Stereochemistry of the radical polymerization of vinyl pentafluorobenzoate

The free-radical polymerization of vinyl pentafluorobenzoate (VPBz) was carried out under various conditions in order to compare its stereochemistry to that of the vinyl benzoate (VBz) polymerization using similar conditions. Contrary to the stereochemistry of the radical polymerization of VBz, VPBz favors syndiotactic propagation. The poly(VPBz) obtained in hexafluoroisopropanol (HFIP) with nBu₃B-air at -30°C has a triad syndiotacticity (rr) of 52% which achieved the highest syndiotacticity reported for the radical polymerization of vinyl esters. The stereochemistry difference for the VPBz polymerization was ascribed to the electron-withdrawing effect of the fluorine on the aromatic ring. The solvent effect of enhancing the rr specificity in HFIP may be related to the hydrogen-bonding between HFIP and VPBz or the growing species. It was also found that the glass transition temperatures (T_g) of the VPBz polymers apparently increased with an increase in their diad syndiotacticities (r): the T_g of poly(VPBz) with r=72% was 79°C, which was 25°C higher than that of poly(VPBz) with r=56% obtained in toluene.