Cross-linkable poly(phthalazinone ether ketone sulfone) hearing tetrafluorostyrene groups (PPEKS - FSt) has been prepared by copolycondensation reaction for optical waveguide applications. The resulting amorphous polymer exhibits good solubility in some common polar organic solvents (e.g., N,N'-dimethylacetamide, N-methyl-2-pyrrolidinone, chloroform) at room temperature, and can be easily spin-coated into thin films with good optical quality. The glass transition temperature (T-g) and the temperature of 1% weight loss (1% T-d) are 261 degrees C and 494 degrees C, respectively, which could be further increased by 31 degrees C and 14 degrees C upon thermal cross-linking. The cross-linked polymer thin films exhibit high refractive index (similar to 1.65, TE mode), high thermo-optic coefficient value (dn/dT) (- 1.455 x 10(-4)/degrees C, TE mode), low optical loss (less than 0.24 dB/cm at 1310 nm) and relatively low birefringence (similar to 0.007). (c) 2007 Elsevier Ltd. All rights reserved.