Intrachain cyclization via postmodification of the internal alkenes of periodic ADMET copolymers: The sequence matters

Article abstract of DOI:10.1021/ma5013732

We demonstrate that monomer sequence is important to regulate the vinyl copolymer thermal properties by intrachain cyclization. This is exemplified by the spontaneous intrachain cyclization with the formation of γ-butyrolactone by dihydroxylation of the internal alkenes of a designed periodic copolymer. A structurally symmetric α,ω-diene monomer 1 containing two tail-to-tail connected ethyl acrylate units was synthesized via a two-step approach. The acyclic diene metathesis (ADMET) polymerization of monomer 1 was conducted with a Hoveyda-Grubbs second generation catalyst in the presence of p-benzoquinone to get well-defined polymer (P1) with high molecular weights as revealed by GPC, NMR, and MALDI-TOF-MS characterizations. Dihydroxylation of P1 using hydrogen peroxide as the oxidant was successfully conducted to afford HP1. Characterization of HP1 by NMR and IR spectra indicated that it contained γ-butyrolactone units in the polymer main chain. Control experiments with four different polymers indicated that the formation of this ring structure was the outcome of the specific sequence of vinyl alcohol-ethyl acrylate formed by dihydroxylation of P1. The cyclization efficiency of P1 was calculated to be 77%. The T_gs of all the modified polymers were increased compared to those of the unsaturated samples; significantly, the T_g of HP1 was drastically elevated by 160 °C as compared to that of P1.

Full text of DOI:10.1021/ma5013732

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Article
pubs.acs.org/Macromolecules
Intrachain Cyclization via Postmodification of the Internal Alkenes of
Periodic ADMET Copolymers: The Sequence Matters
Zi-Long Li, An Lv, Fu-Sheng Du, and Zi-Chen Li*
Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Polymer Chemistry & Physics of Ministry of
Education, Department of Polymer Science & Engineering, College of Chemistry and Molecular Engineering, Peking University,
Beijing 100871, China
S
* Supporting Information
ABSTRACT: We demonstrate that monomer sequence is
important to regulate the vinyl copolymer thermal properties
by intrachain cyclization. This is exemplified by the
spontaneous intrachain cyclization with the formation of γ-
butyrolactone by dihydroxylation of the internal alkenes of a
designed periodic copolymer. A structurally symmetric α,ω-
diene monomer 1 containing two tail-to-tail connected ethyl
acrylate units was synthesized via a two-step approach. The
acyclic diene metathesis (ADMET) polymerization of
monomer 1 was conducted with a Hoveyda−Grubbs second
generation catalyst in the presence of p-benzoquinone to get
well-defined polymer (P1) with high molecular weights as
revealed by GPC, NMR, and MALDI-TOF-MS characterizations. Dihydroxylation of P1 using hydrogen peroxide as the oxidant
was successfully conducted to afford HP1. Characterization of HP1 by NMR and IR spectra indicated that it contained γ-
butyrolactone units in the polymer main chain. Control experiments with four different polymers indicated that the formation of
this ring structure was the outcome of the specific sequence of vinyl alcohol−ethyl acrylate formed by dihydroxylation of P1. The
cyclization efficiency of P1 was calculated to be 77%. The Tgs of all the modified polymers were increased compared to those of
the unsaturated samples; significantly, the Tg of HP1 was drastically elevated by 160 °C as compared to that of P1.
performance thermoplastics via anionic random copolymeriza-
tion of styrene and diene derivatives followed by intrachain
Friedel−Crafts cyclization.8−10 In these contributions, olefins
are involved in cyclization processes, and successful post-
modifications are guaranteed by sequence control, thus leading
to property variation of the final polymers.
