Tryptophan containing covalently cross-linked polymeric gels with fluorescence and pH-induced reversible sol–gel transition properties

Abstract

pH-reversible covalently cross-linked polymeric gels from a side-chain polymer with multi-amino groups and a difunctional aldehyde cross-linker have been prepared by employing dynamic covalent chemistry in the form of reversible imine (CN) bond formation. Two different series of dual pH- and thermo-responsive amino acid based copolymers with primary amine groups located at different positions and various chemical compositions are synthesized by reversible addition fragmentation chain transfer (RAFT) copolymerization of tert-butyl carbamate (Boc)-tryptophan methacryloyloxyethyl ester (Boc-Trp-HEMA) with 2-(2-methoxyethoxy)ethyl methacrylate (MEO₂MA) and N,N-(dimethylamino)ethyl methacrylate (DMAEMA) respectively, followed by Boc-group expulsion. The pH-induced thermoresponsive properties of the copolymers have been observed with the high content of thermoresponsive MEO₂MA/DMAEMA segments (∼80%) in the copolymer. The tryptophanyl moiety present in the copolymer chain provided florescence characteristics, which were studied by florescence spectroscopy. Copolymers with free amino groups were employed for cross-linking with a difunctionalized aldehyde cross-linker, 4-formylphenyl 4′-formylbenzoate (FPFB), to form dynamic covalent gels through the formation of CN bonds in the network. Rheological measurements have been conducted with various ratios of CHO/NH₂ from 1 to 0.33 for homopolymer gels and copolymer gels with different amino acid contents (20–80%), which showed a plateau of storage modulus (G′) over a wide range of angular frequencies indicating the independent nature of G′ with respect to angular frequency, and the mechanical properties of dynamic gels are a function of cross-linking density. The pH-dependent reversible sol–gel transition has also been investigated.