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New photoinitiators for free radical and cationic photopolymerization under near ultraviolet and LED irradiation

Time: 2018-02-08

Jing Zhang and colleagues from Institut de Science des Matériaux de MulhouseIS2M in France, developed three new photoinitiators which can be used for near ultraviolet and LED UV,  it is (2,2’-bithiophen-5-yl)(4-(N,N’-dimethylaminophenyl)ketone(THBP),5,10-dimethoxybenzo[j]fluoranthene (NANA) and 6,6’-(((1E,1’E)-(2,5-bis(octyloxy)-1,4-phenylene)bis(ethene-2,1-diyl))bis(4,1-phenylene))bis(1,3,5-triazine-2,4-diamine)(T1). When they form a composite photoinitiator (the iodonium salt (or at the same time with N- vinylcarbazole) or amine / alkyl halides combination), epoxy for cationic polymerization of methyl acrylate or air under the condition of free radical polymerization, showed excellent ability to lead.


Figure 1:A schematic diagram of the chemical structure of photoinitiators (THBP, NANA and T1)


Figure 2: A schematic diagram of the chemical structure of the auxiliaries and monomers used in the study


The absorption spectrum of THBP or NANA in acetonitrile and T1 in N, N- two methyl formamide is shown in Figure 3.

For THBP and NANA, the maximum absorption at UV wavelength range (i.e., THBP, lambda max = 381nm, 381nm - 29400M - 1cm - epsilon 1; NANA, max = lambda 387nm, epsilon 387nm ~ 6700M - 1cm - 1); for T1, it showed a better light absorption (i.e. lambda = 416nm, Max, 416nm - 70400M - 1cm - epsilon 1). These three photoinitiators have shown a good match for both near ultraviolet and LED, as well as the emission spectra of mercury lamps.

Figure 3: THBP or NANA in acetonitrile, and T1 in N, vis and N- two absorption spectra of UV methyl formamide


For THBP, its excellent absorption performance is closely related to the charge shift transition of HOMO to LUMO. HOMO and LUMO respectively represent the highest occupying molecular orbits and the lowest unoccupied molecular orbits, respectively.For NANA, the charge shift transition of HOMO to LUMO is also observed. The near - position naphthalene structure is shown in order to absorb the electronic functional groups, while the others are shown for the electronic functional groups. For T1, HOMO and LUMO are in the non localized state, and their better absorption may be due to the highly enhanced pi electron delocalization in the structure.


Under different UV light sources, the ring opening cationic polymerization of EPOX in air environment, the THBP/Iod or NANA/Iod photoinitiator system is very effective under the LED condition of 405nm (the final conversion rate is around 60%), and the finger dry coating can be obtained. The T1/Iod photoinitiation system is not so effective (the final double key conversion rate is only 33%). After adding NVK, the initiation efficiency of T1/Iod is greatly improved (see T1/Iod/NVK vs T1/Iod; figure 4C, curve 2 vs curve 1). The addition of NVK does not have much impact on THBP/Iod or NANA/Iod systems, probably because the efficiency of these two systems is very efficient.



Three photoinitiators can initiate free radical polymerization of methacrylate system (bis-GMA/TEGDMA mixture 70%/30%) under air irradiation or film mulching under different UV light sources (FIGS. 5 and table 2). The addition of Iod greatly improved the aggregation effect, and the conversion rates were 48% and 60% respectively (405nm LED irradiation; figure 5a, curve 3 and curve 4).


To sum up, three new photoinitiators, THBP, NANA and T1, are used for cationic polymerization and free radical polymerization of methacrylate in near UV and LED light sources. The iodonium salt, N- vinyl carbazole, and amine or 2,4,6- three - (three chloromethyl) -1,3,5- three triazine, some photoinitiator system showed excellent polymerization efficiency