A bicyclo[4.2.0]octene-derived monomer provides completely linear alternating copolymers via alternating ring-opening metathesis polymerization (AROMP)

Article abstract of DOI:10.1021/ma5012039

Strained bicyclic carbomethoxy olefins were utilized as substrates in alternating ring-opening metathesis polymerization and found to provide low-dispersity polymers with novel backbones. The polymerization of methyl bicyclo[4.2.0]oct-7-ene-7-carboxylate with cyclohexene in the presence of the fast-initiating Grubbs catalyst (H₂IMes)(3-Br-Pyr)₂Cl₂Ru = CHPh leads to a completely linear as well as alternating copolymer, as demonstrated by NMR spectroscopy, isotopic labeling, and gel permeation chromatography. In contrast, intramolecular chain-transfer reactions were observed with [5.2.0] and [3.2.0] bicyclic carbomethoxy olefins, although to a lesser extent than with the previously reported monocyclic cyclobutenecarboxylic ester monomers [Song, A.; Parker, K. A.; Sampson, N. S. J. Am. Chem. Soc. 2009, 131, 3444]. Inclusion of cyclohexyl rings fused to the copolymer backbone minimizes intramolecular chain-transfer reactions and provides a framework for creating alternating functionality in a one-step polymerization.

Full text of DOI:10.1021/ma5012039

Article
pubs.acs.org/Macromolecules
A Bicyclo[4.2.0]octene-Derived Monomer Provides Completely Linear
Alternating Copolymers via Alternating Ring-Opening Metathesis
Polymerization (AROMP)
Li Tan, Kathlyn A. Parker,* and Nicole S. Sampson*
Department of Chemistry, Stony Brook University, Stony Brook, New York 11794-3400, United States
*
S Supporting Information
ABSTRACT: Strained bicyclic carbomethoxy olefins were
utilized as substrates in alternating ring-opening metathesis
polymerization and found to provide low-dispersity polymers
with novel backbones. The polymerization of methyl
bicyclo[4.2.0]oct-7-ene-7-carboxylate with cyclohexene in the
presence of the fast-initiating Grubbs catalyst (H IMes)(3-Br-
2
Pyr) Cl RuCHPh leads to a completely linear as well as
2
2
alternating copolymer, as demonstrated by NMR spectroscopy,
isotopic labeling, and gel permeation chromatography. In contrast, intramolecular chain-transfer reactions were observed with
[
5.2.0] and [3.2.0] bicyclic carbomethoxy olefins, although to a lesser extent than with the previously reported monocyclic
cyclobutenecarboxylic ester monomers [Song, A.; Parker, K. A.; Sampson, N. S. J. Am. Chem. Soc. 2009, 131, 3444]. Inclusion of
cyclohexyl rings fused to the copolymer backbone minimizes intramolecular chain-transfer reactions and provides a framework
for creating alternating functionality in a one-step polymerization.
2
3−25
n recent decades, copolymers have been widely studied for
Buchmeiser
have focused on the design of asymmetric
1
−3
I
their uses in the biomedical and material sciences.
Block
catalysts that provide alternating selectivity for different pairs of
monomers. Typically variation in steric bulk about the catalyst
results in alternating reactivity of very strained and moderately
strained, but more sterically demanding, monomers. Despite
impressive catalyst designs, more than a 10-fold excess of the
moderately strained monomer is required to maintain
copolymers, already established as thermoplastic elastomers,
detergents, cosmetics, and pharmaceutical preparations, prom-
ise to contribute to new applications based on nanoscale
4
structures, membranes, and drug and gene delivery. Much less
explored are alternating copolymers. For applications in which
5
−9
22
two different polymer-borne moieties must interact
(e.g., as
alternation.
in organic light-emitting diodes and solar cells), alternating
copolymers should impose consistently optimal positioning of
the participating substituents.
Monomer-design approaches take advantage of the different
properties of two monomers such as polarity, electron density
26
27
28
and steric hindrance, and acid−base interactions and, in the
latter two cases, provide perfectly alternating copolymers
without use of excess monomer. Herein, we describe new
pairs of monomers that generate, sequentially, two different
ruthenium carbenes. One of these pairs efficiently provides
completely alternating, linear copolymers.
Alternating copolymers are generally synthesized by radical
polymerization in which the alternating order of addition of
monomers at the end of the growing chain is kinetically
1
0−12
controlled.
However, the conditions required for radical
propagation are not compatible with a number of functional
groups that might be desired in the polymer target.
We discovered that cyclobutene-1-carboxylate esters undergo
ring-opening metathesis (ROM), but they do not undergo
13−15
Furthermore, with some exceptions,
such polymerizations
27
ROMP. Nonetheless, in solution with cyclohexene (which
also does not ROMP on its own), the cyclobutenecarboxylic
esters participate in an alternating ring-opening metathesis
polymerization (AROMP or altROMP). The high fidelity of
the alternation in the chain extending steps can be attributed to
the complementary reactivities of the two intermediate
ruthenium carbenes. There is a high kinetic barrier to
cyclobutene ester homopolymerization, and the low ring strain
of the cyclohexene monomer does not overcome the entropic
are generally not “living”. Therefore, they do not afford
polymer products with narrow molecular weight distributions
(
low-molar-mass dispersities or D s) that are advantageous for
M
certain applications.
Living polymerizations based on the functional group-
tolerant ruthenium metathesis catalysts have the potential to
29
provide alternating copolymers that have low D s and that bear
M
a variety of functional groups. In metathesis, alternation of the
incorporation of two monomers requires alternation of the
affinities of the monomer A and monomer B to the living metal
alkylidene. There are few solutions to this problem and
consequently few examples of completely alternating metathesis
Received: June 10, 2014
Revised: September 1, 2014
Published: September 23, 2014
1
6−18
19−22
copolymers. Rooney,
Chen,
and Blechert and
©
2014 American Chemical Society
6572
dx.doi.org/10.1021/ma5012039 | Macromolecules 2014, 47, 6572−6579

Macromolecules
Article
2
7
penalty for its homopolymerization. Moreover, the sub-
stitution of the cyclobutene alkene provides regiochemical and
stereochemical control of the polymerization.
deionized water (50 mL) and Et
2
O (25 mL). The Et
2
O layer was
washed with H O (5 × 25 mL) and dried over anhydrous Na SO .
