Copolymerization and Characterization of Vinylaromatics with Fluorinated Styrenes

We have studied the ability of aromatic fluorocarbons with different levels of fluorination to interact with aromatic hydrocarbons via pi-pi stacking interactions. To observe this behavior, we investigated the radical copolymerization behavior of styrene and 1-vinylnaphthalene (M₁) with fluorinated styrenes (M₂) by determining the monomer reactivity ratios using the Fineman-Ross, Kelen-Tüdos and nonlinear least square curve-fitting methods. To aid in the study, fluorinated styrenes [2,3,5,6-tetrafluorostyrene (TEFS), 2,4,6-trifluorostyrene (TFS), 2,4-difluorostyrene (DFS) and 4-fluorostyrene (4FS)] and 1-vinylnaphthalene (1VN) were synthesized using either Ni-catalyzed Kumada cross-coupling of aryl Grignard reagents with vinyl bromide, or reaction of the aryl Grignard reagents with acetaldehyde followed by dehydration using P₂O₅.

The copolymerization of styrene (St) with 2,3,4,5,6-pentafluorostyrene (PFS, r_Str_PFS = 0.17, 0.16, 0.048 and 0.069 in bulk at 70 °C, in toluene at 70 °C, in bulk at 25 °C and in toluene at 25 °C, respectively) generates copolymers that tend to be alternating, with the alternating tendency increasing with decreasing temperature, according to the monomer reactivity ratios from the nonlinear least square fit of the Mayo-Lewis plot; the alternating tendency is independent of dilution or solvent. In contrast, the copolymerizations of styrene with 4FS (r_Str_4FS = 0.62 at 70 °C in bulk; r_Str_4FS = 0.69 at 25 °C under redox conditions) give ideal copolymerizations, with the monomer reactivity ratios being independent of temperature. The glass transition temperatures (T_g) of the St-PFS copolymers are elevated relative to their mole-average values, whereas the T_g values of the St-4FS copolymers correspond to their mole-average values. The water and ethylene glycol contact angles of the St-PFS are between those of the corresponding homopolymers and increase with increasing content of fluorinated comonomer. The same behavior is observed for the water contact angles of the St-4FS copolymers.

The copolymerization behavior of 1VN with different fluorinated styrenes varies steadily from a tendency to alternate to a tendency to form random copolymers as the number of fluorine atoms on the aromatic ring decreases from five to three. For example, the product of the monomer reactivity ratios at 70 °C of 1VN with PFS is r_1VNr_PFS = 0.11; 1VN with TEFS is r_1VNr_TEFS = 0.30; and 1VN with TFS is r_1VNr_TFS = 0.90. For the 1VN-TEFS copolymerization, r_1VNr_TEFS = 0.13 at 25 °C using t-butylperoxy pivalate as the initiator. If the fluorinated styrene contains three or fewer fluorine atoms, its radical copolymerization with 1VN gives ideal copolymerizations: r_1VNr_TFS = 0.90; r_1VNr_DFS = 0.74; and r_1VNr_4FS = 0.85. Thus, the products of the monomer reactivity ratios correlate with the strength and/or extent of the pi-pi stacking interactions between aromatic hydrocarbons and aromatic fluorocarbons, which presumably increase with increasing fluorination. The T_g values of the 1VN-PFS and 1VN-TEFS copolymers are elevated relative to their mole-average values, whereas those of the 1VN-TFS, 1VN-DFS and 1VN-4FS copolymers occur at their mole-average values. The water contact angles of these copolymer films are intermediate to those of the corresponding homopolymers and increase with increasing content of fluorinated comonomer.