Methods: Samples were prepared with pentaerythritol tetrakis(3-mercaptopropionate) (PETMP) and 1,9-decadiyne (DDY) with 3% initiator l-Hydroxy-Cyclohexyl-Phenyl-Ketone. Photopolymerization behavior and conversions of cured resins were measured with Fourier-transform infrared spectroscopy. DMA in tensile mode was used to determine rubbery moduli and glass transition temperatures.
Results: The thiol-yne reaction between PETMP and DDY was determined to be exclusively step-growth under stoichiometrically balanced conditions. The rate constant of the second addition of the thiol to the vinyl sulfide intermediate was determined to be approximately three times that of the initial thiol-alkyne addition. Analysis of initial rates demonstrated that the addition of the thiol to the alkyne is thiol controlled (0.1 order with respect to alkynes and 0.8 order with respect to thiols). The resulting polymer network exhibited a narrow glass transition indicative of the regular network architecture achieved with step-growth polymerization. These materials possessed increased rubbery moduli and glass transition temperatures relative to analogous thiol-ene resins, indicating increased crosslink density. Volume shrinkage was determined to be 21 mL/mol alkyne.
Conclusions: The thiol-yne photopolymerization addresses an intrinsic shortcoming of thiol-ene materials (low crosslink density) while maintaining the step-growth mechanism and all the advantages inherent therein. Thus, the thiol-yne photopolymerization promises to become an important tool in photopolymer development.
Supported by NSF Grants CTS-0626023 and EEC-444771 and NIH Grant DE10959.