Patent Application
20240043369
Disclosed herein are cyclobutane-based crosslinking compounds that, when incorporated into acrylate-based polymeric materials, can produce toughened acrylate polymer networks. Also disclosed herein are polymers comprising the crosslinkers, methods of preparing toughened polymer networks using the crosslinkers, and methods of using the polymer networks.
The lifetime and utility of acrylate-based polymer networks are determined by their ability to stretch repeatedly without breaking. At sufficiently high strains, the network breaks by forming a crack that subsequently propagates through the material to the point of failure. Tearing at the macroscopic level is resisted at the molecular level by polymer chains within the network that need to break in order for the crack to propagate. The scission of covalent chains occurs through a chemical reaction—typically homolytic bond scission that is accelerated by the tension in overstretched chains at the propagating crack front.
It is possible to design and incorporate a small fraction of mechanically scissile functional groups (mechanophores) whose reactivity dominates polymer chain scission events. In such systems, the mechanical properties of the network might be expected to reflect the force-coupled reactivity of the mechanophores. When mechanophores are embedded into the middle of each elastically active network strand (
Disclosed herein are cyclobutane-based mechanophore cross-linkers that break via force-triggered cycloreversion. When incorporated into acrylate-based polymers, these cross-linkers produce acrylate-based polymer networks that are up to 9 times tougher than conventional analogs. As demonstrated further in the Examples, the response is attributed to a combination of long, strong primary polymer strands and cross-linker scission forces that are ˜ 5-fold smaller than control cross-linkers at the same timescales. The enhanced toughness comes without the hysteresis associated with noncovalent cross-linking, and it is observed in multiple different acrylate elastomers, in fatigue as well as constant displacement rate tension, and in a gel as well as elastomers.
Accordingly, disclosed herein is an acrylate polymer comprising a moiety of formula (I) or a moiety of formula (II):
In some embodiments, the acrylate polymer comprises a moiety of formula (I). In some embodiments, R1 is methyl. In some embodiments, m is 4. In some embodiments, the moiety of formula (I) is:
In some embodiments, the acrylate polymer comprises a moiety of formula (II). In some embodiments, R2 is bromo. In some embodiments, n is 4. In some embodiments, the moiety of formula (II) is:
In some embodiments, the acrylate polymer comprises at least one (meth)acrylate monomer selected from an alkyl (meth)acrylate, a hydroxyalkyl (meth)acrylate, an alkoxyalkyl (meth)acrylate, a cycloalkyl (meth)acrylate, and an aromatic (meth)acrylate.
In some embodiments, the acrylate polymer comprises an alkyl (meth)acrylate monomer selected from methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, n-butyl (meth)acrylate, iso-butyl (meth)acrylate, tert-butyl (meth)acrylate, n-hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, iso-decyl (meth)acrylate, heptadecyl (meth)acrylate, dodecyl (meth)acrylate, 2-propylheptyl (meth)acrylate, and stearyl (meth)acrylate.
In some embodiments, the acrylate polymer comprises a hydroxyalkyl (meth)acrylate monomer selected from hydroxymethyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate.
In some embodiments, the acrylate polymer comprises an alkoxyalkyl (meth)acrylate selected from 2-methoxyethyl (meth)acrylate, 2-ethoxyethyl (meth)acrylate, 1-methyl-2-methoxyethyl (meth)acrylate, ethylene glycol methyl ether (meth)acrylate, diethylene glycol methyl ether (meth)acrylate, and triethylene glycol methyl ether (meth)acrylate.
In some embodiments, the acrylate polymer comprises a cycloalkyl (meth)acrylate selected from cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-cyclohexylethyl (meth)acrylate, 3-cyclohexylpropyl (meth)acrylate, 2-norbornyl (meth)acrylate, and isobornyl (meth)acrylate.
In some embodiments, the acrylate polymer comprises at least two different (meth)acrylate monomers.
In another aspect, disclosed herein is a compound of formula (III) or formula (IV):
wherein:
In some embodiments, the compound is a compound of formula (III). In some embodiments, R1 is methyl. In some embodiments, R3 is hydrogen. In some embodiments, m is 4. In some embodiments, the compound of formula (III) is:
In some embodiments, the compound is a compound of formula (IV). In some embodiments, R2 is bromo. In some embodiments, R3 is hydrogen. In some embodiments, n is 4. In some embodiments, the compound of formula (IV) is:
In another aspect, disclosed herein is an acrylate polymer prepared by:
In some embodiments, the controlled radical polymerization is reversible addition-fragmentation chain transfer (RAFT). In some embodiments, the first mixture further comprises a photoinitiator and a chain transfer agent. In some embodiments, the first mixture comprises at least two different (meth)acrylate monomers. In some embodiments, the compound of formula (III) or formula (IV) is:
In another aspect, disclosed herein is a method of toughening an acrylate-based polymeric material, comprising:
wherein:
In some embodiments, the incorporating step comprises:
In some embodiments, the polymerizing step comprises reversible addition-fragmentation chain transfer (RAFT) polymerization. In some embodiments, the polymerizing step comprises reacting the (meth)acrylate monomer with a photoinitiator and a chain transfer agent. In some embodiments, the polymerizing step comprises polymerizing a mixture of at least two different (meth)acrylate monomers.
Other aspects and embodiments of the disclosure will become apparent in light of the following description and the drawings.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
Disclosed herein are cyclobutane-based crosslinking compounds that, when incorporated into acrylate-based polymeric materials, can produce toughened acrylate polymer networks. Also disclosed herein are polymers comprising the crosslinkers, methods of preparing toughened polymer networks using the crosslinkers, and methods of using the crosslinked polymers.
Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs.
Definitions of specific terms, including certain functional groups and chemical terms, are described in more detail below. For purposes of this disclosure, the chemical elements are identified in accordance with the Periodic Table of the Elements, CAS version, Handbook of Chemistry and Physics, 75th Ed., inside cover, and specific functional groups are generally defined as described therein. Additionally, general principles of organic chemistry, as well as specific functional moieties and reactivity, are described in Sorrell, Organic Chemistry, 2nd edition, University Science Books, Sausalito, 2006; Smith, March's Advanced Organic Chemistry: Reactions, Mechanism, and Structure, 7th Edition, John Wiley & Sons, Inc., New York, 2013; Larock, Comprehensive Organic Transformations, 3rd Edition, John Wiley & Sons, Inc., New York, 2018; and Carruthers, Some Modern Methods of Organic Synthesis, 3rd Edition, Cambridge University Press, Cambridge, 1987; the entire contents of each of which are incorporated herein by reference.
As used herein, the term “alkyl” refers to a radical of a straight or branched saturated hydrocarbon chain. The alkyl chain can include, e.g., from 1 to 24 carbon atoms (C1-C24 alkyl), 1 to 16 carbon atoms (C1-C16 alkyl), 1 to 14 carbon atoms (C1-C14 alkyl), 1 to 12 carbon atoms (C1-C12 alkyl), 1 to 10 carbon atoms (C1-C10 alkyl), 1 to 8 carbon atoms (C1-C8 alkyl), 1 to 6 carbon atoms (C1-C6 alkyl), 1 to 4 carbon atoms (C1-C4 alkyl), 1 to 3 carbon atoms (C1-C3 alkyl), or 1 to 2 carbon atoms (C1-C2 alkyl). Representative examples of alkyl include, but are not limited to, methyl, ethyl, n-propyl, iso-propyl, n-butyl, sec-butyl, iso-butyl, tert-butyl, n-pentyl, isopentyl, neopentyl, n-hexyl, 3-methylhexyl, 2,2-dimethylpentyl, 2,3-dimethylpentyl, n-heptyl, n-octyl, n-nonyl, n-decyl, n-undecyl, and n-dodecyl.
As used herein, the term “alkoxy” refers to an alkyl group, as defined herein, appended to the parent molecular moiety through an oxygen atom. Representative examples of alkoxy include, but are not limited to, methoxy, ethoxy, n-propoxy, isopropoxy, n-butoxy, and tert-butoxy.
As used herein, the term “alkoxyalkyl” refers to an alkyl group, as defined herein, in which at least one hydrogen atom (e.g., one hydrogen atom) is replaced with an alkoxy group, as defined herein. Representative examples of alkoxyalkyl include, but are not limited to, methoxymethyl, 2-methoxyethyl, and 2-ethoxyethyl.
As used herein, the term “aryl” refers to a radical of a monocyclic, bicyclic, or tricyclic 4n+2 aromatic ring system (e.g., having 6, 10, or 14π electrons shared in a cyclic array) having 6-14 ring carbon atoms and zero heteroatoms (“C6-C14 aryl”). In some embodiments, an aryl group has six ring carbon atoms (“C6 aryl,” i.e., phenyl). In some embodiments, an aryl group has ten ring carbon atoms (“C10 aryl,” e.g., naphthyl such as 1-naphthyl and 2-naphthyl). In some embodiments, an aryl group has fourteen ring carbon atoms (“C14 aryl,” e.g., anthracenyl and phenanthrenyl).
As used herein, the term “cyano” refers to a group —CN.
As used herein, the term “cycloalkyl” refers to a radical of a saturated carbocyclic ring system containing three to ten carbon atoms and zero heteroatoms. The cycloalkyl may be monocyclic, bicyclic, bridged, fused, or spirocyclic. Representative examples of cycloalkyl include, but are not limited to, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononyl, cyclodecyl, adamantyl, bicyclo[2.2.1]heptanyl, bicyclo[3.2.1]octanyl, and bicyclo[5.2.0]nonanyl.
