Frustrated Lewis pairs (FLPs) created a buzz in the chemistry community a decade ago when they were introduced as a metal-free catalytic system. A research team in Scotland has now taken the concept into unexplored territory by incorporating FLPs into polymers for the first time. Among other attributes, the reactive FLPs provide a new strategy for forming dynamic cross-linked networks that give polymer gels self-healing properties.
“This is an exciting development and a very clever application of the notion of FLPs to access materials with unique properties and characteristics,” comments Douglas W. Stephan of the University of Toronto, who wasn’t involved in the new work.
Stephan and coworkers introduced the FLP idea in 2006, discovering that when an electron-rich Lewis base attempts to share a spare pair of electrons with an electron-deficient Lewis acid, and when the base and acid are dressed with bulky substituent groups or are separated by a spacer group, their ability to fully neutralize each other and form a strong bond is denied. The pair is said to become “frustrated.”
The unquenched pair creates an energetic mismatch between the lone electron pair on the base and the empty acceptor orbital on the acid. This situation gives the FLP the ability to work cooperatively to bind with and pull apart small molecules such as H2, NO, and CO2, reminiscent of organometallic catalysts. In particular, several research groups have shown that FLPs are useful metal-free catalysts—for example, in hydrogenating organic molecules such as olefins, imines, alkynes, and ketones.
Michael P. Shaver and his group at the University of Edinburgh wondered whether this unique chemistry could enable triggerable responses in polymers. Lewis pairs have been incorporated into polymers before. But Shaver’s team designed frustrated polystyrenes containing bulky phosphine Lewis base side groups or bulky borane Lewis acid side groups. When the two polymers are mixed, the sterically encumbered side groups prevent acid-base interactions and block the soluble polymer chains from cross-linking. The team found that adding diethyl azodicarboxylate as a small, reactive substrate enables the acid-base pairs to interact and rapidly form a polymer gel network (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.7b07725).
The researchers show that the network formation can be tuned by controlling the amount of boron and phosphorus side groups, creating a dynamic gel. For example, when cut in pieces, the solvent-swollen gels can heal the wounds to form a uniform gel again within minutes.
“FLPs represent one of the most interesting new areas of chemistry to emerge in recent years,” says polymer chemist Ian Manners of the University of Bristol. “This work by Shaver and coworkers takes the field in a new direction.” Manners says the design principle might prove useful for capturing environmentally relevant small molecules such as CO2, but the advantages of using FLP interactions over existing strategies for cross-linking and self-healing are not yet clear. “Time will tell,” Manners adds. “It is nevertheless very interesting exploratory work and may represent the first steps in what could become an important new research area.”