Investigation of thiol-ene photopolymerizations and their unique applications

Thiol-ene photopolymerizations are reactions between multifunctional thiol and vinyl (ene) monomers. Thiol-ene polymerizations exhibit the typical advantages of photopolymerizations in that they polymerize rapidly, do not require solvents for processing, are optically clear, and have excellent mechanical properties; however, the polymerization proceeds by a free radical step growth mechanism. Due to the versatile chemistry and unique reaction mechanism, thiol-enes exhibit several advantageous properties distinct from radical polymerizations. They are characterized by slow molecular weight evolution leading to gelation at high conversions, are not significantly inhibited by oxygen, and can be initiated without the use of added photoinitiator molecules. The reaction mechanism and kinetics are not fully understood and many novel polymer applications continue to be developed.; This thesis has focused on determining the primary initiation, propagation, and termination aspects of thiol-ene photopolymerizations to gain a deeper understanding of the polymerization mechanism and kinetics. A variety of different thiol-ene systems exhibiting the traditional step growth mechanism, and combinations of step growth and chain growth mechanisms were studied. A comprehensive model was developed based on the fundamental initiation, propagation, and termination mechanisms. Utilizing model predictions and FTIR experiments, several aspects of thiol-ene photopolymerization kinetics were investigated. Termination was shown to be extremely rapid and dominated by radical-radical recombinations. The propagation kinetics were shown to be strongly controlled by the ratio of propagation to chain transfer kinetic parameters. These parameters control rate limiting steps and individual thiol and ene reaction orders. The analytical expressions developed for the model were used to design kinetic experiments to quantify kinetic parameters directly.; Several of the unique polymer properties of thiol-ene polymerizations were investigated. The chemical versatility of thiol-ene reactions was demonstrated by conducting polymerizations with a multitude of different thiol and ene functional group chemistries. A novel polymerization scheme was developed to form polymer derived ceramic materials, and thin polymer films exposed to ambient oxygen were shown to be readily attainable in thiol-ene systems due to minimization of oxygen inhibition. Initiatorless photopolymerizations were shown to proceed readily for all of the thiol-ene systems studied. Initiatorless polymerizations were initiated with both 365 and 254 nm light. 365 nm light was utilized to produce extremely thick polymers while 254 nm light enables rapid curing of initiatorless systems. The initiation mechanism in the absence of photoinitiators was also investigated in conjunction with FTIR experiments and modeling predictions. Due to their slow molecular weight evolution, thiol-enes were shown to be applicable to polymer stabilized ferroelectric liquid crystals, where a host thiol-ene polymer nanostructure for FLCs was developed.