Mechanistic studies on Ni(dppe)Cl2-catalyzed chain-growth polymerizations: Evidence for rate-determining reductive elimination

Article abstract of DOI:10.1021/ja904197q

The mechanisms for Ni(dppe)Cl₂-catalyzed chain-growth polymerization of 4-bromo-2,5-bis-(hexyloxy)phenylmagnesium chloride and 5-bromo-4-hexylthiophen-2-ylmagnesium chloride were investigated. Rate studies utilizing IR spectroscopy and gas chromatography revealed that both polymerizations exhibit a first-order dependence on the catalyst concentration but a zeroth-order dependence on the monomer concentration. ³¹P NMR spectroscopic studies of the reactive organometallic intermediates suggest that the resting states are unsymmetrical Ni^II-biaryl and Ni ^II-bithiophene complexes. In combination, the data implicate reductive elimination as the rate-determining step for both monomers. Additionally, LiCl was found to have no effect on the rate-determining step or molecular weight distribution in the arene polymerization.

Full text of DOI:10.1021/ja904197q

Published on Web 10/28/2009
Mechanistic Studies on Ni(dppe)Cl2-Catalyzed Chain-Growth
Polymerizations: Evidence for Rate-Determining Reductive
Elimination
Erica L. Lanni and Anne J. McNeil*
Department of Chemistry and Macromolecular Science and Engineering Program, UniVersity of
Michigan, 930 North UniVersity AVenue, Ann Arbor, Michigan 48109-1055
Received May 23, 2009; Revised Manuscript Received October 1, 2009; E-mail: ajmcneil@umich.edu
Abstract: The mechanisms for Ni(dppe)Cl2-catalyzed chain-growth polymerization of 4-bromo-2,5-bis-
(hexyloxy)phenylmagnesium chloride and 5-bromo-4-hexylthiophen-2-ylmagnesium chloride were inves-
tigated. Rate studies utilizing IR spectroscopy and gas chromatography revealed that both polymerizations
exhibit a first-order dependence on the catalyst concentration but a zeroth-order dependence on the
monomer concentration. 31P NMR spectroscopic studies of the reactive organometallic intermediates suggest
that the resting states are unsymmetrical NiII-biaryl and NiII-bithiophene complexes. In combination, the
data implicate reductive elimination as the rate-determining step for both monomers. Additionally, LiCl was
found to have no effect on the rate-determining step or molecular weight distribution in the arene
polymerization.
zuki,7 Heck,8 and Negishi9 couplings) typically proceed through
Introduction
step-growth mechanisms, leading to broad molecular weight
Organic π-conjugated polymers are the active components
of numerous emerging technologies, including thin-film solar
cells1 and light-emitting diodes.2 The predominant cross-
coupling-based polymerization methods3 used to synthesize
these materials (e.g., the Sonogashira,4 Kumada,5 Stille,6 Su-
distributions and limited control over the copolymer micro-
structure. In 2004, Yokozawa10 and McCullough11 simulta-
neously reported chain-growth syntheses of poly(3-hexylthio-
phene) utilizing Ni-catalyzed cross-coupling reactions.
Yokozawa12 and McCullough13 independently proposed a novel
mechanistic pathway for this polymerization in which the key
step is formation of an associated Ni0-arene π complex after
reductive elimination. Subsequent intracomplex oxidative ad-
dition was suggested to occur faster than dissociation, leading
to successive monomer additions at the chain end. Although
Ni0-arene π complexes are known,14 this mechanistic hypoth-
esis remains speculative.
(1) (a) Dennler, G.; Scharber, M. C.; Brabec, C. J. AdV. Mater. 2009, 21,
1323–1338. (b) Hoppe, H.; Sariciftci, N. S. In PhotoresponsiVe
Polymers II; Marder, S. R., Lee, K.-S., Eds.; Advances in Polymer
Science, Vol. 214; Springer-Verlag: Berlin, 2008; pp 1-86. (c)
Thompson, B. C.; Fre´chet, J. M. J. Angew. Chem., Int. Ed. 2008, 47,
58–77. (d) Coakley, K. M.; McGehee, M. D. Chem. Mater. 2004, 16,
4533–4542.
(2) (a) Grimsdale, A. C.; Chan, K. L.; Martin, R. E.; Jokisz, P. G.; Holmes,
A. B. Chem. ReV. 2009, 109, 897–1091. (b) Perepichka, I. F.;
Perepichka, D. F.; Meng, H.; Wudl, F. AdV. Mater. 2005, 17, 2281–
2305. (c) Kulkarni, A. P.; Tonzola, C. J.; Babel, A.; Jenekhe, S. A.
Chem. Mater. 2004, 16, 4556–4573.
If broadly applicable, this chain-growth method has the
potential to provide access to polymers with controlled molec-
ular weights,15 narrow molecular weight distributions, and well-
defined microstructures.16 This method has since been modified
to polymerize a small set of other monomers in solution17 and
(3) (a) Cheng, Y.-J.; Luh, T.-Y. J. Organomet. Chem. 2004, 689, 4137–
4148. (b) Babudri, F.; Farinola, G. M.; Naso, F. J. Mater. Chem. 2004,
14, 11–34. (c) Yamamoto, T. J. Organomet. Chem. 2002, 653, 195–
199.
(9) (a) Yamamoto, T.; Osakada, K.; Wakabayashi, T.; Yamamoto, A.
Makromol. Chem., Rapid Commun. 1985, 6, 671–674. (b) Also see:
Chen, T.-A.; Wu, X.; Rieke, R. D. J. Am. Chem. Soc. 1995, 117, 233–
244.
(4) (a) Sanechika, K.; Yamamoto, T.; Yamamoto, A. Bull. Chem. Soc.
Jpn. 1984, 57, 752–755. (b) For a recent review, see: Bunz, U. H. F.
Macromol. Rapid Commun. 2009, 30, 772–805.
(5) (a) Yamamoto, T.; Hayashi, Y.; Yamamoto, A. Bull. Chem. Soc. Jpn.
1978, 51, 2091–2097. (b) For a recent review, see: Miyakoshi, R.;
Yokoyama, A.; Yokozawa, T. J. Polym. Sci., Part A: Polym. Chem.
2008, 46, 753–765.
(10) (a) Yokoyama, A.; Miyakoshi, R.; Yokozawa, T. Macromolecules
2004, 37, 1169–1171. (b) Miyakoshi, R.; Yokoyama, A.; Yokozawa,
T. Macromol. Rapid Commun. 2004, 25, 1663–1666.
(11) Sheina, E. E.; Liu, J.; Iovu, M. C.; Laird, D. W.; McCullough, R. D.
Macromolecules 2004, 37, 3526–3528.
(6) (a) Bao, Z.; Chan, W.; Yu, L. Chem. Mater. 1993, 5, 2–3. (b) Also
see: Bao, Z.; Chan, W. K.; Yu, L. J. Am. Chem. Soc. 1995, 117,
12426–12435.
(12) (a) Miyakoshi, R.; Yokoyama, A.; Yokozawa, T. J. Am. Chem. Soc.
2005, 127, 17542–17547. (b) Adachi, I.; Miyakoshi, R.; Yokoyama,
A.; Yokozawa, T. Macromolecules 2006, 39, 7793–7795. (c) Yokoya-
ma, A.; Yokozawa, T. Macromolecules 2007, 40, 4093–4101. (d)
Yokozawa, T.; Yokoyama, A. Chem. ReV. [Online early access]. DOI:
10.1021/cr900041c. Published Online: Sept 16, 2009. (e) Also see ref
5b.
(7) (a) Rehahn, M.; Schlu¨ter, A. D.; Wegner, G.; Feast, W. J. Polymer
1989, 30, 1060–1062. (b) For a recent review, see: Sakamoto, J.;
Rehahn, M.; Wegner, G.; Schlu¨ter, A. D. Macromol. Rapid Commun.
2009, 30, 653–687.
(8) (a) Heitz, W.; Bru¨gging, W.; Freund, L.; Gailberger, M.; Greiner, A.;
Jung, H.; Kampschulte, U.; Niessner, N.; Osan, F.; Schmidt, H.-W.;
Wicker, M. Makromol. Chem. 1988, 189, 119–127. (b) For a recent
review, see: Lee, Y.; Liang, Y.; Yu, L. Synlett 2006, 2879–2893.
(13) (a) Iovu, M. C.; Sheina, E. E.; Gil, R. R.; McCullough, R. D.
Macromolecules 2005, 38, 8649–8656. (b) Osaka, I.; McCullough,
R. D. Acc. Chem. Res. 2008, 41, 1202–1214.
9
10.1021/ja904197q CCC: $40.75  2009 American Chemical Society
J. AM. CHEM. SOC. 2009, 131, 16573–16579 16573

A R T I C L E S
Lanni and McNeil
on surfaces,18 including 2,5-bis(hexyloxy)phenylene,19 9,9-
dioctylfluorene,20 2,3-dihexylthienopyrazine,21 N-octylcarba-
zole,20b 3-alkoxythiophene,22 and N-hexylpyrrole.23 However,
without mechanistic data, each monomer has required empirical
development of unique reaction conditions to achieve chain
growth. Preliminary attempts at preparing simple block copoly-
mers have highlighted the challenges involved when each
monomer requires highly specific conditions.23,24 For example,
Yokozawa reported that the sequence of monomer addition had
a significant effect on the molecular weight distribution in the
synthesis of poly(2,5-bis(hexyloxy)benzene-b-N-hexylpyrrole).23
He suggested that the excess 1,2-bis(diphenylphosphino)ethane
(dppe) ligand, which is required for chain-growth polymerization
of the pyrrole, interfered with the phenylene polymerization.
However, the mechanistic influences of the ligand and other
additives that are reported to promote chain growth have not
been explored. In order to rationally expand this methodology
to other monomers and copolymerizations, a detailed under-
standing of the reaction mechanism, particularly the roles of
ligand, monomer, and additives, is essential.
To date, the few mechanistic studies that have been performed
on these Ni-catalyzed chain-growth polymerizations have
focused solely on thiophenes.12,13,25,26 Most notably, rate studies
by McCullough on the polymerization of thiophene catalyzed
by Ni(dppp)Cl2 [dppp ) 1,3-bis(diphenylphosphino)propane]
found that the reaction is first-order in monomer, suggesting
rate-determining transmetalation.13a Given the narrow substrate
scope, we sought to elucidate the mechanistic influences of both
the monomer and the ligand structure. Herein we report the
results of rate and spectroscopic studies of the polymerization
of 2,5-bis(hexyloxy)phenylene and 3-hexylthiophene using
Ni(dppe)Cl2, a frequent alternative to Ni(dppp)Cl2.12b,19,23 We
provide strong evidence for rate-determining reductive elimina-
tion and identify NiII-biaryl and NiII-bithiophene complexes
as the catalyst resting states. Furthermore, we show that LiCl,
an additive reported to be beneficial in controlled polymeriza-
tions of 2,5-bis(hexyloxy)phenylene,19 has no effect on the rate-
determining step or the molecular weight distribution under our
reaction conditions. These results, combined with the rate data
previously reported by McCullough for Ni(dppp)Cl2-catalyzed
polymerization,13a suggest that the ligand structure has a strong
influence on the polymerization mechanism.
(14) (a) Yoshikai, N.; Matsuda, H.; Nakamura, E. J. Am. Chem. Soc. 2008,
130, 15258–15259. (b) Johnson, S. A.; Huff, C. W.; Mustafa, F.;
Saliba, M. J. Am. Chem. Soc. 2008, 130, 17278–17280. (c) Ates¸in,
T. A.; Li, T.; Lachaize, S.; Garc´ıa, J. J.; Jones, W. D. Organometallics
2008, 27, 3811–3817. (d) Garc´ıa, J. J.; Brunkan, N. M.; Jones, W. D.
J. Am. Chem. Soc. 2002, 124, 9547–9555. (e) Braun, T.; Cronin, L.;
Higgitt, C. L.; McGrady, J. E.; Perutz, R. N.; Reinhold, M. New
J. Chem. 2001, 25, 19–21. (f) Bach, I.; Po¨rschke, K.-R.; Goddard, R.;
Kopiske, C.; Kru¨ger, C.; Rufin´ska, A.; Seevogel, K. Organometallics
1996, 15, 4959–4966. (g) Stanger, A.; Shazar, A. J. Organomet. Chem.
1993, 458, 233–236. (h) Boese, R.; Stanger, A.; Stellberg, P.; Shazar,
A. Angew. Chem., Int. Ed. 1993, 32, 1475–1477. (i) Stanger, A.;
Vollhardt, K. P. C. Organometallics 1992, 11, 317–320. (j) Stanger,
A.; Boese, R. J. Organomet. Chem. 1992, 430, 235–243. (k) Scott,
F.; Kru¨ger, C.; Betz, P. J. Organomet. Chem. 1990, 387, 113–121. (l)
Choe, S.-B.; Klabunde, K. J. J. Organomet. Chem. 1989, 359, 409–
418. (m) Benn, R.; Mynott, R.; Topalovic´, I.; Scott, F. Organometallics
1989, 8, 2299–2305. (n) Brezinski, M. M.; Klabunde, K. J. Organo-
metallics 1983, 2, 1116–1123. (o) Brauer, D. J.; Kru¨ger, C. Inorg.
Chem. 1977, 16, 884–891. (p) Jonas, K. J. Organomet. Chem. 1974,
78, 273–279.
(15) Hiorns, R. C.; Bettignies, R.; Leroy, J.; Bailly, S.; Firon, M.; Sentein,
C.; Khoukh, A.; Preud’homme, H.; Dagron-Lartigau, C. AdV. Funct.
Mater. 2006, 16, 2263–2273.
(16) For syntheses of end-functionalized polymers via this method, see:
(a) Urien, M.; Erothu, H.; Cloutet, E.; Hiorns, R. C.; Vignau, L.;
Cramail, H. Macromolecules 2008, 41, 7033–7040. (b) Hiorns, R. C.;
Khoukh, A.; Gourdet, B.; Dagron-Lartigau, C. Polym. Int. 2006, 55,
608–620. (c) Jeffries-EL, M.; Sauve´, G.; McCullough, R. D. Macro-
molecules 2005, 38, 10346–10352. (d) Iovu, M. C.; Jeffries-EL, M.;
Sheina, E. E.; Cooper, J. R.; McCullough, R. D. Polymer 2005, 46,
8582–8586. (e) Jeffries-EL, M.; Sauve´, G.; McCullough, R. D. AdV.
Mater. 2004, 16, 1017–1019. For examples of controlled microstructure
polymers via this method, see: (f) Wu, P.-T.; Ren, G.; Li, C.;
Mezzenga, R.; Jenekhe, S. A. Macromolecules 2009, 42, 2317–2320.
(g) Ouhib, F.; Khoukh, A.; Ledeuil, J.-B.; Martinez, H.; Desbrie`res,
J.; Dagron-Lartigau, C. Macromolecules 2008, 41, 9736–9743. (h)
Ohshimizu, K.; Ueda, M. Macromolecules 2008, 41, 5289–5294. (i)
Zhang, Y.; Tajima, K.; Hirota, K.; Hashimoto, K. J. Am. Chem. Soc.
2008, 130, 7812–7813.
Results
Grignard Metathesis. Monomer 2a was generated in situ from
1 via Grignard metathesis (GRIM) with i-PrMgCl (eq 1):27 In
(17) For polymerization of thiophene derivatives via this method, see: (a)
Benanti, T. L.; Kalaydjian, A.; Venkataraman, D. Macromolecules
2008, 41, 8312–8315. (b) Li, Y.; Xue, L.; Xia, H.; Xu, B.; Wen, S.;
Tian, W. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 3970–3984.
(c) Ouhib, F.; Dkhissi, A.; Iratçabal, P.; Hiorns, R. C.; Khoukh, A.;
Desbrie`res, J.; Pouchan, C.; Dagron-Lartigau, C. J. Polym. Sci., Part
A: Polym. Chem. 2008, 46, 7505–7516. (d) Vallat, P.; Lamps, J.-P.;
Schosseler, F.; Rawiso, M.; Catala, J.-M. Macromolecules 2007, 40,
2600–2602.
(18) (a) Sontag, S. K.; Marshall, N.; Locklin, J. Chem. Commun. 2009,
3354–3356. (b) Khanduyeva, N.; Senkovskyy, V.; Beryozkina, T.;
Horecha, M.; Stamm, M.; Uhrich, C.; Riede, M.; Leo, K.; Kiriy, A.
J. Am. Chem. Soc. 2009, 131, 153–161. (c) Beryozkina, T.; Boyko,
K.; Khanduyeva, N.; Senkovskyy, V.; Horecha, M.; Oertel, U.; Simon,
F.; Stamm, M.; Kiriy, A. Angew. Chem., Int. Ed. 2009, 48, 2695–
2698. (d) Khanduyeva, N.; Senkovskyy, V.; Beryozkina, T.; Bo-
charova, V.; Simon, F.; Nitschke, M.; Stamm, M.; Gro¨tzschel, R.;
Kiriy, A. Macromolecules 2008, 41, 7383–7389. (e) Senkovskyy, V.;
Khanduyeva, N.; Komber, H.; Oertel, U.; Stamm, M.; Kuckling, D.;
Kiriy, A. J. Am. Chem. Soc. 2007, 129, 6626–6632.
the presence of 1 equiv of LiCl, rate studies demonstrated that
the reaction is 4 times faster than in the absence of salt [see the
Supporting Information (SI)]. Furthermore, a peak shift was
observed in the aromatic region of the product’s no-D NMR
spectrum depending on the presence and absence of LiCl (Figure
1A). These results suggest that a mixed aggregate (2b) between
LiCl and the ArMgCl is formed.28 The aggregation state (e.g.,
1:1 mixed dimer vs 2:2 mixed tetramer) for this species was
not determined, but Knochel has suggested that related aryl
(19) Miyakoshi, R.; Shimono, K.; Yokoyama, A.; Yokozawa, T. J. Am.
Chem. Soc. 2006, 128, 16012–16013.
(20) (a) Huang, L.; Wu, S.; Qu, Y.; Geng, Y.; Wang, F. Macromolecules
2008, 41, 8944–8947. (b) Stefan, M. C.; Javier, A. E.; Osaka, I.;
McCullough, R. D. Macromolecules 2009, 42, 30–32.
(21) Wen, L.; Duck, B. C.; Dastoor, P. C.; Rasmussen, S. C. Macromol-
ecules 2008, 41, 4576–4578.
(22) (a) Sheina, E. E.; Khersonsky, S. M.; Jones, E. G.; McCullough, R. D.
Chem. Mater. 2005, 17, 3317–3319. (b) Also see ref 12b.
(24) Miyakoshi, R.; Yokoyama, A.; Yokozawa, T. Chem. Lett. 2008, 37,
1022–1023.
(23) Yokoyama, A.; Kato, A.; Miyakoshi, R.; Yokozawa, T. Macromol-
ecules 2008, 41, 7271–7273.
(25) Beryozkina, T.; Senkovskyy, V.; Kaul, E.; Kiriy, A. Macromolecules
2008, 41, 7817–7823.
9
16574 J. AM. CHEM. SOC. VOL. 131, NO. 45, 2009

Ni(dppe)Cl2-Catalyzed Chain-Growth Polymerizations
A R T I C L E S
Figure 1. 1H NMR spectra of (A) 2a and 2b and (B) 2b with byproducts
[(*) quenched monomer; (0) 1,4-bis(hexyloxy)benzene].
Grignards form 1:1 mixed dimers with LiCl in THF.27f Though
2a or 2b are the major products, several minor products (<10%)
were frequently observed in the aromatic region of the no-D
1H NMR spectrum (Figure 1B). These products were identified
by independent synthesis and coinjection into the NMR sample
(see the SI). THF adducts 3 and 4 were unexpected; however,
a related coupling reaction between electron-rich aryl Grignards
and THF has previously been reported and was suggested to
proceed through a radical pathway.29 Importantly, these byprod-
ucts were not consumed during the polymerization; however,
monomers 2a and 2b were titrated immediately prior to each
kinetic run to account for their formation (see the SI).
Figure 2. (A) Plot of initial rate vs [monomer] for the polymerization of
2a in THF at 0 °C ([Ni(dppe)Cl2] ) 0.0015 M). The curve depicts an
unweighted least-squares fit to the expression initial rate ) a[monomer]n
that gave a ) 22 ( 1 and n ) 0.06 ( 0.04. (B) Plot of initial rate vs
[catalyst] for the polymerization of 2a in THF at 0 °C ([2a] ) 0.20 M).
The curve depicts an unweighted least-squares fit to the expression initial
rate ) a[catalyst]n that gave a ) (1.3 ( 0.1) × 104 and n ) 1.01 ( 0.01.
Monomer 6 was generated in situ from 5 via GRIM with
i-PrMgCl (eq 2). H NMR spectroscopic analysis revealed an
80:20 ratio of regioisomers. Unlike the case of monomers 2a
and 2b, no byproducts were observed after the GRIM reaction.
Figure 3. (A) Plot of initial rate vs [monomer] for the polymerization of
6 in THF at 0 °C ([Ni(dppe)Cl2] ) 0.00025 M). The curve depicts an
unweighted least-squares fit to the expression initial rate ) a[monomer]n
that gave a ) 10 ( 3 and n ) 0.05 ( 0.09. (B) Plot of initial rate vs
[catalyst] for the polymerization of 6 in THF at 0 °C ([6] ) 0.10 M). The
curve depicts an unweighted least-squares fit to the expression initial rate
) a[catalyst]n that gave a ) (4.9 ( 0.8) × 104 and n ) 1.02 ( 0.02.
1
For the polymerization of monomer 2a with Ni(dppe)Cl2, a
plot of the initial rate versus [monomer] showed a zeroth-order
dependence, while a plot of initial rate versus [catalyst] displayed
a first-order dependence (Figure 2A,B). Similarly, for the
polymerization of 6 by Ni(dppe)Cl2, the reaction was zeroth-
order in monomer and first-order in catalyst (Figure 3A,B).
These data eliminate transmetalation as a plausible rate-
determining step because it would exhibit a first-order depen-
dence on [monomer]. However, these rate studies were not able
to distinguish between rate-limiting reductive elimination and
intracomplex oxidative addition because both cases would
exhibit zero- and first-order dependencies with respect to
[monomer] and [catalyst], respectively. We used NMR spec-
troscopic studies to characterize the catalyst structure in the
resting state to differentiate between these two steps.
Rate Studies. Rate studies were carried out to ascertain the
rate-determining step in the Ni(dppe)Cl2-catalyzed chain-growth
polymerizations of 2a and 6 (eqs 3 and 4). Polymerization of
Spectroscopic Studies. 31P NMR spectroscopic studies were
used to identify the catalyst resting state in the chain-growth
polymerizations of 2a and 6. These studies were performed on
samples with higher catalyst concentrations than in the poly-
merizations in order to obtain sufficient signal (see the SI).
According to the proposed catalytic cycles (Scheme 1), the
resting states would be complexes I and IV if oxidative addition
were rate-limiting and complexes III and VI if reductive
elimination were rate-limiting.
2a was monitored by in situ IR spectroscopy, while GC analysis
of aliquots was used to monitor the polymerization of 6 relative
to an internal standard. Because of the insolubility of Ni-
(dppe)Cl2, we found it convenient to initiate this precatalyst with
5-7 equiv of monomer before starting the rate studies (see the
SI for details);30 preinitiation also avoided any potential
complications resulting from sluggish Ni(dppe)Cl2 reduction.
During the polymerization of 2a, the 31P NMR spectrum
revealed two proximate, broad doublets (JPP ) 11 Hz; Figure
9
J. AM. CHEM. SOC. VOL. 131, NO. 45, 2009 16575

A R T I C L E S
Lanni and McNeil
Scheme 1. Proposed Mechanism for the Chain-Growth
Polymerizations of 2 and 6
[catalyst] and zeroth-order rate dependence on [monomer],
support reductive elimination as the rate-determining step in
the chain-growth polymerizations of 2a and 6.
Interestingly, different Ni complexes were observed in the
31P NMR spectra when the polymerizations of 2a and 6 were
complete. After 2a was consumed, two doublets appeared, which
we hypothesized were from complex II (JPP ) 25 Hz; Figure
5A). We synthesized a related NiII model complex (8), which
showed a similar spectrum (JPP ) 15 Hz; Figure 5B), supporting
this assignment. After 6 was consumed, the 31P NMR spectrum
also showed two new doublets (JPP ) 36 Hz; Figure 5C), which
we hypothesized were from complex V. The proximate, lower
intensity doublets were again attributed to regioisomeric NiII
complexes, since both regioisomers of 6 were consumed under
these conditions. These results are consistent with the proposed
catalytic cycle, since the reaction should stall at complexes II
and V when the monomers are consumed. It should be noted
that complexes III and VI could not be isolated because of this
facile conversion to II and V once polymerization was com-
plete.32
Role of LiCl. Yokozawa reported that LiCl accelerated the
Ni(dppe)Cl2-catalyzed polymerization of 2b (eq 5) and led to a
narrower molecular weight distribution (PDI).19 We anticipated
that to produce such a rate acceleration, the LiCl must not only
aggregate with the monomer but also change the rate-determin-
ing step, since (1) the polymerization rate was shown to be
independent of [monomer] for 2a and (2) transmetalation with
either 2a or aggregate 2b should result in the same NiII-biaryl
complex. Instead, initial rate measurements on the polymeri-
zation of 2b gave zeroth- and first-order dependencies in
[monomer] and [catalyst], respectively (Figure 6A,B). Moreover,
the absolute initial rates were nearly identical to the rates without
LiCl (Figure 2A,B), indicating that LiCl has no effect on the
rate. Initial rates were also measured for polymerizations with
excess LiCl to determine whether rate acceleration could be
caused by nonaggregated salt. As evident in Figure 7A, the rate
remained unchanged with more than 1 equiv of LiCl. Further
evidence came from temperature-dependent rate data, which
provided nearly identical activation parameters (Figure 7B). In
the presence of LiCl, the activation enthalpy and entropy were
4A). The coupling constant is consistent with a NiII species,14d
and the small ∆δ is suggestive of complex III (with two similar
phosphorus nuclei). We synthesized a model Ni0-anthracene
π complex (7) for comparison (Figure 4B). This π complex
exhibited a relatively large coupling constant (JPP ) 68 Hz)
compared to the observed resting state. On the basis of this data,
we have assigned the catalyst resting state as complex III.
∆Hq ) 18.4 ( 0.7 kcal/mol and ∆Sq ) 0 ( 3 cal mol-1 K-1
,
respectively, while in the absence of LiCl, the values were ∆Hq
) 18 ( 1 kcal/mol and ∆Sq ) -3 ( 5 cal mol-1 K-1. Finally,
31P NMR spectroscopic studies of the catalyst resting state
(26) (a) Lamps, J.-P.; Catala, J.-M. Macromolecules 2009, 42, 7282–7284.
(b) Mao, Y.; Wang, Y.; Lucht, B. L. J. Polym. Sci., Part A: Polym.
Chem. 2004, 42, 5538–5547.
(27) (a) Shi, L.; Chu, Y.; Knochel, P.; Mayr, H. Org. Lett. 2009, 11, 3502–
3505. (b) Shi, L.; Chu, Y.; Knochel, P.; Mayr, H. J. Org. Chem. 2009,
74, 2760–2764. (c) Shi, L.; Chu, Y.; Knochel, P.; Mayr, H. Angew.
Chem., Int. Ed. 2008, 47, 202–204. (d) Krasovskiy, A.; Straub, B. F.;
Knochel, P. Angew. Chem., Int. Ed. 2006, 45, 159–162. (e) Hauk, D.;
Lang, S.; Murso, A. Org. Process Res. DeV. 2006, 10, 733–738. (f)
Krasovskiy, A.; Knochel, P. Angew. Chem., Int. Ed. 2004, 43, 3333–
3336. (g) Knochel, P.; Dohle, W.; Gommermann, N.; Kneisel, F. F.;
Kopp, F.; Korn, T.; Sapountzis, I.; Vu, V. A. Angew. Chem., Int. Ed.
2003, 42, 4302–4320. (h) Boymond, L.; Rottla¨nder, M.; Cahiez, G.;
Knochel, P. Angew. Chem., Int. Ed. 1998, 37, 1701–1703.
During the polymerization of 6, the 31P NMR spectrum also
revealed two proximate signals (JPP ) 24 Hz; Figure 4C), which
we have assigned to complex VI by analogy to 2a. However,
the spectrum clearly shows additional, related species. Since
both regioisomers of 6 are consumed at these low [monomer]/
[catalyst] ratios (see the SI), we have tentatively attributed these
peaks to regioisomeric Ni0-bithiophene complexes. These
results, combined with the first-order rate dependence on
9
16576 J. AM. CHEM. SOC. VOL. 131, NO. 45, 2009

Ni(dppe)Cl2-Catalyzed Chain-Growth Polymerizations
A R T I C L E S
Figure 4. 31P NMR spectra for (A) the resting state during polymerization of 2a, (B) Ni0-anthracene complex 7, and (C) the resting state during polymerization
of 6. We have tentatively assigned the peaks labeled with * to Ni(dppe)2X2.31
Figure 5. 31P NMR spectra (A) after consumption of 2a, (B) for complex 8, and (C) after consumption of 6.
Figure 6. (A) Plot of initial rate vs [monomer] for the polymerization of
2b in THF at 0 °C ([Ni(dppe)Cl2] ) 0.0015 M). The curve depicts an
unweighted least-squares fit to the expression initial rate ) a[monomer]n
that gave a ) 14.2 ( 0.5 and n ) -0.19 ( 0.02. (B) Plot of initial rate vs
[catalyst] for the polymerization of 2b in THF at 0 °C ([2b] ) 0.20 M).
The curve depicts an unweighted least-squares fit to the expression initial
rate ) a[catalyst]n that gave a ) (1.3 ( 0.2) × 104 and n ) 1.01 ( 0.03.
Figure 7. (A) Plot of initial rates vs [LiCl] for the Ni(dppe)Cl2-catalyzed
polymerization of 2b in THF at 0 °C ([2b] ) 0.20 M, [Ni(dppe)Cl2] )
0.0015 M). The curve depicts an unweighted least-squares fit to the
expression initial rate ) a[LiCl]n that gave a ) 20 ( 1 and n ) 0.02 (
0.05. (B) Plot of ln(kh/kBT) vs 1/T for the polymerization of 2a (O) and 2b
(b) in THF ([2a] ) [2b] ) 0.20 M, [Ni(dppe)Cl2] ) 0.001 M). The curves
depict unweighted least-squares fits to the formula ln(kh/kBT) ) -∆Hq/RT
+ ∆Sq/R that provided ∆Hq ) 18 ( 1 kcal/mol and ∆Sq ) -3 ( 5 cal
mol-1 K-1 for 2a and ∆Hq ) 18.4 ( 0.7 kcal/mol and ∆Sq ) 0 ( 3 cal
mol-1 K-1 for 2b.
showed two proximate doublets (JPP ) 9 Hz; Figure 8A),
consistent with complex III and rate-limiting reductive elimina-
tion. Complex II was observed once conversion of monomer
(2b) was complete (Figure 8B). Altogether, these data imply
that there is no substantive effect of LiCl on the absolute rate
and the rate-determining step.
the rate studies. In contrast, Yokozawa initiated his catalyst in
situ, where the influence of LiCl on the initiation rate may be
significant. This hypothesis is supported by the identification
of a monomer-LiCl mixed aggregate (2b) that would be
involved in initiation.
Comparing plots of PDI versus conversion for the polymer-
izations of 2a and 2b revealed that in contrast to the report by
Yokozawa,19 LiCl had no significant effect on the PDI of the
resulting polymers (Figure 9A,B). In this case, however, a subtle
difference between the two reports may be playing an important
role. In chain-growth polymerizations, the relative rate of
initiation versus propagation influences the molecular weight
distribution.33 We avoided this relative rate issue by preinitiating
the Ni(dppe)Cl2 with 5-7 equiv of monomer before beginning
Discussion
Despite the general utility of Ni-catalyzed cross-coupling
reactions34 in both small-molecule35 and polymer syntheses,3,5,9,36
the operative mechanisms are still highly debated.37 Moreover,
the extrapolation of small-molecule-based mechanistic studies
to polymerizations is not straightforward. Yokozawa12 and
McCullough13 independently proposed a new mechanistic
9
J. AM. CHEM. SOC. VOL. 131, NO. 45, 2009 16577

A R T I C L E S
Lanni and McNeil
reductiVe elimination for the polymerization of monomers 2a,
2b, and 6 using Ni(dppe)Cl2. Interestingly, the monomer
structure (arene vs thiophene) had no influence on the rate-
determining step of the catalytic cycle. Notably, McCullough
found evidence for a rate-determining transmetalation in the
polymerization of 6 using a different catalyst, Ni(dppp)Cl2.13a
In combination, these results point to a significant mechanistic
influence of the ligand on the polymerization and suggest that
alternative ligand structures may lead to catalysts with improved
reactivities.38,39 Finally, though LiCl formed a mixed aggregate
with the arene monomer, our rate and spectroscopic studies
showed that this additive has no effect on either the polymer-
ization rate or mechanism. Nevertheless, the role of LiCl in the
initiation step may be significant, and future studies are needed
to address this issue.
Figure 8. 31P NMR spectra for (A) the resting state during polymerization
of 2b and (B) after consumption of 2b. We have tentatively assigned the
peak labeled with * to Ni(dppe)2X2.31
Chain Growth via Ni0 π Complexes? The structure40 and
reactivity41 of Ni0-olefin π complexes has been widely
documented. For example, van der Boom recently demonstrated
that alkene coordination to Ni0 is kinetically preferred over
oxidative addition of aryl-I and aryl-Br bonds.42 In addition,
they only observed products resulting from intracomplex
oxidative addition after alkene coordination. Far fewer studies
for Ni0-arene π complexes have been reported; recent theoreti-
cal and kinetic isotope effect studies by Nakamura have
suggested that arene π complexation to Ni0 is the first irrevers-
ible step in cross-coupling reactions of o-chloro- and o-
bromotoluene with Grignards.14a Evidence of an intermediate
Ni0-arene π complex in the chain-growth polymerization has
only been circumstantial: (1) Kiriy indirectly probed the
existence of a π complex by examining whether the chain-
growth mechanism depends on monomer size.25 A decrease in
chain-growth behavior was observed for terthiophene relative
to thiophene, suggesting that detrimental chain-transfer and
termination processes become more prevalent with larger
distances between the C-C bond-forming site and the reactive
end group. (2) McCullough observed an unexpected double-
substitution reaction to generate thiophene trimers when a 2:1
Figure 9. Mn (b) and PDI (O) vs conversion for the Ni(dppe)Cl2-catalyzed
polymerizations of (A) 2a and (B) 2b in THF at 0 °C ([2a] ) 0.10 M; [2b]
) 0.20 M; [Ni(dppe)Cl2] ) 0.0015 M).
pathway for this polymerization in which the key step is the
formation of an associated Ni0-arene π complex (e.g., com-
plexes I and IV). Subsequent intracomplex oxidative addition
leads to chain growth. Although Ni0-arene π complexes have
precedent,14 their role in the polymerization mechanism remains
uncertain. Both the potential of this method to provide access
to novel well-defined polymers and its current limitations
motivated us to explore the mechanism in more detail, particu-
larly the influence of the ligand, monomer, and additives, with
the aim of generating improved catalysts.
Mechanism. Through a combination of rate and spectroscopic
studies, we have found evidence supporting a rate-determining
(37) (a) Jin, L.; Zhang, H.; Li, P.; Sowa, J. R., Jr.; Lei, A. J. Am. Chem.
Soc. 2009, 131, 9892–9893. (b) Amatore, C.; Jutand, A. Organome-
tallics 1988, 7, 2203–2214. (c) Colon, I.; Kelsey, D. R. J. Org. Chem.
1986, 51, 2627–2637. (d) Semmelhack, M. F.; Helquist, P.; Jones,
L. D.; Keller, L.; Mendelson, L.; Ryono, L. S.; Smith, J. G.; Stauffer,
R. D. J. Am. Chem. Soc. 1981, 103, 6460–6471. (e) Smith, G.; Kochi,
J. K. J. Organomet. Chem. 1980, 198, 199–214. (f) Tsou, T. T.; Kochi,
J. K. J. Am. Chem. Soc. 1979, 101, 6319–6332. (g) Tsou, T. T.; Kochi,
J. K. J. Am. Chem. Soc. 1979, 101, 7547–7560. (h) Tsou, T. T.; Kochi,
J. K. J. Am. Chem. Soc. 1978, 100, 1634–1635. (i) Morrell, D. G.;
Kochi, J. K. J. Am. Chem. Soc. 1975, 97, 7262–7270.
(28) For an X-ray crystal structure of a related mixed aggregate, see:
Buttrus, N. H.; Eaborn, C.; El-Kheli, M. N. A.; Hitchcock, P. B.; Smith,
J. D.; Sullivan, A. C.; Tavakkoli, K. J. Chem. Soc., Dalton Trans.
1988, 381–391.
(29) Inoue, A.; Shinokubo, H.; Oshima, K. Synlett 1999, 1582–1584.
(30) It should be noted that we obtained lower polymerization rates when
using commercial batches of Ni(dppe)Cl2 that contained impurities
observable by 31P NMR spectroscopy.
(31) Jarrett, P. S.; Sadler, P. J. Inorg. Chem. 1991, 30, 2098–2104.
(32) NiII-biaryl complexes with chelating phosphine ligands have previ-
ously been reported to be unstable toward isolation as a result of facile
reductive elimination. For example, see: Coronas, J. M.; Muller, G.;
Rocamora, M.; Miravitlles, C.; Solans, X. J. Chem. Soc., Dalton Trans.
1985, 2333–2341.
(38) For an example of the influence of the ligand (dppe vs dppp) on
reductive elimination in NiII dimethyl complexes, see: Kohara, T.;
Yamamoto, T.; Yamamoto, A. J. Organomet. Chem. 1980, 192, 265–
274.
(39) For examples of alternative initiators, see: (a) Doubina, N.; Ho, A.;
Jen, A. K.-Y.; Luscombe, C. K. Macromolecules 2009, 42, 7670-
7677. (b) Bronstein, H. A.; Luscombe, C. K. J. Am. Chem. Soc. 2009,
131, 12894–12895. (c) Also see ref 18e.
(33) Odian, G. In Principles of Polymerization, 4th ed.; Wiley: Hoboken,
NJ, 2004; pp 422-436.
(34) (a) Tamao, K.; Sumitani, K.; Kumada, M. J. Am. Chem. Soc. 1972,
94, 4374–4376. (b) Corriu, R. J. P.; Masse, J. P. Chem. Commun.
1972, 144. (c) Tamao, K.; Sumitani, K.; Kiso, Y.; Zembayashi, M.;
Fujioka, A.; Kodama, S.; Nakajima, I.; Minato, A.; Kumada, M. Bull.
Chem. Soc. Jpn. 1976, 49, 1958–1969. (d) Tamao, K.; Kodama, S.;
Nakajima, I.; Kumada, M.; Minato, A.; Suzuki, K. Tetrahedron 1982,
38, 3347–3354.
(40) (a) Massera, C.; Frenking, G. Organometallics 2003, 22, 2758–2765.
(b) Tolman, C. A.; Seidel, W. C.; Gosser, L. W. Organometallics 1983,
2, 1391–1396. (c) Tolman, C. A. J. Am. Chem. Soc. 1974, 96, 2780–
2789. (d) Tolman, C. A.; Seidel, W. C. J. Am. Chem. Soc. 1974, 96,
2774–2780. (e) Brauer, D. J.; Kru¨ger, C. J. Organomet. Chem. 1974,
77, 423–438. (f) Tolman, C. A.; Seidel, W. C.; Gerlach, D. H. J. Am.
Chem. Soc. 1972, 94, 2669–2676. (g) Cheng, P.-T.; Cook, C. D.;
Nyburg, S. C.; Wan, K. Y. Inorg. Chem. 1971, 10, 2210–2213. (h)
Ittel, S. D. Inorg. Chem. 1977, 16, 2589–2597.
(35) (a) Takahashi, T.; Kanno, K. In Modern Organonickel Chemistry;
Tamaru, Y., Ed.; Wiley-VCH: Weinheim, Germany, 2005; pp 41-
55. (b) Phapale, V. B.; Ca´rdenas, D. J. Chem. Soc. ReV. 2009, 38,
1598–1607. (c) Hassan, J.; Se´vignon, M.; Gozzi, C.; Schulz, E.;
Lemaire, M. Chem. ReV. 2002, 102, 1359–1469.
(41) For leading references, see: Johnson, J. B.; Rovis, T. Angew. Chem.,
Int. Ed. 2008, 47, 840–871.
(36) (a) Yamamoto, T.; Koizumi, T. Polymer 2007, 48, 5449–5472. (b)
Yamamoto, T. Synlett 2003, 425–450.
(42) Zenkina, O. V.; Karton, A.; Freeman, D.; Shimon, L. J. W.; Martin,
J. M. L.; van der Boom, M. E. Inorg. Chem. 2008, 47, 5114–5121.
9
16578 J. AM. CHEM. SOC. VOL. 131, NO. 45, 2009

Ni(dppe)Cl2-Catalyzed Chain-Growth Polymerizations
A R T I C L E S
ratio of monomer to catalyst was used.11 Such preferential
double substitutions have also been observed in Pd-catalyzed
cross-coupling reactions of small molecules43 and polymers.44
Interestingly, Kumada observed a similar preferential Ni-
catalyzed double alkylation in 1976 when using bifunctional
arenes (e.g., 1,4-dichlorobenzene) despite having a 2-fold excess
of the arene reagent relative to the alkyl Grignard.34c He
suggested that such substrates undergo a “mechanistically
different” reaction but provided no further explanation.
Our observation of a rate-determining reductive elimination
and McCullough’s observation of a rate-determining trans-
metalation indicate that the Ni0 π complex, if formed, is only
a fleeting, post-rate-limiting intermediate. Moreover, our
extensive spectroscopic studies identified the catalyst species
both during and after polymerization, and neither was
consistent with a Ni0 π complex. As a result, the mechanistic
underpinnings of the chain-growth nature of these polymer-
izations remain unclear, and further studies are necessary to
probe both the existence and catalytic relevance of the
proposed Ni0 π complexes.
of poly(2,5-bis(hexyloxy)phenylene) and poly(3-hexylthio-
phene). These results, combined with the data from
McCullough13a using Ni(dppp)Cl2, suggest that the ligand
has a strong influence over the rate-determining step.
NiII-biaryl and NiII-bithiophene complexes, though unstable
to isolation, were identified as the active catalyst resting
states. These studies also revealed that the role of LiCl is
complex and that this additive may be unnecessary under
certain reaction conditions. By addressing the mechanistic
influences of monomer and catalyst structure as well as the
role of additives, these results provide a strong foundation
for future studies aimed at preparing novel polymers and
developing improved catalysts. In addition, we are now in a
position to explore the more complex yet intriguing copo-
lymerization mechanisms.
Acknowledgment. We thank the donors of the American
Chemical Society Petroleum Research Fund (47661-G7) and the
University of Michigan for direct support of this work. E.L.L. thanks
the NSF for a predoctoral fellowship.
Supporting Information Available: Experimental details and
spectroscopic data. This material is available free of charge via
the Internet at http://pubs.acs.org.
Conclusion
Rate and spectroscopic studies support a rate-limiting
reductive elimination for the Ni(dppe)Cl2-catalyzed syntheses
JA904197Q
(43) (a) Weber, S. K.; Galbrecht, F.; Scherf, U. Org. Lett. 2006, 8, 4039–
4041. (b) Sinclair, D. J.; Sherburn, M. S. J. Org. Chem. 2005, 70,
3730–3733. (c) Dong, C.-G.; Hu, Q.-S. J. Am. Chem. Soc. 2005, 127,
10006–10007.
(44) Yokoyama, A.; Suzuki, H.; Kubota, Y.; Ohuchi, K.; Higashimura, H.;
Yokozawa, T. J. Am. Chem. Soc. 2007, 129, 7236–7237.
9
J. AM. CHEM. SOC. VOL. 131, NO. 45, 2009 16579