The goal of fabricating materials with better properties and understanding the underlying structure-property-performance relationship has continuously driven research efforts and motivated our work in this thesis. Unique block copolymers have been strategically designed and employed in concert with commercially available resins to achieve blend materials with well-controlled nano-structures and excellent mechanical properties. We first carried out a model system study of the phase behavior between iPP and a series of synthesized copolymers that are potentially miscible with iPP according to the conformation asymmetry theory. Though they are not miscible with iPP as predicted by theory because of density mismatch, their marginal immiscibility imparted very low interfacial tensions with iPP, producing blends with nano-sized dispersed droplets and excellent optical transparency. More interestingly, 5 wt% of these copolymers raised the elongation at break from 20% for neat PP to more than 300%, which is attributable to greatly reduced interparticle distance and cavitation induced shear yielding as evidenced in electron microscopy studies. With the knowledge learned from the model study, we proceeded to strategically design ‘amphiphilic’ block copolymers (BCPs) to explore the application of block copolymer micelles for toughening of semi-crystalline iPP matrix. When melt blended with iPP, these polyolefin block copolymers were uniformly dispersed as sub-100nm micelles. Moreover, these excellent toughening agents increased the tensile toughness by 20 times with merely 5 wt% addition and improved the impact strength by 12 times with 10 wt% addition, and more importantly, no significant deterioration in the elastic modulus or tensile strength was observed. Electron microscopy revealed coexistence of the cavitated micelles and shear band structure in the matrix of the BCP modified blends, suggesting a cavitation induced shear yielding toughening mechanism. A well-established theory was employed to model the dependence of toughening performance on the modifier size and an optimal size range was identified where particle cavitation and matrix shear yielding can occur simultaneously so that maximum toughness can be achieved. Lastly, we targeted the grand PP/PE recycling challenge faced by the global society using iPP-PE block copolymers synthesized by our Cornell collaborators. The compatibilizing performance of the iPP-PE block copolymers was evaluated from two perspectives: blend morphology studied with electron microscopy and interfacial adhesion studied with model T-peel testing. Then the mechanical properties of compatibilized blends were measured with tensile testing. The iPP-PE diblock copolymers with high molecular weights and multiblock copolymers with moderate molecular weights are shown to be exceptional compatibilizers, significantly reducing the droplet size in the blend morphology and leading to PE cohesive failure during peel testing. To explain the molecular weight and architecture dependence, we have invoked two mechanisms concerning cocrystallization in diblocks and interlocked entanglements in multiblocks. The finer blend morphology and enhanced interfacial adhesion translate into excellent blend mechanical properties. Tough blends can be obtained with as little as 0.5 wt% BCP, an amazing result that demonstrates the amazing interfacial activity of these BCP species.