Phenylthioether thiophene-based oxime esters as novel photoinitiators for free radical photopolymerization under LED irradiation wavelength exposure

Four new photoinitiators (PIs) containing phenylthioether-substituted thiophene groups as chromophores and oxime ester groups as initiating groups were designed and synthesized. These compounds exhibit excellent absorption under exposure to light-emitting diode (LED) irradiation wavelength (i.e., 365–425 nm). Moreover, they have a higher radical generation efficiency than the commercial PI O-benzoyl-α-oxooxime (i.e., OXE 01). The mechanism by which these oxime esters generate radicals was investigated via steady-state photolysis, high performance liquid chromatography, and online 1H NMR. These PIs can efficiently initiate the polymerization of acrylate monomer. Acetyl oxime esters show higher conversions than OXE 01 under LED irradiation at 365–425 nm. The methyl substituent on the benzene ring, the volume effect of the large alkyl group, and the acetyl substitution on the oxime ester all contribute to high photoinitiation efficiency. Owing to their excellent absorption and efficient photoinitiation properties, these optimized PIs have great potential in photocuring applications.