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

Abstract

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 chain-based 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.