We prepared a side-chain phenol-functionalized poly(ether sulfone) (P1) from a one-pot reaction of a 4,4'-dihydroxybenzophenone (DHBP)-based poly(ether sulfone), poly(oxy-1,4-phenylenecarbonyl-1,4-phenyleneoxy-1,4-phenylene-sulfonyl-1,4-phenylene (DHBP-PES)), with 9,10-dihydro-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and phenol in the presence of sulphuric acid. The phenol linkages of P1 act as reacting sites for epoxy resins. Subsequently, flexible and light-yellow transparent films of epoxy thermosets can be achieved from the curing of P1 with cresol novolac epoxy (CNE) and diglycidyl ether of bisphenol A (DGEBA). The thermoset based on P1 and CNE (P1/CNE) shows a high T-g value (241 degrees C), a low coefficient of thermal expansion (44 ppm/degrees C), and flame retardancy (VTM-0). The moderate-to-high molecular weight of P1 is responsible for the characteristics high T-g and flexibility, which are rarely seen in epoxy thermosets based on a low-molecular weight curing agent. (C) 2013 Elsevier Ltd. All rights reserved.