1Reaction with Mechanical Force

2014/1/27 Kiyoshi Aoki (B4)

0. Introduction
what kind of mechanical force?
⋅compression
cut⋅extension⋅ultrasoundone example (how to use ultrasound)mechanophore (cf. p24)
retro DAanthracene

maleimide

C. W. Bielawski, et al. J. Am. Chem. Soc. 2011, 133, 7180.
ultrasound induced retro DA reation
PMA backbone
ultrasound in CH3CN, 9 ºC, 3 hOOnPMA

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ultrasound ( how to generate mechanical stress )Chem. Rev. 2009, 109, 5755.
The mechanism of converting ultrasound waves to energy in the form of force on a polymer chain is through aprocess called cavitation.
Cavitation is defined as the nucleation, growth, and collapse of bubbles in a liquid.
Polymer chains near a collapsing bubble experience solvodynamic forces as the proximal chain end is pulled
toward the void creating by the imploding bubble.

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no rxn without PMA
mechanophore
forceforce

C. W. Bielawski, et al. J. Am. Chem. Soc. 2011, 133, 7180.
Thermally prohibited retro Diels-Alder reaction was achieved. (cf. 110 ºC, 24 h)blue

When heating (60ºC, 48 h), recombination wasobserved.
*Purpose of investigation
for reaction with mechanical force
There are two reaction types.
active use of mechanical force
to achieve otherwise impossible reaction
passive use of mechanical force

GPC traces
black: before sonication
blue: after sonication
green: after heating

for material scientific application(damage responsive polymer)

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1. active use of mechanical force(1) triazole to azide and alkyne (unclicking)
C. W. Bielawski, et al. Science 2011, 333, 1606.

Ultrasound, CH3CN, 0 ºC, 2 h, quant.
CuI, CH3CN 80 ºC, 19 h ,86%

GPC trace

Triazole, although remarkably inert toward chemical and thermal
perturbation, undergoes cycloreversion through the applicationof mechanical force.
function as mechanicall labile protecting groupred

black: before sonication
red: after sonication
blue: after reclick
green: only heating(258 ºC, 19 h)
cf. 力学的エネルギーで”逆”クリック！（chem-station)

5*Regiochemical Effects
C. W. Bielawski, et al. J. Am. Chem. Soc. 2012, 134, 9882.
The rate of cycloreversion was found to be 20% greater for the 1,5-disubstituted regioisomer than for its 1,4congener.
This is explained byincreased molecular compliance(steric interaction) for TS geometry
anddirect bond activation.
The relative regiochemistry of
the polymer attachments
on a mechanophore must be considered‚
when evaluating or designingmechanochemical systems.

1,4
1,5

method: Irradiating acetonitrile solutions of these polymers with ultrasound6(2) violating Woodward-Hoffman rules
biasing reaction pathways with mechanical force
S. R. White, J. S. Moore, et al. Nature 2007, 446, 423.
PEG: polyethylene glycolOnbenzocyclobutene

6-9 ºCultrasoundendo
N-(1-Pyrenyl)maleimide

The trans and cis isomers of a 1,2disubstituted benzocyclobutene
undergo an ultrasound-induced
electrocyclic ring opening in a formallyconrotatory and formally disrotatory
process, respectively,that yield identical products.
(not following Woodward-Hoffman rules)
cf．ウッドワード・ホフマン則を打ち破る『力学的活性化』（chem-station)

7(3)mechanical reconfiguration of stereoisomer (binol derivatives)
C. W. Bielawski, et al. J. Am. Chem. Soc. 2010, 132, 3256.

CH3CN, 24 h
Thermally restricted isomerization barriers can be surmounted by force.
Upon heating these same materials to temperatures exceeding 250 ºC for longer than 72 h
no isomerization was observed.

