Characterization of benzoyl-substituted group 8 metallocenes as photochemical initiators for the anionic polymerization of vinylic monomers
Sanderson, Cynthia Therese
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This thesis demonstrates that ferrocene, ruthenocene, and their benzoylsubstituted analogs are photoinitiators for the anionic polymerization of ethyl 2- cyanoacrylate and evaluates the effectiveness of these photoinitiators based on the rates of polymerization as determined by attenuated-total-reflectance Fouriertransform infrared spectroscopy (ATR-FTIR). While all of these compounds are effective photoinitiators, the mechanism of photoinitiation depends on which metal is present and whether or not the compound contains electron-withdrawing benzoyl groups. The parent compounds exhibit solvent insensitive ligand field transitions in the visible/near UV region of the spectrum, and resist photoinduced ring loss. Photoinitiation is thereby shown to occur via a photoactive chargetransfer- to-solvent complex between the metallocene and the cyanoacrylate monomer which results in the oxidation of the unsubstituted metallocene and reduction of the electrophilic monomer. In contrast, addition of one or more benzoyl groups to the cyclopentadienyl rings of the metallocene causes the mixing of charge-transfer character (metal-to-ligand) into the ligand field transitions of the parent compound. This charge-transfer character is manifested by an increase in the intensity of the electronic transition, which is accompanied by a shift to lower energy as compared to the transitions of the parent compounds. The nature of this charge transfer character has been studied using resonance Raman spectroscopy, and it is shown that the assignment of metal-toligand charge transfer is accurate for all of the benzoyl-substituted metallocenes studied. In the case of 1,1’-dibenzoylferrocene, this metal-to-ligand charge transfer character is responsible for photoinduced ring loss which occurs upon irradiation into the low-energy electronic transition. This photoreaction has been studied using an on-line photolysis procedure which allows the identification of short-lived photoproducts by mass spectrometry. For the ruthenium containing compounds, addition of a benzoyl group to one or both of the cyclopentadienyl rings causes similar spectral changes as those seen in the case of the iron analogs; however, there is no indication that the primary photochemical reaction is ring loss. Rather, photoinitiation occurs via the same charge-transfer-to-solvent mechanism as seen for the unsubstituted metallocenes. Finally, this manuscript attempts to expand the use of these photoinitiators to another vinylic monomer, methyl methacrylate, which is also susceptible to anionic attack.