Rapid bonding/debonding on demand: Reversibly cross-linked functional polymers via diels-alder chemistry

Article abstract of DOI:10.1021/ma100945b

The synthesis of a novel poly(methyl methacrylate) (PMMA) chain bearing Cp functionality at both chain ends and a trifunctional pyridinyldithioformate linker molecule, which are able to rapidly and reversibly cross-link on demand within a highly accessibl

Full text of DOI:10.1021/ma100945b

Macromolecules 2010, 43, 5515–5520 5515
DOI: 10.1021/ma100945b
utilized. Beyond this temperature, an effective retro-DA
reaction sets in, leading to a quantitative reformation of the
starting materials.
Rapid Bonding/Debonding on Demand:
Reversibly Cross-Linked Functional Polymers via
Diels-Alder Chemistry
A characteristic of DA chemistry that renders it highly
attractive in such technologies is the wide variety of diene-
dienophile pairs that can potentially be utilized, which allows
one to fine-tune the temperature profile in which bonding
and debonding take place. To date, particularly ultrarapid
hetero-DA chemistry has not been reported in the prepara-
tion of reversible cross-linked polymeric structures. Thus, we
herein report the synthesis of a novel poly(methyl metha-
crylate) (PMMA) chain bearing Cp functionality at both
chain ends and a trifunctional pyridinyldithioformate linker
molecule (see Scheme 1), which are able to rapidly and
reversibly cross-link on demand within a highly accessible
temperature range.
Andrew J. Inglis,† Leena Nebhani,† Ozcan Altintas,†
Friedrich Georg Schmidt,‡ and
Christopher Barner-Kowollik*,†
†Preparative Macromolecular Chemistry, Institut fu€r
Technische Chemie und Polymerchemie, Karlsruhe Institute of
Technology (KIT), Engesserstr. 18, 76128 Karlsruhe, Germany,
‡
and Evonik Degussa GmbH, Paul-Baumann-Strasse 1,
45764 Marl, Germany
Received April 29, 2010
Revised Manuscript Received June 4, 2010
Experimental Part. Materials. The ATRP initiator 1,2-
bis(bromoisobutyryloxy)ethane was synthesized according
to the literature.25 Methyl methacrylate (MMA, Acros) was
passed through a short column of basic alumina and stored
at -19 °C prior to use. Copper(I) bromide (Fluka) was puri-
fied by sequential washing with sulfurous acid, acetic acid,
and ethanol, followed by drying under reduced pressure.
Copper(II) bromide (Fluka), 2,20-bipyridyl (bpy, Sigma-
Aldrich), 4-bromomethylbenzoic acid (Acros), 2-(chloromethyl)-
pyridine hydrochloride (Aldrich), 1,8-diazabicycloundec-7-
ene (DBU, Fluka), N,N0-dicyclohexylcarbodiimide (DCC,
Aldrich), 4-(dimethylamino)pyridine (DMAP, Aldrich),
nickelocene (Strem), potassium tert-butoxide (Merck), tri-
butylphosphine (Alridch), sodium iodide (Fluka), sodium
phenylsulfinate (Aldrich), elemental sulfur, p-toluenesulfonic
acid (Aldrich), trifluoroacetic acid (TFA, Sigma-Aldrich),
and 1,1,1-tris(hydroxymethyl)propane were all purchased at
the highest available purity and used as received.
Introduction. The design of reversibly linked and cross-
linked polymeric materials that are able to change their physi-
cal characteristics rapidly, within a well-defined temperature
interval and in a cyclic fashion, continues to be a highly inves-
tigated area in materials research.1,2 Lying at the core of
such research is the exploitation of reversible covalent bonds
within a polymer network. To date, there has been a wealth
of chemistries that have been investigated within this con-
text, such as thiol-driven sulfide coupling,3 photoreversible
olefin cycloadditions,4 carbene dimerization,5 nucleophilic
addition of isocyanates with imidazoles,6 and thermally clea-
vable alkoxyamines.7 However, the most widely investigated
technique by far has been Diels-Alder (DA) chemistry.8-14
The use of DA cycloadditions (most specifically those between
various maleimide and furan derivatives) in such technology
has greatly been facilitated by both the commercial avail-
ability and chemical accessibility of such functional groups.
