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Abstract
The second-generation click chemistry, sulfur-fluoride exchange (SuFEx) reaction was established on the silicon centers can activate the exchange of S-F bonds for S-O bonds. We found phosphorus-fluoride and boron-fluoride bonds are reactive as well. This work first proved boron-fluoride exchange reaction (BorFEx) by a set of small molecule studies. Difluoroborane (R-BF2), generated via in situ from trifluoroborates (R-BF3K), was able to react with bis silyl ethers and form stable cyclic boronic esters. Then we found one particular application for BorFEx in polymer synthesis. Vitrimer is nowadays a hot topic in polymeric research area. Thermosets usually have a better mechanical property and solvent, thermal resistance rather than thermoplastics. Conversely, thermoplastics have the recyclability to be deformed and processed multiple times. We used BorFEx to design a vitrimer with cyclic boronic esters on the backbone and proved it has great mechanical properties and reprocessability as well. Coincidentally, transesterification reaction on dialkyl phosphite was found to be another excellent dynamic covalent exchange reaction catalyzed by organic bases, while we explored the possibility of the phosphorus-fluoride exchange reaction. Herein, we introduce dialkyl phosphite as a dynamic cross-linker in the polymeric network, which performs the exchange reaction with hydroxyl groups on poly(ethylene-co-vinyl alcohol) (EVOH) backbone through nucleophilic substitution transesterification. The vitrimer samples were proved to be thermally stable, and certain blends of EVOH and vitrimer successfully melt-processed in an extruder on a large scale.