Monitoring the Nitroxide-Mediated Polymerization of Styrene Using Gel Permeation Chromatography and Proton NMR

This laboratory exercise is designed to give students experience in controlled radical polymerization (CRP) techniques, with an emphasis on monitoring the conversion of monomer into polymer as the reaction proceeds. Specifically, nitroxide-mediated polymerization (NMP) is used to synthesize polystyrene, with reaction-mixture aliquots removed and analyzed via gel permeation chromatography (GPC) and ¹H NMR at varying time intervals. Students obtain and analyze data from ¹H NMR during the laboratory period, which allows them to calculate the percent monomer conversion at each time interval. Number-average molecular weight (M_n) and polydispersity index (PDI) values are determined by GPC analysis, which is performed by the instructor or teaching assistant and the data given to the students. From the experimental data, Mn values show a linear relationship to percent monomer conversion, consistent with controlled polymerization. Additionally, kinetic plots, which show ln([M]₀/[M]_t) as a function of reaction time where [M]_t is the monomer concentration at time t and [M]₀ is the initial monomer concentration, are also linear after a brief induction period. This experiment is designed for advanced undergraduate students with little or no experience in polymer synthesis.