Isomerization polymerization of 4-alkylcyclopentenes catalyzed by Pd complexes: Hydrocarbon polymers with isotactic-type stereochemistry and liquid-crystalline properties

Article abstract of DOI:10.1021/ja904150w

(Chemical Equation Presented) Isomerization polymerization of 4-alkylcyclopentenes catalyzed by Pd-diimine complexes produces the polymers with trans-1,3-disubstituted cyclopentane rings located regularly along the polymer chain. The polymers with isotact

Full text of DOI:10.1021/ja904150w

Published on Web 07/15/2009
Isomerization Polymerization of 4-Alkylcyclopentenes Catalyzed by Pd
Complexes: Hydrocarbon Polymers with Isotactic-Type Stereochemistry and
Liquid-Crystalline Properties
Takeshi Okada, Daisuke Takeuchi, Atsushi Shishido, Tomiki Ikeda, and Kohtaro Osakada*
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Yokohama 226-8503, Japan
Received May 22, 2009; E-mail: kosakada@res.titech.ac.jp
The physical properties of poly(R-olefins) vary to a large degree
depending on the alkyl substituents derived from the monomer and
the relative stereochemistry of the neighboring repeating units
(
tacticity). Polymers with five- or six-membered rings in the
1
monomer units have unique thermal and optical properties. Early
transition-metal complex-catalyzed polymerization of cyclic olefins
and cyclopolymerization of dienes provide these polymers ef-
ficiently, although polymerization with regulated tacticity is rare.
Ni- or Pd-diimine complexes catalyze the polymerization of
2
3
R-olefins and cyclopolymerization of dienes with isotactic selectiv-
4
ity when a C
2
-symmetric ligand is employed. The polymerization
5
reactions, including cyclopolymerization of monoterminal dienes,
involve isomerization of the growing polymer end via a chain-
walking process. In this paper, we report polymerization of
4-alkylcyclopentenes catalyzed by Pd complexes to afford polymers
with 1,3-disubstituted five-membered rings formed via olefin
6,7
insertion and chain walking. The polymer with regulated tacticity
8
exhibited liquid-crystalline properties.
2
Pd-diimine complexes 1a and 1b, with C2V- and C -symmetric
5
ligands, respectively, promote the polymerization of 4-methylcy-
clopentene (I-1) in the presence of NaBARF (BARF ) [{3,5-
-
(
CF
3
)
2
C
6
H
3
}
4
B] ) to produce poly(I-1) (eq 1):
Figure 1. 13C{ H} NMR spectra (125 MHz, CDCl
run 1 in Table 1), (ii) poly(I-1) (run 2), (iii) poly(I-3) (run 3), (iv) poly(I-4)
run 4), and (v) poly(I-10) (run 5).
1
at 50 °C) of (i) poly(I-1)
3
(
(
diffraction peak at 2θ ) 17.92° and a halo pattern, indicating the
coexistence of the crystalline and amorphous domains. The isotactic
polymer obtained using 1b/NaBARF exhibited only a single sharp
diffraction peak at 2θ ) 18.42° and thus contained the crystalline
domain almost exclusively at room temperature.
a
Table 1. Polymerization of 4-Alkylcyclopentenes
run
olefin
Pd
time
conv. (%)
M
n
w n
M /M
b
,
c
b
,
c
1
2
3
4
5
I-1
I-1
I-3
I-4
I-10
1a
1b
1b
1b
1b
3 h
5 min
1 h
1.5 h
36 h
quant.
quant.
quant.
66
10100
1.90
-
The 13C{ H} NMR spectrum of poly(I-1) obtained using 1a/
1
c
e
e
e
,
d
8200
3000
3000
5700
e
e
e
2.18
3.38
2.38
NaBARF [Figure 1(i)] exhibits three peaks (39.32, 39.26, and 39.22
9
2 1
ppm) for the CH (b ) carbon between two CH carbons, which
56
corresponds to the triad structures with respect to the orientation
of three neighboring five-membered rings. The ratio among the
peaks (31:47:22) suggests a random or atactic-type arrangement
of the five-membered rings. Poly(I-1) obtained using 1b/NaBARF,
on the other hand, gives rise primarily to the signal at the lowest
field position (39.32 ppm) [Figure 1(ii)]. It is assigned to three
monomer units with the same stereochemistry, an rr triad, indicating
a highly threo-diisotactic structure of the polymer (rr > 93%).
