Synthesis and Stereocomplexation of New Enantiomeric Stereo Periodical Copolymers Poly(l-lactic acid–l-lactic acid–d-lactic acid) and Poly(d-lactic acid–d-lactic acid–l-lactic acid)

Abstract

New enantiomeric stereo periodical copolymers (SPCPs) of lactic acids, poly(l-lactic acid–l-lactic acid–d-lactic acid) [P(LLA-LLA-DLA)] and poly(d-lactic acid–d-lactic acid–l-lactic acid) [P(DLA-DLA-LLA)] as typical and most simple examples of enantiomeric SPCPs, which cannot be synthesized by catalytic stereoselective polymerization of lactide or lactic acid, were synthesized by preparing the stereosequence-controlled trimers and their polycondensation. A new type of stereocomplex (SC) formation between the synthesized enantiomeric SPCPs was reported for the first time. Unblended P(LLA-LLA-DLA) and P(DLA-DLA-LLA) had two types of crystalline forms, tentatively named α- and β-forms, for solvent evaporation (or purification) and melt-crystallization, respectively. Interestingly, the melting temperature values of SC crystallites of P(LLA-LLA-DLA)/P(DLA-DLA-LLA) blend (143.4–144.0 °C) were between those of α-form (157.6–158.0 °C) and β-form (119.0–119.5 °C) homocrystallites of unblended P(LLA-LLA-DLA) and P(DLA-DLA-LLA), in marked contrast with the results reported for enantiomeric poly(l-lactic acid) and poly(d-lactic acid) homopolymers. The radial growth rate of spherulites (G) of the P(LLA-LLA-DLA)/P(DLA-DLA-LLA) blend with SC crystallites (6.6 × 10–2 μm min–1) at crystallization temperature = 100 °C was much higher than those of unblended P(LLA-LLA-DLA) and P(DLA-DLA-LLA) with β-form homocrystallite spherulites (1.8 × 10-2 and 8.6 × 10-3 μm min-1, respectively). The G values of unblended and blend specimens were 1–4 orders of magnitude lower than those of isotactic and syndiotactic poly(lactic acid)s. The present study strongly suggests the probability of new types of SC formation between enantiomeric SPCPs with a wide variety of stereosequences and is expected to pave the way to widen the physical properties and biodegradation behavior and rate of PLA-based materials by synthesis and stereocomplexation of various types of SPCPs.