The contents of poly(L-lactic acid) (PLLA) prepared by direct condensation polymerization without using a catalyst were studied. 1H NMR and mass spectrometry analyses suggested that PLLA contained cyclic oligo(L-lactic acid) (c-OLLA) with 3–20 repeat units. Notably, only c-OLLA was extracted and isolated using hexane or cyclohexane at 4°C; thus the hydrophobicity, topology, and temperature dependence of the solubility of the obtained PLLA enabled the selective extraction of c-OLLA. The effect of cyclic compounds on direct polycondensation and the potential for c-OLLA to form molecular inclusion complexes were also discussed. 1. Introduction Poly(L-lactic acid) (PLLA) has attracted attention for its carbon neutrality and biodegradability. Various grades of PLLA, including some with molecular weights greater than one hundred thousand, are now industrially produced. Generally, these polymers are regarded as linear molecules and are synthesized by the ring-opening polymerization of L,L-lactide [1] or the direct condensation polymerization of L-lactic acid with catalyst [2]. On the other hand, methods for the synthesis of cyclic poly(L-lactic acid) (c-PLLA) have been reported recently. For example, c-PLLA with molecular weights of approximately 4,000–39,000 (the number of repeat units; 60–540) was synthesized using an alumatrane-inspired catalyst [3]. Shin et al. also prepared c-PLLA by the zwitterionic polymerization of lactide using the N-heterocyclic carbene 1,3-dimesitylimidazol-2-ylidene (IMes) as the catalyst and investigated the crystallinity of the polymer [4]. Cyclic esters, also known as lactones, are found in living systems. For example, 3-methyl-4-octanolide is present in oak trees [5], while exaltolide with a 16-membered ring is known as a musk perfume compound [6]. In the field of medicine, cyclic oligoesters with 14–16 repeat units are generally known as macrolides, which exhibit antibacterial activity [7–9]. In addition, cyclic oligo(L-lactic acid) (c-OLLA), which is composed of lactic acid, is recognized as an antitumor material, and studies to elucidate the antitumor activity of c-OLLA have been conducted [10, 11]. OLLA that includes both linear and cyclic compounds suppresses the growth of cancer cells in vivo by directly affecting the glycolytic system. However, it is not clear whether it is the linear or cyclic compounds that affect the cancer cells, because there is no description for the preparation of OLLA which does not include linear OLLA (l-OLLA). Therefore, in order to elucidate the function of the cyclic compounds, the isolation
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