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The side-chain-substituted poly(lactic acid)s (substituted PLAs) have attracted much attention as novel bio-based polymers, but their possibility as crystalline polymeric materials still remains unknown. In this study, the crystallization behavior and solid-state structure are comprehensively investigated for poly(l-2-hydroxy-3-methylbutanoic acid) [P(L-2H3MB)] with M_n = 6.2 × 10⁴, i.e., stereoregular substituted PLA having isopropyl side chains. The equilibrium melting temperature of P(L-2H3MB) is 240 °C, which is significantly higher than that of poly(l-lactic acid) (PLLA) (≈200 °C). In addition, the isothermal and nonisothermal crystallization behaviors show that the crystallization of P(L-2H3MB) is extremely faster than that of PLLA. X-ray diffraction results suggest that P(L-2H3MB) has two or more crystal modifications, but only one modification appears with a high crystallinity (≈60%) when melt-crystallized at 155–180 …