5 and HZSM-5 sanq)les is either unaffected or shows an increase while the concentration of the hydroxyl groups reduced to 25-40% with the increase in pretreatment tenq)erature fi om 300 to 700°C. Similarly, while the concentration of the... 

Figure 22 shows that the rate of reaction is not directly proportional to ethylene pressure. When ethylene was adsorbed only on the active sites but its concentration in the gas phase was negligible, no polymerization was observed even after 68 hours at room temperature. Polymerization therefore does not occur by migration and interaction of the ethylene molecules adsorbed on active sites. It is possible that the physically adsorbed ethylene participates in the reaction. However, no simple rate expression could be found to fit the observed dependence on ethylene pressure. 

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. 

In 1957, D. B. Ludlum and coworkers at the E. I. DuPont Company published an early paper on the characteristics of the Mulheim catalyst [11], Ludlum investigated the polymerization of ethylene in a high-boiling inert solvent (decalin) using TiCl and tri-isobutylaluminum (TIBA), Tri-n-hexyl aluminum (THA) or triethylaluminum (TEAL) at 100 C and 1-1.5 atm of ethylene pressure. The rate of ethylene uptake was found to decrease shortly after polymerization was initiated and Lunlum postulated that this was due to the encapsulation of polymerization sites by the polyethylene. The polymerization was also found to be first-order with regard to the ethylene pressure and the concentration of TiCl. The effect of the Al/Ti ratio on catalyst activity and the type of aliuninum alkyl used (TIBA or TEAL) in the polymerization are shown in Figure 2.1. 

In a classic 1978 paper [5,6], L.L. Bohm reported on the experimental parameters needed to establish steady-state polymerization conditions in order to eliminate monomer transport phenomena from the experimental results. As pointed out by Bohm, suspension or slurry polymerization takes place if the polymerization temperature is lower than the polyethylene solubility temperature and, therefore, the semicrystalline polymer precipitates from the suspension medium as the polymerization proceeds. The important physical process is the mass transfer of ethylene, comonomer and hydrogen (chain transfer reagent used to control polymer molecular weight) from the gas phase through the suspension medium and into the growing polymer particle to the active site. In order to obtain correct kinetic results, concentration gradients and temperature gradients within the polymer particle need to be removed from the polymerization process to achieve the necessary steady-state polymerization conditions. 

The polymerization of ethylene was carried out in an identical way with these heterogeneous catalysts as with the homogeneous systems. Typical results are given in Table XII and show that the Si-0 ligand enhances the activity of the transition metal site for polymerization. Some of the higher activities are minimum values since the concentration of ethylene in the diluent is well below equilibrium concentrations and with these conditions the process is diffusion controlled. 

Finally the ESR spectrum of Nb(7r-allyl)4/alumina was unaffected by the addition of ethylene gas to the ESR sample tube. It is assumed that polyethylene is produced in this process since polymer can be isolated from larger scale reactions under similar conditions. The accepted mechanism for the ethylene growth reaction postulates a steady-state concentration of a a-bonded transition metal-hydrocarbon species which would be expected to modify the ESR spectrum of the supported complex. A possible explanation for the failure to detect a change in the ESR spectrum may be that only a small number of the niobium sites are active for polymerization. Although further experiments are needed to verify this proposition, it is consistent with IR data and radiochemical studies of similar catalyst systems (41, 42, 43). 

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