Asymmetric repetitive Mukaiyama aldol reaction between bis(trimethylsilylketene thioacetal) and dialdehydes proceeded in the presence of chiral oxazaborolidinone to afford optically active poly(beta -hydroxy thioester)s. Degree of asymmetric induction during polymerization was evaluated both by a model reaction and chiral HPLC analysis of degradation products of the chiral polymers.