As used herein, the term “halo” or “halogen” refers to F, Cl, Br, or I.
As used herein, the term “haloalkyl” refers to an alkyl group, as defined herein, in which at least one hydrogen atom (e.g., one, two, three, four, five, six, seven or eight hydrogen atoms) is replaced with a halogen. In some embodiments, each hydrogen atom of the alkyl group is replaced with a halogen. Representative examples of haloalkyl include, but are not limited to, fluoromethyl, difluoromethyl, trifluoromethyl, 2-fluoroethyl, 2,2,2-trifluoroethyl, and 3,3,3-trifluoropropyl.
As used herein, the term “hydroxy” refers to an —OH group.
As used herein, the term “hydroxyalkyl” refers to an alkyl group, as defined herein, in which at least one hydrogen atom (e.g., one hydrogen atom) is replaced with a hydroxy group. Representative examples of hydroxyalkyl include, but are not limited to, hydroxymethyl, 2-hydroxyethyl, 3-hydroxypropyl, and 4-hydroxybutyl.
As used herein, the term “(meth)” designates optional methyl substitution. Thus, a term such as “(meth)acrylates” denotes both methacrylates and acrylates. For example, the term “methyl (meth)acrylate” refers to methyl acrylate and methyl methacrylate.
As used herein, the term “nitro” refers to a group —NO2.
Disclosed herein are crosslinking compounds, including compounds of formula (III) and compounds of formula (IV):
The compounds of formula (III) and compounds of formula (IV) include a cyclobutane moiety. For example, these compounds include a cis-diaryl substituted cyclobutane-based mechanophore. When incorporated into polymeric materials, these mechanophores react under tension by means of a force-coupled [2+2] cycloreversion to form two cinnamates (see, e.g.,
In some embodiments, the compound is a compound of formula (III). In some embodiments, R1 is C1-C12 alkyl. In some embodiments, R1 is methyl. In some embodiments, R3 is hydrogen. In some embodiments, R3 is methyl. In some embodiments, m is 1. In some embodiments, m is 2. In some embodiments, m is 3. In some embodiments, m is 4. In some embodiments, m is 5. In some embodiments, m is 6. In some embodiments, the compound of formula (III) is:
In some embodiments, the compound is a compound of formula (IV). In some embodiments, R2 is halo. In some embodiments, R2 is bromo. In some embodiments, R1 is methyl. In some embodiments, R3 is hydrogen. In some embodiments, R3 is methyl. In some embodiments, n is 1. In some embodiments, n is 2. In some embodiments, n is 3. In some embodiments, n is 4. In some embodiments, n is 5. In some embodiments, n is 6. In some embodiments, the compound of formula (III) is:
Crosslinking compounds of formula (III) and/or formula (IV) can be incorporated into acrylate-based polymers to crosslink such polymers. Once incorporated into an acrylate-based polymer, the polymer (i.e., a crosslinked acrylate polymer) will include a moiety of formula (I) or a moiety of formula (II):
In some embodiments, the when the compound of formula (III) is used as the crosslinker, the polymer will comprise a moiety of formula (I). In some embodiments, R1 is C1-C12 alkyl. In some embodiments, R1 is methyl. In some embodiments, R3 is hydrogen. In some embodiments, R3 is methyl. In some embodiments, m is 1. In some embodiments, m is 2. In some embodiments, m is 3. In some embodiments, m is 4. In some embodiments, m is 5. In some embodiments, m is 6. In some embodiments, the moiety of formula (I) is:
In some embodiments, when the compound of formula (IV) is used as the crosslinker, the polymer will comprise a moiety of formula (II). In some embodiments, R2 is halo. In some embodiments, R2 is bromo. In some embodiments, R1 is methyl. In some embodiments, R3 is hydrogen. In some embodiments, R3 is methyl. In some embodiments, n is 1. In some embodiments, n is 2. In some embodiments, n is 3. In some embodiments, n is 4. In some embodiments, n is 5. In some embodiments, n is 6. In some embodiments, the moiety of formula (II) is:
The acrylate polymers described herein can include one or more (meth)acrylate monomers. Any (meth)acrylate monomers can be used. In some embodiments, the polymers include at least one (meth)acrylate monomer selected from an alkyl (meth)acrylate, a hydroxyalkyl (meth)acrylate, an alkoxyalkyl (meth)acrylate, a cycloalkyl (meth)acrylate, and an aromatic (meth)acrylate.
For example, in some embodiments, the polymer comprises an alkyl (meth)acrylate monomer. Exemplary alkyl (meth)acrylates include, but are not limited to, methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, n-butyl (meth)acrylate, iso-butyl (meth)acrylate, tert-butyl (meth)acrylate, n-hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, iso-decyl (meth)acrylate, heptadecyl (meth)acrylate, dodecyl (meth)acrylate, 2-propylheptyl (meth)acrylate, and stearyl (meth)acrylate.
In some embodiments, the polymer comprises a hydroxyalkyl (meth)acrylate monomer. Exemplary hydroxyalkyl (meth)acrylate monomers include, but are not limited to, hydroxymethyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate.
In some embodiments, the polymer comprises an alkoxyalkyl (meth)acrylate monomer. Exemplary alkoxyalkyl (meth)acrylate monomers include, but are not limited to, 2-methoxyethyl (meth)acrylate, 2-ethoxyethyl (meth)acrylate, 1-methyl-2-methoxyethyl (meth)acrylate, ethylene glycol methyl ether (meth)acrylate, diethylene glycol methyl ether (meth)acrylate, and triethylene glycol methyl ether (meth)acrylate.
In some embodiments, the polymer comprises a cycloalkyl (meth)acrylate monomer. Exemplary cycloalkyl (meth)acrylate monomers include, but are not limited to, cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-cyclohexylethyl (meth)acrylate, 3-cyclohexylpropyl (meth)acrylate, 2-norbornyl (meth)acrylate, and isobornyl (meth)acrylate.
The polymers can include at least two different (meth)acrylate monomers, such as any of the monomers described herein. For polymer can be a block copolymer (e.g., a diblock copolymer or a triblock copolymer), a random copolymer, a graft copolymer, or the like.
The crosslinked acrylate polymers described herein can be prepared by a number of methods, including controlled radical polymerization methods, such as reversible addition fragmentation chain transfer polymerization (RAFT), atom transfer radical polymerization (ATRP), stable free radical polymerization (SFRP), nitroxide-mediated polymerization (NMP), and the like. In some embodiments, the acrylate polymers are prepared by reversible addition-fragmentation chain transfer (RAFT).
For example, in some embodiments, the acrylate polymer can be prepared by:
The controlled radical polymerization (e.g., RAFT) reaction is conducted using an initiator, which is an agent capable of producing a free radical. In some embodiments, the controlled radical polymerization (e.g., RAFT) is conducted using a photoinitiator. An initiator is a compound that decomposes into radicals which subsequently react with a monomer to initiate a free-radical polymerization reaction. Photoinitiators decompose by photochemical processes. Typical examples of photoinitiators include, but are not limited to: benzil, benzoin, acetophenone, benzophenone, camphorquinone, or derivatives thereof, monoacyl and bisacyl phosphine oxides; and α-ketoesters including α-ketoglutaric acid, ethyl pyruvate, and the like. Combinations of photoinitiators can also be used. In some embodiments, the photoinitiator is an α-ketoester. In some embodiments, the photoinitiator is α-ketoglutaric acid.
