Alkyl borane-mediated metal-free ring-opening (co)polymerizations of oxygenated monomers

Sensibility to environmental concerns and the actual demand for polymeric materials free of any metal contaminants in most applications have directed research towards significant breakthroughs in organocatalytic polymerizations. The overarching challenge is to develop new and efficient organocatalysts for extending the scope and to improve the performance of organocatalytic polymerizations. Since 2016 commercially available alkyl boranes, especially triethyl borane (TEB), have been discovered as exceptional Lewis acids that served to generate ate complexes by combination with chain ends on the one hand and to activate epoxides on the other. This double role of boranes has received widespread attention especially in oxygenated polymer synthesis. Lewis pairs consisting of alkyl boranes combined with an onium salt or organic base has indeed demonstrated unprecedented versatility for (co)polymerizations of oxygenated monomers such as epoxides, oxetanes, cyclic esters and with CO2, COS, isocyanates, or cyclic anhydrides, producing a variety of oxygenated polymers. In this review, we take TEB-mediated polymerization systems as the main line of emerging area, summarize the progress comprehensively made to promote the rapid development of organocatalytic polymerizations of oxygenated polymers by these systems, and propose key challenges in organocatalytic synthesis in the future.