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

• Published in: Polymer chemistry 2014-01, Vol.5 (11), p.3624-3633
• Main Authors: Roy, Saswati Ghosh, Bauri, Kamal, Pal, Sunirmal, De, Priyadarsi
• Format: Article
• Language: English
• Subjects: Aldehydes; Amino acids; Copolymers; Covalence; Crosslinking; Dynamics; Gels; Sol gel process
• ISSN: 1759-9954; 1759-9962
• DOI: 10.1039/c3py01691a

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 (C&z.dbd; N) 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 (MEO sub(2)MA) 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 MEO sub(2)MA/DMAEMA segments ( similar to 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 C&z.dbd; N bonds in the network. Rheological measurements have been conducted with various ratios of CHO/NH sub(2) 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.