Synthesis of Bipyridyl-Branched Polyoxazoline and Its Gelation by Means of Metal Coordination

Abstract

ABSTRACT:Polyoxazoline hydrogel was prepared through the coordination of the metal ions to 2,2'-bipyridyl-branched poly(N-acetylethylenimine) (PAET), which was synthesized from the partially hydrolyzed PAE! by the reaction with 3-{4-(4'-methyl-2,2'-bipyridyl)}propanoic acid in the presence of dicyclohexylcarbodiimide. Concentrated solution of 2,2'-bipyridyl-branched PAEI gave polyoxazoline gel in a good yield by the treatment with metal salts such as iron(II) sulfate, or ruthenium(III) trichloride. The resulting colored gel was highly swollen in water and stable at ambient temperature for handling. In the case of nickel(II) or cobalt(II) ions, the formed intermolecular cross-linking points via the coordination (gel state) were labile due to the rapid ligand exchange reaction of the cross-linking from intermolecular to intramolecular manner. The nickel(II) and cobalt(II) gels became soluble in a large amount of water within a few hours. A series of PAEis having varying amounts of the functional groups were prepared and subjected to the cross-linking reaction by the coordination with iron(II). The swelling degree and the stability of the gel in water depended on the content of the functional groups in the prepolymers. Water uptake was up to 56 multiples of its own weight in dry state.KEY WORDS Ring-Opening Polymerization / Polyoxazoline / Bipyridyl / Metal Coordination/ Cross-Linking/ Hydrogel / Water Swelling Property/ Cross-linking through the interaction between metal ions and polymeric ligands has been of interest for the preparation of networks. Most of these investigations focused on their physical properties in the concentrated solution. The complex formation of the metal ion with the polymer bearing the ligand groups was first claimed by Kuhn and his coworkers. 1 They discussed on the network formation from poly(vinyl alcohol) fiber by the treatment with Cu(II) salt with respect to the rheological measurement.Polymers containing carboxylic acids or hydroxyl groups such as poly(acrylic acid),2 poly{bis( carboxylatophenoxy)phosphazene}, 3 polysaccharide, 4 -6 and cellulose derivatives 7 are known to form stable hydrogels by the treatment with metal salts under the specific conditions (pH, concentration, ionic strength, etc.). A large number of ions (Ca(Il), 3 Cu(II), 2 • 3 Al(III), 3 • 7 Cr(III), 8 Ti(III), 6 • 7 and borate 4 · 5 ) are used as a cross-linking agent. These gels have been used widely in a field of industrial chemistry. However, the mechanism of the cross-linking has not been fully established. The stability and the physical properties often depend on their specific conditions for their preparations. In these systems, it is difficult to control the swelling properties and to reveal the nature of the

Poly(2‐oxazoline) Hydrogels: State‐of‐the‐Art and Emerging Applications

“…One of the most prolific periods of research on innovative cross-linking chemistry for PAOx was in the late 1980s and early 1990s single-handedly by Chujo et al who published at least a dozen articles [21][22][23][24][25][26][27][28][29][30][31][32] revealing new ways of forming crosslinked networks based on PAOx. Their work has been reviewed before (e.g., in the review articles mentioned earlier [18,[45][46][47] ) and so will not be discussed here other than to mention that their work was almost exclusively focused on the synthesis of the poly mers and cross-linking chemistry and the stability of the gels without investigation of potential applications (at least in the open literature).…”

“…Perhaps the earliest example of a PAOx hydrogel was from Litt who presented at a 2012 symposium on PAOx his early work in the 1960s on using thiol‐ene photochemistry to cross‐link functional PAOx, although there is no peer reviewed account of this work. Later in the 1980s and early 1990s, Chujo et al published a large body of innovative work on PAOx hydrogels based on covalent, dynamic covalent, and supramolecular cross‐linking . These reports appeared, however, before the field of biomaterials became widespread and so their materials were not used with biological systems.…”

“…Later in the 1980s and early 1990s, Chujo et al published a large body of innovative work on PAOx hydrogels based on covalent, dynamic covalent, and supramolecular cross-linking. [21][22][23][24][25][26][27][28][29][30][31][32] These reports appeared, however, before the field of biomaterials became widespread and so their materials were not used with biological systems.…”

Strategies for the Synthesis of Poly(2‐Oxazoline)‐Based Hydrogels

“…Saegusa and co‐workers32 reacted statistical pMeOx ‐ co ‐ pEI with 3‐[4‐(4′‐methyl‐2‐2′‐bipyridyl)]propanoic acid to form bipyridyl‐bearing copoly(2‐oxazoline)s (Scheme ). A concentrated solution of these polymers formed stable hydrogels through the coordination of metals to the bipyridyl pendant groups after the addition of metal salts such as FeSO 4 and RuCl 3 .…”

Reversible polypeptide hydrogels from asymmetric telechelics with temperature-dependent and Ni2+-dependent connectors

“…For these latter systems, network formation can be influenced along different kinetic pathways, depending on the order in which the blocks A and B are triggered to assemble or disassemble, and short loops are excluded due to the orthogonality. [16][17][18][19] An alternative that is gaining popularity is to construct triblock copolymers exclusively from (natural) amino acids, and to use a biosynthetic approach, exploiting the genetic code to obtain the desired macromolecule. 11,12 An orthogonal pair is conveniently obtained, e.g., when A has temperature-dependent solubility, while B undergoes electrostatically driven binding, i.e., an interaction involving ions.…”