Bulky substituents change polyureas from being highly stable to readily degradable.

Bulky substituents change polyureas from being highly stable to readily degradable.

Polyureas That Love To Break Down

Polyureas are relatively inexpensive and easy to synthesize, plus they tend to be highly resistant to breaking down, making them useful for coatings, adhesives, and other applications requiring durability. Two professors from the University of Illinois, Urbana-Champaign, have now changed the game for the polymers by designing and synthesizing a class of easily hydrolyzable polyureas. They report that adding a bulky substituent such as a tert-butyl group to polyureas destabilizes the polymer backbone and makes it more susceptible to hydrolysis, causing degradation in a few days. The new polymers could be useful for drug delivery, tissue engineering, controlled release, and other applications in which polymer breakdown in aqueous solution is desirable. Modifying the size and nature of the bulky substituents could also enable tight control over the degradation rate, the researchers note, although this research group has not confirmed that yet. 

Other types of hydrolyzable polymers exist, but the new polyureas are more readily synthesized than most, without catalysts or generation of by-products, the group says. However, the toxicity of the breakdown products will have to be evaluated before biomedical, food, or agricultural applications can be considered.

As i mentioned, Hydrolyzable polymers are widely used materials that have found numerous applications in biomedical, agricultural, plastic, and packaging industrials. They usually contain ester and other hydrolyzable bonds, such as anhydride, acetal, ketal, or imine, in their backbone structures. 

With this latest report comes the upgrades to polyurea with the first design of hydrolyzable polyureas bearing dynamic hindered urea bonds (HUBs) that can reversibly dissociate to bulky amines and isocyanates, the latter of which can be further hydrolyzed by water, driving the equilibrium to facilitate the degradation of polyureas. 

In the form of either linear polymers or cross-linked gels, polyureas bearing 1-tert-butyl-1-ethyl urea bonds that show high dynamicity (high bond dissociation rate) can be completely degraded by water under mild conditions. Given the simplicity and low cost for the production of polyureas by simply mixing multifunctional bulky amines and isocyanates, the versatility of the structures, and HUB-bearing degradation profiles' tunability polyureas, these materials are potential of very broad applications.

Polymers with transient stability in aqueous solution, also known as hydrolyzable polymers, have been applied in many biomedical applications, such as in the design of drug delivery systems.

These applications usually require short functioning time and complete degradation and clearance of materials after their use. Hydrolyzable polymers have also been applied in the design of controlled release systems in the agriculture and food industries and used as degradable, environmentally-friendly plastics and packaging materials. 

Besides polyesters, a class of widely used, conventional hydrolyzable materials, a large variety of other hydrolyzable polymers bearing anhydride, orthoester, acetal, ketal, aminal, hemiaminal, imine, phosphoester, and phosphazene bonds have also been reported. Syntheses of these polymers usually involve ondensation or ring-opening polymerization, and these syntheses typically involve removal of byproducts and use of high reaction temperature and/or metal catalysts, which complicates materials preparation. 

In this study, we report the design of polyureas bearing hindered urea bonds (HUBs) as potentially one of the least expensive degradable polymers that can be easily synthesized by mixing multifunctional bulky amines and isocyanates, expanding the family of hydrolyzable polymers.

Polyureas are commonly used as fiber, coating, and adhesive materials. They can be readily synthesized via addition reaction of widely available, di- or multifunctional isocyanates and amines that do not require the use of catalysts and extreme reaction conditions and do not produce any byproducts. 

Urea is one of the most stable chemical bonds against further reactions, including hydrolysis, due to its dual amide structure's conjugation stabilization effects. However, urea bonds can be destabilized by incorporating bulky substituents to one of its nitrogen atoms, through disturbing the orbital coplanarity of the amide bonds that diminishes the conjugation effect Urea bonds bearing a bulky substituent, or HUBs, can reversibly dissociate into isocyanate and amines and show the interesting dynamic property. The fast reversible reactions between HUBs and isocyanates/amines have been the basis in our recent design of self-healing polyureas. 

Because isocyanates can be subject to hydrolysis in aqueous solution to form amines and carbon dioxide, an irreversible process that shifts the equilibrium to favor the HUB dissociation reaction and eventually lead to irreversible and complete degradation of HUBs, we reason that HUBs can be used to design easily available hydrolyzable polymers potentially for the numerous applications above-mentioned. 

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