We present a study of the structure of Tetra-PEG model networks, using proton multiple-quantum NMR at low field in combination with computer simulations. Tetra-PEG is a novel high-performance hydrogel designed by combination of two symmetric tetra-arm macromonomers. In contrast to conventional hydrogels, which are highly heterogeneous due to fixed concentration fluctuations, Tetra-PEG exhibits a much less heterogeneous microstructure as indicated by previous light and small-angle neutron scattering studies. Here, the local-scale connectivity inhomogeneities, i.e., the sol and dangling polymer chains as well as the typical network connectivity defects resulting from the AÀB reaction of four-arm macromonomers, are quantified experimentally for the first time, studying as-prepared Tetra-PEG hydrogels, cross-linked at different polymer concentrations and stoichiometries. To this end, we developed a novel approach for the analysis of double-quantum buildup curves consisting of well-distinguishable components with different segmental dynamic order parameters, benefitting from the superb large-scale homogeneity of the samples. As a model for each component we suggest different connectivity modes between the macromonomers-mainly regular single links and double links between individual stars as well as other network defects with lower order parameters. To support the model, we report results from computer simulations with the bond-fluctuation model, which confirm the concentrationdependent trends of the network and double-link fractions.

After decades of efforts by many researchers, we have succeeded in realizing a near-ideal polymer network. This network, the Tetra network, is made by cross-end-coupling of tetra-arm polymer modules. The mechanical energy dissipation was extremely low (tan δ ≈ 10(-4) ). The macroscopic stress-strain relationship of the Tetra network was in good agreement with that of microscopic elastic blobs. The maximum breaking strength was extremely high (≥27 MPa). These results indicate that the Tetra network is closer to an ideal polymer network than any other conventional model networks. Because the Tetra network can be treated as uniformly packed elastic blobs, it should help apply the knowledge of single polymer chains seamlessly to the design of polymer materials and help further develop the theory of rubber elasticity.

Polymer gels formed from A-B type of coupling reaction were investigated vigorously as a "model network" because the mesh size of the network was defined by the size and shape of the building blocks. However, the formation of these networks has an inherent difficulty in homogeneous mixing of mutually reactive building blocks, leading to the formation of inhomogeneous network structures. Recently, we have designed and fabricated the Tetra-PEG gel by combining two well-defined symmetrical tetra-arm polymers of the same size. Although this gel was formed by simply mixing two polymer solutions, the structure was extremely homogeneous. In this study, we investigated the gelation kinetics of Tetra-PEG gel in detail. The reaction kinetics corresponded well with the theoretical prediction, suggesting the homogeneous reacting system. This homogeneous reacting system may contribute to the homogeneous network structure of Tetra-PEG gel.

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