Introduction to chemistry for covalent adaptable networks

When Tobolsky coined the term “chemorheology” in 1944 to describe chemical reactions occurring within elongated rubbers to relax stress, this phenomenon was portrayed as theoretically interesting but ultimately destructive from a practical perspective.¹ By 1966, James Craven at DuPont recognized that thermally reversible covalent cross-links could in fact be useful by combining insolubility and remoldability, and envisioned applications such as gaskets, diaphragms, adhesives, coatings, and shoe uppers.² While the dynamic bonds described by Tobolsky and Craven—including Diels–Alder cycloaddition, polysulfide exchange, siloxane metathesis, and urethane cleavage—continue to be mainstays of the field, since then, a wealth of different reversible covalent chemistries have been harnessed to create covalent adaptable networks (CANs), enabling new applications and insights. The distinction between associative and dissociative exchange mechanisms came to the fore when Bowman, in 2005, and Leibler, in 2011, demonstrated efficient associative exchange mechanisms that enable topology reconfiguration without loss of network integrity. The field of CANs has become a chemical playground where molecular reactivity and mechanistic principles meet polymer properties. We are excited to present this special themed collection of Polymer Chemistry dedicated to chemical advances in CANs.

A key goal in the field, explored by several contributions to this themed collection, is uncovering the chemical design principles that will allow rational manipulation of the viscoelastic properties of CANs. Tao Xie and coworkers (DOI: 10.1039/c9py01891c) separately tuned the crosslink density and hydroxyl content in a polycaprolactone network and discovered that stress relaxation in the absence of intentionally introduced hydroxyl groups arises from adventitious water associated with the catalyst. Baochun Guo and coworkers (DOI: 10.1039/c9py01826c) also examine the effect of crosslink density by systematically varying the molecular weight of aldehyde-terminated polybutadiene in an imine vitrimer. Like Xie, they observe faster stress relaxation for networks with lower crosslink density, but counterintuitively, flow activation energies are increased with the decrease in crosslink density. This surprising observation was explained by the fact that lower crosslink density decreases the number of bonds that must exchange to achieve topology rearrangement (affecting relaxation time), but also decreases the concentration of exchangeable groups (affecting activation energy). Hillmyer, Coates and coworkers (DOI: 10.1039/c9py01957j) also study imine CANs, and notice dramatic effects of the prepolymer sequence (gradient vs. statistical) on the high-temperature viscoelastic behavior, without affecting the tensile properties below T_g. This study was enabled by a modular alternating ring-opening copolymerization of anhydrides and epoxides, using a vanillin-derived epoxide as the aldehyde partner for cross-linking, and provides materials that may be rapidly recycled by mechanical or chemical processes. Guerre, Winne, Du Prez and coworkers (DOI: 10.1039/D0PY00114G) studied the effect of the polymer matrix on the viscoelastic behavior of vinylogous urethane vitrimers, and could tune relaxation times over two orders of magnitude based on the molecular weight and polarity of the prepolymer. Interestingly, in this study, decreasing the crosslink density tended to increase both the flow relaxation time and activation energy. In a complementary study, Kalow and coworkers (DOI: 10.1039/d0py00233j) studied the effect of cross-linker structure on a PDMS vitrimer based on conjugate addition–elimination of thiols. Varying the reactivity of the electrophilic component enabled manipulation of the stress relaxation rate without affecting the flow activation energy.

Both associative and dissociative mechanisms have advantages in different contexts; while vitrimers have improved solvent resistance and more gradual changes in viscosity as a function of temperature, the high viscosity could be a limitation in some contexts. In contrast, the viscosity drop associated with the gel temperature in a dissociative network may be preferred for applications requiring extrusion. CANs that can exchange by both dissociative and associative mechanisms are an intriguing opportunity to achieve the best of both worlds. Bowman and coworkers (DOI: 10.1039/d0py00091d) show that CANs derived from thiol addition to succinic anhydride favor a reversible ring-closing/addition pathway in the absence of excess thiols or a good catalyst, but can be readily modified to favor an associative transthioesterification mechanism. This detailed study was enabled by dielectric spectroscopy to measure changes in network polarization as a function of temperature and frequency. This data-rich technique provides insights distinct from mechanical analysis and, as Bowman and coworkers show, has much to offer to the CAN field.

