Cyclopolymerization of dimethyl dipropargylmalonate in supercritical carbon dioxide to give a highly regular polyene containing predominantly five-membered rings

Article abstract of DOI:10.1039/b510046a

Poly(dimethyl dipropargylmalonate) synthesized in scCO₂ contained more than 95% five-membered rings, i.e., highly regular polyene was produced, which shows carbon dioxide as a reaction medium plays a significant role in determining a highly selective polymer structure. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b510046a

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Cyclopolymerization of dimethyl dipropargylmalonate in supercritical
carbon dioxide to give a highly regular polyene containing
predominantly five-membered rings{
Ji Won Pack, Young-Jae Hur, Hwayong Kim and Youn-Woo Lee*
Received (in Cambridge, UK) 14th July 2005, Accepted 30th August 2005
First published as an Advance Article on the web 21st September 2005
DOI: 10.1039/b510046a
Poly(dimethyl dipropargylmalonate) synthesized in scCO2
contained more than 95% five-membered rings, i.e., highly
regular polyene was produced, which shows carbon dioxide as
a reaction medium plays a significant role in determining a
highly selective polymer structure.
Cyclopolymerization leading to only five-membered rings has
8
also been accomplished using Mo(CO) . All these approaches
6
have been focused on the effect of initiator types on the properties
of the polymer. Unfortunately, the literature has paid little
4b,5a
attention to the effect of solvent on their structure.
Chemical
interactions of scCO with substrates and/or catalysts offer an
2
The use of supercritical carbon dioxide (scCO ) as a solvent for
2
as yet largely unexplored potential for activity and selectivity
9
control.
polymerization is attractive because of its low toxicity, cost and
1
nonflamability. The literature has reflected an intensive interest in
We report for the first time the cyclopolymerization of dimethyl
2
2
.
metal-catalyzed olefin metathesis polymerization in scCO
diproparglymalonate (DMDPM) by MoCl -based metathesis
5
However, few examples of substituted acetylene polymerization
3
in scCO have been reported. 1,6-Heptadiyne is a very interesting
catalysts in scCO , which encourages the regioselective addition
2
2
of the first triple bond to an alkylidene. This results in an
example of substituted acetylenes, which can be susceptible to ring-
forming polymerization to give a new type of conjugated polymer
a-substituted metallacyclobutene and therefore polyenes that
contain predominately five-membered rings in the polymer
4–6
backbone system. The cyclopolymerization of 1,6-heptadiynes
containing appropriate substituents at the 4-position has been
investigated over the past four decades by both a classical
methathesis catalyst and a well-defined alkylidene initiator,
producing a mixture of five- and six-membered rings as a con-
sequence of the first terminal triple bond adding to an alkylidene
to give either an a- or a b-substituted metallacylcobutene inter-
backbone occur due to the chemical interaction of scCO with
2
the transition-metal metathesis catalysts.
Full details of general polymerization procedures in supercritical
1
0
fluid conditions was described in our previous reports. DMDPM
1
1
is completely soluble in scCO2. However, poly(DMDPM) is
essentially insoluble in scCO even at low molecular weight
5 8000), in the range of temperature and pressure investigated
in the work. For comparison, the polymerizations of DMDPM
were carried out in various solvents, such as 1,4-dioxane, CH Cl
2
(M
n
5,6
mediate (a- or b-addition, respectively; eqn (1)).
2
2
and 1,1,1,2-tetrafluoroethane (R134a) to investigate the solvent
effect on the polymerization. The resulting polymer is highly
soluble in 1,4-dioxane and CH Cl , while it exhibits very low
2
2
ð1Þ
12
solubility in liquid R134a. Based on these measurements the
polymerization of DMDPM in scCO and in R134a began
2
homogeneously, but became heterogeneous as the polymer
precipitated. In all of these solvents cyclopolymerization was
effectively carried out by MoCl
optimized polymerization results are summarized in Table 1.
n-Bu) Sn has been known to be an excellent cocatalyst for the
5
-based metathesis catalysts and the
Schrock’s group has reported that well-defined high oxidation-
state molybdenum carbenes cyclopolymerize 1,6-heptadiynes in a
living manner to produce polyenes that contain either a mixture of
(
4
13
4
6
polymerization of mono- and disubstituted acetylenes. In this
five- and six-membered rings or exclusively six-membered rings.
7
Recently, Nuyken and co-workers have reported cyclopolymer-
study, however, (n-Bu) Sn had little effect on the polymer yield
4
and the number average molecular weight (M ). WCl exhibited
n
6
ization with both well-defined molybdenum-based initiators such
markedly less effective catalytic activity than MoCl (results not
5
as Mo(N-2,6-i-Pr
2
C
6
H
3
)(CHCMe Ph)(OC(CH
2
3
)
3
)
2
–quinuclidine
–n-Bu Sn–
presented here). As a cocatalyst of WCl
on the polymer yield and M , which is consistent with previous
reports. The polymerization initiated by MoCl /(n-Bu) Sn in 1,4-
6 4
, (n-Bu) Sn had no effect
and with classical methathesis catalysts such as MoCl
5
4
n
ethanol–quinuclidine, to produce .95% five-membered rings.
