Polyanhydrides

First reported in the early 20th century, polyanhydrides have been extensively studied. Generally, polyanhydrides are characterized by the highly hydrolytic nature. Various diacid (dicarboxylic acid) monomers can be used to produce polyanhydrides. Historically, there were three main production strategies: bulk melt condensatioin, ring-opening polymerization and reaction between dibasic acid and diacid chlorides. Bulk melt condensation involves acetylation of dicarboxylic acid monomers, which produces short acetyl-terminated prepolymers, which is later placed under high temperature and vacuum to produce higher molecular weight polymers. Ring-opening polymerization is thought to involve smaller cyclic polymers that subsequently undergo ring-opening and further polymerization. Polyanhydrides can be categorized into aliphatic, aromatic and unsaturated, depending on the monomers used to produce the polymers.

Figure 1. Core structure of polyanhydrides Unlike the other polymers described earlier, polyanhydrides are not suitable for thermoplastic production due to their highly hydrolytic nature. Instead, polyanhydrides are widely used in applications where quick degradation is essential. For example, polyanhydrides have been used in medical applications, especially drug delivery. Once in contact with bodily fluids, the polymer disintegrates into diacid monomers, which are completely removed from the body within a relatively short period of time. [2] By encapsulating delicate pharmaceutical agents, such as proteins, in a auto-degradable scaffold, more efficient drug delivery is possible. Researchers have explored the possibility of using polyanhydride-based microspheres as drug carriers. More recent research has also explored possible applications such as oral delivery of protein-based drugs, such as insulin, which normally requires intravenous injections due to their fragile nature. [2]