A convergence of photo-bergman cyclization and intramolecular chain collapse towards polymeric nanoparticles

Article abstract of DOI:10.1002/pola.25013

Employing enediynes as crosslinking precursors, a novel yet efficient strategy, namely photo-triggered Bergman cyclization, was integrated with intramolecular chain collapse to yield polymeric nanoparticles with the size regime below 20 nm. Enediyne motif was designed delicately to possess a high photo-reactivity, with the double bond locked in a methyl benzoate ring while triple bonds substituted with phenyls. Single electron transfer-living radical polymerization was conducted to provide linear acrylate copolymers with controlled molecular weights and narrow polydispersities. Poly(butylarylate-co- 5) went through UV-irradiation with a concurrent Bergman cyclization, resulting in well-defined ultrafine polymeric nanoparticles. Results from NMR, Raman scattering, photoluminescence and UV-vis spectra corroborated the presence of conjugative structures in the polymeric nanoparticles, indicating the occurrence of photo-induced Bergman cyclization. A series of other acrylate-based nanoparticles were investigated to confirm the applicability of such a unique strategy in thermal sensitive but UV-stable polymeric structures, making photo-Bergman cyclization a promising tool towards polymeric nanoparticles.

Full text of DOI:10.1002/pola.25013

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A Convergence of Photo-Bergman Cyclization and Intramolecular Chain
Collapse Towards Polymeric Nanoparticles
Benchuan Zhu, Guannan Qian, Yuli Xiao, Sheng Deng, Meng Wang, Aiguo Hu
Shanghai Key Laboratory of Advanced Polymeric Materials, School of Materials Science and Engineering,
East China University of Science and Technology, Shanghai 200237, China
Correspondence to: A. Hu (E-mail: hagmhsn@ecust.edu.cn)
Received 12 August 2011; accepted 2 September 2011; published online 6 October 2011
DOI: 10.1002/pola.25013
ABSTRACT: Employing enediynes as crosslinking precursors, a
novel yet efficient strategy, namely photo-triggered Bergman
cyclization, was integrated with intramolecular chain collapse
to yield polymeric nanoparticles with the size regime below 20
nm. Enediyne motif was designed delicately to possess a high
photo-reactivity, with the double bond locked in a methyl ben-
zoate ring while triple bonds substituted with phenyls. Single
electron transfer-living radical polymerization was conducted
to provide linear acrylate copolymers with controlled molecular
weights and narrow polydispersities. Poly(butylarylate-co-5)
cence and UV-vis spectra corroborated the presence of
conjugative structures in the polymeric nanoparticles, indicat-
ing the occurrence of photo-induced Bergman cyclization. A se-
ries of other acrylate-based nanoparticles were investigated to
confirm the applicability of such a unique strategy in thermal
sensitive but UV-stable polymeric structures, making photo-
Bergman cyclization a promising tool towards polymeric nano-
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particles.
2011 Wiley Periodicals, Inc. J Polym Sci Part A:
Polym Chem 49: 5330–5338, 2011
went through UV-irradiation with
a
concurrent Bergman
KEYWORDS: Bergman cyclization; copolymerization; gel perme-
ation chromatography (GPC); intramolecular chain collapse;
photoploymerization; polymeric nanoparticles; SET-LRP
cyclization, resulting in well-defined ultrafine polymeric nano-
particles. Results from NMR, Raman scattering, photolumines-
INTRODUCTION Avid interest has been devoted to the ex-
ploitation of advanced organic chemistry in polymer science
and materials recently.1,2 The main driving force is that the
synthetic organic chemistry has the capability to exquisitely
control the composition of small molecules, thus fabricate
well-defined materials. One of the classic examples is the for-
mation of ultra-fine polymeric nanoparticles through intra-
molecular cross-linking or single chain collapse. This is not
surprising since the intramolecular chain collapse technique
provides a facile strategy for preparing polymeric nanopar-
ticles within the critical size regime of 1–20 nm, which
showed multiple applications ranging from catalysts and
drug delivering systems,3–8 light- and energy-harvesting,9
and nanoporous low dielectric constant materials for micro
electric applications.10 A key requirement for an effective
intramolecular chain collapse involves an easy and controlla-
ble incorporation of appropriate cross-linking precursors
into an individual polymer chain. So far, several typical pre-
cursors and synthetic tools have been designed and applied
to induce such a coil-to-globule transition, resulting in the so
called unimolecular nanoparticles.10–27 Vinyl functionaliza-
tions, such as acrylate,10 as well as benzocyclobutane
(BCB)12 were successfully investigated to form such kinds of
nanoparticles through free-radical cross-linking. In the BCB
system, no initiator or catalystꢀwas required except for the
high heating temperature (250 C) to trigger the crosslinking
without side product formation. Meanwhile, benzosulfones
were utilized as precursors to form the same stable benzocy-
clooctadiene cross-linked entities as obtained from BCB,
resulting in polyelectrolyte nanoparticles16 or 3D architec-
tures.19 It should be noted that ultra-dilute reaction conditions
or a continuous addition technique must be requested to favor
intramolecular coupling against intermolecular cross-linking.
