Isocyanate-Free Route to Caprolactam-Blocked Oligomeric Isocyanates via Carbonylbiscaprolactam- (CBC-) Mediated End Group Conversion

Abstract

Blocked isocyanate functional oligomers and liquid rubbers are prepared by a novel, solvent-free carbonylbiscaprolactam (CBC) mediated end group conversion reaction of amine-terminated oligodimethylsiloxanes (PDMS) and oligopropyleneoxides (PPO), with molar masses varying between 400 and 3140 g/mol, and of hydroxy-terminated oligoethylene oxide (PEG), oligopropyleneoxide (PPO) and oligotetrahydrofuran (PTHF), with molar masses of around 1000 g/mol. The key reaction is the carbonylbiscaprolactam (CBC) mediated amino or hydroxy end group conversion yielding carbamoyl caprolactam functional polymers, without requiring addition of either phosgene or isocyanates. The quantitative CBC conversion of amine end groups occurred in bulk at 100 °C in the absence of catalysts, yielding N-carbamoyl caprolactam terminated oligomers and caprolactam (ring elimination, pathway RE). The reaction of hydroxy end groups at 100 to 150 °C in the presence of catalysts such as zirconium alcoholates, magnesium bromide or dibutyltindilaurate (DBTDL) produced N-carbamoyl caprolactam end groups via nucleophilic attack of the hydroxy group at one of the CBC caprolactam rings and subsequent ring opening (ring opening, pathway RO). The CBC reactions were monitored by means of ReactIR, 1H NMR and 13C NMR spectroscopy. The molecular masses of the oligomers and liquid rubbers with caprolactam-blocked isocyanate end groups were measured by means of MALDI−ToF mass spectroscopy and size exclusion chromatography (SEC). The thermal behavior and deblocking temperatures of the caprolactam-blocked isocyanates obtained were examined by means of thermogravimetric analysis (TGA).