In some embodiments, the controlled radical polymerization (e.g., RAFT) is conducted using a chain transfer agent. Representative chain transfer agents include, but are not limited to: trithiocarbonates such as 3,5-bis(2-dodecylthiocarbonothioylthio-1-oxopropoxy) benzic, 3-butenyl 2-(dodecylthiocarbonothioylthio)-2-methyl propionate, 2-(2-carboxyethylsulfanylthiocarbonylsulfanyl)-proprionic acid, 4-((((2-carboxyethyl)thio) carbonothioyl)thio)-4-cyanopentanoic acid, 2-cyanobutan-2-yl 4-chloro-3,5-dimethyl-1H-pyrazole-1-carbodithioate, 2-cyanobutanyl-2-yl 3,5-di methyl-1H-pyrazole-1-carbodithioate, 4-cyano-4-[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid, 2-(butylthiocarbonothioylthio)propanoic acid, 4-cyano-4-(ethylcarbonothioylthio) pentanoic acid, 4-cyano-4-[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanol, cyanomethyl (3,5-Dimethyl-1H-pyrazole)-carbodithioate, cyanomethyl dodecyl trithiocarbonate, cyanomethyl [3-(trimethoxysilyl)propyl] trithiocarbonate, 2-cyano-2-propyl dodecyl trithiocarbonate, S,S-di benzyl trithiocarbonate, 2-(dodecyl-thiocarbonothioylthio)-2-methylpropionic acid, 2-(dodecyl-thiocarbonothioyl-thio)-2-methylpropionic acid, 3-azido-1-propanol ester, 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid, N-hydroxysuccinimide ester of 4-cyano-4-[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid, pentafluorophenyl ester of 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid, 2-(dodecylthiocarbonothioylthio)propionic acid, methyl 2-(dodecyl)-2-methylpropionate, pentaerythritol tetrakis[2-(dodecylthiocarbonothioylthio)-2-methylpropionate], phthalimidomethyl butyl trithiocarbonate, poly(acrylic acid) having a 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid end, poly(ethylene glycol)bis[2-(dodecylthiocarbonothioylthio)-2-methylpropionate], poly(ethylene glycol) methyl ether 4-cyano-4-[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoate, poly(ethylene glycol) methyl ether (4-cyano-4-pentanoate dodecyl trithiocarbonate), poly(ethylene glycol)methyl ether (4-cyano-4-pentanoate dodecyl trithiocarbonate), poly (ethylene glycol)methyl ether (4-cyano-4-pentanoate dodecyl trithiocarbonate), poly(ethylene glycol) methyl ether 2-(dodecylthiocarbonothioylthio)-2-methylpropionate, poly(ethylene glycol)methyl ether 2-(dodecylthiocarbonothioylthio)-2-methylpropionate, poly(ethylene glycol)methyl ether (2-methyl-2-propionic acid dodecyl trithiocarbonate)L-lactide) 4-cyano-4-[(dodecylsulfanyl-thiocarbonyl)sulfanyl] pentonate, poly(L-lactide) 4-cyano-4-[(dodecylsulfanyl-thiocarbonyl)sulfanyl] pentonate, poly(D,L-lactide), 4-cyano-4-[(dodecylsulfanyl-thiocarbonyl)sulfanyl] pentonate, polystyrene with an end of 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid. or 1,1,1-tris[(dodecylthiocarbonothioylthio)-2-methylpropionate] ethane; dithiocarbamates such as benzyl 1H-pyrrole-1-carbodithioate, cyanomethyl diphenylcarbamodithioate, cyanomethyl methyl(phenyl)carbamodithioate, cyanomethyl methyl(4-pyridyl)carbamodithioate, 2-cyanopropan-2-yl N-methyl-(pyridin-4-yl)carbamodithioate, methyl 2-[methyl(4-pyridinyl)carbamothioylthio] propionate, 1-succinimidyl-4-cyano-4-[N-methyl-N-(4-pyridyl)carbamothioylthio] pentanoate; dithioabenzoates such as benzyl benzodithioate, cyanomethyl benzodithioate, 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid, N-succinimidyl ester of 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid, 2-cyano-2-propyl benzodithioate, 2-cyano-2-propyl4-cyanobenzodithioate, ethyl 2-(4-methoxyphenylcarbonothioylthio)acetate, ethyl 2-methyl-2-(phenylthiocarbonylthio)propionate, ethyl 2-(phenylcarbonothioylthio)-2-phenylacetate, ethyl 2-(phenylcarbonothioylthio) propionate, 1-(methoxycarbonyl)ethyl benzodithioate, 2-(4-methoxyphenylcarbonothioylthio) ethanoic acid, 2-nitro-5-(2-propynyloxy)benzyl 4-cyano-4-(phenylcarbonothioylthio)pentanoate, 2-(phenylcarbonothioylthio) propanoic acid, or 2-phenyl-2-propyl benzodithioate; and switchable RAFT agents such as cyanomethyl methyl(4-pyridyl)carbamodithioate, 2-cyanopropan-2-yl N-methyl-N-(pyridin-4-yl)carbamodithioate, methyl 2-[methyl(4-pyridinyl)carbamothioylthio]propionate, or 1-succinimidyl-4-cyano-4-[N-methyl-N-(4-pyridyl) carbamothioylthio] pentanoate. In some embodiments, the chain transfer agent is 4-cyano-4-[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid.
The crosslinking compounds disclosed herein are mechanically weak, yet produce acrylate polymers that have improved strength and tear resistance. Accordingly, disclosed herein is a method of toughening an acrylate-based polymeric material, comprising incorporating a moiety of formula (I) or formula (II) into the acrylate-based polymeric material. The moiety of formula (I) or formula (II) can be incorporated, for example, by crosslinking an acrylate polymer with a compound of formula (III) or a compound of formula (IV).
The toughened acrylate polymer networks can be used in any application in which it would be useful or desirable to use a mechanically strong polymeric material. For example, acrylate-based polymeric materials are often used in additive manufacturing processes (i.e., 3D printing), in materials used in artificial joints, and in cosmetic applications such as nail polish.
Materials and Methods
Materials. Lab general solvents (dichloromethane, acetonitrile, hexane, ethyl acetate, acetone, tetrahydrofuran, dioxane, methanol, dimethyl formaldehyde) were purchased from VWR or Sigma Aldrich. Hydrogen was purchased from Airgas. 4-bromo-cinnamic acid, 4-(dimethylamino)pyridine (DMAP), 3-(3-dimethylaminopropyl)-1-ethyl-carbodiimide hydrochloride (EDC·HCl), copper (I) iodide (CuI), Sodium iodide (NaI), trans-N,N-dimethylcyclohexane-1,2-diamine, 3-Butyn-1-ol, bis(triphenylphosphine)palladium(II) dichloride, diisopropylamine, palladium on carbon (10 wt %), acrylic acid, 4-hydroxybutyl acrylate, adipic acid, 2-hydroxyethyl acrylate, pyridinium p-toluenesulfonate (PPTS), 2-methoxypropene, triethylamine (TEA), Grubbs II catalyst, 4-pentenoic anhydride, 9-oxabicyclo[6.1.0]non-4-ene, ethyl acrylate, 2-methoxylethyl acrylate, 4-Cyano-4-[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid, α-ketoglutaric acid, and propylene carbonate were purchased from Sigma Aldrich, or Alfa Aesar, or ChemImpex, or Ambeed Inc., and used without further purification. Borosilicate glass plates, low friction transparent FEP tape (0.0035″ thick), low friction Polytetrafluoroethylene (PTFE) tape (0.012″ thick) was purchased from McMASTER-CARR.
General Methods. 1H NMR and 13C NMR spectra were collected on a 500 MHz Bruker spectrometer. 1H shifts are reported as chemical shift, multiplicity, coupling constant if applicable, and relative integral. Multiplicities are reported as: singlet (s), doublet (d), doublet of doublets (dd), doublet of triplets (dt), doublet of doublet of doublets (ddd), doublet of doublet of triplets (ddt), triplet (t), triplet of doublets (td), quartet (q), multiplet (m), or broad (br). High-resolution mass spectra were collected on an Agilent LCMS-TOF-DART at Duke University's Mass Spectrometry Facility.
Flash chromatography was performed using Silicycle SiliaFlash@ F60 gel (40-63 μm particle size, 230-400 mesh) and medium pressure liquid chromatography (MPLC) was performed on a Teledyne ISCO CombiFlash Rf 200.
Thermogravimetric analysis (TGA) was performed on a TA instrument (TGA 550) using 100 μL open platinum pans under nitrogen flow. A heating rate of 20° C./min. from 25° C. to 800° C. was utilized to obtain volatility and decomposition temperatures. Differential scanning calorimetry (DSC) was performed on a TA instrument (DSC2500). The sample of 4˜8 mg was placed in a non-hermetic Tzero aluminum pan and an empty pan was used as a reference pan. The DSC experiment was performed in a heat-cool cycle. Detailed parameters used for different experiments are mentioned herein.
Gel permeation chromatography (GPC) was performed on in-line two columns (Agilent PLgel 105 Å, 7.5×300 mm, 5 μm, part number PL1110-6550) at room temperature using inhibitor free THE at a flow rate of 1.0 mL/min. The flow rate was set using an Agilent 1260 Infinity Isocratic pump, molecular weights were calculated using in line Wyatt Optilab T-rEX refractive index detector and Wyatt miniDAWN TREOS multiangle light scattering detector, and UV absorbance was measured with an in-line Agilent 1260 Infinity UV detector. The UV detector monitored 190 to 800 nm with step of 2.0 nm and slit width of 4.0 nm. The refractive index increment (dn/dc) values were determined by using on-line 100% mass recovery assumption calculations built into Wyatt Astra software using injections of known concentration and mass. Before GPC analysis, 1-2 mg/mL in THE solutions were filtered through a 0.2 μm pore size PTFE syringe filters.
The AFM pulling experiments were conducted in toluene at an ambient temperature (˜23° C.) in the same manner as described previously (Wu et al. J. Am. Chem. Soc. 132, 15936-15938 (2010); Klukovich et al. Nat. Chem. 5, 110-114 (2013); Wang et al. Nat. Chem. 7, 323-327 (2015); Wang et al. J. Org. Chem. 80, 11895-11898 (2015); Kouznetsova et al. ChemPhysChem 18, 1486-1489 (2017)) using a homemade AFM, which was constructed using a Bruker (previously Digital Instruments) Multimode AFM head mounted on top of a piezoelectric positioner (Physik Instrumente, GmbH), similar to the one described in detail previously (Kouznetsova 2017). Sharp Microlever silicon probes (MSNL) were purchased from Bruker (Camarillo, CA) and the force curves used for analysis were obtained with rectangular-shaped cantilevers (205 μm×15 μm, nominal tip radius˜2 nm, nominal spring constant k˜0.02 N/m, frequency ˜15 kHz). Multiple probes of the same type were used throughout the course of the experiments. The spring constant of each cantilever was calibrated in air, using the thermal noise method, based on the energy equipartition theorem as described previously (Oberhauser et al. Nature 393, 181-185 (1998)). Cantilever tips were prepared by soaking in piranha solution for ˜15 min at room temperature. Silicon surfaces were prepared by soaking ˜30 min in hot piranha solution, followed by washing with DI-water and drying under a stream of nitrogen. The surface and cantilever were then placed in a UVO cleaner (ozone produced through UV light) for 15 min. After ozonolysis, the cantilever was mounted and ˜20 μL of a ˜0.1-0.05 mg mL−1 polymer solution was added to the silicon surface and allowed to dry. Measurements were carried out in a fluid cell with scanning set for a series of constant velocity approaching/retracting cycles.
