Chemistry Reference
In-Depth Information
COOH
COOH
+
COOH
HOOC
truxillic acid
truxinic acid
Schmidt and coworkers [
120
,
121
] studied the reaction mechanism and came to the following
conclusions:
1. Photo-dimerization of cinnamic acid and its esters is controlled by the crystal lattice.
2. Dimerizations are possible when olefinic double bonds of the two neighboring molecules in the
crystals are 4.1
˚
or less apart.
3. Dimerizations are not possible when the double bonds are 4.7
˚
or more apart.
The dimerization reaction takes place upon irradiation with light of a wavelength longer than
300 nm. It was demonstrated subsequently [
122
], however, on poly(vinyl cinnamate) that the adducts
dissociate again upon irradiation with light of 254 nm. Photo-dimerization and formation of
cyclobutane groups were demonstrated to be enhanced as a result of sensitized irradiation [
123
].
When dealing with poly(vinyl cinnamate), it is reasonable to assume that the degree of order in the
relationship of one cinnamic group to another is much lower than is found in a crystal lattice of
cinnamic acid. On the other hand, it should be higher than in solutions of cinnamic acid, where the
groups are far enough apart so that very little photo-dimerization takes place. Photocross-linking of
poly(vinyl cinnamate) can include the following reactions [
124
]:
1. Truxinic acid type dimerization in irradiated poly(vinyl cinnamate) that can occur intramolecu-
larly. It can be shown as follows:
O
O
O
O
This is accompanied by formation of both folded and parallel chains
2. Truxillic acid type intermolecular dimerization in irradiated poly(vinyl cinnamate), on the other
hand, can be illustrated as follows:
