Bulk copolymerization of vinylidene fluoride (or 1;1-difluoroethylene (VDF)) with 4,5,5-trifluoro-4-ene pentyl acetate (FAc) initiated by di-tert-butyl peroxide is presented. A series of nine copolymerization reactions was investigated from initial [VDF](0)/[FAc](0) molar ratios ranging from 32/68 to 95/5. Both these comonomers copolymerized in this range of copolymerization. Moreover, these comonomers homopolymerized separately in these conditions. The copolymer compositions of these random-type copolymers were calculated by means of H-1 and F-19 NMR spectroscopies and allowed one to quantify; the respective amount of each monomeric unit in the copolymer. From the Tidwell,and Mortimer method, the reactivity ratios, ri, of both comonomers were determined showing a higher incorporation of FAc in the copolymer (r(FAc) = 3.26 +/- 1.49 and r(VDF) = 0.17 +/- 0.10 at 120 degrees C), Alfrey-Price's Q:and e values of trifluorovinyl acetoxy monomer FAc were calculated to be 0.060 (from Q(VDF) = 0.008) or 0.040 (from Q(VDF) = 0.015) and +1.14 (vs e(VDF) = 0.40) or +1.23 (vs e(VDF) = 0.50), respectively, indicating that FAc is an electron-accepting monomer. The normalized monomer-diad and -triad fractions as a function of the polymer composition were obtained from the comonomer sequence distribution theory and this was evidenced by F-19 NMR analysis.