To avoid the harsh conditions, isocyanates were then selected
as cross-linkers to go through a much more moderate, room
temperature condition.20 In addition to the covalent bonding,
multiple hydrogen bonding was also employed to produce
metastable supramolecular polymeric nanoparicles.22 This
technique was later expanded with the combination of the liv-
ing radical polymerization and ‘‘click’’ chemistry to produce
supramolecular polymeric nanoparticles under UV irradia-
tion.23 As functionalized polymeric nanoparticles have been
regarded as versatile building blocks for scores of nanotechno-
logical structures, much more diversified precursors and
milder reaction conditions are still in urgent need.
Intrigued by the multiple applications of polymeric nanopar-
ticles, we successfully expanded thermal Bergman cyclization
Additional Supporting Information may be found in the online version of this article.
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into the ‘‘tool box’’ of preparing single chain polymeric nano-
particles in our previous work.21 Bergman cyclization is the
intramolecular cyclization of enediyne derivatives to generate
a highly reactive aromatic 1,4-diradical intermediate.28 Sig-
nificant amount of aromatic oligomers or polymers will be
obtained even with large excess of radical terminators, such
as 1,4-cyclohexadiene. Bergman cyclization is reckoned to be
an attractive and promising synthetic tool for material sci-
ence29–31 as the polymerization of aromatic 1,4-diradical can
give polyphenylenes and polynaphthalenes without the need
of exogenous catalyst or reagent. The structural variability of
enediyne monomers also allows introduction of multi func-
tionalities through substitution, leading to soluble and proc-
essable aromatic polymers. Hitherto, Bergman cyclization has
been reported to fabricate aromatic polymers and carbona-
ceous materials, such as poly-p-phenylenes, polynaphtha-
lenes, conjugated brush polymers, carbon nanotubes and
glassy carbons.32–41
However, the harsh reaction condition (>200 ꢀC) previously
encountered may suppress the universality of Bergman cycli-
zation in the field of polymers and materials, since the high
temperature required will go against most of the thermal
sensitive polymers, such as poly(t-butyl acrylate). To solve
this problem, herein we attempt to investigate the capability
of an alternative synthetic method, namely photo-induced
Bergman cyclization,28 to form unimolecular nanoparticles.
SCHEME 1 Synthesis of enediyne-containing G-1 (5) and G-2
(11) dendrimers.
reduction, resulting in uncyclized substances,58 whereas
others may even suffer from a cis, trans-isomerization of the
double bond.59 It’s already been well-documented that both
the types of photolytic products and the yield of the Berg-
man-type product were dependent on the substituents on
the triple bonds. Specifically, the photoreduction products
were eliminated when the double bond was confined in a
benzene ring along with the triple bonds were substituted
with phenyl groups. Herein, 1,2-bis(2-phenylethynyl)ben-
zene60 and its derivatives were taken for our ideal models.