The different tacticities of poly(I-1) formed by different catalysts
a
Reaction conditions: Pd complex (0.010 mmol), NaBARF (0.012
b
mmol), [monomer]/[Pd] ) 300, CH
GPC (o-dichlorobenzene) measurements on poly(I-1) (runs 1 and 2)
were not conducted because of low solubility. Estimated by H NMR.
GPC (o-dichlorobenzene).
2 2 3
Cl solvent, r.t. GPC (CHCl ).
c
d
1
e
Scheme 1 summarizes a plausible mechanism for the polymer-
ization of I-1. Coordination and insertion of the monomer produce
a cyclopentylpalladium intermediate (A). It does not undergo direct
insertion of the monomer into the Pd-secondary alkyl bond but
causes isomerization via chain walking to produce the cyclopen-
10
affected the crystallographic results. XRD measurements (Cu KR)
on the polymer obtained using 1a/NaBARF showed an intense
1
0852 9 J. AM. CHEM. SOC. 2009, 131, 10852–10853
10.1021/ja904150w CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
3
tylmethyl palladium species (B). Insertion of a new monomer into
the Pd-primary alkyl bond leads to further polymer growth.
Formation of a trans five-membered ring in the polymer is explained
by assuming preferential coordination of the monomer to Pd at the
less-hindered side followed by smooth insertion and chain walking.
Scheme 1. Proposed Mechanism for the Polymerization of I-1
Figure 2. Polarizing optical micrograph of poly(I-4) (run 4 in Table 1).
In summary, the polymerization of 4-alkylcyclopentenes cata-
lyzed by Pd complexes affords polymers with trans-1,3-disubsti-
tuted cyclopentane rings located at regulated intervals along the
2
linear polymer chain. The C -symmetric catalyst produced polymers
Scheme 2 depicts a possible coordination mode of the monomer
to the Pd center of 1b activated by NaBARF. Steric repulsion
between the bulky naphthyl substituent and the monomer renders
coordination in C more favorable than that in D. Repetitive
coordination in C and insertion of the coordinated monomer would
result in formation of the isotactic polymer.
with an isotactic structure that exhibit high crystallinity at low
temperature and liquid-crystalline properties upon heating.
Acknowledgment. The authors are grateful to the Institute for
Molecular Science for the NMR measurements. This work was
supported by a Grant-in-Aid for Scientific Research on Priority
Areas (18750094) from the Ministry of Education, Science, Sports,
and Culture, Japan. T.O. acknowledges a JSPS Research Fellowship
for a Young Scientist.
Scheme 2. Selective Coordination of I-1
Supporting Information Available: Experimental procedures and
NMR spectra, DSC profiles, polarized optical micrographs, and XRD
data of polymers. This material is available free of charge via the
Internet at http://pubs.acs.org.
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2
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2
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10
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(
(
2
007, 46, 6141.
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(
(
n
M ) 10100 and 4500), however, undergoes only an irreversible
glass transition and does not show liquid-crystalline behavior.
8
Recently, Naga et al. reported the formation of liquid-crystalline
(8) For the report of liquid-crystal polyolefins, see: Naga, N.; Yabe, T.;
Sawaguchi, A.; Sone, M.; Noguchi, K.; Murase, S. Macromolecules 2008,
hydrocarbon polymers having five-membered rings from the
cyclopolymerization of 1,5-hexadiene, although the stereochemical
structure of the polymer has not been clarified and is probably not
regulated.
4
1, 7448.
1
3
1
(9) Determined from the C{ H} NMR spectrum at 230 MHz.
(
10) The Supporting Information contains complete diffraction and DSC data.
JA904150W
J. AM. CHEM. SOC. 9 VOL. 131, NO. 31, 2009 10853