Rheological measurements were conducted on an Anton Paar MCR 302 rheometer with a 8 mm parallel plate geometry. Uniaxial tensile tests and tearing energy measurements were performed on a TA Instruments RSA III Dynamic Mechanical Analyzer (force resolution: 0.0001 N, displacement resolution: 1 m) at Duke University's Shared Material Instrument Facility (SMIF). Fatigue tests were performed on a Test Resources 910LX25 dynamic & fatigue test machine (force resolution: 0.0001 N, displacement resolution: 1 m).
Synthesis of 1b. Compound 1a was synthesized as previously reported (Wang et al. J. Am. Chem. Soc. 143, 3714-3718 (2021)). To a 250 mL round bottom flask (RBF), compound 1a (7 g, 15.4 mmol) was mixed with 150 mL acetonitrile. N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC-HCl) (8.9 g, 46.4 mmol) was added portion-wise. The solid first dissolved, and then the solution became cloudy again. 4-dimethylaminopyridine (DMAP) (750 mg, 6 mmol) and MeOH (2.5 mL, 62 mmol) were then added to the solution. The reaction was stirred at room temperature (r.t.) overnight. After the reaction completed, the solution was concentrated using a rotary evaporator and diluted with 200 mL ethyl acetate. The solution was washed with deionized (DI) water (150 mL×2) and brine (150 mL×1). EA phase was collected and dried with MgSO4. After filtration, the solution was concentrated onto silica. Column chromatography (SiO2, 0˜ 40% ethyl acetate (EtOAC)/hexane gradient eluent) gave compound 1b as a white solid (6.9 g). 1H NMR (500 MHz, CDCl3): δ 7.27-7.26 (d, J=8.4 Hz, 4H), 6.79-6.77 (d, J=8.4 Hz, 4H), 4.34-4.33 (m, 2H), 3.76-3.75 (m, 8H). 13C NMR (126 MHz, CDCl3) δ 172.62, 138.21, 133.21, 129.55, 120.77, 54.05, 44.41, 42.79. HRMS-ESI (m/z): [M+H]+ calculated for C20H18Br2O4, 480.9647; observed 480.9643.
Synthesis of 1c. To a 300 mL pressure vessel charged with a stir bar, compound 1b (6.9 g, 14.3 mmol) was mixed with 35 mL dioxane. CuI (273 mg, 1.43 mmol) and NaI (8.63 g, 57.2 mmol) were added. The mixture was purged with N2 for 10 mins. Trans-N,N-dimethylcyclohexane-1,2-diamine (448.5 μL, 2.86 mmol) was added using micropipette. The vessel was capped with PTFE cap and heated to 110° C. The reaction was stirred for 24 h. The vessel was cooled to room temperature and the mixture was poured onto a short silica plug (ethyl acetate as eluent) to give compound 1c as thick pale-yellow oil (˜8.5 g). NMR spectrum showed clean product and the compound was directly used in next step without further purification. 1H NMR (500 MHz, CDCl3): δ 7.47-7.46 (d, J=8.4 Hz, 4H), 6.67-6.65 (d, J=8.3 Hz, 4H), 4.32-4.21 (m, 2H), 3.74-3.73 (m, 8H). 13C NMR (126 MHz, CDCl3) δ 173.84, 138.08, 137.48, 128.77, 91.38, 52.44, 45.11, 42.43.
Synthesis of 1d. To a 500 mL RBF charged with a stir bar, compound 1c (˜8.5 g, ˜ 14.3 mmol) was mixed with 150 mL Dioxane. Diisopropyl amine (12 mL, 85.8 mmol) was added. The mixture was purged with N2 for 10 mins. Under nitrogen atmosphere, CuI (545 mg, 2.86 mmol, 0.2 equiv. to 1c) and Pd(PPh3)2Cl2 (1 g, 1.43 mmol, 0.1 equiv. to 1c) were added sequentially. The solution turned dark immediately when Pd(PPh3)2Cl2 was added. The reaction was stirred under N2 atmosphere overnight. After the reaction was finished, the solution was filtered with celite. The filtrate was diluted with EA and washed with dilute (1˜2%) HCl DI water solution (150 mL×2) and brine (150 mL×1). EA phase was collected and dried over MgSO4. Column chromatography (SiO2, 0˜ 80% EtOAC/hexane gradient eluent) gave compound 1d as thick yellow oil (6.1 g). The yellow color was likely due to the metal residual, which can be removed by QuadraPure® TU. If not pure by NMR, repeat the chromatography. 1H NMR (500 MHz, DMSO): δ 7.11-7.09 (d, J=8.4 Hz, 4H), 7.01-6.99 (d, J=8.5 Hz, 4H), 4.86-4.84 (t, J=5.6 Hz, 2H), 4.25-4.24 (m, 2H), 4.00-3.98 (m, 2H), 3.64 (s, 6H), 3.55-3.51 (td, J=6.9, 5.6 Hz, 4H), 2.49-2.47 (t, 4H). 13C NMR (126 MHz, DMSO) δ 172.04, 139.85, 129.97, 128.57, 120.37, 88.42, 81.09, 60.66, 52.46, 45.15, 42.36, 23.78. HRMS-ESI (m/z): [M+H]+ calculated for C28H28O6, 461.1959; observed 461.1958.
Synthesis of 1e. To a 500 mL three-neck RBF charged with a stir bar, compound 1d (6.1 g, 13.2 mmol) was dissolved in 300 mL tetrahydrofuran (THF). The solution was purged with N2 for 10 min. Under nitrogen atmosphere, Pd/C (10% Pd) (2 g) was added portion-wise. A large amount of Pd/C was added because some ligands that carried over from the last step can poison Pd/C. The mixture was purged with N2 for another 10 min. The hydrogen balloon was connected to the RBF. The mixture was purged with hydrogen for 10 min and was allowed to react under hydrogen atmosphere for 24 h. After the reaction was finished, the solution was filtered with celite. The filtrate was loaded onto silica by rotary-evaporating the solvent. Column chromatography (SiO2, 0˜80% EtOAC/hexane gradient eluent) gave compound 1e as thick colorless oil (3.5 g). 1H NMR (500 MHz, CDCl3): δ 6.91-6.90 (d, J=8.2 Hz, 4H), 6.82-6.80 (d, J=8.1 Hz, 4H), 4.34-4.32 (m, 2H), 3.82-3.81 (m, 2H), 3.74 (s, 6H), 3.60-3.58 (t, J=6.4 Hz, 4H), 2.51-2.48 (t, J=7.5 Hz, 4H), 1.60-1.45 (m, 12H). 13C NMR (126 MHz, CDCl3) δ 173.76, 143.51, 136.07, 128.14, 127.08, 65.66, 53.54, 46.61, 43.37, 35.83, 33.00, 29.10. HRMS-ESI (m/z): [M+H]+ calculated for C28H36O6, 469.2585; observed 469.2590. [M+Na]+ calculated for C28H36O6 491.2404; observed 491.2406.
Synthesis of C1. To a 100 mL round bottom flask (RBF), acrylic acid (461 mg, 6.4 mmol), compound 1e (1 g, 2.13 mmol), DMAP (104.3 mg, 0.85 mmol) were dissolved in 20 mL DCM. EDC-HCl (1.23 g, 6.4 mmol) was added portion-wise at the end. The reaction was stirred at r.t. overnight. After the reaction completed, the solution was concentrated using rotary evaporator and diluted with 200 mL ethyl acetate. The solution was washed with DI water (150 mL×2) and brine (150 mL×1). EA phase was collected and dried with MgSO4. After filtration, the solution was concentrated onto silica. Column chromatography (SiO2, 0˜40% EtOAC/hexane gradient eluent) gave compound C1 (0.75 g). Crosslinker C1 can self-crosslink at high concentration under vacuum, thus it was directly prepared as a stock solution (200˜300 mg/mL) in dioxane. 1H NMR (500 MHz, CDCl3): δ 6.91-6.89 (d, J=8.1 Hz, 4H), 6.82-6.81 (d, J=8.1 Hz, 4H), 6.40-6.36 (dd, J=17.3, 1.5 Hz, 2H), 6.13-6.07 (dd, J=17.3, 10.4 Hz, 2H), 5.82-5.80 (dd, J=10.5, 1.5 Hz, 2H), 4.34-4.33 (m, 2H), 4.13-4.10 (t, 4H), 3.83-3.80 (m, 2H), 3.74 (s, 6H), 2.52-2.48 (t, 4H), 1.62-1.57 (p, 8H). 13C NMR (126 MHz, CDCl3) δ 172.44, 168.45, 140.09, 136.21, 131.07, 128.69, 128.14, 127.94, 65.19, 52.27, 44.80, 42.96, 35.75, 28.12, 27.64. HRMS-ESI (m/z): [M+H]+ calculated for C34H40O8, 577.2796; observed 577.2798. [M+Na]+ calculated for C34H40O8 599.2615; observed 599.2612.
