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The technology of photoinduced polymerizations is widely applied in several
industrial fields. Important advantages offered by this technology are: high speed,
reduced release of volatiles since no solvents are used, low cost of equipments.
Also good mechanical properties, inertness to solvents, resistance to abrasion etc
could be achieved through a high degree of cross linking in the obtained polymer.
In the polymerization reaction the photoinitiator plays a key role since the efficiency
of the initiation step is closely dependent on the cleavage properties of the involved
excited state (singlet or triplet) and on the reactivity of the radicals arising from this
cleavage.
In pigmented systems, used for instance in the UV inks, however, the pigments
absorb UV radiation and compete with the photoinitiator for the available energy. In
order to absorb the UV light still transmitted through those "windows", tipically in the
region of 300 - 400 nm, where the pigments show minimum absorption,
photoinitiators with redshifted UV absorption are needed. Several types of
substitutes have been introduced on the aryl group of the aromatic ketones used as
photoinitiators in order to achieve a red shift in the UV absorption spectra. Among
these a thioether group proved to be very effective because not only it redshifts the
maximum of absorption but also ináreases the lifetimes of the excited states.. Thus
the rates of polimerization are increased in comparison to parent compounds with
no substitution 1]. Unfortunately in the practical application these photoinitiators
often give off an unpleasant smell after curing probably due to the development of
volatile thiols produced after the splitting of the C-S bond.