To illustrate two very recent examples, low molecular weight
star-shaped molecules were prepared by Aumsuwan and
Urban15 in which a maleimide-furan cycloadduct formed
the linkage between the arms of the star and its core. From a
1
Characterization. H NMR spectroscopy was performed
using a Bruker AM 250 spectrometer at 250 MHz for hydro-
gen nuclei and 100 MHz for carbon nuclei. All samples were
dissolved in either CDCl3 or DMSO d6. The δ-scale is refe-
renced to the internal standard trimethylsilane (TMS, δ =
0.00 ppm).
Size exclusion chromatography (SEC) measurements were
performed on a Polymer Laboratories (Varian) PL-GPC 50
Plus Integrated System, comprising an autosampler, a PLgel
5 μm bead-size guard column (50 ꢀ 7.5 mm), one PLgel 5 μm
Mixed E column (300 ꢀ 7.5 mm), three PLgel 5 μm Mixed C
columns (300 ꢀ 7.5 mm), and a differential refractive index
detector using THF as the eluent at 35 °C with a flow rate of
1 mL min-1. The SEC system was calibrated using linear
poly(styrene) standards ranging from 160 to 6 ꢀ 106 g mol-1
and linear poly(methyl methacrylate) standards ranging
from 700 to 2 ꢀ 106 g mol-1. Molecular weights relative to
PMMA are reported in the current contribution.
Mass spectra were recorded on a LXQ mass spectrometer
(ThermoFisher Scientific) equipped with an atmospheric
pressure ionization source operating in the nebulizer-assisted
electrospray mode. The instrument was calibrated in the m/z
range 195-1822 using a standard comprising caffeine, Met-
Arg-Phe-Ala acetate (MRFA), and a mixture of fluorinated
phosphazenes (Ultramark 1621) (all from Aldrich). A con-
stant spray voltage of 4.5 kV and a dimensionless sweep
gas flow rate of 2 and a dimensionless sheath gas flow rate of
1
variable-temperature H NMR and ATR-IR spectroscopic
analysis, the forward star-forming DA reaction was deter-
mined to proceed at 40 °C. The stars could subsequently be
unmade through a 90 °C retro-DA reaction. In the other
example, Syrett et al. synthesized maleimide- and furan-
capped polymers via atom transfer radical polymerization
(ATRP) which could be linked (60 °C, 24 h) and unlinked
(110 °C, 24 h).16
We have recently reported a particularly rapid version of
hetero-DA chemistry where a highly electron-deficient di-
thioester undergoes a [4 þ 2] cycloaddition with cyclopenta-
diene (Cp) derivatives quantitatively within a few minutes
under ambient conditions.17 The rapidity of the reaction has
since been employed in the facile modular construction of block
copolymers17,18 and in surface modifications.19,20 Although
we have already explored the quantitative reversion of such
chemistry by thermal treatment,21,22 it is with a recent report
that we truly investigated its reversibility within the context
of color switching in polymeric materials.23 While the for-
ward reaction is swift, the resulting cycloadducts are tempe-
rature stable up to a relatively moderate temperature range
(60-90 °C), depending on the dithioester-Cp combination
*Corresponding author. E-mail: christopher.barner-kowollik@kit.edu.
r 2010 American Chemical Society
Published on Web 06/10/2010
pubs.acs.org/Macromolecules

5516 Macromolecules, Vol. 43, No. 13, 2010
Communication
Scheme 1. Synthetic Pathway to the Trilinker (Top) and r,ω-Functional PMMA-Cp2 (Bottom), Which Are the Two Principal Components in the
Bonding/Debonding on Demand Systema
a The R,ω-functionalization of the PMMA proceeds via our recently reported nickelocene procedure.24
12 were applied. The capillary voltage, the tube lens offset
voltage, and the capillary temperature were set to 60 V, 110 V,
and 275 °C, respectively. The LXQ was coupled to a Series
1200 HPLC system (Agilent) that consisted of a solvent
degasser (G1322A), a binary pump (G1312A), and a high-
performance autosampler (G1367B), followed by a thermo-
stated column compartment (G1316A). Separation was per-
formed on two mixed bed GPC columns (Polymer Labora-
tories, Mesopore 250 ꢀ 4.6 mm, particle diameter 3 μm) with
precolumn (Mesopore 50 ꢀ 4.6 mm) operating at 30 °C. THF
at a flow rate of 0.3 mL min-1 was used as the eluent. The
mass spectrometer was coupled to the column in parallel to
an RI detector (G1362A with SS420x A/D) in a setup des-
cribed previously.26 A 0.27 mL min-1 aliquot of the eluent
was directed through the RI detector and 30 μL min-1
infused into the electrospray source after postcolumn addi-
tion of a 0.1 mM solution of sodium iodide in methanol at
20 μL min-1 by a microflow HPLC syringe pump (Teledyne
ISCO, Model 100DM). The polymer solutions (20 μL) with a
concentration of close to 3 mg mL-1 were injected into the
HPLC system.