To a 500 mL RBF, adipic acid (4 g, 27.3 mmol), EDC-HCl (10.5 g, 55 mmol), DMAP (0.67 g, 5.5 mmol) were added sequentially to 200 mL dichloromethane (DCM). 4-hydroxybutyl acrylate (10 g, 111 mmol) was added at the end. The reaction was stirred at r.t. for overnight. After the reaction completed, the solution was concentrated using a rotary evaporator and diluted with 200 mL ethyl acetate. The solution was washed with DI water (150 mL×2) and brine (150 mL×1). EA phase was collected and dried with MgSO4. After filtration, the solution was concentrated onto silica. Column chromatography (SiO2, 0˜40% EtOAC/hexane gradient eluent) gave compound C2 (˜6 g). Crosslinker C2 can self-crosslink at high concentration under vacuum, thus it was directly prepared as a stock solution (200˜300 mg/mL) in dioxane. 1H NMR (500 MHz, CDCl3): δ 6.34-6.40 (dd, J=17.4, 1.4 Hz, 2H), 6.13-6.07 (dd, J=17.3, 10.4 Hz, 2H), 5.83-5.80 (dd, J=10.4, 1.5 Hz, 2H), 4.18-4.18 (t, J=6.0 Hz, 4H), 4.08-4.10 (t, J=6.1 Hz, 4H), 2.33-2.30 (m, 4H), 1.76-1.63 (m, 12H). 13C NMR (126 MHz, CDCl3) δ 173.41, 166.29, 130.85, 128.55, 64.10, 63.95, 33.96, 25.42, 24.48. HRMS-ESI (m/z): [M+H]+ calculated for C20H30O8, 399.2013; observed 399.2013. [M+Na]+ calculated for C20H30O8 421.1833; observed 421.1836
To a 500 mL round bottom flask (RBF), 1a (2 g, 4.4 mmol), EDC-HCl (1.69 g, 8.9 mmol), DMAP (108 mg, 0.88 mmol) were added sequentially to 50 mL acetonitrile. 4-hydroxybutyl acrylate (1.4 g, 9.7 mmol) was added at the end. The reaction was stirred at r.t. overnight. After the reaction completed, the solution was concentrated using a rotary evaporator and diluted with 200 mL ethyl acetate. The solution was washed with DI water (150 mL×2) and brine (150 mL×1). EA phase was collected and dried with MgSO4. After filtration, the solution was concentrated onto silica. Column chromatography (SiO2, 0˜40% EtOAC/hexane gradient eluent) gave compound C3 (˜2 g). Crosslinker C3 can self-crosslink at high concentration under vacuum, thus it was directly prepared as a stock solution (200˜300 mg/mL) in dioxane. 1H NMR (500 MHz, CDCl3): δ 7.28-7.26 (d, J=8.3 Hz, 4H), 6.81-6.79 (d, J=8.2 Hz, 4H), 6.42-6.38 (dd, J=17.4, 1.4 Hz, 2H), 6.15-6.09 (dd, J=17.3, 10.4 Hz, 2H), 5.84-5.82 (dd, J=10.4, 1.4 Hz, 2H), 4.33-4.32 (m, 2H), 4.20-4.17 (m, 8H), 3.76-3.74 (m, 2H), 1.78-1.71 (m, 8H). 13C NMR (126 MHz, CDCl3) δ172.16, 166.26, 137.38, 131.47, 130.93, 129.54, 128.50, 120.74, 64.81, 64.00, 44.50, 43.41, 25.38, 25.36. HRMS-ESI (m/z): [M+H]+ calculated for C32H34Br2O8, 705.0693; observed 705.0681. [M+NH4]+ calculated for C32H34Br2O8 722.0959; observed 722.0950. [M+Na]+ calculated for C32H34Br2O8 727.0513; observed 727.0539.
To a 50 mL round bottom flask (RBF) charged with a stir bar, 2-hydroxyethyl acrylate (5 g, 4.95 mL, 43 mmol) and pyridinium p-toluenesulfonate (PPTS) (500 mg, 2 mmol) were added sequentially to 15 mL THF. 2-Methoxypropene (1.67 g, 2.22 mL, 23 mmol) solution in 15 mL THE was added dropwise at 0° C. The reaction was allowed to react overnight. After the reaction was finished, a few drops of TEA were added to the solution, and the solution was concentrated onto silica. Column chromatography (SiO2, 0˜ 40% EtOAC/hexane gradient eluent, 1% TEA was added) gave compound C4 (˜3 g). Crosslinker C4 can self-crosslink at high concentration under vacuum, thus it was directly prepared as a stock solution (˜200 mg/mL) in DCM. 1H NMR (500 MHz, CDCl3): δ 6.40-6.36 (dd, J=17.3, 1.5 Hz, 2H), 6.13-6.08 (dd, J=17.4, 10.4 Hz, 2H), 5.82-5.77 (dd, J=10.5, 1.5 Hz, 2H), 4.29-4.19 (m, 4H), 3.67-3.65 (m, 4H), 1.35 (s, 6H). 13C NMR (126 MHz, CDCl3) δ 166.19, 131.03, 128.37, 100.29, 63.93, 58.97, 24.84. HRMS-ESI (m/z): [M+Na]+ calculated for C13H20O6 295.1152; observed 295.1152.
Synthesis of 1g. Compound if was synthesized as previously reported (Bowser 2021). To a 25 mL RBF charged with a stir bar, compound if (500 mg, 0.68 mmol) and 3-Butyn-lol (124 μL, 115 mg, 1.6 mmol) were mixed with 8.2 mL TEA. The mixture was purged with N2 for 10 mins. Under nitrogen atmosphere, CuI (2.6 mg, 13.6 μmol, 0.02 equiv. to 10) and Pd(PPh3)2Cl2 (19.2 mg, 27.4 μmol, 0.04 equiv. to if) were added sequentially. The reaction was heated to 60° C. and stirred under N2 atmosphere for overnight. After the reaction was finished, TEA was evaporated using rotary evaporator. The mixture was diluted with EA and washed with dilute (1˜2%) HCl DI water solution (150 mL×2) and brine (150 mL×1). EA phase was collected and dried over MgSO4. Column chromatography (SiO2, 0˜80% EtOAC/hexane gradient eluent) gave compound 1g as thick yellow oil (347 mg). The yellow color was likely due to the metal residual, which can be removed by QuadraPure® TU. 1H NMR (500 MHz, CDCl3): δ 7.17-7.15 (d, J=8.2 Hz, 4H), 6.85-6.83 (d, J=8.0 Hz, 4H), 5.35-5.31 (m, 2H), 4.37-4.36 (m, 2H), 4.16-4.05 (m, 4H), 3.80-3.74 (m, 6H), 2.66-2.63 (t, J=6.2 Hz, 4H), 2.06-2.00 (m, 4H), 1.66-1.60 (m, 4H), 1.36-1.26 (m, 24H). 13C NMR (126 MHz, CDCl3) δ 172.32, 138.70, 131.58, 130.97, 127.85, 121.48, 86.48, 65.64, 61.27, 44.82, 43.48, 32.19, 29.77, 29.54, 29.44, 29.00, 28.77, 28.19, 26.41, 23.96. HRMS-ESI (m/z): [M+H]+ calculated for C46H60O6, 709.4463; observed 709.4467.
Synthesis of 1h. To a 50 mL three-neck RBF charged with a stir bar, compound 1g (347 mg, 0.49 mmol) was dissolved in 25 mL THF. The solution was purged with N2 for 10 min. Under nitrogen atmosphere, Pd/C (10% Pd) (30 mg) was added portion-wise. The mixture was purged with N2 for another 10 min. The hydrogen balloon was connected to the RBF. The mixture was purged with hydrogen for 10 seconds and was allowed to react under hydrogen atmosphere for 48 hours. After the reaction was finished, the solution was filtered with celite. The filtrate was loaded onto silica by rotary-evaporating the solvent. Column chromatography (SiO2, 0˜80% EtOAC/hexane gradient eluent) gave compound 1h as thick colorless oil (312 mg). 1H NMR (500 MHz, CDCl3): δ 6.91-6.89 (d, J=7.9 Hz, 4H), 6.82-6.81 (d, J=8.0 Hz, 4H), 4.34-4.33 (m, 2H), 4.15-4.06 (m, 4H), 3.80-3.78 (m, 2H), 3.60-3.57 (t, J=6.4 Hz, 4H), 2.51-2.48 (t, J=7.4 Hz, 4H), 1.65-1.46 (m, 16H), 1.37-1.28 (m, 30H). 13C NMR (126 MHz, CDCl3) δ 172.71, 140.29, 136.37, 128.09, 127.92, 65.32, 62.84, 44.72, 43.63, 35.16, 32.22, 29.21, 29.16, 28.84, 28.72, 28.51, 28.13, 27.65, 27.63, 27.48, 25.90. HRMS-ESI (m/z): [M+H]+ calculated for C46H70O6, 719.5245; observed 719.5249.