2
2
4
After filtration, the extract was concentrated and the residue was
purified by flash column chromatography to yield a mixture of
bicyclo[2.2.1]heptanes as a colorless oil (16.75 g, 87%). H NMR (500
The molecular weight homogeneity of the copolymers
resulting from our cyclohexene/1-cyclobutene ester pair was
limited by “backbiting” reactions, intramolecular chain-transfers
that lead to the formation of cyclic polymers, shortened chains,
and compromised molar-mass dispersities (D s). Indeed, we
discovered that the use of the Hoveyda−Grubbs II catalysts,
which favor cyclizations, resulted in the exclusive formation
1
MHz, CDCl ): δ 4.09 (d, J = 10 Hz, 1H), 2.78 (td, J = 15 Hz, 10 Hz,
3
1
H), 2.71 (s, 1H), 2.68 (d, J = 10 Hz, 1H), 2.12 (d, J = 10 Hz, 1H),
1
.76 (m, 1H), 1.64 (m, 1H), 1.40−1.20 (m, 3H).
Bicyclo[2.2.1]hept-2-ene-2-carboxylic Acid. In a screw-top vial
M
under an N atmosphere, KOH (5.89 g, 10.5 mmol) was dissolved in
2
deionized H O (4 mL). A sample of 2-bromo-3-
2
(
within the limits of detection) of cyclic alternating
(bromodifluoromethyl)bicyclo[2.2.1]heptanes (0.776 g, 2.66 mmol)
was added to the solution, and the mixture was heated at 130 °C in a
microwave reactor at 20 bar for 2 h. Then the solution was cooled and
3
0
copolymers. Recently, we observed complete inhibition of
backbiting upon introduction of bulky side chains into the
AROMP monomers. However, the increased steric hindrance
near the double bond slows the polymerization propagation
rate, resulting in shorter polymers and thus limits the utility of
washed with CHCl (2 × 4 mL), and the pH of the basic extract was
3
adjusted to 2 with 3 N aqueous HCl. The aqueous solution was
extracted with CHCl (3 × 4 mL), and the combined CHCl solution
3
3
was dried over anhydrous Na SO and concentrated in vacuo to give a
9
2
4
this approach.
brown oil. Purification by flash column chromatography (95:5/
Cognizant of the desirability of high-molecular-weight linear
polymers with narrow molecular weight distributions, we set
out to find a pair of monomers that would give longer and
linear AROMP polymers with low-molar-mass dispersities.
Here we report the results of this search to date.
CH Cl :MeOH) yielded bicyclo[2.2.1]hept-2-ene-2-carboxylic acid as
a colorless oil (160 mg, 50%).
2
2
Methyl Bicyclo[2.2.1]hept-2-ene-2-carboxylate, 2. The pro-
3
3
cedure of Mathias was followed. Bicyclo[2.2.1]hept-2-ene-2-carbox-
ylic acid (40 mg, 0.29 mmol) was treated with N,N-diisopropyl-O-
methylisourea (180 mg, 1.16 mmol) in dry ether (5 mL) and stirred
for 48 h. The urea byproduct precipitated at −20 °C, and it was
removed by filtration. Removal of the solvent afforded a yellow oil
which was purified by flash column chromatography (90:10/
EXPERIMENTAL METHODS
All metathesis reactions were performed under an N atmosphere.
■
2
hexane:CH
MHz, CD Cl
2
Cl
) to yield ester 2 (26 mg, 58%). 1H NMR (400
2
Solvents, e.g. CH Cl and benzene, were purified and dried in a
2
2
2
GlassContour solvent push-still system. Deuterated solvents for all
ring-opening reactions were degassed and filtered through basic
alumina before use. [(H IMes)(PCy )(Cl) RuCHPh] and ethyl 1-
2
): δ 6.91 (d, J = 2.8 Hz, 1H), 3.72 (s, 3H), 3.26 (s, 1H),
3.02 (s, 1H), 1.75 (m, 2H), 1.48 (d, J = 8.4 Hz, 1H), 1.20 (d, J = 8.4
Hz, 1H). 13C NMR (100 MHz, CDCl
48.0, 43.4, 41.8, 24.5, 24.4.
): δ 164.9, 146.6, 140.5, 51.0,
2
3
2
3
bromocyclobutane carboxylate were purchased from Aldrich. Cyclo-
hexene-d10 was purchased from CDN Isotope Inc. The synthesis of
Grubbs III catalyst, [(H IMes)(3-Br-Pyr) Cl RuCHPh], was
General Procedure for the Synthesis of Bicyclo[n.2.0]
Monomers. These monomers were prepared according to Snider’s
2
2
2
3
4
31
approach; the purification of monomer 4 was modified as noted. To
a 50 mL flask with anhydrous AlCl powder under an N atmosphere
performed according to the procedure of Love et al. A fresh stock
solution of the Grubbs III catalyst (0.02 M for AROMP or 0.03 M for
ROM, AROM-1, and AROM-2) and fresh stock solutions of
monomers 3 and 4 (0.17−1.0 M for AROMP, depending on the
desired length, or 0.03 M for ROM, AROM-1, and AROM-2) were
prepared in CD Cl for each of the NMR experiments.
3
2
was added dry benzene and methyl propiolate. The mixture was stirred
until a homogeneous yellow solution formed. Cycloalkene was added,
and the resulting mixture was stirred for 7 days. The reaction mixture
was cooled in an ice bath and quenched with saturated NH Cl
4
2
2
solution. The precipitate was removed by filtering the resulting
mixture through a pad of Celite. The filtrate was extracted with three
Mallinckrodt silica gel 60 (230−400 mesh) was used for column
chromatography. Analytical thin layer chromatography (TLC) was
performed on precoated silica gel plates (60F254), chromatography on
silica gel-60 (230−400 mesh), and Combi-Flash chromatography on
RediSep normal phase silica columns (silica gel-60, 230−400 mesh).
