Polymerization, activation pressure effects

Under the applied temperature and pressure conchtions the formed radicals lead to very fast ethylene polymerization (typically 20% ethylene conversion in 40 s). The concentration of ethene and the reaction pressure both affect the rate of ethene polymerization. This pressure effect is generally treated in terms of a volume of activation, similar to the energy of activation. On a molecular level one can understand this pronounced pressure effect on the reaction rate in the sense that the pressure promotes the configuration change necessary for the reactants to reacdi the transition state. The contribution of pressure to the reaction rate of PE... 

Our discussion here explores active connections between the potential distribution theorem (PDT) and the theory of polymer solutions. In Chapter 4 we have already derived the Flory-Huggins model in broad form, and discussed its basis in a van der Waals model of solution thermodynamics. That derivation highlighted the origins of composition, temperature, and pressure effects on the Flory-Huggins interaction parameter. We recall that this theory is based upon a van der Waals treatment of solutions with the additional assumptions of zero volume of mixing and more technical approximations such as Eq. (4.45), p. 81. Considering a system of a polymer (p) of polymerization index M dissolved in a solvent (s), the Rory-Huggins model is... 

As in the previous section, hydrogen increases the concentration of the active sites. Moreover, we think that the polymerization is preceded by the formation of a titanium hydride complex. If this is correct, hydrogen also increases the overall catalytic activity. The effects of hydrogen pressure on catalytic activity and molecular weight of SPS are summarized in Table 17.7. 

Asymmetric induction polymerization, MA with optically active monomers, 318, 383 Atropine, MA copolymer reaction, 443 Auramine O, EMA resin interactions, 433 Azelaic acid, 151 Azeotrpic copolymer index, 291 Azeotropic copolymerizations, pressure effects, 291 Azepine, acryloyl derivative MA copolymerization, 382... 

Table II summarizes the yields obtained from the CONGAS computer output variable study of the gas phase polymerization of propylene. The reactor is assumed to be a perfect backmix type. The base case for this comparison corresponds to the most active BASF TiC 3 operated at almost the same conditions used by Wisseroth, 80 C and 400 psig. Agitation speed is assumed to have no effect on yield provided there is sufficient mixing. The variable study is divided into two parts for discussion catalyst parameters and reactor conditions. The catalyst is characterized by kg , X, and d7. Percent solubles is not considered because there is presently so little kinetic data to describe this. The reactor conditions chosen for study are those that have some significant effect on the kinetics temperature, pressure, and gas composition.

Organic peroxides, which readily decompose into free radicals under the effect of thermal energy, are used under high pressures as initiators for radical polymerizations. The measurement of the influence of pressure on the rate of decomposition gives rise to the determination of the activation volume, which, in turn, allows conclusions to be drawn on the decomposition mechanism and the transition state. 

Methyl branching also occurs in the preparation of many polymers. Ordinarily methyl branches are present in only trace amounts, barely detectable by the most sensitive 13C-NMR(69), and they have no effect on polymer properties. But if the polymerization is carried out at atmospheric pressure or less, methyl branching becomes much more common (69). It may originate from isomerizarion at the active site. This isomerization is probably not favored but does occur very rarely (Scheme 2). Low pressure decreases the propagation rate, allowing each chain to spend more time around the active site, thus increasing the probability of such side reactions. Internal... 

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. 

These results, opposite to those observed for the cyclopentadienyl early transition metal systems, can be accounted for in terms of the known influence of the cocatalyst concentration on the possible eliminations, alkyl transfer pathways, and other deactivation processes [45,46]. Under similar conditions, the polymerization of ethylene at high pressure leads to a considerable increase in activity and produces polymers of higher molecular weight than at atmospheric pressure (entries 6 and 3). This effect is a consequence of the rate of insertion, which is proportional to the monomer concentration in solution. 

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