Effect of heterocyclic substituents on the thermo/pH/CO2-responsiveness of multistimuli-responsive homopolymers

Novel Temperature/pH/CO2/Redox-Quadruple-Responsive Ferrocene-Containing Homopolymers and Their Self-Assembly Behavior

“…Due to the hydrophobicity of Fc, these polymers are insoluble in neutral water, but when acid is added, the protonation of amino groups can increase the hydrophilicity of the polymers; thus, the polymers gradually become soluble. Deprotonation of the amino group by adding base can result in phase separation between the polymer and solution, at which point the pH is recorded as the critical pH Figure A shows the pH-dependent transmittance of P­(Fc-N-EOMA) 48 , P­(Fc-N-EO 2 MA) 47 , P­(Fc-N-Et-EO 2 MA) 45 , and P­(Fc-N-MA) 44 , and the obtained critical pH was 4.65, 5.5, and 5.3, as summarized in Figure B.…”

“…CO 2 can form chemical equilibrium with weak carbonic acid in aqueous solution, which is a kind of unique trigger with nontoxicity, low cost, and biocompatibility. , The pH of a CO 2 -saturated solution is about 3.9, which is below the critical pH of P­(Fc-N-EO 2 MA) 47 (critical pH = 5.7, C = 0.5 wt %). Therefore, by bubbling CO 2 into the turbid solution of P­(Fc-N-EO 2 MA) 47 (pH = 6.0 >critical pH), on the one hand, the pH of the solution is reduced, and on the other hand, CO 2 can form into ammonium bicarbonate with the amine group, − which greatly improves the hydrophilicity of the polymer, so P­(Fc-N-EO 2 MA) 47 becomes soluble, as shown in Figure , the transmittance of the solution increases from 0.5 to 94% immediately (<30 s), and the pH of the solution is 5.5, which is below the critical pH. Under the same conditions, with D 2 O as the solvent, 1 H NMR showed that the chemical shift of amine group moved to low field (3.42 and 2.92 ppm) compared with that of the common amine group (around 2.50 ppm), which proved the formation of quaternary ammonium (Figure S9).…”

“…Subsequently, CO 2 is expelled by pumping with N 2 into the solution, it is found that the solution becomes turbid again (pH = 5.9 >critical pH) after bubbling with N 2 for 5 min at 50 °C. One can see that the removal of CO 2 is relatively slow, mainly because of the interaction force between carbonic acid and amine groups, and it becomes slower with the number of cycles . During this cycle, the D h and zeta potential of the solution also change.…”

“…The most typical representative of them is poly­( N , N -dimethylaminoethyl methacrylate) (PDMAEMA), which has a triple stimuli response of temperature/pH/CO 2 . Inspired by this homopolymer, some novel multistimuli-responsive homopolymers have been prepared. − However, compared with multistimuli-responsive copolymers, they are few in number and types, and most of types are temperature/pH/CO 2 . − This is because small adjustments in molecular structure of a homopolymer can lead to large changes of property. Some responsive groups, such as ferrocene (Fc), are relatively hydrophobic, and a large number of hydrophilic groups need to be introduced to maintain the balance of macromolecule, which increases the difficulty of design and the complexity of molecule.…”

“…Between them, copolymers have been studied and applied more because of their simple design principle and wide application scope. − Their types are very rich, for example, temperature/pH, − temperature/pH/redox, , O 2 /CO 2 , temperature/pH/light, , temperature/pH/oxidation/light, , and temperature/pH/CO 2 /oxidation. , However, this kind of copolymers usually has some shortcomings: (1) the synthesis or purification steps of copolymers are usually complicated, and the molecular controllability is poor; (2) the response range of a copolymer is generally broad, and the sensitivity is relatively low; (3) due to the incompatibility of each block, it is difficult to systematically study the relationship between different responsiveness and its structure. However, multistimuli-responsive homopolymers, prepared by precisely setting the spatial arrangement of different responsive groups in the monomer first and then polymerization, can greatly alleviate the abovementioned shortcomings. − …”