2-D NMR Studies of Chain-End Structures in Poly(Hexafluoropropylene Oxide) Fluoropolymers

Abstract

2-D NMR techniques were used to study the structures of poly (hexafluoropropylene oxide) (PHFPO) and its mechanism of polymerization. Model compounds K4, containing four hexafluorpropylene oxide repeat units, and E4 and E2, containing four and two ether oxygens respectively, were analyzed to interpret the resonance patterns in the NMR spectra from the polymers. Selective 19F-13C gradient HSQC experiments provided information about the C-F attachments within monomer units of the backbone and chain ends. A series of 19F-19F selective COSY 2-D NMR experiments were performed to determine the connections and arrangements of each hexafluoropropylene oxide unit. When selective experiments were combined with selective inversion pulses to remove most of the JFF couplings in the f1 dimension, greatly simplified 19F-19F COSY 2-D NMR spectra were obtained. This made it possible to resolve signals from various stereo sequences and monomer inversions. Resonance chain-end structures detected in the polymer have been identified and attributed to the various mechanisms for initiation and termination of polymerization. Unambiguous evidence of inverse addition and isopropyl end-group structures was obtained.