8*the combination of ultrasound-induced isomerization and enzymatic resolution
C. W. Bielawski, et al. Angew. Chem., Int. Ed. 2012, 51, 1640.
stereoselective enzymatic hydrolysis
no polymer chain→unreactive underultrasound
90% yield
98% ee25 ºC, 48 hactive use of mechanical force
developing field (few example)
need polymer
cf. 「引っ張って光学分割」(chem-station)

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2. passive use of mechanical force
for material scientific application (damage responsive polymer)(1) Reaction with production of reactive species(i) strengthening of a polymer*gem - dichlorocyclopropane
mechanically facilitated electrocyclic ring openings of gem - dichlorocyclopropanes
S.L. Craig, et al. J. Am. Chem. Soc. 2009, 131, 10818.

polybutadiene
backbone

polybutadiene backbone
Products were characterized by quantitative 1H NMR spectroscopy.
*gem-dibromocyclopropane
S.L. Craig, et al. J. Am. Chem. Soc. 2010, 132, 15936.
use Single-molecule force spectroscopy
allyl bromide10*Mechanochemical strengthening of a synthetic polymer
S. L. Craig, et al. Nat. Chem. 2013, 5, 757.
concept

TBA SA: the di-tetrabutylammonium
salt of sebacic acidgeneration of allylic bromide2(Bu) 4N+

nucleophilic substitution reactions
OBrBr

Stress activation

BrBr

1closed8O

BrOOO

TBA SA

Crosslinking

8OO

2(Bu) 4N+ BrO

1open

activated mechanophoremechanophore

Stress activation

BrCrosslinking

black: mechanophore red: activated mechanophore blue: crosslinker
backbone:polybutadiene11TBA SAultrasound in THF

Support for this crosslinking was provided by infrared spectroscopy.

12Elastic Modulus(MPa)polymer 1closed

7.6 ± 4.5

polymer 1open

7.8 ± 3.3+TBA SA

1.7 ± 0.5+TBA SA(extruded)

150 ± 85

*gem-difluorocyclopropanes (gDFCs) ~different reactivity~
Trapping a Diradical Transition Stateby Mechanochemical Polymer Extension
S. L. Craig, et al. Science 2010, 329, 1057.

6 to 9 ºC

achieved on demand strengthening
backbone: polybutadiene*Note that the trans isomers are more stable than the cis isomers.

Radical trapping support
diradical formation.
cis
The tension-induced stabilization of the 1,3-diradical in the polymer affords
the opportunity to reactively probe this trapped transition state structure.
→provides a tool for the study of reactive intermediates(another example of active use of mechanical force)

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The s-trans/ s-trans diradical is formed regardless of the cis ortrans arrangement.
When coupled to the applied force, the ring- opened diradical islower in energy than the ring- closed cyclopropanes.

14*epoxides
hypothesis
Epoxides also undergo mechanically triggered ring-opening
to carbonyl ylides?
electrocyclic ring openings of epoxides and the influence of polymer backbone
S.L. Craig, et al. J. Am. Chem. Soc. 2012, 134, 9577.
cf. regiochemical effect (p5)ultrasoundelectrocyclic
ring-opening

When epoxidized polybutadiene was sonicated,
there was no observable small molecule addition to the polymer.
In contrast, when epoxidized polynorbornene wassubjected to the same conditions,addition of ylide trapping reagents was observed.

polybutadienepolynorbornene

The cyclopentyl groups increase the efficiency of force
transduction? (not steric effect)cf. The critical force for reaction of gem-dihalocyclopropane (gDHC) (cf.p9) drops by about one-third
in the polynorbornene scaffold relative to polybutadiene.
S. L. Craig, et al. Nat. Chem. 2013, 5, 110.
PMA is often used because it is robust.

15~ self healing ~cf. Y. Wang, Self-healing polymers and composites (Lit. Seminar)

repairing crack or failure
Three primary conceptual approaches to self healing

Annu. Rev. Mater. Res. 2010, 40, 179.

16a. Capsule - Based Healing System
S. R. White, J. S. Moore, et al. Nature 2001, 409, 794
use of Grubbs catalyst in an epoxy matrix
2.5% (by weight) Grubbs’ catalyst and 10% (by weight) microcapsules
microcapsules consists of Urea Formaldehyde Resin
The material incorporates a microencapsulated healing agent
that is released upon crack intrusion.
Polymerization of the healing agent is then triggered by contact
with an embedded catalyst, bonding the crack faces.
After failure, the crack allowed to heat at rt with nomanual intervention (48 h).

