Kinetics of Anionic Living Copolymerization of Isoprene and Styrene Using in Situ NIR Spectroscopy: Temperature Effects on Monomer Sequence and Morphology

Abstract

The living anionic copolymerization of isoprene (I) and styrene (S) can afford a variety of different polymer microstructures that strongly depend on experimental parameters such as solvent, counterion, and temperature. In this work, in situ near-infrared (NIR) spectroscopy was employed as a versatile and fast method to track the conversion of the individual monomers in a nonpolar (cyclohexane, CyH) and a coordinative solvent mixture (CyH with 1.5%vol tetrahydrofuran (THF)). For the first time, in situ monitoring of the copolymerization is performed by deriving the individual monomer signals from the superimposed spectra using the molar attenuation coefficients of each component in the wavenumber range of 5900–6250 cm–1. The polymerization in nonpolar solvents features a unique kinetic behavior and is known to result in tapered block copolymers. Kinetic rate constants and the corresponding activation energies were determined in CyH, covering a broad temperature range of 10–60 °C. Paralleling the experimental studies, the polymerization was also simulated in silico by feeding the measured kinetic rate constants into a kinetic Monte Carlo simulation (kMC). This combination of in situ monitoring and kMC simulation allows to reduce reaction times, which is especially desired in multiblock syntheses. The observed differences in activation energies aid in understanding the temperature dependence of the reactivity ratios. Thus, temperature can be used as an external parameter to adjust the gradient and the size of the polystyrene block. The peculiar temperature dependence of the gradient affects the resulting bulk morphology. This led to a surprising partial change from the lamellar to the tetragonally cylindrical or perforated lamellar structure at the identical isoprene/styrene composition, only caused by a change of the polymerization temperature. In situ NIR probing is established as a fast and accurate method for real-time copolymerization monitoring that enables tracking complex copolymerization procedures, such as multiblock formation with a temporal resolution exceeding current standards set by 1H NMR kinetics.