Mechanisms of radiation induced cationic polymerization in the presence of onium salts

Abstract

Cationic polymerization of various monomers in the presence of onium salts were induced by hv, EB and γ irradiation. The mechanism for the initiation process involves the photoreduction of onium salts by a direct photolysis or by an indirect redox reaction from organic free radicals or solvated electrons depending on the reduction potentials of the onium salts. For EB and γ irradiation only solvated electrons were capable of reducing the onium salts with reduction potentials lower than approximately −100 kJ/mol. An enhanced production of protons and/or carbenium ions takes place if the reduction potentials of the onium salts are higher than −60 kJ/mol. This paper will give some indications of useful onium structures that fulfill the needs in EB and γ induced cationic polymerization. Typical examples are fragmenting type of dialkylphenacyl and cyclic ringopening phenacylic sulfonium salts. The influence of typical “polymer or monomer backbone” structural groups, such as esters and ethers on the proton formation under high energy irradiation, was studied by UV spectroscopy at 540 nm. The formation of acid was monitored in the presence of various onium salts, and α-naphtylred was used as an indicator. By comparing aromatic versus aliphatic structural group influences on the generation of protons and carbenium ions a good correlation was found between experimental data and theoretical calculations on nucleophilicity, electron charge density distributions and electron scavenging effects by the use of simplified Hückel calculations (SHMO).