Precise isomerization polymerization of alkenylcyclohexanes: Stereoregular polymers containing six-membered rings along the polymer chain

Article abstract of DOI:10.1021/ja2043968

Pd and Ni diimine complexes catalyze the isomerization polymerization of alkenylcyclohexanes to afford polymers composed of alternating trans-cyclohexane-1,4-diyl rings and oligomethylene spacers with high selectivity. The melting points of the polymers vary from 130 to 226 °C depending on length of the oligomethylene spacer.

Full text of DOI:10.1021/ja2043968

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pubs.acs.org/JACS
Precise Isomerization Polymerization of Alkenylcyclohexanes:
Stereoregular Polymers Containing Six-Membered Rings along the
Polymer Chain
Daisuke Takeuchi*
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Yokohama 226-8503, Japan
S Supporting Information
b
structure. The mechanism and stereoselectivity of the reaction
are also discussed.
ABSTRACT: Pd and Ni diimine complexes catalyze the
isomerization polymerization of alkenylcyclohexanes to afford
polymers composed of alternating trans-cyclohexane-1,4-diyl
rings and oligomethylene spacers with high selectivity. The
melting points of the polymers vary from 130 to 226 °C
depending on length of the oligomethylene spacer.
The Pd and Ni complexes 1ꢀ3 catalyze the polymerization of
olefins having cyclohexyl groups (I-m; m = 0ꢀ3, 9) in the pre-
sence of NaBARF (BARF = B{C6H3(CF3)2-3,5}4ꢀ) or modified
methylaluminoxane (MMAO) to produce polymers formulated as
ꢀ[CH2CH2(CH2)mC6H10]nꢀ (poly-I-m), as shown in eq 1.
The results of the polymerization are summarized in Table 1.
The polymer of vinylcyclohexane, ꢀ[CH2CH2C6H10]nꢀ (poly-
I-0), obtained from the reaction with a monomer-to-catalyst
molar ratio of 300:1 (run 1) was practically insoluble in organic
solvents and could not be characterized by NMR or gel-permea-
tion chromatography (GPC) measurements. Poly-I-0 obtained
using [I-0]/[1] = 10:1 was slightly soluble in C2D2Cl4 at 130 °C,
probably because of its lower molecular weight in comparison
with that from the 300:1 reaction. The 13C{1H} NMR spectrum
of the fraction soluble in C2D2Cl4 exhibited three sharp signals at
δ 38.6, 35.0, and 33.8 (Figure 1i). The peak positions as well as
their comparison with polymers of olefins with longer alkyl
groups indicate the structure proposed in Figure 1i. Minor signals
may be assigned to repeating units having other structures and/
or the terminal groups of the polymer. The polymer of I-1 was
obtained from a reaction in which [I-1]/[1] = 300:1 (run 3).
GPC measurements at 152 °C (soluble fraction) showed Mn to
be 18 000. Catalysts 2 and 3 also polymerized I-1 to form the
corresponding polymer (runs 4 and 5). Poly-I-1 obtained from a
reaction in which [I-1]/[1] = 10:1 had sufficient solubility to
enable 13C{1H} NMR measurements. The 13C{1H} NMR
spectrum in C2D2Cl4 at 130 °C showed four sharp signals almost
exclusively (Figure 1ii). Signals of the CH and CH2 carbons in
the six-membered ring (δ 38.1 and 33.8) were at positions similar
to those for the ethyleneꢀbutadiene copolymer having trans-
cyclohexane-1,4-diyl groups (δ 38.45 and 33.94).7 Thus, the six-
membered rings in poly-I-1 as well as in poly-I-0 have trans-1,4-
disubstituted structures with high selectivity. The chemical shifts
of the CH2 carbon signals in the trimethylene spacer of poly-I-1
(δ 38.3 and 24.4) were in good agreement with those of the
model compound 1,3-dicyclohexylpropane (δ 38 and 25).8
Table 1 shows a summary of the results of the polymerization
of alkenylcyclohexanes having longer oligomethylene spacers,
namely, I-2, I-3, and I-9. The polymerizations of I-2 and I-3
proceeded smoothly to give polymers with molecular weights
(Mn) of 9500 and 11 000, respectively (runs 6 and 7). The
