Polymer blends are used to access unique combinations of properties beyond those of neat homopolymers. Blends confer flexibility in tailoring a specific material to a given application, and in some cases, they can lend improved properties compared to their constituent materials. Some examples of blend-synergistic properties in the literature include toughness enhancement, increased chemical resistance, increased modulus, and improved processability. Given the breadth of properties that can be improved by blends, they are employed extensively in commercial products, with more than a third of all polymer resins used in blends (Utracki, 2003). Most polymer pairs are immiscible, thus their blends require compatibilization to aid dispersion in the melt state and to transfer stress across interfaces in the solid state. Block copolymers have proven to be successful compatibilizers, in both premade and reactively formed systems. This thesis focuses mainly on reactive systems. The reaction at immiscible polymer interfaces is kinetically limited and most reactions are too slow for applications, so general methods of increasing interfacial reaction rate have been investigated. This work also seeks to find new tools for measuring localization and conversion in polymer blends, with the ultimate goal of making useful, economical materials, and understanding the resulting structures. This thesis attempts to further our knowledge of compatibilization of polyolefin blends in particular. Chapter 2 attempts to create facile reactive compatibilization schemes for polyolefins with poly(methyl methacrylate). Chapters 3 and 4 examine the use of catalysts to increase interfacial reaction rate between functional polyethylene and polylactide. Chapter 3 demonstrates stannous octoate catalyst is localized at the interface, and blends show better compatibilization than those with a more active but non-localized tin chloride dihydrate catalyst. Chapter 4 uses cobalt octoate catalyst to increase interfacial reaction rate by ~90-fold and the extension at break of polylactide majority blends to ~200%. Structural dependence of copolymers on compatibilization efficiency in polypropylene/polyethylene blends is investigated in Chapter 5. Finally, a small scale coextruder is created using a dual-bore capillary rheometer, with the potential to examine the effect of flow on copolymer localization, catalyst localization, and interfacial reaction rate (Chapter 6).