75% recovery in toughness
high cost of Grubbs catalystROMP
DCPD: dicyclopentadiene17b. Vascular-Based Healing System

S.R. White, J. S. Moore, et al. Nat. Mater. 2007, 6, 581.

three-dimensional microvascular network*There is no mechanophore appliedin two strategies to date.

c. Intrinsic Healing Systems
cf. Y. Wang, Self-healing polymers and composites (Lit. Seminar)18(ii) dicyano-substituted cyclobutane (self healing approach)
Masked Cyanoacrylates Unveiled by Mechanical ForceJ. S. Moore, et al. J. Am. Chem. Soc. 2010, 132, 4558.

cyanoacrylates are known to autopolymerize→ self healing application

trapping experiments

19(2)Reaction with Activation of Catalytic Speices
mechanocatalyst(a) NHC and Grubbs catalyst

Strategy

Activating catalysts with mechanical force
R. P. Sijbesma, et al. Nat. Chem. 2009, 1, 133.
(i) Mechanochemical activation of silver–NHC complexes
for catalysis of transesterification (proof of concept)

Silver(I) complexes of NHCs have been used astransesterification catalysts that can be activated thermally.
R. M. Waymouth, J. L. Hedrick, et al. J. Org. Chem. 2005, 70, 2391.
Silver(I) complexes with polymer-functionalized NHC ligands
break when exposed to ultrasound.
R. P. Sijbesma, et al. J. Am. Chem. Soc. 2008, 130, 14968.

1 h, 4-8 ºC, 65%

ligand with polymer chain

(ii) Mechanochemical activation of a catalytic ring-opening metathesis polymerization (ROMP)

ROMP

20 ºC, 2 h, 85-95%(Cl ligands have been omitted for clarity.)*for self healing application(cf. strategy a (p16) without capsule)20

21(b) Mechanical Activation of Catalysts for Anionic Polymerization Reactions
C. W. Bielawski, et al. J. Am. Chem. Soc. 2010, 132, 16631.

type(i) (p23)[(1)(PyP 66)2] 0.025mol%
CH3CN, 4 ºC, 22 h, 42%

PM:PMA (MkDa)

22(c) mechanophore with acid-releasing capability
S. R. White, J. S. Moore, et al. J. Am. Chem. Soc. 2012, 134, 12446.
Previous Results

This time results

J. S. Moore, S. L. Craig et al. J. Mater. Chem. 2011, 21, 8454.

(cited by Angew. Chem., Int. Ed. 2013, 52, 3806.)

cf. p9
Aromatization provides the driving force to elimination.
compress at 88-352 MPa using KBr pellet press(high pressures are required)pink

The color becomes pink when acid is present.

monotonic rise in activation against applied pressure

exposing a piece of insoluble compressed polymer to a pH
indicator solution

23(3) rearrangement
stress relaxation
expanding the concept of mechanophore(i) allyl sulfide groups in Cross-Linked Polymers
C. N. Bowman, et al. Science 2005, 308, 1615.
use light to produce radicals

thiol-ene polymerization(radical mechanism)mechanophore24*Allyl sulfide groups in the network are essential tostress/strain relaxation.
radicals via photocleavage of residual photoinitiatorin the polymer matrix*termination reaction

mechanophore ~definition~
bonds or chemical functionalities that are especially susceptible toundergoing a chemical change caused by mechanical stress
J. S. Moore, et al. (2013). Self-healing polymers. Wiley
functional groups that respond to mechanical perturbation
in a controlled manner
C. W. Bielawski, et al. Polym. Int. 2013, 62, 2.

