Theories of Emulsion Polymerization

Kinetics and Mechanisms. Early researchers misunderstood the fast reaction rates and high molecular weights of emulsion polymerization (11). In 1945 the first recognized quaHtative theory of emulsion polymerization was presented (12). This mechanism for classic emulsion preparation was quantified (13) and the polymerization separated into three stages. 

The increase in iV, and therefore in the rate as well, with initial soap concentration is thus explained. Quantitative results agree approximately with the predicted three-fifths power dependence. The prediction of an increase in polymerization rate with also has been confirmed by experiments at variable initiator concentrations.t Most important of all, the actual number of particles N calculated from Eq. (35) agrees within a factor of two with that observed. It is thus apparent that the theory of emulsion polymerization developed by Harkins and by Smith and Ewart has enjoyed spectacular success in accounting for the unique features of the emulsion polymerization process. 

Although there have been several references on the theory of emulsion polymerization kinetics, it is surprising that its commercial importance and multiphase kinetics have not generated more interest. Smith and Ewart (46) predicted a constant rate of polymerization per particle, based on initiation in the water phase and three ranges of radical concentration per particle. Van der Hoff (55) confirmed this for concentrations... 

The simple Harkins model is the basis of most quantitative theories of emulsion polymerization. 

These examples illustrate the great variety of current ideas. A generally accepted theory of emulsion polymerization, based on a non-uniform composition of the monomer—polymer particles, is presently being developed [152-155]. 

The most widely accepted theories of emulsion polymerization (5, ) point out that new latex particles are generated only as long as soap In excess of the particle surface adsorption requirements is present In the system. After sufficient particle surface area has developed to adsorb the soap in the aqueous... 

Most papers involving the theory of emulsion polymerization begin with a brief discussion of the classical work of Smith-Ewart (1). Their Case II theory, tested extensively with styrene batch reactions, yields the following prediction for polymerization rate (R ) and peurticle number (N). ... 

Emulsion Polymerization. Emulsion SBR was commercialized and produced in quantity while the theory of the mechanism was being debated. Harkins was among the earliest researchers to describe the mechanism (16) others were Mark (17) and Flory (18). The theory of emulsion polymerization kinetics by Smith and Ewart is still valid, for the most part, within the framework of monomers of limited solubility (19). There is general agreement in the modem theory of emulsion polymerization that the process proceeds in three distinct phases, as elucidated by Harkins (20) nudeation (initiation), growth (propagation), and completion (termination). 

This assumption is supported by other observations. The most important of these observations was the increase of Obf, when the dose rate was enhanced at different stages of polymerization. In all these cases, the increase of Ubf followed rather exactly the square root law (ubf I ). According to the theory of emulsion polymerization, extended by Gerrens (J6), this behavior must be expected for the case of the gel effect. The high molecular weight of PM A is another support for our assumption. 

The original theory of emulsion polymerization is based on the quahtative picture of Harkins (1947) and the quantitative treatment of Smith and Ewart (1948). The essential ingredients in an emulsion polymerization system are water, a monomer (not miscible with water), an emulsifier, and an initiator which produces free radicals in the aqueous phase. Monomers for emulsion polymerization should be nearly insoluble in the dispersing medium but not completely insoluble. A slight solubility is necessary as this will allow the transport of monomer from the emulsified monomer reservoirs to the reaction loci (explained later). 

According to O Donnell et al. [130], the emulsion polymerization of vinyl acetate follows the Smith-Ewart theory of emulsion polymerization [131] because the rate of polymerization is independent of the total amount of monomer present, the rate is a function of the 0.6th power of the emulsifer concentration, and the rate of emulsion polymerization is a function of the 0.7th power of the initiator concentration instead of the expected 0.4th power. In this work poly(vinyl alcohol), 88% hydrolyzed with a medium molecular weight (i.e., Du Font s Elvanol 52-22), was used as the only externally added emulsifier. Light-scattering studies indicated that this emulsifier formed no aggregates in the aqueous solution. These latter observations may, however, have been made at room temperature and not at the reaction temperature [1]. The conversion versus time curve was essentially linear up to 80% conversion. 

Since the solubility of vinyl acetate is 2.1% at 50°C and 3.5% at 70°C [15], deviations from the Smith-Ewart treatment are not entirely surprising. The water solubility of vinyl acetate was one of the significant factors in the deviation from the conventional theory of emulsion polymerization. Another factor is the reactivity of the vinyl acetate radicals toward other materials present in the system such as the surfactants. 

The Smith-Ewart kinetic theory of emulsion polymerization is simple and provides a rational and accurate description of the polymerization process for monomers such as styrene, butadiene, and isoprene, which have very limited solubility in water (less than 0.1%). However, there are a number of exceptions. For example, as we indicated earlier, large particles (> 0.1 to 0.5 cm diameter) may and can contain more than one growing chain simultaneously for appreciable lengths of time. Some initiation in, followed by polymer precipitation from the aqueous phase may occur for monomers with appreciable water solubility (1 to 10%), such as vinyl chloride. The characteristic dependence of polymerization rate on emulsifier concentration and hence N may be altered quantitatively by the absorption of emulsifier by these particles. Polymerization may actually be taking place near the outer surface of a growing particle due to chain transfer to the emulsifier. 

Free-radical Initiated Processes.—A great deal of work has appeared concerning emulsion copolymerizations such as that of Khanna and Noonan who have described the syntheses of copolymers of acrylic esters with uniform composition by continuous polymerization processes. Emulsion processes will not be considered further here except to mention that the kinetics and current problems in the theory of emulsion polymerization have been reviewed as have many other aspects of this field. ... 

Unsaturated polyesters for laminates Debye, light scattering of polymer solutions Flory, viscosity of polymer solutions Harkins, theory of emulsion polymerization Weissenberg, normal stresses in polymer flow Silicones... 

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