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

Metal-Free Preparation of Linear and Cross-Linked Polydicyclopentadiene

Metal-free ring-opening metathesis polymerization (ROMP) utilizes organic photoredox mediators as alternatives to traditional metal-based ROMP initiators to allow the preparation of polymers without residual metal contamination. Herein we report studies exploring the use of <i>endo</i>-dicyclopentadiene (DCPD), a common ROMP monomer, to form linear polyDCPD and copolymers with norbornene. Subsequent cross-linking of the linear polyDCPD using thiol–ene chemistry allows for a completely metal-free preparation of cross-linked polyDCPD. Furthermore, the examination of a number of structurally related monomers offers insights into mechanistic details of this polymerization and demonstrates new monomers that can be utilized for metal-free ROMP

Modular Elastomer Photoresins for Digital Light Processing Additive Manufacturing

A series of photoresins suitable for the production of elastomeric objects via digital light processing additive manufacturing are reported. Notably, the printing procedure is readily accessible using only entry-level equipment under ambient conditions using visible light projection. The photoresin formulations were found to be modular in nature, and straightforward adjustments to the resin components enabled access to a range of compositions and mechanical properties. Collectively, the series includes silicones, hydrogels, and hybrids thereof. Printed test specimens displayed maximum elongations of up to 472% under tensile load, a tunable swelling behavior in water, and Shore A hardness values from 13.7 to 33.3. A combination of the resins was used to print a functional multimaterial three-armed pneumatic gripper. These photoresins could be transformative to advanced prototyping applications such as simulated human tissues, stimuli-responsive materials, wearable devices, and soft robotics

Modular Elastomer Photoresins for Digital Light Processing Additive Manufacturing

A series of photoresins suitable for the production of elastomeric objects via digital light processing additive manufacturing are reported. Notably, the printing procedure is readily accessible using only entry-level equipment under ambient conditions using visible light projection. The photoresin formulations were found to be modular in nature, and straightforward adjustments to the resin components enabled access to a range of compositions and mechanical properties. Collectively, the series includes silicones, hydrogels, and hybrids thereof. Printed test specimens displayed maximum elongations of up to 472% under tensile load, a tunable swelling behavior in water, and Shore A hardness values from 13.7 to 33.3. A combination of the resins was used to print a functional multimaterial three-armed pneumatic gripper. These photoresins could be transformative to advanced prototyping applications such as simulated human tissues, stimuli-responsive materials, wearable devices, and soft robotics

Metal-Free Ring-Opening Metathesis Polymerization

We have developed a method to achieve ring-opening metathesis polymerization (ROMP) mediated by oxidation of organic initiators in the absence of any transition metals. Radical cations, generated via one-electron oxidation of vinyl ethers, were found to react with norbornene to give polymeric species with microstructures essentially identical to those traditionally obtained via metal-mediated ROMP. We found that vinyl ether oxidation could be accomplished under mild conditions using an organic photoredox mediator. This led to high yields of polymer and generally good correlation between <i>M</i>_n values and initial monomer to catalyst loadings. Moreover, temporal control over reinitiation of polymer growth was achieved during on/off cycles of light exposure. This method demonstrates the first metal-free method for controlled ROMP

Organocatalyzed Anodic Oxidation of Aldehydes

A method for the catalytic formation of electroauxiliaries and subsequent anodic oxidation has been developed. The process interfaces N-heterocyclic carbene-based organocatalysis with electro-organic synthesis to achieve direct oxidation of catalytically generated electroactive intermediates. We demonstrate the applicability of this method as a one-pot conversion of aldehydes to esters for a broad range of aldehyde and alcohol substrates. Furthermore, the anodic oxidation reactions are very clean, producing only H₂ gas as a result of cathodic reduction

Comparison of Mechanochemical Chain Scission Rates for Linear versus Three-Arm Star Polymers in Strong Acoustic Fields

The effect of star versus linear polymer architecture on the rates of mechanochemically induced bond scission has been explored. We determined rate constants for chain scission of parent linear and star polymers, from which daughter fragments were cleanly resolved. These studies confirm a mechanistic interpretation of star polymer chain scission that is governed by the spanning rather than total molecular weight. We further demonstrate the preserved rate of site-selective mechanophore activation across two different polymer structures. Specifically, we observed consistent activation rate constants from three-arm star and linear polymer analogues, despite the <i>M</i>_n of the star polymer being 1.5 times greater than that of the linear system

Additive Manufacturing by Heating at a Patterned Photothermal Interface

Direct additive manufacturing (AM) of commercial silicones is an unmet need with high demand. We report a new technology, heating at a patterned photothermal interface (HAPPI), which achieves AM of commercial thermoset resins without any chemical modifications. HAPPI integrates desirable aspects of stereolithography with the thermally driven chemical modalities of commercial silicone formulations. In this way, HAPPI combines the geometric advantages of vat photopolymerization with the materials properties of, for example, injection molded silicones. We describe the realization of the new technology, HAPPI printing using a commercial Sylgard 184 polydimethylsiloxane resin, comparative analyses of material properties, and demonstration of HAPPI in targeted applications

Facile Synthesis of Fluorine-Substituted Polylactides and Their Amphiphilic Block Copolymers

We report the facile synthesis of 3-trifluoromethyl-6-methyl-1,4-dioxane-2,5-dione and ring-opening polymerization of the fluoro-lactide monomer to prepare polylactides composed of trifluoromethyl and methyl pendent groups on each repeat unit (FPLA). Molecular weights of the prepared polymers correlated well with the initial molar ratio of monomer to initiator and were found to range from 6.6 to 22.5 kDa as determined by ¹H NMR spectroscopy. GPC analysis revealed an <i>M</i>_n of up to 16.5 kDa. ¹H, ¹³C, and ¹⁹F NMR spectroscopy were consistent with the structures of the lactide monomer isomers, and ¹H NMR analysis was consistent with polymer backbones of alternating trifluoromethyl- and methyl-substituted lactate constituents. Glass transition temperature (<i>T</i>_g) and decomposition temperature (<i>T</i>_d) of the new FPLA were found to be 39 and 225 °C by DSC and TGA, respectively. Additionally, we prepared amphiphilic block copolymers of FPLA and poly­(ethylene glycol) (PEG). Specifically, FPLA-<i>b</i>-PEG diblocks and FPLA–PEG–FPLA triblocks were synthesized by using PEG monomethyl ether (mPEG) or PEG as alcohol initiators, respectively. We observed the formation of vesicles or wormlike micelles from the particles of FPLA–PEG–FPLA in dilute aqueous solution by transmission electron microscopy (TEM), suggesting potential applications for drug delivery