Varian Inova400, Inova500, Inova600 and Bruker Nanobay 400,
Avance III 500, Avance III 700, Avance III-HD 850 MHz NMR
instruments were used for analysis. Chemical shifts are denoted in
ppm (δ) and calibrated from residual undeuterated solvents. The
degree of polymerization (DP) of linear polymers was assessed by
portions of Et
brine and dried over anhydrous MgSO
O, and the combined Et
O extract was washed with
2
2
. The solvent was removed in
4
vacuo, and the crude product was subjected to flash column
chromatography.
Methyl bicyclo[3.2.0]hept-6-ene-6-carboxylate, 3. AlCl (160
3
mg, 1.21 mmol, 0.5 equiv), methyl propiolate (204 mg, 2.42 mmol),
and cyclopentene (170 μL, 2.90 mmol, 1.2 equiv) were stirred for 7
days in dry benzene (50 mL). Flash column chromatography of the
1
comparing the H NMR integration of the polymer alkene protons to
crude product (30:70/hexane:CH
2
Cl
yield). H NMR (400 MHz, acetone-d
3H), 3.33 (br dd, J = 3.2 Hz, 7.6 Hz, 1H), 3.06 (br dd, J = 3.6 Hz, 8.0
2
) yielded ester 3 (175 mg, 48%
1
that of the phenyl end group. Molecular weights and molar mass
dispersities were measured with a gel phase chromatography system
constructed from a Shimadzu pump coupled to a Shimadzu UV
detector. CH Cl served as the eluent with a flow rate of 0.700 mL/
6
): δ 6.62 (br s, 1H), 3.69 (s,
13
Hz, 1H), 1.78−1.21 (m, 6H). C NMR (100 MHz, CDCl
): δ 162.5,
Methyl Bicyclo[4.2.0]oct-7-ene-7-carboxylate, 4. AlCl (227
3
3
147.5, 138.2, 51.1, 46.9, 44.5, 25.5, 25.4, 22.9.
2
2
min on an American Polymer Standards column (Phenogel 5 μ MXL
GPC column, Phenomenex). All GPCs were calibrated with
poly(styrene) standards at 30 °C.
mg, 1.72 mmol, 0.86 equiv), methyl propiolate (168 mg, 2.00 mmol),
and cyclohexene (263 μL, 2.60 mmol, 1.30 equiv) were stirred for 7
days in dry benzene (5 mL) and yielded a mixture of compound 4 and
isomer methyl (E)-3-(cyclohex-1-enyl)propenoate in a ratio of
1:0.11−0.23. The mixture was treated with m-chloroperoxybenzoic
acid (m-CPBA) to epoxidize the isomeric byproduct, followed by flash
3
2,33
Bicyclo[2.2.1]hept-2-ene-2-carboxylic Acid.
A modifica-
32
tion of the two-step procedure of Elsheimer was followed. Safety
warning: CF Br has a very low boiling point, and addition of CF Br to
2
2
2
2
norbornene is very exothermic. Therefore, for a large-scale reaction, this
procedure should be carried out carefully behind a safety shield.
column chromatography (30:70/hexane:CH
Cl
ester 4 (210 mg, 65% yield). H NMR (400 MHz, acetone-d
6
) to provide bicyclic
2
2
1
2
-Bromo-3-(bromodifluoromethyl)bicyclo[2.2.1]heptanes.
): δ 6.89
To a mixture of norbornene (6.10 g, 65 mmol), CuCl (0.99 g, 1
mmol), ethanolamine (3.00 g, 50 mmol), and tert-butyl alcohol (7.40
g, 100 mmol) in a 50 mL flask, CF Br was slowly added (27.30 g, 130
(d, J = 1.2 Hz, 1H), 3.71 (s, 3H), 3.07−3.04 (m, 1H), 2.80−2.77 (m,
13
1H), 2.00−1.30 (m, 8H). C NMR (100 MHz, CDCl ): δ 162.4,
3
150.1, 141.4, 50.7, 39.8, 38.1, 23.5, 23.3, 18.6, 18.0.
2
2
mmol). The resulting mixture was protected from light and stirred at
reflux (80−85 °C) for 48 h. Then it was cooled and diluted with
Methyl Bicyclo[5.2.0]non-8-ene-8-carboxylate, 5. AlCl (1.00
g, 7.56 mmol, 0.5 equiv), methyl propiolate (1.27 g, 15.2 mmol), and
3
6
573
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Macromolecules
Article
cycloheptene (2.16 mL, 18.2 mmol, 1.2 equiv) were stirred for 7 days
in dry benzene (50 mL). Flash column chromatography of the crude
equiv) were mixed in CD Cl . After 50 min, cyclohexene-d 6-d10
2
2
10
(19.7 mg, 24.2 μL, 40 equiv) was added. The NMR tube was spun for
6 h at 35 °C to reach >95% consumption of monomer 4. The product
was purified by flash column chromatography (97:3/CH Cl :acetone)
1
product (30:70/hexane:CH Cl ) yielded 5 (1.40 g, 50% yield). H
2
2
NMR (400 MHz, CDCl ): δ 6.70 (d, J = 1 Hz, 1H), 3.61 (s, 3H), 2.99
3
2
2
13
to yield poly(4-alt-6-d ) (18 mg, 60%). 1H NMR (600 MHz,
(
m, 1H), 2.77 (m, 1H), 1.77−1.21 (m, 10H). C NMR (100 MHz,
CDCl ): δ 162.6, 148.9, 140.5, 50.6, 46.9, 45.4, 31.6, 29.1, 28.6, 27.9,
10 15
CD Cl ): δ 7.35−7.4 (m, 5H), 5.78 (m, 15H), 5.42 (m, 2H), 3.70−
3
2
2
2
7.7.
3.60 (m, 45H), 2.82 (m, 23H), 2.25 (m, 27H), 1.78−1.20 (m, 164H).