However, the yield of the cyclized product under photo-irra-
diation in our experiment was as low as been reported else-
where.61 Considering that an extended conjugative effect and
wider range of UV absorption might somewhat enhance the
photo-reactivity of the enediynes (Supporting Information
Fig. S1), we incorporated the double bond into a ring of
methyl benzoate instead of benzene. Accordingly, enediyne 3
was synthesized immediately employing Pd-mediated Sono-
gashira coupling reaction (Scheme 1). Both 3 and 1,2-bis(2-
phenyl ethynyl)benzene were UV-triggered in THF for 6 h
with the same amount. The reaction mixtures were concen-
trated under vacuum to yield the products, which were later
characterized with 1H NMR without further purification. There’s
no significant change for 1,2-bis(2-phenylethynyl)benzene
before and after UV irradiation in the corresponding 1H NMR
spectrum. Whereas for 3, apparent new peaks around 8.1 and
8.9 ppm were presented after UV-irradiation, indicating the for-
mation of naphthyl (Supporting Information Fig. S2). Hereby a
novel yet efficient cross-linker, which not only possessed high
reactivity but also required only two high-yield steps from a
commercially available starting material, was exploited.
Photo-induced Bergman cyclization was uncovered in the
1990s by Sugiura,42 Nicolaou,43 and Wender.44 Later, Turro
and Nicolaou revealed that simple artificial enediynes could
also undergo cycloaromatization upon photo-irradiation.45
Subsequently, plenty of arene-fused or nonbenzenoid, locked
or acyclic enediynes were explored and investigated under
photoirradiation.46–56
Photo-Bergman
cyclization was
deemed to be similar in mechanism to that of the thermal
induced. However, most of the work aimed at cycloaromati-
zation products and treated polymers formed during the
procedures as by-products without further investigation. As
we have been acknowledged, only one report was issued
employing photo Bergman cyclization to yield a sparingly
soluble polymeric materials by Zaleski et al.57 They excited
metalloenediynes in solid state with a 785 nm diode laser
where both the metal ions and the laser acted as stimuli.
While here, employing enediynes as cross-linkers, we report
for the first time the utilization of a pure photo-triggered
Bergman cyclization route in ultra-dilute solution to fabricate
single chain polymeric nanoparticles with interior conjugated
oligomers. A couple of vinyl monomers are used to further
verify its applicability, including methyl acrylate (MA), ethyl
acrylate (EA), butyl acrylate (BA), and tert-butylacrylate
(tBA).
RESULTS AND DISCUSSION
The design of an appropriate parent compound turns out to
be very essential in virtue of the fact that upon photo-irradi-
ation, enediynes would likely go through a Bergman-type
rearrangement to yield benzene derivatives as expected from
the thermal Bergman cyclization, or a triple bond photo-
As most of the precedents involving photo-Bergman cycliza-
tion were carried out mainly to obtain benzene or
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FIGURE 1 1H (left) and 13C (right) NMR spectrum of compound 3 (a) and its polymeric products (b) after UV irradiation.
naphthalene rather than their oligomers or polymers, they
were conducted particularly with hydrogen donors such as
1,4-cyclohexadiene in a solvent or with a solvent bearing
good H-donatability, with THF and iso-propanol (i-PrOH)
being the routines. However, such kinds of H-donors and sol-
vents will quench the biradical formed during the Bergman
cyclization immediately, making polymerization almost
impossible. Therefore, to perform an effective chain collapse
within a single chain, the solvent circumstance must be cho-
sen prudently. Toluene was regarded as a good candidate for
a couple of reasons. First of all, according to its UV absorp-
tion spectrum, toluene is able to filter out most of the short
wavelengths below 280 nm that might cause polymer degra-
dation. Second, toluene is a good solvent for most of the
hydrophobic polymers. The most important thing is that pro-
tons bearing on toluene are less reactive than those of THF
or i-PrOH, thus making them harder to trap the biradical im-
mediately. Enediyne 3 was UV-triggered in toluene to con-
firm our hypothesis. Polymeric products were obtained via a
flash chromatography (silica gel, hexane: ethyl acetate 5/1)
and characterized with NMR, ESI-MS, GPC, UV-vis, and
Raman. The photolytic product showed broad rather than
distinct peaks from 6.5 to 8.5 ppm assigned to the protons
on arene, so did the methyl ester part locating around 3.9
ppm (Fig. 1). That might be ascribed to longer relaxation
time of the conjugated structure upon biradical coupling. As
can be seen in the 13C NMR spectra, peaks at 96.6, 94.4,
87.9, and 87.5 ppm corresponding to the triple bond totally
disappeared after the reaction. Further characterization with
Raman scattering presented an extensive intensity reduction
for the triple bond (2,200 cmꢁ1) after photo-Bergman cycli-
zation (Fig. 2). The existence of the peak around 2,200 cmꢁ1
suggested an incomplete photo-Bergman cyclization, how-
ever, it did little harm on the intramolecular chain collapse
(vide infra). A main peak around 1500 Da along with two
small peaks at 809, 439 Da showed up in GPC trace (vs. PS
FIGURE 2 Raman scattering of compound 3 before (left) and after (right) UV irradiation.
