Phosphonium-based ionic liquids as a new class of radical initiators and their use in gas-free frontal polymerization

Two new radical initiators have been synthesized (tetrabutylphosphonium persulfate, TBPPS, and trihexyltetradecylphosphonium persulfate, TETDPPS) and their kinetic parameters of thermal dissociation have been calculated by DSC. It was found that they give rise to radical species without gas evolution in thermal conditions that are analogous to those of Aliquat persulfate (APS), and the well-known benzoyl peroxide (BPO) and 2,2'-azobisisobutyronitrile (AIBN). However, it has been established by TGA that the new initiators are much more thermally stable than the other mentioned above toward thermal degradation resulting in undesired evolution of volatile products. This finding is particularly important when radical polymerizations have to be performed in batch and nonstirred reactors in which the difficulty of gas removal may result in bubble-containing materials thus not directly useful in practical applications. Moreover, all the above initiators have been tested in frontal polymerization experiments. All samples obtained in the presence of TBPPS or TETDPPS contained no bubbles. The maximum temperatures reached in the presence of TBPPS or TETDPPS were lower than those recorded with AIBN or BPO but front velocities were in the same range, thus indicating that the same reaction times can be used but the possibility of polymer degradation is now limited. Also, the minimum molar concentration of radical initiator for a front to self-sustain is extremely low, it being a fifth of that of both AIBN and BPO and a half of those of APS; this finding may result in important positive consequences on the molecular mass when an uncrosslinked polymer has to be prepared.