Despite the tremendous progress made in the field of biomedical engineering, many challenges still remain to be addressed-especially in the design of new materials. The biodegradable synthetic polymers currently used for biomedical applications are almost exclusively based on short aliphatic moieties, such as lactides, glycolides, e-caprolactone, and sebacic acid, which all display relatively "hard" mechanical properties that adjust poorly to those of tissues, and therefore cause considerable stress mismatches at the interface responsible for necrosis or abnormal regeneration. [1] Materials based on bile acids show great promise for drug delivery [2] and controlled release [3] applications, and their rigid steroidal backbone and amphiphilicity appear to make them candidates of choice for fine-tuning the mechanical and interfacial properties of synthetic degradable polymers. However, reports on main-chain bile acid based polyesters, polyamides, and polyurethanes are still scarce in the literature, and their synthesis constitutes a real challenge, especially when higher molecular weights are required. Most techniques used to date rely on the use of toxic coupling agents.
Ring-opening polymerization (ROP) is a very versatile technique that has been applied to the synthesis of polyesters with a controlled molecular weight. In virtually all cases, small strained cycles (3-8-membered rings) are used and enthalpy drives the polymerization. However, macrocycles, including those based on esters and alkenes, could be polymerized, and afforded appreciably high molecular weights, depending on the conditions. [4] In these cases, polymerization is driven by entropy, as described by the Jacobson-Stockmayer theory for ring-chain equilibria. [5] Entropy-driven ring-opening polymerization (ED-ROP) therefore appears to be a method of choice for the synthesis of high-molecular-weight polyesters based on bile acids.