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dynamic volumes, which was able to be distinguished by
GPC analysis. As shown in Figure 4, the retention time of the
polymeric nanoparticles was much longer than that of the
linear copolymers. The products obtained after 6 hours’ irra-
diation, as expected, were even longer in retention time than
that from the 3 h. Statistically, the apparent molecular weight
decreased about 19% after 3 h UV irradiation (entry 1)
while 26% after 6 h (entry 2). Whereas another longer irra-
diation time (>6 h) caused no further apparent molecular
weight change in our experimental systems, thus subsequent
photo-Bergman cyclizations were all performed for 6 h. Vari-
ant fractions of enediyne moieties were incorporated into
the linear copolymers to tailor the nanoparticles. However,
no clear relationship between the ratio of the cross-linker
and the decrease fraction in the apparent molecular weight
of polymeric nanoparticles was disclosed here.
More detailed information about the polymeric nanoparticles
was given afterwards by analyzing PBA-5 (entry 2). The
results from GPC, 1H NMR, Raman, and UV-vis were inte-
grated to present some useful structural information. Similar
to what has been observed for compound 3, the resonance
of aromatic protons in linear copolymers ranging from 6.5 to
FIGURE 3 Photoluminescence emission spectra of compound 3
(solid line) and its polymeric products (dashed line) after UV
irradiation.
in THF), which was later reckoned as tetramers, trimers and
dimmers, respectively. It should be noted that, as the oligom-
ers have more compact structures and thus smaller hydrody-
namic values when compared to PS, the actual molecular
weights of the oligomers ought to be higher than the values
obtained from GPC. The cyclized conjugative structure was
also evidenced by a red shift from 456 to 516 nm in Photo-
luminescence spectra (Fig. 3) after the irradiation. The for-
mation of these oligomers (Scheme 2) dropped a hint that
it’s enough for us to investigate the intramolecular chain col-
lapse with this photo-initiated Bergman cyclization.
1
8.5 ppm became board after irradiation in H NMR (Support-
ing Information Fig. S3). The vagueness caused here could
also be ascribed to the hindrance arose from the rest part of
the polymer structures besides the longer relaxation time of
the conjugated structure formed during photo-Bergman cy-
clization. Significantly, no substantial change of integration
value of the four methylene protons on enediyne moieties
(around 4.4 ppm) compared to backbone methane protons
provided strong evidence that the enediyne moieties didn’t
break off during the photo-irradiation. Other characteriza-
tions, including Raman scattering, UV-vis spectroscopy, and
PL emission spectroscopy (Supporting Information Figs. S4–
S6), also displayed an analogous change to that of compound
3. Control experiments (entry 7) were done to dig a further
insight into the intramolecular chain collapse. BA and tBA
were copolymerized respectively with 2-(acryloyloxy)ethyl
benzoate and then irradiated under the same reaction condi-
tions. There was no detectable change in the apparent
molecular weight of these cases, thus demonstrating that
photo-degradation of the polymer backbone didn’t occur in
our systems and the intramolecular chain collapse was
caused by photo-Bergman cyclization of the enediynes.
