Exclusive Stereocomplex Crystallizationof Linearand Multiarm Star-Shaped High-Molecular-Weight Stereo Diblock Poly(lacticacid)s.

Linear, 3-arm, and 6-arm star-shapedstereo diblock copolymersof l- and d-lactic acid (PLLA-b-PDLA) with high molecular weights (MWs) were synthesized via two-stepring-opening polymerization (ROP) with 1-dodechanol, glycerol, and d-sorbitol as the initiators, respectively. The chemical structure,nonisothermal and isothermal crystallization kinetics, crystallinestructure, lamellar morphology, and mechanical thermal propertiesof PLLA-b-PDLAs with different macromolecular topologieswere investigated. Compared to the high-molecular-weight (MW) poly(l-lactic acid)/poly(d-lactic acid) (PLLA/PDLA) racemicblends, PLLA-b-PDLAs exhibit faster crystallizationupon cooling and isothermal melt crystallization; they crystallizeexclusively in stereocomplex (sc) crystallites under all of the conditionsinvestigated. This is attributable to the enhanced interactions betweenenantiomeric blocks linked covalently. Macromolecular topology influencesthe crystallization kinetics and crystalline structure of PLLA-b-PDLAs significantly. The crystallization temperature uponcooling, melting temperature, degree of crystallinity, spheruliticgrowth rate, crystallite size, long period, and crystalline layerthickness of PLLA-b-PDLA decrease with increasingbranching number because of the retarding effect of branching on thecrystallization rate and crystallizability. Because of the formationof high-melting-point sc crystallites, both the linear and star-shapedPLLA-b-PDLAs exhibit better thermal resistance andhigher storage moduli at high temperature than does homocrystallinePLLA. [ABSTRACT FROM AUTHOR]