Emulsion polymerization Smith-Ewart behavior

Panajkar and Rao (1979) have reported a ratber extensive study of the y radiation-initiated polymerization of vinylidene chloride in emulsion. With sodium lauryl sulfate as the emulsifier smooth polymerization-time curves at high rates were obtained, up to more than 9 conversion. Between 45 and 60% conversion, the linear region, the rate was 0.3 order with respect to tbe emulsifier concentration. The molecular weights were found to increase with conversion and values up to 79,000 were obtained. Some reasons for the departure from Smith Ewart behavior were suggested. Earlier. Hummel el al. (1967) had presented some interesting data on a closely related system, a similar rate-time behavior was observed and a tentative explanation proposed. Both discussions were based on tbe insolubility of the polymer in its own monomer. 

A polymerization with n very large (102-6) indicating suspension-like (bulk polymerization) kinetic behavior, and a particle nucleation mechanism residing outside the monomer droplets, which delineates an emulsion process. Smith-Ewart case III systems are examples of this type of behavior provided they have evolved from case I and/or case II polymerization at low conversions, which is common. 

In an emulsion polymerization of styrene in a 50 m CSTR, the feed contains 5.845 mol of styrene per liter. For an average conversion of 75% in the reactor and assuming the Smith-Ewart behavior is followed, estimate ... 

Emulsion Polymerization. Emulsion and suspension reactions are doubly heterogeneous the polymer is insoluble in the monomer and both are insoluble in water. Suspension reactions are similar in behavior to slurry reactors. Oil-soluble initiators are used, so the monomer—polymer droplet is like a small mass reaction. Emulsion polymerizations are more complex. Because the monomer is insoluble in the polymer particle, the simple Smith-Ewart theory does not apply (34). 

In the case of more water-soluble monomers and (amphiphilic) macromonomers, the Smith-Ewart [16] expression does not satisfactorily describe the particle nucleation. The HUFT [9,10] theory, however, satisfactorily describes the polymerization behavior or the particle nucleation of such unsaturated hydrophilic and amphiphilic monomers. The HUFT approach implies that primary particles are formed in the aqueous phase by precipitation of oligomer radicals above a critical chain length. The basic principals of the HUFT theory is that formation of primary particles will take place up to a point where the rate of formation of radicals in the aqueous phase is equal to the rate of disappearance of radicals by capture of radicals by particles already formed. Stabilization of primary particles in emulsifier-free emulsion polymerization may be achieved if the monomer (or macromonomer) contains surface active groups. Besides, the charged radical fragments of initiator increases the colloidal stability of the polymer particles. 

The kinetic behavior of emulsion polymerization is greatly affected by radical desorption from polymer particles. This has been shown by Dgelstad et al. (1969)> Litt et al. (1970), Harada et nl. (1971), Friis and Nyhagen (1973), and Nomura et al (1971). It is believed that the deviation of the kinetic behavior of the emulsion polymerization of water-soluble monomers such as vinyl acetate and vinyl chloride from the Smith and Ewart (1949) Case 2 kinetic theory is mainly due to dominant desorption of... 

The value of n in Eq. (6.230) is of critical importance in determining the rate of polymerization in stage II. Three cases — designated 1, 2, 3 — corresponding, respectively, to n < 0.5, n = 0.5, and n > 0.5 can be distinguished based on the work of Smith and Ewart [69] and others [70-74]. The kinetic treatment given above conforms to Case 2 (n = 0.5), which is the predominant behavior for emulsion polymerizations. It occurs when desorption of radicals does not occur or is negligible compared to the... 

Smith and Ewart (13a. 13b) quantified the Harkins theory by the equation R = k MpN/2 where Rp is the rate of propagation, kp is the rate constant for propagation, M is the monomer concentration in growing chain particles, and N the number of polymer particles per unit volume. If M is the constant, this equation is reduced to R = k N. Thus, the rate of emulsion polymerization should solely be a function of the number of polymer particles. In actuality, the reaction rate increases up to 20-25% conversion because of the increase in the number of growing radical chains then the rate steadies as does the number of polymer particles up to 70-80% conversion. Beyond this point, the rate drops off because of low monomer concentration. Thus, as Talamini (13c. 13d) has noted, available evidence indicates that emulsion polymerization of vinyl chloride does not resemble true emulsion polymerization as described by Smith and Ewart, but shows the general behavior of heterogeneous polymerization. 

A Smith-Ewart case 1 behavior in 1) can be observed during emulsion polymerization of monomers with a high chain-transfer rate constant such as vinyl chloride because the monomer radicals have a high tendency to escape from the particles. Especially for vinyl chloride emulsion polsunerization, Ugelstad and Hansen (113) derived the following equation 18 to calculate n, which predicts dependence, h a... 

The time-dependent behavior of emulsion polymerization arises due to variation in monomer concentration, changes in the number of polymer particles Nt, or both. We have aheady observed that N, changes due to nucleation in stage I of emulsion polymerization and this normally ends at about 10-15% conversion. However, when the monomer-to-water ratio MIW) is high or the monomer is more than sparingly soluble, the constancy of N, cannot be assiuned up to conversions as large as 50%. If the monomer droplets are sufficiently small, they also become the loci of particle formation and, in such circumstances, the Smith-Ewart theory is inadequate to explain the experimental phenomena. We now present the outline of a mathematical model of emulsion polymerization that is... 

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