General Procedure for NMR Scale AROMP Reactions. All
NMR AROMP of 4 and 6, Poly(4-alt-6) . Monomer 4 (49.7 mg,
34
kinetic experiments were performed at least twice, and preparative
polymerization experiments were performed three times. Under an N2
atmosphere, a solution of monomer A (cyclobutene derivative) in
CD Cl (300 μL) was added to the NMR tube. Then 300 μL of
Grubbs III stock solution (C = 0.02 M) was added to the NMR tube.
After complete mixing of the solution, NMR spectra were acquired at
300 μmol, 50 equiv) and Grubbs III catalyst (5.3 mg, 6.0 μmol, 1
equiv) were mixed in CD Cl , and cyclohexene 6 (49.2 mg, 60.6 μL,
2
2
100 equiv) was added after 30 min. The NMR tube was spun for 6 h at
1
2
2
35 °C to reach 68% conversion of monomer 4. Partial H NMR of
crude poly(4-alt-6)34 (600 MHz, CD Cl ): δ 7.35−7.14 (m, 5H), 6.89
2
2
(d, J = 1.2 Hz, 8H), 6.50 (m, 34H), 5.79 (m, 32H), 5.28 (m, 41H),
1
2
5 °C until the catalyst had reacted with monomer A as determined by
the disappearance of its α alkylidene proton signal. Monomer B
cyclohexene 6) was added to the NMR tube. After no further
3.70−3.60 (m, 150H), 3.07−3.04 (m, 10H). (Partial H NMR
spectroscopic data are reported due to incomplete polymerization and
significant upfield overlap of 4 and 6 with the new peaks from the
polymer.)
(
propagation occurred, the reaction was quenched with ethyl vinyl
ether and stirred for 1 h. Solvent was evaporated, and polymer was
purified by chromatography over silica gel (97:3/CH Cl :acetone).
NMR AROMP of 4 and 6, Poly(4-alt-6) . Monomer 4 (49.7 mg,
36
2
2
300 μmol, 50 equiv) and Grubbs III catalyst (5.3 mg, 6.0 μmol, 1
equiv) were mixed in CD Cl . After 30 min, cyclohexene 6 (49.2 mg,
Yield was determined by assuming 100% conversion of monomer A.
NMR AROMP of 2 and 6. Monomer 2 (8.3 mg, 60 μmol, 10
equiv) and Grubbs III catalyst (5.3 mg, 6.0 μmol, 1 equiv) were mixed
2
2
60.6 μL, 100 equiv) was added. The NMR tube was spun for 2 h at 60
°C to reach 72% conversion of monomer 4. The product was purified
by flash column chromatography (97:3/CH Cl :acetone) to yield
1
in CD Cl . The reaction was followed by H NMR spectroscopy at 25
2
2
2
2
1
°
C for 5 h before the temperature was elevated to 50 °C. Cyclohexene
poly(4-alt-6)36 (36 mg, 65%). H NMR (600 MHz, CD Cl ): δ 7.35−
2
2
6
(12 μL, 120 μmol, 20 equiv) was added. No change in the alkylidene
7.14 (m, 5H), 6.50 (m, 36H), 5.79 (m, 34H), 5.28 (m, 55H), 3.70−
3.60 (m, 129H), 2.75 (m, 53H), 2.30−2.10 (m, 142H), 1.95−1.20 (m,
589H).
peak of the catalyst was observed within 300 min.
NMR AROMP of 3 and 6, Poly(3-alt-6) . Monomer 3 (23.8 mg,
50 μmol, 25 equiv) and Grubbs III catalyst (5.3 mg, 6.0 μmol, 1
13
1
NMR AROMP of 5 and 6, Poly(5-alt-6)10. Monomer 5 (54.8 mg,
300 μmol, 50 equiv) and Grubbs III catalyst (5.30 mg, 6.00 μmol, 1
equiv) were mixed in CD Cl , and cyclohexene 6 (49.0 mg, 60 μL, 100
equiv) were mixed. Cyclohexene 6 (24.5 mg, 30 μL, 50 equiv) was
added 30 min later. The NMR tube was spun for 19 h at 25 °C. Flash
column chromatography (97:3/CH Cl :acetone) of the crude product
2
2
2
2
equiv) was added after 50 min. The NMR tube was spun for 72 h at 35
1
1
yielded poly(3-alt-6)13 (15 mg, 43%). H NMR (600 MHz, CD Cl ):
°C until no further propagation was observed by H NMR
2
2
δ 7.35−7.14 (m, 5H), 6.60 (m, 13H), 5.32 (m, 27H), 3.68−3.60 (m,
spectroscopy. The crude product was subjected to flash column
chromatography (97:3/CH Cl :acetone) to yield poly(5-alt-6) (11
5
4H), 3.20−3.10 (m, 14H), 2.0−2.3 (m, 10H), 2.60−1.00 (m, 267H).
2
2
10
1
NMR AROMP of 3 and 6-d , Poly(3-alt-6-d ) . Monomer 3
mg, 14%). H NMR (600 MHz, CD Cl ): δ 7.35−7.12 (m, 5H), 6.58
10
10 6
2
2
(
1
9.5 mg, 50 μmol, 10 equiv) and Grubbs III catalyst (5.3 mg, 6.0 μmol,
equiv) were mixed. Cyclohexene 6-d10 (9.8 mg, 12 μL, 20 equiv) was
(m, 10H), 5.42 (m, 19H), 5.28 (m, 10H), 3.70−3.60 (m, 36H), 3.10−
3.00 (m, 10H), 2.77−1.95 (m, 254H).
added 30 min later. The NMR tube was spun for 8 h at 25 °C. The
General Procedure for Ring-Opening Metathesis. Under an
crude product was purified by flash column chromatography (97:3/
N atmosphere, a solution of monomer A (cyclobutene derivative, 1, 3,
2
1
CH Cl :acetone) to yield poly(3-alt-6-d ) (4.8 mg, 40%). H NMR
4, or 5, [A] = 0.03 M) in CD Cl (300 μL) was added to an NMR
2
2
10
6
2
2
(
1
600 MHz, CD Cl ): δ 7.35−7.14 (m, 5H), 7.02 (dd, J = 8.1, 5.7 Hz,
tube. Then 300 μL of the stock solution of Grubbs III catalyst (C =
2
2
H), 5.32 (m, 6H), 3.68−3.60 (m, 22H), 3.20−1.00 (m, 80H).
