Two azide functionalized polymers were synthesized by free radical copolymerization. Each new polymer was effectively cross-linked with a small molecule cross-linker by a thermally activated reaction at 100 °C. This cross-linking method was compatible with plastic substrates for flexible electronic applications. The new method did not use any catalyst and initiator, and did not produce any by-product which could stay in the dielectric layer. The cross-linked thin-films showed good solvent resistance and smooth surfaces. The dielectric characteristics of the cross-linked polymer films were evaluated via quantitative capacitance and leakage current measurements in a metal–insulator–metal (MIM) test structure. The cross-linked thin-films showed significantly reduced leakage current density compared with uncross-linked thin-films in MIM testing devices.