Hyperbranched polyethylenes containing covalently tethered polyhedral oligomeric silsesquioxane (POSS) nanoparticles were synthesized in this work by chain walking ethylene copolymerization with a POSS macromonomer bearing a polar acrylate group, acryloisobutyl-POSS. The unique hyperbranched chain topology of these polymers was achieved owing to the chain walking mechanism of the Pd-diimine catalyst, [(ArNd

C(Me)-(Me)CdNAr)Pd(CH3)(NtCMe)]SbF6 (Ar ) 2,6-(iPr)2C6H3). Regardless of its bulky structure and polar nature, the acryloisobutyl-POSS macromonomer was successfully copolymerized to give a range of POSSethylene copolymers with the POSS macromonomer content up to 35 wt %. A systematic study of the effects of covalent POSS incorporation on the polymer properties was undertaken using techniques including gel permeation chromatography with on-line viscometry (GPC-VIS), X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and rheometry. It was found from GPC-VIS measurements that the covalent incorporation of the high-mass POSS nanoparticles reduces significantly the intrinsic viscosity of the copolymers compared to homopolyethylenes of the same molecular weight, owing to the highly compact spherical cage structure of the POSS nanoparticles. Thermal studies confirm that the POSS incorporation enhances significantly the thermal oxidative stability of the polymers in air, and the copolymer glass transition temperature increases with POSS macromonomer content. The XRD study showed aggregation of POSS nanoparticles in the

copolymers, leading to the formation of crystalline POSS domains. Rheological measurements demonstrate that the covalently tethered POSS nanoparticles greatly reinforce polymer rheological properties. In particular, gellike rheological behavior was observed in the POSS copolymers. This gelation behavior is attributed to the aggregation/interactions of POSS nanoparticles, which lead to the construction of a physical network system throughout the polymer nanocomposite materials.