Synthesis of 1i. To a 25 mL RBF charged with a stir bar, compound 1h (312 mg, 0.43 mmol) and DMAP (10 mg, 82.9 μmol) were dissolved in 10 mL DCM. 4-pentenoic anhydride (166.5 μL, 166 mg, 0.91 mmol, 2.1 equiv. to 1 h) was added dropwise. The reaction was stirred overnight. After the reaction was finished, the solution was loaded onto silica by evaporating solvent. Column chromatography (SiO2, 0˜20% EtOAc/hexane gradient eluent) gave compound 1i as colorless oil (291 mg). 1H NMR (500 MHz, CDCl3): δ 6.90-6.89 (d, J=7.9 Hz, 4H), 6.83-6.81 (d, J=7.9 Hz, 4H), 5.77-5.85 (ddt, J=16.4, 10.2, 6.0 Hz, 2H), 5.06-4.79 (m, 4H), 4.33-4.34 (m, 2H), 4.15-4.06 (qt, J=10.8, 7.0 Hz, 4H), 4.05-4.03 (m, 4H), 3.79-3.77 (m, 2H), 2.50-2.48 (m, 4H), 2.41-2.33 (m, 8H), 1.67-1.61 (p, J=7.0 Hz, 4H), 1.57-1.53 (m, 8H), 1.37-1.30 (m, 32H). 13C NMR (126 MHz, CDCl3) δ 173.24, 172.67, 139.98, 136.84, 136.50, 128.08, 127.97, 115.61, 65.33, 64.34, 44.67, 43.75, 34.96, 33.69, 29.20, 29.16, 29.02, 28.84, 28.72, 28.51, 28.17, 28.13, 27.65, 27.62, 25.90. HRMS-ESI (m/z): [M+H]+ calculated for C56H82O8, 883.6083; observed 883.6078. [M+NH4]+ calculated for C56H82O8 900.6348; observed 900.6342. [M+Na]+ calculated for C56H82O8 905.5902; observed 905.5900.
Synthesis of 1j. To a 500 mL RBF charged with a stir bar, compound 1i (291 mg, 0.33 mmol) was dissolved in 200 mL DCM. Grubbs 2nd catalyst (14 mg, 16.5 μmol, 0.05 equiv. to 1i) was then added. The solution was refluxed overnight and quenched with several drops of vinyl ethyl ether. The solution was loaded onto silica gel by evaporating the solvent. Column chromatography (SiO2, 0˜20% EtOAc/hexane gradient eluent) gave compound 1j as colorless oil (220 mg). 1H NMR (500 MHz, CDCl3): δ 6.88-6.85 (m, 4H), 6.80-6.77 (m, 4H), 5.46-5.39 (m, 2H), 4.33-4.32 (m, 2H), 4.16-4.07 (m, 4H), 4.03-4.00 (m, 4H), 3.82-3.80 (m, 2H), 2.50-2.47 (t, J=7.2 Hz, 4H), 2.35-2.30 (m, 8H), 1.67-1.62 (p, J=7.0 Hz, 4H), 1.57-1.45 (m, 8H), 1.37-1.30 (m, 32H). 13C NMR (126 MHz, CDCl3) δ 173.19, 172.70, 139.86, 136.30, 129.64, 127.98, 65.34, 64.27, 44.88, 43.19, 34.81, 34.30, 29.21, 29.17, 28.85, 28.73, 28.53, 28.14, 28.01, 27.95, 27.67, 27.64, 27.56, 25.91. HRMS-ESI (m/z): [M+H]+ calculated for C54H78O8, 855.5770; observed 855.5777.
Synthesis of P1. A 2 mL crimp top vial was charged with 1j (46.3 mg, 0.4 equiv.) and freshly distilled 9-oxabicyclo[6.1.0]non-4-ene (10.4 mg, 0.6 equiv.) under N2 (g). A stock solution of Grubbs Catalyst 2nd Generation in dry DCM (0.66 mg/mL) was prepared and sparged with N2 (g) for 10 minutes. Then, the stock solution (0.1 mL) containing the Grubbs Catalyst (1/1500 equiv.) was added via an air-tight syringe to dissolve the monomers to the concentration of 1 M and initiate the polymerization. After 16 hours, the polymerization was quenched with 5 drops of ethyl vinyl ether and then precipitated into methanol to give the crude polymer. Polymers were purified via two additional precipitations into MeOH and one reverse precipitation from DCM. The polymer was dried on a high vacuum for at least 1 hour prior to use. 1H NMR (500 MHz, CDCl3): δ 6.90-6.88 (m, 4H), 6.83-6.81 (m, 4H), 5.51-5.44 (m, 4.7H), 4.34-4.33 (m, 2H), 4.15-4.10 (m, 8H), 3.78-3.77 (m, 2H), 2.94-2.88 (m, 2.9H), 2.50-2.47 (m, 4.3H), 2.36-2.12 (m, 13.7H), 1.67-1.54 (m, 20H), 1.37-1.29 (m, 33H). Mn=354 kDa, PDI=1.38.
To a 100 mL round bottom flask (RBF), acrylic acid (240 mg, 3.3 mmol), compound 1h (800 mg, 1.11 mmol), DMAP (54.3 mg, 0.44 mmol) were dissolved in 20 mL DCM. EDC-HCl (640 mg, 6.4 mmol) was added portion-wise at the end. The reaction was stirred at r.t. overnight. After the reaction was completed, the solution was concentrated onto silica. Column chromatography (SiO2, 0˜ 40% EtOAC/hexane gradient eluent) gave compound C5 (340 mg). Crosslinker C5 can self-crosslink at high concentration under vacuum, thus it was directly prepared as a stock solution (200˜ 300 mg/mL) in DCM. 1H NMR (500 MHz, CDCl3): δ 6.91-6.89 (d, J=8.1 Hz, 4H), 6.83-6.81 (d, J=8.1 Hz, 4H), 6.40-6.36 (dd, J=17.3, 1.5 Hz, 2H), 6.13-6.07 (dd, J=17.3, 10.4 Hz, 2H), 5.82-5.79 (dd, J=10.5, 1.5 Hz, 2H), 4.34-4.33 (m, 2H), 4.16-4.06 (m, 8H), 3.79-3.78 (m, 2H), 2.52-2.49 (t, 4H), 1.66-1.58 (m, 12H), 1.34-1.30 (m, 32H). 13C NMR (126 MHz, CDCl3) δ 172.65, 166.39, 139.95, 136.48, 130.67, 128.68, 128.08, 127.97, 65.32, 64.50, 44.68, 43.72, 34.95, 29.20, 29.16, 28.84, 28.73, 28.51, 28.13, 28.12, 27.65, 27.63, 25.90. HRMS-ESI (m/z): [M+H]+ calculated for C52H74O8, 827.5457; observed 827.5456. [M+NH4]+ calculated for C52H74O8 844.5722; observed 844.5719. [M+Na]+ calculated for C52H74O8 849.5276; observed 849.5277.
To a 25 mL RBF charged with a stir bar, compound 1e (200 mg, 0.43 mmol) and DMAP (31 mg, 256 μmol) were dissolved in 10 mL DCM. Acetic anhydride (101 μL, 109 mg, 1.07 mmol, 2.5 equiv. to 1e) was added dropwise. The reaction was stirred overnight. After the reaction was finished, the solution was loaded onto silica by evaporating solvent. Column chromatography (SiO2, 0˜40% EtOAc/hexane gradient eluent) gave compound 1k as colorless oil (101 mg). 1H NMR (500 MHz, CDCl3): δ 6.91-6.89 (d, J=8.0 Hz, 4H), 6.82-6.81 (d, J=8.2 Hz, 4H), 4.34-4.33 (m, 2H), 4.03-4.01 (m, 4H), 3.81-3.80 (m, 2H), 3.74 (s, 6H), 2.51-2.48 (m, 4H), 2.03 (s, 6H), 1.56-1.53 (m, 8H). 13C NMR (126 MHz, CDCl3) δ 173.16, 171.31, 140.10, 136.21, 128.14, 127.94, 64.45, 52.28, 44.80, 43.50, 34.98, 28.14, 27.66, 21.14. HRMS-ESI (m/z): [M+H]+ calculated for C32H40O8, 553.2796; observed 553.2796. [M+Na]+ calculated for C32H40O8 575.2614; observed 575.2614.
