Mechanophore-linked addition polymers

Article abstract of DOI:10.1021/ja076189x

The use of mechanical energy to increase reaction rates and alter the distribution of products has gained considerable interest of late. However, the discovery of new mechanophores (i.e., mechanochemically reactive units) is currently time-consuming, expensive, low-yielding, and a process that is not easily scaled to large quantities. Here we show that mechanophore-linked addition polymers are easily prepared using bifunctional initiators with a living radical polymerization method. The mechanophore is positioned close to the center of the polymer, where ultrasound-generated forces are the largest. Since these forces are strongly dependent on molecular weight, the use of controlled polymerization enables fine-tuning of the mechanical activity so that mechanophore reactions are initiated while minimizing chain scission. The approach is illustrated first with the synthesis and investigation of a 1,2-disubstituted benzocyclobutene mechanophore that is incorporated into the center of a polymethacrylate (PMA) chain. Selected molecular weights are probed using ultrasound to illustrate that the 4π electrocyclic ring opening of the benzocyclobutene link is stress-induced. To demonstrate the broad applicability of this method for mechanophore screening, we also report a new spiropyran-linked PMA that undergoes an ultrasound-induced 6π-electron electrocyclic ring opening. The straightforward synthesis of mechanophore-linked addition polymers presented here shows considerable promise for the investigation of new mechanophores and will lead to a greater understanding of mechanochemical reactivity within polymer systems. Copyright

