Synthesis of novel "rod-coil" brush polymers with conjugated backbones through bergman cyclization

Article abstract of DOI:10.1021/ma902176j

This work reports synthesis of "rod-coil" brush polymers with rigid conjugated backbone. "Grafting through" strategy was employed via combination of ring-opening polymerization (ROP) and Bergman cyclization polymerization. Enediyne-containing macromonomer

Full text of DOI:10.1021/ma902176j

Macromolecules 2010, 43, 909–913 909
DOI: 10.1021/ma902176j
Synthesis of Novel “Rod-Coil” Brush Polymers with Conjugated
Backbones through Bergman Cyclization
Xin Cheng, Jianguo Ma, Jian Zhi, Xi Yang, and Aiguo Hu*
Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering,
East China University of Science and Technology, Shanghai 200237, China
Received October 1, 2009; Revised Manuscript Received November 23, 2009
ABSTRACT: This work reports synthesis of “rod-coil” brush polymers with rigid conjugated backbone.
“Grafting through” strategy was employed via combination of ring-opening polymerization (ROP) and
Bergman cyclization polymerization. Enediyne-containing macromonomers were first synthesized through
ROP of caprolactone with dual-functional initiators conceiving free hydroxy groups and dialkynylbenzene
moieties. After protection of terminal free hydroxy group of PCL chain and removal of trimethylsilyl
protecting group of enediyne unit, the macromonomers were subjected to thermal Bergman cyclization under
vacuum. The brush polymers obtained were characterized with GPC and NMR, IR, and UV-vis
spectroscopy. The conjugated backbones were revealed as copolymers of naphthalene and indenylmethylene
according to NMR analysis.
Introduction
applications in batteries, capacitors, light-emitting diodes, and
transistors. Development of new synthetic strategies for metal-free
Brush polymers as a special grafted polymer with a densely
side chains distribution have draw much attention not only
for their peculiar architectures related to properties but also for
their potential applications.1-5 As an important kind of nano-
scopic macromolecule, brush polymers have been well investi-
gated since the concept of hyperbranched polymers was first
presented in theoretical work by Flory in 1953.6 A variety
of elegant synthetic methods have been developed in recent years;
in general, there are three strategies for synthesis of brush
polymers: “grafting from” (grafting side chains from the back-
bone), “grafting onto” (attachment of side chains to the back-
bone), and “grafting through” (homo- and copolymerization of
macromonomers).1,5,7 Extending synthetic organic chemistry,
like “click chemistry”8 and multiple hydrogen-bonding interac-
tions9,10 together with living polymerization, especially living
radical polymerizations developed in past decade, into the mole-
cular design in polymer chemistry has facilitated the synthesis of
polymers with various architectures.11
Conjugated polymers are a class of polymers with unique thermal
and electrical properties.12-20 There are several families of conju-
gated polymers with respect to their backbones, including poly-
(p-phenylene) (PPP), polyacetylene (PA), poly(phenylenevinylene)
(PPV), poly(phenyleneethynylene) (PPE), polypyrrole (PPy), and
polythiophene (PT). Because of the high rigidity and crystallinity of
backbone, pristine conjugated polymers are typically difficult to
melt and insoluble in common solvents. Introducing a pendant
moiety onto conjugated polymer to form brush polymer is a generic
way to improve the solubility and modify the electrical property of
these rodlike polymers. “Grafting through” is the method of choice
in most cases for the synthesis of these brush polymers. After
introduction of flexible groups (either polymer or long alkyl chain),
macromonomers were subjected to transition-metal-catalyzed
cross-coupling reaction or coordination polymerization to form
brush polymers. The residual metal complexes in the final polymers,
however, seriously affected their electrical properties for wide
conjugated polymers is challenging. We noted that polyphenylene
and polynaphthalene have been synthesized via an alternative way
through thermal Bergman cyclization reported by Tour et al.21,22 In
many cases, polymers were obtained as insoluble tar.