0.03 M) was added to the NMR tube. After complete mixing of the
1
13
Alternating ring-opening polymerization of monomer 4 and 6 was
solution, the reaction was closely monitored by H NMR or C NMR
spectroscopy.
carried out at different temperatures ranging from 25 to 60 °C to
optimize reaction conditions.
Procedure for Alternating Ring-Opening Metathesis
(AROM-1, BA Dimer Synthesis). A solution of monomer A (3 or
4, [A] = 0.03 M) in CD Cl (300 μL, 18.86 μmol) was added to an
NMR AROMP of 4 and 6, Poly(4-alt-6) . Monomer 4 (19.9 mg,
20 μmol, 20 equiv) and Grubbs III catalyst (5.3 mg, 6.0 μmol, 1
16
1
2
2
equiv) were mixed in CD Cl . Cyclohexene 6 (19.7 mg, 24 μL, 40
NMR tube that had been flushed with N . Then 300 μL of the stock
2
2
2
equiv) was added after 50 min. The NMR tube was spun for 8 h at 25
C to reach 90% consumption of monomer 4. The crude product was
subjected to flash column chromatography (97:3/CH Cl :acetone) to
solution of Grubbs III catalyst (C = 0.03 M) was added to the NMR
°
tube. After complete mixing of the solution, the reaction was followed
1
13
2
2
by H NMR or C NMR spectroscopy until >90% of the catalyst
(10−12 h) was consumed as determined by disappearance of the Ru
alkylidene proton or carbon resonance of the Grubbs III catalyst at
19.1 or 316.1 ppm. Then cyclohexene 6 was added in 10-fold excess,
and the reaction was monitored until the Ru alkylidene proton
resonance at 19.0 ppm disappeared. The reaction was terminated with
ethyl vinyl ether, and the crude mixture was subjected to silica
chromatography (100% CH Cl ). For the products of the 3−6
1
yield poly(4-alt-6)16 (16 mg, 53%). H NMR (600 MHz, CD Cl ): δ
2
2
7
3
2
1
5
.35−7.14 (m, 5H), 6.52 (m, 16H), 5.79 (m, 15H), 5.31 (m, 26H),
.69−3.59 (m, 45H), 2.60 (m, 23H), 2.25 (m, 60H), 2.00−1.22 (m,
66H). 13C NMR (100 MHz, CDCl ): δ 169.4, 142.4, 141.9, 136.3,
3
31.2, 130.7, 130.6, 130.4, 130.2, 130.2, 130.0, 129.1, 128.4, 51.5, 51.5,
1.4, 43.9, 42.8, 37.3, 28.7, 28.1, 21.8, 21.7.
NMR AROMP of 4 and 6, Poly(4-alt-6) . Monomer 4 (19.9 mg,
16
2
2
120 μmol, 20 equiv) and Grubbs III catalyst (5.3 mg, 6.0 μmol, 1
AROM-1 experiment, partially purified fractions were characterized by
1
13
equiv) were mixed in CD Cl , and cyclohexene 6 (19.7 mg, 24.2 μL,
mass spectrometry, H NMR, C NMR, and HSQC spectroscopy
2
2
4
3
0 equiv) was added after 50 min. The NMR tube was spun for 6 h at
5 °C to reach >95% consumption of monomer 4. The crude product
(Supporting Information). Fraction I was a white solid identified as E-
1
stilbene. H NMR (850 MHz, CD Cl ): δ 7.53 (dd, J = 8.1, 0.9 Hz,
2
2
1
3
was subjected to flash column chromatography (97:3/
CH Cl :acetone) to yield poly(4-alt-6) (20 mg, 65%). H NMR
2H), 7.36 (t, J = 7.7 Hz, 2H), 7.28−7.23 (m, 2H), 7.13 (s, 1H).
NMR (214 MHz, CD Cl ): δ 137.9, 129.2, 129.1, 128.2, 127.0. ESI
(M/Z) [M + H] 180.1. Fraction II contained Ph-(3-alt-6) -Ph as the
C
1
2
2
16
2
2
+
(
1
2
600 MHz, CDCl ): δ 7.35−7.14 (m, 6H), 6.52 (m, 16H), 5.79 (m,
3
1
5H), 5.50 (m, 1H), 5.31 (m, 17H), 3.69−3.59 (m, 47H), 2.78 (m,
major component (Supporting Information). Fraction III contained
3H), 2.10 (m, 64H), 1.98−1.20 (m, 263H).
cyc-(3-alt-6) as the major component (Supporting Information).
1
NMR AROMP of 4 and 6, Poly(4-alt-6-d ) . Monomer 4 (19.9
Procedure for Sequential Alternating Ring-Opening Meta-
thesis (AROM-2, BA′BA Tetramer Synthesis). A solution of
10 15
mg, 120 μmol, 20 equiv) and Grubbs III catalyst (5.3 mg, 6.0 μmol, 1
6
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monomer A (3 or 4, [A] = 0.03 M) in CD Cl (300 μL, 18.86 μmol)
2
2
was added to an NMR tube that had been flushed with N . Then 300
2
μL of the stock solution of Grubbs III catalyst (C = 0.03 M) was added
to the NMR tube. After complete mixing of the solution, the reaction
1
was followed by H NMR spectroscopy until >90% of the catalyst
(
10−12 h) was consumed as determined by disappearance of the Ru
alkylidene proton of the Grubbs III catalyst at 19.1 ppm; then
cyclohexene 6 was added in 10-fold excess. Generation of [Ru]-B-A
was monitored by the appearance of a multiplet resonance at 19.0
ppm. When the formation of [Ru]-B-A was complete as judged by the
integrated intensity of the resonance, 1 equiv of monomer A′ was
added to form [Ru]-A′-B-A and then [Ru]-B-A′-B-A. The reaction
was monitored until the Ru alkylidene proton resonance at 19.0 ppm
disappeared or the intensity was constant. Then the reaction was
terminated with ethyl vinyl ether.
