Synthesis and characterization of dendritic star-shaped Poly(ε-caprolactone)-block-Poly(L-lactide) block copolymers

Dendritic star-shaped poly(E-caprolactone)block-poly(L-lactide) (PCL-b-PLLA) diblock copolymers were svnthesized via sequential ring-opening polymerization. In the first step, an aliphatic dendritic polyester containing 16 terminal hydroxyl groups was used as the core molecule to initiate the ring-opening polymerization of epsilon-caprolactone, which was catalyzed by stannous(II) octanoate, to obtain dendritic star-shaped poly (epsilon-caprolactone) (PCL) terminated with hydroxyls, which was used further to initiate the ring-opening polymerization Of L-lactide to form the dendritic star-shaped diblock copolymers. The dendritic star-shaped polymers (PCL-b-PLLA) were characterized with nuclear magnetic resonance spectroscopy and gel permeation chromatography. The results showed that the arm length of the dendritic star-shaped polymers could be well controlled in terms of the molar ratios of the initiators (i.e., the aliphatic dendritic polyester and star-shaped PCL) to the monomers (i.e., epsilon-caprolactone and L-lactide). The crystalline structure and thermal properties of the dendritic star-shaped polymers were investigated with Xray diffraction and differential scanning calorimetry. The X-ray diffraction indicated that the formation of the dendritic star-shaped topological structure did not affect the structure of the crystals of PCL and poly(L-lactide) (PLLA) blocks. The thermal analyses showed that the crystallization rate of the PCL blocks in the block copolymers was greatly reduced compared to that in the parent dendritic star-shaped PCL. This observation could be attributed to the confinement of the dendritic core and PLLA blocks upon the crystallization of the PCL blocks. (C) 2007 Wiley Periodicals, Inc.