The addition reaction of alcohols and aromatic carbodiimides and its application to produce thermo-reversible polymer networks

Abstract

The addition reaction of primary alcohols onto aromatic carbodiimides giving iso-ureas was studied in bulk at various temperatures. A side reaction at 60 °C was found to occur between carbodiimide and iso-urea yielding N,N′-substituted polyguanidine oligomers. The occurrence of this side reaction coincides with a reduced conversion of the alcohol; the carbodiimide was always fully converted to either iso-urea or polyguanidine. The formation of the polyguanidine was reversible at higher temperatures to reform iso-urea and carbodiimide. The latter reaction was complete at about 130 °C. The thermo-reversibility of the guanidine bond was utilized to produce thermo-reversible self-healing polymer networks by reaction of polyether diol and a 1.5 to 2-fold molar excess of aromatic biscarbodiimide. The materials were soft and elastic and showed a glass transition at −60 °C. The guanidine linkages in the polymers started to unzip above 60 °C as was indicated by a drop in the E-modulus, at about 130 °C sufficient guanidine crosslinks had unzipped to provide the polymer liquid-like properties. The guanidine crosslinks were reformed after cooling to ambient temperature giving back the polymer network.