Network Preparation
Fresh acrylate monomer was prepared by passing it through a basic aluminum oxide pad to remove the inhibitor before every network preparation. Here we use a pair of elastomers (E1 and E2) with a stoichiometry of [M]:[C]:[CTA]:[PI]=1:1/50:1/1200:1/2000 as an example. Other elastomers were prepared in the same manner. A pre-gel solution was prepared by mixing 2-methoxyethyl acrylate (M) (7 g, 1 equiv.), 4-Cyano-4-(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid (CTA) (18.1 mg, 1/1200 equiv.) and α-ketoglutaric acid (PI) (3.93 mg, 1/2000 equiv.) in a 45 mL scintillation vial. The pre-gel solution was sonicated for 5 minutes to obtain a homogeneous solution. The pre-gel solution was separately transferred into two 20 mL scintillation vials (3 mL each) with a pipette. The desired amount of different stock solutions of crosslinkers (200˜300 mg/mL) was separately added to these two vials to obtain two solutions with the same ratio of [M]: [C]: [CTA]: [PI]=1:1/50:1/1200:1/2000. Since the stock solution of crosslinkers may have had different concentrations, the desired amount of solvent was added to ensure the same concentration for both solutions. These two vials with different crosslinkers were put onto a rotary evaporator to remove the volatile component. Afterward, the solutions were purged with nitrogen for 3 minutes. Each solution was passed through a 1 m pore size PTFE syringe filter to remove dust and transferred into a “sandwich” mold (
Network Characterization
Rheology. Network films were cut to size with an 8 mm diameter biopsy punch. Rheological measurements were conducted on an Anton Paar MCR 302 rheometer with an 8 mm parallel plate geometry. Frequency sweep measurements were conducted at 22-23° C. with a constant 0.5% shear strain, well within the linear viscoelastic regime based on initial strain sweeps. Constant temperature was maintained with a Peltier temperature control stage. Three samples from different positions in the network film were punched out for shear moduli measurement. Frequency sweep data are shown in
Sol fraction and equilibrium swelling ratio. After rheology tests, the disc samples (×3) were weighted and submerged in ethyl acetate solvent to extract sol fractions. The solvent was changed every 24 hours until the mass of the dry samples stops changing. The mass of the dry samples usually reaches constant after three times of solvent exchange. The sol fractions were calculated by (mass loss)/(initial mass)×100%. The dry samples were then submerged in DMF to reach equilibrium swelling. The equilibrium swelling ratios were calculated by (mass of the swollen sample)/(mass of the dry sample). The results of sol fraction and equilibrium swelling ratios are shown in
Differential Scanning Calorimetry (DSC). Samples of E1 and E2 were performed following this procedure (−50° C. to 50° C., 10° C./min; isothermal 50° C., 3 min; 50 to −50° C., 10° C./min; isothermal −50° C., 3 min; −50° C. to 50° C., 10° C./min; isothermal 50° C., 3 min; 50 to −50° C., 10° C./min), wherein the thermal transitions for the heating cycle were recorded. The glass transition temperature (Tg) was determined by the inflection point of the heat capacity with temperature sweep (second cycle). The Tg of E2 is consistent with reported data (Chen et al. J. Mater. Chem. A 7, 17334-17344 (2019)) of linear poly 2-methoxyethyl acrylate, where Tg of E1 is slightly higher, but still well below room temperature (
Tearing energy and fatigue threshold. The fatigue tests were conducted following the previous method (Wang et al. J. Am. Chem. Soc. 143, 3714-3718 (2021)). Samples for tearing energy were cut with a razor blade as a 15×20 mm rectangle. This was reduced to a 20×3 mm area after clamping. Typically, each measurement used 3 un-notched samples and 5 notched samples; for notched samples, a ˜5 mm cut was made in the center of one side of the piece, perpendicular to the edge. The exact thickness and width were measured with calipers before each test. Samples were loaded into the clamps at a gauge length of ˜2 mm, then stretched to a force of about 0.01 N, which resulted in an initial gauge length of 2.5+0.5 mm. Unnotched samples and notched samples were pulled to failure at a constant strain rate of 0.2% s−1. Tearing energy was calculated using the Rivlin-Thomas method (Rivlin et al. J. Polym. Sci. 10, 291-318 (1953)) where the strain energy is obtained by integrating the un-notched stress-strain curve to the strain at which the crack of the notched samples began to propagate. Results are shown in
The fatigue tests were conducted based on a method described by Suo, Zhao, and coworkers (Grandbois et al. Science 283, 1727-1730 (1999); Wang et al. Science 374, 193-196 (2021)). Samples for the fatigue test were cut with a razor blade as a 30 mm×30 mm rectangle. This was reduced to around 30×3 mm area after clamping. Acrylic sheets and Buna-N rubber spacers were used to achieve a wide and firm grip for these samples (
Thermal stability of E1 and its cyclobutane crosslinks. Elastomer E1 prepared with a stoichiometry [M]: [C]: [CTA]: [PI]=1:1/50:1/1200:1/2000 was submerged in DMF to reach equilibrium swelling (
Compound 1k was synthesized from 1e to mimic the crosslinks in E1. Thermogravimetric analysis (TGA) (
The approach is shown in
Experiments were started with a stoichiometry of [M]: [C]: [CTA]: [PI]=1:1/50:1/1200:1/2000. The reactivity of alkyl acrylates is largely independent of the character of the alkyl group (Haehnel et al. Macromolecules 47, 3483-3496 (2014)). Thus, polymerizing the same pre-gel solution with C1 and C2 will lead to effectively identical networks that differ only in the mechanically coupled reactivity of the cross-linker. This was verified through several characterizations. First, shear moduli of E1 and E2 (blue and red in
Although these two elastomers have similar network connectivity, it was noticed that their unnotched films break very differently when stretched either by hand (not shown) or by dynamic mechanical analysis (DMA) (
The enhanced toughness brought about by replacing C2 with C1 is not limited to networks made from a 2-methoxyethyl acrylate co-monomer, to the elastomer state, or to the specific characterization test—although the magnitude of the effect does change. For example, the same trend observed in tearing is also observed in fatigue testing, which minimizes contributions to the tearing resistance from the energy that is dissipated rather than stored elastically during network stretching (hysteresis) by slowing down the crack growth rate with cyclic loading (Lake et al. J. Appl. Polym. Sci. 9, 1233-1251 (1965); Tang et al. Extrem. Mech. Lett. 10, 24-31 (2017)). As shown in
The results raise the important question of why the effect of introducing the same scission reaction in side-chain crosslinked networks is almost the exact opposite of what is observed in the end-linked networks of
Toughening mechanism of E1. The consequences of the topology are proposed in
Random network cross-linking comprises a complex mixture of intramolecular and intermolecular junctions and loops (Wang et al. Chem. Sci. 10, 5332-5337 (2019)). The programmed cycloreversion of intramolecular C1 releases hidden length in a manner that is similar to noncovalent domain unfolding (Chung et al. Nat. Mater. 13, 1055-1062 (2014); Fang et al. Nat. Commun. 4, (2013)) and covalent reactive strand extension (RSE) (Wang et al. Science 374, 193-196 (2021)), while that of intermolecular C1 deviates the crack and increases pathway tortuosity. The effect of C1 is reminiscent of the toughening effects observed in most dynamic side-chain crosslinked network systems (Sun et al. Nature 489, 133-136 (2012); Neal et al. J. Am. Chem. Soc. 137, 4846-4850 (2015); Sakai et al. ACS Macro Lett. 9, 1108-1113 (2020); Du et al. Nat. Commun. 2022 131 13, 1-9 (2022)). The dynamic nature of the crosslinking makes networks formed by such “sacrificial bonds” prone to greater hysteresis and viscous energy dissipation during use. Although the magnitude of the overall toughening observed from the use of C1 is smaller than from the addition of weak, dynamic cross-linking (Sun 2012; Du 2022), it comes with negligible hysteresis (
Based on the image in
Elastomers of differing Np were prepared with either C1 or C2 and the moduli and swelling were again indistinguishable between mechanophore and control networks for all Np(
The reactivity-enabled toughening afforded by C1 provides a mechanism to mitigate an otherwise intrinsic tradeoff in polymer network optimization, namely the inverse correlation of modulus and toughness. At fixed Np≈1400, increasing the cross-linker content ([C]:[M]=1:200, 1:100, 1:50) leads to high modulus materials. As expected, this increase in modulus is accompanied by a significant drop of T in the control elastomer E2, with a scaling exponent of −1.7 that is similar to that reported in other side-chain crosslinked systems (
The decrease of T for E1 with higher cross-linking (
How the reactivity of the cross-linker, relative to the primary chain, influences the effect of cross-linker mechanochemical reactivities on I′ was postulated. Another mechanophore cross-linker C3 of intermediate strength (
Finally, a cross-linker that reproduces the mechanochemical reactivity of C1, but which leads to molecular extension rather than scission, was tested. A cross-linker C5 (synthetic details provided in the supplementary materials), in which initial cyclobutane scission analogous to that found in C1 leads to the release of local stored length, but complete scission of the cross-linker needs subsequent homolytic bond scission in a manner analogous to C2 was synthesized. The new cross-linker C5 was incorporated into elastomer E5 following the same procedure as employed to make E1 and E2. Consistent with the models proposed above, E5 displays at best a very modest toughening effect compared to E2 (
Results presented above suggest that the crosslinker needs to be weaker than the primary chain to achieve this toughening effect, but it is not known what relative strength between the crosslinker and the primary chain could give maximum effect. An “intermediate” crosslinker C3 (synthesis of C3 is shown in the section chemical synthesis) was synthesized based on a precursor of C1. Albeit no direct SMFS characterization on this specific cyclobutane, similar cis-diester substituted cyclobutanes characterized by SMFS show activation forces of ˜2 nN on the timescales of milliseconds (Wang et al. Nat. Commun. 7, 13433 (2016)), making C3 still a preferential site for breaking in networks compared to the primary chains. Elastomer E3 was made from C3 in the same way as E1 and E2, and they have similar shear moduli and stress-strain curves (
To show that the chain transfer agent has good control over the molecular weight of primary chains in the networks, an experiment shown in
GPC traces of PA1 and PA2 prepared with 4 different stoichiometries are shown in
Elastomers reported in
To investigate whether the C1 can be activated by cyclic tensile loading close to the strain-at-break, cyclic loading tests were performed to maximum strains of 0.7, 0.73, 0.76, and 0.79 for E1 prepared with a stoichiometry of [M]: [C]: [CTA]: [PI]=1:1/50:1/1200:1/2000. The results are shown in
General simulation details. The Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) package (Plimpton et al. J. Comput. Phys. 117, 1-19 (1995)) was used with the coarse-grained bead-spring model (Grest et al. Phys. Rev. A 33, 3628 (1986); Kremer et al. J. Chem. Phys. 92, 5057 (1990)) of polymer chains. Monomers (beads) are interacting via the Lennard-Jones (LJ) potential (Johnson et al. Mol. Phys. 78, 591-618 (1993); Frenkel, D. & Smit, B. Understanding Molecular Simulation: Monte Carlo Simulations in Various Ensembles. (Academic Press, 2002)).
