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With a view to fabricating high glass transition temperature (Tg) graded-index plastic optical fiber (GI POF) by the coextrusion process, core material consisting of copolymer of 2,2,2-trichloroethyl methacrylate (TCEMA) and N-cyclohexyl maleimide (cHMI) (P(TCEMA-co-cHMI)), and diphenyl sulfide (DPS) as dopant was newly developed. Although homo polymer of TCEMA (PTCEMA) possesses high Tg and high transparency, it was found that PTCEMA began depolymerization at ca. 200° C, which means that PTCEMA-based GI POF fabricated by the coextrusion process has low Tg due to depolymerized monomer. By copolymerizing TCEMA with cHMI (5 mol%), the rate of weight loss at constant temperature T = 230°C was drastically decreased (15 wt%/hr). Moreover, the Tg linearly increased from 130° C to 197° C with increasing cHMI composition ratio from 0 to 62 wt%, respectively. Scattering loss of the copolymer was increased with the increase of cHMI composition ratio due to the refractive index difference between PTCEMA and PcHMI. It was found that the increment of the scattering loss was suppressed significantly when the copolymer includes DPS (120 dB/km decrease by adding 4 wt% DPS for the copolymer including 5 mol% cHMI). From these results, P(TCEMA-co-cHMI)-based core material was thought to be a promising candidate for high Tg GI POF.