Radiation-induced Ionic Reactions: Cyclopentadiene Polymerization

Abstract

RECENT investigations^1–3 on the radiation-induced polymerization of certain monomers have provided evidence for the production of ionic as opposed to radical initiation. The polymerization of such monomers can be initiated by Friedel–Crafts catalysts, and cyclopentadiene⁴ falls into such a class. Pure, rigorously dried (silica-gel treated) cyclopentadiene was irradiated with cobalt-60 γ radiation (dose-rate 2.27 × 10¹⁷ eV g⁻¹ min⁻¹). At − 78.5° C a gel was formed after conversion of 1.6 per cent of the monomer to polymer. The results of carbon, hydrogen analysis fitted an empirical formula of C₅H₆. The gelling dose was 8.5 × 10¹⁷ eV g⁻¹, and the subsequent increased rate of polymer formation corresponded to the removal of 17,760±1,521 (S.D.) monomer molecules per 100 eV. This was linear with dose up to at least 40 per cent conversion. At 0° C the yields were lower and suffered a high degree of irreproducibility (G(-monomer) range of 1,518–12,750). At 22° C a dimer impurity could not be excluded owing to its spontaneous thermal formation⁴, but polymerization did ensue in one experiment with an overall (G(-monomer) yield of 10,600 at 15 per cent conversion. In the absence of silicagel drying the yields were substantially reduced. The efficiency of the reaction at low temperature, and the necessity for rigorous drying² to attain high yield are criteria for the non-participation of radical processes.