13 October 2013Year: 2013
Photoinitiators (PI) are a very important part in a light curable system. They have a big
influence on the performance and the economy of the reaction as they influence the usable
wavelength, curing rate, double bond conversion, as well as polymer properties. Initiators
either directly absorb a photon and form radicals (type I) or work as a bimolecular system
(type II). The efficiency of the initiation reaction depends on the process of radical formation,
e.g. the interaction of benzophenone and tertiary amines in the well-known type II system, or
the photophysical properties of the acylphosphine oxide (type I). Another important factor are
the follow-up reactions of the formed primary radicals and their activity towards monomers.
There is a broad range of analytical techniques in use today to study photoinitiators, such as
differential scanning calorimetry (DSC), IR spectroscopy, laser-flash photolysis, NMR, and
GC-MS to name a few. A lot of these techniques only work on bulk properties or are indirect
measurements. The advantage using the here described magnetic resonance techniques is
that they provide direct evidence of the primary radicals and their follow-up products. With
time-resolved EPR (TR-EPR) it is possible to follow the generated radicals on a sub-μs
timescale and also to determine kinetic information, e.g. of additions to monomers.
Furthermore, it is possible to extract magnetic properties (g factor, hyperfine coupling
constants – hfcs) of short-lived radicals, which can help to unambiguously determine the
radical structure. Chemically induced dynamic nuclear polarization (CIDNP), on the other
hand is an NMR technique, which detects the polarized follow-up products of radical
reactions. These polarizations stem from the interaction of the initial radical pair and can give
information about reaction pathways and secondary reactions. Moreover, CIDNP enhances
the sensitivity of NMR by typically three orders of magnitude, making it possible to detect
small amounts of reaction products.
As examples for recent applications of these techniques we present the two following
systems: an acylgermane initiator1 currently used in dental filling materials, and a system
consisting of benzophenone and benzaldoxime ester.2 The magnetic resonance experiments
were performed in solution with typically 10 mM of PI. For irradiation Nd:YAG lasers at 355
nm were used. For further information see reference 2.
2013 Conference Elucidating Reaction Mechanisms of Photoinitiators by Magnetic Resonance