Synthesis and characterization of carboxylate-containing polymers for biomedical applications

Abstract

Polysaccharides, one of three types of natural polymer, are used by living organisms for energy storage, structural support, and other vital applications. In particular, acidic polysaccharides such as alginic acid and hyaluronan play important roles in lubrication and water storage. Isolation of polysaccharides is complicated by the difficulty of purification and lack of batch-to-batch consistency. Polysaccharide synthesis has also proved challenging due to their stereochemical complexity and high density of functional groups with similar reactivity. One solution is the synthesis of mimics that possess many of the properties of natural polysaccharides, including a rigid pyranose or furanose backbone and numerous hydroxyl or carboxylic acid groups. Synthesized polysaccharide mimics include polymers with ether linkages not seen in nature, non-ether linked sugar monomers, and polymers created from non-carbohydrate sources. These mimics have been used in various applications, including biolubrication, membrane synthesis, and DNA delivery. As a first approach to a polysaccharide mimic, a polymer with a rigid cyclic backbone and several pendant water-soluble functional groups was sought. High molecular weight ester-functionalized norbornene polymers were synthesized by ring-opening metathesis polymerization of 5-norbornene-2-ethyl ester. The resulting polymers were saponified and hydroxylated along the polymer backbone without a reduction in chain length. The chemical, thermal, and mechanical properties of the various polymers were measured, indicating high thermal stability and strong solid-like character. Even in their saponified and hydroxylated form, the polymers possessed very limited water solubility. Poly-amido-saccharides, a new type of carbohydrate polymer mimetic, were synthesized by the controlled anionic ring opening polymerization of a beta-lactam glucose monomer. The primary alcohol was oxidized to investigate the effects of an ionizable carboxylic acid group, such as those found in natural polysaccharides containing glucuronic acid. Circular dichroism revealed an ordered helical secondary structure in solution that was lost following ring opening. The oxidized poly-amido-saccharides were shown to stabilize lysozyme towards freezing and dehydration stresses better than currently used methods. Glycopolymers containing the poly-amido-saccharides on a poly(acrylamide) backbone were also synthesized and characterized as a model for more complex systems.