Enhanced visible radiation photopolymerization of dimethacrylates with the three component thioxanthone (CPTXO)–amine–iodonium salt system
Photo-DSC and UV-visible spectroscopy were used to study the photo-curing kinetics and mechanism of the photopolymerization of dimethacrylates using three-component initiation systems consisting of 1-chloro-4-propoxy-9H-thioxanthen-9-one (CPXTO), diphenyl iodonium hexafluorophosphate (Ph2IPF6), and the aromatic N,N,3,5-tetramethyl aniline (TMA) or the aromatic-like N,N-dimethylbenzylamine or aliphatic triethylamine. The effect of monomers with different backbone flexibilities on the curing kinetics were also investigated, using aliphatic triethylene glycol dimethacrylate (TEGDMA), aliphatic nona-ethylene glycol dimethacrylate and aromatic diethoxylated bisphenol-A dimethacrylate. Photo-DSC cure kinetics studies showed that the fastest polymerization occurred when all three components were present for CPTXO/amine/Ph2IPF6 systems, and the maximum polymerization rate followed the trend: CPTXO/amine/Ph2IPF6 ≫ CPTXO/Ph2IPF6 ≥ CPTXO/amine, irrespective of the monomer or amine used. UV-visible spectroscopy studies revealed that for the TEGDMA/CPTXO/TMA, TEGDMA/CPTXO/Ph2IPF6, and TEGDMA/CPTXO/TMA/Ph2IPF6 systems, the CPTXO was rapidly photobleached at its absorption maximum (near 385 nm) during the photo-DSC timescale with photobleaching rates which followed the trend: CPTXO/TMA ≫ CPTXO/Ph2IPF6 ≈ CPTXO/TMA/Ph2IPF6. At the same time, a new absorption at 500 nm appeared for the systems containing CPTXO and Ph2IPF6, and the rate of this photodarkening behavior was greater for the CPTXO/Ph2IPF6 system compared with CPTXO/TMA/Ph2IPF6, – no photodarkening at 500 nm was found for the CPTXO/TMA system. Based on the curing kinetics, photobleaching studies and the thermodynamic feasibility it is proposed that for the three-component CPTXO/amine/Ph2IPF6 system, the reaction involves two main pathways. The first is the irreversible oxidation of the excited CPTXO molecule by Ph2IPF6 with the formation of a thioxanthone radical cation and a phenyl radical by an electron-transfer mechanism, followed by H-transfer between a thioxanthone radical cation and amine (or monomer) to form an aminoalkyl radical and to also regenerate CPTXO. The second pathway involves the reduction of the excited CPTXO molecule by the amine to form ketyl and aminoalkyl radicals, followed by the irreversible oxidation of the amine and the ketyl radicals by the iodonium salt, to form an initiating radical and also to regenerate CPTXO. Due to the non-reversible oxidation of the CPTXO and amine radicals by the iodonium salt (which prevents the back-electron transfer of amine to the excited CPTXO), due to the CPTXO regeneration reactions and due to the removal of the potentially terminating CPTXO radical, the efficiency of the CPTXO/TMA/Ph2IPF6 photoinitiating system is dramatically enhanced.