Composition drift

Composition drift occurs during the process of free radical copolymerization causing variation in the instantaneous mole fraction of a monomer added to copolymer, therefore altering the chemical composition of the copolymer over the period of conversion.

[1] The degree of composition drift is directly affected by the reactivity ratios of each monomer in the copolymer system.

Both the Mayo-Lewis equation and plot of the equation make evident that as monomer conversion increases, the copolymer composition will drift as the preferences for monomers change due to the interaction between reactivity ratios and the instantaneous concentration of each monomer.

[2] Composition drift in some degree will occur unless the reactivity ratios for both monomers are equal to 1.

This causes equal rates of consumption for copolymer formation and leads to random copolymerization.

[5] Binary copolymerization resembles distillation of a bicomponent liquid mixture with reactivity ratios corresponding to the ratio of vapor pressures of the pure components in the latter case.

Distillation terminology is also borrowed for the case of azeotropic compositions in copolymer systems.

The equation for the azeotropic concentration is shown below at the given reactivity ratios for each monomer species:[6]

The azeotropic concentrations are unstable operation points, as any small change in temperature will cause a shift in molar concentration through reactivity ratio effects and cause subsequent composition drift.

[7] Azeotropic points occur when the feed monomers have reactivity ratios that are both less than 1 or both greater than 1.

For commercial applications, copolymer composition must be consistent across the aggregate.