The synthesis of enediyne containing acrylate 5 and its
copolymers with BA was depicted in Scheme 1. Acrylate
monomer species were selected because of the multiple
mature synthetic strategies, such as SET-LRP, to precisely
control over the molecular weight and the polydispersity
(PDI).62 The copolymerization of 5 with BA was conducted
in a binary solvent of acetone and water (9/1, vol).63 Small
amount of hydrazine hydrate was added as a reducing agent
to reduce the Cu (II) generated during the polymerization,
affording highly controllable molecular weight as well as nar-
row PDI.64 The incorporation of the enediyne moieties was
evidenced by the appearance of aromatic protons between
6.8 and 8.5 ppm in the 1H NMR analysis (Supporting Infor-
mation Fig. S3). The incorporation ratios of the enediyne
monomers calculated from 1H NMR analysis were found to
be the same as that of the feeding amount, representing a
comparative reactivity of the monomers during a SET-LRP
technique. Molecular weights, PDIs and the amount of ene-
diynes in each copolymer chain were detailed in Table 1.
Linear precursor PBA-5 was then subjected to UV-irradiation
in toluene (0.06 mg/mL). To check the efficiency of the
photo-reaction, Bergman cyclization was carried out for 3 h
and 6 h, respectively. After the UV exposure, the copolymers
were assumed to be compact globules with conjugated
cross-linkers, resulting in an apparent decrease in the hydro-
SCHEME 2 Formation of oligomeric products upon irradiation
of enediyne 3 in toluene.
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TABLE 1 Comparison of Apparent Molecular Weight and PDI of Starting Linear Polymers and
Corresponding Nanoparticlesa
Linear Polymer
Nanoparticle
EDY%b
15
Mw
PDId
1.27
Mw
PDId
1.23
DMw%c
d
d
Entry
1e
Copolymer
PBA-5
27,200
21,900
19.49
2
PBA-5
15
10
10
20
20
0
27,200
21,400
11,900
21,700
38,800
22,000
26,800
19,800
27,900
22,300
1.27
1.22
1.24
1.23
1.37
1.32
1.27
1.26
1.20
1.34
20,100
16,800
8,200
17,100
27,500
–
1.12
1.15
1.29
1.13
1.29
–
26.10
21.50
31.09
21.20
29.12
NA
3
PBA-5
4
PBA-5
5
PBA-5
6
PBA-5
7
PBA-benzoate
PMA-5
8
15
20
17
0
20,100
13,000
18,000
–
1.11
1.19
1.14
–
25.00
34.34
35.48
NA
9
PEA-5
10
11
a
PtBA-5
PtBA-benzoate
All reactions were run in toluene under ultra-dilute condition at room temperature for
specified.
6 h unless
b
Molar fraction of enediyne (EDY) 5 moiety in copolymer calculated through NMR analysis.
c
Change of apparent molecular weight.
d
Weight average molecular weight and polydispersity index (PDI) measured by GPC using poly(styrene)
as calibration standard.
e
Irradiation for 3 h.
Based on the above results, it can be addressed that photo-
triggered Bergman cyclization is a feasible and mild pathway
for intramolecular chain collapse. To inspect its generality,
other acrylate monomers, MA, EA, and tBA (entries 8–10)
were copolymerized with 15% mol feed ratio of enediyne 5.
However, the fraction of enediyne moieties in each linear co-
polymer, as shown in Table 1, was slightly higher than 15%,
which, in another word, suggested monomers MA, EA, and
tBA were slightly less reactive than enediyne 5. Further
photo-irradiation of the linear copolymers fabricated more
compact nanoparticles with narrow PDIs (Table 1, entries
8–10).
