Free radical polymerization activation energy

A series of simulations were performed to study the effect of variables such as initiator concentration, initiator half-life and activation energy on the optimum temperature and optimum time. It was assumed that initially the polymerization mixture contained S volume percent monomer, the rest of the mixture being solvent and polymer formed earlier. It was required to reduce the monomer concentration from S volume percent to 0.S volume percent in the minimum possible time. The kinetic and tbeimodyamnic parameters used are similar to those of free radical polymerization of MMA. The parameter values are given in Appendix B. 

As done in Chapter 5, the effect of temperature can be determined using average activation of the various steps. Again, the rates of all single step reactions increase as the temperature increases but the overall result may be different for complex reactions. For free radical polymerizations the activation energies are generally of the order Ei>Ei E > El. Remembering that the description of the specific rate constant is... 

Many polymerizations are carried out at temperatures between 0 and 100°C. Initiation at the required rates under these conditions is confined to compounds with activation energies for thermal homolysis in the range 1(X)-165 kJ/mol. If the decomposition process is endothermic, the activation energy can be considered to be approximately equal to the dissociation energy of the bond which is being split. It can be expected, then, that useful initiators will contain a relatively weak bond. (The normal C—C sigma bond dissociation energy is of the order of 350 kJ/mol, and alkanes must be heated to 3(X)-500°C to yield radicals at the rates required in free-radical polymerizations.)... 

For a given tube radius there exists a particular wall temperature that gives maximum conversions in free-radical polymerizations. This can be seen qualitatively from the following considerations. If the tube wall is loo cool, the initiator will be slowly decomposed and some of it will leave the reactor unconsumed. However, the activation energy for initiator decomposition exceeds that for consumption of monomer (Section 6.16.1), and the initiator can be entirely decomposed at low monomer conversions if the wall temperature is too high for the particular reaction system [2]. 

Free radical polymerization offers a convenient approach toward the design and synthesis of special polymers for almost every area. In a free radical addition polymerization, the growing chain end bears an unpaired electron. A free radical is usually formed by the decomposition of a relatively unstable material called initiator. The free radical is capable of reacting to open the double bond of a vinyl monomer and add to it, with an electron remaining unpaired. The energy of activation for the propagation is 2-5 kcal/mol that indicates an extremely fast reaction (for condensation reaction this is 30 to 60 kcal/mol). Thus, in a very short time (usually a few seconds or less) many more monomers add successively... 

Table 2 Energies of activation for propagation ( p) and termination ( t) in free-radical polymerization...

In general, a polymerization process model consists of material balances (component rate equations), energy balances, and additional set of equations to calculate polymer properties (e.g., molecular weight moment equations). The kinetic equations for a typical linear addition polymerization process include initiation or catalytic site activation, chain propagation, chain termination, and chain transfer reactions. The typical reactions that occur in a homogeneous free radical polymerization of vinyl monomers and coordination polymerization of olefins are illustrated in Table 2. 

From this relation it can be seen that the free-radical polymerization will be of first order in monomer, M, and the effective rate coefficient, will have a temperature dependence that will depend on the activation energies E, Ep and of the elementary reactions ... 

Though ionic polymerization resembles free-radical polymerization in terms of initiation, propagation, transfer, and termination reactions, the kinetics of ionic polymerizations are significantly diflFerent from free-radical polymerizations. In sharp contrast to free-radical polymerizations, the initiation reactions in ionic polymerizations have very low activation energies, chain termination by mutual destruction of growing species is nonexistent, and solvent effects are much more pronounced, as the nature of solvent determines whether the chain centers are ion pairs, free ions, or both. No such solvent role is encountered in free-radical polymerization. The overall result of these features is to make the kinetics of ionic polymerization much more complex than the kinetics of free-radical polymerization. 

Free-radical polymerization can be initiated by a wide range of energy sources. In principle, methacrylates can be cured not only by thermal activation, but also by ultraviolet and electron radiation. 

Turning to the comparison between the rate constants for the chain propagation in the free radical polymerization of methyl and butyl acrylayes, it can be observed that both these reactions should occur with the same entropy decrease, because identical double bonds are involved. From the experimental data by Melville and Bickel (1 3) and by Bengough and Melville (14) relative to butyl acrylate, 4 pairs of activation energy and entropy can be calculated, which are collected in Table IV. It is evident that the experimental activation entropy which is closest to the calculated ASp for alkyl acrylates (i.e. the ASp value reported for methyl acrylate in Table III) is -12.+. f j/mol K, whereas all the other activation entropies seem to be too high. The rate constant calculated at JO°C from... 

Table IV - Experimental activation energy and entropy and rate constant at 30°C for the chain propagation in the free radical polymerization of butyl acrylate.

Polymerization in bulk, that is, of undiluted monomer, minimizes any contamination of the product. Bulk polymerization is difficult to control, however, due to the high exothermicity and high activation energies of free-radical polymerization and the tendency toward the gel effect in some cases. 

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. 

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