This study demonstrated that poly(styrene-co-N-maleimide isobutyl polyhedral oligosilsesquioxane (POSS)) (P(S-co-NMIP)) was successfully prepared using free radical polymerization. For this purpose, firstly, N-maleimide isobutyl POSS (NMIP) was synthesized using aminopropyllsobutyl POSS (POSS-NH₂) and maleic anhydride. Secondly, P(S-co-NMIP) was synthesized using styrene, NMIP, and 2,2-azobis(isobutyronitrile) as initiator in tetrahydrofuran for 24 h at 75°C to give copolymer. The synthesized polymer and compounds were characterized by proton nuclear magnetic resonance, gel permeation chromatography, and Fourier transform infrared spectroscopy. Thermal behaviors of P(S-co-NMIP) were analyzed using thermogravimetric and differential scanning calorimetric analyses. The apparent activation energies (Es) for thermal degradation of P(S-co-NMIP) were obtained by integral methods (Flynn–Wall–Ozawa (FWO) and Kissinger). P(S-co-NMIP) was heated thermogravimetrically under various heating rates such as 5, 10, 15, and 20°C min^–1 at a temperature range of 30–1000°C to determine their thermal degradation mechanisms. The values of E for P(S-co-NMIP) were found to be 127.5 ± 2.3 and 134.4 ± 14.8 kJ mol^–1 for FWO and Kissinger methods, respectively. Also, the values of E of synthesized copolymer (P(S-co-NMIP)) were compared with styrene-co-maleic anhydride copolymer in literature.