INTRODUCTION
■
Postpolymerization modification is one of the most important
methods to introduce functional groups into polymers and
subsequently regulate polymer properties.1 The modification
can be done in the polymer side chains, in polymer backbones,
and at the polymer ends, leading to a variety of functional
polymers with tunable properties from one type of polymer
platform. Selective and quantitative modifications are highly
desired to yield homogeneous polymers with predesigned
structures. Chemical modification of polymer backbone is much
more challenging than those of side chains or end groups, since
the reactivity of internal functional group in the polymer
backbone is significantly decreased and the steric repulsion
cannot be overlooked. One important approach of post-
modification of polymer backbone is the intrachain cyclization
of polymers with defined microstructures, where sequence
distribution plays a key role. Indeed, microstructure control
including sequence regulation has attracted much attention and
is definitely a significant parameter in polymer design that leads
to polymers with complex structures and sophisticated
functions.2−6 For example, Grubbs et al. reported the
quantitative cyclization of 1,2-polydiene via dynamic ring-
closing metathesis to afford stereoregular cyclopentene−
methylene alternating copolymer.7 Recently, Kamigaito et al.
developed a method to obtain thermally and optically high-
Olefin metathesis is one of the most powerful tools to
synthesize unsaturated polyolefins. Acyclic diene metathesis
(ADMET) polymerizations of structurally symmetric diene
monomers have been systematically investigated to generate
periodic vinyl copolymers.11−15 Pioneered by Wagener, a series
of precision polyethylenes have been synthesized by this
method, and the effects of branch type, frequency, and
regularity on the morphology changes of the resulting polymers
have been elucidated.16,17 Moreover, the related materials have
found wide applications in energy18,19 and biomedical20,21
fields. Recently, our group also reported a new family of
sequence-regulated vinyl copolymers via ADMET polymer-
ization.22−25 Generally, the internal alkenes of the ADMET
polymers are exhaustively hydrogenated to obtain saturated
carbon−carbon backbones. However, if the internal alkenes can
Received: July 3, 2014
Revised: August 8, 2014
Published: August 19, 2014
© 2014 American Chemical Society
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be quantitatively transformed to other entities by other efficient
chemical approaches, one or two new monomer units other
than ethylene will be incorporated into the repeating unit of the
ADMET polymers. This strategy can thus lead to new densely
functionalized periodic vinyl copolymers with defined micro-
structures, the property of which can be tuned due to such
monomer sequence variation, thus deepening our under-
standing of the structure−property relationship of polymers.
Internal alkenes are functional moieties that appear in natural
small molecules26−28 (e.g., plant oil) and polymers29 (e.g.,
rubber). Countless fine chemicals and valuable polymeric
materials are produced through functionalization of these
alkene groups. For example, vulcanization serves as an
indispensable step to generate undistorted rubber. Basically,
chemical transformation of internal double bonds to other
functional groups that used in organic chemistry can be applied
in postmodification of polymers containing these groups as
well. However, polymer substrates face challenges such as
incomplete modification, side reactions, and purification
difficulty.30−34 For typical polymers containing internal alkenes
such as polybutadiene, chemical modifications under harsh
conditions (e.g., halogenation and oxidation) were investigated
decades ago, but the poor controllability (e.g., degree of
functionalization) and severe side reactions (e.g., chain scission
and coupling) prevented their applications in industry.35−37
Later on, hydroboration/oxidation of the internal alkenes
within the polymers by ring-opening metathesis polymerization
using 9-BBN were reported, but only one hydroxyl group was
incorporated with ambiguous regioselectivity for a single
butadiene unit.38−40 Very recently, a few contributions have
been reported to describe the postmodification of unsaturated
polymers containing butadiene units under mild conditions to
generate functionalized polyolefins.41−46 These excellent works
shed light on functionalization of well-known unsaturated
polymeric materials.
Inspired by the previous work, in this study, we provide the
first example of postmodification of the internal alkene groups
of periodic ADMET polymers, with an aim to elucidate the
importance of monomer sequence on regulating the polymer
properties. To this end, we synthesized a new type of periodic
vinyl copolymer of butadiene and ethyl acrylate with a 1:2
sequence via the ADMET polymerization of a designed
structurally symmetric α,ω-diene monomer containing two
tail-to-tail connected ethyl acrylate units. Upon highly efficient
dihydroxylation with hydrogen peroxide, we found that
spontaneous intramolecular γ-lactonization occurred due to
the formation of chemically reactive vinyl alcohol−ethyl
acrylate sequence (see Scheme 1). As a result, the obtained
polyolefin after modification was densely functionalized with γ-
butyrolactone units, which exhibited drastically elevated glass
transition temperature.
