Chemistry Reference
In-Depth Information
Many organic compounds decompose or cleave into radicals upon irradiation with light of an
appropriate wavelength [
38
,
39
]. Because the reactions are strictly light and not heat induced, it is
possible to carry out the polymerizations at low temperatures. In addition, by employing narrow
wavelength bands that only excite the photoinitiators, it is possible to stop the reaction by merely
blocking out the light. Among the compounds that decompose readily are peroxides, azo compounds,
disulfide, ketones, and aldehydes. A photodecomposition of a disulfide can be illustrated as follows:
hν
2
S
S
S
Today, many commercially prepared photoinitiators are available. Some consists of aromatic
ketones that cleave by the Norrish reaction or are photoreduced to form free radicals. There are also
numerous other two and three component photoinitiating systems. There are also those that decom-
pose by irradiation with visible light and make it possible to initiate the reactions with longer
Many others can be found in the literature.
As an example can be sited the work by Barner-Kowollik and coworkers studied the
photoinitiation process in methyl methacrylate polymerization, using high-resolution electro spray-
mass spectrometry [
40
]. The polymerization was conducted using a pulsed laser at temperatures
0
C in the presence of the photoinitiators 2,2-dimethoxy-2-phenylacetophenone, benzoin, benzil,
benzoin ethyl ether, and 2,2-azobisisobutylnitrile. They identified the termination products, both
combination and disproportionation with high accuracy. Both the benzoyl and acetal fragments
generated as a result of 2,2-dimethoxy-2-phenylacetophenone photocleavage were found to initiate
and highly likely terminate the polymerization. Both the benzoyl and ether fragments produced as a
result of benzoin photocleavage were found to act as initiating and probable terminating species,
indicating that the ether radical fragment does not act exclusively as a terminating species.
3.2.5
Initiation of Polymerization with Radioactive
Sources and Electron Beams
Different radioactive sources can initiate free-radical polymerizations of vinyl monomers. They can be
emitters of gamma rays, beta rays, or alpha particles.Most useful are strong gamma emitters, such as
60
Co
or
90
Sr. Electron beams from electrostatic accelerators are also efficient initiators. The products from
irradiation by radioactive sources or by electron beams are similar to but not identical to the products of
irradiation by ultraviolet light. Irradiation by ionizing radiation causes the excited monomer molecules to
decompose into free radicals. Ionic species also form from initial electron captures. No sensitizers or
extraneous initiating materials are required. It is commonly accepted that free radicals and ions are the
initial products and that they act as intermediate species in these reactions. There is still insufficient
information, however, on the exact nature of all of these species [
38
,
39
]. The polymerizations are
predominantly by a free-radical mechanismwith somemonomers and by an ionic onewith others [
38
,
39
].
3.3 Capture of Free Radicals by Monomers
Once the initiating radical is formed, there is competition between addition to the monomer and all
other possible secondary reactions. A secondary reaction, such as a recombination of fragments, as
shown above, can be caused by the cage effect of the solvent molecules [
41
]. Other reactions can take
