The Polymerization of Tetrahydrofuran Initiated by Niobium(V) and Tantalum(V) Halides

TThe halides MX(5) (M = Nb, Ta, X = F, Cl, Br) (1) are effective in promoting the polymerization of tetrahydrofuran (thf). The polymerization reactions promoted by MCl(5) and MBr(5) proceed via the ionic species 3, which forms from MX(5)(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 [TaF(6)](-) anion is not formed when the polymerization is initiated by TaF(5). The activities of the promoters increase according to the series TaBr(5) < NbCl(5) < TaCl(5) < TaF(5). The latter possesses high activity and produces high molar mass poly(tetrahydrofuran) (67100 g/mol).