Chelation-assisted CuAAC in star-shaped polymers enables fast self-healing at low temperatures

Abstract

The achievement of self-healing (SH) under ambient conditions (low temperature, no external input of energy) still presents a significant area of research, and is strongly linked to fast and efficient cross-linking reactions. We investigate here fast cross-linking reactions of star-shaped polymers containing copper chelating moieties (picolinazide) at the end of each arm, able to promote a “click”-reaction through chelation of the Cu(I)-catalyst and thus strongly increasing cross-linking rates. The synthetic preparation and cross-linking kinetics of a low molecular weight model system (p-carboxylic-acid-methylester-picolinazide and phenylacetylene) were investigated by utilizing different catalysts (CuBr, CuBr(PPh₃)₃, Cu(MeCN)₄PF₆ and CuOAc) and applying in situ NMR experiments. The most efficient catalyst systems (CuBr, CuBr(PPh₃)₃, CuF(PPH₃)₃) were used to monitor the cross-linking of three-arm star polymers bearing the carboxylic acid-methylester-picolinazide moiety on each arm via melt-rheology studies, in turn enabling self-healing. The complete cross-linking of the components can be observed within 71 minutes even at low temperatures (10 °C), thus generating a highly efficient low-temperature SH-system. Self-healing of a polymeric material at room temperature was demonstrated, consisting of a star-shaped picolinazido-telechelic PIB, an encapsulated multivalent alkyne embedded within a high molecular-weight PIB matrix, together with CuBr(PPh₃)₃ and a fluorogenic dye, the latter acting as sensing tool for the proceeding click network formation. A damage-induced increase in the fluorescence intensity due to the click activation of the fluorogenic dye at room-temperature and the formation of a polymer network was thus proven. We envision that this highly enhanced cross-linking speed will facilitate applications of self-healing polymers under low temperature conditions.