Chemistry Reference
In-Depth Information
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dimerization of this type would be accompanied by formation of folded chains.
Reactions of formations of folded and parallel chains are similar with the exception that the
reacting cinnamic groups are further apart in folded chains and come together only by virtue of chain
folding. Chains located parallel to each other but at the right distance can also conceivably yield
truxinic acid type dimerization. This would be similar to the arrangements in crystal lattices.
Formation of a truxillic dimer, like in reaction 2, shown above, requires favorable folding or two
different chains. Also, there is accompanying possibility that the double bonds may simply polymer-
ize by a chain propagating reaction [
125
]. This was observed with some cinnamate esters [
125
].
Attempts were made to determine the reaction products of photocross-linked poly(vinyl cinnamate)
by first hydrolyzing it, and then by isolating and identifying the acids. The results showed that
a
-
truxillic acid does form. Formation of
-truxillic acid, however, was not demonstrated. In addition,
among the reaction products there is also a large quantity of unreacted cinnamic acid. This indicates
that only a small portion of the double bonds participate in the reaction. Also, it should not be
forgotten that only small changes in unsaturation in polymer molecules, as a result of cross-linking,
can have a profound effect on solubility.
Studies [
126
] of electronic structures in photo-isomerization and photo-dimerization of cinnamic
acid showed that phosphorescence of cinnamic groups occurs at about 20,000/cm. Also, it was
demonstrated when photosensitizers are present, the critical distance between donor, sensitizer, and
acceptor molecules (cinnamic acid) is about 10
˚
[
126
]. Although all the details of incipient
photocross-linking of poly(vinyl cinnamate) have to date still not been fully worked out, most accept
that all three mechanisms take place. These are: dimerizations to truxillic and truxinic acid type
structures and polymerizations through the double bonds. The excited states of the molecules can be
produced by direct irradiation and also through intersystem crossing from an appropriate photosensi-
tizer [
126
].
Several criterions were derived from proper selection of sensitizers [
127
,
128
]. These are:
b
1. The triplet state must be at the energy level close to 50-55 kcal/mole for the cinnamate moiety.
2. The quantum yield of the ratio of phosphorescence to fluorescence should be higher than unity.
3. The mean lifetime of a triplet state must be greater than 0.01 s. The photosensitizing activity
(characterized by the triplet state) of derivatives of cinnamic acid is beyond the phosphorescence
of the cinnamate group (at about 20,000/cm, as stated above). An energy transfer diagram for poly
(vinyl cinnamate) photosensitization with a sensitizer like 2-nitrofluorene was published [
128
].
The rate of dimerization obeys first-order kinetics. In addition, polymers consisting of flexible
segments exhibit higher rates of photo-dimerization than do those composed of rigid segments.
