Browsing by Author "Liu, Shu"

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    Molecular necklaces: polyester rotaxanes
    Liu, Shu (Virginia Tech, 1995-01-05)
    Polyrotaxanes consisting of 30-60 membered aliphatic crown ether macrocycles whose cavities are pierced by polysebacate chains were synthesized by several polymerization approaches including transesterification polymerization, the acid chloride method, and interfacial polymerization. The polyrotaxanes were purified by multiple reprecipitations into good solvents for the crown ethers. In some cases the threaded macrocycles are constrained onto polymer chains by the incorporation of monofunctional blocking groups at polymer chain ends, or by the copolymerization of a difunctional blocking group with other monomers. The compositions and the physical properties of the polyrotaxanes were determined by a variety of characterization techniques including NMR, UV, VPO, GPC, DSC, TGA, and intrinsic viscosity measurements. Significant amounts, up to 51 mass %, of the macrocyclic components were incorporated. Because of incorporation of flexible and polar macrocycles, polyrotaxanes display special behavior in solution and in the solid state. In solution the linear components of polyrotaxanes are stiffened by the threaded macrocycles, resulting in increased hydrodynamic volumes. The solubilities of linear polymers in polar solvents are enhanced by the incorporated crown ethers. The glass transitions are also affected by the crown ether component. Due to the movement of the macrocycles along the backbone, the macrocycles are able to aggregate and crystallize without dethreading from the polysebacate backbones. The threading and dethreading processes were systematically studied in the diol/diacid chloride system. Due to the hydrogen bonding between the cyclic and linear species, the macrocycle contents of unblocked polyrotaxanes are significantly affected by the feed ratio of macrocycle to linear monomers up to a value of 2 but are independent of reaction time and the length of diol monomers. The macrocycle content of polyrotaxanes increases non-linearly with the size of the macrocycles, presumably due to changes in the fraction of threadable conformations of the macrocycles. Although some macrocycles near the unblocked polymer chain ends are apparently susceptible to the dethreading from polymer chains, most of the macrocycles are prevented on resonable time scales, e.g., months in solution, from the dethreading by the entanglement of cyclic and linear species.
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    Regioselective Synthesis of Polysaccharide-based Polyelectrolytes
    Liu, Shu (Virginia Tech, 2018-01-12)
    Polysaccharides are one of the most abundant and diverse families of natural polymers, and have an incredibly wide range of natural functions including structural reinforcement, energy storage, aqueous rheology modification, and communication and identity. Application of native polysaccharides like cellulose as sustainable materials is limited by some inherent drawbacks such as insolubility in common solvents including water, and poor dimensional stability. To increase their functionality and utility, researchers have sought to tailor the chemical and physical properties of cellulose and other polysaccharides using a variety of chemical modification techniques, resulting in a number of important, useful commercial derivatives. Because of their greater biocompatibility and biodegradability, and low immunogenicity, naturally derived cationic polymers including cationic polysaccharide derivatives are very attractive candidates for biomedical applications, due to the fact that they are capable of binding with anionic biomolecules, such as nucleic acids and certain proteins, via electrostatic interactions. However, there are relatively few practical synthetic methods reported for their preparation. We demonstrated a useful and efficient strategy for cationic polysaccharide salt preparation by reaction of 6-bromo-6-deoxypolysaccharides such as 6-bromo-6-deoxycellulose esters with pyridine or 1-methylimidazole exclusively at the C-6 position, resulting in high degrees of substitution (DSs). These permanently cationic polysaccharide derivatives have been demonstrated to dissolve readily in water, and bind strongly with a hydrophilic and anionic surface. Availability of these cationic polysaccharides will facilitate structure-property relationship studies for biomedical uses including drug delivery and bioelectronics applications. We also extended the chemistry, reacting 6-imidazolo-6-deoxycellulose with propane sultone, leading to a new synthetic pathway to zwitterionic cellulose derivatives. In addition to cationic and zwitterionic derivatives, we found a simple, efficient route to carboxyl-containing polysaccharide derivatives from curdlan esters via regioselective ring-opening reactions catalyzed by triphenylphosphine (Ph3P) under mild conditions. Curdlan, a polysaccharide used by the food industry and in biomedical applications, was employed as starting material for preparing these carboxyl-containing derivatives by a reaction sequence of bromination, azide displacement and ring-opening reaction with cyclic anhydrides, affording high conversions. These modification techniques have been demonstrated to display essentially complete regio- and chemo-selectivity at C-6. These novel polysaccharide-based materials starting from abundant and inexpensive curdlan are promising for some applications such as amorphous solid dispersion (ASD) oral drug delivery.