Fourier-transform near-infrared (FT-NIR) fiber-optic spectroscopy was successfully used to monitor the anionic polymerization of ethylene oxide (EO). Kinetic data are provided for the polymerization of EO with the sec-BuLi/ButP4 initiating system under different reaction conditions. In addition, the influence of different initiators and reaction conditions on the polymerization of EO is investigated. Online monitoring using NIR spectroscopy reveals an unexpected induction period present in EO homopolymerizations as well as in the synthesis of PEO containing block copolymers with [Li/ButP4]+ counterions. The resulting polymers are characterized by size exclusion chromatography (SEC). A low-molecular-weight polystyrene-block-poly(ethylene oxide) (PS-b-PEO) diblock copolymer was synthesized to gain more insight into the observed induction period by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-ToF MS) on samples taken during EO polymerization. The induction period is believed to be a result of different factors involved in the formation of active centers, for example, the break up of lithium alkoxide aggregates by the phosphazene base ButP4, and chain length effects. It depends on reaction temperature, concentration of the phosphazene base ButP4, as well as the structure of the initiator.
SEC traces for EO homopolymerizations using different initiating systems at 50 °C.