Bergman cyclization is an intramolecular cyclization of enediyne
compounds first studied by Bergman et al. three decades ago.23 It
was later found that some naturally occurred antibiotics like
calicheamicin, dynemicin A, esperamicin A1, namenamicin, and
shishijimicin A conceive a enediyne “warhead”. Bergman cycli-
zation of the enediyne moiety is initiated in vivo to generate a
1,4-phenylene diradical, which can cause DNA cleavage or cross-
linking. The high cytotoxicity of these compounds is promising for
anticancer applications. Following this line, a great deal of research
work has been conducted on elucidation of the mechanism of
Bergman cyclization, designing and total synthesis of bio-similar
compounds with less complicated structures and lower toxicity.24-32
Despite these activities, there were some discrete reports on applica-
tions of Bergman cyclization in polymer chemistry and material
science. Diradicals from Bergman cyclization were used to initiate
free radical polymerization of functionalized olefins.33,34 Poly-
naphthalene from Bergman cyclization of bis(o-diynylarene) are
good precursors for glassy carbon and surface functionality to
solubilize carbon nano-onion in organic solvents.35-37 Polystyrene
with pendant enediyne group cured under high temperature showed
high resistance to plasma etching.38 Bergman cyclization could be
triggered with a variety of stimuli, including heat, UV, acid, and
organometallic catalysts. Together with structural variability of
enediyne precursors, Bergman cyclization is prone to show
wide applications in synthetic chemistry. However, exploration of
Bergman cyclization in the polymer synthesis is still rare. Herein, we
wish to report our work utilizing Bergman cyclization to synthesize
“rod-coil” conjugated polymers with polycaprolactone as flexible
side chains.
Experimental Section
Materials. ε-Caprolactone (ε-CL; 99%, Acros Organics) was
dried over calcium hydride (CaH2), distilled under reduced
*Corresponding author. E-mail: hagmhsn@ecust.edu.cn.
r 2009 American Chemical Society
Published on Web 12/09/2009
pubs.acs.org/Macromolecules

910 Macromolecules, Vol. 43, No. 2, 2010
Cheng et al.
Scheme 1. Bergman Cyclization and Related Polymerization
copper(I) iodide (0.07 g, 0.35 mmol), and but-3-yn-1-ol (0.97 g,
13.84 mmol). The mixture was then stirred at 80 °C for 20 h under a
nitrogen atmosphere. After being cooled to room temperature, the
solvent was removed in vacuo. The residue was dissolved in ethyl
acetate, washed with saturated NH4Cl and NaCl solution, and
dried over anhydrous MgSO4. LC separation on silica gel column
(ethyl acetate/petroleum ether = 1:5, Rf = 0.5) gave pure product
as a brown-red oil (1.51 g, 54%). 1H NMR (CDCl3, δ, ppm): 7.46
(m, Ph-H, 1H), 7.39 (m, Ph-H, 1H), 7.23 (m, Ph-H, 2H),
39,40
˚
pressure, and stored over activated 4 A molecular sieves.
3
3
3.82(m, JH-H = 6.2 Hz, JH-H = 6.3 Hz, -CH2OH-, 2H),
Tetrahydrofuran (THF), toluene, and triethylamine (Et3N)
were distilled over sodium or calcium hydride prior to use.
Stannous(II) 2-ethylhexanoate (SnOct2) (Sigma) and other re-
agents were commercial chemicals and used as received. All
reactions were performed with dried Schlenk techniques under
an atmosphere of nitrogen unless otherwise noted.
2.73 (t, 3JH-H = 6.0 Hz, -CH2CH2OH, 2H), 2.14 (m, 3JH-H
6.7 Hz, -OH, 1H), 0.27 (s, Si-CH3, 9H).
=
Typical Procedure for the Synthesis of Polycaprolactone
(PCL, 3a). All glassware and needles were dried overnight in
an oven at 140 °C prior to use, and all reactions were performed
under a dry nitrogen atmosphere. Compound 2a (0.6 g,
2.5 mmol) was first dissolved in THF and dried over anhydrous
MgSO4 and then transferred to a Schlenk flask. After removal of
THF in vacuo,39 ε-caprolactone (1.42 g, 12.5 mmol), Sn(Oct)2
(2 wt % CL), and toluene (3 mL) were added under nitrogen.
