Nature of the initiation, propagation, and chain transfer steps during the formation of ethylene oligomers catalyzed by PdMe2(N,N,N′,N′-tetramethylethylenediamine)/B(C 6F5)3

Article abstract of DOI:10.1021/om971060p

Reaction of equimolar quantities of PdMe₂(Me₂-NCH₂CH₂NMe₂) and B(C₆F₅)₃ in the presence of ethylene results in the formation of [PdMe(C₂H₄)(Me

Full text of DOI:10.1021/om971060p

2382
Organometallics 1998, 17, 2382-2384
Na tu r e of th e In itia tion , P r op a ga tion , a n d Ch a in
Tr a n sfer Step s d u r in g th e F or m a tion of Eth ylen e
Oligom er s Ca ta lyzed by
P d Me2(N,N,N′N′-tetr a m eth yleth ylen ed ia m in e)/B(C6F 5)3
Sylvie Y. Desjardins, Alison A. Way, Michael C. Murray, Dan Adirim, and
Michael C. Baird*
Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada
Received December 3, 1997
Summary: Reaction of equimolar quantities of PdMe2(Me2-
NCH2CH2NMe2) and B(C6F5)3 in the presence of ethylene
results in the formation of [PdMe(C2H4)(Me2N-
CH2CH2NMe2)]+[BMe(C6F5)3]-; the latter catalyzes the
oligomerization of ethylene to highly branched products,
which are apparently formed via both reversible â-elimi-
nation-reinsertion processes and the incorporation of
short chain oligomeric products.
system facilitates â-hydrogen transfer relative to propa-
gation reactions and produces highly branched ethylene
oligomers of sufficiently low molecular weights that
useful NMR data may be obtained.
Reaction of equimolar quantities of PdMe2(tmeda) and
B(C6F5)3 in CD2Cl2 at 193 K was monitored by 1H NMR
spectroscopy and proceeds cleanly as in eq 1. The Pd-
PdMe2(tmeda) + B(C6F5)3 f
There is currently great interest in Ziegler catalysts,
based on cationic nickel and palladium compounds
containing bulky diimine ligands, that polymerize eth-
ylene to polyethylenes containing highly branched mi-
crostructures.1 Although the branching is thought to
result from a series of reversible â-H elimination and
reinsertion steps, which allow the metal to migrate
along the polymer chain (chain walking), there has been,
as yet, little research into the mechanism(s) of the
overall process.1,2
We now report the acquisition of useful mechanistic
information via in situ NMR monitoring of a polymer-
ization process; we have also identified end groups and
the types of unsaturation in the products of polymeri-
zation via detailed NMR analyses of polymers of low
molecular weight. Since the presence of bulky ligands
in catalyst systems is known to inhibit chain transfer
reactions, resulting in the formation of high molecular
weight polymers,1,2 we have utilized the new, sterically
undemanding cationic catalytic system obtained when
PdMe2(tmeda)3 (tmeda ) N,N,N′N′-tetramethylethyl-
enediamine) is activated with B(C6F5)3. This catalyst
[PdMe(CD2Cl2)(tmeda)]+ + [BMe(C6F5)3]- (1)
Me resonances of PdMe2(tmeda)3b disappeared, giving
rise to new methyl resonances at δ 0.48 and 0.33 (br)
attributable to [PdMe(CD2Cl2)(tmeda)]+ and free [BMe-
(C6F5)3]-, respectively. Thus, the borate anion does not
coordinate to the cationic metal center via the methyl
group, as commonly observed in group 4 d0 systems.4
