On kinetic features of photo- or γ-induced polymerization in p-diethynylbenzene crystals in the temperature range 4.2–300K

Abstract

The stereoregular radical polymerization in photo- or γ-irradiated crystals of p-diethynylbenzene (DEB) has been found to proceed in the temperature range of 4.2–300K [J. Low Temp. Phys. 139, 675 (2005)]. We investigate the kinetics of this process. The polymerization of acetylene monomers due to the system of conjugated bonds formation results in the shift of a crystal absorption band from UV to visible. When compared with gravimetrical data on the polymer yield, it allows the direct detection of polymerization process in a crystal. The monomer radicals initiating the polymerization process, as well as propagating macroradicals, are detected by the ESR method. The rates of both radical formation and chemical reaction of polymerization are retarded, as is inherent to solid-phase processes, already at low yields. Thus we are applying a mechanism that pertains to the first stages of the processes only. It is proved that both photo- and γ-induced polymerization have chain character in the temperature range 77–300K but it is most probably not chained at 4.2K and, for photo-polymerization, every act of monomer addition to the polymer needs an extra quantum of light. The kinetic chain length (the number of added monomer molecules per radical) turn out to be around of 200 at 300K and of 20 at 77K. It is interesting to note that polymer just formed is able to be modified—the radicals newly created in a polymer chain are able to add monomer molecules, forming, in such a way, a branched polymer. This process, of course, is unchained, too.