Thiol−Isocyanate−Ene Ternary Networks by Sequential and Simultaneous Thiol Click Reactions

Thiol−isocyanate−ene ternary networks with systematic variations (100/100/0, 100/80/20, 100/60/40, 100/40/60, 100/20/80, and 100/0/100) were prepared by sequential and simultaneous thiol−ene and thiol−isocyanate click reactions. The thiol−isocyanate coupling reaction was triggered thermally or photolytically to control the sequence with the thiol−ene photopolymerization. Triethyl amine (TEA) and 2,2-dimethoxy-2-phenyl acetophenone (DMPA) were used for the sequential thermally induced thiol−isocyanate coupling and photochemically initiated thiol−ene reaction, respectively. A thermally stable photolatent base catalyst (tributylamine·tetraphenylborate salt, TBA·HBPh4) capable of in situ generation of tributylamine by UV light was used with isopropylthioxanthone (ITX) for the simultaneous thiol−isocyanate/thiol−ene curing systems. The kinetics of the hybrid networks investigated using real-time IR indicate that both thiol−isocyanate and thiol−ene reactions were quantitatively rapid and efficient (>90% of conversion in a matter of minutes and seconds, respectively). The Tg of the thiourethane/thiol−ene hybrid networks progressively increases (−5 to 35 °C by DSC) as a function of the thiourethane content due to the higher extent of hydrogen bonding, also resulting in enhanced mechanical properties. Highly uniform and dense network structures exhibiting narrow full width at half-maximum (∼10 °C) were obtained for both the sequential and the simultaneous thiol click reactions, resulting in identical thermal properties that are independent of the sequence of the curing processes.