Crosslinking of elastomeric vinylidene fluoride copolymers with nucleophiles

The monomer sequence CF2CF(CF3)CH2CF2CF2CF(CF3) has been identified by 19FNMR as the selectively base‐sensitive site of poly(vinylidene fluoride/hexafluoropropylene). Similar studies of other vinylidene fluoride (VF2) copolymers with either tetrafluoroethylene (TFE) or perfluoro(methyl vinyl ether) (PMVE) show that the generalized base‐sensitive site of VF2 copolymers which contain one or more perfluorinated monomers can be formulated as: magnified image Depending on the nature of A, treatment with bases can lead to elimination of HF or the elements of CF3OH. In perfluoro(methyl vinyl ether)‐containing copolymers a major pathway of elimination from the intermediate carbanion involves loss of −OCF3, which decomposes to COF2 and −F. With benzylamine, COF2 was efficiently trapped as dibenzylurea by the excess base. The structural unit of PMVE is therefore substantially degraded when such copolymers are subjected to basic curatives. The use of PMVE in fluoroelastomer technology is therefore reserved for TFE‐based ultra‐high performance perfluoroelastomers, and for peroxide‐curable hydrofluoroelastomers designed for excellent low‐temperature service. The relative reactivity of HFP‐VF2‐HFP and TFE‐VF2‐TFE sites toward basic nucleophiles was determined in solution at room temperature. Whereas the two sites undergo HF elimination with similar ease, subsequent nucleophilic attack on the formed unsaturation by unreacted base or another nucleophile clearly differentiates these two sites. This differential reactivity is consistent with the differing vulcanization rates and vulcanizate stabilities of poly(VF2/HFP) and poly(VF2/HFP/TFE).