Rhodium-mediated stereoselective polymerization of "carbenes"

Article abstract of DOI:10.1021/ja058722j

Unprecedented rhodium-catalyzed stereoselective polymerization of "carbenes" from ethyl diazoacetate (EDA) to give high molecular mass poly(ethyl 2-ylidene-acetate) is described. The mononuclear, neutral [(N,O-ligand)M¹(cod)] (M = Rh, Ir) catalytic precursors for this reaction are characterized by (among others) single-crystal X-ray diffraction. These species mediate formation of a new type of polymers from EDA: carbon-chain polymers functionalized with a polar substituent at each carbon of the polymer backbone. The polymers are obtained as white powders with surprisingly sharp NMR resonances. Solution and solid state NMR data for these new polymers reveal a highly stereoregular polymer, with a high degree of crystallinity. The polymer is likely syndiotactic. Material properties are very different from those of atactic poly(diethyl fumarate) polymer obtained by radical polymerization of diethyl fumarate. Other diazoacetates are also polymerized. Further studies are underway to reveal possible applications of these new materials.

Full text of DOI:10.1021/ja058722j

Published on Web 07/07/2006
Rhodium-Mediated Stereoselective Polymerization of
“Carbenes”
†
†
†
†
Dennis G. H. Hetterscheid, Coen Hendriksen, Wojciech I. Dzik, Jan M. M. Smits,
†
†
‡
‡
Ernst R. H. van Eck, Alan E. Rowan, Vincenzo Busico, Michele Vacatello,
‡
§
|
Valeria Van Axel Castelli, Annalaura Segre, Erica Jellema,
|
,|
Tom G. Bloemberg, and Bas de Bruin*
Contribution from the Institute for Molecules and Materials (IMM), Radboud UniVersity
Nijmegen, ToernooiVeld 1, 6525 ED Nijmegen, The Netherlands, Dipartimento di Chimica,
UniVersit a` di Napoli “Federico II”, Via Cintia, 80126 Naples, Italy, Istituto di Metodologie
Chimiche, CNR, 00016 Montelibretti, Italy, and Department of Homogeneous Catalysis,
Van ‘t Hoff Institute for Molecular Sciences (HIMS), UniVersity of Amsterdam,
Nieuwe Achtergracht 166, 1018 WV, Amsterdam, The Netherlands
Received January 3, 2006; E-mail: bdebruin@science.uva.nl
Abstract: Unprecedented rhodium-catalyzed stereoselective polymerization of “carbenes” from ethyl
diazoacetate (EDA) to give high molecular mass poly(ethyl 2-ylidene-acetate) is described. The mononuclear,
I
neutral [(N,O-ligand)M (cod)] (M ) Rh, Ir) catalytic precursors for this reaction are characterized by (among
others) single-crystal X-ray diffraction. These species mediate formation of a new type of polymers from
EDA: carbon-chain polymers functionalized with a polar substituent at each carbon of the polymer backbone.
The polymers are obtained as white powders with surprisingly sharp NMR resonances. Solution and solid
state NMR data for these new polymers reveal a highly stereoregular polymer, with a high degree of
crystallinity. The polymer is likely syndiotactic. Material properties are very different from those of atactic
poly(diethyl fumarate) polymer obtained by radical polymerization of diethyl fumarate. Other diazoacetates
are also polymerized. Further studies are underway to reveal possible applications of these new materials.
Introduction
synthesis. The most frequently observed reactions include
carbene dimerization to form olefins, carbene transfer to olefins
The thermal decomposition of diazomethane (reported around
in cyclopropanation reactions, and carbene insertion into O-H,
N-H, and C-H bonds. The reactions are often mediated by
transition metals such as platinum, copper, rhodium acetates,
and ruthenium catalysts, which have been reported to give
efficient and, in many cases, stereoselective conversion of the
carbenoid to more or less sophisticated products.7 In this
framework, though, only three reports describe the polymeri-
zation (oligomerization) of R-carbonyl stabilized “carbenes”
from diazocarbonyl compounds. This concerns Cu- and Pd-
mediated formation of low molecular mass polymers (oligomers)
from alkyl diazoacetates and related diazocarbonyls (number
1
1900) is the first known route to polymethylene. Neither this
2
explosive process, however, nor the milder catalytic versions
ever found large scale practical applications, and it was more
than half a century later that transition metal mediated ethene
polymerization gave easy access to linear polyethylene.
Unlike the inherently unstable diazoalkanes, diazocarbonyl
compounds such as diazoacetates (N2CHCO2R) are reasonably
stable, safe (even in large scale/industrial synthesis), easy to
3
4
,5
6
prepare, and extensively used as carbene precursors in organic
†
‡
§
|
Radboud University Nijmegen.
Universit a` di Napoli “Federico II”.
Istituto di Metodologie Chimiche.
University of Amsterdam.
8,9
average degree of polymerization up to ca. 100). These
materials are notable because they carry a polar functionality
(
1) (a) von Pechmann, H. Ber. Dtsch. Chem. Ges. 1898, 31, 2643. (b)
Bamberger, E.; Tchirner Ber. Dtsch. Chem. Ges. 1900, 33, 956.
