Addition of bulky substituents weakens urea bond
Addition of bulky substituents weakens urea bond

Scientists have developed a cheap hydrolyzable polymer that can be designed to degrade over time, and which could offer a viable alternative to those used in a range of biomedical applications, such as in the design of drug delivery systems, tissue engineering, surgical sutures and transient electronics, and in degradable/compostable packaging materials, coatings and adhesive materials.

The researchers, from the University of Illinois at Urbana-Champaign in the US, showed how it was possible to reverse the characteristics of polyurea, a key bonding material, developing a class of hindered urea bond (HUB) containing polymeric materials – or poly(hindered urea)s (PHUs). As reported in the Journal of the American Chemical Society [Ying, H. and Cheng, J., J. Am. Chem. Soc. (2014) DOI: 10.1021/ja5093437], this urea bond is very inert, so the polymer is extremely stable and so can be used in long-lasting applications.

The new PHUs have significant benefits over other hydrolyzable polymers, especially as they can be created with cheap chemical precursors in ambient conditions using simple and clean chemistry with no catalyst or by-products. This allows for the control of the copolymer recipe for particular needs without complex synthesizing. As researcher Jianjun Cheng said, “PHUs can be completely hydrolyzed within a few days. Since ‘hindrance’ is the cause of the bond destabilization, the hydrolysis kinetics of PHUs can be easily tuned as needed for a specific application.”

Polyurea typically contain ester and other hydrolyzable bonds in their backbone structures. Here, the team demonstrated the potential of PHUs for the design of water degradable polymeric materials that can be easily synthesized by mixing multifunctional bulky amines and isocyanates. They previously found that urea bonds with bulky substituents can form reversible equilibrium with isocyanate and amine under ambient conditions. As water can react with isocyanate, they figured that it could shift the chemical equilibrium and degrade the urea bond, leading them to explore the hydrolysis behaviors of hindered polyurea.

The findings demonstrate these highly inert materials could become dynamic and degradable with simple structure modification, while for biomaterials it offers a new type of polymers that are an improvement over existing ones in terms of cost, facile synthesis and high kinetic tunability. However, it is important to gain a better understanding of the HUB hydrolysis behaviors, and the researchers hope to investigate changes of hydrolysis kinetics under various environmental conditions, as well as further applications in biomaterials and packaging.