Chemistry Reference
In-Depth Information
1.4 The Thiol-ene Reaction: A New Strategy for
Photo-curable Materials from Vegetable Oils
The thiol-ene photo-reaction is a unique class of photo-polymerization that is
advantageous with regard to the polymerization process and also the per-
formance of the resulting polymers.
82-84
AsshowninFigure1.19,thethiol-ene
reaction proceeds in a free-radical step-growth polymerization manner
(Propagation A), and the excess acrylate continues to polymerize through
homopolymerization (Propagation B).
51,84
In thiol-ene polymerizations a broad
range of physical properties are achieved because a wide variety of enes such as
acrylate, vinyl ether, allyl ether, vinyl acetate, and alkene are used. Thiol-ene
photo-polymerization occurs through a step-growth reaction mechanism that
offers many advantages such as delayed gel points, uniform structures, low
polymerization shrinkage and reduced stress,
85-87
which make thiol-ene poly-
mers high-performance materials,
88-90
especially for photo-curing coatings.
91,92
The unsaturated nature of triglycerides obtained from vegetable oils make
them good candidates for thiol-ene coupling. The fatty compounds can be
transformed into polymeric networks by adding multifunctional thiols
under UV or thermal radical conditions; the low thermal stability of lipidic
compounds justifies the use of UV irradiation instead of thermal activation
in the thiol-ene coupling reaction.
62,93-95
The thiol-ene photo-induced
coupling reaction is a powerful method for the chemical modification of
triglycerides or lipidic compounds to prepare vegetable oil derivates.
96,97
The
Auvergne group
98
demonstrated an ecient thiol addition onto vegetable
oils, leading to bio-based polyols (Figure 1.20). The most important feature
of the reaction was the influence of the number of double bonds per chain in
the vegetable oil on the thiol grafting yield. Indeed, disulfide formation
and intermolecular recombination occurred. In the case of oleic acid
functionalization, transesterification was also detected. Despite these side-
reactions, byproducts were found to exhibit alcohol functional groups.
Interestingly, the photo-reaction can therefore be performed under mild
hv
R-SH
I
I
Initiation:
R-S
S
R-S
R'
R
R'
Propagation A:
S
S
R-S
R-SH
R
R'
R
R'
R'
R'
Propagation B:
S
R'
S
R
R'
R
Figure 1.19 Proposed mechanism for the thiol-ene photo-reaction.
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