High molecular weight (MW), 3-arm star poly(methyl methacrylate) (PMMA) with a narrow MW distribution (Mn=570,000 g/mol, PDI=1.36) was successfully synthesized by activators regenerated by electron transfer (ARGET) atom transfer radical polymerization (ATRP). The polymerization was carried out with a trifunctional initiator/CuBr2/N,N,N',N",N" -pentamethyldiethylenetriamine (PMDETA) initiator/catalyst system in the presence of a tin(II) 2-ethylhexanoate [Sn(EH)2] reducing agent at 90 ℃. The concentration of the copper catalyst was as low as 30 ppm, and a high initiation efficiency of the initiating sites was obtained. The chain-end functionality of the high MW, 3-arm star PMMA was confirmed by a chain extension experiment with styrene via ARGET ATRP, using the same catalyst system.