Anionic polymerization of methyl methacrylate (MMA) was initiated with the disodium salt of dihydroxy-poly(tetra-methylene oxide) (PTMO) in tetrahydrofuran in the presence and the absence of a crown ether. The resulting block copolymer of poly(methyl methacrylate) (PMMA) and PTMO was characterized by1H NMR spectroscopy, gel permeation chromatography, and solvent extractions. In the absence of the crown ether, a linear triblock copolymer of PMMA-PTMO-PMMA was predominantly obtained, showing a unimodal and relatively narrow molecular weight distribution. In the presence of the crown ether, a linear triblock copolymer and a PTMO grafted block copolymer due to the transesterification reaction between PTMO and a methoxy group in MMA were produced with an increase in polymer yield. The stereosequence distributions of the PMMA part in the copolymers were essentially atactic and the addition of the crown ether increased the fraction of the rr triad.