Lifetime of Polymer Radicals

In general, radicals have to terminate bimolecularly. Polymer radicals may react inframolecularly with another radical situated at the same polymer molecule or znfermolecularly with a radical at another polymer molecule. With a large number of radicals at the same macromolecule, such as are produced at the high dose rate of pulse radiolysis and at low polymer concentrations, the former process is usually favored (Fig. 9.1). 

The kinetics of the bimolecular decay of poly(vinyl alcohol) (Ulanski et al. 1994) and poly(vinyl methyl ether) radicals (Janik et al. 2000b) have been studied in some detail (cf. Fig. 9.2). The OH radicals formed during the pulse generate on the (coil-shaped) polymer a non-random distribution of radicals. First, the radicals which are very close to one another recombine. The intrinsic bimolecular rate constant for such a process can be much faster than that of the decay of an equal concentration of randomly distributed low molecular weight radicals. As the number of close-by radicals decreases, the intrinsic rate constant drops, and the lifetime of the polymer radicals increases considerably. Now, the bimolecular decay of the polymer radicals becomes much slower than that of the corresponding low molecular weight radicals. While in the case of low molecular weight radicals the bimolecular rate constant is independent of the 

---

Bruk et al. [716] described the low-temperature radiation polymerization of crystalline TFE in detail. It has been established that three solid-phase postpolymerization reactions can take place when irradiated specimens are heated above the melting point low-temperature polymerization (in the interval 77 to IlOK), slow polymerization close to the melting point (in the interval 128 to 138 K), and rapid polymerization during melting of the crystal (142 K). Tabata et al. [717] have found that a significant post-polymerization takes place even in the liquid phase. Kinetic analysis has been made of the in-source and post-polymerizations [718,719]. Post-polymerization is explained by a long lifetime of polymer radicals in the hquid phase at —78 °C due to the slow combination rate of the polymer radicals caused by their rod-like shape. 

---