Unprecedented isospecific 3,4-polymerization of isoprene by cationic rare earth metal alkyl species resulting from a binuclear precursor

Article abstract of DOI:10.1021/ja055397r

The isospecific 3,4-polymerization of isoprene has been achieved for the first time by use of a combination of a binuclear rare earth metal dialkyl complex, such as [Me₂Si(C₅Me₄)(μ-PCy)YCH₂SiMe₃]₂ (Cy = cyclohexyl), and an equimolar amount of [Ph₃C][B(C₆F₅)₄] as a catalyst system. A DFT calculation suggested that a binuclear monocationic monoalkyl species, such as [Me₂Si(C₅Me₄)(μ-PCy)Y(μ-CH₂SiMe₃)Y(μ-PCy)(C₅Me₄)SiMe₂]⁺, in which the alkyl group bridges the two metal centers, could be the true catalyst species. Copyright

Full text of DOI:10.1021/ja055397r

Published on Web 09/30/2005
Unprecedented Isospecific 3,4-Polymerization of Isoprene by Cationic Rare
Earth Metal Alkyl Species Resulting from a Binuclear Precursor
Lixin Zhang, Yi Luo, and Zhaomin Hou*
Organometallic Chemistry Laboratory, RIKEN (The Institute of Physical and Chemical Research), Hirosawa 2-1,
Wako, Saitama 351-0198, Japan, and PRESTO, Japan Science and Technology Agency (JST), Japan
Received August 8, 2005; E-mail: houz@riken.jp
Chart 1. Bi- and Mononuclear Rare Earth Metal Dialkyl Complexes
Cationic alkyl complexes of group 4 metals have been the subject
of intensive investigations over the past two decades because of
their crucial importance in catalytic olefin polymerization.1 In
contrast, cationic alkyl species of rare earth (group 3 and lanthanide)
metals are much less developed.2,3 However, recent studies have
shown that cationic rare earth metal alkyls are highly efficient olefin
polymerization catalysts3 and can show unique activities that differ
from those of group 4 metals.3a,b,e Treatment of a neutral di- or
trialkyl precursor with an equimolar amount of a borate compound
proved to be a convenient route to generate a cationic trivalent rare
earth metal alkyl species. Due to the lack of appropriate precursors,
the cationic rare earth metal alkyls reported so far were limited
solely to mononuclear species, while a binuclear cationic rare earth
metal alkyl species remained unknown to date.5
Table 1. Polymerization of Isoprene under Various Conditionsa
microstructure (%)c
b
d
Tp
t
yield
Mn
105)
Tg/Tm
C)
b
run
cat.
(
°
C) (h) (%)
(
×
Mw/Mn
3,4
mm mmmm
(
°
During our investigations on new organo rare earth metal
catalysts,4 we have synthesized several silylene-linked cyclopen-
tadienyl-phosphido rare earth metal alkyl complexes, such as 1
and 2 (Chart 1).6 These complexes adopt a relatively robust
binuclear structure through the phosphido bridges, in which each
of the two metal centers bears a terminal alkyl ligand. In view of
the novel activity observed recently for the cationic alkyl species
generated from the mononuclear rare earth metal dialkyl complexes,
such as 3 and 4 in olefin polymerization,3a,b,e we became interested
in the olefin polymerization behavior of the cationic alkyl species
generated from the binuclear complexes 1 and 2. In this Com-
munication, we report that such binuclear cationic alkyl species
can show unprecedented regio- and stereoselectivity in the polym-
erization of isoprene, which afford isotactic 3,4-polyisoprene with
extremely high regio- and stereoselectivity (3,4-selectivity: 100%,
mmmm > 99%). As far as we are aware, this is the first example
of isospecific 3,4-polymerization of isoprene. Previously, various
metal catalysts or initiators have been reported for the polymeri-
zation of isoprene, but most of them yielded predominantly 1,4-
polyisoprene and none were reported to show isospecific 3,4-
selectivity.7,8
1
2
3
4
5
6
7
8
9
1
A
B
C
25
2
2
2
2
2
0
25
25
25
25
80 0.3
0
0
7.9
<5e
n.d.f
1+A
92 1.0
1.8
99
80
96
30 42/-
80 28/148
1+A -10 16 41 4.3; 0.3 1.6; 1.4 100
1+A -20 48 50 3.2; 0.3 1.7; 1.4 100 100 >99 30/154
1+A
25
2
100 1.2
1.3
1.6
1.6
1.6
1.3
1.4
1.6
1.06
99
100
80
96
30 41/-
80 31/138
1+A -10 16 60 3.7
10 1+A -20 48 87 5.0
100 100 >99 33/162
11g 1+A
12 1+B
13 1+C
14 2+A
15 4+A
25
25
25
25
25
2
2
2
2
2
65 0.8
85 1.4
63 1.1
17 1.3
100 0.9
99
99
99
89
66h
25
40
38
38/-
n.d.
