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

The addition reaction of primary alcohols onto aromatic carbodiimides giving iso -ureas was studied in bulk at various temperatures. A side reaction at 60 degrees 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 degrees C. The thermo-reversibility of the guanidine bond was utilized to produce thermoreversible 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 degrees C. The guanidine linkages in the polymers started to unzip above 60 degrees C as was indicated by a drop in the E-modulus, at about 130 degrees 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.