Polymerization ideal emulsion

The progression of an ideal emulsion polymerization is considered in three different intervals after forming primary radicals and low-molecular weight oligomers within the water phase. In the first stage (Interval I), the polymerization progresses within the micelle structure. The oligomeric radicals react with the individual monomer molecules within the micelles to form short polymer chains with an ion radical on one end. This leads to the formation of a new phase (i.e., polymer latex particles swollen with the monomer) in the polymerization medium. 

Figure 1 The typical tendencies for the variation of monomer conversion by the polymerization time and for the variation of polymerization rate by the monomer conversion in the ideal emulsion polymerization process.

The rate of an ideal emulsion polymerization is given by Eqn (4). In this expression [/] is the initiator concentration, [ ] is the emulsifier concentration, and [M] is the concentration of monomer within the forming latex particles. This value is constant for a long reaction period until all the monomer droplets disappear within the water phase. 

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). 

The kinetic analysis here is based on quantitative considerations of the ideal emulsion polymerization systems which have been described qualitatively in the preceding sections. The treatment centers only around stage I and stage II (Fig. 6.18), as no general theory for stage III is available. The treatment applies to styrene-like monomers, meaning those monomers with low water solubility and those in which monomer and polymer are completely miscible over all ranges of composition. 

Figure 10.5 Representation of stages of an ideal emulsion polymerization. (-0) An emulsifier molecule (M), a monomer molecule (P) a polymer molecule and (R j a free radical, (a) Prior to initiation (b) polymerization stage 1 shortly after initiation (c) polymerization stage 2 all emulsifier micelles consumed (d) polymerization stage 3 monomer droplets disappear and (e) end polymerization.

An emulsion polymerization reaction follows three conventional steps, namely, initiation, propagation, and termination. These steps can be described by the conventional reactions that are valid for any free radical polymerization. Smith and Ewart [10] proposed that a forming latex particle in an ideal emulsion polymeriza-... 

Emulsion polymerization is used for 10-15% of global polymer production, including such industrially important polymers as poly(acrylonitrile-butadiene-styrene) (ABS), poly (styrene), poly(methyl methacrylate), and polyvinyl acetate.38 These are made from aqueous solutions with high concentrations of suspended solids. The important components have unsaturated carbon-carbon double bonds. These systems are ideal for Raman spectroscopy and a challenge for other approaches, though NIR spectroscopy has been used. 

The "ideal" concept of emulsion polymerization was built on the assumption that the monomer was water insoluble and that in the absence of chain transfer, the number average degree of polymerization, Xj can be related to the rate processes of initiation and propagation by the steady-state relationship Xjj = 2 Rp/Rj. Since Ri and Rp are both constant and termination is assumed to be Instantaneous during the constant rate period described by Smith-Ewart kinetics, the above equation predicts the generation of constant molecular weight polymer. Data has been obtained which agrees with Smith-Ewart but there is... 

The foregoing mechanism is amenable to mathematical analysis, with the salient results that during ideal interval II polymerization, the rate of reaction is proportional to and while DP depends on and [1] . (Here [I] is the initiator molar concentration and S is the weight concentration of surfactant.) In conventional solution, suspension, or low-conversion bulk free-radical reactions, the rate of polymerization depends on [1] / while DP is proportional to [I]". In these cases DPp cannot be increased at given [M] without decreasing Rp. In emulsion polymerization, however, both Rp and DP can be changed in parallel by controlling the soap concentration. 

It has been pointed out that the degree of ideality of an emulsion polymerization is determined by the magnitude of parameter a, which in turn determines the value of the subdivision factor, z. Since... 

A number of polymerization techniques are used in the transformation of monomers into plastics (Chapter 10). These include bulk, solution, suspension, and emulsion polymerization processes. Each of these polymerization techniques has its advantages and disadvantages and may be more appropriate for the production of certain types of polymer materials. For example, bulk polymerization is ideally suited for making pure polymer products, as in the manufacture of optical-grade poly(methyl methacrylate) or impact-resistant polystyrene, because of rninimal contamination of the product. On the other hand, solution polymerization finds ready application when the end use of the polymer requires a solution, as in certain adhesives and coating processes. 

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 ... 

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