UV-vis spectroscopy was performed using a Cary 300 Bio
UV/vis photospectrometer (Varian) equipped with a tempe-
rature-controlled sample cell. Absorption was measured in
toluene solution (3 mg mL-1) at a wavelength of 522 nm at
temperatures ranging from 30 to 100 °C (using a 0.5 °C min-1
heating rate).
Synthesis of Bis(cyclopentadienyl) Poly(methyl methacrylate)
(PMMA-Cp2). Methyl methacrylate (MMA), 1,2-bis(bromo-
isobutyryloxy)ethane (initiator), copper(I) bromide (CuIBr),
copper(II) bromide (CuIIBr), and 2,20-bipyridine (bpy) were
added to a round-bottom flask in the ratio 50/1/0.105/0.01250/
0.25. Acetone was subsequently added such that the resulting
mixture was 50 vol % acetone. Nitrogen was then bubbled
through the mixture for 40 min to remove residual oxygen.
The mixture was subsequently sealed under nitrogen and
placed in a thermostated oil bath set to 50 °C. After 2 h, the
polymerization was stopped by cooling the mixture in an ice
bath and exposure to oxygen. The mixture was then passed
through a short column of neutral alumina to remove the
copper catalyst. R,ω-functional poly(methyl methacrylate)
(PMMA-Br2) was isolated by 2-fold precipitation in cold
n-hexane. GPC (THF): Mn = 3500 g mol-1, PDI =1.2.
According to our previously reported nickelocene Cp-
functionalization procedure,24 R,ω-functional PMMA-Br2,
sodium iodide, tributylphosphine, and nickelocene in the
ratio 1/12/4/8 were dissolved, under nitrogen, in anhydrous
THF such that the solution was 0.1 M with respect to the
polymer. The ambient temperature reaction was allowed
to proceed for at least 12 h before the reaction mixture was
passed through a short column of basic alumina. PMMA-
Cp2 was the isolated by a 2-fold precipitation in cold
n-hexane.
Synthesis of 4-((Pyridine-2-carbonothioylthio)methyl)benzoic
Acid. Step I: A mixture of 2-(chloromethyl)pyridine hydro-
chloride (10 mmol, 1.64 g), sodium phenylsulfinate (15 mmol,
2.46 g), and a catalytic amount of tetrapropylammonium
bromide (2 mmol, 0.53 g), DBU (10 mmol, 1.52 g) in CH3CN
(10 mL), was refluxed for 12 h. The solvent was subsequently
removed under vacuum, and the residue was dissolved in CH2-
Cl2, washed with brine, and dried over MgSO4 and the sol-
vent removed. The crude product was obtained in quantitative

Communication
Macromolecules, Vol. 43, No. 13, 2010 5517
yield, as a white solid and was used without purification in
the next step.
Step II: A mixture of sulfone (8.51 mmol, 1.9 g) (from step I)
and elemental sulfur (24.3 mmol, 0.78 g) was placed into
THF (10 mL) under magnetic stirring. After addition of t-BuOK
(24.3 mmol, 2.72 g), the color of the solution changed to dark
brown. The reaction was stirred at ambient temperature for
12 h. Subsequently, 4-methylbenzoic acid (12.15 mmol, 2.6 g)
in THF (10 mL) was added dropwise to the solution, and
stirring was continued for 5 h. The color changed to reddish-
pink. The product was washed with aqueous HCl (2 N) and
extracted in CH2Cl2. The crude product was finally purified
by extraction with acetone (45% yield).
1H NMR (DMSO-d6, 250 MHz): δ/ppm = 4.63 (s, 2H,
S-CH2), 7.51-7.54 (m, 2H, ArH), 7.67-7.71 (m, 1H, pyrH),
7.87-7.91 (m, 2H, pyrH), 7.95-8.26 (m, 2H, ArH), 8.62-
8.64 (m, 1H, pyrH), 13.0 (s, 1H, -COOH). 13C NMR (DMSO-
d6, 100 MHz): δ/ppm = 40.1, 121.89, 127.87, 129.42, 129.5,
1
137.78, 140.67, 148.34, 155.37, 166.94, 226.09. The H and
13C NMR spectra can be found in the Supporting Informa-
tion (Figures S1 and S2).