Glass transition temperatures (Tg) were also measured by
differential scanning calorimetry (DSC) to indicate the mor-
phology change from coil to globular. Diverse linear acrylate
backbones exhibited different Tgs (ꢁ15.82 ꢀC for PBA-5,
13.28 ꢀC for PEA-5, 37.04 ꢀC for PMA-5, 60.38 ꢀC for PtBA-
5). Meanwhile, large exothermic peaks were observed above
300 ꢀC for all the linear copolymers, relating to the corre-
sponding thermal Bergman cyclization. While for PtBA-5, an
additional endothermic peak showed up at 238 ꢀC, as a
result of the thermal degradation of the tBA moieties. All the
polymeric nanoparticles exhibited neither measurable glass
transition temperatures nor characteristic exothermal peak
of Bergman cyclization in DSC curves (Supporting Informa-
tion Fig. S7). The disappearance of the Tg must be ascribed
to the decreased segmental chain mobility for the highly
cross-linked nanoparticles. While the vanishing of character-
istic exothermal peaks of Bergman cyclization indicated the
complete consumption of the enediyne moieties. In all these
DSC curves, unexpected exothermal peaks around 215 ꢀC
appeared for polymeric nanoparticles. At current stage, we
can only speculate that other reactions (e.g., formation of
five-membered rings65) besides Bergman cyclization may
have taken place since the mechanism of photo-Bergman cy-
clization remains poorly understood.
AFM is a powerful method to characterize the assembly of
polymer chains.66–69 After the photo-crosslinking, the as-pre-
pared nanoparticles were dissolved in THF and spin-coated
onto a hexamethyldisilazane modified silica wafer at the
speed of 2,000 rpm for 30 s. Figure 5 displayed tens of well-
defined spherical-like polymeric nanoparticles. Taking the
shape of the particles as half an ellipsoid, nanoparticles in
FIGURE 4 GPC traces of linear P(BA-r-5) (solid line) and corre-
sponding nanoparticles for 3 hr irradiation (dashed line, entry
1) and 6 hr irradiation (dotted line, entry 2)
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FIGURE 5 AFM phase image (left) and 3D height image (right) of nanoparticles (entry 6) on silicon wafer.
Figure 5 would possess a calculated diameter of ꢂ10 nm
following the Meijer’s method,22 which was slightly larger,
yet in the same order of magnitude, than that deduced from
GPC. The deviation in diameters might be caused by the
large extent of rigid structures in the nanoparticles that
made them irregular balls rather than half ellipsoids.
Synthesis of Methyl 3,4-dibromobenzoate (2)
Compound 2 was synthesized according to a literature pro-
cedure.71 The crude product was recrystallized from ethyl
acetate-hexane to give pure white crystals.
1H NMR (CDCl3, d ppm): 8.27 (d, 4J ¼ 1.9 Hz, Ph-H, 1H,),
7.81 (dd, 3J ¼ 8.3 Hz, 4J ¼ 1.9 Hz, Ph-H, 1H), 7.70 (d, 3J ¼
8.3 Hz, Ph-H, 1H), 3.9 (s, OCH3, 3H).
EXPERIMENTAL
Materials
Synthesis of Methyl 3,4-bis(2-phenylethynyl)benzoate (3)
Compound 2 (0.75 g, 2.5 mmol), PdCl2(PPh3)2 (0.183 g, 0.25
mmol), CuI (0.143 g, 0.75 mmol), and KI (1.245 g, 7.5 mmol)
were mixed with a mixture solvent Et3N/DMF (1/5, 10 mL)
in a 50 mL schlenk flask under nitrogen. Subsequent freeze-
pump-thaw techniques were conducted for three times, and
then phenylacetylene (1.02 g, 10 mmol) was added under
nitrogen. The solution was later subjected to an 80 ꢀC oil
bath overnight. After dilution with ethyl acetate, the organic
layer was washed with 1 N HCl, brine and dried over MgSO4.
The crude product was purified through silica gel column
chromatography (ethyl acetate/petroleum ether ¼ 1:20, Rf ¼
0.6) to give the pure product as a brown-red powder
(0.61 g, 73%).
All of the reactions and manipulations were carried out with
Schlenk techniques under Nitrogen unless otherwise men-
tioned. Tetrahydrofuran (THF) and toluene were distilled
over Na under nitrogen before use. N,N-dimethylforamide
(DMF) and triethylamine (Et3N) were distilled over Calcium
hydride under nitrogen before use. MA, EA, BA, and tBA
were washed with a 5% aqueous NaOH solution to remove
the inhibitor and distilled under vacuum before use. Tris[2-
(dimethylamino)ethyl]amine (Me6-TREN) was synthesized
according to literature procedures.70 All other reagents were
commercially available and used as received.
