Critical Effects of Branch Numbers at the Cross-Link Point on the Relaxation Behaviors of Transesterification Vitrimers

Understanding the Topology Freezing Temperature of Vitrimer-Like Materials through Complementary Structural and Rheological Analyses for Phase-Separated Network

Vitrimers are sustainable cross-linked polymers characterized by an associative bond exchange mechanism within their network. A well-known feature of vitrimers is the Arrhenius dependence of the viscosity or relaxation time. Another important aspect is the existence of a topology-freezing temperature (Tv), which represents a transition between the viscoelastic solid state and the malleable viscoelastic liquid state. Various methods, including viscosity-temperature plots and temperature-ramp creep (or dilatometry), have been proposed for determining the Tv. In this study, we complementarily employ X-ray scattering-based structural analysis and rheological analysis to assign Tv in phase-separated vitrimer-like materials undergoing trans-N-alkylation bond exchange. Note that the trans-N-alkylation progresses via the dissociative bond exchange pathway, whereas our previous studies demonstrated that the temperature-dependence of relaxation time followed the Arrhenius dependence, which was the reason for the classification as a vitrimer-like material. Specifically, we identify Tv as the temperature at which an anomalous increase in domain distance occurs during the rubbery state in the structural analysis. In the rheological analysis, Tv corresponds to the transition temperature marking the shift from the Williams-Landel-Ferry dependence to the Arrhenius dependence in the shift factors used to create master curves for frequency sweep rheology data. Importantly, both methods yield nearly the same Tv, validating the accuracy of the proposed assignment and, thus, providing valuable insights into the specific properties of vitrimers.

Seamless, Self-Transformation of Thermoplastic Polyesters into Vitrimers Through Bond Exchange-Triggered Cross-Linking

In this study, a seamless, self-transformation system of linear thermoplastic polyesters into the sustainable cross-linked polymers, vitrimers, is demonstrated. The key is the use of polyesters bearing abundant hydroxyl side groups, which are synthesized via the reaction using dithiol molecules bearing ester units and diepoxy molecules. The polymerization reaction progresses efficiently at relatively low temperature due to the click nature of the thiol-epoxy reaction, which provides the hydroxyl side groups along the polyester chain. The tin catalyst (stannous octoate) is added in the initial polymerization, and the catalyst also works to cross-link the polyesters via intermolecular transesterification bond exchange simply by heating at high temperatures. By adjusting the degrees of cross-linking, the mechanical properties as well as the thermal properties are well tuned. The bond exchange can still be activated in the final cross-linked sample; and thus, the material behaves as vitrimers, exhibiting mechanical recyclability. The application of a new type of hot melt adhesive, where the post-coating tuning/enhancement of adhesion strength is realized, is also demonstrated. On the whole, the present system is very simple but proposes a new application window of bond exchange concept into self-transformation polymers.

Trapping bond exchange phenomenon revealed for off-stoichiometry cross-linking of phase-separated vitrimer-like materials

Vitrimer materials combined with nano-phase separated structures have attracted attention, expanding the tuning range of physical properties, such as flow and creep properties. We recently demonstrated a preparation of vitrimer-like materials with phase-separated nanodomains in which dissociative bond exchange via trans-N-alkylation of quaternized pyridine was operated. In this study, we demonstrate a new finding about the bond exchange mechanism: that is, the trapping bond exchange phenomenon. The component polymer is a poly(acrylate) containing pyridine side groups randomly along the chain, which is cross-linked by diiodo molecules via pyridine-iodo quaternization, where the quaternized pyridines are aggregated to form nano-size domains. When the cross-linking reaction is performed at an off-stoichiometric pyridine : iodo ratio (i.e., an excess of pyridine groups), free pyridine groups are located in the matrix phase. Since the bond exchange in the present system progresses in an inter-domain manner, the dissociated unit bearing pendant iodo is trapped by the free pyridine groups in the matrix, which generates other small aggregates. This trapping phenomenon greatly affects the relaxation and creep properties, which are very different from those found in conventional knowledge about vitrimer physics.

Polymer Networks with Cubic, Mixed Pd(II) and Pt(II) M6L12 Metal-Organic Cage Junctions: Synthesis and Stress Relaxation Behavior

Metal-organic cages/polyhedra (MOCs) are versatile building blocks for advanced polymer networks with properties that synergistically blend those of traditional polymers and crystalline frameworks. Nevertheless, constructing polyMOCs from very stable Pt(II)-based MOCs or mixtures of metal ions such as Pd(II) and Pt(II) has not, to our knowledge, been demonstrated, nor has exploration of how the dynamics of metal-ligand exchange at the MOC level may impact bulk polyMOC energy dissipation. Here, we introduce a new class of polymer metal-organic cage (polyMOC) gels featuring polyethylene glycol (PEG) strands of varied length cross-linked through bis-pyridyl-carbazole-based M6L12 cubes, where M is Pd(II), Pt(II), or mixtures thereof. We show that, while polyMOCs with varied Pd(II) content have similar network structures, their average stress-relaxation rates are tunable over 3 orders of magnitude due to differences in Pd(II)- and Pt(II)-ligand exchange rates at the M6L12 junction level. Moreover, mixed-metal polyMOCs display relaxation times indicative of intrajunction cooperative interactions, which stands in contrast to previous materials based on point metal junctions. Altogether, this work (1) introduces a novel MOC architecture for polyMOC design, (2) shows that polyMOCs can be prepared from mixtures of Pd(II)/Pt(II), and (3) demonstrates that polyMOCs display unique relaxation behavior due to their multivalent junctions, offering a strategy for controlling polyMOC properties independently of their polymer components.

Postpolymerization Modification by Nucleophilic Addition to Styrenic Carbodiimides

Carbodiimides are electrophilic functional groups that react with select nucleophiles under mild conditions. However, their potential as platforms for postpolymerization modification has been relatively underexplored. We describe the synthesis and radical polymerization of a styrenic carbodiimide which undergoes rapid nucleophilic addition with primary and secondary alkyl amines under ambient conditions, even in the presence of other protic nucleophiles. The monomer is amenable to both free and controlled radical (co)polymerization, and we further demonstrate the utility of this approach by preparing covalent adaptable networks through guanylation of the styrenic carbodiimide with difunctional amines. These materials exhibit a variation in relaxation times according to both the guanidine structure and concentration, providing a facile means for tuning dynamic behavior.