Synthesis of the Trifunctional Linker. 1,1,1-Tris(hydroxy-
methyl)propane (TMP) (0.072 g, 0.536 mmol), 4-(dimethyl-
amino)pyridinium-4-toluenesulfonate (DPTS) (0.118 g, 0.402
mmol), and (dimethylamino)pyridine (DMAP) (0.025 g,
0.201 mmol) were dissolved in 10 mL of CH2Cl2. 4-((Pyridine-
2-carbonothioylthio)methyl)benzoic acid (0.581 g, 2.01 mmol)
was dissolved in 3 mL of DMF and then added to the solu-
tion. After 10 min, dicyclohexylcarbodiimide (DCC) (0.622 g,
3.01 mmol) in 2 mL of dichloromethane was added to this
solution. The reaction mixture was stirred overnight at ambi-
ent temperature. It was subsequently filtered and evaporated,
and the remaining product was purified by column chromato-
graphy over silica gel eluting with hexane/ethyl acetate (6:4)
to obtain pure red solid trilinker, yielding 0.301 g (59%).
1H NMR (250, CDCl3): δ (ppm) 8.61-8.59 (d, 3H, pyrH),
8.32-8.29 (d, 3H, pyrH), 7.94-7.91 (d, 6H, ArH), 7.83-7.76
(m, 3H, pyrH), 7.50-7.47 (m, 3H, pyrH), 7.44-7.41 (d, 6H,
ArH), 4.57 (s, 6H, S-CH2), 4.47 (s, 6H, O-CH2), 1.79-1.70
(q, 2H, CH2CH3), 1.07-1.00 (t, 3H, CH2CH3). 13C NMR
(CDCl3, 100 MHz): δ/ppm=6.6 (CH2CH3), 22.76 (CH2CH3),
24.59 (Cq), 39.71 (CH2O), 64.06 (CH2S), 121.30 (Car), 125.89
(Car,py), 128.47 (Car), 128.86 (Car), 135.99 (Car,py), 140.02
(Car), 146.39 (Car,py), 155.14 (Car,py), 164.44 (CdO), 226.09
(CdS). ESI-MSþNa (m/z) calcd 970.12; found 970.08. The
1H NMR and MS spectra of the trilinker are depicted in
Figure 2. The 13C NMR spectrum can be found in the Sup-
porting Information (please refer to Figure S3, which also
contains the peak assignments).
Cross-Linking (Bonding) between PMMA-Cp2 and Tri-
functional Linker. PMMA-Cp2 and trifunctional linker were
mixed in 1:1 ratio with respect to functional groups in chloro-
form such that the concentration of polymer was 0.05 M.
1.5 equiv of trifluoroacetic acid (TFA) was added, and the
resulting mixture shaken at ambient temperature for 10 min.
The solvent was removed and the residue directly analyzed
by SEC prior to gelation.
De-Cross-Linking (Debonding) between PMMA-Cp2 and
Trifunctional Linker. The resulting cross-linked polymer
was placed in toluene and heated to above 80 °C for 5 min.
During this time, the colorless toluene turned pink in color,
indicative of the release of the trifunctional linker via a retro-
Diels-Alder cycloaddition and the polymer dissolved.
Re-Cross-Linking (Rebonding) between PMMA-Cp2 and
Trifunctional Linker. The non-cross-linked polymer was
dissolved in chloroform as described above and additional
TFA added (1.5 equiv). The solvent was removed within the
Figure 1. ESI-MS spectra of the starting R,ω-functional PMMA-Br2
(a) and the corresponding R,ω-functional PMMA-Cp2 (b). Note that
both a single and double charged distribution ofthe polymeric materials
are visible. Minor impurities due to side products formed in the ATRP
process (bimolecular termination and elimination) are also visible.
space of 1 min, and the residue was directly analyzed by SEC
prior to gelation.
Solid-State Cross-Linking. PMMA-Cp2 and trifunctional
linker were mixed in 1:1 ratio with respect to functional
groups. Upon the addition of 1.5 equiv of TFA, the mixture
was thoroughly mixed with a mortar and pestle. Fully cross-
linked (i.e., completely insoluble) material resulted within
the space of seconds.