Scheme 1. Postmodification of 1:2 Sequenced Butadiene and
Ethyl Acrylate Copolymers Formed by the ADMET of a
Structurally Symmetric α,ω-Diene Monomer
(TEA, ≥99.0%), hydrogen peroxide (H2O2, 30% solution), and
potassium carbonate (K2CO3, ≥99.0%) were purchased from
Sinopharm Chem. Reagent, Co., Ltd. 1,9-Decadiene (98%, TCI) was
distilled over sodium and stored at 4 °C before use. Toluene, acetone,
ethanol (EtOH), chloroform (CHCl3), and dichloromethane
(CH2Cl2) were purchased from Beijing Chem. Works. CH2Cl2 was
distilled over calcium hydride and degassed upon sonication for 2 min
before use.
Synthesis of Ethyl 1-Allyl-2-Oxocyclopentanecarboxylate.47
K2CO3 (33 g, 0.24 mol), acetone (75 mL), ethyl 2-oxocyclopentane-
carboxylate (9.36 g, 0.06 mol), and allyl bromide (10.38 mL, 0.12 mol)
were sequentially transferred into a 250 mL round-bottom flask
equipped with a reflux condenser. The flask was placed in an oil bath
set at 60 °C and refluxed for 20 h. After cooling to room temperature,
the mixture was vacuum filtered to remove KBr and K2CO3. The
organic filtrate was concentrated and purified by vacuum distillation to
obtain the pure product as a colorless oil in 98% yield. 1H NMR (400
MHz, CDCl3), δ (TMS, ppm): 5.70 (ddt, J = 17.3, 10.1, 7.3 Hz, 1H),
5.08 (m, 2H), 4.15 (m, 2H), 2.19 (m, 8H), 1.24 (t, J = 7.1 Hz, 3H).
13C NMR (100 MHz, CDCl3): δ 213.80, 170.52, 132.99, 118.60,
61.01, 59.56, 37.69, 37.54, 31.89, 19.27, 13.81. ESI-MS: [M + H]+ =
C11H17O3, calcd: 197.117 22, found: 197.117 16; [M + Na]+
=
C11H16O3Na, calcd: 219.099 17, found: 219.099 00. The NMR spectra
are shown in Figure S1 of the Supporting Information.
Synthesis of Monomer 1.48 To a 100 mL round-bottom flask
containing 15.6 mL of EtOH was slowly added Na (1.38 g, 0.06 mol).
After the mixture was cooled to room temperature, ethyl 1-allyl-2-
oxocyclopentanecarboxylate (11.76 g, 0.06 mol) was added to the
solution. The reaction mixture was heated under gentle reflux with
stirring for 8 h. Then, allyl bromide (5.7 mL) was dropwise added to
the hot solution. The solution was heated to reflux with stirring for
another 8 h, during which time NaBr precipitated out. After cooling to
room temperature, dilute HCl solution was added, and the aqueous
layer was separated and extracted once with ethyl acetate. The extract
was combined with the organic layer, sequentially washed with dilute
Na2CO3 solution, deionized water, and brine, and dried over
anhydrous Na2SO4. After filtration and rotary evaporation, the dark
red crude product was purified by silica gel column chromatography
(petroleum ether/ethyl acetate = 20/1) to obtain 1 as a colorless oil in
76% yield. 1H NMR (400 MHz, CDCl3), δ (TMS, ppm): 5.72 (ddt, J
= 16.8, 10.1, 6.7 Hz, 2H), 5.03 (dd, J = 20.0, 9.7 Hz, 4H), 4.13 (q, J =
7.1 Hz, 4H), 2.37 (m, 4H), 2.28−1.37 (m, 6H), 1.23 (m, 6H). 13C
NMR (100 MHz, CDCl3): δ 174.85 (d, J = 9.7 Hz), 135.14 (d, J = 3.8
Hz), 116.60 (d, J = 6.8 Hz), 60.00, 44.95 (dd, J = 22.4, 9.6 Hz), 36.19,
29.08 (d, J = 13.3 Hz), 14.17. ESI-MS: [M + Na]+ = C16H26O4Na,
calcd: 305.172 33, found: 305.171 72.
