The anionic ring-opening polymerization of L-lactide was initiated by dipotassium-polyisobutylene-alcoholate telechelic polymer to yield poly(L-lactide)-block-polyisobutylene-block-poly(L-lactide) triblock copolymer, a partially biodegradable thermoplastic elastomer. The pure triblock copolymer was obtained by gradient column chromatography on silica gel. The molar mass and molar mass distribution of the block copolymer was ascertained by SEC and quantitative H-1 NMR spectroscopy. It showed two glass transitions and microphase separation.