A simple and mild route to highly fluorinated model polymers

We have developed a simple and mild method for the fluorination of polybutadiene based on the addition of perfluoroalkyl iodides (R_fI) to carbon-carbon double bonds. Triethylborane (Et₃B) was utilized to initiate this free radical addition to model polybutadiene (PBD) homopolymers and a polystyrene-polybutadiene block copolymer at room temperature. Optimized reaction conditions led to consumption of more than 95% of the double bonds and preservation of narrow molecular weight distribution after the modification. We propose that the reaction undergoes a cyclization pathway rather than open-chain addition and that five-member ring structures are formed during the addition of R_fI to 1,2-PBD. From ¹H NMR spectroscopy, we estimate that 83% of the double bonds in the 1,2-PBD cyclized with their neighbors. This agrees well with a theoretical prediction by Flory for random irreversible cyclization between neighboring polymer repeat units. We also demonstrate the selective fluorination of the 1,2-PBD block in a polystyrene-block-1,2-polybutadiene (PS-b-1,2-PBD) copolymer. In contrast to the fluorinated homopolymers, subsequent hydrogenolysis of this fluorinated PS-b-1,2-PBD copolymer gave a soluble material. The ¹H NMR spectrum and elemental analysis confirmed the complete hydrogenolysis. Preliminary physical characterization was performed by differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), contact angle measurements, and small-angle X-ray scattering (SAXS). The glass transition temperature (T_g) of the fluorinated 1,2-PBD increases by 75 °C, removal of the iodine in the fluorinated PS-b-1,2-PBD copolymer increases the thermal stability by ca. 100 °C, and all fluorinated polymers exhibit very low critical surface tensions (14-16 mN/m).