Multiresponsive Spiropyran-Based Copolymers Synthesized by Atom Transfer Radical Polymerization

Multiresponsive spiropyran-based random copolymers were synthesized by atom transfer radical polymerization. The polymers were prepared by the copolymerization of 2-(dimethylamino)ethyl methacrylate (DMAEMA) with the photochromic monomer 1′,3′,3′-trimethyl-6-methacryloyloxyspiro(2 H -1-benzopyran-2,2′-indoline) (SP). The solvatochromic, pH, temperature, and light responsive behavior of the PDMAEMA- co -PSP polymers in solution was monitored by UV/vis spectroscopy. The copolymers exhibited “reverse photochromism” and stabilized the planar zwitterionic form of the chromophore, leading to “negative solvatochromism” upon increasing the polarity of the solvent, in contrast to a poly(methyl methacrylate)- co -PSP analogue which stabilized a nonpolar photoisomer and exhibited negligible sensitivity to the polarity of the surrounding medium, before irradiation. Moreover, the PDMAEMA- co -PSP copolymers exhibited a reversible pH-responsive character in aqueous media; the addition of a strong acid induced the SP-to-merocyanine (MC) isomerization, the formation of [MC−OH] + and [SP−NH] + species, and the disappearance of the H-type aggregates, whereas the initial SP moieties were recovered in alkaline media. The chromophore content and the photoinduced MC-to-SP isomerization affected the transition temperature of the PDMAEMA- co -PSP polymers. The lower critical solution temperature (LCST) of the copolymers increased with their content in hydrophilic MC moieties, while the MC-to-SP photoinduced isomerization decreased significantly the LCST due to the hydrophobic character of the SP units. Finally, the copolymers exhibited a first-order photoinduced bleaching of the chromophore units in water and acetonitrile, with a slower decoloration rate in the aqueous medium due to the effective stabilization of the bipolar MC form in the polar environment.