Design, synthesis and applications of thermosensitive linear poly(ether amide)s with unconventional cluster luminescence and tunable LCST

Abstract

The design of stimuli-responsive polymers with both tunable lower critical solution temperature (LCST) and unconventional cluster luminescence represents a significant challenge in materials science. In this study, thermosensitive linear poly(ether amide)s with cluster fluorescence and LCST tunability were prepared via phosphazene base (t-BuP₂)-catalyzed oxa-Michael addition polymerization of N,N′-methylene-bis(acrylamide) (MBA) with diols containing different numbers of carbon atoms. The fluorescence properties and LCST tunable behavior of the linear poly(ether amide)s were investigated by ultraviolet-visible (UV-Vis) and fluorescence spectroscopy. The fluorescence results demonstrate that the resulting linear poly(ether amide)s exhibit aggregation-induced enhanced emission (AEE) and excitation-dependent emission properties due to cluster fluorescence, which is formed by the aggregation of nonconjugated chromophores, such as amide, ether, and hydroxyl groups. It is noteworthy that the fluorescent polymers display temperature-responsive and LCST tunable characteristics, which can be attributed to the interactions between the hydrophobic carbon chains and the hydrophilic ether, amide, and hydroxyl groups present in the linear polyether amides. Moreover, linear poly(ether amide)s display distinctive LCST-dependent fluorescence characteristics, exhibiting a pronounced shift in fluorescence intensity with temperature that closely aligns with their cloud point temperature (T_cp) values. The intrinsic fluorescence and temperature response of linear poly(ether amide)s were exploited to demonstrate their potential as a versatile tool for security tamper-evident labels, physiological cell imaging, and traceable and controlled release of doxorubicin. This study provides an LCST-tunable strategy for linear nonconjugated fluorescent polymers, which enriches and develops the synthesis, mechanism and application studies of stimuli-responsive nonconjugated fluorescent polymers.