RESULTS AND DISCUSSION
■
Design and Synthesis of Monomers. We noted that
using Grubbs III catalyst for the incorporation of a ring into the
propagating chain backbone limits backbiting in the case of the
well-investigated norbornene ROMP. Therefore, we decided
to examine norbornene ester 2 as a partner for cyclohexene in
AROMP. We prepared monomer 2 by a modification of the
35
Figure 2. Kinetic monitoring of ring-opening metathesis of monomers
1
and 3−5. Monomer and Grubbs III catalyst were mixed in a 1:1
ratio, [A] = [Grubbs III] = 0.03 M. Percent conversion was
determined by 1H NMR spectroscopy and integration of the Ru
32
method of Elsheimer (see Experimental Methods). However,
when we subjected monomer 2 to AROMP conditions with
cyclohexene 6, we observed no polymerization. Furthermore,
when the catalyst was mixed with monomer 2, no ring-opened
product was observed at all. We attributed the loss of ROMP
activity of monomer 2 to the steric hindrance posed by the
combination of the bridging methylene group and the ester.
We next tested bicyclic monomers 3−5, designed for their
ring strain and lower steric hindrance around the alkenes
alkylidene α proton resonance relative to the methyl ester resonances
(
Supporting Information). t1/2 were obtained from the plot, monomer
1: t1/2 = 40 min, monomer 3: t1/2 = 25 min, monomer 4: t1/2 = 100
min; monomer 5: t1/2 = 300 min. Each experiment was performed at
least twice, and data from representative experiments are shown.
opening. Thus, upon addition of Grubbs III catalyst, monomer
3
has the fastest ring-opening rate, in accordance with the
36−38
predicted ring strains.
(
Figure 1). These monomers were prepared by Lewis acid
Alternating Copolymers. When subjected to Grubbs III
catalyst in the absence of cyclohexene 6, each of the three
bicyclic monomers 3, 4, and 5 underwent ring-opening;
however, no polymerization could be detected. Thus, we
established that these monomers are suitable for the
preparation of alternating copolymers because their rates of
homopolymerization are zero. All three monomers produced
copolymers with cyclohexene 6 in the presence of Grubbs III
catalyst. The rates of polymerization varied substantially, and all
were slower than the rate of AROMP between cyclobutene
1
ester 1 and cyclohexene 6 as determined by H NMR
spectroscopy (Table 1). Lengths of polymers were determined
Figure 1. Monomers employed in alternating ring-opening metathesis
polymerization.
by integration of alkene proton peaks (H1, H3, and H4, Figure
1
3
) relative to that of the phenyl end group in the H NMR
catalyzed [2 + 2] cycloaddition according to Snider’s
spectrum. Copolymerization of monomer 3 or 5 with 6 yielded
much shorter polymers than did that of monomer 4 with 6
under the same conditions (Table 1).
3
4
approach. For practicality, we developed a simplified
purification procedure for monomer 4 to remove the isomeric
byproduct (see Experimental Methods).
1
Furthermore, the H NMR spectrum of each of the polymers
Relative Kinetics of Ring-Opening Metathesis (ROM).
First, we undertook kinetic monitoring of the initial ring-
opening metathesis (ROM) reactions for each of these
is consistent with an alternating backbone structure (Support-
ing Information) in which the olefin bearing a carbomethoxy
substituent has an E configuration. For example, in the
spectrum of poly(4-alt-6) , the proton resonance for the
1
monomers by H NMR spectroscopy (Figure 2). In each of
13
the experiments, an equimolar amount of monomer A (the
bicycloalkene ester) and Grubbs III catalyst was mixed in
CD Cl . The disappearance of the alkylidene signal of the
catalyst at 19.1 ppm was followed by H NMR spectroscopy;
the signal was integrated relative to the methyl ester signals
between 3.8 and 3.5 ppm. Under the conditions of the
experiment, 50% of monomer 3 was ring opened in 25 min,
whereas 50% of monomer 1 was ring-opened in 40 min. Under
the same conditions, monomer 4 underwent 50% ring-opening
in 100 min and monomer 5 required 300 min for 50% ring-
carbomethoxy-substituted olefin (H1), which is derived from
cyclohexene 6, has an integration identical to that of the H3
alkene resonance at 5.8 ppm which is derived from monomer 4
(Figure 3a). Likewise, the analogous proton resonances in
poly(3-alt-6) and poly(5-alt-6) have nearly identical integra-
2
2
1
n
n
tion values. Moreover, characterization of poly(4-alt-6) by
n
HSQC spectroscopy confirmed that the carbomethoxy-
substituted olefin is a single stereoisomer; there is a single
H1 signal at 6.5 ppm that correlates with C1 (Supporting
Information). Comparison of model compound chemical shifts
6
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Table 1. AROMP Applications of Sterically Hindered
Monomers with Cyclohexene
shorter polymers than did monomer 4 (Table 1). Monomer 4
provided polymers with up to 35−36 AB repeats. Reaction
conditions for poly(4-alt-6) were examined, and the best yield
n
temp
(°C)
time
(h)
a
b
was obtained in CH Cl , at temperatures between 35 and 60
entry
A
B
[A]:[B]:[Ru]
% conv DP[AB]
2 2
°
C. The GPC elution profile of poly(4-alt-6) displayed a
n
1
2
3
4
5
6
7
8
9
1
2
3
3
4
4
4
4
4
5
6
6
6
10:20:1
10:20:1
25:50:1
20:40:1
20:40:1
20:40:1
20:40:1
50:100:1
50:100:1
50:100:1
25
25
25
25
25
35
35
35
60
35
3
3
98
0
10
monomodal molecular weight distribution (Supporting In-
formation). This distribution is consistent with the absence of
cyclic polymer. Likewise, the dispersity (DM = 2.0 ± 0.1) of
poly(4-alt-6)n was significantly smaller than that of the
previously reported poly(1-alt-6) (D = ∼5), which displayed
c
19
6
NA
80
96
97
85
68
72
85
13
6
6-d10
6
8
16
16
15
34
36
10
n
M
6
8
27
a bimodal molecular weight distribution. The molecular
weight profile is consistent with the absence of intramolecular
chain transfer for the AROMP of monomer 4 and cyclohexene.