Results and Discussion. In the current study, reversibly cross-
linked polymeric structures were generated utilizing rapid hetero-
Diels-Alder (HDA) chemistry. Unlike the majority of reports,
in which maleimide and furan DA chemistry is used as the
reversible linking mechanism, the present use of the highly
reactive Cp moiety and pyridinyldithioformate allows for facile
ambient temperature conjugations to be performed within the
space of several minutes at ambient temperatures rather than
several hours at elevated temperatures. Herein, R,ω-Cp-func-
tional PMMA is reversibly and rapidly cross-linked by reaction
with a tripyridinyldithioformate functional linker molecule.
Bis(cyclopentadienyl) poly(methyl methacrylate) (PMMA-
Cp2) was synthesized from the corresponding dibromo pre-
cursor. Dibromo poly(methyl methacrylate) (PMMA-Br2)
was obtained via ATRP of methyl methacrylate using the
difunctional initiator 1,2-bis(bromoisobutryloxy)ethane. Sub-
sequently, the PMMA-Br2 was transformed into the targeted

5518 Macromolecules, Vol. 43, No. 13, 2010
Communication
Figure 2. 1H NMR spectrum (a) and ESI-MS spectrum (b) of the trilinker. The 13C NMR spectrum of the trilinker can be found in the Supporting
Information (see Figure S3).
PMMA-Cp2 via nucleophilic substitution of the bromide
moieties with cyclopentadienyl moieties via our facile and
recently reported nickelocene pathway.24 The quantitative
conversion of PMMA-Br2 into PMMA-Cp2 was monitored
via ESI-MS, which is a very convenient technique in deter-
mining low molecular weight polymer end-group functiona-
lization. Figure 1 depicts the comparison of the ESI-MS
spectra of the starting PMMA-Br2 and PMMA-Cp2. As can
be observed, a shift of the dominant series of signals to lower
m/z values is complete and consistent with the targeted trans-
formation. All minor species, which are associated with various
minor byproduct of the ATRP process such as bimolecular
coupling and elimination species,24 are carried through the
substitution process. A comparison of the experimentally
observed and the theoretically calculated m/z values for the
dominant series is presented in Table S1 in the Supporting
Figure 3. Structure of the linkages formed during the HDA reaction.
For simplification, not all regio-/stereoisomers are depicted.
Information.
The dienophilic trifunctional linker is based upon the di-
thioester molecule 4-((pyridine-2-carbonothioylthio)methyl)-
are formed via the HDA reaction are depicted in Figure 3.
Within a maximum time frame of 30 min at ambient tempe-
ratures completely cross-linked structures were synthesized,
as indicated by their lack of solubility. When the cross-linking
reaction was performed in the solid state, fully cross-linked
material was formed within the space of seconds. Therefore,
in order to perform a SEC analysis to visualize the changes
in molecular weight that occur, the crude reaction mixtures
were quickly diluted with THF prior to the gel point (<10 min)
and directly analyzed. Figure 4a depicts the SEC distribution
of the starting trifunctional linker and PMMA-Cp2 having a
molecular weight of 990 and 3500 g mol-1, respectively.
After a reaction time of 10 min, the molecular weight of the
system markedly increases to 12 700 g mol-1. It is also noted
that the polydispersity of the system increases from 1.21 to
2.57. Both of these observations are indicative of the pro-
gression of rapid cross-linking reactions.
Aside from the rapid nature of the above cycloadditions,
another advantage of the use of the reactive Cp moiety is the
reduction in temperature at which the retro-HDA reaction
occurs. In previous investigations, open-chain dienes were
utilized, the HDA adducts of which underwent reversion at
temperatures exceeding 120 °C.21,22 At such temperatures,
the liberated dithioesters readily decompose which precludes
such systems to be used in a continually reversible fashion.
