Pyridines as retarding agents in photoinduced thermal frontal cationic polymerization of epoxydes

Photoinduced thermal frontal cationic polymerization of a diepoxide was found to compete with spontaneous thermal cationic polymerization, due to the unstability of 2,4,6-triphenylpyrylium salt used as coinitiator. In order to delay the spontaneous thermal polymerization, pyridine derivatives were introduced as retarding agents. It is found that the effect of the amine depends on both the steric hindrance and its pKa. Strong bases have important retarding effect on the polymerization. However, when the nitrogen lone pair is easily accessible, a strong inhibition of the cationic polymerization also takes place. This inhibitory effect is suppressed for sterically hindered pyridines. For example, 2,6-lutidine exhibits a nice retarding effect without any detrimental effect on the final conversion of the epoxide. Further information is given by the determination of the activation energy for the polymerization reaction in the presence of different pyridines, confirming the predominant role of the pKa. Apart the thermal polymerization, it is found that the photopolymerization is also slightly affected by the presence of pyridines in terms of rate of photopolymerization, albeit no decrease in conversion was noticed in this case. Finally, photoinduced thermal frontal polymerization was performed in the presence of 2,6-lutidine, showing that 2,6-lutidine effectively acts as retarding agent in this case. The velocity of the thermal front was found to be reduced in the presence of pyridine derivative although without any effect on the heat evolved.