To gain properties that cannot be achieved with a single type of cross-link, several contributions to this collection explore the use of multiple distinct cross-link classes in a single material. Williams and coworkers (DOI: 10.1039/c9py01787a) synthesize catalyst-free epoxy vitrimers with physical cross-links based on alkyl chains. The strength of the physical association was readily tuned by changing the chain length of the alkylamine component, with an intermediate chain length providing the most significant increase in plateau modulus and stress relaxation time. In an example of combining reversible and irreversible covalent bonds, Yagci and coworkers (DOI: 10.1039/c9py01056d) achieve simultaneous direct and inverse vulcanization of benzoxazine, polybutadiene, and elemental sulfur in a simple melt process to create flexible and recyclable thermosets. Tournilhac, Gresil and coworkers (DOI: 10.1039/D0PY00342E) discovered that epoxy networks prepared with excess diepoxide relative to dicarboxylic acid include both dynamic ester and non-dynamic ether bonds; FTIR studies during gelation suggest that the latter bonds form through ring-opening epoxide homopolymerization once the diacid is consumed. Usefully, the thermomechanical properties and flow behavior of these epoxy networks can be tuned over a wide range using only the monomer stoichiometry. Rather than incorporating both dynamic and non-dynamic bonds in a network at the outset, Otsuka and coworkers (DOI: 10.1039/d0py00048e) install dynamic bonds in pre-formed, non-dynamic amine–epoxy networks using boronic acids, which selectively complex with the diethanolamine groups. This strategy allows them to coat epoxy resins with light-responsive dyes and create robust adhesives between epoxy resins and dissimilar bulk materials. As an alternative to combining two types of cross-links in the same network, Konkolewicz and coworkers (DOI: 10.1039/c9py01387c) demonstrate the advantages of interpenetrating networks (IPNs), using polymers capable of H-bonding and Diels–Alder cross-links. In addition to improved modularity, which enables facile tuning of the network properties, the IPN has higher stress at break and improved self-healing properties relative to a compositionally comparable single network.

Several contributions to this collection demonstrate that advances in the chemistry of CANs can enable applications to areas far beyond the realm of shoe uppers. Ji and coworkers (DOI: 10.1039/D0PY00075B) review the field of polymer actuators based on CANs, encompassing shape-memory polymers, liquid crystalline elastomers, electroactive elastomers, and bilayer actuators. The authors highlight how CANs enable unique capabilities in actuators, such as reprogrammability, self-healing, and sophisticated shape manipulation, and call for new strategies to increase the output stress of these materials. Ojha and coworkers (DOI: 10.1039/c9py01807g) take advantage of the stability of polyester CANs in various solvents, including acidic and basic media, and selective swelling in toluene to separate toluene from azeotropic mixtures. Miller and coworkers (DOI: 10.1039/d0py00016g) incorporate thermoreversible Diels–Alder cross-linkers in poly(ionic liquid) networks, which can recover both mechanical properties and ionic conductivity after damage and re-healing.

We thank all of the authors for sharing their contributions; the scientific and international scope of this collection is representative of the CAN field. However, as Polymer Chemistry Editor-in-Chief Christopher Barner-Kowollik has noted in his editorials for the emerging investigators issues, the gender imbalance in this collection reflects the work remaining to bridge the gender gap. We hope that readers enjoy this themed collection, and that it serves not only as a resource for researchers in the CAN field, but also a call to researchers from diverse backgrounds to join the vibrant CAN community.

Notes and references

1. A. V. Tobolsky, I. B. Prettyman and J. H. Dillon, J. Appl. Phys., 1944, 15, 380–395.
2. J. M. Craven, US Pat, US3435003A, 1969.

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