Characterization
1H NMR (400 MHz) and 13C NMR (100 MHz) measurements
were performed on a Bruker AVANCE 400 FT-NMR spec-
trometer with CDCl3 as solvent. Gel permeation chromatogra-
phy (GPC) was conducted at 40 ꢀC in THF on a WATERS
1515 system equipped with a series of PS gel columns. The
molecular weights of both the polymers and polymeric nano-
particles were calculated in relative to linear polystyrene
(PS) standards. Differential scanning calorimetry (DSC) stud-
ies were carried out on a Mettler-Toledo thermal analysis
workstation equipped with a model 822e DSC module. Glass
transition temperatures were measured under a constant
1H NMR (CDCl3, d ppm): 8.24 (s, PhAH, 1H), 7.97 (d, 3J ¼
8.0 Hz, PhAH, 1H), 7.59 (m, PhAH, 5H), 7.37 (m, PhAH, 6H),
3.95 (s, OCH3, 3H). 13C NMR (CDCl3, d ppm): 166.0, 132.9,
131.8, 131.8, 131.8, 130.1, 129.5, 129.0, 128.7, 128.5, 128.5,
126.1, 123.0, 122.8, 96.6, 94.4, 87.9, 87.5, 52.4. MS: m/z
calcd. for C24H16O2(MþH)þ: 337.1; found: 337.1.
Synthesis of 3,4-Bis(2-phenylethynyl)benzoic acid (4)
To a stirring solution of 3 (1.38 g, 4.1 mmol) in CH2Cl2/
CH3OH (150 mL, v/v ꢂ9:1) was added a methanolic solution
of NaOH (1.31 g, 32.8 mmol). After 15 min, the solution
became cloudy and the sodium salt of the carboxylic acid
started to precipitate. The solvents were removed under vac-
uum after stirring overnight. The reaction mixture was dis-
solved in water and acidified to pH ¼ 2–3 with dilute HCl,
whereupon the crude product was extracted with ethyl ace-
tate and dried over anhydrous MgSO4. Further concentration
over a rotary evaporator under a high vacuum afforded acid
4 as a pale yellow solid (1.24 g, 93%).
ꢀ
nitrogen flow with a heating rate of 10 C/min. Mass spectra
were obtained from a Micromass LCTTM mass spectrometer
employing ESI method. UV analyses were performed using a
UNICO UV-21-2 PCS spectrometer in THF at room tempera-
ture. AFM images of nanoparticles were acquired on a
Multimode Nanoscope
V
scanning probe microscopy
system using the tapping mode. Fluorescence spectra
were recorded on
spectrophotometer.
a Varian Cary eclipse fluorescence
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1H NMR (CDCl3, d ppm): 8.31 (d, 4J ¼ 1.6, PhAH, 1H), 8.03
(dd, 3J ¼ 8.2 Hz, 4J ¼ 1.7 Hz, PhAH, 1H), 7.66 (d, 3J ¼ 8.1
Hz, PhAH, 1H), 7.59 (m, PhAH, 4H), 7.37 (m, PhAH, 6H).
polymerization. The copolymer was obtained after precipita-
tion in methanol or methanol/water ¼ 1:1 (vol).
General Procedure for Formation of Nanoparticles via
UV Triggered Intramolecular Chain Collapse
Synthesis of 2-(Acryloyloxy)ethyl 3,4-bis
A solution of polymer 6 (30 mg) in toluene (450 mL) was
added to a quartz UV cuvette and sparged with nitrogen. The
UV irradiation was conducted in a photo reactor box (Beijing
Trusttech, China) using a medium-pressure Hg lamp. The
reaction was stirred for 6 h under nitrogen atmosphere.
(2-phenylethynyl)benzoate (5)
To a solution of 4 (0.99 g, 3.08 mmol) in DMF was added 1-
ethyl-3-[3-dimethylaminopropyl] carbodiimide (0.71 g, 3.70
mmol), N,N-dimethylaminopyridine (0.08 g, 0.62 mmol) and
2-hydroxyethyl acrylate (1.29 mL, 12.32 mmol) under nitro-
gen. The mixture was stirred at room temperature overnight.
