Counterion Sensitive Behavior In RAFT Polymerization Of Histamine-based Ionic Monomers In Dilute Aqueous Solution At Or Below 25℃

As a natural ionic amino acid, histidine is the only one whose imidazole residue can serve either as an acid or as a base in physiological p H range, which exhibit broad bioactivities in proteins, i.e., enzymatic catalyses. Recent reports demonstrated crucial roles of imidazole/ imidazolium residues for biological and other advanced materials. Accordingly, almost all reversible deactivation radical polymerization(RDRP) were explored for the controlled synthesis of imidazole-/imidazolium-polymers in organic solvents. However, a majority of imidazoliums containing hydrophilic halogen ions are insoluble in organic solvents. Although many studies were involved in RDRP synthesis of polyelectrolyte in water, the counter-ion effects on reaction kinetics is still unrevealed. Therefore, exploring effective approach for the controlled synthesis of polyelectrolyte in water is increasingly imperative, and the effect of counter-ion on the polymerization waits to be comprehended. This paper describes the counter-anion effect during RAFT polymerization of ionic monomers in dilute aqueous solutions at or below room temperature, and there is some progress as follows:1. Histamine-based monomer is synthesized, and the polymerization is studied. First the ionic-based monomers that contain counter-anions of Cl-, BF4-and Tf O- are synthesized, and the polymerization reactions are commenced on the irradiation with weak visible light. The polymerization rates followed the sequence: HA-Tf OH > HA-HBF4> HA-HCl, while chain growth exhibited counter-ion sensitive stepwise linear kinetics. GPC measurement, UV-Visible Spectra measurement and iterative reaction indicate that stepwise linear kinetics is not induced by the loss of CTA chain-ends. The conductivity measurement of monomers and resultant reaction solutions suggests that residual monomer concentration when stepwise linear kinetics occurred is approximately equal to the critical monomer concentration in molar conductivity measurement, so the stepwise linear kinetics are induced by the dissociation of ion pairing. These studies indicate that the aqueous RAFT Polymerization of ionic monomers is sensitive to the counter-ions. Moreover, the counter-anion sensitive kinetics was elucidated on varying the HA-HCl concentrations. The results show that the conversion increased with adding monomer concentrations, and stepwise linear kinetics was obvious in [HA-HCl]0≤1.1 M; GPC measurement, UV-Visible Spectra measurement and conductivity measurement indicate that stepwise linear kinetics are induced by the dissociation of ion pairing. The conductivity measurement and the polymerization with different monomer concentrations indicate that the aqueous RAFT Polymerization of ionic monomers is sensitive to the counter-ions, and stepwise linear kinetics are induced by the dissociation of ion pairing.2. The influence of changing solution media on polymerization is studied. Based on the above work, Counter-anion effects on reaction kinetics are studied via changing solution media such as addition of non-ionic organic solvents. Indeed, in dilute solutions at a [HA-HCl]0=0.6 M, the propagation is faster in 30:70 methanol/water(e.g. 93% conversion in 95 min) than in 10:90 methanol/water(e.g. 75% conversion in 100 min). GPC measurement indicates that stepwise linear kinetics are not induced by the loss of CTA chain-ends. While the conductivity measurement indicates that ion association is enhanced with adding methanol, leading to the increase of polymerization. We also add strong electrolytes to change solution media, and propagation is faster in solution with higher Na Cl concentration. There was report that adding Na Cl enhanced the ionic strength of the ionic monomer and screened charge around ionic propagation polymer, leading to faster propagation in sodium acrylate solution with higher Na Cl concentration. These studies indicate that the polymerization can be regulated via adjusting the aqueous solution media, and the aqueous RAFT Polymerization of ionic monomers is sensitive to the counter-ions.3. Strikingly, negative regulation occurred in dilute aqueous solution. The influence of temperature on polymerization is studied via changing solution temperature. The propagation is significantly speeded upon cooling and the initialization period is prolonged. Conductivity measurement of monomers and resultant reaction solutions suggests that the ion-association of the ionic monomer and ionic polymer is enhanced with the decreasing of solution temperature, leading to the increase of polymerization. These studies indicate that the aqueous RAFT Polymerization of ionic monomers is sensitive to the counter-ions.4. The sensitive behavior of RAFT polymerization of ionic monomers in dilute aqueous solution is obvious. These findings shed new insight into the controlled radical polymerization of ionic monomers toward fast synthesis of a structure-tunable polyelectrolyte.In summary, the aqueous RAFT Polymerization of ionic monomers is sensitive to the counter-ions, and the polymerization can be adjusted by changing the associate degree of ion-association of ionic monomer. These findings shed new insight into protein in situ conjugation in dilute aqueous solution at or below room temperature.