(6) (a) Clarck, J. D.; Shah, A. S.; Peterson, J. C. Thermochim. Acta 2002,
392-392, 177-186. (b) Clarck, J. D.; Shah, A. S.; Peterson, J. C.; Patelis,
L.; Kersten, R. J. A.; Heemskerk, A. H. Thermochim. Acta 2002, 386, 73-
79. (c) Clarck, J. D.; Shah, A. S.; Peterson, J. C.; Patelis, L.; Kersten, R.
J. A.; Heemskerk, A. H.; Grogan, M.; Camden, S. Thermochim. Acta 2002,
386, 65-72. (d) Clarck, J. D.; Heise, J. D.; Shah, A. S.; Peterson, J. C.;
Chou, S. K.; Levine, J.; Karakas, A. M.; Ma, Y.; Ng, K.-Y.; Patelis, L.;
Springer, J. R.; Stano, D. R.; Wettach, R. H.; Dutra, G. A. Org. Process
Res. DeV. 2004, 8, 176-185.
(7) (a) Merlic, C. A.; Zechman, A. L. Synthesis 2003, 1137. (b) Ye, T.;
McKervey, A. Chem. ReV. 1994, 94, 1092. (c) Maas, G. Chem. Soc. ReV.
2004, 33, 183.
(8) Liu, L.; Song, Y.; Li, H. Polym. Int. 2002, 51, 1047.
(9) (a) Ihara, E.; Haida, N.; Iio, M.; Inoue, K. Macromolecules 2003, 36, 36.
(b) Ihara, E.; Fujioka, M.; Haida, N.; Itoh, T.; Inoue, K. Macromolecules
2005, 38, 2101.
(2) (a) Feltzin, J.; Restaino, A. J.; Mesrobian, R. B. J. Am. Chem. Soc. 1955,
7
1
1
7, 206-210. (b) Bahwn, C. E. H.; Ledwith, A.; Matthies, P. J. Pol. Sci.
959, 34, 93-108. (c) Cowel, G. W.; Ledwith, A. Q. ReV. Chem. Soc.
970, 24 (1), 119-167 and references therein.
(
3) For recent studies on polymethylene synthesis, see the following. (a) Via
decomposition of diazomethane on TM surfaces: Bai, D.; Jennings, G. K.
J. Am. Chem. Soc. 2005, 127, 3048. (b) Via boron-catalyzed polymerization
of sulfoxonium ylides: Busch, B. B.; Paz, M..M.; Shea, K. J.; Staiger, C.
L.; Stoddard, J. M.; Walker, J. R.; Zhou, X.-Z.; Zhu, H. J. Am. Chem. Soc.
2
002, 124, 3636-3646 and references therein.
(
4) Ziegler-Natta catalyst: (a) Ziegler, K.; Breil, H.; Martin, H.; Holzkamp,
R. German Patent 973726, 1953. (b) Natta, G.; Pino, P.; Mazzanti, G. U.S.
Patent 3715344, 1954. (c), Natta, G. J. Polym. Sci. 1955, 16, 143-154.
5) Phillips catalyst: Hogan, J. P.; Banks, R. L. U.S. Pattent 2,825,721, 1985.
(
(b) McDaniel, M. P. AdV. Catal. 1995, 33, 47-98.
9746
9
J. AM. CHEM. SOC. 2006, 128, 9746-9752
10.1021/ja058722j CCC: $33.50 © 2006 American Chemical Society

Rh-Mediated Polymerization of “Carbenes”
A R T I C L E S
Scheme 1. Synthesis of Catalysts 1a,b and the Structure of 2-4
(amenable to further transformations) at each main chain C
atomsa structure which is not accessible via Ziegler-Natta
4
catalysis and had only been obtained so far from dialkyl
10
maleates or fumarates by free radical processes, with no control
on the resultant polymer stereochemistry. As such, carbene
polymerization seems an attractive route to prepare function-
alized polymers. However, stabilized diazocarbonyl compounds
are notoriously more difficult to polymerize than their nonsub-
stituted diazoalkyl analogues. Presently, there are no catalysts
known that produce high molecular mass or stereoregular
polymethylenes with functional side groups. As a consequence
of their low molecular mass, materials obtained thus far (i.e. in
the above Cu and Pd mediated reactions) are rather unattractive
viscous oils with very broad NMR resonances.
In this article, we report the stereoselective polymerization
of carbenes generated from alkyl diazoacetates to high molecular
mass poly(alkyl 2-ylideneacetate)s (PEA; Mw up to 190 kDa as
white solids with sharp NMR resonances) in the presence of
rhodium-based catalysts.
Figure 1. X-ray structures of 1a, 2a, and 3.
