Influence of alkyl chain lengths and positions on visible-light absorbing triphenylamine oxime esters for free radical photopolymerization

To investigate the effect of alkyl chain lengths on photoreactivity, various triphenylamine-based oxime esters (WH-1-5) with different alkyl chain lengths at the R2 and R3 positions, which have not been reported before, were synthesized in this study. That is, an alkane segment in WH-1 at R2 is methyl and R3 is methyl, WH-2 at R2/ R3 are butyl/methyl, WH-3 at R2/R3 are undecane/methyl, WH-4 at R2/R3 are pentadecane/methyl, WH-5 at R2/R3 are methyl/butyl. In addition, the reference compound TP-1M, with hydrogen and methyl at the R2 and R3 positions, was also prepared for comparison. Compared to the reference compound TP-1M, all the alkyl chainbased compounds exhibited better dispersity in the testing monomer, trimethylolpropane triacrylate (TMPTA). Additionally, the maximum absorption wavelengths of these oxime esters range from 336 to 354 nm, exhibiting a slightly bule-shifted absorption compared to the reference compound TP-1M. Furthermore, the photochemical reactions were studied in detail through steady-state photolysis, electron spin resonance (ESR), and cyclic voltammetry (CV). Finally, the photoreactivity of the photoinitiation systems was studied by photo-DSC instrument under various exposure wavelengths (UV lamp, LED@365 nm and LED@405 nm). Among all, WH-5 based formulation exhibits the best double bond conversion efficiency whether upon UV lamp (43 %) or LED@405 nm (46 %). WH-5 based formulation also showed good final double conversion under LED@405 nm within relative lower light intensity. This study thoroughly examined and discussed the effect of positions, length, and dispersity of alkyl chains in triphenylamine oxime esters.