polymerization of I-9 also produced the corresponding polymer
ydrocarbon polymers whose main chain contains three- to
H
six-membered rings1 have advantages due to their higher
melting or glass-transition temperatures2 and optical transpar-
ency in comparison with common polyolefins. Coordination po-
lymerization of cyclopentene and norbornene provides the major
route to polymers having cyclic repeating units.1,3 However, such
polymerization is mostly limited to that of monomers with five-
membered rings. Six-membered cyclic monomers, such as cyclohex-
ene, are much less reactive than five-membered-ring monomers. We
recently found that the polymerization of 4-alkylcyclopentenes via
chain-walking reactions catalyzed by Pd diimine complexes yields
products composed of trans-1,3-cyclopentane rings and oligomethy-
lene spacers and that this process involves selective insertion of the
monomer into the CH2ꢀPd bond of the growing polymer end
rather than the CHꢀPd bond.4 Further studies revealed that alkylcy-
clohexenes do not undergo similar polymerization because of the
poor reactivity of their six-membered cyclic olefins. The isomeriza-
tion of alkenes with a cycloalkyl substituent may also yield polymers
with cyclic groups in the chain. Such reactions, however, have not
been reported to date. For example, the coordination polymerization
of vinylcyclohexane usually produces a polymer with cyclohexyl
substituents bonded to the main chain.5,6 Here we report that
Pd and Ni complexes catalyze the isomerization polymerization
of alkenylcyclohexanes to afford polymers with an unexpected
Received: May 13, 2011
Published: June 27, 2011
r
2011 American Chemical Society
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Table 1. Polymerization of Alkenylcycloalkanesa
monomer
b
b
run
([monomer]/[cat.])
cat.
yield (%)
Mn
Mw/Mn
d
d
d
1c
2c
3
I-0 (300)
I-0 (300)
I-1 (300)
I-1 (300)
I-1 (100)
I-2 (100)
I-3 (100)
I-9 (100)
II-0 (100)
II-1 (100)
1
2
1
2
3
1
1
1
1
1
64
ꢀ
ꢀ
d
quant
86
ꢀ
ꢀ
18000
21000
14000
9500
2.48
1.79
1.71
1.71
1.86
2.65
4
97
5e
6f
7f
8f
9f
quant
80
80
11000
11000
75
trace
trace
10f
a Reaction conditions: cat. (0.010 mmol), cocat. = NaBARF (0.012 mmol),
solvent = CH2Cl2 (1.5 mL), at rt for 20 min, unless otherwise noted.
b Determined by GPC detected by FTIR based on a polyethylene standard
using o-dichlorobenzene as the eluent at 152 °C. c Reaction time = 1 h. d Not
available because of the low solubility of the polymer. e Reaction conditions:
cat. (0.010 mmol), cocat. = MMAO ([Al]/[3] = 300), solvent = toluene
(2 mL) at rt for 24 h. f Solvent = CH2Cl2 (0.5 mL).
Figure 2. DSC profiles of (i) poly-I-1, (ii) poly-I-2, (iii) poly-I-3, and
(iv) poly-I-9.
that the rate-determining step of the polymerization is not coordi-
nation/insertion of the monomer but isomerization of the growing
polymer end by chain walking. Chain-walking reactions in olefin
polymerization using Pd catalysts have been proposed to be much
faster than insertion.9 Kinetic results independent of the monomer
concentration are rare in coordination polymerization using Pd
catalysts.10,11
Analogous isomerization polymerizations of monomers with
five-membered rings, such as vinylcyclopentane (II-0) and
allylcyclopentane (II-1), yielded only trace amounts of unchar-
acterized polymers (runs 9 and 10), although they were expected to
give polymers via the reactions of 4-ethyl- and 4-propylcyclopentenes
catalyzed by Pd complexes.4 In contrast, 3-methylcyclohexene did
not polymerize in the presence of the Pd catalysts. The Pd-catalyzed
polymerization of 4-alkylcyclopentenes afforded polymers composed
of alternating trans-cyclopentane-1,3-diyl groups and oligomethylene
spacers, whereas polymers containing trans-cyclohexane-1,4-diyl
groups and oligomethylene spacers were obtained from alkenylcy-
clohexane rather than alkylcyclohexene.