Table 1. Selected Bond Lengths (Å) and Angles (deg) Observed
for Complexes 1-3
Results and Discussion
param
1a (M
)
Rh)
2a (M
)
Rh)
2b (M
)
Ir)
3 (M ) Rh)
M-O1
2.042(2)
2.109(2)
2.095(3)
2.088(3)
2.124(3)
2.115(3)
2.0593(11)
2.1558(14)
2.0986(15)
2.1155(15)
2.1049(17)
2.1432(17)
2.0548(15)
2.152(2)
2.092(2)
2.108(2)
2.099(2)
2.127(3)
2.0597(13)
2.0857(16)
2.1028(19)
2.084(2)
2.1370(19)
2.127(2)
Synthesis and Characterization of the Catalysts. The
M (L-prolinate)(cod)] complexes 1a (M ) Rh) and 1b (M )
M-N
I
[
M-C1
M-C2
M-C5
M-C6
Ir) (cod ) Z,Z-1,5-cyclooctadiene) were prepared by reaction
I
of the [(cod)M (Cl)]2 precursors with in situ deprotonated
L-proline in methanol at room temperature (Scheme 1). The
I
O1-M-N
O1-M-C1
N-M-C1
81.88(8)
163.61(18)
96.58(12)
79.61(5)
155.49(6)
98.84(7)
79.40(7)
155.74(8)
100.00(9)
80.00(6)
164.13(7)
100.45(7)
benzyl analogues [M (N-benzyl-L-prolinate)(cod)] (2a, M ) Rh;
2
b, M ) Ir) and the nonchiral picolinate and quinolinate Rh
analogues 3 and 4 were prepared via similar routes. The
structures of 1-3 were determined by single-crystal X-ray
diffraction (Figure 1; Table 1). The reported structure of 4 is
12
transfer reactions using diazoacetates as carbene precursors.
1
1
very similar.
II
II
It has always been assumed that the Rh -Rh bond of these
catalysts stays intact during catalysis, but no synergistic effects
have so far been proven. Whereas these species have a formal
Complexes 1-4 all adopt a square-planar geometry. The
picolinate ligand in 3 is planar and causes little steric hindrance
to the two trans vacant sites of the Rh center. The aliphatic
prolinate ring of 1a on the other hand is bent, thus shielding
one of the two vacant sites somewhat from its environment.
This effect is more pronounced for the N-benzyl-L-prolinate
analogue 2a. The Ir species 2b is isostructural to its Rh analogue
II
I-
III
Rh oxidation state, they can also be considered as Rh Rh
I
species. Therefore, it is quite possible that mononuclear Rh ,
III
II
Rh , and/or Rh oxidation state species are the actual catalyti-
cally active species. In fact, Bergman, Tilley, and co-workers
II
described in a recent paper that mononuclear Rh species can
2
a. The M-ligand bond lengths of 1-4 are quite comparable.
indeed function as active catalysts for carbene transfer reac-
The variations in the M-N bond lengths in the order NPy <
13
tions. This work inspired us to investigate carbene transfer
2
NHR2 < NR3 are as expected for sp and secondary and tertiary
I
reactions mediated by mononuclear Rh species.
3
sp hybridized nitrogen atoms.
Initially we aimed at catalytic cyclopropanation of olefins,
using ethyl diazoacetate (EDA) as a carbene precursor and the
Polymerization of Carbenes from Diazoacetates. Doyle-
II
II
type dinuclear tetracarboxylate-bridged Rh -Rh species have
been extensively studied as catalysts for a variety of carbene
(
12) See for example: Davies, H. M. L.; Beckwith, R. E. J. Chem. ReV. 2003,
1
03, 2861 and references therein.
(10) Toyoda, N.; Yoshida, M.; Otsu, T. Polym. J. 1983, 15, 255.
11) Leipoldt, J. G.; Grobler, E. C. Inorg. Chim. Acta 1983, 72, 17.
(13) Krumper, J. R.; Gerisch, M.; Suh, J. M.; Bergman, R. G.; Tilley, T. D. J.
Org. Chem. 2003, 68, 9705.
(
J. AM. CHEM. SOC.
9
VOL. 128, NO. 30, 2006 9747

A R T I C L E S
Hetterscheid et al.
Scheme 3. Preparation of Diazocarbonyl Compounds from
Scheme 2. Stereoregular Polymerization of Ethyl Diazoacetate
Dioxenones
contributes to somewhat broader weight distributions (Table 2,
entries 3 and 4). Therefore, we consider the obtained polydis-
persities to be in agreement with a nonliving polymerization
process at a single center active species.
Table 2. Polymerization of EDA with Catalysts 1-4a
reacn
time
yield,
%b
M
w
,
Quite remarkably, 1b (the iridium analogue of 1a) does not
produce any PEA polymer from EDA. Instead, 1b catalyzes
quantitative carbene dimerization to yield a mixture of diethyl
fumarate and diethyl maleate (in a ratio of 1:9). Iridium complex
entry
catalyst
T,
°
C
solvent
kDa
w n
M /M
1
2
3
4
5
6
7
8
9
1a
1a
1a
1a
1a
1a
1a
1a
1a
2a
-20 CHCl3
CHCl3
7 days
3 days
14 h
30 min
14 h
14 h
14 h
14 h
14 h
14 h
14 h
14 h
14 h
50
35
45
5
30
10
40
35
25
25
0
190
169
133
49
100
82
143
130
120
154
3.3
3.0
2.4
2.0
2.2
2.2
2.5
2.4
2.7
2.7
0
20 CHCl3
20 CHCl3
40 CHCl3
20 MeCN
20
20 THF
20
20 CHCl3
20 CHCl3
20 CHCl3
20 CHCl3
20 CHCl3
2b on the other hand, containing a weaker coordinating benzyl-
functionalized N-donor, does produce PEA in low yields (4%).
