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Ítem Síntesis de polietileno con ramificaciones muy largas y poco frecuentes(Sena, 2015-12-03) Delgado, Lina M.Este proyecto se realizó por medio de una polimerización vía “acyclic diene metathesis polymerization (ADMET)” en presencia del catalizador de Grubbs de primera generación con el fin de sintetizar polietileno (PE) lineal de baja densidad (LLDPE), el cual a diferencia del LLDPE comercial presenta ramificaciones en lugares específicos de la cadena carbonada. En este proyecto se obtuvieron tres materiales, dos de ellos con ramificaciones de 17 y 21 carbonos las cuales aparecen cada 21 carbonos, mientras el tercero de ellos se sintetizó con una ramificación de 21 carbonos la cual aparece cada 39 carbonos.
Ítem Hierarchical Acrylic Acid Aggregate Morphologies Produce Strain-Hardening in Precise Polyethylene-Based Copolymers(American Chemical Society, 2015-05-01) Cordaro S, JosephWe report tensile testing and in situ X-ray scattering measurements of a homologous series of precise poly(ethylene-co-acrylic acid) copolymers (pxAA). The number of backbone carbons (x) between pendant acrylic acid groups along the polyethylene chain (x = 9, 15, 21) has a pronounced effect on both their tensile properties as well as their morphologies during deformation. The semicrystalline precise copolymer (p21AA) displays yielding behavior similar to polyethylene. Also, strain hardening in p21AA coincides with the originally isotropic acid-rich layered structures strongly aligning with acid layers perpendicular to the strain direction, demonstrating the facile nature of the H-bonding within the acid aggregates. When the alkyl spacer is only nine carbons (p9AA), the precise copolymer withstands strains of >1000% without failing, because the liquid-like assembly of acid aggregates permits the acid groups to exchange without developing substantial anisotropy in the structure. Both p21AA and p9AA maintain...Ítem Precision Long-Chain Branched Polyethylene via Acyclic Diene Metathesis Polymerization(American Chemical Society -, 2015-10-21) Li, HongA series of polyethylenes containing 21-carbon alkyl branches have been synthesized by acyclic diene metathesis (ADMET) polymerization. These 21-carbon alkyl branches are precisely placed on every 15th, 19th, 21st, 23rd, and 39th carbon along the polymer backbone. Precision of primary structures of all polymers is verified by 1H and 13C NMR spectroscopy. All polymers present well-defined melting profiles, even at a high branch incorporation (13.3% mol). The melting temperature increases as the branch frequency decreases, similar to what we observed for short-chain branched polyethylenes. These observations together with a good linear relationship derived from Flory's theory suggest the exclusion of 21-carbon side chains from polyethylene crystal units.