Theoretical study of the photochemical initiation in nitroxide-mediated photopolymerization

Nitroxide-mediated photopolymerization (NMP(2)) is a promising novel route to initiate radical polymerization. In NMP(2), alkoxyamines bounded to a monomer are attached to a chromophore. Upon light absorption, the excitation energy is transferred from the chromophore to the alkoxyamine moiety, inducing the cleavage of the oxygen-carbon bond and thus initiating the polymerization. The NMP(2) mechanism depends strongly on several factors like the type of chromophore, the monomer, the connectivity pattern, etc. This complexity makes it difficult to design new NMP(2) initiators with increased polymerization efficiency and selectivity. In the present article, we characterize by means of quantum mechanical calculations the main steps of the NMP(2) initiation for alkoxyamines attached to aromatic ketones. We show how the excitation energy can be transferred from the chromophore to the alkoxyamine moiety, and present two easily computed parameters which can account for the selectivity of the O-C bond photocleaveage. Finally, using results obtained for a series of isomers, we give some rules that may help the design of more efficient NMP(2) initiators.