25(ii) Siloxane ~ mechanophore or not? ~T. J. McCarthy, et al. J. Am. Chem. Soc. 2012, 134, 2024.
*Use Siloxane EquilibrationE ≒ 0.4MPaachieve self healing
introduced as mechanophore in Moore's review

J. S. Moore, et al. (2013). Self-healing polymers. Wiley

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Mini Summary~ passive use of mechanical force ~production of reactive species (strengthening, self healing)
production of catalytic species (mechanocatalyst, polymerization to self healing)
rearrangement (damage relaxation)

27(4) breaking hydrogen bond ( and supramolecular network ) (cf. mechanocatalyst)
cf. Y. Wang, Self-healing polymers and composites (Lit. Seminar)
UPy motif
Titin ~ Biomimetic approach ~Z. Guan, et al. J. Am. Chem. Soc. 2009, 131, 8766.
Titin: skeletal muscle protein in Myofibril
It functions as a molecular spring which is responsible for the passive elasticity of muscle.
the ability to absorb energy by the reversible rupture of intramolecular secondary interactions, followed byrefolding induced recovery
The load-bearing region has six hydrogen bonds between β strands.

cited from Angew. Chem., Int. Ed. 2011, 50, 9026.

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the quadruple hydrogen bonding 2-ureido-4[1H]-pyrimidone (UPy) motif as the modular domain- forming mimic

the polymer combines high toughness andself-healing like properties in one material.

NHONONNH 2
H
2-ureido-4[1H]-pyrimidone
UPy

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at rtºC×stress relaxation

stiff

proposed mechanismorYoung's modulus ~200MPa
Deformation could gradually recover with time(at rt) or upon heat treatment. (80 ºC, 30s)

or

auto
recovery30*use UPy to develop self healing supramolecular block copolymer
Z. Guan, et al. Angew. Chem., Int. Ed. 2012, 51, 10561.

use poly(n-butyl acrylate) (PBA) as the soft blockand polystyrene (PS) as the hard blockThe supramolecular healing motifs located within
the soft phase remain dynamic and reversible,
providing self- healing capability.
utilizing a 2,6-diisopropyl- phenyl group todisrupt π –π stacking between UPy dimers
samples show the characteristic stress-strain
behavior of thermoplastic elastomers

E 21.1MPa
after cutting, gently pressed together for 1minand left for 18 h at 45 ºC
recovery of tensile strength (> 90 %) andstrain at break (ca. 75 %)31

Summary
theme: Reaction with Mechanical Force
active use of mechanical force

key characters of Reaction with Mechanical Forcepassive use of mechanical force

mechanophore
polymer backbone(1) active use of mechanical force
regiochemical effectdeprotection (unclicking) cf. retro DA
vias reaction pathway (violating WH rules)
reconfiguration of stereoisomer (atropisomer)
stabilize intermediate (radical trapping)(2) passive use of mechanical force
production of reactive species (strengthening, self healing)
production of catalytic species (mechanocatalyst, polymerization to self healing)
rearrangement (damage relaxation)
breaking hydrogen bond (damage relaxation, self healing)other application: damage sensing

32$SSHQGL[VHOHFWHGother reactions*damage sensing(i)Spiropyran
S. R. White, J. S. Moore, et al. J. Am. Chem. Soc. 2007, 129, 13808.

cited from Chem. Soc. Rev. 2013, 42, 7130.

condition: US, CH3CN, 6-9 ºC
Spiropyran mechanophore undergoes 6π−electron electrocyclic ring opening.
The colorless polymer solution turned pink.
Exposure to ambient light for 40 min at room temperature caused the color to disappear.
Application in damage sensing

33*application in solid state
S. R. White, J. S. Moore, et al. Nature 2009, 459, 68.

find mechanophore in solution by using US↓
application in solid state

reversible ring-opening reaction
under tensile stress
Colour change could be reversed after 6 h ofexposure to fluorescent room light.
If it fails, it can not be repaired.

OO

Polymer:PMAnPMA

The spiropyran mechanophore can function asa molecular force sensor, providing visible detection andmapping of mechanical stresses within bulk polymeric materials.

cf.力を加えると変色するプラスチック（chem-station)

34(ii)1,2-dioxetane
response to strain by emitting lightR. P. Sijbesma, et al. Nat. Chem. 2012, 4, 559.
cycloreversion process that generated electronically excited ketone intermediates
The 1,2-dioxetane unit is well known as an efficient source of electronically excited products
when it is activated thermally.
Acc. Chem. Res. 1974, 7, 97.
On relaxation, blue light (at 420 nm) is emitted.

Application
crosslinking (by ketone formation)
damage sensing

use film

35ba+ acceptor(doping)

Colour tuning
were achieved.