According to the radical generation mechanisms, radical photoinitiators can be divided into three categories:
- Type I: Cleavable photo initiators
- Type II: Bimolecular photo initiating system
- Two photon active photo initiators
Upon irradiation, Type-I photoinitiators such as Irgacure 2959 (I2959) undergo cleavage from the excited triplet state and generate two radicals for initiating polymerization.
The initiating mechanism of Type-II systems (e.g., benzophenone/ tertiary amine) is more complex. The exited benzophenone initiates a fast electron transfer from the lone pair of tertiary amine followed by a slow proton transfer process, providing the H-donor radical for initiating polymerization. Compared to Type-I photoini- tiators, Type-II systems are much less efficient due to the bimolecular process, back electron transfer and especially the solvent cage effect in aqueous solutions.
The 3rd type of initiation essentially have highly conjugated π-systems, good coplanarity and strong donor/acceptor groups. In the simplest case (i.e., radical polymerization), the initiation mechanism is currently accepted as: after intra- and inter- molecular charge transfer interactions between the two-photon excited initiator and monomer, radicals are formed by electron transfer to initiate polymerization.
The above content comes from BioNanoMat 2014; 15(3-4): 49–70