n.d.
n.d.
n.d.
a Conditions: C6H5Cl, 10 mL; neutral complex, 2.5 × 10-5 mol;
[isoprene]0/[complex]0 ) 600, [complex]0/[activator]0 ) 1:1, unless oth-
erwise noted. A ) [Ph3C][B(C6F5)4], B ) [PhMe2NH][B(C6F5)4], C )
B(C6F5)3. In runs 5-7, isoprene was added to a mixture of 1 and A. In
runs 8-15, an activator A, B, or C was added to a mixture of a neutral
complex and isoprene. b Determined by GPC against polystyrene standard.
c Determined by H and 13C NMR. d Determined by DSC. e The trans-1,4
1
unit is a major component. f n.d. ) not determined. g Toluene was used as
a solvent. h The 1,4-unit: 34%.
The binuclear Y and Lu dialkyl complexes 1 and 2 were easily
obtained in 60-70% isolated yields by reaction of Me2Si(C5Me4H)-
(PHCy) (Cy ) cyclohexyl) with an equimolar amount of Ln(CH2-
SiMe3)3(THF)2 in hexane.6 1H NMR monitoring of the reaction of
1 with an equimolar amount of [Ph3C][B(C6F5)4] (A) in C6D6 or
C6D5Cl showed instant disappearance of the signals for the neutral
complex 1 and appearance of new signals assignable to Ph3CCH2-
SiMe3. However, the possible resulting cationic Y alkyl species
could not be assigned because of the complexity of the spectrum.
Nevertheless, addition of 600 equiv of isoprene to a mixture of 1
and A in C6H5Cl led to rapid polymerization of isoprene, which
yielded regiospecifically 3,4-polyisoprene (99%) with isotactic-rich
stereo microstructures (mm ≈ 80%, mmmm ≈ 30%) and a relatively
narrow molecular weight distribution (Mw/Mn ) 1.8) (Table 1, run
5). This is in sharp contrast with the reaction of either the neutral
dialkyl complex 1 or the borate compound A with isoprene. In the
case of 1, no reaction was observed under the similar conditions
(Table 1, run 1), while the borate A alone caused cationic isoprene
polymerization and yielded a polymer with 1,4-rich microstructures
and a very broad molecular weight distribution (Mw/Mn ) 7.9)
(Table 1, run 2). When the polymerization was carried out at low
temperatures by use of a combination of 1 and A as a catalyst,
further high regio- and stereoselectivity (3,4-selectivity 100%,
mmmm up to 99%) were achieved (Table 1, runs 6 and 7).
Unfortunately, however, the GPC curves of the polymers obtained
at low temperatures became bimodal. These results suggest that
the cationic alkyl species generated in the reaction of 1 with A is
unstable and could decompose or change to other species.9 To trap
and utilize more efficiently the cationic alkyl species prior to its
9
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J. AM. CHEM. SOC. 2005, 127, 14562-14563
10.1021/ja055397r CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. A Possible Polymerization Mechanism
isoprene molecule would follow the same way at the Y2 center.