benzoic acid, the synthesis of which has been appropriated
from a two-step procedure reported in the literature.27 In the
first step, commercially available 2-(chloromethyl)pyridine
hydrochloride is transformed into the corresponding sulfone
by reaction with sodium phenylsulfinate in the presence of
DBU and tetrapropylammonium bromide. The targeted
sulfone was isolated in quantitative yields and used without
further purification for the next step. The second step was
carried out by reacting the above sulfone with sulfur (S8) in
the presence of potassium tert-butoxide, and subsequent
alkylation with 4-methylbenzoic acid yielded the desired
4-((pyridine-2-carbonothioylthio)methyl) benzoic acid. The
1H and 13C NMR of 4-((pyridine-2-carbonothioylthio)methyl)-
benzoic acid are shown in Figures S1 and S2 in the Sup-
porting Information. The trifunctional linker was obtained
via esterification of 1,1,1-tris(hydroxymethyl)propane with
4-((pyridine-2-carbonothioylthio)methyl))benzoic acid in the
presence of DCC, 4-(dimethylamino)pyridinium-4-toluene-
sulfonate (DPTS), and DMAP. The trifunctional linker was
purified via column chromatography, and the purity was deter-
mined by 1H NMR spectroscopy and ESI-MS (Figure 2).
In order to generate cross-linked structures under ambient
conditions, the trilinker core was reacted with PMMA-Cp2
in a 1:1 ratio with respect to functional groups using chloro-
form as the solvent and TFA as a catalyst. The linkages that

Communication
Macromolecules, Vol. 43, No. 13, 2010 5519
Figure 4. (a) Visualizing the changes in molecular weight in the cross-linking system by SEC. (b) Graphical representation of the bonding/debonding
on demand, which occurs within minutes in either direction. (c) UV/vis monitoring of the onset of debonding (i.e., the reaction solution becomes
colored again) at close to 80 °C (λ=522 nm).
Table 1. Characterization of the Bonded and Debonded Polymer
Systems, Demonstrating the Switching Behavior in Both the
Observed Molecular Weight and the Polydispersity Index
starting trilinker molecule. Performing the same reaction
quantitatively inside a UV/vis spectrophotometer allowed
for the absorbance of the solution to be monitored as a
function of temperature. Figure 4c depicts the results of such
an analysis, and it is observed that a sharp increase in the
absorbance of the system occurs at around 80 °C (the solu-
tion returns the color of the dithioester), corresponding to
the rapid onset of debonding.
polymer
Mn,SEC/g mol-1
PDI
PMMA-Cp2
bond
debond
bond
3500
12700
3800
1.20
2.57
1.26
2.14
10200
Conclusions. The reversibility of rapid HDA reactions has
been exploited for the generation of a highly reversibly
stimuli-responsive cross-linked polymeric network. The am-
bient temperature cross-linking of PMMA (bonding) could
be reversed upon thermal treatment (debonding) and subse-
quently re-formed (rebonding) again at ambient tempera-
ture. Although accessibility to the HDA functionalities is not
the same as the widely used maleimide/furan derivatives, the
provision and continued development of such systems will
ultimately lead to significant advances in such fields as
coatings, adhesives, and self-healing polymeric materials.
In the present study, the retro- or debonding reactions were
performed at temperatures between 80 and 100 °C. Referring
back to Figure 4a, it is noted that heating the highly linked
structure for no more than 5 min at >80 °C results in debond-
ing taking place. One can clearly observe not only the reduc-
tion in molecular weight of the system (Mn =3800 g mol-1
)
but also an increase in the signal representing the trilinker
molecule, indicating its release into solution. The final part of
Figure 4a depicts an attempt at rebonding the above debon-
ded sample. The debonded polymer/trilinker mixture was
dissolved in fresh chloroform, and a catalytic amount of
TFA was added. Analysis of the reaction mixture via SEC
Acknowledgment. C.B.-K. acknowledges funding from the
Karlsruhe Institute of Technology (KIT) in the context of the
Excellence Initiative for leadingGerman universities, the German
ResearchCouncil(DFG), andthe MinistryofScience andArtsof
€
the state of Baden-Wurttemberg. C.B.-K. is additionally grateful
for continued support from Evonik Degussa.
reveals that an increase in molecular weight (10 200 g mol-1
)
and polydispersity (2.14) occurred, thus confirming the abi-
lity of the system to bond, debond, and rebond on demand.
The system is graphically depicted in Figure 4b. Further-
more, the molecular weight data for the above study are in-
cluded in Table 1.
The onset of debonding was also monitored via UV/vis
spectroscopy. In an initial qualitative trial, fully cross-linked
material was suspended in toluene. Upon heating with a
hand-held heat gun, the material gradually dissolved and the
toluene solution turned pink, indicating the release of the
Supporting Information Available: 1H and 13C NMR spec-
tra of 4-((pyridine-2-carbonothioylthio)methyl)benzoic acid,
the 13C NMR spectrum of the trilinker, and individual peak assign-
ments for the mass spectra depicted in Figure 1. This material is
available free of charge via the Internet at http://pubs.acs.org.

