Radiation-induced polymerization ionic mechanisms

The most important feature of ionizing radiations is, as the term implies, ionization to give ionic intermediates in irradiated systems. Though radiation-induced radical polymerization had long been studied, it is only a decade since radiation-induced ionic polymerization was first found. In 1957, Davison et al. obtained polymer from isobutene, which is known not to be polymerized by radical catalysts, by irradiating at low temperature with y-rays (7). Before long, the radiation-induced polymerization of styrene was proved to proceed as an ionic mechanism in suitable solvents (2,3,4). Since these pioneering researches, the study of the chemical kinetics of radiation-induced ionic polymerization has been extended to several vinyl, diene and cyclic monomers. 

Hence, in ionic radiation-induced polymerization the exponent of the rate of the absorbed dose can vary from 0.5 to 1.0 depending on the termination mechanism. Moreover, this order is an indication of the purity of the reaction system. A conclusion follows which is... 

The main features of the ionic mechanism in radiation-induced polymerization are given below. 

Although radical cations are generated in some electron-irradiated monomers (e.g., vinyl ethers or epoxies), efficient cationic polymerization is not observed. Under certain conditions (addition of iodonium, sulfonium, or sulfoxonium salts, cationic polymerization with the use of electron beam irradiation can be induced. Several studies on radiation cross-linking of elastomers support the concept of ionic mechanism. ... 

The solid n-hexadecene-1 system has also been studied.76 It was concluded that the experimental results were incompatible with a free-radical reaction and an ionic mechanism was proposed. An enhancement in the polymerization yield by a factor of 2 on going from the liquid at 20 °C. to the solid at 0°C. was observed. Solid-state ionic polymerizations induced by high-energy radiation have been reviewed by Magat.76... 

Sufficient experimental data from several laboratories now exist to describe the conditions under which the radiation-induced ionic propagation of many pure liquid vinyl monomers can be observed. The kinetic data and electrical conductivity measurements establish the ionic nature of the reaction scavenger studies appear to establish the preponderant role played by the carbonium ion in propagating the polymerization. On the basis of a single propagating species, it is possible to write a simple mechanism to describe the process. Limiting values of several of the kinetic rate constants can be estimated, notably the rate constant for reaction between a bare carbonium ion and a vinyl double bond. These rate constants are compared with similar constants arrived at in chemically initiated free radical, carbonium ion and carbanion polymerization. Several shortcomings of the present scheme are discussed. 

There are other phenomena that do not present unconditional proof of the mechanism. The activation energies of polymerizations in the solid state are often unusually low (see below). Since no activation energy is needed with a radiation-induced start reaction, these low activation energies can also be due to the special conditions in the crystal. The same applies to solid state polymerizations of monomers that can only be polymerized ionically in the fluid phase. 

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