Silacyclobutane as “Carbanion Pump” in Anionic Polymerization. III. Synthesis of Di- and Tri-block Copolymer by “Diphenylsilacyclobutane–Potassium tert-Butoxide System”

Abstract

Efficiency of formation of carbanion from a combination of an alkaline metal tert-butoxide and a silacycle (carbanion pump efficiency) was studied in the presence of diphenylethylene. Combination of a silacyclobutane with potassium counter ion was the most effective among the silacycles studied (1:1 molar ratio). Sodium and lithium counter ions gave low efficiency. When 2-times 1,1-diphenylethylene was used, diphenylsilacyclobutane with potassium counter ion was found superior to dimethylsilacyclobutane–potassium, and produced (tert-butoxy)diphenyl(5,5-diphenylhexyl)silane as the major trapped product of initially formed carbanion. However, (tert-butoxy)diphenylpropylsilane and diphenylpropylsilanol were also formed as non-trapped products in 1 to 27% depending on the reaction conditions, especially when tetrahydrofuran–hexane (9:1) was used. Hydrogen elimination from initially formed intermediate was suggested. With 4-times diphenylethylene, or reaction in tetrahydrofuran–hexane (1:1), (tert-butoxy)(5,5-diphenylhexyl)silane was produced in 95% yield after treatment with methyl iodide, accompanied by only 1–2% (tert-butoxy)diphenylpropylsilane as non-trapped product, and tert-butoxy anion was converted to carbanion in as high as 97% yield. This carbanion pump system was applied in the syntheses of poly(ethylene oxide)-block-polystyrene, poly(ethylene oxide)-block-polystyrene-block-poly(ethylene oxide), poly(ethylene oxide)-block-poly(methyl methacrylate), and poly(ethylene oxide)-block-poly(methyl methacrylate)-block-poly(ethylene oxide).