5520 Macromolecules, Vol. 43, No. 13, 2010
Communication
References and Notes
(14) Reinecke, M.; Ritter, H. Makromol. Chem. 1993, 194, 2385–2393.
(15) Aumsuwan, N.; Urban, M. W. Polymer 2009, 50, 33–36.
(16) Syrett, J. A.; Mantovani, G.; Barton, W. R. S.; Price, D.; Haddleton,
D. M. Polym. Chem. 2010, 1, 102–106.
(17) Inglis, A. J.; Sinnwell, S.; Stenzel, M. H.; Barner-Kowollik, C.
Angew. Chem., Int. Ed. 2009, 48, 2411–2414.
(18) Inglis, A. J.; Stenzel, M. H.; Barner-Kowollik, C. Macromol. Rapid
Commun. 2009, 30, 1792–1798.
(19) Nebhani, L.; Gerstel, P.; Atanasova, P.; Bruns, M.; Barner-Kowollik,
C. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 7090–7095.
(20) Nebhani, L.; Schmiedl, D.; Barner, L.; Barner-Kowollik, C. Adv.
Funct. Mater. 2010, DOI: 10.1002/adfm.200902330.
(1) Engle, L. P.; Wagener, K. B. J. Macromol. Sci., Rev. Macromol.
Chem. Phys. 1993, C33, 239–257.
(2) Kloxin, C. J.; Scott, T. F.; Adzima, B. J.; Bowman, C. N. Macro-
molecules 2010, 43, 2643–2653.
(3) Furusho, Y.; Oku, T.; Hasegawa, T.; Tsuboi, A.; Kihara, N.;
Takata, T. Chem.;Eur. J. 2003, 9, 2895–2903.
(4) Chen, Y.; Chen, K. H. J. Polym. Sci., Part A: Polym. Chem. 1997,
35, 613–624.
(5) Kamplain, J. W.; Bielawski, C. W. Chem. Commun. 2006, 1727–1729.
(6) Chang, J. Y.; Do, S. K.; Han, M. J. Polymer 2001, 42, 7589–7594.
(7) Themistou, E.; Patrickios, C. S. Macromolecules 2006, 39, 73–80.
(8) Chen, X. X.; Dam, M. A.; Ono, K.; Mal, A.; Shen, H. B.; Nutt,
S. R.; Sheran, K.; Wudl, F. Science 2002, 295, 1698–1702.
(9) Gandini, A.; Silvestre, A. J. D.; Coelho, D. J. Polym. Sci., Part A:
Polym. Chem. 2010, 48, 2053–2056.
(21) Inglis, A. J.; Sinnwell, S.; Davis, T. P.; Barner-Kowollik, C.;
Stenzel, M. H. Macromolecules 2008, 41, 4120–4126.
(22) Sinnwell, S.; Synatschke, C. V.; Junkers, T.; Stenzel, M. H.; Barner-
Kowollik, C. Macromolecules 2008, 41, 7904–7912.
€
(23) Paulohrl, T.; Inglis, A. J.; Barner-Kowollik, C. Adv. Mater. 2010,
(10) Gheneim, R.; Perez-Berumen, C.; Gandini, A. Macromolecules
2002, 35, 7246–7253.
DOI: 10.1002/adma.201000361.
€
(24) Inglis, A. J.; Paulohrl, T.; Barner-Kowollik, C. Macromolecules
(11) Goiti, E.; Heatley, F.; Huglin, M. B.; Rego, J. M. Eur. Polym. J.
2004, 40, 1451–1460.
2010, 43, 33–36.
(25) Karanam, S.; Goossens, H.; Klumperman, B.; Lemstra, P. Macro-
(12) Imai, Y.; Itoh, H.; Naka, K.; Chujo, Y. Macromolecules 2000, 33,
4343–4346.
(13) Jones, J. R.; Liotta, C. L.; Collard, D. M.; Schiraldi, D. A.
Macromolecules 1999, 32, 5786–5792.
molecules 2003, 36, 8304–8311.
(26) Gruendling, T.; Guilhaus, M.; Barner-Kowollik, C. Anal. Chem.
2008, 80, 6915–6927.
(27) Abrunhosa, I.; Gulea, M.; Masson, S. Synthesis 2004, 928–934.