The polymerizations were carried out in a 110 °C oil bath. After
24 h, the mixture was evaporated under reduced pressure and
poured into cold methanol. The polymer was collected after
filtration and dried at room temperature in vacuo for 4 h as a
light-brown powder. 1H NMR (CDCl3, δ, ppm): 7.43 (m,
Ph-H, 1H), 7.38 (m, Ph-H, 1H), 7.22 (m, Ph-H, 2H),
4.28 (m, -CtC-CH2-CH2, 2H), 4.06 (m, -CH2-O-CO-
in PCL), 2.80 (m, -CtC-CH2-, 2H), 2.30 (m, -O-CO-
CH2- in PCL), 1.64 (m, -CH2-CH2-O-CO- in PCL),
1.40 (m, -O-CO-CH2-CH2- in PCL), 1.29 (m, -O-CO-
CH2-CH2-CH2- in PCL), 0.27 (s, Si-CH3, 9H).
Acetylated PCL. A mixture of polymer 3 and acetic anhydride
was stirred at 80 °C for 6-8 h. The product was obtained after
removal of the excess acetic anhydride in vacuo. 1H NMR
(CDCl3, δ, ppm): 7.45 (m, Ph-H, 1H), 7.39 (m, Ph-H, 1H),
7.23 (m, Ph-H, 2H), 4.29 (m, -CtC-CH2-CH2, 2H), 4.07 (m,
-CH2-O-CO- in PCL), 2.82 (m, -CtC-CH2-, 2H), 2.31
(m, -O-CO-CH2- in PCL), 2.05 (m, -O-CO-CH3 in PCL,
3H), 1.67 (m, -CH2-CH2-O-CO- in PCL), 1.41 (m,
-O-CO-CH2-CH2- in PCL), 1.37 (m, -O-CO-CH2-
CH2-CH2- in PCL), 0.28 (s, Si-CH3, 9H).
Deprotection of the Acetylated PCL (4). To a solution of
acetylated PCL (0.3 g) in THF (3 mL) was added tetrabutyl-
ammonium fluoride (0.44 g, 10 equiv to TMS group).45 After
stirring for 1.5 h at room temperature, the solution was passed
through a short silica gel column and eluted with THF to get the
product. Note: this reaction was run in a dark environment.
1H NMR (CDCl3, δ, ppm): 7.25-7.47 (m, Ph-H, 4H), 4.28 (m,
-CtC-CH2-CH2, 2H), 4.06 (m, -CH2-O-CO- in PCL),
3.27 (s, Ph-CtC-H, 1H), 2.82 (m, -CtC-CH2-, 2H), 2.31
(m, -O-CO-CH2- in PCL), 2.05 (m, -O-CO-CH3 in PCL,
3H), 1.67 (m, -CH2-CH2-O-CO- in PCL), 1.41 (m,
-O-CO-CH2-CH2- in PCL), 1.37(m, -O-CO-CH2-
CH2-CH2- in PCL).
“Bottle-Brush” Polymer 5 through Bergman Cyclization. Poly-
mer 4 was transferred into a tube and sealed under vacuum. The
tube was then embedded in a refluxing diphenyl ether (or diethyl
succinate) bath. After 6 h of heating, a brownish oil was
obtained inside the tube, which was proven to be brush polymer
with high polydispersity. The high molecular weight fraction
was obtained either by repeat precipitation in a mixture solvent
of THF and methanol or dialysis in THF with dialysis mem-
branes (MWCO: 50 000).
Characterization. 1H NMR (400 MHz) and 13C NMR
(100 MHz) spectra were recorded in chloroform-d (CDCl3) on
an Ultra Shield 400 spectrometer (Bruker BioSpin AG, Magnet
System 400 MHz/54 mm). The apparent Mn, Mw, and PDI were
determined by gel permeation chromatography (GPC) on a
Waters 1515 equipped with a series of PS gel columns, using
THF as an eluent at 40 °C with a PS calibration. Onset and peak
temperature of the model compounds were studied with differ-
ential scanning calorimetry (DSC) using a Pyris Diamond
thermal analysis workstation equipped with a model 822e
DSC module under a constant nitrogen flow. Fourier transform
infrared analysis (FTIR) was recorded from KBr pellets on a
Nicolet Magna 5700 FTIR spectrometer. UV analyses were
performed on a UNICO UV-21-2 PCS spectrometer from
CH2Cl2 solutions at room temperature.
