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The classical metallocenes, ferrocene and ruthenocene, dissolve readily in neat ethyl 2-cyanoacrylate (CA) monomer. The electronic spectra of the resulting solutions display a near-ultraviolet absorption band assigned as a charge-transfer-to-solvent (metallocene → CA) transition. Irradiation into this band causes the one-electron oxidation of the metallocene to the corresponding metallocenium cation accompanied by reduction of CA to its radical anion. Addition of the latter species to CA initiates the anionic polymerization of the electrophilic monomer. The progress of photoinitiated polymerization was monitored in real time by attenuated total reflectance infrared spectroscopy.

Spectroscopic, photochemical, and photoinitiation studies are reported for a series of monobenzoylferrocenes and 1,1‘-dibenzoylferrocenes. These nonionic metallocene complexes absorb strongly in the ultraviolet and visible wavelength regions owing to electronic transitions of mixed ligand field/charge-transfer character. Irradiation of the complexes in methanol results in ring−metal cleavage to yield a benzoyl-substituted cyclopentadienide carbanion and the corresponding half-sandwich iron(II) complex. This process occurs with an appreciable quantum efficiency (>0.30 at 546 nm) for 1,1‘-dibenzoylferrocenes; in contrast, monobenzoylferrocenes are appreciably less photosensitive. When the solvent is neat ethyl α-cyanoacrylate, the photogenerated carbanion initiates rapid anionic polymerization of this electrophilic monomer. Several factors that can influence the performance of benzoyl-substituted ferrocenes as anionic photoinitiators are considered.

It is shown that visible light irradiation of trans-[Cr(NH 3 ) 2 (NCS) 4 ] - dissolved in ethyl α-cyanoacrylate initiates anionic polymerization. The results suggest that ligand field excitation of judiciously selected transition metal complexes represents a promising strategy for the controlled release of anionic initiators.