Synthesis of Copolymers by Alternating ROMP (AROMP)

Full text of DOI:10.1021/ja809661k

Published on Web 02/18/2009
Synthesis of Copolymers by Alternating ROMP (AROMP)
Airong Song, Kathlyn A. Parker,* and Nicole S. Sampson*
Department of Chemistry, Stony Brook UniVersity, Stony Brook, New York 11794-3400
Received December 10, 2008; E-mail: kathlyn.parker@stonybrook.edu; nicole.sampson@stonybrook.edu
Scheme 1. Alternating Ring-Opening Metathesis Polymerization,
AROMP
Copolymers are employed in applications ranging from the
biomedical to the electronic.1 Among the most commonly used are
block copolymers that require phase separation of the two blocks
for their function, e.g., drug delivery,2 and random copolymers in
which two functional moieties communicate, e.g., organic light
emitting diodes.3 Regularly alternating polymers should allow
optimal positioning of functional substituents and be useful in a
variety of applications.
Alternating polymers are generally synthesized by radical po-
lymerization with kinetic control of the order of monomer incor-
poration.4 However, there are isolated examples of their synthesis
by ring opening metathesis polymerization (ROMP). Early on, it
was reported that the ROMP of racemic 1-methylbicyclo[2.2.1]hept-
2-ene with ReCl5 gave a polymer in which the two enantiomers
alternate.5 More recently, several reports of alternating polymers
as the products of ROMP have appeared. With the exception of
Grubbs’s ring opening insertion metathesis (ROIMP) approach, in
which alternation is controlled by equilibration,6 these rely on the
pairing of a bulky but strained monomer with an unhindered and
Table 1. AROMP Polymers Synthesizeda
rxn
time (h)
%
A
B
[Ru] (M)
[A]:[B]:[Ru]
product
convb
2a
2a
2a
2a
2a
2a
2a
2a
2b
2a
4a
4a
4a
4a
4a
4a
4a-D10
4a-D10
4a
0.05
0.01
0.01
0.01
0.01
0.005
0.01
0.01
0.01
0.01
3:6:1
6
3
3
3
3
6
3
3
4
4
(2a-4a)3
(2a-4a)10
(2a-4a)20
(2a-4a)50
(2a-4a)100
(2a-4a)200
(2a-4a-D10)20
(2a-4a-D10)20
(2b-4a)20
(2a-4b)20
74c
98
98
98
97
73
97
97
96
95
10:20:1
20:40:1
50:100:1
100:200:1
200:400:1
20:24:1
20:160:1
20:40:1
20:40:1
only slightly strained monomer. In all cases except one,7
a
significant excess of one of the monomers is required for high levels
of alternation.8
We now describe the highly alternating polymerization of
cyclobutene 1-carboxylic esters with cyclohexene derivatives with
precatalyst 1 [(H2IMes)(3-Br-pyr)2Cl2Ru)CHPh].9 The success of
this reaction derives from the combination of two monomers neither
of which forms a homopolymer under the ROMP reaction
conditions.
Our laboratory recently developed cyclobutene 1-carboxamides
as monomers that undergo ruthenium-catalyzed ring-opening me-
tathesis to yield translationally invariant polymers.10 During the
course of extending the ROMP to cyclobutenecarboxylic acid
derivatives, we observed that cyclobutene methyl ester 2 underwent
ring-opening metathesis without polymerization to afford, with 10
mol % of catalyst, ∼10% of the R-methylene ester 2a1 (Scheme
1). As in the ring-opening metathesis of 1-substituted cyclobutene
amides,10 this reaction is regiospecific. However with ester 2a, the
key enoic ruthenium carbene 3 does not react with additional
substrate; rather, it survives to react with the quenching agent,
providing ester 2a1.
Cyclohexene is a ring-opening metathesis inactive substrate with
ruthenium catalysts.11 However, it undergoes ring-opening cross
metathesis with acrylates.12 On the basis of this result and the
observation noted above, we postulated that ester 2a and cyclo-
hexene, subjected together to an active ruthenium catalyst, would
undergoalternatingring-openingmetathesispolymerization(AROMP).
To test this premise, we examined the fate of a mixture of
cyclobutene ester 2a and cyclohexene (4a) in the presence of
precatalyst 1. Indeed, polymerization occurred with 74-98%
conversion (Table 1). Taken together with the regioselective ROM
(but not ROMP) of ester 2a and the lack of reactivity of
cyclohexene, this result strongly suggested that AROMP had
4b
a All ROMP reactions were performed in CD2Cl2 and monitored by
1H NMR spectroscopy at rt. b Percent conversion determined by
integration of 1H NMR spectra unless specified otherwise. c Reaction
was performed in CH2Cl2, and the isolated yield was determined after
flash column chromatography purification.
1
occurred. H NMR spectroscopic analysis (Figure S1) of each of
the polymers revealed the phenyl protons and two sets of vinyl
protons. The ratio of the signal for the protons on the disubstituted,
nonconjugated olefin (δ ) 5.4 ppm) to the signal for the protons
on the trisubstituted, conjugated olefin (δ ) 6.8 ppm) was ∼2:1.
This result is consistent with polymer structure (2a-4a)n which
contains nearly equal numbers of repeating units A and B generated
from monomers 2a and 4a, respectively. 1H-1H gCOSY spectros-
copy of (2a-4a)20 clearly showed internal connectivity between
repeating units A and B (Figure S3), further establishing the
alternating nature of the polymer backbone.
To ascertain with greater accuracy the extent of alternation in
the sequence of monomer units, we undertook an isotopic labeling
experiment. Cyclohexene-D10, 4a-D10, and cyclobutene 2a were
subjected to AROMP, the 1H NMR spectra of the crude polymers
acquired, and the intensities of the olefinic peaks integrated against
the phenyl end group (Figure 1). As expected, for a deuterated
alternating AB copolymer (2a-4a-D10)20, the relative intensity of
the signal at δ ) 5.4 ppm was reduced to half of that in the spectrum
of polymer (2a-4a)20 (Figure 2A). This halved intensity is consistent
9
3444 J. AM. CHEM. SOC. 2009, 131, 3444–3445
10.1021/ja809661k CCC: $40.75
2009 American Chemical Society

