Spotlight on Methyl tert-Butyl Ether─Underrated or Overlooked? Unveiling Its Role for Living Anionic Polymerization

The moderately polar solvent methyl tert-butyl ether (MTBE) was investigated as the reaction medium for the carbanionic polymerization of the 1,3-dienes isoprene and β-farnesene. Key characteristics of MTBE, e.g., 50 times longer half-life of n-butyllithium in MTBE compared to that in tetrahydrofuran, and polymerization rate constants were determined. In situ 1H NMR kinetics showed a polymerization rate in MTBE in between the commonly utilized solvents cyclohexane and tetrahydrofuran for diene polymerization. Remarkably, the living polydienyllithium chains in MTBE are predominantly present as nonaggregated unimers. The copolymerization of styrene and isoprene in cyclohexane with an increasing MTBE fraction was investigated via in situ near-infrared kinetics. The gradient structure successively changes to random monomer incorporation and ultimately yields an inverted yet still almost random incorporation in pure MTBE, reflected by the reactivity ratios rSMTBE = 1.82 and rIMTBE = 0.55. A spotlight is placed on the great potential of this highly available, albeit rarely used solvent for anionic polymerization.