Stereocomplexes of A-B-A triblock copolymers based on poly(L-lactide) and poly(D-lactide) A blocks

A series of A-B-A triblock copolymers was prepared by a Sn(II) 2-ethylhexanoate catalyzed chain extension of various telechelic soft segments, with L,L-lactide (LLA). Poly(epsilon-caprolactone)s, P epsilon CL, poly(ethylene glycol)s, PEG, and a poly(dimethyl siloxane), PDMS, served as soft segments. The lengths of both soft and hard segments (poly-LLA, PLLA, or poly-D,D-lactide, PDLA) were varied, For the lactide blocks, average lengths of 25, 50, and 100 lactic acid units were selected. An analogous series of A-B-A triblock copolymers was prepared with DLA. Compositions and block lengths of these triblock copolymers were characterized by H-1 NMR spectroscopy. Furthermore, stereocomplexes (racemates) of PLLA- and PDLA-based triblock copolymers were prepared to find out if the attractive forces between PLLA and PDLA blocks or the repulsive forces between incompatible soft segments dominate the morphology. Both neat triblock copolymers and stereocomplexes were characterized by differential scanning calorimtery measurements by wide-angle X-ray diffraction. The influence of block lengths on glass transition (T-g) and melting temperatures (T-m) of soft and hard segments was studied in detail, Stereocomplexes of all triblock combinations were obtained, even when the soft segments were incompatible.