Polymerization, activation kinetics

The first generation of catalysts based on titanium tetrachloride or different modifications of titanium trichloride and ethyl-, isobutyl-, isoprenyl-, or chloride containing aluminum compounds, Ziegler catalysts, (Table 1) was characterized by low polymerization activity. Kinetic studies and applications of various methods have helped to define the nature of the active center . ... 

The so-called Klenow fragment of DNA polymerase 1 of E. coli (Chapter 14, section Al) contains the 5 -3 -polymerization and the 3 -5 -exonuclease domains. Detailed pre-steady state kinetics have been made of the polymerization and exonuclease activities.39-43 The editing site is 35 A away from the polymerization site.32 The mechanism of the polymerization activity (Figure 13.7) is very similar to that for hydrolysis (Figure 13.8). The key to both is the presence of two metal ions, 3.9 A apart, that stabilize the developing charges on the transition state and metal-bound HO- or RO ions (see Chapter 2, section B7).44,45... 

On the other hand, there is no doubt that diffusion through the silica does affect polymerization activity (although probably not the kinetic profile). This is discussed in Section IV. 

In addition to these two studies the polymerization kinetics of three different Nd-compounds which were activated by DIBAH and EASC were comparatively studied. In this investigation a Nd alcoholate [NdA = neodymium(III) neopentanolate], a Nd phosphate [NdP = neodymium(III) 2-ethyl-hexyl-phosphate] and a Nd carboxylate (NdV) were compared with a special focus on the variation of the molar ratios of zzdibah/hncI and ci/ Nd [272]. For each of these ternary catalyst systems the polymerization activities depend... 

In addition to the patent literature available on the production of BR in the gas-phase there is some scientific literature which mainly refers to the modeling of reaction kinetics. Details on the experimental procedure for the determination of the macroscopic kinetics of the Nd-mediated gas-phase polymerization of BD in a stirred-tank reactor are reported [568,569]. Special emphasis is given to video microscopy of individual supported catalyst particles, individual particle growth and particle size distribution (PSD). These studies reveal that individual particles differ in polymerization activity [536,537,570,571]. Reactor performance and PSD are modeled on the... 

Cyano-7,7,8,8-tetrachloro-p-xylene(CTCX) is prepared similarly by gas-phase dechlorination of 2,5-bistrichloromethylbenzonitrile on a copper mesh 59). CTCX also readily undergoes spontaneous polymerization, the kinetics of which is approximately first order with respect to monomer at monomer concentrations higher than 2-3 x 10 3 mol/1 whereas it is second order below this monomer concentration. The apparent first-order rate constant has been determined as 1,3 x 10s 1 at 30 °C at an initial monomer concentration of 1.0 x 10-2 mol/1. The apparent activation energy of the polymerization is 8.8 kcal/mol. In addition, the temperature at which the apparent rate constant is 5.6 x 10-5 s 1 has been found to be 15 °C for CTCX. The tendency of CTCX for homopolymerization is higher than that of HCX (30 °C) and much lower than that of QM (—68 °C). 

Aggregation of centres is a thermodynamic category manifested in the process kinetics. By changes of temperature, component concentration, and medium (polarity and solvent power of the solvent), equilibria are shifted to new values. The number of inactive centres is generally changed. Therefore termination by aggregation of centres does not necessarily mean final and total loss of polymerizing activity. This usually occurs by other, irreversible processes. 

When the Cr02Cl2 adsorbed as chromate, such as on silica that had been calcined at 400 °C, normal polymerization activity was observed at 100 °C and a concentration of ethylene of 1.0 mol L-1 in isobutane. Indeed, the activity was nearly identical to that of Cr03/silica activated at 400 °C. The kinetics profile of the polymerization reaction was also the same, as shown in Figure 7. The polymer FILMI, MW, and MW breadth were also almost the same, as was the UV-vis reflectance spectrum. In contrast, the chlorochromate catalysts were not active for ethylene polymerization under these conditions. Thus, the monochromate species... 

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. 

One of the most interesting results of this approach of using "two-valent" chromium species is the effect on catalyst activity. Yields as high as 16 kg g 1 h 1 were obtained, which was more than twice that of the chromium oxide parent, and many times more than that of the chromium alkyl when deposited on silica. The induction time of chromium oxide was eliminated, and also the declining kinetics profile of the chromium alkyl catalyst. That is, the hybrid catalyst seemed to have incorporated the best aspects of both parents to yield unusually high polymerization activity. 

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