ABSTRACT
Diffusion of particles in the polymer matrix occurs much more slowly than in liquids.
Since the rate constant of a diffusionally controlled bimolecular reaction depends on the
viscosity, the rate constants of such reactions depend on the molecular mobility of a polymer
matrix (see monographs [1-4]). These rapid reactions occur in the polymer matrix much more
slowly than in the liquid. For example, recombination and disproportionation reactions of
free radicals occur rapidly, and their rate is limited by the rate of the reactant encounter. The
reaction with sufficient activation energy is not limited by diffusion. Hence, one can expect
that the rate constant of such a reaction will be the same in the liquid and solid polymer
matrix. Indeed, the process of a bimolecular reaction in the liquid or solid phase occurs in
accordance with the following general scheme [4,5]:
Aþ B kD
A B!k Products
The observed rate constant is kobs¼ kkD(kþ vD)1. For the fast reactions with k vD the rate constant is kobs¼ kD. In the case of a slow reaction with k vD the rate constant is kobs¼ kKAB, where KAB¼ kD/vD is the equilibrium constant of formation of cage pairs A and B in the solvent or solid polymer matrix. The equilibrium constant KAB should not
depend on the molecular mobility. According to this scheme, the rate constant of a slow
bimolecular reaction kobs¼ kKAB(kobs kD) should be the same in a hydrocarbon solution and the nonpolar polymer matrix. However, it was found experimentally that several slow
free radical reactions occur more slowly in the polymer matrix than in the solvent. A few
examples are given in Table 19.1.
