31P and 19F NMR spectroscopic studies on ring-opening polymerization of 1,6-anhydro-2,3,4-tri-O-benzyl-β-D-glucopyranose by PF5 catalyst

In order to elucidate the catalytic behavior of phosphorus pentafluoride in the polymerization of anhydro sugars, ¹³P and ¹⁹F NMR spectra were measured on a reaction mixture of 1,6-anhydro-2,3,4-tri-O-benzyl-β-D -glucopyranose (LGTBE) and PF₅ with different mole ratios in a temperature range of ?40 to ?80°C. In the ³¹P NMR spectrum measured at low temperatures, there was a total of 16 peaks, which consisted of a broad quintet, a septet, and a sharp quartet, being assigned to the PF₄O-group, to PF, and to POF₃, respectively. These fluoro compounds were also determined by the ¹⁹F NMR spectrum of the reaction mixture. The concentration of PF ions was found to correspond to that of oxonium ions, which are assumed to be actual propagating species, by determining both the concentration of PF from ¹⁹F NMR spectrum and the degree of polymerization of 2,3,4-tri-O-benzyl-α-D -glucopyranan obtained. Formation of the PF₅: LGTBE complex was observed from the ³¹P NMR spectrum of the polymerization system at ?80°C, which exhibits a broad sextet as well as absorptions due to POF₃, PF₄O–, and PF. To confirm the PF₅:LGTBE complex, the NMR measurement of the PF₅: tetrahydropyran complex was carried out. A polymerization mechanism of LGTBE by PF₅ catalyst is discussed on the basis of the NMR measurement of the polymerization system.