The kinetics of two thiol-ene systems were investigated. The bulk polymerization kinetics of a model system 1,8-octane dithiol (ODT) and styrene (STY) and a network system comprised of pentaerythritol tetra-3-mercaptoproprionate (PTM) and divinyl benzene 960 (DVB 960) were studied with respect to the effects of stoichiometry, temperature and initiator concentration. Rate expressions were derived for the ODT / STY system.
The reaction kinetics of the two systems were monitored by real time FT-NIR and FT-Raman spectroscopies. The NIR was chosen to follow the disappearance of the ene (C=C) moiety. FT-Raman spectroscopy was used to track the conversion of the thiol (SH) group. The C=C band was also monitored by FT-Raman spectroscopy.
For the ODT / STY system the effect of stoichiometry with respect to the rate of reaction indicated that formulations with high thiol concentration exhibited a higher initial rate of reaction of thiol and those with high STY formulations exhibited a lower initial rate of reaction of STY.
The effect of temperature on the ODT / STY reaction mechanism showed that there is an increase in the rate of reaction with an increase in temperature for all the formulations and stoichiometries investigated. The homopropagation of styrene was favoured by an increase in the temperature.
The effect of the initiator on the ODT / STY system was shown to have a first order dependence which was consistent with the derived rate expressions for the removal of radicals by some impurity. However, a range of different termination processes may be involved.
Structural characterisation of a 1:1 ODT/STY formulation by HPLC, GPC, NMR and GC / MS provided conclusive evidence of the formation of STY-STY linkages, in agreement with the kinetic analyses. Confirmation that the two thiol groups react independently was also found from the presence of products with two STY-STY ends remaining at the final stages of the reaction.
Many similarities were found in the kinetics of the model system and network system. The effect of stoichiometry, temperature and initiator concentration were observed to demonstrate similar trends to that of the model system. Viscosity and diffusional factors were thought responsible for the differences in the kinetics of the PTM / DVB 960 system. However, the mechanisms of reaction were shown to be similar.
The gel point of the PTM / DVB 960 system was determined by two rheological methods. A steady shear test experiment was used to determine the point of gelation by locating the asymptotic behaviour of shear viscosity, while a multiwave test was used to determine the gel point by measuring G', G" and tan δ. The Flory-Stockmayer theory of gelation was used to predict the conversion of C=C at the gel point from FT-NIR data. In this way, the calculated time coinciding with the conversion resulting in a 3D network was related to the experimental measured time to gelation. The measured gel times were thought to be prolonged due to cyclisations and looping occurring.