Photodimerization and Polymerization of PEG Derivatives through Radical Coupling using Photochemistry of Dithiocarbamate

This article reports the development of a novel polymerization based on the photochemistry of benzyl N , N -diethydithiocarbamate (BDC), which dissociates to a pair of radicals, benzyl radicals and dithiocarbamyl radicals, by ultraviolet light (UV) irradiation. Upon UV irradiation to a N , N -dimethylformamide (DMF) solution with extremely thin thickness containing mono-BDC-derivatized poly(ethylene glycol) (PEG) dimerization of the PEG derivative occurred due to bibenzyl bond formation by radical coupling between benzyl radicals generated in the terminal of the PEG derivative. The reaction rate was about 90% by several tenth minutes of irradiation. On the other hand, UV irradiation of a DMF solution of bis-BDC-derivatized PEG produced high molecular weight PEG derivatives by inter-molecularly coupling between diradicals, which have benzyl radicals at both terminals of the PEG derivative. The products had alternative structure of PEG chain and bibenzyl bond. The chain length of the produced polymer grew with irradiation time. The reaction rate rapidly increased at higher light intensities, and further increased at shorter optical distance of the solution to reach about 90% by several tenth minutes of irradiation. Possibility and limitation of this polymerization method was discussed.