Thermodynamic, kinetic, and mechanistic studies of the thermal guanidine metathesis reaction.

We describe studies of the thermal guanidine metathesis (TGM) reaction, a reversible transformation that results in exchange of N-substituents of the guanidine functional group. By comparing the effects of discrete structural variations, we find that steric congestion is an important factor in determining both the equilibrium guanidine composition and the reaction kinetics. The alkyl versus aryl nature of N-substitution also plays an essential role in the reaction rate, up to the point that minimal TGM reactivity is observed when the guanidine contains wholly alkyl substituents. Furthermore, we demonstrate that TGM occurs under thermodynamic control and present evidence that it proceeds by a dissociative mechanism, supported by direct observation of a carbodiimide intermediate.

Mechanochemical Release of N-Heterocyclic Carbenes from Flex-Activated Mechanophores.

We have discovered a new flex-activated mechanophore that releases an N-heterocyclic carbene (NHC) under mechanical load. The mechanophore design is based upon NHC-carbodiimide (NHC-CDI) adducts and demonstrates an important first step toward flex-activated designs capable of further downstream reactivities. Since the flex-activation is non-destructive to the main polymer chains, the material can be subjected to multiple compression cycles to achieve iterative increases in the activation percentage of mechanophores. Two different NHC structures were demonstrated, signifying the potential modularity of the mechanophore design.

Poly(allyl alcohol) Homo- and Block Polymers by Postpolymerization Reduction of an Activated Polyacrylamide.

Direct polymerization of allyl alcohol generally results in low molar mass oligomers or highly branched or cross-linked structures, and the properties and applications of linear, high molar mass poly(allyl alcohol) (PAA) are relatively unexplored. Such macromolecular materials that cannot otherwise be obtained directly can be accessed by postpolymerization modification strategies. Herein we describe the synthesis and characterization of linear, high molar mass PAA by borohydride reduction of a new activated polyacrylamide. The facile polymerization and mild reduction conditions enable the preparation of PAA of targeted molar masses and low dispersity, as well as PAA block polymers via chemoselective reduction of the activated amide moiety.

Successive mechanochemical activation and small molecule release in an elastomeric material.

We have developed a mechanochemically responsive material capable of successively releasing small organic molecules from a cross-linked network upon repeated compressions. The use of a flex activated mechanophore that does not lead to main chain scission and an elastomeric polyurethane enabled consecutive compressions with incremental increases in the % mechanophore activation. Additionally, we examined the effect of multiple applications of compressive stress on both mechanophore activity and the mechanical behavior of the elastomeric matrix in which the mechanophore is embedded.

"Flex-activated" mechanophores: using polymer mechanochemistry to direct bond bending activation.

We describe studies in mechanochemical transduction that probe the activation of bonds orthogonal to an elongated polymer main chain. Compression of mechanophore-cross-linked materials resulted in the release of small molecules via cleavage of covalent bonds that were not integral components of the elongated polymer segments. The reactivity is proposed to arise from the distribution of force through the cross-linking units of the polymer network and subsequent bond bending motions that are consistent with the geometric changes in the overall reaction. This departure from contemporary polymer mechanochemistry, in which activation is achieved primarily by force-induced bond elongation, is a first step toward mechanophores capable of releasing side-chain functionalities without inherently compromising the overall macromolecular architecture.

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.

Excitation of "forbidden" guided-wave plasmon polariton modes via direct reflectance using a low refractive index polymer coupling layer.

A surface plasmon polariton (SPP) is an excitation resulting from the coupling of light to a surface charge oscillation at a metal-dielectric interface. The excitation and detection of SPPs is foundational to the operating mechanism of a number of important technologies, most of which require SPP excitation via direct reflectance, commonly achieved via Attenuated Total Reflection (ATR) using the Kretschmann configuration. As a result, the accessible modes are fundamentally high-loss "leaky modes," presenting a critical performance barrier. Recently, our group provided the first demonstration of "forbidden," or guided-wave plasmon polariton modes (GW-PPMs), collective modes of a MIM structure with oscillatory electric field amplitude in the central insulator layer with up to an order of magnitude larger propagation lengths than those of traditional SPPs. However, in that work, GW-PPMs were accessed by indirect reflectance using Otto configuration ATR, making them of limited applied relevance. In this paper, we demonstrate a technique for direct reflectance excitation and detection of GW-PPMs. Specifically, we replace the air gap used in traditional Otto ATR with a low refractive index polymer coupling layer, mirroring a technique previously demonstrated to access Long-Range Surface Plasmon Polariton modes. We fit experimental ATR data using a robust theoretical model to confirm the character of the modes, as well as to explore the potential of this approach to enable advantageous propagation lengths. The ability to excite GW-PPMs using a device configuration that does not require an air gap could potentially enable transformative performance enhancements in a number of critical technologies.

Thermal Guanidine Metathesis for Covalent Adaptable Networks.

We demonstrate that a dynamic chemical reaction that we term thermal guanidine metathesis (TGM) can serve as the basis for covalent adaptable network (CAN) materials. CANs are a class of cross-linked polymers that transition from thermoset to thermoplastic-like rheological behavior upon significant activation of reversible exchange reactions within the network and thus can be reprocessed. Small molecule studies indicate the TGM reaction proceeds by a dissociative mechanism, and guanidine-cross-linked network polymers can be reprocessed at elevated temperature. These TGM-based CANs exhibit dynamic behavior, such as dissolution in the presence of monofunctional exchange partners and stress relaxation above Tg. Additionally, differences in the activation energies obtained by small molecule kinetic studies and stress relaxation analysis are consistent with key predictions of the Semenov-Rubinstein model of thermoreversible gelation of highly cross-linked networks.

Activated Polyacrylamides as Versatile Substrates for Postpolymerization Modification.

The displacement of an activated leaving group in polymeric repeat units is a powerful method of postpolymerization modification. This strategy enables the synthesis of polymers otherwise unobtainable by direct polymerization as well as the preparation of a diverse array of macromolecular structures. We demonstrate that the activation of acrylamide through the introduction of two tert-butyloxycarbamate (Boc) groups followed by radical polymerization leads to a new class of activated polyacrylamides analogous to well-known activated polyacrylates. Transamidation of poly(di(Boc)-acrylamide) utilizing primary amines proceeds to high conversion under mild conditions, and the products can be readily purified. Less nucleophilic secondary amines and alcohols require more forcing conditions. We demonstrate the utility of this approach by preparing copolymers capable of on-demand gel formation and the synthesis of block polymers using controlled radical polymerization.

3D-printed mechanochromic materials.

We describe the preparation and characterization of photo- and mechanochromic 3D-printed structures using a commercial fused filament fabrication printer. Three spiropyran-containing poly(ε-caprolactone) (PCL) polymers were each filamentized and used to print single- and multicomponent tensile testing specimens that would be difficult, if not impossible, to prepare using traditional manufacturing techniques. It was determined that the filament production and printing process did not degrade the spiropyran units or polymer chains and that the mechanical properties of the specimens prepared with the custom filament were in good agreement with those from commercial PCL filament. In addition to printing photochromic and dual photo- and mechanochromic PCL materials, we also prepare PCL containing a spiropyran unit that is selectively activated by mechanical impetus. Multicomponent specimens containing two different responsive spiropyrans enabled selective activation of different regions within the specimen depending on the stimulus applied to the material. By taking advantage of the unique capabilities of 3D printing, we also demonstrate rapid modification of a prototype force sensor that enables the assessment of peak load by simple visual assessment of mechanochromism.