The NMR spectrum also exhibited broadened reso-
nances at ca. δ 2.32 and 3.18 attributable to nonequiva-
lent CH2 hydrogens and at ca. δ 2.45 (6H), 2.62 (3H),
and 2.72 (3H) attributed to the N-Me groups. Since
the N-Me groups appeared in at least three environ-
ments, the cationic complex is formulated as in eq 1
rather than as the more symmetric µ-methyl complex
[Pd2(µ-Me)2(tmeda)2]2+. The spectrum broadened sig-
nificantly on raising the temperature over 200 K,
consistent with ensuing rapid ring inversion.3
Ethylene is oligomerized by “[PdMe(tmeda)][BMe-
(C6F5)3]”, formed in situ in toluene. In a typical experi-
ment, the oligomer obtained over 2 h at ∼7 atm at 298
K (TON ) 1250 g of oligomer/mol of catalyst/h) was a
low molecular weight (Mw ) 265, Mn ) 240), highly
branched material with low polydispersity (1.1). Con-
sistent with the very narrow molecular weight distribu-
tion, lower oligomers were not detected in the crude
catalytic mixture by GC-MS analysis, although a large
number of compounds in the molecular weight range
250-300 were detected. While molecular ions and
cracking patterns showed that the oligomeric products
were olefinic rather than saturated, more definitive
(1) (a) Killian, C. M.; Tempel, D. J .; J ohnson, L. K.; Brookhart, M.
J . Am. Chem. Soc. 1996, 118, 11664. (b) J ohnson, L. K.; Mecking, S.;
Brookhart, M. J . Am. Chem. Soc. 1996, 118, 267. (c) J ohnson, L. K.;
Killian, K. M.; Brookhart, M. J . Am.Chem. Soc. 1995, 117, 6414. (d)
McLain, S. J .; McCord, E. F.; J ohnson, L. K.; Ittel, S. D.; Nelson, S.
D.; Halfhill, A. M. J .; Teasley, M. F.; Tempel, D. J .; Killian, C.;
Brookhart, M. S. Polym. Prepr. (Am. Chem. Soc. Div. Polym. Chem.)
1997, 38, 772.
(2) Deng, L.; Woo, T. K.; Cavallo, L.; Margl, P. M.; Ziegler, T. J .
Am. Chem. Soc. 1997, 119, 6177. (b) Deng, L.; Margl, P.; Ziegler, T. J .
Am. Chem. Soc. 1997, 119, 1094. (c) Froese, R. D. J .; Musaev, D. G.;
Morokuma, K. J . Am. Chem. Soc. 1997, 119, 1581. (d) Stro¨mberg, S.;
Zetterberg, K.; Siegbahn, P. E. M. J . Chem. Soc., Dalton Trans. 1997,
4147.
(3) (a) De Graaf, W.; Boersma, J .; Smeets, W. J . J .; Spek, A. L.; van
Koten, G. Organometallics 1989, 8, 2907. (b) The 1H NMR spectrum
of PdMe2(tmeda) in CD2Cl2 at 193 K exhibits singlets at δ -0.47 and
2.32 attributable to the Pd-Me and N-Me groups, respectively, and
broadened doublets at δ 2.12 and 2.75 attributable to nonequivalent
CH2 hydrogens of the puckered five-membered ring.3a At higher
temperatures, ring inversion becomes rapid on the NMR time scale
and exchange of the CH2 hydrogens occurs to give a singlet at δ 2.49
(298 K).
characterization was achieved by H and 13C{1H}NMR
1
spectroscopy.5
(4) See, for instance: (a) J ia, L.; Yang, X.; Stern, C. L.; Marks, T. J .
Organometallics 1997, 16, 842. (b) Giardello, M. A.; Eisen, M. S.; Stern,
C. L.; Marks, T. J . J . Am. Chem. Soc. 1995, 117, 12114. (c) Wang, Q.;
Quyoum, R.; Gillis, D. J .; J eremic, D.; Baird, M. C. J . Organomet.
Chem. 1997, 527, 7. (d) Sarsfield, M. J .; Ewart, S. W.; Tremblay, T.
L.; Baird, M. C. J . Chem. Soc., Dalton Trans. 1997, 3097.
S0276-7333(97)01060-1 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 05/16/1998

