Synthesis and characterization of well-defined star PLLA with a POSS core and their microspheres for controlled release

In this paper, a series of hybrid star PLLA (sPLLA) with different arm lengths was synthesized via the hydrosilylation between octakis(dimethylsiloxy) silsesquioxane (Q₈M₈) and functionalized PLLA macromolecules with vinyl end groups (mPLLA). mPLLA was synthesized by ring-opening polymerization of l-lactide using 2-hydroxyethylmethacryl as an initiator in the presence of stannous 2-ethylhexanoate as a catalyst. The obtained sPLLA has low polydispersity with polydispersity index values from 1.29 to 1.30. The arm numbers of sPLLA vary from 5 to 7 and decrease with the increase in the length of the mPLLA arm due to the steric hindrance, which are estimated by ¹H NMR analysis. The branched structure of sPLLA is also evidenced by the lower intrinsic viscosity when compared with the linear mPLLA with similar molecular weight. Both the glass transition temperatures (T _g’s) and melting temperatures (T _m’s) of sPLLAs are higher than those of the mPLLA arms. The incorporation of polyhedral oligomeric silsesquioxane (POSS) does not change the crystalline structure of PLLA, while the crystallinity of sPLLA is enhanced as the result that the POSS core acts as a heterogeneous nucleating agent in the matrix to promote the crystallization ability of PLLA. High-resolution transmission electron microscopy observation suggests that POSS disperses in the crystalline PLLA matrix as 5–20 nm aggregates. Microspheres of sPLLA with mean diameter 1 to 2 μm were prepared via emulsion solvent evaporation method. The sPLLA microspheres have higher loading capacity and encapsulation efficiency and lower drug release rate than mPLLA microspheres.