Effects of 1°, 2°, and 3° Thiols onThiol–Ene Reactions: Polymerization Kinetics and MechanicalBehavior
2020
The effect of thiol substitution
in radical thiol–ene reactions
has been studied by using model, monofunctional thiols as well as
multifunctional thiol monomers along with the assessment of their
subsequent polymerization reactions and polymer mechanical behavior.
FT-IR was used to monitor the polymerization rate and quantify the
overall conversion. While the total conversion was observed to range
from 70% to 100%, the polymerization rate was found to decrease by
as much as 10-fold as the thiol substitution was changed from primary
to tertiary. Analogous multi-thiol monomers of similar structure but
varying substitution were synthesized to observe the effect of substitution
type on polymerization kinetics and polymer behavior. Methylation
at the α-carbon was varied from primary to tertiary to observe
these differences. Mechanical properties were assessed by using dynamic
mechanical analysis and water sorption experiments, where the glass
transition temperatures were found to be within 1–2 °C
as thiol substitution varied. Furthermore, primary thiol films absorbed
1–3% more water than secondary thiol films. Resin shelf stability
experiments were performed by using rheometry to measure storage time-dependent
viscosity changes, and it was found that secondary thiol films remained
relatively stable for up to 100 times longer than their primary counterparts.
It was concluded that while there are differences under relatively
slow initiation conditions, at typical initiation rates all three
thiol substitutions may be made to react at similar rates for both
monofunctional and polymeric systems.
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