Polymerization, activation experiments

In a quest to increase the efficiency of olefin polymerization catalysts and their selectivity in the orientation of the polymerization, the highly effective Group IV metallocene catalysts, M(Cp)2(L)2, have been studied, since they all display high fluxionality. Following methide abstraction, the metallocene catalysts of general formula M(Cp-derivatives)2(CH3)2 (M= Ti, Zr, Hf), were turned into highly reactive M+-CH3 cationic species. The activation parameters for the methide abstraction, derived from variable temperature NMR experiments, establish a correlation between the enthalpies of methide abstraction, the chemical shift in the resulting cation, and the ethylene polymerization activities [149]. 

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. 

Polymerization Activity. In a few preliminary experiments listed in Table I, the catalysts prepared in separate reduction and activation steps were compared with catalysts prepared by mixing in one step under various conditions— viz., at ultimate dilution or under the normal conditions of the reducing step. Catalysts prepared by such one-step procedures proved to have a diminished— often strongly diminished—activity compared with two-step catalysts of about the same overall composition. 

The polystyrene produced by catalyst 25 is most likely produced by the titanium center, as shown by the results of fhe confrol experiments carried out with Ti(NMe2)4 as a catalyst—which was found to exhibit a styrene polymerization activity comparable to that observed for 25 under identical conditions. 

In consecutive screening experiments for polymerization activity [vide infra) it was found that the triphenylphosphane donor was not easily removed. Therefore, methyl nickel precatalysts were synthesized starting from (tmeda)NiMe2 [31] as the nickel reagent, resulting in the formation of several compounds of which (N,O)2Ni and an oxygen-bridged dimer (N,O)Ni-Me 2 were identified by NMR and FAB-mass spectroscopy. However, this mixture of Ni hydroxy imine com-... 

The effect of high-temperature reduction on polymerization activity is shown in Table 2 for one experiment. Cr/silica was activated in air at... 

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. 

Partial reduction and poisoning experiments were performed with a Cr/silica-titania activated at 600 °C. The polymerization activity is shown in the upper part of Figure 20 as a function of the reduction temperature in CO. CO treatment at 25 °C had no effect no reduction occurred and full activity was observed. The catalyst remained unchanged from its original orange color. However, when the catalyst was treated with CO at 300 °C,... 

In some experiments, the selectivity of the poison decreased with increasing reaction temperature. That is, the catalyst became more tolerant of some of the poisons, and more had to be added to achieve an equivalent loss in activity. Most likely, the poison blocks a site by adsorption onto it, and at higher temperatures it is not held as tightly. In general, these plots show the difficulty of using titration by poisons to quantify the active-site density. In every case, even the most severe one, the poisoning is probably not 100% selective, so again the value calculated can serve only as an upper limit. This indicates once more that only a small fraction of the total chromium accounts for the observed polymerization activity. 

In one experiment, a catalyst containing divalent chromium was killed by prior exposure to a large excess of CO at 100 °C, followed by flushing to remove unadsorbed CO. Then this catalyst was physically mixed with an equal amount of another catalyst containing hexavalent chromium and not exposed to CO [377]. The mixture of the two catalysts was introduced into the reactor and tested for its polymerization activity. It was found to exhibit activity comparable to that of the catalyst containing the... 

The usual preference for chromate may be shifted slightly toward dichromate by the presence of titania, although this suggestion has not been investigated much [76,102]. It is not known whether the increase in observed activity is a consequence, as seems likely, of an increase in the number of active sites, or whether the reactivity of existing sites is enhanced by the Ti-associated acidity. It seems clear, however, that the Ti-associated sites are more easily reduced, which means a faster development of polymerization rate. Experiments by Marsden [474], who used temperature programmed reduction, and also other data [103], indicate that a second, and more easily reducible, Cr(VI) species is formed by the presence of titania. Indeed, some of the behavior modification that accompanies the addition of titania is shared with other acidic supports as well. 

Although titania is very useful as a promoter for Cr/silica catalysts, it is a poor catalyst support itself. It does not have the porosity necessary for polymerization, but, more significantly, it also does not seem to provide an adequate chemical environment for the chromium. In one experiment, a high-surface-area (100 m2 g 1) TiC>2 carrier was impregnated with the usual 0.4 Cr atoms nm 2, followed by calcination at 400 and at 500 °C. A small amount of Cr(VI) was stabilized, and when tested for polymerization activity the catalyst provided very low yields of polyethylene. Whereas Cr/silica-titania produces polymers of lower MW than those made by Cr/silica, Cr/titania yielded polymers of higher MW. Indeed, it produced ultrahigh-MW polymer (UHMW PE). This finding is consistent with the view that it is the acidity created when titania is combined with silica that is important, and not the titania itself. 

In another experiment, the Cr(II) /silica-titania catalyst was exposed to a small amount of water vapor in N2 at 350° after reduction in CO at the same temperature. This treatment also completely stopped the in situ branching response, although it did not change the polymerization activity. 