C O M M U N I C A T I O N S
proceeded with high conversion (>95%) at room temperature in
4 h to yield (2b-4a)20 (Table 1). This type of product is viewed as
a precursor to polymers with a variety of side chains because of
the electrophilicity of phenyl esters. Next, we investigated the effect
of substituents on the cyclohexene. Neither 1-methylcyclohexene
nor 1-methoxy-cyclohexene generated an AROMP polymer with
2a. We inferred that increased substitution at the alkene prevents
addition. Indeed, 4-(methoxymethyl)-cyclohexene, 4b, underwent
AROMP with 2a to generate the corresponding alternating polymer
(2a-4b)20, with 95% conversion in 4 h (Table 1). The regiochemistry
of metathesis could not be determined in the polymerization of 4b
and is most likely random. 4-Substituted cyclohexenes are attractive
monomers for AROMP because they are readily available through
Diels-Alder chemistry.
1
Figure 1. Alkene region of H NMR spectra (CD2Cl2) of polymers (2a-
In conclusion, we have demonstrated that synthetically accessible,
select monomer pairs undergo AROMP with the reactive precatalyst
1 to form (AB)n heteropolymers with an alternating backbone and
alternating functionality. The regiocontrol of heteropolymer forma-
tion derives from the inability of the cyclobutene ester and
cyclohexene monomers to undergo homopolymerization in com-
bination with the favorable kinetics of cross polymerization.
4a)20 and (2a-4a-D10)20. Proton integrations and assignments are indicated
above the peaks.
Acknowledgment. We acknowledge NIH grants R01HD38519,
S10RR021008 (N.S.S.) and R01GM74776 (K.A.P.), an NYSTAR
grant (FDP C040076, N.S.S.), and NSF Grant CHE0131146
(NMR). We thank Dr. F. Picart for his assistance with NMR
spectroscopy.
Supporting Information Available: Experimental procedures and
characterization data. This information is available free of charge via
the Internet at http://pubs.acs.org.
Figure 2. Possible substructures generated in the copolymerization of 2a
with cyclohexene-D10. Red carbons are perdeuterated. Blue carbons bear
hydrogen.
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