Synthesis. 3-(2-(2-(Trimethylsilyl)ethynyl)phenyl)prop-2-yn-
1-ol (2a). This compound was synthesized according to a
literature procedure with minor modification.41 To a triethyl-
amine (15 mL) solution of 3-(2-bromophenyl)propargyl alcohol
(3.17 g, 15 mmol) was added Pd(PPh3)2Cl2 (0.53 g, 0.75 mmol),
copper(I) iodide (0.09 g, 0.45 mmol), and trimethylsilylacetylene
(2.21 g, 23 mmol). The mixture was then stirred at 90 °C for 20 h
in seal tube. After cooled to room temperature, volatile compo-
nents were removed in vacuo. The residue was dissolved in ethyl
acetate, washed with saturated NH4Cl and NaCl solution, and
dried over anhydrous MgSO4. LC separation on silica gel
column (ethyl acetate/petroleum ether = 1:5, Rf = 0.5), giving
pure product as a brown-red oil (1.23 g, 36%). 1H NMR
(CDCl3, δ, ppm): 7.47 (m, Ph-H, 1H), 7.43 (m, Ph-H, 1H),
7.28 (m, Ph-H, 1H), 7.24 (m, Ph-H, 1H), 4.54 (d, -CH2OH-,
2H), 1.64 (m, -OH, 1H), 0.27 (s, Si-CH3, 9H).
(2-(2-Bromophenyl)ethynyl)trimethylsilane. This compound
was synthesized according to a literature procedure with
minor modification.42 To a mixture of 1-bromo-2-iodobenzene
(1) (2.83 g, 10 mmol), Pd(PPh3)2Cl2 (0.21 g, 0.3 mmol), and
copper(I) iodide (0.06 g, 0.3 mmol) in dry Et3N (15 mL),
trimethylsilylacetylene (1.18 g, 12 mmol) was added under a
nitrogen atmosphere. The reaction mixture was stirred over-
night at room temperature. The mixture was concentrated in
vacuo, then dissolved in ethyl acetate, and washed with saturated
NaHCO3 solution and saturated NaCl solution. The organic
layer was dried over anhydrous MgSO4 and concentrated. The
crude product was purified by silica gel column chromatogra-
phy eluted with petroleum ether (Rf = 0.8) to give pure product
as a yellow oil (2.52 g, 100%). 1H NMR (CDCl3, δ, ppm): 7.37
3
4
(d, JH-H = 8.0 Hz, JH-H = 1.0 Hz, Ph-H, 1H), 7.29
3
4
(d, JH-H = 7.7 Hz, JH-H = 1.7 Hz, Ph-H, 1H), 7.04 (m,
3JH-H = 7.6 Hz, 3JH-H = 7.6 Hz, 4JH-H = 1.2 Hz, Ph-H, 1H),
3
3
4
6.96 (m, JH-H = 8.0 Hz, JH-H = 7.6 Hz, JH-H = 1.7 Hz,
Ph-H, 1H), 0.09 (s, Si-CH3, 9H).
Results and Discussion
4-(2-(2-(Trimethylsilyl)ethynyl)phenyl)but-3-yn-1-ol (2b). This
compound was synthesized according to a literature procedure
with minor modification.43,44 To a triethylamine (15 mL) solu-
tion of the above-mentioned o-TMSA-substituted bromobenzene
(2.92 g, 11.5 mmol) was added Pd(PPh3)2Cl2 (0.24 g, 0.35 mmol),
Synthesis of PCL Macromomomer (4). Compound 2 was
chosen for this study due to its structural feature and facial
preparation. Hydroxy group can be easily modified with a
variety of reactions to introduce functional groups onto this

Article
Macromolecules, Vol. 43, No. 2, 2010 911
Figure 1. DSC curves of model compounds.
molecule. In this study, we used the hydroxy group to initiate
ring-opening polymerization of cyclic lactone to introduce
a flexible PCL chain onto enediyne compound. Trimethyl-
silyacetylene group is a synthetic equivalent of ethyne, and
the steric effect of trimethylsilyl (TMS) group also inhibits
unwanted Bergman cyclization or oxidative coupling of
terminal alkyne during preparation and storage. DSC stu-
dies for model compounds revealed that with TMS group
thermal Bergman cyclization could not take place until it was
heated up to 260 °C; however, after removal of TMS group,
Bergman cyclization took place at much lower temperature
(Figure 1).