Intrinsic Rates of Chain Propagation. Monomer 3 was
selected for comparison with monomer 4 in order to study how
their structures affect the initiation and propagation rates and
the extents of polymerization. To preclude the possibility that
impurities in monomer 3 inherited from synthesis (see
Experimental Methods) deactivated the catalyst, monomer 3
was also treated with m-CPBA and reisolated. This sample was
utilized in AROMP and compared with monomer 3
synthesized otherwise. No difference in activity or polymer
length was observed.
6-d10
8
6
6
6
8
2
10
24
a
1
Percent conversion determined by integration of H NMR spectra of
b
1
monomer A unless specified otherwise. DP
was determined by H
[
AB]
NMR spectroscopy with integration relative to the phenyl end group
and represents the average numbers of AB dyads incorporated in linear
c
1
copolymers. % conv could not be determined by H NMR
spectroscopy due to overlap of alkene and polymer peaks.
Then we undertook kinetic monitoring of AROM-1
(
formation of BA dimer) with monomers 3 or 4 and
13
cyclohexene 6 by C NMR spectroscopy. In both reactions,
we observed formation of [Ru]-A upon addition of the A
monomer to Grubbs III catalyst (Figure 4, [Ru] + 3 and [Ru] +
4
) and disappearance of Ru catalyst. In the case of [Ru]-4, the
formation of a single Ru carbene (314.5 ppm) was observed.
Addition of cyclohexene cleanly yielded [Ru]-6-4 (338.0 ppm,
Figure 4, 6 + [Ru]-4) in 1.5 h, and there was no further change
in the NMR spectrum over 30 h.
In the case of [Ru]-3, two Ru carbene species were produced
(311.5 and 311.8 ppm). After the addition of cyclohexene, we
observed regeneration of the Grubbs III catalyst (316.1 ppm) in
addition to the [Ru]-6-3 carbene (337.8 ppm, Figure 4, 6 +
[Ru]-3) during the first 2 h of reaction. Within 5 h of
cyclohexene addition, both the [Ru]-6-3 and Ru catalyst
carbene resonances disappeared, and no new carbene
resonances appeared.
Figure 3. Alternating ring-opening metathesis polymerization
1
(
AROMP) of monomers 4 and 6. The region of H NMR spectra
in which backbone olefinic hydrogen resonances of poly(4-alt-6)n
appear is shown. (a) Polymer product prepared from cyclohexene and
dissolved in CDCl . The ratio of H1:H3:H4 is 1:1:1. (b) Polymer
3
product prepared from cyclohexene-d10 and dissolved in CD Cl . The
2
2
ratio of H1:H3:H4 is 0:1:0. poly(4-alt-6-d ) was dissolved in CD Cl
1
0
n
2
2
instead of CDCl3 to allow accurate integration of the phenyl
resonances.
We analyzed products of the AROM-1 reaction and obtained
Ph-(3-alt-6) -Ph, E-stilbene, and cyc-(3-alt-6) (Scheme 1a).
1
1
with the H1 alkene chemical shift further confirmed that the E
configuration was obtained.
Further evidence for the alternating structure was obtained
Hence, backbiting or cross-metathesis can occur very early in
the reaction. An independent experiment in which Ru catalyst
was mixed with monomer 3 for 18 h showed that [Ru]-3
remains intact. Thus, the Ru enoic carbene is not reactive with
itself, and side reactions must occur after cyclohexene addition.
Formation of stilbene suggests that if the desired propagation
pathway is kinetically less favorable, the Ru alkylidene ([Ru]-6-
3) species undergoes cross-metathesis in an intra- or
intermolecular reaction with the styrene end group. In contrast,
no regeneration of Grubbs III catalyst was observed in the (4-
3
9,40
for the poly(4-alt-6)n copolymers by experiments with
1
cyclohexene-d , 6-d . H NMR analysis of the deuterium-
10
10
labeled copolymer poly(4-alt-6-d ) indicates a complete loss
1
0 n
of the carbomethoxy-substituted olefin (H1) resonance at 6.5
ppm and the H4 alkene resonance at 5.3 ppm (Figure 3b).
Complete loss of the H1 and H4 resonances upon deuteration
is consistent with a rigorously alternating AB linear scaffold.
Their loss indicates the absence of AA dyads that can form
alt-6) experiment (Figure 4, 6 + [Ru]-4). The [Ru]-6-4
1
27
upon backbiting during the AROMP. Likewise, in the
experiments with deuterated cyclohexene, the integrated ratio
of H3 versus the methyl ester remains at 1:3, suggesting that no
BB dyad is formed. Therefore, introduction of a cyclohexyl ring
fused to the cyclobutene ester monomer provides access to
linear, alternating copolymers.
carbene remained stable for 2 days under the reaction
conditions. This result is consistent with our observations
that monomer 4 provides a longer and backbiting-free linear
alternating copolymer via AROMP.
We also carried out double AROM (AROM-2) experiments
with monomers 3 and 4 to compare their behavior in systems
with longer chains (Scheme 1 and Figure 5). In these
experiments, we mixed monomer A and catalyst in a 1:1 ratio
in an NMR tube to form [Ru]-A, and we monitored the
All three monomers 3−5 produced rigorously alternating
copolymers. However, at the same concentrations and
monomer/catalyst ratios, monomers 3 and 5 both generated
6
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Figure 4. Alternating ring-opening metathesis (AROM-1) of monomers 3 and 6 and monomers 4 and 6. The carbene regions of 13C NMR spectra
of (3-alt-6) (left) and (4-alt-6) (right) are shown. In the top spectra, [Ru] catalyst was mixed with 3 or 4 in CD Cl for 10−12 h. Cyclohexene was
1
1
2
2
added after >90% of Ru catalyst was consumed. The 6 + [Ru]-3 reaction was monitored for 300 min, and 6 + [Ru]-4 reaction was monitored for 30
h. The 13C NMR spectrum of 6 + [Ru]-3 was acquired 30−50 min after addition of cyclohexene, and the C NMR spectrum of 6 + [Ru]-4 was
acquired 50−70 min after addition of cyclohexene. Each experiment was performed at least twice, and data from representative experiments are
shown.
