Synthesis and dissociation of amine-blocked diisocyanates and polyurethane prepolymers

Substituted N-methylanilines are shown to act as blocking agents for toluenediisocyanate. N-methylaniline-, N-methyl-p-anisidine- and N-methyl-p-nitroaniline-blocked toluene diisocyanates have been prepared and characterized by FTIR, H-1 NMR and C-13 NMR spectroscopies, and nitrogen content analysis. A new method for determining the minimum deblocking temperature of the blocked isocyanate is described. The method has advantages in that it can be used to find the minimum deblocking temperature of even non-volatile blocking agents. The minimum deblocking temperature of the adducts is found to be in the following order: N-methyl-p-anisidine-TDI adduct<N-methyaniline-TDI adduct<N-methyl-p-nitroaniline-TDI adduct. The anilines exhibit the same trend when they block a polyurethane prepolymer prepared using polypropylene glycol of molecular weight 2000 g mol(-1) and tolylene-2,6-diisocyanate. The deblocking temperatures are lower in the case of blocked prepolymers than in the blocked adducts. The blocked adducts and prepolymers are reacted with pyromellitic dianhydride (PMDA) in dimethylpropylene urea (DMPU) and the evolution of carbon dioxide is monitored to study the completion of imidization. The reaction time is in accordance with the deblocking ability of the adducts. The regeneration of the blocking agent is confirmed by gas chromatography. (C) 2002 Society of Chemical Industry.