Azodioxides as Inhibitors and Retarders in Photoinitiated Cationic Polymerization

The cyclic azodioxide, 1,4,4-trimethyl-2,3-diazabicyclo[3.2.2]non-2-ene 2,3-dioxide (5), displays the unique ability to function as a thermally reversible inhibitor for the UV-induced photoinitiated cationic polymerizations of epoxide and vinyl ether monomers. At room temperature and below, the photopolymerizations are suppressed due to the capture of photogenerated initiating protons by the cis-azodioxide group, which is both basic and serves as a bidentate coordinating group. However, at higher temperatures, the azodioxide group reverts to its ring-opened dinitroso isomer and, as a consequence, loses its ability to coordinate to protons. Protons released during this process subsequently initiate the cationic polymerizations of the monomers. Reversible inhibitors such as 5 have several potential applications such as in the design of one-component photoactivatable structural and assembly adhesives and in composites.