13
Scheme 1. (a) Alternating Ring-Opening Metathesis (AROM-1) of 3 or 4 with Cyclohexene To Form BA Dimer and Proposed
a
Intra- and Intermolecular Cross-Metathesis for (3-alt-6) ; (b) Double Alternating Ring-Opening Metathesis (AROM-2) To
1
b
Form BA′BA Tetramer
aCarbenes (circled) were monitored by 13C NMR spectroscopy (Figure 4). The proton resonances monitored are colored red (Figure 5).
b
1
reactions by H NMR spectroscopy. The mixtures were allowed
1 min) than with the corresponding enoic carbene from
monomer 4 (t1/2 = 43 ± 5 min) (Figure 5 and Supporting
Information). When the formation of Ru alkylidene ([Ru]-B-
A) was complete as judged by integration of the resonance at
19.0 ppm, 1 equiv of monomer A′ was added to investigate the
rate of ring-opening catalyzed by [Ru]-B-A. We examined all
four cases of double ROM: ROM of 3 with [Ru]-6-3 and with
[Ru]-6-4 and ROM of 4 with [Ru]-6-4 and with [Ru]-6-3. The
disappearance of the monomer A′ alkene signal at 6.7 ppm and
that of the [Ru]-B-A alkylidene signal at 19.0 ppm were
to react for 10−12 h to ensure nearly complete conversion to
Ru]-A (the benzylidene proton signal at 19.1 ppm was
[
reduced to less than 10% of its original intensity). At this time,
0 equiv of cyclohexene (monomer B) was added to generate
Ru]-B-A (Figure 5). The formation of [Ru]-B-A was
monitored by the appearance of a new multiplet resonance at
9.0 ppm corresponding to the alkylidene proton (Supporting
Information). We found that cyclohexene 6 reacts with ring-
opened monomer 3 enoic carbene 1.5 times faster (t1/2 = 28 ±
1
[
1
6
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appears to hinder propagation and to favor cross-metathesis;
both effects lead to the premature termination of polymer-
ization.
CONCLUSIONS
■
We have demonstrated that alternating copolymers are
synthetically accessible via AROMP with bicyclic carbomethoxy
olefin monomers 3−5 and cyclohexene. These AROMP-active
monomers do not self-metathesize regardless of monomer feed
ratios or concentrations. Thus, formation of homopolymeric
2
0,21,26,49,50
blocks, as is observed in some other systems,
does
not occur in the 3−5/6 AROMP systems. Ruthenium-catalyzed
ring-opening metathesis of cyclohexene and methyl
bicyclo[4.2.0]oct-7-ene-7-carboxylate, 4, provides rigorously
linear and alternating copolymers free of backbiting and chain
transfer. The resulting polymer backbone constrained by a cis-
1,2-substituted cyclohexane provides an exciting alternative to
traditional norbornyl-derived polymers for the preparation of
functional alternating copolymers.
Figure 5. Kinetic monitoring of double ring-opening metathesis
(
1
(
6
AROM-2) reactions of monomer A (3 or 4) with [Ru]-6-A (Scheme
b). Time zero corresponds to the addition of 1 equiv of monomer A′
3 or 4) to 1 equiv of [Ru]-6-A in the presence of excess cyclohexene
. Mole fraction of Ru alkylidene at 19.0 ppm was determined by
integration of the Ru alkylidene resonance relative to the methyl ester
resonances between 3.5 and 3.8 ppm. The mole percent of [Ru]-6-A at
time zero was only 30−42% due to the low concentrations, and thus,
the extended reaction times used in the initial AROM to generate
ASSOCIATED CONTENT
■
*
S
Supporting Information
[
Ru]-6-A. Each experiment was performed at least twice, and data
Figures showing characterization of monomers and polymer,
GPC, 1D- and 2D-NMR spectra. This material is available free
of charge via the Internet at http://pubs.acs.org.
from representative experiments are shown.
monitored as a function of time. Both signals disappeared at the
same rate as measured by comparison of the integrals with that
of the signal for ester peaks. We found that reaction of
monomer 3 with [Ru]-6-4 (t1/2 = 26 ± 1 min) was 27% faster
than its reaction with [Ru]-6-3 (t1/2 = 33 ± 1 min). In the case
of the reaction of monomer 4 with [Ru]-6-A, the rate with
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail kathlyn.parker@stonybrook.edu (K.A.P.).
*E-mail nicole.sampson@stonybrook.edu (N.S.S.).
Notes
[
Ru]-6-4 (t1/2 = 41 ± 4 min) is 17% faster than the rate with
The authors declare the following competing financial
interest(s): We have no competing interests except for National
Institutes of Health Funding: R01GM097971 & R01GM74776.
[
Ru]-6-3 (t1/2 = 48 ± 2 min). Moreover, oligomers derived
from monomer 3 as either A or A′ failed to completely convert
to [Ru]-6-A′-6-A; this result is consistent with the premise that
competing cross-metathesis reactions dominate the reaction
ACKNOWLEDGMENTS
■
(
Figure 5).
We thank the National Institutes of Health (NIH) for grants
R01GM097971/R01HD38519 (N.S.S.) and R01GM74776
K.A.P). We also thank Dr. Francis Picart and James Marecek
Although the rates of the second ROM did not vary widely,
the ROM reaction appears to be very sensitive to long-range
polymer structure, i.e., the presence of a five-membered ring in
the backbone one position removed from the living [Ru]
species reduces the reaction rate of either bicyclic monomer
with the unhindered [Ru]-alkylidene. The rates of reaction
indicate that the backbone containing a six-membered ring
(
for their assistance with NMR spectroscopy.
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