Synthesis of thermoresponsive nonconjugated fluorescent branched poly(ether amide)s via oxa-Michael addition polymerization

With the aim of providing a straightforward method for the synthesis of stimuli-responsive nonconjugated fluorescent polymers suitable for biological applications, this study describes the one-pot synthesis of novel thermoresponsive nonconjugated fluorescent branched poly(ether amide)s via the oxa-Michael addition polymerization of N,N′-methylenebis(acrylamide) with triols, i.e., trimethylolpropane (TMP) and trimethylolethane (TME), using the phosphazene base t-BuP2 as a catalyst at room temperature. The structure of the obtained branched poly(ether amide)s was confirmed by performing nuclear magnetic resonance and Fourier transform infrared spectroscopy analyses. The temperature-dependent phase transition behavior of the branched poly(ether amide)s was investigated by ultraviolet–visible spectroscopy, and the results indicated that these polymers had a lower critical solution temperature (LCST) ranging from 18.6°C to 55.2 °C, which could be tuned by changing the polymer concentration or the feed ratio of TMP to TME. The fluorescence analysis results showed that the emission intensities of the branched poly(ether amide)s increased with increasing ratio of TME to TMP, and the aggregation of the polymers induced emission features due to the hydrogen bonding interactions of the unconventional amide chromophore. Moreover, in the temperature range of 15 to 60 °C, the nonconjugated fluorescent branched poly(ether amide)s with LCST exhibited LCST-dependent fluorescent behavior, i.e., the temperature at which the fluorescence intensity and position changed was close to the LCST. Most importantly, the thermoresponsive nonconjugated fluorescent branched poly(ether amide)s showed low cell toxicity and bright cell imaging, which are suitable for biological applications.