Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Polyaspartic acid (PASA) is a biodegradable, water-soluble polyaminoacid with potential to replace many non-biodegradable polymers. It is used as a pure homopolymer and in various copolymers. In nature, PASA has been found in as fragments of larger proteins with length up to 50 amino acids, but as of 2004 had not been isolated as a pure homo polymeric material from any natural source. The first isolation of synthetic oligomeric sodium polyaspartate, obtained by thermal polycondensation of aspartic acid, was done by Hugo Schiff in late 19th century. Later it was proposed that thermal polymerization process leads through polysuccinimide intermediate. Polyaspartic acid is currently produced on the industrial scale and is available as commercial product in forms of acid and sodium polyaspartate.
Properties and structure
PASA is a yellow liquid miscible with water. Due to presence of carboxylic groups it is polyelectrolyte with anionic character. Naturally occurring PASA fragments consists of α,-linked L-aspartatic acid. In contrast, the repeating unit of synthetic polyaspartic acid may exist in four different isomeric forms depending on the stereochemistry of starting material (D- and L-aspartic acid) and synthetic procedure leading to α and β links.
There are currently many different synthetic protocols available leading to PASA. In the simplest and the oldest approach aspartic acid is heated to high temperature resulting in water release and the formation of polysuccinimide. In the subsequent step this polymer is reacted with sodium hydroxide in water which yields partial cleavage of the succinimide ring. In this process sodium-DL-(α,β)-poly(aspartate) with 30% α-linkages and 70% β-linkages randomly distributed along the polymer chain, and racemized chiral center of aspartic acid is produced. There were many catalysts reported for improving thermal polymerization method. Main benefits from their application is increasing of the conversion rate and higher molecular weight of the product. Polyaspartic acid can also be synthesized by polymerization of maleic anhydride in presence of ammonium hydroxide. High control over repeating unit isomers can be achieved by polymerization of N-carboxyanhydrides (NCA) derivatives, by polymerization of aspartic acid esters or by application of enzyme catalyzed reaction. Pure homopolymers, D- or L- PASA with α- or β-links only, can be synthesized using those methods.
Polyaspartic acid and its derivatives are environmentally friendly, biodegradable alternative to traditional polyanionic materials, in particular as replacement for polyacrylic acid. PASA has ability to inhibit deposition of calcium carbonate, calcium sulfate, barium sulfate and calcium phosphate salts and can be used as an antiscalining agent in cooling water systems, water desalination processes, and waste water treatment operations. In addition and due to its ability to chelate metal ions it provides corrosion inhibition. It can act as a super-swelling material in diaper/feminine-hygiene products and food packaging. It can also be used as biodegradable detergent and dispersant for various applications. There is a broad interest in this material from biomedical and material research community.
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