Both the molecular weight (Mw ) 12 kDa) and the polydisper-
sity (Mw/Mn ) 1.6) of the polymer obtained with 2b are much
lower than those of the polymers obtained with the Rh species.
A variety of diazocarbonyl “monomers” are readily obtained
via cheap and easy routes. Addition of nucleophiles to di-
oxenones, followed by diazo transfer to the resulting â-keto
esters and base-assisted elimination of acetic acid, provides a
CH2Cl2
C7H8
1
1
1
1
1
0
1
2
3
4
1b (M ) Ir)
2b (M ) Ir)
4
15
15
12
132
147
1.6
2.5
2.9
3
4
14 h
7
,14
convenient route to such species (Scheme 3).
Preliminary
a
Conditions: 0.037 mmol of catalysts; 1.84 mmol of EDA, 6 mL of
results reveal that treatment of thus obtained crude 3-butenyl-
diazoacetate (Nu ) -OCH2CH2CHdCH2) and 3-butynyldiazo-
acetate (Nu ) -OCH2CH2CtCH) with 1a yields the corre-
sponding polymers, albeit in low yields (2-8%). Copolymers
from 3-butenyldiazoacetate and EDA (1:10) are obtained in
higher yields (32%). Polymerization of n-butyldiazoacetate (Nu
b
solvent. After precipitation with MeOH.
I
chiral Rh complex 1a as a catalytic precursor. Cyclopropanation
of styrene is indeed mediated by 1a but only in low yields
(
∼12%) with hardly any cis/trans specificity (53:47) and without
any enantioselectivity. As expected, 1a also catalyzes carbene
)
-OBu) gives a yield of ∼20%. Clearly, the scope of the
dimerization yielding diethyl maleate and diethyl fumarate in a
8:42 ratio. However, while 1a mediates full conversion of
EDA, this yields at most 40% diethyl maleate and diethyl
fumarate according to NMR. The remaining material (∼60%)
proved to be the new poly(ethyl 2-ylideneacetate) polymer
catalytic process introduced herein is not limited to EDA, and
we are currently investigating other diazocarbonyl compounds
as carbene precursors in Rh-mediated polymerization reactions.
Characterization of Stereoregular PEA. Quite surprisingly,
the new high molecular weight PEA polymers obtained with
5
(
PEA, Scheme 2). Formation of PEA was further investigated
1
13
catalysts 1-4 reveal sharp resonances in solution H and
C
using the catalysts 1-4.
NMR spectra indicative for stereoregular polymers. The low
molecular weight (Mw ∼ 1.2 kDa) PEA polymers (oligomers)
obtained with Pd-based catalysts reported by Ihara and co-
workers are viscous oils which show very broad NMR
Treating EDA with a catalytic amount of 1-4 (2 mol %)
gave PEA in isolated yields between 10% and 50% depending
on the reaction conditions (Table 2). The polymer was separated
from the reaction mixture by quenching with methanol, yielding
PEA as a white powder. The highest yields in polymer were
observed with 1a in chloroform or dichloromethane. In other
solvents (e.g., THF, acetonitrile, or toluene) yields were lower,
possibly due to partial catalyst precipitation.
Molecular masses of the new PEA polymers were determined
from size exclusion chromatography (SEC) measurements
calibrated with matrix-assisted laser light scattering (MALLS).
At 20 °C, with all four rhodium-based catalysts high molecular
mass polymers were obtained (Mw typically in the range 120-
55 kDa) within 14 h. Not unexpectedly, shortening the reaction
time leads to formation of lower weight polymers in lower yields
Table 2, entry 4). Decreasing the reaction temperature and
increasing the reaction time leads to an increase of the polymer
molecular mass (Mw up to 190 kDa for polymerization with 1a
in chloroform at -20 °C).
9a
resonances. Apart from the possibility that these polymers are
atactic, the broadening of the NMR signals of these low
molecular weight polymers could also be caused by chain-end
effects. For none of the PEA samples obtained with the Rh and
Ir catalysts 1-4 we were able to detect polymer chain-end
signals or indirect chain-end effects with NMR. To investigate
tacticity effects, we prepared atactic PEA by radical polymer-
9a,10
ization (initiated by AIBN) of diethyl fumarate.
These atactic
PEA samples have a larger molecular weight (Mw ) 6.1 kDa)
but still reveal very broad NMR resonances. NMR spectra of
the new stereoregular PEA polymers obtained with catalysts
1
1
-4 are clearly different from those of atactic PEA (Figure 2).
Even at 600 MHz for H NMR and 150 MHz for C NMR,
(
1
13
the main chain methine signal of stereoregular PEA appears as
a highly symmetrical singlet, with a half-height width of 11
and 20 Hz, respectively. A weak shoulder at higher frequency
The polydispersities of the polymers obtained with the Rh
catalysts are all slightly higher than 2.0. The polymerization
experiments were run with full conversion of EDA, which
(
14) (a) Clemens, R. J.; Hyatt, J. A. J. Org. Chem. 1985, 50, 2431. (b) Li, G.-
Y.; Che, C.-M. Org. Lett. 2004, 6, 1621.
