Brush macromolecules (BMs) are unique because of their distinct properties from linear or other nonlinear polymers, relying on side chain steric repulsions and backbone stiffness. BMs with densely grafted side chains have been synthesized either by cationic, anionic, or controlled radical polymerization, yet CO 2 -epoxide immortal alternating copolymerization has rarely been applied for the synthesis of BMs via the "grafting-from" approach. Here we report synthesis of BMs by alternating copolymerization of CO 2 and propylene oxide (PO) initiated from poly(acrylic acid) as a multifunctional macroinitiator. The copolymerization afforded ultrahigh-molecular-weight BMs with poly(propylene carbonate) (PPC) side chains (molecular weight >10 6 ), and the side chain length was tunable via further CO 2 -PO alternating copolymerization initiated from hydroxy end groups of the resulting side chain PPCs. These BMs were directly observed by atomic force microscopy, demonstrating that the BMs have ellipsoidal morphologies with 20-50 nm. Furthermore, the BMs were thermally decomposable at around 240 °C.