EXPERIMENTAL SECTION
■
Materials. The following chemicals were used as received: ethyl 2-
oxocyclopentanecarboxylate (98%, Acros), ethyl 2-methyl-4-pente-
noate (>98.0%, TCI), Grubbs first, second, and third generation (G-I,
G-II, and G-III) catalysts (Aldrich), Hoveyda−Grubbs first and second
generation (HG-I and HG-II) catalysts (Aldrich), 2,3-dichloro-5,6-
dicyano-p-benzoquinone (DDQ, 98%, Aldrich), p-benzoquinone (BQ,
≥98.0%), trifluoroacetic acid (TFA, 99%, Acros), trifluoroacetic
anhydride (TFAA, 99%, Acros), 4-methylbenzenesulfonhydrazide
(TSH, 97%, Acros) ethyl vinyl ether (98%, stabilized with 0.1%
N,N-diethylaniline, Alfa Aesar), and tripropylamine (TPA, 98%, Alfa
Aesar). Sodium (Na, >99.5%), allyl bromide (≥97.0%), triethylamine
Synthesis of Diethyl 2,7-Dimethyl-4-octenedioate (2). Ethyl
2-methyl-4-pentenoate (142 mg, 1 mmol), BQ (1.1 mg, 10 μmol), and
HG-II catalyst (3.1 mg, 5 μmol) were sequentially added into a 25 mL
Schlenk flask equipped with a Teflon valve. Then, 1.0 mL of CH2Cl2
(degassed by sonication for 2 min) was added to dissolve the mixture
to obtain a homogeneous blue solution. The flask was connected to a
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reflux condenser that equipped with an anhydrous CaCl2 loaded
drying tube. Subsequently, the flask was placed in an oil bath at 60 °C.
Nitrogen gas was continuously purged in through the Teflon valve. As
the reaction proceeded, CH2Cl2 gradually evaporated, and the mixture
became heterogeneous in that blue precipitate coexisted with a small
amount of liquid. The mixture was dissolved in CHCl3 (5 mL) and
filtered through a pad of silica. The silica gel was washed with CHCl3,
and the combined organic phase was evaporated to generate pale
yellow oil in 98% yield. 1H NMR (400 MHz, CDCl3), δ (TMS, ppm):
5.44 (m, 2H), 4.12 (q, J = 7.1 Hz, 4H), 2.28 (m, 6H), 1.25 (t, J = 7.1
Hz, 6H), 1.13 (dd, J = 11.0, 6.9 Hz, 6H). 13C NMR (100 MHz,
CDCl3): δ 175.77 (d, J = 1.6 Hz), 129.29 (d, J = 1.9 Hz), 128.30, 59.97
(d, J = 7.3 Hz), 39.43 (d, J = 1.6 Hz), 36.45 (d, J = 4.9 Hz), 16.30 (d, J
= 5.5 Hz), 14.09. ESI-MS: [M + H]+ = C14H25O4, calcd: 257.174 74,
found: 257.174 73; [M + Na]+ = C14H24O4Na, calcd: 279.156 68,
found: 279.156 53. The NMR spectra are shown in Figure S2.
Other model metathesis reactions of ethyl 2-methyl-4-pentenoate
were carried out under similar conditions with different inhibitors and
tiny bubbles were observed during that time. The reaction was then
stopped upon cooling to generate an opaque solution with white
precipitates. An equal supply of TSH and TPA was readded, and the
reaction was allowed to proceed for another 12 h at 130 °C. The
mixture was washed with water for three times, and then toluene in the
organic layer was removed in vacuo. Chloroform (1 mL × 3) was
utilized to dissolve the crude hydrogenated product. The viscous
solution was poured into cold petroleum ether (100 mL) to generate a
pale brown suspension, and a pale brown precipitate was formed after
the suspension being stored at 4 °C overnight. P2 was obtained in 90%
yield after filtration and vacuum dryness.
