Activation energy emulsion polymerization

Figure 4. Styrene emulsion polymerization—variation of the propagation constant with conversion during adiabatic polymerization of 395-A latex particles (kp values normalized to 75 °C with an activation energy of 7700 cal/gmol and in L/mol s)...

From the discussion above, it is clear that there is no evidence for catalysis of persulfate initiation in emulsion polymerization systems. However, many ionic reactions have been shown to be subject to large catalytic effects in the presence of emulsifier micelles (Fendler and Fendler, 1975) so that the question arises as to whether there are any radical reactions that are subject to micellar catalysis and whether this phenomenon plays any part in any emulsion polymerization systems, Prima fade evidence that uiicellar catalysis may be important when emulsified monomer is allowed to polymerize thermally is provided by the work of Asahara et al. (1970, 1973) who find that several emulsifiers decrease the energy of activation for thermal initiation of alkyl methacrylate and styrene, [n particular, the energy of activation for thermal initiation of styrene emulsified with sodium tetrapropylene benzene solfonate was reported as S3 kl mol. much lower than any value determined in bulk. Hui and Hamielec s value of ] IS kj tnol (1972) seems to be representative of the data available on thermal initiation in bulk. The ctmclusions of Asahara et al. are based on observations of the temperature dependence of the degree of polymerization and are open to several objections. 

An alternative method of calculating the energy of activation for initiation in emulsion polymerization was introduced by Bartholome et al. 

Ballantine (4) observed that the y-induced emulsion polymerization of styrene is about 100 times faster and yields higher molecular weights (up to 2 X 10 ) than the y-induced bulk polymerization. He explains the large difference in reaction rates by the high radical yield (G/ value) of water, as compared with the G/j value of styrene. An over-all activation energy of 3.7 kcal. per mole was calculated from the temperature dependence of the reaction. Allen et al. (1) prepared and grafted polystyrene and poly (vinyl acetate) dispersions under the influence of y-radiation. Mezhirova et al. (28) found a temperature-independent reaction rate of the y-induced emulsion polymerization of styrene. 

Hence, in order to ensure a relatively high rate of radiation-induced polymerization in the emulsion, low dose rates (0.1-1 Gy/s) and low absorbed doses are required. For all these reasons, radiation emulsion polymerization is particularly advantageous from an economical standpoint. Its activation energy, just as for other processes of radical radiation-induced polymerization, is 15-35 kJ/mol. The molecular weight of polymers increases with temperature, as in the case of typical free-radical processes (to a certain extent). This increase is due to an increase in kp with temperature, whereas k, does not depend on the temperature. 

The hydrogen peroxide itself undergoes S, reduction to hydroxyl ion and hydroxyl radical, the latter being able to initiate a polymerization reaction, as schematically indicated in the second part of the equation. Certain redox systems require a comparatively low activation energy, and if such a system is employed in an emulsion polymerization, the reaction can proceed at a temperature of, for instance, 0°C with a satisfactory rate. 

Often times, polymeric emulsions are formed. All emulsions are thermodynamically unstable because their Interfacial area is orders of magnitude greater than the Interfacial area of the corresponding coagulated systems. A so-called "stable emulsion" is in reality a meta-stable system. The input of a certain activation energy is necessary for coagulation to occur, and the... 

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