ABSTRACTA novel free radical photoinitiator, 3‐allyl‐1‐[2‐hydroxy‐3‐(thioxanthen‐9‐one‐2‐yl)oxypropyl]imidazolium chloride ([AIPTX]Cl), is synthesized by the addition reaction of 2‐(2,3‐epoxy)propoxylthioxanthone (ETX) with a heterocyclic compound imidazole firstly, and the achieved intermediate 1‐[2‐hydroxy‐3‐(thioxanthen‐9‐one‐2‐yl)oxypropyl]imidazole (IPTX) is then reacted with allyl chloride. IPTX is chosen to evaluate the photoefficiency of [AIPTX]Cl. FTIR and 1H‐NMR confirm the structures of [AIPTX]Cl and IPTX. UV‐Vis spectra of the two photoinitiators are similar and both exhibit the maximal absorption about 400 nm. Fluorescence spectra show [AIPTX]Cl/IMZ has slightly higher fluorescence intensity than IPTX system. Photopolymerization studies indicate that [AIPTX]Cl/IMZ is more efficient for the polymerization of water‐soluble monomer than IPTX. Moreover, due to its advantages of water solubility and polymerizability, [AIPTX]Cl is an environmental‐friendly photoinitiator and has potential for application in UV‐curing systems. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40659.