A new dendritic heteroarm star copolymer that contains multi-alternating arms of poly(ethylene oxide-tetrahydrofuran) (P(EO-THF)) and poly(methyl methacrylate) (PMMA) on a dendritic polyester core has been synthesized by a ‘core-first’ approach by combination of sequential cationic ring-opening polymerization (CROP) and reversible addition–fragmentation transfer (RAFT) polymerization initiated by a dendritic macroinitiator (3) capped with multi-alternating terminal carboxylic acid groups (used directly to initiate the ROP of THF in the presence of EO as a polymerization promoter to attain P(EO-THF) arms) and dithiobenzoate groups (used to initiate RAFT polymerization of MMA to attain PMMA arms). The structures of the products were confirmed by NMR spectroscopy, GPC-MALLS, and DSC measurements.