Synthesis and Characterization of Temperature and Light-Responsive Donor−Acceptor Stenhouse Adduct-Polymer Conjugates

Abstract

Materials with the remarkable ability to undergo spontaneous property changes in response to external stimuli hold great promise in the field of polymer research and hold the key to numerous potential applications. For instance, specific organic chromophores alter their chemical/physical characteristics when exposed to light, making them well-suited for use in polymer systems. This work adopted a photochromic group known as the donor-acceptor Stenhouse adduct (DASA) by effectively incorporating it into different series of polyacrylate and polyacrylamide copolymers. These copolymers were well-selected for their thermo-responsive behavior in aqueous solutions, allowing further investigations into their tunability by light-induced isomerization of the corresponding chromophoric moiety. In a first step, a PMMA-DASA copolymer library was prepared with varying chromophore content between 1 and 11 mol%. Therefore, a free-radical copolymerization involving MMA (methyl methacrylate) and an activated ester acrylate was performed. The grafted chromophore was subsequently generated via two consecutive polymer analogous reactions first with an amine and then with an activated furan precursor. This designed procedure provided access to well-defined stimuli-responsive polymers with high molecular weights and precisely targeted chromophoric content. All copolymers exhibited an upper critical solution temperature (UCST) in ethanol-water mixtures. A notable correlation between the amount of chromophore and the concentration of ethanol (ranging from 65% to 95% by volume) was revealed. When compared to PMMA, the copolymers generally displayed a decrease in UCST up to an ethanol concentration of 80%. Higher concentrations, however, caused an increase in the UCST. By subjecting the polymer solutions to visible light (520 nm) and inducing the photoswitching of the DASA group, the resulting hydrophilic structure caused a decrease of UCST values od all copolymers. After 2 h of irradiation, a sample containing 10.7 mol% dye reached the dUCST maximum of 20 °C. Overall, the UCST of the PMMA-DASA copolymers was hardly affected by the light-induced solubility change. Despite this process being irreversible in ethanol-water mixtures, heating in dichloromethane was proven to revert the samples to their initial UCST prior to light irradiation. The investigation on UCST behavior was extended to polymers showing a lower critical solution temperature (LCST) upon heating. By adopting the synthetic protocol, two PNIPAM and PDEA statistically arranged copolymer series with different amounts of incorporated DASA side groups were successfully prepared. The solubility of all polymers appeared to decrease in aqueous solution compared to the homogenous PNIPAM/PDEA samples. The correlation between the phase transition point and the amount of DASA side group was clearly demonstrated by measuring the LCST values, showing a nearly linear dependency. Following a 4 h duration of irradiation (white light bulb) the copolymer solutions exhibited a shift towards higher LCST. In the case of PNIPAM, the maximum deltaLCST of 11.6 °C was observed for a sample including 11.5 mol% DASA content. The presence of 10 mol% chromophoric group in PDEA resulted in deltaLCST of 9.5 °C. Thus, the thermo-responsive property of the polyacrylamides was significantly influenced by the photoswitching nature of the incorporated side group. Finally, PDEA samples including DASA and azobenzene as second chromophoric side group were synthesized by RAFT polymerization. All polymer solutions underwent a sequence of light irradiation (lambda1 = 570 nm; lambda2 = 365 nm). The procedure proved that it is possible to switch the chromophore orthogonally, resulting in two independent deltaLCSTs to increased values as compared to PDEA. Concluding, the chemical nature of the DASA chromophore has proven to be a highly ef-fective stimuli-responsive group for tuning the thermo-responsive properties of various polyacrylates and polyacrylamides. The functional group have been demonstrated to be very promising for smart materials based on polymers. Noteworthy, the DASA switching behavior is accompanied by a high loss of color intensity. During the practical work on the LCST shifts, the highest polarity was easily recognizable macroscopically in all measure-ments by discoloration. This effect gives DASA also a great potential for several further applications.