After the insertion of the second isoprene, the interaction between
Y1 and the SiMe3 group could be replaced by that between Y1
and a CdC double bond in the side chain (1f). Therefore, the
coordination and insertion of the following isoprene monomers
could take place always at the Y2 metal center in the same fashion,
and thus afford isotactic 3,4-polyisoprene selectively.
Acknowledgment. This work was partly supported by a Grant-
in-Aid for Scientific Research on Priority Areas (No. 14078224,
“Reaction Control of Dynamic Complexes”) from the Ministry of
Education, Culture, Sports, Science and Technology of Japan.
Computational resources were provided by RIKEN Advanced
Center for Computing and Communication. Dr. Tardif is gratefully
acknowledged for assistance in the synthesis of 1 and 2.
Supporting Information Available: Detailed experimental pro-
cedures, GPC, NMR, DSC, and powder XRD data of selected polymers,
and computations (PDF). This material is available free of charge via
the Internet at http://pubs.acs.org.
decomposition, the polymerization reaction was then carried out
by addition of the activator A to a mixture of the neutral dialkyl 1
and isoprene, so that the resulting cationic metal alkyl species could
have a chance to react immediately with isoprene, although this
might risk undesired isoprene polymerization caused by A. By use
of this improved experimental procedure, a significant increase in
catalytic activity was achieved without loss of the selectivity, and
more remarkably, the bimodal problem observed above was also
solved successfully (Table 1, runs 8-10 versus 5-7). No polym-
erization caused by A was observed, suggesting that the reaction
of A (or a reaction intermediate of A and isoprene) with 1 to give
an active cationic Y alkyl species is extremely fast and should be
much faster than its reaction with isoprene. Thus, when the
polymerization was carried out at -20 °C by addition of A to a
mixture of 1 (1 molar equiv) and isoprene (600 equiv) in C6H5Cl,
a polyisoprene polymer with almost perfect isotactic 3,4-micro-
structure (3,4-selectivity 100%, mmmm > 99%), high molecular
weight (Mn ) 5 × 105), and unimodal narrow molecular weight
distribution (Mw/Mn ) 1.6) was obtained (Table 1, run 10). No
References
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1
evidence for other structures was observed in H and 13C NMR.
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Since the true active species in the present catalyst system is
extremely unstable, it is very difficult to obtain experimentally a
piece of information about its structure and its interaction with
isoprene. A theoretical study was therefore carried out, which shed
some light on the mechanistic aspects of the polymerization process.
A possible mechanism based on DFT calculation is shown in
Scheme 1. The reaction of the dialkyl complex 1 with an equimolar
amount of A should yield straightforwardly the corresponding
binuclear monocationic monoalkyl species, such as 1a. A DFT
calculation suggested that the remaining alkyl group, CH2SiMe3,
in 1a prefers bridging the two metal centers, Y1 and Y2, via the
methylene carbon and yielding simultaneously an agostic interaction
with Y1 via a Me group, thus leading to 1b.10 These new
interactions make 1b 6.90 kcal/mol more stable than 1a. Because
of the agostic interaction between Y1 and the SiMe3 group, the
coordination of an incoming isoprene molecule could occur
selectively at Y2 in a 4,3-fashion to afford 1c, which after 4,3-
insertion of isoprene would afford 1d. The newly inserted isoprene
unit in 1d could also bridge the two metal centers via its methylene
end, while the agostic interaction between Y1 and the SiMe3 group
remains. Hence, the coordination and insertion of the second
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temperature might be very short, and further decomposition to give an
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however, it might survive for a longer time to initiate isoprene polym-
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(10) Attempts to confirm the alkyl bridge by 13C NMR analysis of the in situ
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because of the poor solubility and instability of the organometallic species.
JA055397R
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