Olefin polymerization catalyzed by Ni and Pd complexes
involves the isomerization of the growing polymer end, as
proposed by Brookhart and co-workers.12 The details have been
investigated by their research group as well as others.13 We
reported the polymerization of dienes, trienes, and cycloolefins
using a Pd catalyst, in which the preferential insertion of the
monomer into the CH2ꢀPd bond rather than the CHꢀPd
bonds enables apparent selective chain walking.4,14 The polym-
erization in this study also involves the selective insertion of vinyl
groups of monomers into a CHꢀPd bond of the six-membered
ring because the polymers are composed of 1,4-disubstituted six-
membered rings and oligomethylene spacers.
Scheme 1 shows the polymer growth mechanism proposed for
I-3. The polymerizations of the other monomers also proceed via
the same mechanism. The 4-alkylcyclohexylpalladium intermedi-
ate (A) undergoes selective 2,1-insertion of the vinyl group of the
monomer into the CHꢀPd bond. The 1,2-insertion does not
take place, probably because of the steric repulsion between the
cyclohexyl group attached to Pd and the monomer substituent.
Intermediate B does not undergo direct insertion of a new
monomer10 but instead undergoes a chain-walking reaction via
β-hydrogen elimination and reinsertion of the olefin-terminated
polymer, giving the new intermediate C and then intermediate
Figure 1. 13C{1H} NMR spectra (C2D2Cl4, 130 °C) of (i) poly-I-0, (ii)
poly-I-1, (iii) poly-I-3, and (iv) poly-I-9. Ratios in parentheses correspond to
the initial monomer-to-catalyst molar ratio in the polymerization reaction.
in 75% yield in 20 min (run 8). The 13C{1H} NMR spectra of
poly-I-3 and poly-I-9 (Figure 1iii,iv) exhibited signals due to
trans six-membered-ring carbons and CH2 carbons attached to
the ring at positions similar to those for poly-I-1. All of these
results indicate a structure composed of trans-1,4-disubstituted six-
membered rings and oligomethylene spacers, regardless of the length
of the spacer. Differential scanning calorimetry (DSC) measure-
ments on poly-I-1ꢀ3 and poly-I-9 showed melting transitions
(Figure 2), but poly-I-0 did not show a melting point in the range
30ꢀ300 °C. Increasing the number of carbons in the alkyl group of
the monomer lowered the melting point of the polymer.
The polymerizations of I-0, I-1, and I-3 catalyzed by 1/NaBARF
obeyed zeroth-order kinetics with respect to the monomer concen-
tration. The kinetic constants were found to be 6.9 ꢁ 10ꢀ6, 4.0 ꢁ
10ꢀ5, and2.9ꢁ 10ꢀ5 Msꢀ1, respectively, at 0 °C. The results suggest
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COMMUNICATION
Scheme 1. Plausible Mechanism of Polymerization of I-3
In this work, we have presented isomerization polymerization
occurring with high selectivity during the insertion of the
monomer. The 2,1-insertion of the monomer takes place selec-
tively, in contrast to the polymerization of R-olefins using the
same catalyst. Here the monomer inserts into a CHꢀPd bond at
the 4-position of the cyclohexyl group rather than into the other
CHꢀPd bonds of the growing polymer chain. This mechanism is
in contrast to those for the isomerization polymerizations
reported to date, which show either apparent selectivity for inser-
tion of the monomer into the CH2ꢀPd bond instead of the
CHꢀPd bond or no such selectivity. The steric requirements of
β-hydrogen elimination during the polymerization result in a
single stereoisomeric polymer containing trans-cyclohexane-1,4-
diyl groups.17 This selective isomerization polymerization has
been developed for the copolymerization of alkenylcyclohexanes
as well as the copolymerization of these monomers with ethylene
and R-olefins, the results of which will be presented in the future.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures and
b
NMR spectra of polymers. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
dtakeuch@res.titech.ac.jp
’ ACKNOWLEDGMENT
The author is grateful to Prof. Kohtaro Osakada of Tokyo
Institute of Technology for helpful discussions and suggestions.
This work was supported by a Grant-in-Aid for Young Scientist
(22685012) for Scientific Research from the Ministry of Educa-
tion, Science, Sports and Culture, Japan.
Scheme 2. Possible Intermediates Formed in Going from C
to A0 in Scheme 1
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