The polymerization of tetrahydrofuran initiated by niobium(V) and tantalum(V) halides

The halides MX5 (M = Nb, Ta, X = F, Cl, Br) (1) are effective in promoting the polymerization of tetrahydrofuran (thf). The polymerization reactions promoted by MCl5 and MBr5 proceed via the ionic species 3, which forms from MX5(thf) (2) by attack of the incoming monomer to the metal-bonded thf. The metal-bonded thf of 3 is not involved in the immediately successive steps of the reaction. The [TaF6](-) anion is not formed when the polymerization is initiated by TaF5. The activities of the promoters increase according to the series TaBr5 < NbCl5 < TaCl5 < TaF5. The latter possesses high activity and produces high molar mass poly(tetrahydrofuran) (67100 g/mol). ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).