9748 J. AM. CHEM. SOC.
9
VOL. 128, NO. 30, 2006

Rh-Mediated Polymerization of “Carbenes”
A R T I C L E S
fine structures for the methylene (δ ) 59.6-63.7 ppm) and
methyl (δ ) 12.8-14.6 ppm) peaks (Supporting Information,
Figure S6 and S7).
By deconvolution of the methine resonance in a quantitative
1
3
single pulse C MAS NMR spectrum (recycle time, 60 s), the
relative abundance of the crystalline phase was estimated to be
1
3
slightly above 70%. In a single pulse C MAS NMR spectrum
run with very short recycle time (1 s), so as to minimize the
contribution of more rigid, ordered structures, which generally
1
6
tend to have a longer T1 relaxation time, the methylene and
methyl resonances turned out to be clearly stronger than the
methine and carbonyl ones, which suggests some conformational
disorder of the ethyl residue even in the crystalline domains.
Powder X-ray diffraction spectra of stereoregular PEA reveal
strong but relatively broad peaks (Supporting Information,
Figure S9). This seems in good agreement with a high degree
of crystallinity but with substantial disorder of the ethyl residues.
Stereoregular PEA can in principle be isotactic or syndiotactic.
Preliminary molecular mechanics (MM2) calculations on model
compounds indicated that low-energy conformations with a two-
backbone-bond conformational repeat are found for both
polymers in the TG (trans-gauche) and GT domains; this is
consistent with a shift to lower frequency of the methine peak
for the ordered phase relative to the disordered one in the CP/
MAS NMR spectrum. The H-C-CdO torsions are all close
to anti (anti ∼ 160°) in the isotactic polymer, whereas their
sequence in the syndiotactic polymer is ...syn, syn, anti, anti,
syn, syn... (syn ∼ 30°, anti ∼ 170°), with the syn, anti or anti,
syn arrangements flanking the main chain T bonds. The SCF-
MO calculated NMR spectra are compatible with the experi-
mental one for both structures; therefore additional information
is needed for a definitive assignment of the configuration. We
Figure 2. 13C NMR spectra of atactic PEA obtained with radical
polymerization of diethyl fumarate (top) and stereoregular PEA obtained
by Rh-mediated polymerization of EDA (catalyst 1a).
recently prepared carbonyl C-labeled samples of the new
stereoregular PEA polymers, and we are currently investigating
13
their configuration by 2-dimensional carbon-carbon correlation
17
experiments (DOQSY). Analyzing the results of these experi-
ments is not straightforward and requires elaborate data-fitting.18
These results will be published in a subsequent paper.
(
a) Thermal Analysis. Stereoregular PEA as obtained by
precipitation from the reaction mixture with MeOH is largely
amorphous. However, slow evaporation of the solvent from a
solution of stereoregular PEA in chloroform results in a
“
crystallized” brittle film of the polymer. These “amorphous”
and crystallized samples of stereoregular PEA have different
properties.
Figure 3. 13C CP/MAS NMR spectrum of a typical PEA sample.
Thermogravimetric measurements (TGA) reveal that both the
amorphous and crystallized PEA samples decompose above 300
1
of the H NMR signal is possibly traceable to stereo defects (in
°
C, but DSC data for these samples at lower temperatures are
all cases well below 10%, as estimated by peak deconvolution).
Solid-state NMR is consistent with the above conclusion and
clearly different from each other. The amorphous samples
showed a reversible first-order phase transition at 100 °C/75
1
5
provides more information. In the CP/MAS NMR spectrum
Figure 3), the methine signal reveals two sharp resonances (δ
43.73 and 45.30 ppm) in an approximate 1:1 ratio, partly
overlapped by a weaker and broader peak at higher frequency
δ ) 47.8 ppm). The former can be attributed to a crystalline
phase, while the latter represents an amorphous fraction. Fourier
transformation with a resolution enhancing window function
revealed a similar splitting for the carbonyl peak (1:1 sharp
doublet at δ ) 171.0 and 171.8 ppm) and more complicated
°
C on heating/cooling (Supporting Information, Figure S11).
(
)
In the first heating/cooling cycle these transitions are very weak
-
1
but become stronger (∆h ) 21 J g ) in a second cycle after
annealing the polymer for a few seconds at 240 °C.
(
Samples which were first crystallized from CHCl3 reveal quite
pronounced (∆h ) 24 J/g) and sharp DSC transitions from the
beginning, without the need to anneal the polymer (see Figure
(
16) Fyfe, C. A.; Lyerla, J. R.; Volksen, W.; Yannoni, C. S. Macromolecules
1
979, 12, 757-761.
(
15) Bovey, F. A.; Mirau, P. A. In NMR of Polymers; Academic Press: San
(17) Schmidt-Rohr, K. Macromolecules 1996, 29, 3975-3981.
(18) Van Beek, J. D.; Meier, B. H. J. Magn. Res. 2006, 178, 106-120.