Full text of DOI:10.1021/ja076189x

Published on Web 10/24/2007
Mechanophore-Linked Addition Polymers
Stephanie L. Potisek, Douglas A. Davis, Nancy R. Sottos,† Scott R. White,‡ and Jeffrey S. Moore*
Department of Chemistry & Beckman Institute for AdVanced Science and Technology, UniVersity of Illinois at
UrbanasChampaign, Urbana, Illinois 61801
Received August 16, 2007; E-mail: jsmoore@uiuc.edu
Scheme 1. Synthesis of Benzocyclobutene Mechanophore-Linked
Addition Polymer PMA-BCB-PMA and Subsequent
Mechanochemical Reaction: (a) Cu(0), Me6TREN, Methyl
Acrylate, DMSO, 25 °C; (b) N-(1-Pyrene)maleimide, CH3CN, BHT,
Ar, 8.7 W/cm2, 6-9 °C
There is growing interest in the use of mechanical energy to
alter the molecular and supramolecular structure of polymers to
create stress-responsive materials.1a-l Chemical reactions that are
accelerated by force remain poorly understood, and there is a need
for the rapid discovery of new mechanophores (i.e., stress-sensitive
units). Screening putative mechanophores, however, is a slow
process that requires a high molecular weight polymer having a
single testable unit positioned near the midpoint of the chain, the
location where stress under elongation is greatest. Here we show
that the required mechanophore-linked addition polymers are easily
prepared using bifunctional initiators and a living polymerization
method. The approach is demonstrated with benzocyclobutene1k
and spiropyran mechanophores that undergo stress-induced 4π and
6π electrocyclic ring opening, respectively. Mechanophore-linked
addition polymers thus show considerable promise for rapidly
identifying new mechanophores and will lead to a greater, molec-
ular-level understanding of mechanochemical transduction in
polymeric materials.
raphy (GPC) using a refractive index (RI) detector. The remainder
of the sonicated solution was analyzed by preparatory GPC having
both UV (set to 345 nm) and RI detectors.
Single electron transfer living radical polymerization (SET-LRP)2
was employed for the synthesis of mechanophore-linked polymers,
as this method has been shown to generate high molecular weight
macromolecules with narrow polydispersity indices (PDIs). cis-
1,2-Bis(R-bromopropionyloxy)-1,2-dihydrobenzocyclobutene (1),
capable of initiating bidirectional SET-LRP, was synthesized and
used to produce a series of benzocyclobutene (BCB)-linked PMAs
(Scheme 1). Polymerizations were performed at room temperature
in DMSO with Cu(0) catalyst and a hexamethylated tris(2-
aminoethyl)amine (Me6TREN) ligand. Low (18 kDa), medium (91
kDa), and high (287 kDa) molecular weight BCB-linked PMAs
(PMA-BCB-PMA) with PDIs around 1.3 were synthesized and
used to investigate the ultrasound-induced electrocyclic ring opening
reaction. Mechanochemical activation was analyzed by trapping
the intermediate ortho-quinodimethide with UV-active N-(1-pyrene)-
maleimide via cycloaddition (Scheme 1).1k PMA end-functionalized
with a BCB unit (PMA-BCB) was prepared as a mechanochemical
control polymer since ultrasound-generated forces at the chain ends
are minimal. Specifically, the monofunctional initiator cis-1-
acetoxy-2-(R-bromopropionyloxy) 1,2-dihydrobenzocyclobutene was
used to produce a PMA-BCB with a PDI of 1.3 and molecular
weight of 190 kDa. This control polymer dispels the notion that
the chemical changes are thermally induced, rather than the result
of mechanical force.
Figure 1. (A-C) Preparatory GPC chromatograms of PMA-BCB-PMA.
The upper pair of traces corresponds to analysis before sonication; the lower
pair, after sonication. The dotted pink trace is the UV signal; the blue trace
is the RI signal. (D) Overlay of the analytical GPC traces before (solid
light blue) and after (dotted dark blue) sonication.
The BCB-containing polymers and PMA homopolymer were
subjected to an acoustic field to probe for mechanical activity. Each
polymer was dissolved in CH3CN with a large excess of N-(1-
pyrene)maleimide and radical trap 2,6-di-tert-butyl-4-methylphenol
(BHT) and exposed to pulsed sonication3 for 45 min under Ar at
6-9 °C. Aliquots were withdrawn at the beginning and end of each
experiment and analyzed by analytical gel permeation chromatog-
No UV signal was present in the GPC trace of the sonicated
low molecular weight PMA-BCB-PMA, indicating no reaction
(Figure 1A). This low molecular weight polymer exhibited no chain
cleavage, as indicated by the superposition of analytical GPC traces
collected before and after sonication (Figure 1D). This result is
consistent with the findings of Vijayalakshmi and Madras that the
lower molecular weight threshold for chain cleavage of PMA by
ultrasound is 87 kDa.4 The GPC trace of the intermediate molecular
weight sample (91 kDa) PMA-BCB-PMA exhibited a marked
† Department of Materials Science and Engineering and Beckman Institute.
‡ Department of Aerospace Engineering and Beckman Institute.
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10.1021/ja076189x CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
room temperature caused the color to disappear (Figure 2A),
consistent with the known photolytic reversion to the closed
form 2.
A monofunctional spiropyran initiator was synthesized and used
to prepare an end-terminated polymer (PMA-SP) with a PDI of
1.3 and molecular weight of 183 kDa. In great contrast to PMA-
SP-PMA, the sonicated end-functionalized solution was colorless
and showed no visible absorption following ultrasound exposure
(Figure 2B). These experiments suggest that the observed ring
opening for PMA-SP-PMA is mechanically induced and not the
result of thermal activation.
Figure 2. (A) UV spectrum of PMA-SP-PMA in CH3CN (7.5 mg/mL)
before sonication (blue trace), after 18 min pulsed sonication (dashed pink
trace), and after 40 min exposure to ambient light (dotted green trace). (B)
UV spectrum of end-functionalized PMA-SP control before sonication (blue
trace) and after 18 min pulsed sonication (dashed pink trace).
In conclusion, we have developed a method for synthesizing
mechanophore-linked addition polymers and have demonstrated
their value in probing mechanochemical activity. The bidirectional
living method used here is capable of placing the mechanophore
near the center of the polymer where the ultrasound-generated forces
are the largest. Control experiments with terminally linked mecha-
nophores rule out the possibility of thermally induced reactions.
We have also established the utility of the PMA backbone as an
effective polymer for investigating mechanophores via ultrasound.
The value of this method was demonstrated by its application to a
spiropyran mechanophore, which underwent a mechanochemical
6π-electron electrocyclic ring opening. A thorough study of the
mechanochromic properties of these polymers is now underway
and will be reported in due course.
increase in the UV signal following sonication (Figure 1B). These
observations together with the appropriate controls5 suggest that
incorporation of N-(1-pyrene)maleimide occurs via a mecha-
nochemically induced electrocyclic ring opening of the benzocy-
clobutene linker and that the reaction is not due to thermal
activation, consistent with our previous findings.1k Little chain
scission was observed with PMA-BCB-PMA of intermediate
molecular weight (Figure 1D), indicating that the mechanically
induced electrocyclic reaction takes place competitively or possibly
in preference to chain scission.6 The high molecular weight PMA-
BCB-PMA sample (287 kDa) also showed a strong UV signal
following ultrasound exposure (Figure 1C). Since the RI signal is
proportional to the mass of the polymer, while the UV signal is
proportional to moles of polymer molecules, a smaller UV/RI ratio
is observed for the 287 kDa sample compared to the 91 kDa sample.
The final molecular weight of the cleaved polymer was found to
be 85 kDa (Figure 1D, trace c), a value near the reported4 molecular
weight threshold for chain scission.7 Although nearly complete chain
scission is observed (Figure 1D), N-(1-pyrene)maleimide is still
incorporated into the polymer, suggesting that the cycloaddition
occurs prior to, or competitively with, chain scission. These
observations demonstrate that mechanochemical activation via
ultrasound is not limited to the PEG polymers we studied
previously.1k
Acknowledgment. This material is based on work supported
by the U.S. Army Research Office under Grant W911NF-07-1-
0409 and by the American Chemical Society Petroleum Research
Fund.
Supporting Information Available: Procedures, synthesis, further
control experiments, and thermal experiments of the SP-functionalized
PMAs. This material is available free of charge via the Internet at http://
pubs.acs.org.
References
The ease of preparing mechanophore-linked addition polymers
makes it possible to rapidly screen for new mechanophore activity.
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previously been noted to exhibit mechanochromic properties upon
grinding. Given this observation, we investigated the potential for
ultrasound to cause a stress-induced 6π-electron electrocyclic ring
opening of a spiropyran mechanophore (Scheme 2).
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Scheme 2. Mechanochemical Ring Opening of PMA-SP-PMA
(3) Ultrasound conditions: pulse rate ) 0.5 s on, 1.0 s off, intensity ) 8.7
W/cm2; frequency ) 20 kHz.
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spiropyran-linked PMA 2 (PMA-SP-PMA) having a PDI of 1.2
and molecular weight of 170 kDa. When subjected to pulsed
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