We have prepared a series of cross-linkable oligo-and poly(dialkylfluorene)s by nickel(0)mediated polymerization of 2,7-dibromo-9,9-dialkylfluorene (alkyl ) n-hexyl) and 4-bromostyrene. The resulting fully soluble and processable, styryl-functionalized oligomers and polymers can be cross-linked via the vinyl end-groups by curing at 175-200 °C, consistent with the autopolymerization mechanism of styrene. These relatively mild conditions render the materials insoluble and enable multilayering of polymers in organic light emitting devices. At the same time, the electrical and/or optical properties of the cross-linked polymers are preserved and no deleterious species or undesirable byproducts are produced. Furthermore, the cross-linking allows control of the supramolecular ordering of the planarized rigid rodtype fluorene segments in the polymer backbone that leads to suppression of troublesome excimer/aggregate in the photo-and electrolumenescence.
Several new approaches to biodegradable dendritic aliphatic block copolymers are described, including hyperbranched and dendrimer-like star structures. The hyperbranched polymers were obtained by the co-condensation of different AB 2 macromonomers. The macromonomers were prepared by ringopening polymerization (ROP) of either -caprolactone, L-lactide, or various substituted lactones using the benzyl ester of 2,2′-bis(hydroxymethyl)propionic acid as initiator. Catalytic hydrogenation of the benzyl ester generated the requisite acid functional AB2 macromonomer. The second route utilizes a new type of molecular architecture, denoted as dendrimer-like star polymers. These block copolymers are described by a radial geometry where the different layers or generations are comprised of high molecular weight polymer emanating from a central core. With this architecture, more control in the placement of the different blocks is afforded over the hyperbranched analogue. As a means of imparting desirable mechanical properties to the dendritic copolymers, a series of new substituted lactones were prepared. The use of such monomers prevents crystallization of the poly(lactone), allowing dendritic polyesters with a range of mechanical properties from thermoplastic elastomers to rubber toughened systems, depending on the relative composition of the two components. The synthesis, characterization, and morphology of these new copolymers are discussed.
The high-temperature transition at 340 "C for the homopolymer of p-hydroxybenzoic acid (PHBA) has been shown to display characteristics typical of a plastic crystal or a highly ordered smectic phase transition.For low molecular weight samples of the homopolymer a nematic texture can be induced above this phase transition by modest external shear. However, for high molecular weight samples the material is intractable in this temperature regime. A much higher transition at 445 "C has been identified and characterized by DSC, TMA, and polarized light microscopy, indicating the presence of a nematic mesophase. Processing of the homopolymer above this transition results in fusion of the material.
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