Amphiphilic multiarm star polylactide with hyperbranched polyethylenimine as core: A systematic reinvestigation

The Sn(Oct)2 catalyzed polymerization condition of l-lactide (LLA) and racemic lactide (r-LA) using hyperbranched polyethylenimine (PEI) as the macroinitiator was optimized. Multiarm star polymers bearing PEI core and well-controlled poly(l-lactide) (PLLA) or poly(racemic lactide) (PDLLA) arms were successfully prepared under the optimized condition, including: (1) high concentration of Sn(Oct)2 catalyst was required; (2) with respect to the polymerization of r-LA, the optimal temperature was in the range of 115-130 °C; (3) as for the polymerization of LLA, the optimal temperature was only around 130 °C. Model experiments demonstrated that secondary amine could also effectively initiate the Sn(Oct)2 catalyzed polymerization of LA under the optimized condition, however, its initiation efficiency was usually a little less than 100%, unlike the primary amine and hydroxyl initiators. The results of Gel Permeation Chromatography demonstrated that the obtained multiarm star polymers with PDLLA arms had narrower dispersities than those with PLLA arms. Thermal analyses demonstrated that raising the arm numbers of the stars impaired their thermal stability a little. Stars with PDLLA arm were amorphous. The glass transition temperature of all the PDLLA-based polymers was similar and had no obvious relationship with the arm length, arm number and the molecular architecture. The crystallizability of star polymers with PLLA arm was weaker than that of linear PLLA, and only star polymers with long PLLA arm showed obvious crystallization. The guest encapsulation and release properties of the obtained star polymers were also investigated. It was found that their guest encapsulation capacity had correlation with the PLA arm length, the PEI core size and the degree of quaternization of PEI core, but had no relationship with the type of the PLA arm (PDLLA or PLLA). Whereas the guest release rate was strongly affected by the arm type.