Novel polystyrene nanoparticles were synthesized by the controlled intramolecular crosslinking of linear polymer chains to produce well-defined single-molecule nanoparticles of varying molecular mass, corresponding directly to the original linear precursor chain. These nanoparticles are ideal to study the relaxation dynamics/processes of high molecular mass polymer melts, as the high degree of intramolecular crosslinking potentially inhibits entanglements. Both the nanoparticles and their linear analogs were characterized by measuring their intrinsic viscosity, hydrodynamic radius (Rh), and radius of gyration (R-g). The ratio R-g/R-h was computed to characterize the molecular architecture of the nanoparticles in solution, revealing a shift toward the constant density sphere limit with increasing crosslink density and molecular mass. Further, confirming particulate behavior, Kratky plots obtained from neutron scattering data show a shift toward particle-like nature. The rheological behavior of the particles was found to be strongly dependent on both the extent of intramolecular crosslinking and molecular mass, with a minimal viscosity change at low crosslinking levels and a gel-like behavior evident for a large degree of crosslinking. These and other results suggest the presence of a secondary mode of polymer relaxation/movement besides reptation, which in this case, is influenced by the total number of crosslinked loops present in the nanoparticle. (c) 2006 Wiley Periodicals, Inc.