Aromatic carbodiimides stability

At room temperature aliphatic and aromatic carbodiimides are liquid or solid. They usually can be purified by distillation under reduced pressure or by recrystallization to yield neutral products. On prolonged standing they polymerize to yield basic products. The liquid carbodiimides are less stable than the solid ones. The stability of carbodiimides increases with the degree of branching of the alkyl substituents attached to the nitrogen atom RCH2 < R2CH < R3C. For example, diethylcarbodiimide [14, 21] polymerizes in a few days, whereas diisopropyl- and dicyclohexylcarbodiimides [14] are stable for several months. Unsaturated substituents also cause a marked decrease in stability... 

Aliphatic and aromatic carbodiimides are liquids or solids at room temperature. The stability of substituted dialkylcarbodiimides increases as follows RCH2 < R2CH < R3C."" Dimethylcarbodiimide should be used freshly prepared, but it can be stored for several days below room temperature. Unsaturation in the aliphatic substituents decreases the stability of carbodiimides. Eor example, diallylcarbodiimide is unstable. 

Oligomeric carbodiimides derived from sterically hindered aromatic diisocyanates are used as stabilizers for polyester based polymers because the carbodiimide groups in the oligomers, or in sterically hindered diisocyanates, mop up HCl, which catalyzes the depolymerization of polyesters. The carbodiimide stabilizers are commercially available under the tradename Stabaxol from Rhein Chemie, a subsidiary of Bayer. 

Applications for the Stabaxol stabilizers include thermoplastic polyester urethanes, polyesteramide thermoplastic elastomers, castable polyester urethanes, polyester polyols, monofilament PET fibers, polycarbonates, polycarbonate/PETblends, EVA copolymers and poly(caprolactones). The thermal stabilization of poly(ethylene sulfide) is also accomplished with 4 % hexamethylenebis(t-butyl)carbodiimide and 2 % diphenylacetylene. Also, alternating carbon monoxide/ethylene copolymers are stabilized using aromatic carbodiimides. ... 

In the aromatic series, carbodiimides having a substituent in the o-position are preferred. Examples include N,N -di-t)-tolylcarbodiimide and NJ l -di-2,6-diethylcarbodiimide, the latter being a useful stabilizer for polyester based polyurethanes. ... 

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