Stereocomplexes of Discrete, Isotactic Lactic Acid Oligomers Conjugated with Oligodimethylsiloxanes

Abstract

Discrete length block co-oligomers (BCOs) comprised of a crystalline and an amorphous block are a new class of materials that gives highly ordered lamellar morphologies at small length scales. Here, we show the preparation of discrete, isotactic oligo l- and d-lactic acid (olLA and odLA) homoblocks followed by ligation to oligodimethylsiloxane (oDMS), affording a library of crystalline–amorphous BCOs that vary in molecular weight and composition. Mixing the two enantiomeric BCOs or homoblocks results in the formation of the corresponding stereocomplex. The properties and phase behavior of the isotactic (block co)oligomers and the stereocomplexes thereof are studied using differential scanning calorimetry and small-angle X-ray scattering. A systematic study of the isotactic homoblock lengths and crystal structure confirmed the formation of a 103 helix with a monomeric rise of 0.3 nm, whereas the stereocomplex adopts a 31 helix. The same type of crystal structure was found for the isotactic and stereocomplex of BCOs giving rise to the formation of lamellar morphologies at room temperature as a result of crystallization of the oLA blocks. Distorted lamellar structures were found in BCOs that preorganize into nonlamellar morphologies prior to crystallization. The stereocomplex BCOs shows more crystal defects and a loss of long-range ordering in the microstructure due to the larger driving force for crystallization. Hence, the balance between chain length, block volume, and the crystallization strength are of major importance for the formation of the final structure with the least defects.

Precise modulation of molecular weight distribution for structural engineering

“…1e and Table S1 †), indicating that all these crystals adopt an extended chain conformation (Scheme 1b). 36 Extrapolating the linear t to x ¼ 0 gives the thickness of the amorphous layer (d a ¼ 0.91 nm, mainly the protecting group, assuming 100% crystallinity), and the thickness of the crystalline layer (d c ) can thus be calculated accordingly…”

“…The slope of the linear t, on the other hand, indicates an increase of 0.307 AE 0.003 nm per monomeric residue, which is in good agreement with the crystal unit cell parameters. 36 For polymer lamellar crystals, the relationship between T m and d c can be described by the Thomson-Gibbs equation (eqn (1))…”

“…Discrete polymers (Đ ¼ 1) having a denite composition, sequence, and topology exhibit unique phase behaviors compared to their dispersed analogues, 31,32 revealing high sensitivity of microstructures to the chain length heterogeneity. [33][34][35][36][37][38][39] In this study, we will demonstrate a method for precisely tailoring the molecular weight distribution. By rationally blending discrete oligo-L-lactic acid (oLLA) according to certain distribution functions (such as the Schulz-Zimm distribution, Gaussian distribution, skew-normal distribution, etc.…”

Relationship between crystallization state and degradation behavior of poly(l‐lactide)/four‐armed poly(d,l‐lactide)‐block‐poly(d‐lactide) blends with different poly(d‐lactide) block lengths

“…15 Most recently, Meijer reported that four lactic acid repeat units were needed to obtain SC crystallites. 16 It seems that the minimum block length of PDLA to form SC crystallites may change because of the chemical structure of PDLA. As a result, it is also necessary to identify if the minimum block length of PDLA to form SC crystallites would change in copolymers with outer PDLA blocks to from SC crystallites with a PLLA matrix.…”

“…Hennink reported that a minimum degree of polymerization (DP) of lactic acid oligomer was required to form SC crystallites, and crystallization occurred at DP ≥ 7 in a blend of d ‐ and l ‐lactic acid oligomers 15 . Most recently, Meijer reported that four lactic acid repeat units were needed to obtain SC crystallites 16 . It seems that the minimum block length of PDLA to form SC crystallites may change because of the chemical structure of PDLA.…”