Evidence of isomerization of 5-methylenebicyclo[2.2.1] hept-2-ene cation by13C NMR spectroscopy

Summary Cations obtained by reaction of various protonic acids with 5-Methylenebicyclo 2.2.1] hept-2-ene (5-methylene-2-norbornene) have been studied by C NMR spectroscopy. The isomerization of initial carbocation has been pointed out. A correlation of these results with the structure of the corresponding polymers has been establishedThe cationic polymerization of 5-Methylenebicyclo 2.2.1] hept2-ene (5-methylene-2-norbornene) has been investigated several times and KENNEDY (1974) published a review on this topic. A structure of the polymer obtained by classical cationic initiation has been suggested; this results from infrared and X.R. diffraction studies.In a recent work IVIN et al. (1980) reported the¹³C NMR spectrum of a poly5-methylenebicyclo 2.2.1] hept-2-ene] resulting from a Ziegler-Natta initiation. We obtained the same¹³C NMR spectrum as IVIN et al. when the initiator were TiCl₄, CF₃ COOH, CCl₃COOH and concentrated H₂SO₄ (see fig.1). Chemical shifts and coupling constants fit with the generally accepted structure of the polymers: KENNEDY and MAKOWSKY (1967) suggested an isomerization of the active species: