Remote substituents controlling catalytic polymerization by very active and robust neutral nickel(II) complexes

Article abstract of DOI:10.1002/anie.200352062

Remote control: Substituents strongly affect the catalytic properties of complexes 1 in ethylene polymerization, despite their remoteness from the active center. An appropriate substitution pattern provides very active and robust catalysts.

Full text of DOI:10.1002/anie.200352062

Angewandte
Chemie
Polymerization Catalysis
position of the O donor in neutral nickel(ii) complexes has
been reported to increase catalytic activities substantially,
again in accordance with theoretical calculations.[6c,7a,9] Most
specifically for this class of catalysts, Grubbs and co-workers
have shown that bulky groups in the C3 position of the O-
coordinating phenolate moiety of salicylaldimine ligands
substantially increase polymerization activity. While these
ligands afford highly active catalysts, their syntheses require
multistep procedures with very low yields.[6c,d]
Remote Substituents Controlling Catalytic
Polymerization by Very Active and Robust
Neutral Nickel(ii) Complexes**
Martin A. Zuideveld, Peter Wehrmann, Caroline Röhr,
and Stefan Mecking*
Our particular interest in the design of novel NiII
salicylaldiminato complexes stems from the recent finding
that the known isopropyl-substituted complexes enable the
synthesis of latexes of high-molecular-weight polyethylene,
which are, to date, inaccessible by other techniques.[3d] Such
polyolefin latexes can provide environmentally friendly and
economically attractive coatings, which, for example, can be
stable towards UV light and hydrolysis at the same time in
contrast to current commodity coatings.[10] In view of appli-
cations, a very active catalyst based on conveniently acces-
sible ligands, and that is suited to polymerization in emulsion
to higher-molecular-weight polyethylene is a prerequisite.
Such a system is equally attractive for fundamental studies of
catalytic polymerization in emulsion, in which well-defined
catalyst precursors are also desirable. Our investigations
subject to this report were initiated by the reasoning that an
aryl substituent with strongly electron-withdrawing groups
could provide steric bulk and electron withdrawing properties
at the same time.
More than 70 million tons of polyethylene and polypropylene
are produced annually. The majority is prepared by catalytic
polymerization employing Ziegler or Phillips catalysts based
on early transition metals. More recently, olefin polymer-
ization by complexes of late transition metals has also
received increasing attention.[1] A major motivation is their
higher tolerance towards polar reagents due to a reduced
oxophilicity by comparison to early transition-metal catalysts.
Thus, ethylene and 1-olefins can be copolymerized with
acrylates in a random fashion,[2] and ethylene homo- and
copolymerizations can be carried out in aqueous emulsion to
afford polymer latexes (i.e., aqueous dispersions of polymer
particles of about 50–1000 nm size).[3]
The discovery by Brookhart and co-workers of the unique
catalytic properties of cationic nickel and palladium diimine
catalysts in olefin polymerization has given a strong impulse
to the field.[4] As a result, polymerization with neutral NiII
complexes has received renewed interest, as these catalysts
are expected to be more functional-group tolerant than their
cationic NiII counterparts. However, catalyst activity and
stability over time and the capability to form polymers with
higher molecular weights at the same time are critical issues,
particularly if the effort for catalyst synthesis is also consid-
ered.[5–7] By analogy with the influence of bulky alkyl[4] or
aryl[8] groups in cationic diimine complexes, in neutral NiII k2-
N,O salicylaldiminato complexes bulky isopropyl groups on
the N-aryl moiety retard chain transfer, which is supported by
computational studies by Ziegler and co-workers.[9] Introduc-
tion of electron-withdrawing substituents in the ortho or para
Suzuki coupling[11,8a] provided a convenient synthetic
method for the introduction of electron-withdrawing substi-
tuted aryl groups in the C2 and C6 position of the aniline aryl
ring (Scheme 1). A series of salicylaldimine ligands with
systematically varied electronic properties, 1a–e, resulted
from the condensation of the corresponding substituted
anilines with 3,5-diiodo-salicylaldehyde. The 13C NMR reso-
nances of the compounds were fully assigned by 1H–1H
1
1
COSY, heteronuclear H-13C 2D NMR and H–13C 2D long-
range-coupling NMR spectroscopy. The chemical shifts of the
carbon atom para to the imine function in 1a–e (atom labeled
p in Scheme 1) are d = 126.90, 126.96, 126.55, 126.48, and
=
126.56 ppm, respectively, and for the imine carbon atom, C
[*] Dr. M. A. Zuideveld, Dipl. Chem. P. Wehrmann,
Priv.-Doz. Dr. S. Mecking
N, d = 168.42, 168.05, 166.99, 166.23, and 166.26 ppm were
observed. Although the differences in chemical shifts are
moderate, this trend follows the electron withdrawing/donat-
ing character of the R group and indicates that the electronic
character of the substituents R in 1 indeed affects the
electronic properties of the neighboring aryl ring and the
imine function.
Institut für Makromolekulare Chemie und Freiburger Material-
forschungszentrum der Albert-Ludwigs-Universität Freiburg
Stefan-Meier-Strasse 31, 79104 Freiburg (Germany)
Fax: (+49)761-203-6319
E-mail: stefan.mecking@makro.uni-freiburg.de
Prof. Dr. C. Röhr
Institut für Anorganische Chemie der
Albert-Ludwigs-Universität Freiburg
Albertstrasse 20, 79104 Freiburg (Germany)
Reaction of 1a–e in diethylether with [(tmeda)-
Ni(CH3)2][12]
(tmeda = N,N,N’,N’-tetramethylethylenedi-
[**] We thank BASF AG and the Deutsche Forschungsgemeinschaft
(Me1388/3 in the AM2Net) for financial support. M.A.Z. is grateful
for a research stipend by the Alexander von Humboldt-Foundation.
We thank G. Mörber for technical assistance and C. Stoz for
participation in this research as part of her undergraduate studies.
Catalyst deactivation studies by A. Warmbold, 2D NMR measure-
ments by M. Keller and high-temperature GPC by D. Lilge (Basell
GmbH) are gratefully acknowledged.
amine) in the presence of excess pyridine[3c,6b,6d,6g] afforded
the neutral methylnickel(ii) complexes 2a–e in high yield
(Scheme 1). The molecular structure of 2a and 2c was
determined by single-crystal X-ray crystallography
(Figure 1).[13,14] To our knowledge, these are the first examples
of structurally characterized neutral methylnickel complexes,
which are precursors to very active olefin polymerization
catalysts. Such methyl complexes are of particular interest, in
comparison to the more frequent phenyl complexes[5–7]
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2004, 43, 869 –869
DOI: 10.1002/anie.200352062
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Scheme 1. Synthesis of ligands 1a–e and of methylnickel(ii) complexes 2a–e (py=pyridine).
[(XO)NiPh(L)] (X = N or P), as the methyl group resembles
the growing polymer chain much more closely than a phenyl
group. For example, a hindered activation for polymerization
À
due to slow insertion of ethylene in the Ni C bond of the
catalyst precursor need not be considered.[5e,15] The nickel-
bound methyl group is located in the trans position to the
O donor in both complexes 2a and 2c. A strong steric
shielding of the apical positions of the metal center by the aryl
groups bound to the C2,C6-position of the N-aryl moiety is
evident for both compounds.
Complexes 2a–e, and for comparison also the known
complex 2 f,[3c] were employed as precursors for ethylene
polymerization (Table 1). Alternatively, the mixing of ligands
1a–f with one equivalent of [(tmeda)Ni(CH3)2] in toluene
solution under an ethylene atmosphere afforded an in situ
catalyst (entry 3), which allowed a convenient rapid prescre-
ening of these and other ligands in automated parallel
pressure reactors.[16] The activity of all catalysts with 3,5-
substituted aryl moieties (R,R’ = CF3, Me and OMe; 2a, 2d,
and 2e) is much higher by comparison to the well-known
isopropyl substitution pattern represented by 2 f (entries 1, 7,
8 and 9). Catalysts formed from 2b and 2c, which do not bear
substituents in the 3,5-position (R,R’ = H) or are only
monosubstituted (R = NO2, R’ = H), respectively, appear
much less active. The catalyst stability over time was studied
by monitoring the ethylene uptake by means of a mass flow
meter. Whereas 2a, 2d, 2e, and 2 f remain active for hours at
608C and 10 bar ethylene pressure, 2b and 2c are deactivated
completely within 20 minutes (see Supporting Information).
The initial activities are similar for all five catalysts 2a–e, and
thus the lower average activity of 2b and 2c in the 30 min
experiments given in Table 1 is due to deactivation rather
than to a strong difference in intrinsic activity between the
À
five catalysts. Possibly, a deactivation of 2b and 2c by C H
activation of the hydrogen atom in meta position of the phenyl
rings occurs.[17] Alternatively, the somewhat higher steric bulk
in 2a, 2d, and 2e may prevent possible bimolecular[6d]
deactivation reactions. As for the known isopropyl-substi-
tuted catalysts,[3c] activities of the aryl-substituted catalysts
Figure 1. X-ray crystal structure of 2a (top) and 2c (bottom; ORTEP
presentation with thermal ellipsoids drawn at the 50% probability
level). Hydrogen atoms are omitted for clarity.
870
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Table 1: Results of ethylene polymerization reactions.[a]
Entry Catalyst precursor Amount of cata- Pressure [bar] Polymer
À1 [c]
¯
¯
¯
Mn
TOF [mol(C2H4) Mn [gmol
mol(Ni)À1 hÀ1 [b]
]
Mw/
Branching (per
1000C)[d]
Tm [8C]
[c]
(R/R’)
lyst [mmol]
yield [g]
]
1
2
3
4
5
6
7
8
9
2a (CF3)
2a (CF3)
1a[e] (CF3)
2b (NO2)[i]
2c (H)
40
19
56
40
40
56
40
40
40
19
40
5
5
40
40
5
40
40
40
40
23.4
2.2
7.3
3.0
9.0
41800
8270
9320
5360
16080
3200
42800
44600
7500
1.9”104
1.6”104
3.2”104
1.1”104
2.9”103
4.0”103
1.1”103
1.9”103
1.2”104
1.8”104
5.1
6.7
8.2
2.6
2.3
2.3
2.1
2.5
2.5
3.1
10
10
14
26
52
54
76
79
5
123
124
125
106
78
2c (H)
2.5
68
[f]
2d (Me)
2e (OMe)
2 f
24.0
25.0
4.2
–
[f]
–
127
122
10[g] 2a (CF3)
3.0[h]
5640
14
[a] Reaction conditions: 100 mL of toluene, 508C, reaction time 0.5 h. [b] TOF=average turnover frequency. [c] Determined by GPC versus linear
polyethylene standards. [d] Branches per 1000 carbon atoms (predominantly methyl branches). [e] In situ catalyst (1 + [(tmeda)Ni(CH3)2]).
[f] Completely amorphous polymer. [g] Reaction in aqueous emulsion; reaction time 1 h. [h] Formed as an aqueous polymer dispersion.
[i] Monosubstituted, R=NO2, R’=H.
2a–e are strongly dependant on ethylene concentration
(entries 1, 2, 5 and 6).
Ethylene polymerization with 2a in aqueous emulsion was
investigated. By comparison to polymerization with the
isopropyl-substituted complex 2 f,[3d] activities are increased
five-fold in preliminary experiments without further optimi-
zation. In experiments with reaction times from 1 to 5 h the
polymer yield increased linearly with time, thus demonstrat-
ing the robustness of the catalyst system based on 2a during
polymerization in aqueous emulsion (Figure 2).
Surprisingly, despite their spatial remoteness from the
metal center the nature of the substituents R,R’ has a
dramatic effect on branching and thus crystallinity, and on
polymer molecular weight (Table 1). With 2a a semicrystal-
line, stiff polymer of 50% crystallinity (determined by
differential scanning calorimetry, DSC) is obtained. Com-
plexes 2b and 2c afford polyethylenes with a considerably
higher degree of branching, and correspondingly low crystal-
linities (18% and < 10%, respectively), and with 2d and 2e
an entirely amorphous material is obtained. At the same time,
the molecular weight of the polymer decreases by more than
an order of magnitude going from 2a to 2e. The origin of the
relatively broad molecular weight distributions obtained with
2a is currently unclear. Given the similar intrinsic activities of
all catalysts, the lower molecular weight obtained with 2d and
2e cannot result from a lower rate of chain growth, but must
indeed result from a higher rate of chain transfer. This is also
in reasonable accordance with the higher branching observed:
methyl branching originates from a b-hydride transfer and a
subsequent 2,1-reinsertion of the resulting metal-bound 1-
olefin, and b-hydride transfer is also crucial in chain trans-
fer.[18] Whereas the steric requirements of the substituents
R,R’ are roughly similar in 2a (R,R’ = CF3), 2d (R,R’ = CH3)
and 2e (R,R’ = OCH3) and higher than in 2c (R,R’ = H),[19]
their electron withdrawing character increases in the
sequence 2e ꢀ 2d < 2c < 2b ꢀ 2a.[20] Both the molecular
weights and the branching of the polymers obtained with
these different catalyst precursors vary systematically with the
electronic nature of R rather than their steric requirements.
This indicates that the effect of these remote substituents is
related to their electronic properties. To our knowledge, there
is no precedent for such a pronounced and systematic effect of
such remote substituents on catalytic olefin polymerization by
late transition-metal complexes, despite the enormous recent
research efforts directed towards the latter field.[1] Future
theoretical studies may provide insight on the correlation of
the substitution pattern of the particular ligands and the
reactivity of the metal center.
Figure 2. Polymer yield in polymerization in aqueous emulsion with
catalyst precursor 2a (ethylene pressure: 40 bar. Reaction temperature:
508C. Dotted line given only as a visual aid).
In summary, we present very active neutral methylnick-
el(ii) complexes for ethylene polymerization based on a set of
conveniently accessible ligands. A series of well-defined
catalyst precursors with a systematically varied substitution
pattern reveals a surprising and unprecedented effect of
remote substituents on polymer branching and molecular
weight, despite their spatial remoteness from the catalytically
active center. An appropriate substitution pattern provides a
catalyst that combines the capability to polymerize ethylene
to higher-molecular-weight polymer with a high stability and
activity in polymerization in aqueous emulsion.
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(W. R. Grace), WO 98/42664, 1998; f) F. A. Hicks, M. Brookhart,
Organometallics 2001, 20, 3217 – 3219; g) D. L. Schröder, W.
Keim, M. A. Zuideveld, S. Mecking, Macromolecules 2002, 35,
6071 – 6073; h) E. F. Connor, T. R. Younkin, J. I. Henderson, S. J.
Hwang, W. P. Roberts, J. J. Litzau, R. H. Grubbs, J. Polym. Sci.
Part A 2002, 40, 2842 – 2854.
Experimental data is given in the Supporting Information:
Synthesis and analytical data (1H and 13C NMR data and
elemental analysis) of 1a–e and 2a–e; polymerization proce-
dures; investigation on catalyst stability over time for 2a and
2c and ORTEP plots of 2a and 2c with full labeling schemes.
[7] recent work with k2-P,O phosphinoenolato complexes: a) R.
Soula, J.-P. Broyer, M. F. Llauro, A. Tomov, R. Spitz, J. Claverie,
X. Drujon, J. Malinge, T. Saudemont, Macromolecules 2001, 34,
2438 – 2442; b) V. C. Gibson, A. Tomov, A. J. P. White, D. J.
Williams, Chem. Commun. 2001, 719 – 720.
[8] a) M. Schmid, R. Eberhardt, M. Klinga, M. Leskelä, B. Rieger,
Organometallics 2001, 20, 2321 – 2330; b) L. S. Moody, P. B.
Mackenzie, C. M. Killian, G. G. Lavoie, J. A. Ponasik, Jr.,
A. G. M. Barret, T. W. Smith, J. C. Pearson (Eastman Chemical
Company), WO 00/50470, 2000.
[9] M. S. W. Chan, L. Deng, T. Ziegler, Organometallics 2000, 19,
2741 – 2750.
[10] a) Wäßrige Polymerdispersionen (Ed.: D. Distler), VCH, Wein-
heim, 1999; b) Polymer Dispersions and Their Industrial Appli-
cations (Eds.: D. Urban, K. Takamura), Wiley-VCH, Weinheim,
2002.
Received: June 5, 2003 [Z52062]
Keywords: alkenes · homogeneous catalysis · polymerization ·
.
polymers · substituent effects
[1] Reviews: a) G. J. P. Britovsek, V. C. Gibson, D. F. Wass, Angew.
Chem. 1999, 111, 448 – 468; Angew. Chem. Int. Ed. 1999, 38, 428 –
447; b) V. C. Gibson, Chem. Rev. 2003, 103, 283 – 316; c) S.
Mecking, Coord. Chem. Rev. 2000, 203, 325 – 351; d) S. D. Ittel,
L. K. Johnson, M. Brookhart, Chem. Rev. 2000, 100, 1169 – 1204;
e) S. Mecking, Angew. Chem. 2001, 113, 550 – 557; Angew. Chem.
Int. Ed. 2001, 40, 534 – 540.
[2] a) S. Mecking, L. K. Johnson, L. Wang, M. Brookhart, J. Am.
Chem. Soc. 1998, 120, 888 – 899; b) E. Drent, R. van Dijk, R.
van Ginkel, B. van Oort, R. I. Pugh, Chem. Commun. 2002, 744 –
745.
[11] N. Miyaura, T. Yanagi, A. Suzuki, Synth. Commun. 1981, 11, 513.
[12] W. Kaschube, K. R. Poerschke, G. Wilke, J. Organomet. Chem.
1988, 355, 525 – 532.
[3] a) A. Held, F. M. Bauers, S. Mecking, Chem. Commun. 2000,
301 – 302; b) A. Held, S. Mecking, Chem. Eur. J. 2000, 6, 4623 –
4629; c) F. M. Bauers, S. Mecking, Macromolecules 2001, 34,
1165 – 1171; d) F. M. Bauers, S. Mecking, Angew. Chem. 2001,
113, 3112 – 3115; Angew. Chem. Int. Ed. 2001, 40, 3020 – 3022;
e) F. M. Bauers, M. M. Chowdhry, S. Mecking, Macromolecules
2003, 36, 6711 – 6715; f) F. M. Bauers, R. Thomann, S. Mecking,
J. Am. Chem. Soc. 2003, 125, 8838 – 8840; g) A. Tomov, J.-P.
Broyer, R. Spitz, Macromol. Symp. 2000, 150, 53 – 58; h) R.
Soula, C. Novat, A. Tomov, R. Spitz, J. Claverie, X. Drujon, J.
Malinge, T. Saudemont, Macromolecules 2001, 34, 2022 – 2026;
j) R. Soula, B. Saillard, R. Spitz, J. Claverie, M. F. Llaurro, C.
Monnet, Macromolecules 2002, 35, 1513 – 123; Review: k) S.
Mecking, A. Held, F. M. Bauers, Angew. Chem. 2002, 114, 564 –
582; Angew. Chem. Int. Ed. 2002, 41, 544 – 561; l) S. Mecking, J.
Claverie in Late Transition Metal Polymerization Catalysis (Eds.:
B. Rieger, L. S. Baugh, S. Kacker, S. Striegler), Wiley-VCH,
Weinheim, 2003, pp. 231 – 278.
[13] Crystallographic data for 2a and 2c. 2a: C35H20F12I2N2NiO, Mr
= 1025.04, monoclinic space group P21/c (no. 14), a = 12.840(2),
b = 14.681(2),
c = 20.919(3) Š,
b = 113.049(7)8,
V=
3628.5(9) Š3, Z = 4, 1calcd = 1.8764(5) gcmÀ3, m = 23.30 cmÀ1, R1
(wR2) = 0.035 (0.100), T= 293(2) K, GOF = 1.031. 2c:
¯
C31H24I2N2NiO, M = 753.01, triclinic space group P1 (no. 2),
a = 8.460(3), b = 12.243(4), c = 14.917(5) Š, a = 113.380(5) b =
90.876(5)8, g = 95.868(6), V= 1408.0(8) Š3, Z = 2, 1calcd
=
1.7762(10) gcmÀ3 m = 29.08 cmÀ1
, , R1 (wR2) = 0.045 (0.101),
T= 223(2) K, GOF = 1.01. The intensity data were collected on
a Bruker AXS CCD diffractometer with graphite-monochro-
mated MoKa radiation (l = 0.71070 Š). The structures were
solved by direct methods with SHELXS-97. All hydrogen atoms
were refined anisotropically. The final cycles of full-matrix least-
squares refinement by using SHELXL-97 were based on 6396
(2a) and 6270 (2c) observed reflections (all data) and 485 (2a)
and 340 (2c) variable parameters, respectively. CCDC-214669
(2a) and CCDC-214670 (2c) contain the supplementary crys-
tallographic data for this paper. These data can be obtained free
of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from
the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB21EZ, UK; fax: (+ 44)1223-336-033; or deposit@
ccdc.cam.ac.uk).
[4] L. K. Johnson, C. M. Killian, M. Brookhart, J. Am. Chem. Soc.
1995, 117, 6414 – 6415.
[5] Earlier work on ethylene and 1-olefin polymerisation by neutral
nickel(ii) complexes: a) W. Keim, F. H. Kowaldt, R. Goddard, C.
Krueger, Angew. Chem. 1978, 90, 493; Angew. Chem. Int. Ed.
Engl. 1978, 17, 466 – 467; b) W. Keim, R. Appel, A. Storeck, C.
Krueger, R. Goddard, Angew. Chem. 1981, 93, 91 – 92; Angew.
Chem. Int. Ed. Engl. 1981, 20, 116 – 117; c) V. M. Moehring, G.
Fink, Angew. Chem. 1985, 97, 982 – 984; Angew. Chem. Int. Ed.
Engl. 1985, 24, 1001 – 1003; d) K. A. Ostoja-Starzewski, J. Witte,
Angew. Chem. 1987, 99, 76 – 77; Angew. Chem. Int. Ed. Engl.
1987, 26, 63 – 64; e) U. Klabunde, S. D. Ittel, J. Mol. Catal. 1987,
41, 123 – 134; f) K. Kurtev, A. Tomov, J. Mol. Catal. 1994, 88,
141 – 150.
[14] Selected bond lengths (in Š) and angles for 2a: Ni1-C1 1.931(4);
Ni1-N1 1.896(3); Ni1-O 1.908(3); Ni1-N51 1.897(4); N1-Ni1-O1
92.6(1)8; C1-Ni1-N51 87.5(2)8; N1-Ni-C1 96.3(2); O1-Ni1-N51
83.7(1). 2c: Ni1-C1 1.941(5) Š; Ni1-N1 1.903(4); Ni1-O1
1.920(3); Ni1-N51 1.908(4); N1-Ni1-O1 93.8(1); C1-Ni1-N51
87.3(2); N1-Ni1-C1 95.4(2); O1-Ni1-N51 84.7(2).
[15] a) R. M. Ceder, J. Cubillo, G. Muller, M. Rocamora, J. Sales, J.
Organomet. Chem. 1992, 429, 391 – 401; b) R. Ceder, G. Muller,
J. Sales, J. Vidal, D. Neibecker, I. Tkatchenko, J. Mol. Catal.
1991, 68, 23 – 31; c) W. Keim, A. Behr, B. Gruber, B. Hoffmann,
F. H. Kowaldt, U. Kuerschner, B. Limbaecker, F. P. Sistig,
Organometallics 1986, 5, 2356 – 2359; d) P. Braunstein, J. Pietsch,
Y. Chauvin, S. Mercier, L. Saussine, A. DeCian, J. Fischer, J.
Chem. Soc. Dalton Trans. 1996, 3571 – 3574.
[6] recent work with k2-N,O complexes: a) S. Y. Desjardins, K. J.
Cavell, J. L. Hoare, B. W. Skelton, A. N. Sobolev, A. H. White,
W. Keim, J. Organomet. Chem. 1997, 544, 163 – 174; b) L. K.
Johnson, A. M. A. Bennett, S. D. Ittel, L. Wang, A. Parthasara-
thy, E. Hauptman, R. D. Simpson, J. Feldman, E. B. Coughlin
(DuPont), WO 98/30609, 1998; c) C. Wang, S. Friedrich, T. R.
Younkin, R. T. Li, R. H. Grubbs, D. A. Bansleben, M. W. Day,
Organometallics 1998, 17, 3149 – 3151; d) T. R. Younkin, E. F.
Connor, J. I. Henderson, S. K. Friedrich, R. H. Grubbs, D. A.
Bansleben, Science 2000, 287, 460 – 462; e) D. A. Bansleben, S.
Friedrich, T. R. Younkin, R. H. Grubbs, C. Wang, R. T. Li
[16] A. Tuchbreiter, M. A. Zuideveld, S. Mecking, unpublished
results.
[17] G. G. Hlatky, H. W. Turner, R. R. Eckman, J. Am. Chem. Soc.
1989, 111, 2728 – 2729.
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[18] Theoretical studies indicate, that instead of b-hydride elimina-
tion, transfer to monomer may also be a preferential mode of
chain transfer.[9] However, the factors determining the energy
barriers for both types of reactions appear to be similar.
[19] 2b is omitted in this consideration, being the only monosub-
stituted compound (R = NO2, R’ = H) in the series.
[20] a) F. A. Carey, R. J. Sundberg, Advanced Organic Chemistry.
Part A: Structure and Mechanism, Plenum, New York, 3rd ed.
1990; b) L. P. Hammett, Physical Organic Chemistry, McGraw-
Hill, New York, 2nd ed., 1970; c) C. Hansch, A. Leo, R. W. Taft,
Chem. Rev. 1991, 91, 165 – 195.
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