Addressing polymer topologies is one of the key methods for tailoring polymer properties. Herein, we report for the first time on the core-first synthesis of three-armed star-shaped polymers with adjustable molecular weights via rare earth metal-mediated group transfer polymerization (REM-GTP). Based on the versatility of REM-GTP, enabling polymerization of a broad variety of functional monomers not accessible via conventional techniques, a novel and fast method toward directed polymeric structures was established. Therefore, the trinuclear catalyst was synthesized by 3-fold C-H bond activation of 1,3,5-tris(3,5-dimethy1-4-pyridinyObenzene using Cp2YCH2TMS(THF) as precursor complex. Kinetic investigations in comparison to monometallic cp(2)Y(sym-collidinyl) on the polymerization of diethyl vinylphosphonate (DEVP) and 2-isopropeny1-2-oxazoline (IPOx) evidenced activity of all three metal centers. However, in REM-GTP generally occurring incomplete initiation provoked by the interaction of initiators and monomers, potential impurities, and applied reaction conditions led to a distribution of stars, long linear, and short linear polymers originating from chain growth in three, two, and one direction, respectively. For further visualization PIPOx produced by the trinuclear complex was converted into P(IPOx-graft-2-ethyl-2-oxazoline) using living cationic ring-opening polymerization. AFM scans confirmed the occurrence of the three types of polymer. Additionally, comparable solely linear PDEVP and PIPOx were synthesized by dinuclear complexes generated by C-H bond activation of 1,3-bis(3,5-dimethyl-4-pyridinyl)benzene and 1,4-bis(3,5-dimethyl-4-pyridinyl)benzene using Cp2YCH2TMS(THF) as precursor. In the case of PDEVP, the mass fraction of the low molecular weight polymer, being formed by chain growth in one direction, was accessible via GPC analysis. Further stochastic examinations on the incomplete initiation for multinuclear complexes corroborated our findings accurately.