Polymers P3 and P4 were synthesized similarly using G-I catalyst
according to our previous work.41 Polymer P5 was synthesized by
copolymerization of monomer 1 and 1,9-decadiene (1/1, molar ratio)
with HG-II catalyst and BQ.
All the GPC traces and NMR spectra of P2, P3, P4, and P5 are
shown in Figures S7−S12.
Dihydroxylation and Cyclization. Take the synthesis of HP1 as
an example. Polymer P1 (200 mg) was dissolved in CH2Cl2 (1.0 mL)
containing TEA (101 mg, 1.0 mmol) and TFA (114 mg, 1.0 mmol) to
form solution 1. H2O2 solution (30%, 2.0 mL) was added to CH2Cl2
(2.0 mL) at 0 °C, and then TFAA (0.42 g, 2.0 mmol) was added
dropwise to generate solution 2. Subsequently, solution 2 was added to
solution 1, and the resulting solution was refluxed at 45 °C for 12 h.
To the reaction mixture was dropwise added 12 mol/L HCl solution
(3.0 mL). The solution was heated to reflux with stirring for an
additional 12 h. After cooling, an opaque suspension was generated
and poured into methanol (100 mL). White precipitate was observed
via storage at 4 °C for 24 h. After filtration and vacuum dryness, HP1
was obtained in 85% yield.
HP3, HP4, and HP5 were synthesized similarly. HP3 was
precipitated in cold ethyl ether. HP4 was obtained as follows: The
reaction mixture was extracted with CH2Cl2. Then, the organic layer
was washed with deionized water for three times and concentrated
through rotary evaporation, and the viscous solution was poured into
cold petroleum ether (100 mL) and stored at 4 °C for 24 h. After
filtration and vacuum dryness, HP4 was obtained in 70% yield.
Measurements. 1H and 13C NMR spectra were recorded in either
CDCl3 or DMSO-d6 on a Bruker ARX-400 spectrometer with
tetramethylsilane (TMS) as the internal reference for chemical shifts.
Number-average molecular weights (Mn) and polydispersity indices
(PDI = Mw/Mn) of the polymers were determined by gel permeation
chromatograph (GPC). The measurements were conducted with
DMF (containing 0.02 mol/L LiBr, flow rate: 1 mL/min) as the eluent
at 50 °C with a Waters 1515 isocratic HPLC pump connected to a
Waters 2414 refractive index detector. A family of narrowly dispersed
polystyrenes was utilized as the standards, and Breeze 2 software was
used for data acquisition and spectra manipulation. Infrared (IR)
spectra were recorded on a Bruker Vector-22 Fourier transform
infrared spectrometer. All the samples were dispersed in potassium
bromide (KBr) by grinding, and OPUS/IR software was used to
manipulate the spectra. Electrospray ionization mass spectroscopy
(ESI-MS) characterizations were conducted on a Bruker APEX-IV
Fourier transform mass spectrometer in a positive ion mode. A Bruker
BIFLEX-III MALDI-TOF mass spectrometer was utilized to perform
the matrix-assisted laser desorption/ionization time-of-fight mass
spectroscopy (MALDI-TOF-MS) measurement in a linear mode.
Thermal gravimetric analysis (TGA) was carried out by using Q600-
SDT thermogravimetric analyzer (TA Co., Ltd.) with nitrogen purging
rate set at 100 mL/min. All the measurements were conducted from
50 to 600 °C at a heating rate of 10 °C/min. Calorimetric
measurements were performed using a Q100 differential scanning
calorimeter (TA Co., Ltd.) with nitrogen purging rate set at 50 mL/
min. The program was set to finish two cycles in the temperature
range from −80 to 100 °C for unsaturated polymers and from −20 to
160 °C for modified polymers. The heating/cooling rate was set to 10
°C/min. Data of the endothermic curve were acquired from the
second scan and analyzed with TA Universal Analysis software.
