Polyanhydrides modified

Figure 5 General structure of polyanhydrides. R and R can be varied to modify degradation kinetics and profile. The lower frame shows the structure of P(CPP-SA), a polyanhydride copolymer, used in the Gliadel product. Abbreviations SA, sebacic acid CPP, carboxyphen-oxypropane.

Polyanhydrides have been modified by incorporating amino acids into im-ide bonds. The imide with the terminal carboxylic acids is activated with acetic anhydride and copolymerized with sebacic acid or CCP. Poly(anhydride-imides) increase the mechanical properties of the polyanhydrides. Degradation of poly(anhydride-imide)s is similar to that of polyanhydrides (i.e., surface erosion). Two different cleavable bonds (anhydride and ester) in the polymer chains have been included in polyanhydrides. Carboxylic acid-terminated e-caprolactone oligomers or carboxylic acid-terminated monomers (e.g., salicylic acid) have been polymerized with activated monomers (e.g., SA). 

Some modified boronic acids were allowed to enter a resin-to-resin transfer reaction (RRTR) to participate in a borono-Mannicli MCR with a substrate bound to a different polymer. By doing so the authors avoided a difficult work-up of boronic acids, which often tend to form oligo and polyanhydrides. Thus, starting from the (DEAM-PS)-bound 3-amino boronic acids (438), Ugi reactions yielding dipeptides such as (441) have been performed (Scheme 90). 

The properties of polyanhydrides ean be modified by ehanging polymer composition and structure. This is obtained by realizing copolymers, polymer blends, erosslinking between ehains, partial hydrogenation and reaction with epoxides. Low moleeular weight PLA, PHB and PCL are miscible with polyanhydrides whereas high moleeular weight polyesters are not. 

It is known that the potential monomers or pol5uner building blocks should have at least one (in the case of addition polymerization) or two double bonds in their structure (in the case of condensation pol5nnerization) to get thermoplastic materials. Therefore, triglycerides should be modified of fimctionalized before pol5nnerization. Nevertheless, there are some examples of thermoplastic biomaterials obtained from naturally functionalized castor oil with homogeneous composition and acceptable polymerization yields. The main thermoplastic materials already synthesized from vegetable oils are thermoplastic polyurethanes (TPUs), polyamides (PA), thermoplastic polyesters, polyesteramides and polyanhydrides. 

Polyanhydride structures can be modified by the addition of aminoacids, linked via imide bonds at the amino terminus, so that the carboxylic acid terminus remains available for the interaction with acetic anhydride [459]. These poly(anhydride-imides) degrade in a similar way as the simple polyanhydride polymers. Other modifications that can take place are copolymers linked with esters. In that way, the polymer contains two types of hydrolytically cleavable bonds. In the presence of water, both types of bonds are hydrolyzed, releasing the dicarboxylic acid and the ester [460]. An example of a poly(anhydride-ester) with application in the medicinal field includes sebacic acid and salicylic acid, a therapeutically useful compound. The release of sebacic acid in the body opens up a variety of potential applications. 

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