AbstractOptically active poly(L‐phenyllactic acid) (Ph‐PLLA), poly(L‐lactic acid) (PLLA), and poly(L‐phenyllactic acid‐co‐L‐lactic acid) with weight‐average molecular weight exceeding 6 × 103 g mol−1 were successfully synthesized by acid catalyzed direct polycondensation of L‐phenyllactic acid and/or L‐lactic acid in the presence of 2.5–10 wt % of p‐toluenesulfonic acid. Their physical properties and crystallization behavior were investigated by differential scanning calorimetry, thermogravimetry, and polarimetry. The absolute value of specific optical rotation ([α]) for Ph‐PLLA (−38 deg dm−1 g−1 cm3) was much lower than that of [α] for PLLA (−150 deg dm−1 g−1 cm3), suggesting that the helical nature was reduced by incorporation of bulky phenyl group. PLLA was crystallizable during solvent evaporation, heating from room temperature, and cooling from the melt. Incorporation of a very low content of bulky phenyllactyl units even at 4 mol % suppressed the crystallization of L‐lactyl unit sequences during heating and cooling, though the copolymers were crystallizable for L‐phenylactyl units up to 6 mol % during solvent evaporation. The activation energy of thermal degradation (ΔEtd) for Ph‐PLLA (200 kJ mol−1) was higher than that for PLLA (158 kJ mol−1). The ΔEtd for the copolymers increased with an increase in L‐phenyllactyl unit content. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008