Elucidating Reaction Mechanisms of Photoinitiators by Magnetic Resonance

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.

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.