7.1 Polytetrahydrofuran (Polytetramethylene Glycols)

Polytetrahydrofuran (PTHF) is a polyether obtained by cationic ring opening polymerisation of tetrahydrofuran (THF), a five membered cyclic ether:

The driving force of this polymerisation reaction is the ring strain of the THF cycle. The main contribution to this ring strain is not due to the deviation from the normal angle values of chemical bonds (angular tension), which is relatively low, but the most important contribution is due to the torsional repulsion forces between the hydrogen atoms situated in eclipsate positions (Figure 7.1). The angular tension of the THF cycle is small, around 0.556 kcal/mol [1-8], but the total ring strain is around 4.3 kcal/mol [7], 4-5 times higher than the angular tension. This ring strain of 4.3 kcal/mol is much lower than the ring strain of alkylene oxides, but is high enough to assure the ring opening polymerisation of THF.

Figure 7.1: Torsional repulsion forces of hydrogen atoms in the tetrahydrofuran ring

The cationic catalysts used for THF polymerisation, with the general formula R+X- are: trialkyloxonium salts of superacids, esters of superacids, oxocarbenium salts, Lewis acid - oxirane complexes, superacid anhydrides, (e.g., triflic anhydride), and others [1-38] (Figure 7.2):

Figure 7.2: Cationic catalysts used in THF polymerisation

The ring opening polymerisation of THF has as active centres tertiary oxonium cations and is based on the nucleophilic attack of the oxygen atom of the THF monomer at the ?-carbon atom of the oxonium cation. The ?-carbon atom is activated by the presence of the...