Photoinitiation of multifunctional acrylates via ferrocene-alkyl chloride charge transfer complexes

Abstract

The propensity for ferrocene-alkyl chloride charge transfer complexes (CTCs) to photoinitiate free-radical polymerization of multifunctional acrylates was determined using photodifferential scanning calorimetry. Also, the effects of varying ferrocene (ferrocene, methoxyferrocene, and cyanoferrocene) and alkyl chloride (dichloromethane and benzyl chloride) derivatives were evaluated with regard to the overall polymerization rate and conversion. Furthermore, relative polymerization rates of traditional freeradical Type I and Type II photoinitiators were compared to those of the ferrocene-alkyl chloride CTCs. Semi-empirical quantum mechanical analysis of the complexation reaction was performed using PM3, indicating a thermodynamic preference of complexations involving benzyl chloride, and corroborated the reported complexation mechanism. In order to explain the varying polymerization rates, the association constants for each complex were determined, whereupon complexation of each ferrocene derivative with dichloromethane was found to be more facile than similar complexation with benzyl chloride due to steric considerations. Substituent effects were more pronounced for the benzyl chloride complexes relative to those involving dichloromethane where steric constraints caused deviation from the expected effect. Thus, the cyanoferrocene-benzyl chloride CTC was determined to be the most effective photoinitiator examined with regard to semi-empirical analysis, complexation kinetics, and polymerization rate.

Recent advances on ferrocene-based photoinitiating systems

“…Considering the propensity of ferrocene to form intermolecular charge transfer complexes (CTCs), the possibility to initiate a free radical polymerization (FRP) of acrylates with ferrocene-alkyl/aryl halide CTCs was examined. [273] Interestingly, upon photoexcitation of the CTC in the UV region, a decomposition of the CTC could occur, producing radicals according to the mechanism depicted in the Scheme 6. Notably, theoretical calculations carried out with numerous alkyl and aryl halides revealed that formation of the charge transfer complexes was inducing a geometrical deformation of ferrocene, due to the intercalation of the halogen between the two cyclopentadiene rings.…”

Iron complexes as photoinitiators for radical and cationic polymerization through photoredox catalysis processes

“…Examples include, iron(III) salts, Fe(acac) 3 , ferricyanide salts, tris(oxalato)iron(III), ferric oxalate, tris(thiocyanato)tris(pyridine)iron(III), tris(o‐phenanthroline)iron(III), bis(triethylenetetramine) iron, (tetraphenylporphyrin)iron(II), and Fe(II) alkyltriarylborate salts . Ferrocene/alkyl chloride charge transfer complexes have been often mentioned and iron complexes such as tris(bypyridine)iron were revisited . Ferrocenium salts (e.g., (η 5 ‐cyclopentadienyl η 6 ‐arene) hexafluorophosphate) known as cationic PIs can also work as radical PIs in the presence of oxygen or hydroperoxide .…”

Recent advances on iron-based photoinitiators of polymerization

“…However, it remains an interesting result considering that ferrocene derivatives are mostly examined as initiators for the cationic polymerization of epoxides. [168][169] Figure 34. Chemical structure of chalcones 1-6 and the different additives.…”

“…Photogeneration of reactive initiating species with iron complexes is not a recent topic, [169][170][171] but the development of photoredox catalysts based on iron complexes was only reported in 2015. [29][30][31][32][33][34]172] Compared to iridium or copper complexes that exhibit excited state lifetimes in the microsecond scale, iron complexes possess shorter excited state lifetimes, in the nanosecond scale.…”