Emulsion polymerization propagation rate constants from

The ideal case of the Smith-Ewart treatment actually proposes a rather elegant method for obtaining the absolute value of the propagation rate constant from emulsion polymerization systems, as shown in Eq. (2.26), where N is the number of particles per imit volume ... 

BLE 2.4 Propagation Rate Constants from Emulsion Polymerization ... 

Experiments using no emulsifier were conducted in tbe same stainless steel autoclave equipment described above (Machi et nf.. 1975). Stable latices were obtained, believed to be achieved by hydroxyl end groups and adsorbed hydroxyl ions. As with a number of the experiments with emulsifiers the polyethylene had a considerable cross-linked gel content. Finally, the same group of workers studied tbe radiation-induced emulsion polymerization of ethylene in a flow system (Kodama et al, 1974). Both potassium inyristate and ammonium perfluorooctanoate were used as emulsifiers. At longer residence times (above 0.2 hr) the rate of polymerization was essentially constant. As with the batch system it was assumed that the number of particles remained constant. In this region the rate was found to be proportional to tbe 0.3 power of the potassium myristate concentration and the 0.5 power of the dose rate, not too different from the batch systems. The kinetics was developed and estimates of tbe propagation rate constants obtained. Despite other similarities between the two systems, these were quite different, however, from those extracted from the batch experiment. 

Values for the propagation rate constant can be determined from bulk or solution experiments. Values of k have been published for a wide variety of monomers as a function of temperature. With standard emulsion polymerization recipes the value of [M]p is determined from equilibrium swelling measurements if a free monomer phase is present and by a mass balance if all the monomer is in the polymer particles. One normally assumes that [M] is not dependent on particle size in latexes comprised of different-sized particles. This assumption will be questionable in some systems, especially those involving high-swelling particles. 

The monomer concentration within the forming latex particles does not change for a long period due to the diffusion of monomer from the droplets to the polymerization loci. Therefore, the rate of the propagation reaction does not change and a constant polymerization rate period is observed in a typical emulsion polymerization system. 

Propagation constants for butadiene and isoprene were determined from rate of polymerization per particle in emulsion polymerization. 

The rate of polymerization in emulsion polymerization is proportional to kg-, where kg is the fhain transfer step on the vinyl group (10). Substituting trideuterovinyl acetate for vinyl acetate raised the rate by a factor of 1.76. When the calculation for the isotope effect on rate is done accurately, taking into account the 3% H on the trideuterovinyl, we find that if the effect is purely on k3, the rate should rise by a factor of 1.69 as compared to 1.76 . 02. This is almost within the experimental error. There may be a very slight secondary isotope effect (23,24) on the propagation and re-ini tation rate constants k2 and k, but it cannot be decided from these data. 

In the examples described above, the transition is shown from ideal (n = /2) to nonideal (n > /2) behavior. There are, however, systems for which ideal emulsion polymerization practically cannot be achieved. It is nevertheless possible to describe the kinetics of such systems quantitatively. Recently, Gerrens has obtained values of the propagation and termination rate constants at diflFerent temperatures for vinyltoluene and vinylxylene (28). The termination rate of polymer radicals of these monomers is so low that even at small rates of initiation in small particles, n is larger than /2. From measurements of the reaction rate before and after injection of additional initiator in the polymerizing system it was possible to calculate n both at the original and at the boosted initiation rate with the aid of Equation 5. Consistent results were obtained when the additional amount of initiator was varied. From these rate data, the termination rate constant was found to be 10 and 17 liters mole- sec. at 45° C. for vinyltoluene and vinylxylene, respectively. These values are to be compared with 10 for styrene (Table IV). 

Equation (2.26) leads to a solution for from available knowledge of the rate R, the concentration of monomer in the monomer-polymer particles [M], and the number of particles, N. This method has been applied to several monomers and has been especially useful in the case of the dienes, where the classical method of photoinitiation poses difficulties. Some of these results are shown in Table 2.4 in the form of the usual kinetic parameters. The results obtained for styrene by photoinitiation techniques are included for comparison. It can be seen that the agreement is remarkably good, considering the widely different experimental methods used. Recent studies of the emulsion polymerization of butadiene have shown that the rate constant for propagation is even higher than previously estimated (see Table 2.1) (Weerts et al., 1991). 

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