The construction of nanoscale polymeric objects with complex, well-defined structures and regiochemical functionalities is of great importance, because it enables the fabrication of soft materials with tunable properties. Direct polymerization of macromonomers through covalent bond formation and self-assembly of block copolymers via non-covalent interactions are two typical strategies to afford nanoscopic structures. Molecular brush polymers are composed of densely-grafted side chains along a polymeric backbone. Due to the significant steric repulsion from the side chains, they tend to adopt bottle-brush like conformations, as opposed to linear polymers. "Grafting through" synthesis of molecular brush polymers can provide precise control over the dimensions and functionalities of brush polymers. Shell crosslinked knedel-like nanoparticles (SCKs) are constructed by assembling from amphiphilic block copolymers into micelles, followed by covalent shell crosslinking to further stabilize the nanoparticles and introduce additional functional moieties. SCKs are attractive nanocarriers because of their variable morphologies, compositions and functionalities, which allow for the development of platforms for therapeutic or diagnostic purposes. By utilizing the orthogonal reactivity of the norbornene group and methacrylate group, two distinctly different reactive well-defined linear polymers, and a facile, one-pot synthesis of well-defined molecular brush polymers were studied by selective, orthogonal controlled radical polymerizations (CRPs) and ring-opening metathesis polymerization (ROMP). The living and high efficient characteristics of "grafting-through" strategy were further investigated for the preparation of topology-controlled brush polymers with tunable dimensions of both backbone and side chain lengths. Apart from the fundamental investigation of molecular brush polymers, a series of poly(carboxybetaine) (PCB)- and poly(ethylene glycol) (PEG)-grafted degradable SCKs were developed to evaluate their in vivo pharmacokinetics and biodistributions, aiming to achieve novel therapeutic and diagnostic platforms that may surpass the performance of the conventional PEGylated analogs.