Synthesis and characterization of dendritic star‐shaped poly(ϵ‐caprolactone)‐block‐poly( L ‐lactide) block copolymers

Dendritic star‐shaped poly(ε‐caprolactone)‐block‐poly(L‐lactide) (PCL‐b‐PLLA) diblock copolymers were synthesized 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 ε‐caprolactone, which was catalyzed by stannous(II) octanoate, to obtain dendritic star‐shaped poly(ε‐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., ε‐caprolactone and L‐lactide). The crystalline structure and thermal properties of the dendritic star‐shaped polymers were investigated with X‐ray 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.