5a
5
4
dioxane (see entry 2 in Table 1) resulted in the highest polymer
School of Chemical and Biological Engineering & Institute of Chemical
Processes, Seoul National University, San 56-1, Sillim-dong, Gwanak-
gu, Seoul, 151-744, Korea. E-mail: ywlee@snu.ac.kr; Fax: 82 2 888 1604;
Tel: 82 2 880 1883
21
n
yield (88%) and M (68 000 g mol ), which is similar to the
previous report for the cyclopolymerization of diethyl dipropargyl-
7b
malonate. The polymerization initiated by MoCl -based meta-
5
{
Electronic supplementary information (ESI) available: Details of
2
thesis catalysts in scCO (see entries 7 and 8 in Table 1) resulted in
experimental procedures and the characterization of poly(DMDPM).
See http://dx.doi.org/10.1039/b510046a
ca. 70% polymer yield and ca. 13 000 number average molecular
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5
208 | Chem. Commun., 2005, 5208–5210

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a
Table 1 Cyclopolymerization of dimethyl dipropargylmalonate by transition-metal metathesis catalysts in various solvents
b
5-Rings (%)
c
n
21
/g mol
c
Entry
Solvent
Cat. system
M
MWD
Yield (%)
d
1
1,4-Dioxane
1,4-Dioxane
MoCl
MoCl
MoCl
MoCl
MoCl
MoCl
MoCl
MoCl
5
5
5
5
5
5
5
5
67(3)
70(2)
73(3)
71(1)
65(3)
66(2)
.95(1)
.95(2)
65 000
68 000
62 500
65 000
10 500
11 500
12 500
13 500
3.40
3.31
3.52
3.56
2.42
2.36
2.05
1.95
86
88
80
85
69
71
70
73
b
d
2
/(n-Bu)
/(n-Bu)
/(n-Bu)
/(n-Bu)
4
4
4
4
Sn
Sn
Sn
d
3
CH
CH
2
Cl
Cl
2
2
d
4
2
e
5
CF
CF
3
CH
CH
2
F
F
e
6
3
2
f
7
scCO
scCO
2
f
8
2
Sn
21
a
0
Conditions of polymerization: [M] 5 0.25 mol L , [Cat.] 5 [Cocat.] 5 5 mM; mol ratio of monomer to catalyst 5 50. All spectra were
obtained at 25 uC in a 0.05 M solution of Cr(acac)
carbonyl resonances in the region of 170–175 ppm and quaternary carbon resonances in the region of 40–60 ppm. The average is listed here
3 3
in CDCl at 125 MHz. The percentages of five-membered ring were determined using
c
and the difference between the two within parentheses. Determined by GPC vs. polystyrene standard with the RI detector. Polymerization
d
e
f
was carried out at 40 uC for 24 h. Polymerization was carried out in R134a at 40 uC and 150 bar for 24 h. Polymerization was carried out
at 40 uC and 250 bar for 24 h.
weight, which is lower than those of 1,4-dioxane or CH Cl
2
2
but
indicating the presence of both five- and six-membered rings in the
polymers (see Fig. 2(a) and (b)). The relative amounts of five-
membered rings formed in poly(DMDPM) prepared with a variety
of solvents are listed in Table 1. Common organic solvents and
the type of classical catalysts used appear to play a negligible
role in determining the ratio of five- to six-membered rings. This
observation is in accordance with other polymerization data for
similar to those of R134a.
1
Poly(DMDPM) structure was identified by H and C NMR,
13
1
IR, and UV-visible spectroscopies. Fig. 1 represents the H NMR
spectrum of poly(DMDPM). The new peaks at 6.2–7 ppm are
assigned to the resonances of protons on the conjugated double
bonds. No acetylenic carbon–hydrogen stretching band of
2
1
DMDPM (3310 cm ) was found in the IR spectrum of
poly(DMDPM) (see ESI{). The UV-visible spectrum of the
polymer obtained in THF exhibits a characteristic broad peak at
diethyl dipropargylmalonate with MoCl
typically produced ca. 70% five-membered rings. The C NMR
spectrum of poly(DMDPM) synthesized in scCO , however,
5
-based initiators, which
4b
13
2
3
20–600 nm that is due to a p–p* transition of conjugated polyene
reveals predominantly one carbonyl carbon resonance at
172.5 ppm and one at 57.3 ppm for the quaternary carbon, indicat-
ing that it contains .95% five-membered rings (see Fig. 2(c)).
