| The physical properties of a molecule are dictated by its structure. In polymers, there are many levels of structure, including the chemical nature of the repeat units, molecular weight, molecular weight distribution, chain architecture (linear, branched, etc.), and additionally in block copolymers, composition and block sequence. Polymer scientists are continually working to improve their understanding of these structure-property relationships. To do so, a wide array of polymeric structures is needed, which requires the ongoing development of new synthetic strategies.; This research focuses on the novel syntheses of two types of polymeric architectures that are of interest for structure-property relationship studies: star polymers and ABA-type triblock copolymers containing glassy (A) and semicrystalline (B) blocks. In the first section, well-defined polyisoprene three-armed star polymers were synthesized through a combination of living anionic polymerization and an efficient coupling process using hexafluoropropylene oxide (HFPO) as a multifunctional linking agent. Molecular characterization of the star polymers gave data consistent with the star structure and proposed mechanism of formation. The effects of HFPO concentration and the molecular weight of the arm precursors on the extent of star formation were investigated. Under optimized conditions, at least 97% of the living PI anions were coupled, without the need for subsequent fractionation or purification. In addition, the reaction was extended to both polystyrene and polybutadiene star polymers.; In a second study, block copolymers containing polystyrene and polycyclooctene were synthesized using a ring-opening metathesis polymerization (ROMP)/chain transfer approach. Polystyrene, containing appropriately-placed olefins, was prepared by anionic polymerization and served as a macromolecular chain transfer agent for the ROMP of cyclooctene. These unsaturated polymers were subsequently converted to the corresponding polycyclohexylethylene-b -polyethylene-b-polycyclohexylethylene triblock copolymers using a simple heterogeneous catalytic hydrogenation step. Molecular and morphological characterization of the block copolymers, including relatively high melting temperatures (114-127°C) and levels of crystallinity (17-42%) for polyethylene, was consistent with the absence of branching in the central polycyclooctene and polyethylene blocks. Although none of the samples were particularly well-ordered, a dramatic improvement of both the long-range order and the mechanical properties of a symmetric block copolymer sample was observed after removal of homopolymer contamination. |
