Synthesis of New Thermoresponsive Polymers Possessing the Dense1,2,3-Triazole Backbone

Thermoresponsive water-soluble polymers, aqueous solutions of which undergo lower critical solution temperature(LCST)-type phase separation, have been investigated in detail forseveral decades. To develop LCST-type thermoresponsivepolymers with new polymer backbone, 4-azido-5-hexynamide(AHA) derivatives were designed as monomers for copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) polymerization.AHA derivatives carrying secondary amide side chains, that is, 4-azido-N-methyl-5-hexynamide (M), 4-azido-N-ethyl-5-hexynamide(E), and 4-azido-N-isopropyl-5-hexynamide (iP), werefirstsynthesized and polymerized by CuAAC to obtain polymers(poly(M), poly(E), and poly(iP)). Contrary to our expectation,poly(M), poly(E), and poly(iP) were insoluble in water and many organic solvents presumably because of the formation ofhydrogen bonding between the amide side chains or between the amide side chains and triazole residues in the backbone. Thus,AHA derivatives carrying tertiary amide side chains, that is, 4-azido-N,N-dimethyl-5-hexynamide (MM), 4-azido-N-ethyl-N-methyl-5-hexynamide (ME), 4-azido-N-isopropyl-N-methyl-5-hexynamide (MiP), and 4-azido-N,N-diethyl-5-hexynamide (EE), were alsosynthesized and polymerized to yield polymers (poly(MM), poly(ME), poly(MiP), and poly(EE)). These polymers were soluble ina number of common organic solvents. It is noteworthy that poly(MM) and poly(ME) were also soluble in water. The phaseseparation behavior of 1.0 wt % aqueous solutions of poly(MM) and poly(ME) was then investigated by transmittancemeasurements. These data indicated that poly(ME) was an LCST-type thermoresponsive polymer, whereas poly(MM) was not. Alarge hysteresis was observed in the transmittance measurements for the poly(ME) aqueous solution because of slow rehydrationafter phase separation. The phase separation behavior was investigated preliminarily by differential scanning calorimetry and1HNMR