Spotlight on\nMethyl <i>tert</i>-Butyl EtherUnderrated\nor Overlooked? Unveiling Its Role for Living Anionic Polymerization

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