Unique Architectures Built on Chiral Polycarbodiimides
Siriwardane, Dumindika Aththanayake
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The helix is a very fascinating and unique macrostructure that can be found in biological systems, as well as, throughout nature. The ability to transfer this basic structure into synthetic architecture will offer many advantages for the development of chiral scaffolds important in novel applications. These discoveries include chiral sensing materials, liquid crystalline materials, and amphiphilic block-copolymers for drug delivery applications, to name a few. Therefore, it is important to enhance the synthesis strategy to improve structural control by accessing greater chemical diversity and versatile functionality for these polycarbodiimide systems. Chapter one introduces the basic synthetic routes for ureas, carbodiimide monomers, polymers and strategies utilized for screw sense polymerization to achieve control chiral architectures. It also presents a comprehensive review of the most recent developments in post-polymerization modifications, properties of polycarbodiimides and their potential applications in the material sciences and bio-medical field. Chapter two illustrates the synthesis of novel Ni(II) initiators for screw sense polymerization of chiral monomers and it explained Ni(II) mediated “living” polymerization used in the synthesis of multi-arm star polymers. This star and its linear analogs have been used to conduct kinetic of polymerization and solvent tunable self-assembly behavior studies. Chapter three describes the unusual observation made years ago, that a particular chiral polymer (predominantly P or M helices) synthesized by using chiral BINOL-Ti(IV) catalysts which show specific racemization behavior when heated. This behavior was unique in more than 150 different carbodiimides we have polymerized to date. We postulated that it was due to stereocomplexation formed via racemization between two opposite helical sense. Different characterization techniques and molecular models were employed to investigate this particular observation. By enhancing helical, polycarbodiimide architectures, chapter four introduces the synthesis of hairy polymers which showed micellization in an aqueous medium. These types of amphiphilic graft co-polymers can be utilized in drug delivery applications. The synthesis and optical switching behavior of UV and thermo responsive polycarbodiimides have been illustrated in chapter five. Properties of the polymer were altered through the isomerization behavior, such as its topology and liquid crystallinity. The synthesis of various chiral designs build upon polycarbodiimide helical frameworks are also included to pursue possible future projects.