An amphiphilic block copolymer of silacyclobutane and methacrylic acid (MAA) was synthesized via a living anionic polymerization of 1,1-diethylsilacylcobutane (EtSB). Sequential addition of 1,1-diphenylethylene and t-butyl methacrylate (tBMA) to living poly(EtSB) in the presence of lithium chloride gave poly(EtSB-block-tBMA) with narrow molecular weight distributions. The t-butyl ester groups in the obtained polymer were readily hydrolyzed via heating in 1,4-dioxane in the presence of concentrated aqueous hydrochloric acid. The block copolymer with a short MAA segment was soluble in chloroform and insoluble in methanol and basic water, whereas the block copolymer with a long MAA segment was soluble in methanol and basic water and insoluble in chloroform. The block polymer (EtSB/tBMA = 45/60) formed a monolayer film on the water surface; this was confirmed by surface pressure measurement. (C) 2000 John Wiley & Sons, Inc.