Figure 2. 1H NMR spectra (CDCl3) of (A) as-synthesized PCL, (B)
acetylated PCL, and (C) after removal of trimethylsilyl group.
Poly(ε-caprolactone) 3 was synthesized through ring-
opening polymerization at 110 °C with the feed molar ratio
of caprolactone to initiator from 5:1 to 15:1. The PCL chain
was readily obtained as a powder by precipitation from
methanol to remove the residual monomer and initiator.
GPC showed monomodal trace and low molecular weight
distribution, which revealed living polymerization nature.
The molecular weights calibrated with polystyrene standard
were further corrected with a known equation for PCL
system,20 which were consistent with NMR terminal group
analysis. In the 1H NMR spectra of the obtained polymers,
the major signals around 2.3 and 4.1 ppm were assigned to
the methylene protons adjacent to the ester group of the PCL
main chain, and the remaining high signals in the range from
1.3 to 1.7 ppm were assigned to the inner methylene protons
of the polymer backbone. Four multiplet signals between 7.2
and 7.5 ppm were assigned to the aryl protons in the initiator,
two triplets around 2.8 and 4.3 ppm were assigned to
methylene protons next to triple bond of enediyne moiety
(in the case of 3b, a singlet around 5.0 ppm was assigned to
methylene protons adjacent to triple bond), and one singlet
around 0.3 ppm was assigned to TMS group. The initiating
efficiencies for enediyne-containing alcohol calculated from
molecular weight analysis are between 35% and 50%. The
steric effect of neighboring TMS group could be the reason
for low efficiency.
The free terminal hydroxy groups of 3 were masked with
acetyl group as we found that hydroxy groups could quench
the free radicals generated by Bergman cyclization and
terminate the related polymerization. The protection of the
hydroxy group was achieved via treatment with excess of
acetic anhydride. NMR analysis showed complete protec-
tion of terminal hydroxy group. The polymer was then
subjected to deprotetion of TMS group in a tetrabutylam-
monium fluoride THF solution. The removal of TMS group
was completed in 1.5 h as evidenced by NMR analysis
(Figure 2).
species that could be quencher to radicals, these reactions
were done under vacuum. According to the DSC results
of model compound of 4, Bergman cyclization of 4 could
be triggered at 160 °C. The formation of brush polymer
can be even faster at higher temperature. GPC analysis for
brush polymers revealed broad molecular weight dispersion,
which was due to the step-growth nature of coupling reac-
tion of diradicals generated via Bergman cyclization.21 The
apparent molecular weight of brush polymer varied under
different reaction conditions. There was, however, no clear
relationship between reaction temperature and molecular
weight of brush polymers. The high molecular fraction could
be readily obtained via dialysis or repeat precipitation
(Figure 3). DSC analysis revealed much higher Tg (20.4 °C
for 5a and 22.5 °C for 5b) than free PCL (-61 °C), indicative
of restriction of motion of PCL segments after formation of
brush polymers.
The occurrence of Bergman cyclization was confirmed
through IR analysis. Two adsorption peaks in model com-
pounds were clear seen around 2180 and 2230 cm-1, which
were assigned to internal and terminal triple bonds. Both of
these peaks were absent after Bergman cyclization as shown
in Figure 4, indicative of disappearance of both triple bonds
in enediyne moiety after thermal Bergman cyclization.46 The
reaction at triple bond moieties was further supported by 13C
NMR analysis. Four characteristic peaks between 80 and
105 ppm in macromonomers are completely gone in brush
polymers.
The UV-vis spectra of macromonomers and brush poly-
mers are shown in Figure 5. The adsorption spectra of macro-
monomers showed three well-defined peaks at 237, 263, and
278 nm without notable difference between two types of
macromonomers (3a and 3b) and corresponding model com-
pounds. After Bergman cyclization, all above-mentioned
features in UV-vis spectra disappeared, while an intensive,
broad peak showed up. All these polymers showed strong
adsorption tailing up to 450 nm, which indicated the formation
of longer conjugated system after Bergman cyclization.