Diego, CA, 1969; Chapter 4.
J. AM. CHEM. SOC.
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VOL. 128, NO. 30, 2006 9749

A R T I C L E S
Hetterscheid et al.
Figure 4. DSC 2nd heating and cooling curves of a “crystallized” sample
of stereoregular PEA (entry 3, Table 2, after dissolution in chloroform and
slow evaporation of the solvent).
4). The transitions of the crystallized samples also occur at
higher temperatures (150 °C/110 °C) compared to those of the
amorphous samples (100 °C/75 °C). Since the polymer is a
flexible rubber above 155 °C, whereas it is hard and brittle below
7
0 °C, the transitions at 150 °C/110 °C for crystallized PEA
and 100 °C/75 °C for amorphous PEA could be interpreted as
the softening and glass-transition temperatures, in accord with
reported glass-rubber transitions of atactic PEA (between 65
Figure 5. MALDI-TOF spectra (reflection mode) of PEA obtained with
catalyst 2b (Table 2, entry 12).
10
and 80 °C). The fact that crystallized samples of stereoregular
PEA reveal transitions at higher temperatures compared to the
amorphous samples and the fact that these transitions appear
as real peaks in the DSC could point to a substantial influence
of the glass-rubber transitions by the crystalline domains of
this material.19 However, at this point we cannot exclude an
alternative interpretation of the data, in which the DSC peaks
correspond to crystalline-liquid crystalline transitions instead
of glass-rubber transitions. We are currently investigating this
possibility.
even the amorphous samples reveal transitions at higher
temperatures than atactic PEA. However, since we have no
reference material for glass-rubber transitions of atactic,
isotactic, and syndiotactic carbon-chain polymers which are
substituted at each carbon of the polymer backbone (nor
information excluding the alternative interpretation of the
transitions as crystalline-liquid crystalline transitions), these
data are only an indication for the possible configuration of
stereoregular PEA.
Under a polarization microscope, the crystallized PEA
samples show birefringence. Above the softening temperature,
also the amorphous PEA samples start to show birefringence
but only after physical contact. We take the birefringence as a
further indication for the semicrystalline behavior of stereo-
regular PEA (or perhaps thermotropic liquid-crystalline behavior
in the rubber state).
The reported softening (78-80 °C) and glass transition
temperatures (65 °C) of atactic PEA (obtained by radical
polymerization of diethyl maleate with AIBN)1 are somewhat
lower than the above transitions of our amorphous stereoregular
PEA. For bulky polymers, containing only one repeating side
group/two carbons of the polymer backbone, the highest glass
transition temperatures are usually observed for the atactic and
0
1
9,20
syndiotactic polymers.
(
For poly[methyl(methacrylate)]
for example, quite large differences in glass
transition temperatures are reported for the atactic (∼90 kDa,
Tg ) 105 °C), isotactic (30 kDa, Tg ) 56 °C), and
PMMA)2
1,22
(b) MALDI-TOF. The polymers prepared with Rh catalysts
1
a, 2a, 3, and 4 invariably give high molecular weight PEA
2
2
21b
polymers with rather broad molecular weight distributions (Mw/
Mn ) 2-3; see Table 2). As expected, we were unable to
measure any reliable MALDI-TOF mass spectra from these PEA
samples. However, MALDI-TOF spectra of PEA prepared with
Ir catalyst 2b (Mw ) 12 kDa, Mw/Mn ) 1.6) were readily
obtained, albeit only in the lower mass region of the polymer.
As shown in Figure 5, PEA is clearly built from ethyl
21b
syndiotactic (60 kDa, Tg ) 140 °C) polymers. Van der Waals
forces between polymeric chains in amorphous domains are
stronger for the less symmetrical atactic and syndiotactic
polymers as compared to isotactic polymers. If one reasons in
analogy (and assumes that the transitions belong to glass-rubber
transitions), stereoregular PEA might be syndiotactic because
2
-ylideneacetate (:CHCOOEt) repeating units, resulting in a
(
19) Stevens, M. P. Polymer Chemistry, An introduction, 3rd ed.; Oxford
University Press: New York, 1999.
regular 86 Da spaced repeating pattern. Fragments with an equal
number degree of polymerization (n) give rise to three masses:
(CHCOOEt)n + 25 + x × 86 Da (dominant signals), (CHCOO-
Et)n + 41 + x × 86 Da (weaker), and (CHCOOEt)n + 56 + x
× 86 Da (least intense).
(
20) Burfield, D. R.; Doi, Y. Macromolecules 1983, 16, 702-704.
21) (a) Collins, S.; Ward, D. G. J. Am. Chem. Soc. 1992, 114, 5460. (b) Bolig,
A. D.; Chen, E. Y. X. J. Am. Chem. Soc. 2001, 123, 7943. (c) Deng, H.;
Shiono, T.; Soga, K. Macromolecules 1995, 28, 3067. (d) Cui, C.; Shafir,
A.; Reeder, C. L.; Arnold, J. Organometallics 2003, 22, 3357.
22) Porter, C. E.; Blum, F. D. Macromolecules 2000, 33, 7016.
