Materials that display thermally activated delayed fluorescence (TADF) have recently been identified as the third generation emitters for organic light -emitting diodes (OLEDs). However, there are only a few reported examples of polymeric TADF materials. This study reports a series of polymers with an insulating backbone and varying ratios of 2-(10H-phenothiazin-10-yl)dibenzothiophene-S,S-dioxide as a pendant TADF unit. Steady-state and time -resolved fluorescence spectroscopic data confirm the efficient TADF properties of the polymers. Styrene, as a comonomer, is shown to be a good dispersing unit for the TADF groups, by greatly suppressing the triplet triplet annihilation. Increasing the styrene content within the copolymers results in relatively high triplet energy, small energy splitting between the singlet and triplet states (Delta E-ST), and a strong contribution from delayed fluorescence to the overall emission. Green emitting OLED devices employing these polymers as spin -coated emitting layers give high performance, which is dramatically enhanced in the copolymers compared to the homopolymer. Within the series, Copol with a regiorandom ratio of 37% TADF units:63% styrene units displays the best performance with a maximum external quantum efficiency (EQE) of 20.1% and EQE at 100 cd m(-2) of 5.3%.