1
catalysts and at different reaction temperatures (Scheme 4). The H
NMR spectra of the corresponding products are shown in Figures S3−
S6.
Synthesis of 4,4′-Dimethyltetrahydro-[2,2′-bifuran]-
5,5′(2H,2′H)-dione (3). Compound 2 (51.2 mg, 0.2 mmol) was
dissolved in CH2Cl2 (1.0 mL) containing TEA (20.2 mg, 0.2 mmol)
and TFA (22.8 mg, 0.2 mmol) to form solution 1. H2O2 solution
(30%, 0.5 mL) was added to CH2Cl2 (1.0 mL) at 0 °C, and then
TFAA (84 mg, 0.4 mmol) was added dropwise to generate solution 2.
Subsequently, solution 2 was slowly added to solution 1, and the
resulting solution was refluxed at 45 °C for 12 h. To the reaction
mixture was dropwise added 12 mol/L HCl solution (3.0 mL). The
solution was heated to reflux with stirring for an additional 12 h. After
cooling, the mixture was extracted three times with CH2Cl2. The
organic layer was separated and sequentially washed with dilute
Na2CO3 solution, deionized water, and brine and dried over anhydrous
Na2SO4. After filtration and rotary evaporation, a white solid was
obtained, which was recrystallized from ethyl acetate in 88% yield. Tm
= 147.8 °C (by DSC). 1H NMR (400 MHz, CDCl3), δ (TMS, ppm):
4.53 (m, 2H), 2.77 (m, 2H), 2.09 (m, 4H), 1.30 (dd, J = 7.2, 2.5 Hz,
6H). 13C NMR (100 MHz, CDCl3): δ 179.08 (d, J = 9.9 Hz), 178.37
(d, J = 8.2 Hz), 77.66 (d, J = 7.0 Hz), 77.49 (d, J = 2.2 Hz), 34.98 (d, J
= 5.8 Hz), 33.43 (d, J = 1.6 Hz), 32.70, 32.14, 31.10, 30.67, 15.96 (d, J
= 9.3 Hz), 15.04 (d, J = 1.6 Hz). ESI-MS: [M + H]+ = C10H15O4,
calcd: 199.096 49, found: 199.096 11; [M + Na]+ = C10H14O4Na,
calcd: 221.078 43, found: 221.078 03.
ADMET Polymerization. Take the synthesis of P1 as an example.
Monomer 1 (282 mg, 1.0 mmol), BQ (1.1 mg, 10 μmol), and HG-II
catalyst (3.1 mg, 5 μmol) were transferred sequentially into a 25 mL
Schlenk flask that was equipped with a Teflon valve. Subsequently,
CH2Cl2 (1.0 mL, degassed by sonication for 2 min) was added to
dissolve the mixture to obtain a homogeneous blue solution. The flask
was connected to a reflux condenser which was equipped with an
anhydrous CaCl2 loaded drying tube. Then the flask was placed in an
oil bath at 60 °C. Nitrogen gas was continuously purged in through
the Teflon valve during ADMET polymerization. As the reaction
proceeded, the solvent CH2Cl2 was gradually evaporated, and the
reaction mixture became highly viscous in a couple of hours as the
magnetic bar was sluggish to stir. Polymerization was stopped after 120
h via adding CHCl3 (5 mL) followed by a large excess of ethyl vinyl
ether. The concentrated dark blue polymer solution was poured into
cold petroleum ether (about 100 mL) to generate a pale blue
suspension. A dark blue precipitate was obtained via storage at 4 °C
overnight. After filtration and vacuum dryness, P1 was obtained in
95% yield as a brownish semisolid.