The resulting solution was then washed with brine and
extracted with ethyl acetate. The organic layer was dried
over anhydrous MgSO4 and concentrated under vacuum. Sub-
sequent flash chromatographic purification over silica (1:5
ethyl acetate-petroleum ether, Rf ¼ 0.6) yielded a yellowish
product (0.91 g, 70%).
CONCLUSIONS
We have introduced a novel method to fabricate single chain
polymeric nanoparticles based on photo induced Bergman
cyclization. Enediyne functionalities were easily incorporated
into polymer chains by controllable single electron transfer
living radical copolymerization with different kinds of acry-
late monomers. Both model small molecular biradical cou-
pling and intramolecular photo-crosslinking in polymer
chains were confirmed by various analysis methods. Raman
scattering and 13C NMR revealed the disappearance of CBC
triple bond after certain UV irradiation, which is one of the
most important features of occurring Bergman cyclization,
accompanied with sharp decrease of apparent molecular
weight recorded by GPC. Through AFM characterization, indi-
vidual nanoparticles were presented clearly and distinguish-
able in well defined manner. Furthermore, we expanded this
method to thermal sensitive poly(t-butylacrylate) nanopar-
ticles which could not be accomplished by thermal Bergman
cyclization. The potential applications of these aromatic func-
tionalized polymer nanoparticles are under exploration in
our lab.
1H NMR (CDCl3, d ppm): 8.23 (d, PhAH, 1H), 7.96 (dd, 3J ¼
8.2 Hz, PhAH, 1H), 7.60 (m, PhAH, 5H), 7.37 (m, PhAH, 6H),
2
2
6.45 (m, J ¼ 17.3 Hz, ACH¼¼CHAH, 1H), 6.17 (m, J ¼ 17.3
Hz, ACH¼¼CH2, 1H), 5.87 (m, 2J ¼ 16.7 Hz, ACH¼¼CHAH,
1H), 4.59 (m, PhACO2ACH2A, 2H), 4.53 (m, ACH2CH2A,
2H). 13C NMR (CDCl3, d ppm): 164.8, 164.1, 131.9, 130.8,
130.7, 130.4, 129.2, 128.0, 128.0, 127.8, 127.7, 127.4, 127.4,
126.9, 125.1, 121.9, 121.7, 95.7, 93.4, 86.8, 86.4, 62.0, 61.1.
MS: m/z calcd. for C28H20O4(MþH)þ: 421.1; found: 421.1.
Synthesis of 2-(Acryloyloxy)ethyl benzoate
This compound was synthesized in a manner analogous to
that of 5. The pure product was obtained as a colorless oil.
1H NMR (CDCl3, d ppm): 8.04 (d, 3J ¼ 7.2 Hz, PhAH, 2H),
3
3
7.56 (t, J ¼ 7.4 Hz, PhAH, 1H), 7.44(t, J ¼ 7.82 Hz, PhAH,
2
2
The support of National Natural Science Foundation of China
(20874026, 20704013), Ph. D. Programs Foundation of Minis-
try of Education of China (20100074110002), Shanghai Shu-
guang Project (07SG33), and Shanghai Leading Academic
Discipline Project (B502) is gratefully acknowledged. A. Hu
thank Profs. Shouwu Guo and Lingyong Zhu for their kind help
in AFM and photoluminescence spectroscopy.
2H), 6.43 (m, J ¼ 17.3 Hz, ACH¼¼CHAH, 1H), 6.14 (m, J ¼
17.3 Hz, ACH¼¼CH2, 1H), 5.85 (dd, ACH¼¼CHAH, 1H), 4.55
(m, 3J ¼ 9.3 Hz, PhACO2ACH2A, 2H), 4.50 (m, 3J ¼9.2 Hz,
ACH2CH2A, 2H).
Synthesis of 1,2-Bis(2-phenylethynyl)benzene
This compound was synthesized according to literature pro-
cedure.60 The pure product was obtained by silica gel chro-
matography (petroleum ether, Rf ¼ 0.8) as a colorless
crystal.
REFERENCES AND NOTES
1H NMR (CDCl3, d ppm): 7.46 (m, PhAH, 6H), 7.28 (m,
PhAH, 8H).
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