FT-IR and NMR studies on the sequence distribution in the alternating copolymers ethylene(E)-tetrafluoroethylene (TFE) and ethylene(E)-chlorotrifluoroethylene (CTFE)

Abstract

We have developed an IR method to study the content of monomer sequences in ETFE and ECTFE copolymers that may substitute for NMR spectroscopy in routine analysis. Indeed, the NMR analyses are carried out at 300 °C and 200 °C respectively, risking instrumental damage due to the high temperature. The search for a different technique based only on FT-IR spectroscopy can thereby be justified. We examined NMR and IR spectra of several ETFE and ECTFE polymers with different ethylene molar contents. Combining 19F NMR and elemental analysis data it was possible to determine the structure of ETFE copolymers at the level of monomer triads. The triad monomer distribution can be obtained in the same way for ECTFE copolymers. In this case it was also possible to obtain 1H NMR spectra that can be used to determine the concentration of [E]n sequences directly. In the case of ETFE copolymers this could only be calculated by difference. The IR spectra of both copolymers were carefully analyzed in the CH stretching region, because the CH oscillators are very sensitive to sequence distribution (both for frequency and intensity). The information so obtained considers total ethylene sequences [E]n for n>2. Some weak absorptions in the IR spectrum of the ETFE copolymer due to triad sequences [L.N. Pirozhnaya and L. I. Tarutina, Zh. Prikl. Spektros., 34 (1980) 862; L. I. Tarutina, Zh. Prikl. Spektros., 8 (1968) 653] have been correlated with the corresponding triad content determined through NMR spectroscopy. This work allowed us to discriminate in terms of homogeneity of monomer distribution between copolymers of similar molar composition and to test the reliability of available reactivity ratios.