ABSTRACT
Which is interesting, although with no practical use so far, is the production of PUs in the cationic polymerisation, in the ring opening process of cyclic urethanes. Polymerisation of seven-membered rings takes place at the temperature which is only slightly above the monomer melting point. Six-membered rings would polymerise at 100°C, while the fi ve-membered structures do not polymerise at all-that comes from the positive contribution of the entropy component ∆S to the free enthalpy ∆G = ∆H-T∆S of the polymerisation reaction of cyclic compounds, at least six-membered. The value of the enthalpy component for such reactions is zero (∆H ≈ 0) in practice. The dominant reaction at higher temperatures is polymer degradation (∆G > 0) with cyclisation (equation 70):
This method is applicable in the production of multi-block urethane copolymers, e.g. with poly(tetrahydrofurane), with the use of such initiators like methyl trifl uoromethyl-sulfonate or trifl uoromethylsulfonic anhydride [135, 136].
