A New Associative Diblock Copolymer of Poly(ethylene glycol) and Dense 1,2,3‐Triazole Blocks: Self‐Association Behavior and Thermoresponsiveness in Water

Interaction of Cyclodextrins with Amphiphilic Alternating Cooligomers Possessing the Dense Triazole Backbone

The interaction of cyclodextrins (CDs) with structure-controlled polymers is expected to provide significant insights into macromolecular recognition. However, the interaction of CDs with structure-controlled polymers has been an underexamined issue of investigation. Herein, alternating amphiphilic cooligomers (oligoCnAH, where n denotes the carbon number of alkyl groups; n = 4, 8, and 12) were synthesized by copper(I)-catalyzed azide-alkyne cycloaddition polymerization of heterodimers of 4-azido-5-hexynoic acid (AH) derivatives carrying N-alkylamide and t-butyl (tBu) ester side chains, followed by hydrolysis of the tBu ester, to study the interaction of CDs with oligoCnAH by 1H NMR, nuclear Overhauser effect spectroscopy, and pulse-field-gradient spin-echo NMR. These NMR studies indicated that αCD interacted with oligoC4AH, αCD and βCD interacted with oligoC8AH, and all CDs interacted with oligoC12AH. Based on the equilibrium models proposed, the binding constants were evaluated for the binary mixtures, which showed interaction. Comparing the interactions of the CDs/oligoC12AH binary mixtures with those of the binary mixtures of CDs and alternating copolymers of sodium maleate and dodecyl vinyl ether (polyC12M), it is concluded that oligoC12AH forms less stable micelles than does polyC12M presumably because of the lower molecular weight, the hydrophilic amide groups in the side chain, and the longer interval between neighboring C12 groups in oligoC12AH.

Synthesis of Dense 1,2,3-Triazole Oligomers Consisting Preferentially of 1,5-Disubstituted Units via Ruthenium(II)-Catalyzed Azide-Alkyne Cycloaddition

Ruthenium(II)-catalyzed azide-alkyne cycloaddition (RuAAC) polymerization of t-butyl 4-azido-5-hexynoate (tBuAH), i.e., a heterobifunctional monomer carrying azide and alkyne moieties, was investigated in this study. RuAAC of the monofunctional precursors of tBuAH yielded a dimer possessing a 1,5-disubstituted 1,2,3-triazole moiety. ¹H NMR data showed that the dimer was a mixture of diastereomers. Polymerization of tBuAH using ruthenium(II) (Ru(II)) catalysts produced oligomers of M_w ≈ (2.7-3.6) × 10³ consisting of 1,5-disubstituted 1,2,3-triazole units (1,5-units) as well as 1,4-disubstituted 1,2,3-triazole units (1,4-units). The fractions of 1,5-unit (f_1,5) were roughly estimated to be ca. 0.8 by comparison of signals of the methine and triazole protons in ¹H NMR spectra, indicating that RuAAC proceeded preferentially and thermal Huisgen cycloaddition (HC) somehow took place during the polymerization. The oligomer samples obtained were also characterized by solubility test, size exclusion chromatography (SEC), ultraviolet-visible (UV-Vis) absorption spectroscopy, and thermogravimetric analysis (TGA). The UV-Vis and TGA data indicated that the oligomer samples contained a substantial amount of Ru(II) catalysts. To the best of our knowledge, this is the first report on dense 1,2,3-triazole oligomers consisting of 1,5-units linked via a carbon atom.

Synthesis of New Thermoresponsive Polymers Possessing the Dense 1,2,3-Triazole Backbone

Thermoresponsive water-soluble polymers, aqueous solutions of which undergo lower critical solution temperature (LCST)-type phase separation, have been investigated in detail for several decades. To develop LCST-type thermoresponsive polymers 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), were first synthesized 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 of hydrogen 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 also synthesized and polymerized to yield polymers (poly(MM), poly(ME), poly(MiP), and poly(EE)). These polymers were soluble in a number of common organic solvents. It is noteworthy that poly(MM) and poly(ME) were also soluble in water. The phase separation behavior of 1.0 wt % aqueous solutions of poly(MM) and poly(ME) was then investigated by transmittance measurements. These data indicated that poly(ME) was an LCST-type thermoresponsive polymer, whereas poly(MM) was not. A large hysteresis was observed in the transmittance measurements for the poly(ME) aqueous solution because of slow rehydration after phase separation. The phase separation behavior was investigated preliminarily by differential scanning calorimetry and ¹H NMR.

Synthesis of a New Polyanion Possessing Dense 1,2,3-Triazole Backbone

Polyanions are an important class of water-soluble polymers because polyanions are utilized in a wide range of industrial fields. It is thus a great challenge to develop polyanions with novel structures to make their applications broader. In this study, a new polyanion with a dense 1,2,3-triazole backbone, poly(4-azido-5-hexanoic acid) (poly(AH)), was synthesized by copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) polymerization of t-butyl 4-azido-5-hexanoate followed by hydrolysis of the t-butyl ester groups. Turbidimetric and potentiometric titration data indicated that poly(AH) was well soluble in water under basic conditions (pH < 7) and a weaker polyanion (apparent pKa = 5.4) than polyacrylic acid (apparent pKa = 4.5). Adsorption tests exhibited that sodium salt of poly(AH) (poly(AH)Na) adsorbed most preferably Fe3+ among the four metal ions examined, i.e., Cu2+, Pb2+, Li+, and Fe3+. 1H spin-lattice relaxation time measurements indicated that Fe3+ ions were adsorbed favorably onto the 1,2,3-triazole residues.

Synthesis of Dense 1,2,3-Triazole Polymers Soluble in Common Organic Solvents

Aiming at synthesis of dense 1,2,3-triazole polymers soluble in common organic solvents, a new 3-azido-1-propyne derivative, i.e., t-butyl 4-azido-5-hexynoate (tBuAH), was synthesized and polymerized by copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) and Huisgen cycloaddition (HC). CuAAC polymerization produced poly(tBuAH) composed of 1,4-disubstituted 1,2,3-triazole units (1,4-units), whereas HC polymerization gave poly(tBuAH) composed of 1,4- and 1,5-disubstituted 1,2,3-triazole units (1,4- and 1,5-units). In HC polymerization, the fraction of 1,4-unit (f_1,4) decreased with the permittivity of solvent used. Differential scanning calorimetry data indicated that the melting point of poly(tBuAH) increased from 61 to 89 °C with increasing f_1,4 from 0.38 to 1.0, indicative of higher crystallinity of poly(tBuAH) composed of 1,4-unit. Preliminary steady-state fluorescence study indicated that all the poly(tBuAH) samples of different f_1,4 emitted weak but significant fluorescence in DMF. The maximum of fluorescence band shifted from ca. 350 to ca. 450 nm with varying the excitation wavelength from 300 to 400 nm.