Multiarm Polycarbonate Star Polymers with a Hyperbranched Polyether Core from CO₂ and Common Epoxides

Multiarm star copolymers, consisting of hyperbranched poly­(ethylene oxide) (hbPEO) or poly­(butylene oxide) (hbPBO) polyether copolymers with glycerol branching points as a core, and linear aliphatic polycarbonate arms generated from carbon dioxide (CO₂) and epoxide monomers, were synthesized via a “core-first” approach in two steps. First, hyperbranched polyether polyols were prepared by anionic copolymerization of ethylene oxide or 1,2-butylene oxide with 8–35% glycidol with molecular weights between 800 and 389,000 g·mol^–1. Second, multiple arms were grown via immortal copolymerization of CO₂ with propylene oxide or 1,2-butylene oxide using the polyether polyols as macroinitiators and (R,R)-(salcy)-CoCl as a catalyst in a solvent-free procedure. Molecular weights up to 812,000 g·mol^–1 were obtained for the resulting multiarm polycarbonates, determined by online viscometry with universal calibration and ¹H NMR. Comparing the synthesis of different multiarm star polycarbonates, a combination of a highly reactive macroinitiator with a less reactive epoxide monomer was found to be most suitable to obtain well-defined structures containing up to 88 mol% polycarbonate. The multiarm star copolymers were investigated with respect to their thermal properties, intrinsic viscosity, and potential application as polyols for polyurethane synthesis. Glass transition temperatures in the range from −41 to +25 °C were observed. The intrinsic viscosity could be adjusted between 5.4 and 17.3 cm³·g^–1 by varying the ratio of polyether units and polycarbonate units.