Molecular Origin of the Induction Period in Photoinitiated Cationic Polymerization of Epoxies and Oxetanes

We investigated how the reactivity of epoxide and oxetane monomers in photoinitiated cationic polymerization is strongly influenced by their ability to form intramolecular hydrogen-bonding complexes with the Brønsted acids produced by the photoinitiator. In this paper, we highlight the importance of the thermal stability of the hydrogen-bonded complexes of the protonated monomers. We observed that the glycidyl ether structural motif (and its oxetane analog) is key for the formation of the complexes. Using temperature-controlled Fourier transform infrared (FTIR) with in situ ultraviolet irradiation, we performed a series of real-time FTIR experiments which support the hypothesis that the induction period is due to the thermal stability of the hydrogen-bonded complex. In particular, we focused on the photoinitiated cationic polymerization of bis(1-ethyl(3-oxetanil)methyl) ether ("di-oxetane", DOX) for the thermal stability study because it exhibits a prolonged induction period. Furthermore, it was possible to delay the cationic polymerization of DOX without causing autocatalytic polymerization if the temperature is kept lower than 30 °C. This is because the protonated monomer is stable and propagates very slowly at temperatures below 30 °C. On subsequent heating, the hydrogen-bonded complex of DOX loses its thermal stability, and the autocatalytic cationic polymerization of DOX occurs.