In this study, the thermal degradation behavior of both poly(3,3,4,4,5,5,6,6,7,7,8,8,8-dodecafluoro-5-methyloctyl-4-(acryloyloxy) benzoate), poly(ABCF13) and its graft copolymers onto high-density polyethylene (HDPE) was investigated in detail. The homopolymers and grafted copolymers were synthesized using the bulk-melt polymerization method in the presence of benzoyl peroxide; 10% and 40% of ABCF13 were used in the graft copolymers. The combined analytical technique, Thermo Gravimetric Analysis coupled with Fourier Transform Infrared Spectroscopy (TGA/FTIR), was utilized to understand the obtained products' thermal behavior and formed decomposition chemicals. The thermal tests were carried out in both air and N2 atmospheres. The experimental results showed that the first weight loss of poly(ABCF13) in air and N2 atmospheres started at about 226°C (1.00%), and the primary and early decomposition product was determined as CO2. The weight loss pattern in graft copolymers at lower concentrations closely resembled that of the homopolymer. However, at higher concentrations, the resulting products exhibited a distinct decomposition mechanism characterized by a gradual decrease in trend. This segmented behavior may indicate the coexistence of the homopolymer and grafted copolymers within the matrix. Moreover, the findings showed that the semi-fluorinated grafted unit had a substantial retardation effect on the polymer.