U
b(r)=UFENE(r)+ULJ(r;rc) (S3)
In all simulations, the pressure was held constant (at P=0) using the Nose-Hoover barostat with a damping parameter of 100 τLJ. The temperature was held constant (kT=1 in Lammps units) using the Nose-Hoover thermostat with a damping parameter of 10 τLJ. All molecular dynamics simulations were performed with an integration time-step of 0.01 τLJ.
Network Formation. A precursor melt of M=400 chains with n=64 beads each was equilibrated in the NPT ensemble with P=0 by simulating a long trajectory, 106τLJ, which corresponds to ˜102τR, where τR˜104τLJ is the relaxation time of a chain (Halverson, J. D., Lee, W. B., Grest, G. S., Grosberg, A. Y. & Kremer, K. Molecular dynamics simulation study of nonconcatenated ring polymers in a melt. I. Statics. J. Chem. Phys. 134, 204904 (2011)). The equilibrated melt simulations were instantaneously randomly crosslinked emulating gamma irradiation crosslinking (Leibler et al. Phys. Rev. Lett. 55, 1110 (1985); Grest et al. Macromolecules 23, 4994-5000 (1990)), A snapshot of the equilibrated melt was vulcanized by randomly selecting and bonding monomer pairs at a distance below 1.3σ, which corresponds to the first peak in the melt radial distribution function. The extent of reaction of this vulcanization process was p=0.1. The only restriction imposed on the selection of pairs of monomers for bonding was to exclude any monomers at a curvilinear distance of s=1,2 bonds away from an existing crosslinked monomer, thereby setting the minimal strand length to 2 and the minimum loop length to 4. The networks were equilibrated for 105τLJ at the NPT ensemble at P=0, followed by a production run of 9·105τLJ. The average size of the simulation box was determined from the production run and used for the ensuing pulling simulations.
Network topology. The analysis of the vulcanized network topology was carried out using our in-house code that is based on the analogy between elasticity and electrical resistivity (Gennes et al. J. Phys. Lettres 37, 1-2 (1976)). The network is mapped onto a network of nodes (formed by two monomers bonded by a crosslink) and edges (all strands connected to these nodes). The NetworkX python package (Hagberg et al. in Proceedings of the 7th Python in Science Conference (SciPy 2008) (2008)) is used for identifying the basis set of loops describing the network (Paton, ACM 12, 514-518 (1969)). In the following step, the network is modeled as a collection of resistors (resistivity of strands is proportional to Nx−1) connected at nodes (crosslinks). The voltage of one strand in the largest loop is then set to some finite values and the Kirchhoff circuit laws are then applied by requiring 1) the sum of all currents passing through a node to be zero and 2) the voltage difference along any closed loop is zero. The numerical solution of this set of equations yields the current in each strand in the network, whereby those strands that carry current as elastically active strands are identified.
The topology analysis was applied to the network formed by random crosslinking as described above. The gel fraction of this network is over 99%, with 66% of the mass being elastically active. The average length (i.e., number of bonds) of the elastically active strands is Nx
=11.7.
Calibrating breakable bonds. In the bond scission simulations, the unbreakable FENE bond potentials were replaced by the Morse potential (Morse, Phys. Rev. 34, 57 (1929)) for the breakable bonds,
Two networks were simulated with these two bond potentials: (i) A strong crosslinker network with both primary chain bonds and crosslinker De=120 kT; (ii) A weak crosslinker network with primary chain bonds having De=120 kT and crosslinker De=30 kT.
Simulations of bonds scission upon uniaxial extension. The equilibrated networks are uniaxially stretched by changing the z dimension of the simulation box, Lz, from its unperturbed average size, Lz,0, while the other two dimensions are relaxed at zero pressure by the Nose-Hover barostat. This mimics the strain-control uniaxial stretching experiments. The network is stretched at a constant strain rate of 10−5τ−1 up to the strain (Lz−Lz,0)/Lz,0=6. As shown in
Effect of bond breaking. The effect of breaking a bond on the change of average strand lengths ΔNx,neigh
that are affected by the bond rupture event (neighboring strands of a broken bond) is shown. If the broken bond is a crosslinker, the neighboring strands are defined as the four strands it is connected to. If the broken bond is a primary chain bond, the neighboring strands are defined as the strand it is located on and all other strands directly connected to this strand. The definition of neighboring strands associated with a broken bond is shown in
The distinct effect of breaking either primary chain bonds or crosslinkers and the dominance of these two types of bonds breakage (primary chain bonds or crosslinkers, ΔNx,neigh
due to a broken bond. For the weak crosslinker network, the change of neighboring strand length is about the average strand length of the network (Nx), while the change in the strong crosslinker network is only ˜
Nx
/5. These effects will be magnified for the bridging strand near the crack tip where multiple crosslinkers on its primary chain could break and its length grows by
Nx
for every broken crosslinker. According to the Lakes-Thomas theory, the fracture energy is proportional to the length of these bridging strands. Therefore, the fracture energy of the weak crosslinker network will be increased by breaking crosslinkers in comparison to the strong crosslinker network where the breaking bridging strands are relatively shorter ˜
Nx
Inter- vs. intra-molecular crosslinkers. In the simulated network, the number of intermolecular crosslinkers is 4.5 times higher than the intramolecular ones. As shown in
When considered in combination with quantitative relationships between mechanophore activity and material properties (Wang et al. J. Am. Chem. Soc. 143, 3714-3718 (2021); Wang et al. Science 374, 193-196 (2021)), the work disclosed herein accentuates how the position (along the polymer chain vs. at the cross-links), type (scissile vs. non-scissile), and mechanochemical reactivity of mechanophores work in combination to dictate the mechanical properties of the networks in which they are incorporated. When mechanophores are incorporated along every stress-bearing polymer strand within a network, there is a direct correlation between mechanochemical reactivity and network toughness—the easier it is to break the mechanophore, the easier it is to tear the network (Wang et al. J. Am. Chem. Soc. 143, 3714-3718 (2021)). Replacing strands of scissile mechanophores with analogous strands of non-scissile mechanophores, however, leads to a toughening effect (Wang et al. Science 374, 193-196 (2021)), because the latter release “hidden length” rather than breaking, and the enhanced properties at the single strand level are translated to the macroscopic materials. When mechanophores are incorporated as otherwise conventional side-chain cross-linkers, however, the responses are opposite. Mechanochemically more reactive (weaker) mechanophores lead to tougher rather than more brittle networks (
From these observations, the macroscopic properties of polymer networks can be directly correlated with molecular reactivity and mechanisms. Fracture in cross-linked networks is productively viewed as a sort of “molecular composite,” and that picture has implications for the use of “weak” bonds to toughen covalent polymer networks. Typically, toughening a network through weak cross-linkers involves the use of dynamic interactions, such as ionic bonding (Sun 2012), hydrogen bonding (Neal 2015), the reversible formation of stable radicals (Sakai 2020; Sakai 2020; Watabe et al. Macromolecules 55, 5795-5802 (2022)), or dynamic covalent bonding (Du 2022). Without introducing non-dynamic covalent cross-linkers, all of these systems would eventually lead to a fluid network structure and added, potentially undesirable viscous dissipation on longer timescales or at higher temperatures. The capacity to recombine and/or exchange during or following the scission of dynamic bonds also complicates their toughening mechanism even in hybrid networks that use partial covalent cross-linking to provide long-term shape persistence (Sun 2012; Miquelard-Garnier et al. Polymer 50, 481-490 (2009); Long et al. Macromolecules 47, 7243-7250 (2014)). No such scrambling or reformation is accessible in E1 because C1 decouples its labile mechanochemical reactivity from thermal reversibility. These results demonstrate that preferential bond scission at the propagating crack front alone is sufficient to provide a substantive toughening effect. It is noted that pathway tortuosity at the macroscale, for example in composites, is an established toughening mechanism (Ritchie et al. Int. J. Fract. 100, 55-83 (1999)), and the results observed here indicate that molecular analogues of that behavior likely contribute to the more complex toughening mechanisms at play in reversible networks. For both those dynamic networks and the static covalent networks demonstrated here, the primary chain length effect offers a clear design principle for optimizing reactivity-enabled toughening without losing stiffness.
Toughening effects can also be obtained through other mechanisms, such as dynamic bonding and entanglements. An advantage of toughening through the programmed reactivity of covalent cross-linkers is that the properties of the mechanophore network are indistinguishable from those of the conventional network, with the exception of the preferential scission behavior that occurs only when and where necessary to inhibit material tearing and fatigue; even the primary chain effect has a very modest impact on low-strain mechanical properties relative to the impact on toughness. The utility of C1 shows that these gains can be realized with mechanophores of good thermal stability (Zhang 2017) (
This application claims the benefit of and priority to U.S. Provisional Patent Application No. 63/356,292, filed on Jun. 28, 2022, the contents of which are incorporated by reference herein in their entirety.
This invention was made with government support under CHE-2116298 awarded by the National Science Foundation. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 63356292 | Jun 2022 | US |
