MECHANISMS OF RADIATION-INDUCED CATIONIC POLYMERIZATION IN THE PRESENCE OF ONIUM SALTS

Cationic polymerization of various monomers in the presence of onium s alts were induced by h nu, EB and gamma 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 gamma irradiation only solvated electro ns were capable of reducing the onium salts with reduction potentials lower than approximately -100 kJ/mol. An enhanced production of proton s 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 ind ications of useful onium structures that fulfill the needs in EB and g amma induced cationic polymerization. Typical examples are fragmenting type of dialkylphenacyl and cyclic ringopening phenacylic sulfonium s alts. The influence of typical ''polymer or monomer backbone'' structu ral groups, such as esters and ethers on the proton formation under hi gh energy irradiation, was studied by UV spectroscopy at 540 nm. The f ormation of acid was monitored in the presence of various onium salts, and alpha-naphtylred was used as an indicator. By comparing aromatic versus aliphatic structural group influences on the generation of prot ons and carbenium ions a good correlation was found between experiment al data and theoretical calculations on nucleophilicity, electron char ge density distributions and electron scavenging effects by the use of simplified Huckel calculations (SHMO).