The initial temperature of catalyst activation can also influence the amount of in situ branching obtained in the polymer. This is in agreement with the olefin-generating behavior of the organochromium catalysts (Figures 185 and 192, Table 55). Table 67 shows an experiment in which Cr/silica-titania was activated at 800 °C or at 650 °C, and then it was reduced and tested for polymerization activity with 5 ppm triethylboron cocatalyst. The 800 °C catalyst resulted in significantly lower polymer density than the 650 °C catalyst. This derives from two causes. The 800 °C... 

These annealing results suggest that time and temperature may be considered as somewhat equivalent in their effect on water evolution. A short time at 700 °C is equivalent to a longer time at 600 °C. This comparison was further tested in a series of experiments in which Cr/ silica catalyst was activated at various temperatures for 15 min and also for 20 h. Then each catalyst was tested for polymerization activity, and the resultant polymers were analyzed. 

Conductivity studies of quaternary ammonium salts and polymerization systems, experiments with additions of neutral salts, and investigations on the counterion effect make it possible to state that free ions and ion pairs serve as active centers in the polymerization of nitrogen-containing cycles and also to separate the contributions of the former and the latter to the effective chain propagation rate constant (see Fig. 8). 

Both freshly metalated TMED and aged product contain an active Li-CH2N structure shown by ethylene polymerization activity and trans-metalation reactions. For example metalated TMED reacts slowly with toluene to produce TMED LiCH2. Hydrogenation of 0.5M TMED LiBu complex in n-heptane aged one week at 25 °C gave an 82% recovery of TMED and 18% decomposition products. These experiments also demonstrated that the rearrangement observed by NMR did not involve the skeletal structure of TMED. 

Under identical reaction conditions, no polymerization of the 4-pentenyl-9-BBN derivative was observed. It was concluded that the electron-withdrawing borane moiety must be removed from the double bond by at least three carbon atoms for normal polymerization activity to be observed. No direct rate comparisons between these boron derivatives and comparable a-olefin hydrocarbons were made. However, experiments polymerizing 1-octene with and without added triethylborane showed that the borane had no deleterious side effects on the reaction. 

Metallocene catalysts for the polymerizations of olefins have been known since early 1957 when Natta and co-workers first reacted triethyl aluminum (AlEts) and bis(cyclopentadienyl) titanium dichloride ( -C5H5)2TiCl2 to form a complex that polymerized ethylene. The structure of this complex was described and the polymerization results reported. With ethylene they reported to have made 7 g of crystalline polyethylene in about 8 h at 95°C with 40 atm ethylene pressure in n-heptane. Later in the same year, Breslow and co-workers repeated Natta s experiments. They found that the blue complex described by Natta was a somewhat poor catalyst (in agreement with Natta s findings), but discovered that small amoimts of oxygen in the ethylene boosted polymerization activity. When compared to the heterogeneous Ziegler-Natta catalyst system, these metallocene catalysts were poor with respect to polymerization activity. They were used essentially for mechanistic studies because of their simplicity and ease of structure elucidation. 

According to these experiments the polymerization of olefins is caused by a member (all members ) of the A species, while the B type centres cause some isomerization of the monomer. With catalysts of the same preparative history the specific polymerization activity is not dependent on the Cr concentration up to 1% [61, 62]. The reaction rate (as measured by the decrease of the monomer) is decreasing with tic from ethylene to 1-hexene [22],... 

Sahoo and Mohapatra [66] studied the catalytic effect of the in situ developed Cu(II)-EDTA complex with ammonium persulfate on the surfactant-free emulsionpolymerization of methyl methacrylate. The rate ofpolymerization at 50 °C is proportional to the concentrations of Cu(II), EDTA, ammonium persulfate, and methyl methacrylate to the 0.35, 0.69, 0.57, and 0.75 powers, respectively. In addition, the apparent activation energy and activation energies of the initiator decomposition, propagation, and termination reactions, respectively, are 34.5,26.9,29, and 16 kJ mol. It was proposed that the complex just acts as an effective surfactant in stabilizing the polymethyl methacrylate nanoparticles nucleated during polymerization. Independent experiments are required to verify this speculation and clarify the related stabilization mechanism. 

Sited with chromium in the same group 6B in the Periodic Table of Elements, molybdenum attracts many efforts in the study of ethylene polymerization. As early as in 1950s, Indiana Standard Oil Company discovered supported molybdenum oxide catalyst was active for ethylene polymerization (Field and Feller, 1957). However, the catalyst was then abandoned because of its poor catalytic performance. Our preliminary experiments on the MoO /Si02 catalyst also showed very low reactivity with comparison to the Cr0 y/Si02 catalyst for ethylene polymerization. In order to improve the activity of Mo-based catalyst for ethylene polymerization, the active valence states of molybdenum sites, and the mechanism of the catalytic reaction should be first elucidated. We performed a detailed theoretical study combined with experiments to investigate the active oxidation states of molybdenum and the effects of surface hydroxyl on the polymerization activity of supported Mo-based catalysts (Cao et al., 2010). 

---