Abstract
Non-substituted racemic poly(DL-lactic acid) (PLA) and substituted racemic poly(DL-lactic acid)s or poly(DL-2-hydroxyalkanoic acid)s with different side-chain lengths, i.e., poly(DL-2-hydroxybutanoic acid) (PBA), poly(DL-2-hydroxyhexanoic acid) (PHA), and poly(DL-2-hydroxydecanoic acid) (PDA) were synthesized by acid-catalyzed polycondensation of DL-lactic acid (LA), DL-2-hydroxybutanoic acid (BA), DL-2-hydroxyhexanoic acid (HA), and DL-2-hydroxydecanoic acid (DA), respectively. The hydrolytic degradation behavior was investigated in phosphate-buffered solution at 80 and 37 °C by gravimetry and gel permeation chromatography. It was found that the reactivity of monomers during polycondensation as monitored by the degree of polymerization (DP) decreased in the following order: LA > DA > BA > HA. The hydrolytic degradation rate traced by DP and weight loss at 80 °C decreased in the following order: PLA > PDA > PHA > PBA and that monitored by DP at 37 °C decreased in the following order: PLA > PDA > PBA > PHA. LA and PLA had the highest reactivity during polymerization and hydrolytic degradation rate, respectively, and were followed by DA and PDA. BA, HA, PBA, and PHA had the lowest reactivity during polymerization and hydrolytic degradation rate. The findings of the present study strongly suggest that inter-chain interactions play a major role in the reactivity of non-substituted and substituted LA monomers and degradation rate of the non-substituted and substituted PLA, along with steric hindrance of the side chains as can be expected.
Highlights
Poly(hydroxyalkanoate)s [i.e., poly(hydroxyalkanoic acid)s or poly(DL-2-hydroxyhexanoic acid) (PHA)] such as polylactide [i.e., poly(lactic acid)], poly(glycolide) [i.e., poly(glycolic acid)], poly(3-hydroxybutyrate)[i.e., poly(3-hydroxybutanoic acid)], poly(ε-caprolactone) [i.e., poly(6-hydroxyhexanoic acid)], and their copolymers have been used as scaffolds and pharmaceutical materials whose hydrolytic degradation behavior in vivo should be accurately manipulated [1,2,3,4,5,6,7]
Tsuji et al synthesized optically active phenyl-substituted poly(DL-lactic acid) (PLA) (Figure 1) and its copolymers with lactic acid and investigated their crystallization behavior and thermal properties [51]. They synthesized optically active and crystallizable poly(L-2-hydroxybutyrate) [P(L-2HB)] and poly(D-2-hydroxybutyrate) [P(D-2HB)] and found that stereocomplexation occurs in the blends of P(L-2HB) and P(D-2HB), which enhances the crystallization and hydrolytic/thermal degradation resistance compared to intact P(L-2HB) or
In the first polymerization step, each monomer was polymerized by polycondensation to yield its oligomers at 130 °C at atmospheric pressure for 5 h to have a sufficiently high molecular weight not to be removed under reduced pressure in the second polymerization step
Summary
Poly(hydroxyalkanoate)s [i.e., poly(hydroxyalkanoic acid)s or PHAs] such as polylactide [i.e., poly(lactic acid)], poly(glycolide) [i.e., poly(glycolic acid)], poly(3-hydroxybutyrate)[i.e., poly(3-hydroxybutanoic acid)], poly(ε-caprolactone) [i.e., poly(6-hydroxyhexanoic acid)], and their copolymers have been used as scaffolds and pharmaceutical materials whose hydrolytic degradation behavior in vivo should be accurately manipulated [1,2,3,4,5,6,7]. Tsuji et al synthesized optically active phenyl-substituted PLA (Figure 1) and its copolymers with lactic acid and investigated their crystallization behavior and thermal properties [51]. They synthesized optically active and crystallizable poly(L-2-hydroxybutyrate) [P(L-2HB)] and poly(D-2-hydroxybutyrate) [P(D-2HB)] and found that stereocomplexation occurs in the blends of P(L-2HB) and P(D-2HB), which enhances the crystallization and hydrolytic/thermal degradation resistance compared to intact P(L-2HB) or
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