α,ω‐Diisocyanatocarbodiimides, ‐Polycarbodiimides, and Their Derivatives

Research in the field of low‐molecular weight, oligomeric and polymeric α,ω‐diisocyanatocarbodiimides and ‐polycarbodiimides has been fruitful, not only in connection with these compounds themselves, but also—as so often happens in chemistry—with quite different problems. Novel synthetic methods, discoveries concerning the properties of low‐molecular weight carbodiimides and phosphane imide derivatives, as well as results on the fragmentation reactions of four‐membered heterocyclic compounds containing oxygen, phosphorus, and nitrogen, and a better understanding of the diisocyanate polyaddition process are among the many by‐products of this research. The “high‐ and low‐temperature formation” of polycarbodiimides and the homogeneous and heterogeneous catalysis of this process are described, and the fundamental importance of four‐membered ring fragmentation mechanisms resulting in the formation of phosphane imide derivatives is outlined. Interesting building blocks for the diisocyanate polyaddition and polycondensation processes can be synthesized by many derivatization reactions of oligomeric and high‐molecular weight polycarbodiimides and polyuretonimines. The in situ production of polycarbodiimides via matrix reactions in flexible polyurethane foams leads to a cellular arrangement of the material due to the pronounced symmetrical growth processes. Combination‐foams with increased carbonation tendencies are formed in this way. Attention is drawn to several industrial applications of α,ω‐diisocyanatopolycarbodiimides, of high‐molecular weight cross‐linked polyuretonimines, and of polycarbodiimide foams.