Introducing dimethyl ether as a solvent for anionic polymerization : comprehensive kinetics of dienes and styrene

Abstract

This work aims to shed light on the use of the highly available, yet unexplored dimethyl ether (DME) as a solvent for anionic polymerizations. DME is virtually nontoxic and does not lead to the formation of peroxides. The development of special techniques and setups, for the first time, enabled the use of liquid DME, exhibiting a vapor pressure of 6 bar at 25 °C, as a (co)solvent for the anionic polymerization of isoprene (I), β-myrcene (Myr), β-farnesene (Far), and styrene (S) using sec-butyllithium as an initiator. Propagation constants, kinetic orders, and activation energies of I, Myr, and Far in DME were determined at 0, 10, and 23 °C using in situ 1H NMR kinetics. The use of DME leads to faster monomer consumption compared to the less polar methyl tert-butyl ether (MTBE). Additionally, the effect of increasing amounts of DME on the reactivity ratios of S/I copolymerizations up to [DME]/[Li] = 450 in cyclohexane (CHx) was investigated using in situ near-infrared (NIR) spectroscopy. The necessary amount of polar ether modifier to reach random (i.e., rI ≈ rS ≈ 1) S/I incorporation inversely correlates with the polarity of the solvent, i.e., MTBE < DME < THF. Using in situ 1H NMR kinetics, the same trend could be established for the stability toward n-butyllithium in the respective ethers. NIR kinetics revealed that the polymerization rate of styrene in CHx at 20 °C is accelerated ≈18 times at [DME]/[Li] = 30. Lastly, the effect of Li, Na, K, Rb, and Cs naphthalenides on the microstructure of polyisoprene (PI) synthesized in DME at room temperature was determined and found to be comparable to PI synthesized in 1,4-dioxane.