Formation of carbodiimide
Carbodiimide is formed by a catalyzed irreversible dimerization reaction of isocyanate [67, 68]. It has significant practical utility since it can be used to oligomerize 4,4" MDI; it has found numerous applications including manufacture of cast elastomers, foams, and for making composite laminates. In its simplest expression, its formation is described by Figure 3.17. Polymers formed from carbodiimides can exhibit greater flexibility and less friability versus analogous polymers synthesized from conventional diisocyanates.
Various descriptions and catalytic mechanisms have been put forward to explain the evolution of reactants to products (Fig. 3.18). Significant progress was made with the advent of phospholene oxide catalysts that are highly efficient at forming carbodiimide while minimizing the formation of other products. The carbodiimde is detectable by infrared spectroscopy by the characteristic N = C = N absorption at 2140 cm-1 but may exhibit partial overlap with the isocyanate resonance at 2270 cm-1. Another practical aspect of the formation of carbodiimide results from its unintentional formation in a closed volume (i.e., in a drum for instance) with the concomitant evolution of CO2 resulting in an explosion hazard from pressure buildup.
Carbodiimide chemistry is also notable as part of urethane degradation pathways [71, 72]. When urethanes are heated to high temperatures (ca. 240 - 270 °C) for extended periods of time in an FTIR cell, the characteristic carbodiimide resonance will appear at 2140 cm-1. It was subsequently noticed that the carbodiimide concentration initially increased and subsequently reached an equilibrium concentration. Evolution of isocyanate was also observed but declined with time as did the urethane associated carbonyls between 1715 and 1730 cm-1 coupled with the growth of carbonyl resonance between 1700 and 1710 cm-1. The overlap between the trimer resonance and the urethane resonance makes assignment of carbodiimide trimerization somewhat imprecise, and there is no available mass spectral data. However, the crosslinked behavior of the heated material was noted. It has been postulated that carbodiimide (which clearly forms) is capable of trimerizing according to the reaction given in Fig. 3.19. Additionally, the reaction of two isocyanates and one carbodiimide was also postulated (Fig. 3.20).