Copolymerization incremental composition

One reactant is charged to the reactor in small increments to control the composition distribution of the product. Vinyl copolymerizations discussed in Chapter 13 are typical examples. Incremental addition may also be used to control the reaction exotherm. 

Fig. 24.—Incremental polymer composition (mole fraction Fi) plotted against the monomer composition (mole fraction/i) for ideal copolymerizations (ri — X/r F). Values of r are indicated.

The copolymer composition may drift during the course of an emulsion copolymerization because of differences in monomer reactivity ratios or water solubilities. Various techniques have been developed to produce a uniform copolymer composition. The feed composition may be continuously or periodically enriched in a particular monomer, to compensate for its lower reactivity. A much more common procedure involves pumping the monomers into the reactor at such a rate that the extent of conversion is always very high [>about 90%]. This way, the polymer composition is always that of the last increment of the monomer feed. 

The existence of an azeotropic composition has some practical significance. By conducting a polymerization with the monomer feed ratio equal to the azeotropic composition, a high conversion batch copolymer can be prepared that has no compositional heterogeneity caused by drift in copolymer composition with conversion. Thus, the complex incremental addition protocols that arc otherwise required to achieve this end, are unnecessary. Composition equations and conditions for azeotropic compositions in ternary and quaternary copolymerizations have also been defined. " ... 

Knowledge of /ap is all that is required to calculate the composition of the copolymer formed, assuming that the reactivity ratios for copolymerization are known. In order to predict composition drift. Equations (7.20) and (7.22) must be solved to give /ap as a function of total comonomer conversion, which sinq)ly requires iterative solution of the equations for small increments of conversion over which the copolymer composition can be assumed constant. 

Figure 6.8 Incremental polymer composition Fj versus for ideal copolymerization.

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