Novel method to assess the molecular weights of fluoropolymers by radical copolymerization of vinylidene fluoride with various fluorinated comonomers initiated by a persistent radical

摘要

Batch radical homopolymerization of vinylidene fluoride (VDF) and its copolymerizations with seven fluorinated comonomers such as H _2C=CR_ACF_3 (where R_A stands for H, F, CO_2R (R = H or tBu) or F_2C=CFR_B (R _B: H, CF_3, OCF_3) initiated by ?CF_3 radical generated from a perfluoro-3-ethyl-2,4- dimethyl-3-pentyl persistent radical (PPFR) are presented. That highly perfluorinated branched radical itself was unable to directly initiate the copolymerization in contrast to the generated trifluoromethyl radical that was successful. The microstructures of the resulting PVDFs, poly(VDF-co-H _2C=CR_ACF_3) or poly(VDF-co-F_2C= CFR_B) (co)polymers were assessed by ~(19)F and ~1H NMR spectroscopy. ~(19)F NMR spectra displayed a quintet centered at -61 ppm assigned to a CF_3 end-group which enabled one to determine both the molecular weights of the resulting (co)polymers and the amounts (numbers and molar percentages) of these above comonomers. In all cases, the regioselective radical addition of ?CF_3 onto the methylene site of VDF was noted as well as for 3,3,3-trifluoropropene. In addition, the ~(19)F NMR spectra showed the absence of signals attributed to PPFR or its derivatives. According to [PPFR]_o/([VDF]_o + [comonomers]_o) initial molar ratios, this novel initiating system could produce high molecular weight-fluoropolymers (up to 200 000 g·mol~(-1)). The lower that ratio, the higher their molecular weights as well as the higher their thermal stability (a 10% weight loss under air was noted at 425 C). Although the kinetics of radical copolymerization were not studied, the presence of branched persistent radical did not disturb the reactivities of the comonomers and it was not involved in the primary recombination of radicals in the termination step of the polymerization while the macromolecular recombination was favored. For an initial PPFR concentration lower than 1 mol %, a feed ratio of ca. 20 mol % of 2-trifluoromethacrylic acid, MAF (or tert-butyl 2-trifluoromethacrylate, MAF-TBE) led to alternated poly(VDF-alt-MAF or MAF-TBE) copolymers. High yields (90%) of copolymers based on VDF and 2,3,3,3-tetrafluoroprop-1-ene, hexafluoropropylene (HFP), or perfluoromethyl vinyl ether (PMVE) comonomers were achieved while those from MAF, MAF-TBE, trifluoroethylene, and 3,3,3-trifluoropropene were fair to satisfactory. As expected, for a reasonable amount of HFP and PMVE inserted in the copolymers (>18 mol %), elastomers endowed with low T_g values (-35 and -42 C, respectively) were produced. In addition, the higher the content of VDF, the lower the T_g, though the presence of tBu from MAF-TBE slightly increased the T_g values. Copolymers containing MAF or MAF-TBE exhibited poor thermostability (that arises from the decarboxylation and isobutylene elimination from MAF or MAF-TBE, respectively) in contrast to the other VDF-containing copolymers that showed decomposition from 250 C under air.