Synthesis of Brush Polymer (5). Thermal Bergman cycliza-
tion of macromonomer 4 was conducted in bulk. To exclude

912 Macromolecules, Vol. 43, No. 2, 2010
Cheng et al.
Scheme 2. Synthetic Route for Brush Polymers
Figure 3. GPC traces of (A) PCL, (B) brush polymer, and (C) brush
polymer after dialysis.
Figure 5. UV-vis spectra of 3a (solid line), 3b (solid line), 5a (dotted
line), and 5b (dotted line) in CH2Cl2.
Table 1. Enediyne-Containing Macromonomers and Brush Polymers
PCL macromonomer
brush polymer
entry enediyne [CL]:[2] Mw,GPCa PDIa Mw,GPCa PDIa Mw,GPCa,b PDIa,b
1
2
3
4
5
6
2ac
2ad
2bd
2bc
2bd
2bd
17:1
30:1
8:1
8:1
5:1
13.8 1.43 554.0 13.78 1027.3 7.74
19.5 1.57 112.9 8.90
7.8 1.21 294.2 7.17
7.8 1.21 62.7 8.08
6.0 1.28 110.9 7.42
138.0 4.97
184.2 2.45
72.3 1.72
194.6 4.09
5:1
5.2 1.22 516.2 32.00 661.5 1.28
a Measured by GPC analysis in THF with a PS calibration, molecular
weight in kilodaltons. b After fractionation. c Reaction run at 217 °C.
d Reaction run at 259 °C.
Figure 6. Aromatic region of 1H NMR spectra (CDCl3) of the (A) PCL
4 and (B) brush polymer 5.
The structure of “polynaphthalene” synthesized via Bergman
cyclization was questioned recently.47 It was found that poly-
naphthalenes from Kumada coupling of 1,4-dibromonaphtha-
lene and Bergman cyclization of o-diethynylbenzene were very
different according to TGA, pyrolysis GC-MS, and (CP-MAS)
NMR analysis. Detailed structural analysis showed that poly-
naphthalene from Bergman cyclization consisted of not only
naphthalene but also indene subunits. A radical chain growth
mechanism besides step-growth mechanism was proposed for
the formation of indene moiety. In this mechanism, enediyne is
first attacked by a radical (formed from Bergman cyclization) at
the 1- or 2-position of the alkynyl group to form a vinyl radical
intermediate (intermediate A or B), the subsequent 5-exo-dig
cyclization of A, and the 5-endo-dig cyclization of B give radicals
with an indene moiety, while 6-endo-dig cyclization of A gives
Figure 4. FT-IR spectra of model compound before (A) and after
Bergman cyclization (B).

Article
Macromolecules, Vol. 43, No. 2, 2010 913
radical with a naphthalene moiety. A direct evidence for the
formation of indenyl intermediate, however, is difficult to
find out in this system due to the insolubility of the “poly-
naphthalene”. Thanks to the flexibility of PCL side chain, a
solution NMR spectrum can be obtained for brushed “poly-
naphthalene”. After Bergman cyclization, two new set of peaks
appeared around 8.1 and 6.6 ppm (Figure 6), which were
assigned to naphthalene subunits and indene subunits,48,49
respectively. The main chain of these brush polymers can
thus be rationalized as random conjugated copolymers of
naphthalene and indenylenemethylene.
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We have successfully synthesized brush polymers with con-
jugated backbone via a catalyst-free Bergman cyclization of
enediyne-containing macromonomers. The size distributions of
brush polymers were wide in all cases; higher molecular weight
fractions could be obtained through repeat centrifugation or
dialysis. IR and NMR spectra showed disappearance of acetylene
units after Bergman cyclization; the formation of long conjugated
backbones was further confirmed with UV-vis spectroscopy.
The structure of backbone of brush polymers were rationalized as
a copolymer of naphthalene and indenylenemethylene. All the
brush polymers obtained are readily soluble in common organic
solvents. By utilizing thermal Bergman cyclization, conjugated
polymers could be obtained in metal-free manner. Further
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Acknowledgment. We thank National Natural Science
Foundation of China (No. 20874026; No. 20704013), Shanghai
Pujiang Program (07PJ14024), Shuguang Project (07SG33), New
Century Excellent Talents in University, and Shanghai Leading
Academic Discipline Project (B502) for financial support.
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