Novel acyclic nitroxides for nitroxide-mediated polymerization: Kinetic, electron paramagnetic resonance spectroscopic, X-ray diffraction, and molecular modeling investigations

This work describes the synthesis of new hydrophobic acyclic nitroxides [derived from 2,2,5-trimethyl-4-phenyl-3-azahexane-3-nitroxide (TIPNO)] designed to be employed in radical miniemulsion polymerization. We present the synthetic strategies employed to obtain these different nitroxides and the determination of certain important parameters, such as the dissociation rate constant, the combination rate constant, and the decomposition rate constant, of the corresponding alkoxyamines. All these new nitroxides give good control of the bulk radical polymerization of styrene in comparison with the parent TIPNO. The molecular structure of some nitroxides presented herein has been elucidated by single-crystal X-ray diffraction, and their structural determination helps us to understand better the influence of the nitroxide structure on the activation energy. A molecular modeling study has also been conducted on these new nitroxides, and a good linear correlation between the activation energy and the CNC bond angle for a series of nitroxides with the same type of leaving radical has been found.