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We demonstrate in this paper the novel synthesis of multiarm star polyethylenes of well-defined arm lengths and controllable average arm numbers through the core-first multifunctional “l(fā)iving” polymerization protocol from ethylene stock. These novel star polymers are featured with a core?shell structure, having a hyperbranched polyethylene core joining multiple linear polyethylene arms bearing short branch structures. Utilizing the outstanding features of cationic Pd?diimine catalysts, a three-step synthesis procedure incorporating two sequential Pd?diimine catalyzed ethylene polymerization steps is employed. Hyperbranched polyethylenes bearing different numbers of pendant acryloyl groups (HPE1 and HPE2) were first synthesized with an acetonitrile Pd?diimine catalyst (2) by nonliving chain walking copolymerization of ethylene with 1,4-butanediol diacrylate at elevated concentrations. These two hyperbranched polymers having the specific acryloyl anchoring sites were used as the homogeneous support in the second step for the covalent immobilization of catalyst 2 to generate hyperbranched polyethylenes encapsulating multinuclear covalently tethered Pd?diimine catalysts (HPE-Pd-1 and HPE-Pd-2). Acting as the multifunctional initiating hyperbranched core in the third step, the two multinuclear hyperbranched Pd catalysts initiated and catalyzed successfully ethylene multifunctional “l(fā)iving” polymerization at 400 psi and 5 °C, and led to simultaneous multidirectional arm growth from the hyperbranched core to form two sets of star polymers of very high molecular weights (SPE1 and SPE2 sets, respectively, with number-average molecular weight Mn up to 1,379 kg/mol). The arm growth catalyzed by the tethered Pd centers in the polymerization is confirmed to be “l(fā)iving”, with Mn of both star polymers and the arms increases nearly linearly with polymerization time. Determined essentially by the average numbers of tethered acryloyl groups in HPE1 and HPE2, high average arm numbers (about 21 and 28 per star, respectively) were achieved in the star polymers, along with narrow-distributed tunable arm length (up to about 48 kg/mol). Study on dilute solution properties of these two sets of star polymers confirms their spherical chain conformation and resemblance of rigid spheres and high-generation dendrimers.