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−Llactic 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 αand 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 αform (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 βform 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.

Discrete Diblock Copolymers with Precise Stereoconfiguration

“…Starting from enantiopure L-lactide and D-lactide, chiral monomers consisting of one or two lactic acid residual(s) were prepared through hydrolysis and ring-opening reactions, respectively (Scheme a) Figure S1).…”

“…Starting from enantiopure L-lactide and D-lactide, chiral monomers consisting of one or two lactic acid residual(s) were prepared through hydrolysis and ring-opening reactions, respectively (Scheme 2a). 35 tert-Butyldimethylsilyl (TBDMS) and benzyl (Bn) groups were then installed to orthogonally mask the hydroxyl and carboxylic acid groups (Figure S1). 36 With these building blocks, stereoregular oLA with a discrete number of repeat units can be modularly prepared by consecutively connecting the chiral monomers through the divergent/convergent strategy reported in our earlier studies.…”

“…On the other hand, the 13 C NMR spectra also exhibit distinct patterns due to different chemical environments of the stereoisomers (Figure S5). 35 The peaks ascribed to the carbonyl, methine, and methyl carbons appear at 169, 69, and 16 ppm, respectively. For isotactic or syndiotactic oLA (i.e., L 24 and (LD) 12 ), resonances with singlet peaks were recorded for each carbon (though the chemical shifts are slightly different in these two cases).…”

Synergistic effect of star‐shaped architecture and poly(ethylene glycol) moieties on poly(lactic acid) stereocomplex crystallization behaviors

“…reported that the blend of PDLA and comb‐like nanocrystalline cellulose grafted PLLA exhibited a high stereocomplexation rate and thermal stability. Star‐shaped stereo‐block copolymers were observed to exhibit increased nucleation density during crystallization, which can accelerate the rate of crystallization and enhance stereocomplexation efficiency 45–48 . Compared to linear PLLA/PDLA blends, star‐shaped PLLA/linear PDLA blends have faster crystallization kinetics and a higher degree of crystallinity, which benefit from the flexibility of linear chains and the high nucleation density provided by star‐shaped chains 49 …”

“…Star-shaped stereo-block copolymers were observed to exhibit increased nucleation density during crystallization, which can accelerate the rate of crystallization and enhance stereocomplexation efficiency. [45][46][47][48] Compared to linear PLLA/PDLA blends, star-shaped PLLA/linear PDLA blends have faster crystallization kinetics and a higher degree of crystallinity, which benefit from the flexibility of linear chains and the high nucleation density provided by star-shaped chains. 49 In addition to star-shaped architecture, the incorporation of suitable modifiers on polymer backbones is another effective method to promote scPLA crystallization.…”