Activation energy olefin polymerization

The chain-end stereocontrol for olefin polymerizations leads generally to lower stereoselectivities (differences in activation energy for insertion of the two enantiofaces generally lower than 2 kcal/mol) than the chiral site stereo-control.18131132 For this reason, the corresponding catalytic systems have not reached industrial relevance for propene homopolymerization. However, some of them are widely used for propene copolymerization with ethene. 

If we consider as an example the addition of HC1 to ethylene, we find that whereas the propagation step for polymerization will be exothermic by about 30 kcal mole-1,146 abstraction of H from HC1 by the R—CH2- radical will be endothermic by 5 kcal mole-1. Activation energies for typical polymerization propagation steps are in the range of 6-10 kcal mole-1,147 and that for abstraction from HC1 will have to be greater than the 5 kcal mole-1 endothermicity. These data are at least indicative that radical addition of HC1 will not be favorable experimentally, it is indeed rare, but can be made to occur with excess HC1.148 With HBr the situation is different. Now the hydrogen abstraction is exothermic by about 10 kcal mole-1 and occurs to the exclusion of telomeriza-tion.149 Hydrogen iodide does not add successfully to olefins because now the initial addition of the iodine atom to the double bond is endothermic. 

In the preceding chapter it has been shown that the DFT methods currently available can be used to reproduce relative trends in both reactivities and transition-metal NMR chemical shifts. Thus, NMR/reactivity correlations can be modeled theoretically, at least when relative reactivities are reflected in relative energies on the potential energy surfaces (activation barriers, BDEs). It should in principle also be possible to predict new such correlations. This is done in the following, with the emphasis on olefin polymerization with vanadium-based catalysts. 

The kinetic parameters of the propagation of olefin polymerization on different active centers are compiled in Table 10. Apparently, for both the transition and non-transition metal compounds the insertion of the olefin into Mt—C bonds proceeds with the participation of coordinatively unsaturated metalalkyl compounds via intermediate n-complexes. The higher reactivity of transition metal compounds compared with organoaluminium compounds is primarily due to the lower activation energy of the propagation step when Mt is a transition metal. Many facts indicate that polarization of the Mt—C bond does not determine the reactivity of metalalkyl compounds in olefin addition, e.g. due to the decrease of reactivity in the order... 

Apparently, the reactivity of organometallic compounds in the addition of olefins to Mt—C bonds is determined by the capability of these compounds to coordinate olefins. The formation of intermediate n-complexes ensures further insertion of olefin by a concerted mechanism with a low activation energy. Thus, a high reactivity of active centers, containing a transition metal, comparable to the reactivity of the radical active centers, is achieved. The activation energy of the propagation in olefin polymerization on catalysts containing transition metals (2-6 kcal/mol) does not exceed its value for the radical polymerization (Table 10). 

This olefin polymerizes with TiCl4/Al(i-Bu)3 (Al/Ti = 1/2) to form low molecular weight polymers [185]. Rates are first order in monomer concentration and from the initial values the apparent propagation rate coefficient is ca. 6 x 10 1 mole sec at 50°C, the activation energy being 9.5 kcal mole . This is very similar to the rates observed with propene and butene-1, and suggests that fep has a comparable magnitude. 

Problem 9.1 From the accurate kinetic data that have been obtained [12] for the polymerization of CsH with a-TiCls and Al(CoH5)3 it appears that in the steady state the rate is strictly proportional to the pressure of CsHe- The polymerization rate is also proportional to the amount of a-TiCla and independent of the concentration of Ai(C2H5)3. Suggest a kinetic scheme in conformity with these observations. A qualitative use may be made of the fact that an activation energy of 11-14 kcal/mol has been observed for this polymerization and that no stable complex between a-olefins and Ti has been found. 

The rapidity of the reaction can be seen by the large effect low pressures ( 1 torr) of oxygen can have on the free radical polymerization of a reactive olefin such as styrene [22]. The reaction rate coefficients are expected to be typical for exothermic radical—radical reactions with essentially no activation energy. Thus, if R is alkyl, log(feQ/l mole-1 s-1) would be 9.0 0.5, and be independent of temperature. For simple resonance-stabilized radicals, log(feD/l mole-1 s-1) would be 8.5 0.5. 

Bulk polymerization of vinylene carbonate (VCA) initiated by 60Co y-rays was studied at 30°-110°C at a constant dose rate of 1 - 105 rad/hr. An overall activation energy of 5.0 kcal/mole and a maximum reaction rate of 1 10 3 mole/l-sec were obtained. As has been reported, purification of the monomer is a crucial point because inhibiting impurities are formed during the synthesis. From experiments with chlorine-substituted ethylene and vinylene carbonates, we tentatively conclude that, in addition to mono- and dichloroethylene carbonate, dichloro-vinylene carbonate is mainly responsible for the inhibition. The copolymerization behavior of VC A with some chlorine-substituted olefins was studied. Chlorotrifluoroethylene (CTFE) is an especially suitable comonomer the reactivity ratios found were rVCA = 0.42 and rCXFE = 0.48. 

By studying the changes in the infinred absorption band at 1645 cm (an olefinic double bond stretching band) with time, the rate, order of initial polymerization reaction, activation energy, and the extent of the reaction could... 

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