(
(
9
750 J. AM. CHEM. SOC.
9
VOL. 128, NO. 30, 2006

Rh-Mediated Polymerization of “Carbenes”
A R T I C L E S
Scheme 4. Proposed Formation of PEA via Insertion
Polymerization of “Carbenes” (Propagation Steps)
is somewhat blurred (no clear intensity change upon addition
+
23
of Ag and overlapping signals of Ir-based alternatives).
At this point we cannot entirely exclude the possibility of
organometallic rhodium or iridium complexes being present as
polymer chain ends. PEA samples obtained with catalyst 1a
reveal the presence of 0.09% Rh, which corresponds to
approximately 1 equiv of Rh/polymer chain. Therefore, a final
assignment of the polymer end groups (i.e. Rh/Ir based, only
H-PEA-OH or a mixture of both H-PEA-OH and H-PEA-
H) requires additional data from high-resolution MS spectra
Figure 6. Experimental MALDI-TOF MS spectra of PEA (reflection mode)
without (top, left) and with addition of Ag (bottom, left) as a cationization
+
agent and calculated isotope distributions of H-PEA-H and H-PEA-
+
OH chains with an Ir complex (top, right) and Ag (bottom, right) as charge
carriers.
(
requiring polymer samples with a lower polydispersity) and/
Although the masses of the more intense signals correspond
or NMR characterization of lower molecular weight polymers.
Mechanistic Considerations. At this point we do not
understand all details of the formation of PEA. We need a high
catalyst loading of 2% to obtain complete conversion of EDA.
Half of the starting material converts to diethyl maleate and
diethyl fumarate. Since polymers consisting of about 1000
CHCOOEt repeating units are formed, only about 3% of the
added catalytic precursor becomes really active as a polymer-
ization catalyst. The exact nature of the active species is
therefore unknown, and in part we necessarily rely on (educated)
speculations in the mechanistic considerations below.
+
with [Na] {(H-CHCOOEt)n-H)} (e.g. m/z ) 3467 Da, with
+
n ) 36) and [Na] {(H-(CHCOOEt)n-OH)} (e.g. m/z ) 3483
Da, with n ) 36), these interpretations do not explain their
typical isotope distribution patterns. These patterns are sug-
gestive for the presence of cationic iridium complexes as
charge carriers (Figure 6). The signals could well belong to
III
+
[
3
(
Ir (OH)2(cod)(MeOH)] {(H-(CHCOOEt)n-H)} (e.g. m/z )
III
+
467 Da, with n ) 36) and [Ir (OH)2(cod)(MeOH)] {(H-
CHCOOEt)n-H)} (e.g. m/z ) 3483 Da, with n ) 36), but with
different Ir-bound ligands alternative interpretations are pos-
sible.23 Addition of K salts does not influence the spectrum.
+
The nature of the catalyst has little influence on the obtained
stereoregular PEA. The Rh-based catalytic precursors 1a, 2a,
+
Addition of Ag salts does not change the peak positions but
does influence their relative intensities, leading to significantly
increased intensities of the (CHCOOEt)n + 41 + x × 86 Da
signals (e.g. 3483 Da). Also the isotopic distribution patterns
3
, and 4 produce roughly the same molecular weight PEA with
comparable polydispersities. Only the Ir-based precursor 2b
produces somewhat lower molecular weight PEA. There are
no obvious differences in the properties between the obtained
polymers (including NMR data), suggesting that identical or at
least very similar active species are generated. This likely
involves dissociation of the N,O-ligand from the metal under
catalytic conditions. However, the precursors 1a, 2a, 3, and 4
produce PEA in different yields, thus suggesting that the N,O-
ligands do have an influence on the kinetics of initiation. This
might, for example, involve formation of a reactive metal-
alkyl by attack of a nitrogen donor (as in the Pd-based systems
+
appear to be different in the presence of Ag . The coinci-
dentally unchanged masses with slightly different isotopic
+
distribution patterns are suggestive for the presence of [Ag] {(H-
+
CHCOOEt)n-H)} and [Ag] {H-(CHCOOEt)n-OH}. Consid-
ering the increased intensity of the (CHCOOEt)n + 41 + x ×
+
8
6 Da signals in the presence of Ag , it seems quite safe to
conclude that H-(CHCOOEt)n-OH chains are present in the
polymer samples. The MS data are also suggestive for the
copresence of H-(CHCOOEt)n-H chains, although this picture
9
of Ihara et al.) or oxygen donor of the N,O-ligand to a metal-
(
23) Assuming the presence of iridium, the observed masses could (among
III
+
bound carbene. The MALDI-TOF spectra further suggest the
presence of -H and -OH end groups. Chain growth might thus
start from M-OH and/or M-H species, while termination might
involve reductive elimination of M(H)(Cchain) fragments, pro-
tonation of M-Cchain fragments, and/or oxidation (by e.g. O2).
