A series of 10-butylphenothiazine trimer (3-PTVB) and pentamer (5-PTVB) with in-situ thermally cross-linkable vinyl benzene were synthesized successfully. The TGA thermograms revealed that 3-PTVB and 5-PTVB were stable up to 430 and 370 degrees C, respectively. In the first heating scan of DSC thermograms, 3-PTVB and 5-PTVB showed glass transition temperature (T-g) at 113 degrees C and 95.3 degrees C (143 degrees C). Both 3-PTVB and 5-PTVB showed broad endothermic process in the region of 185-220 degrees C, which is thermally cross-linking temperature. In the second heating process, Tg and endothermic process were not observed. Wavelength of UV-visible maximum absorption of thermally cured 3-PTVB is 340 nm, which is same as the UV-visible maximum absorption of 5-PTVB. The thermally cured films showed no damage nor any discernible change in the UV-visible spectrum after washing with organic solvents such as methylene chloride (MC), chloroform, and toluene. Thermally cured 3-PTVB and 5-PTVB films were electrochemically very stable. The HOMO energy level of 3-PTVB and 5-PTVB were -4.98 and -4.86 eV, respectively. Double layer structured polymer light-emitting diodes based on in-situ thermally cured 3-PTVB and 5-PTVB were fabricated. The maximum luminance efficiency of devices based on 3-PTVB and 5-PTVB were 1.004 cd/A at 14.0 V and 1.074 cd/A at 16.5 V, respectively, which are much higher than that of the device without thermally cured 3-PTVB or 5-PTVB (0.348 cd/A at 15.0 V).