A series of polycarbodiimides was prepared from isocyanates using 1-phenyl-3-methyl-2-phospholene-1 oxide as catalyst. The molecular weight of the series was varied by employing controlled amounts of phenyl isocyanate as a terminating agent. The resultant polymers showed improved processing over very high molecular weight unterminated polymer, while still maintaining good thermal stability and equivalent mechanical properties. A study of the viscoelastic properties of these polymers using modulus–temperature plots showed that the materials underwent a crosslinking reaction at elevated temperatures (200–250°C). These reactions could be explained on the basis of a model, diphenylarbodiimide, and most likely include the formation of carbodiimide dimers and trimers.