Formation of a rhodium-bound carbene is commonly believed
to be a key step in rhodium-catalyzed carbene transfer reactions
others) correspond to the following: [Ir (OH)
2
(cod)(MeOH)] {(H-
III
(cod)(MeOH)]+ {(H-
(
(
(
(
(
CHCOOEt)
CHCOOEt)
CHCOOEt)
CHCOOEt)
CHCOOEt)
n
-H)} (3467 Da for n ) 36); [Ir (OH)
2
II
+
n
-H)} (3483 Da for n ) 36); [Ir (OH)(cod)(MeOH)
2
] {(H-
III
+
n
-H)} (3498 Da for n ) 36); [Ir (OH)
2
(H
(H
2
O)
3
3
] {H-
III
+
n
-H} (3467 Da for n ) 37); [Ir (OH)
2
2
O)
] {H-
III
+
n
-OH} (3483 Da for n ) 37); [Ir (OH)
2
(H
(MeOH)(H
(MeOH)
2
O)
3
(HOMe)] {H-
III
+
2 2
O) ] {H-
+
CHCOOEt)
n
H} (3499 Da for n ) 37); [Ir (OH)
2
III
(
(
(
CHCOOEt)
CHCOOEt)
CHCOOEt)
n
-OH] (3497 Da for n ) 36); [Ir (OMe)
-H] (3483 Da for n ) 36); [Ir (cod)(MeOH)(OH
2
4
]
{H-
+
I
n
2
)] {H-
n
-OH] (3483 Da for n ) 36).
J. AM. CHEM. SOC.
9
VOL. 128, NO. 30, 2006 9751

A R T I C L E S
Hetterscheid et al.
Conclusions
Rh-mediated polymerization of carbenes, generated from ethyl
diazoacetate, results in formation of a new highly stereoregular
polymer with a high molecular mass: poly(ethyl 2-ylidene-
acetate) (PEA). Diazoacetates containing other ester function-
alities can also be polymerized. The polymers contain polar
functional side groups at each carbon of the carbon-chain
backbone.
Figure 7. “Chain-end control” in the stereoselective polymerization of
“
carbenes” from EDA leading to syndiotactic PEA.
PEA is obtained as a white amorphous solid but can be
crystallized to obtain a material with a high degree of crystal-
linity (∼70%). The stereocontrol in the formation of PEA most
likely involves propagation under chain-end control. Therefore,
stereoregular PEA is likely syndiotactic, which seems in
agreement with the relatively high Tg of the material.
with diazoacetates.24 PEA might thus be formed via consecutive
migratory) R-insertions of metal-bound carbenes into the Rh-C
bond of the growing chain (Scheme 4).
a) Stereocontrol. Since no ancillary ligand effect on the
(
(
stereoselectivity was observed, we propose that the insertions
into the Rh-CH(COOEt)P fragment (P ) polymeryl) are
sterically controlled by the configuration of the Rh-bound carbon
atom of the growing chain (propagation under “chain-end
control”). This is illustrated by the Newman projections in
We expect that these new polymers, having a unique structure,
will have special material properties. Further studies, in which
we focus of the structure and properties of these new materials,
are underway.
25
Figure 7. Stereoselective propagation under chain-end control
has been observed in a number of insertion polymerizations,
including the Pd-mediated alternating copolymerization of CO
with styrene.26
Acknowledgment. This work was supported by The Neth-
erlands Organization for Scientific Research (NWO-CW),
the Radboud University of Nijmegen, and the University of
Amsterdam. We thank J. Opsteen, R. Bovee, and Dr. A. Kros
for their assistance with the SEC/MALLS measurements. We
thank P. J. Aarnoutse for her assistance with the MALDI-TOF
measurements.
Migratory carbene insertions involving stereogenic Rh-
CH(COOEt)P methine carbons (*C) in configuration S experi-
*
ence least steric hindrance when the re-face of the rhodium-
carbene is attacked. This yields a new Rh-*CH(COOEt)P
methine carbon in configuration R, which in turn preferentially
attacks the si-face of a carbene (Figure 7). We thus expect
formation of a syndiotactic polymer, having a regular arrange-
ment of alternating R and S configurations concerning the
stereogenic carbons of the polymer backbone.27 This seems in
agreement with the DSC data, but a definitive assignment of
the polymer configuration still requires additional investigations.
Supporting Information Available: Experimental procedures
and crystallographic data for 1a, 2a,b, and 3, details on SEC/
MALLS, NMR, X-ray diffraction, and DSC/TGA polymer
characterization, and MM2 and SCF-MO NMR property
calculations. This material is available free of charge via the
Internet at http://pubs.acs.org.
(
24) Maxwell, J. L.; Brown, K. C.; Bartley, D. W.; Kodadek, T. Science 1992,
2
56, 1544-1547.
JA058722J
(
25) Bovey, F. A.; Tiers, G. V. D. J. Polym. Sci. 1960, 44, 173.
(
26) (a) Corradini, P.; De Rosa, C.; Panunzi, A.; Petrucci, G.; Pino, P. Chimia
1
990, 44 (3), 52-54. (b) Wong, P. K.; Drent, E.; Pino, P.; Corradini, P.;
(27) IUPAC Glossary of Basic Terms in Polymers Science. Pure Appl. Chem.
1996, 68/12, 2287-2311.
Petrucci, G. Eur. Pat. Appl. 410543, 1991.
9752 J. AM. CHEM. SOC.
9
VOL. 128, NO. 30, 2006