Carbon dioxide is a molecule with several potential reactive
coordination sites: the carbon is a Lewis acid center and the
oxygens are weak Lewis bases. The carbon can thus be described
as the electrophilic center of carbon dioxide and the oxygen atoms
13
(
see ESI{). In the C NMR spectrum, two peaks at 123.3 and
2
37.1 ppm for vinylene and quaternary pairs of sp carbons and
1
one resonance at 41.7 ppm for a pair of ring CH groups make a
2
complete assignment of resonances possible (see ESI{).
More information can be obtained about the effect of the
13
solvents on the structure of poly(DMDPM) by C NMR
spectroscopy. Carbon resonances at 172.5 and 57–58 ppm are
characteristic of five-membered rings while resonances at 171.5
14a
as the nucleophilic centers.
Many transition-metal/CO com-
2
14
plexes have been described in the literature. According to
15
13
C
Carmona et al., molybdenum can form a double coordination
with carbon dioxide, in which both carbon dioxide molecules are
bonded ‘‘side-on’’ and the coordinated CLO bonds are staggered
and 44–45 ppm are characteristic of six-membered rings. The
NMR spectra of poly(DMDPM) synthesized in 1,4-dioxane,
CH Cl or R134a reveal two resonances for the carbonyl carbon
2
2
atoms at around 171.5 and 172.5 ppm and two major clusters of
peaks at 57–58 and 44–45 ppm for the quaternary carbons,
1
3
Fig. 2
and quaternary carbon resonances of poly(DMDPM) synthesized in (a)
1,4-dioxane, (b) CH Cl and (c) supercritical carbon dioxide.
C NMR spectra (125 MHz) of the carbonyl carbon resonances
1
Fig. 1
H
NMR spectrum of poly(dimethyl dipropargylmalonate)
).
(
sample: Table 1, entry 7, measured in CDCl
3
2
2
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This type of solvent coordination to metals can strongly affect the
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16
17
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8
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2
7
a
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1
0 J. W. Pack, S. H. Kim, S. Y. Park, Y. -W. Lee and Y. H. Kim,
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2
mechanism of cyclopolymerization in scCO consists of addition of
2
11 The solubility of DMDPM in scCO measured by determining
the first triple bond to an alkylidene to give more favorably an
a-substituted metallacyclobutene intermediate, in which carbon
dioxide acts as coordinating ligands to molybdenum. Intra-
molecular a-addition (cyclization) followed by opening of the
metallacycle leads to the propagating terminal alkylidene (eqn (2)).
the cloud-point pressure at a constant concentration of DMDPM
([DMDPM] 5 6.1 wt%) and various temperatures. The solubilization
0
pressures of DMDPM in CO
0, 55 and 60 uC, respectively.
2 DMDPM is completely soluble in liquid R134a at room temperature
and above, at [DMDPM] 5 6.1 wt%. However, poly(DMDPM)
2
were 73, 83, 88, 93 and 112 bar at 35, 45,
5
1
0
exhibits very low solubility in liquid R134a under 40 uC and 150 bar
when the molecular weight is higher than a few thousand mass units.
13 T. Masuda, T. Hamano, T. Higashimura, T. Ueda and H. Muramatsu,
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1
4 (a) A. Behr, Carbon Dioxide Activation by Metal Complexes, VCH,
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ð2Þ
13
15 The C resonance of CO
2
is shifted from a value of d # 124 for free
dissolved CO to a lower field upon coordination to a transition metal.
2
Values in a range of 150–250 ppm have been reported, and the chemical
shifts may differ widely even for complexes with the same coordination
mode. A concerted rotation of the two carbon dioxide ligands, in which
both molecules rotate in the same direction, has been demonstrated by
variable-temperature NMR spectroscopy. See: R. Alvarez, E. Carmona
and M. L. Poveda, J. Am. Chem. Soc., 1984, 106, 2731; R. Alvarez,
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Intramolecular formation of the five-membered ring must be fast
relative to intermolecular addition of the second triple bond to a
MLC bond, a reaction that would lead to cross-linking. None is
observed.
We gratefully acknowledge the financial support from the
Ministry of Commerce, Industry & Energy and the Energy
Management Corporation. The authors thank Dr Hyun-Nam
Cho for supplying DMDPM used in this study.
16 For example, halocarbon binding favors the useful C-alkylation rather
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Organometallic Chemistry of the Transition Metals, John Wiley and
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Notes and references
1
7 Ligand dependence of molybdenum-catalyzed alkylations: the regio-
1
P. G. Jessop and W. Leitner, Chemical Synthesis Using Supercritical
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2
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210 | Chem. Commun., 2005, 5208–5210
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