Polymer P2 was obtained by hydrogenation of polymer P1; the
detailed procedure is described below. To a 100 mL round-bottom
flask was sequentially added polymer P1 (200 mg), TSH (614 mg, 3.3
mmol), TPA (572 mg, 4.0 mmol), and toluene (30 mL) to obtain a
suspension. The flask was connected to a reflux condenser that
equipped with an anhydrous CaCl2 loaded drying tube. Under
vigorous stirring, the hydrogenation proceeded at 130 °C for 12 h, and
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Scheme 2. Polymers, Metathesis Catalysts, and Olefin Isomerization Inhibitors in This Work
a
RESULTS AND DISCUSSION
Scheme 3. Monomer Synthesis
■
To investigate the effects of microstructure on the alkene
hydroxylation and intrachain cyclization of the ADMET
polymers, we designed and synthesized five types of polymer
samples (P1−P5) as shown in Scheme 2. Polymer P1 was the
primary polymer used to reveal the concept of postmodification
and sequential intramolecular cyclization. Polymers P2−P5 are
control polymer samples to demonstrate the importance of
internal alkene, pendant ester group, and the methylene spacer
length between those two moieties for spontaneous γ-
lactonization after dihydroxylation, respectively.
a
Reagents and conditions: (a) 2.0 equiv of allyl bromide, 4.0 equiv of
K2CO3, acetone, 60 °C, 20 h; (b) 1.0 equiv of EtONa, EtOH, 80 °C, 8
h; 1.1 equiv of allyl bromide, 80 °C, 8 h.
Monomer Synthesis. Monomer 1 is a structurally
symmetric diene monomer containing two tail-to-tail con-
nected ethyl acrylate units. The two terminal alkenes are
separated by a carbon chain which is long enough to guarantee
the ADMET polymerization instead of ring-closing metathesis
(RCM) reaction of this monomer.50 After ADMET polymer-
ization of this monomer, followed by dihydroxylation, a vinyl
alcohol−ethyl acrylate microstructure would be generated in
the polymer main chain. This sequence would potentially
undergo γ-lactonization. We first tried to synthesize this
monomer by direct α-allylation of diethyl adipate using
butyllithium;49 however, multiple allylation caused purification
difficulty. We then tried radical allylation of diethyl meso-2,5-
dibromoadipate with AIBN as an initiator in benzene at 80 °C
for 8 h. Unfortunately, a complex product mixture was
generated as reflected by TLC. Distillation under reduced
pressure was unsuccessful to get the desired monomer. Finally,
we selected a two-step approach as shown in Scheme 3. In the
first step, allylation of ethyl 2-oxocyclopentanecarboxylate with
allyl bromide in the presence of K2CO3 in acetone at 60 °C for
20 h afforded ethyl 1-allyl-2-oxocyclopentane carboxylate in
almost quantitative yield. This compound was then sequentially
treated with sodium ethoxide and allyl bromide to obtain the
targeted monomer 1 in 76% yield. Chemical structure and
Metathesis of Model Compound Ethyl 2-Methyl-4-
pentenoate. To obtain high molecular weight polymers with
minimal structural defects, we carried out the metathesis of
ethyl 2-methyl-4-pentenoate to optimize the appropriate
catalytic system for the following ADMET polymerization
(Scheme 4). In general, for monomers with long methylene
spacers, the G-I catalyst is preferentially chosen for ADMET
polymerization since olefin isomerization can be effectively
suppressed. However, this catalyst exhibits limited catalytic
activity for bulky substituted α-olefins.51 Highly active catalysts
have proved to be powerful for metathesis reaction of
multisubstituted alkenes, but olefin isomerization cannot be
avoided.52−57 Therefore, in this study, we investigated the
dimerization of ethyl 2-methyl-4-pentenoate using five
commercially available metathesis catalysts (see Scheme 2).
The products of the dimerizations were characterized by NMR.
The results revealed that 100% conversion of ethyl 2-methyl-4-
pentenoate was achieved using the HG-II catalyst, and the
dimerized product 2 was obtained in high purity and
quantitative yield by adding benzoquinone. The other four
catalysts showed moderate catalytic activity and high tendency
to cause olefin isomerization even if BQ or DDQ was added
(Figures S3−S6). Hence, the HG-II catalyst was selected for
the following ADMET polymerization of monomer 1.
1
purity of 1 were confirmed by ESI-MS, H NMR, and 13C
Dihydroxylation of compound 2 was performed according to
NMR spectra (Figure 1).
the previously reported procedure.58 Environmentally benign
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