Communications
Organometallics, Vol. 17, No. 12, 1998 2383
resonances associated with internal double bonds of the
types -HCdCH- (δ 132.4-129.5, 124.6-123.5),5 as
suggested by the 1H NMR spectra (see above), while the
presence of relatively weak resonances at δ 139.3 and
114.3 indicated the presence of minor amounts of
1
terminal vinyl end groups, consistent again with the H
NMR data. All 13C assignments were confirmed by 2D
HETCOR NMR experiments.
The occurrence of predominantly internal olefinic
groups of the type RCHdCHR is notable, as apparently
very similar palladium catalysts containing phosphines
rather than TMEDA produce ethylene oligomers con-
taining predominantly trisubstituted (R2CdCHR)
groups.6 Perhaps equally striking is a report that the
corresponding 1,5-cyclooctadiene complex produces only
butenes, the products of ethylene dimerization.6 These
divergent results suggest that in addition to the steric
factors mentioned above, subtle, as yet not understood,
differences in electronic factors can also greatly influ-
ence the mode(s) of insertion and â-elimination pro-
cesses in this type of palladium-based catalysts.
Ethylene coordination and oligomerization were also
F igu r e 1. 13C{1H} NMR spectrum of the ethylene oligo-
mer.
1
The H spectrum exhibited a broad, main chain CH2
resonance at δ 1.27 (52% of total H) and a somewhat
weaker methyl multiplet centered at δ 0.87 (23% of total
H). The presence of internal unsaturation was indicated
by the presence of resonances at δ 2.04 (8% of total H)
attributed to allylic methylene groups of the type -CH2-
CHdCH-, a sharp doublet at δ 1.61 attributed to the
methyl group in CH3HCdCH- (3% of total H), and a
prominent, broad resonance centered at δ 5.4 attributed
to cis- and trans-CHdCH- (4% of total H). Much
weaker resonances at δ 5.82 and 4.95 in a 1:2 ratio,
respectively, are attributable to vinyl end groups,
-CHdCH2. Thus, the majority of the unsaturated
functionalities are internal and involve a methyl group
(CH3CHdCH-), suggesting that termination processes
occur predominantly via â-H transfer from Pd-CH-
(CH3)CH2-P moieties. It is also likely that less steri-
cally hindered terminal olefins are formed but that these
are subsequently incorporated into the growing oligo-
mer, consistent with the GC-MS data discussed above
and an ethylene-1-hexene copolymerization study de-
scribed below. In any case, the high degree of unsat-
uration, the high termination-to-insertion ratio indi-
cated by the low molecular weights obtained, and the
high degree of branching are consistent with the high
rate of â-H elimination and the high degree of branching
predicted theoretically for similar catalyst systems.2
Detailed assignment of the very complex 13C{1H}
NMR spectra of the oligomers is beyond the scope of this
report, but a typical 13C{1H} NMR spectrum exhibited
broad, complex multiplet methine, methylene, and
methyl resonances (Figure 1). The spectrum is clearly
consistent with a highly branched material, including
short chain branches of varying lengths (C-2 to C-6)5
as, in addition to the “main chain” methylene resonance
at δ 29.98, there were resonances of methyl, propyl,
butyl, and longer branches.5 There were also multiple
1
monitored directly by H NMR spectroscopy. On addi-
tion of 1 equiv of ethylene to a 1:1 mixture of PdMe2-
(tmeda) and B(C6F5)3 in CD2Cl2 at 193 K, the cationic
complex [PdMe(CD2Cl2)(tmeda)]+ formed (eq 2), as
1
shown by the H NMR spectrum: δ 0.30 (s, 3H, Pd-
Me), 0.33 (3H, br, B-Me), 4.4 (2H, br, C2H4), 4.7 (2H,
br, C2H4);7 the tmeda resonances were very complex.
[PdMe(CD2Cl2)(tmeda)]+ + C2H4 f
[PdMe(C2H4)(tmeda)]+ + CD2Cl2 (2)
Upon warming to 213 K, the ethylene resonance weak-
ened and disappeared and was replaced by resonances
at δ 1.25-1.29 (CH2) and 0.82-0.88 (Me) attributable
to products of insertion. Thus, migratory insertion of
ethylene into the palladium alkyl bond was already
occurring at this low temperature, although an absence
of olefinic resonances suggests that either â-elimination
was not occurring to a significant extent or low molec-
ular weight, unsaturated products were being incorpo-
rated into the growing polymer chains. As a test of the
latter possibility, a similar ethylene polymerization
1
reaction, also monitored by H NMR spectroscopy, was
carried out in the presence of 1-hexene. Interestingly,
the olefinic resonances of the 1-hexene weakened and
disappeared as the temperature was raised above ∼250
K, and thus, lower weight oligomeric products may well
be incorporated into the final products and could ac-
count for much of the observed branching.
Polymerization catalysts containing 13C-enriched meth-
yl ligands have been used extensively8 to gain mecha-
(6) (a) Wojcinski, L. M.; Getzie, J .; Sen, A. Polym. Prepr. (Am. Chem.
Soc., Div. Polym. Chem.) 1997, 38 (2), 273. (b) The 1H NMR spectra
exhibit broad olefinic resonances at ca. δ 5.2. Wojcinski, L. M.; Getzie,
J .; Sen, A. Abstracts of Papers, 214th National Meeting of the American
Chemical Society, Las Vegas, NV, Fall 1997; Div. of Polym. Sci., paper
0096. (c) Aspects of this work were reported after submission of this
communication, see: Kim, J . S.; Pawlow, J . H.; Wojcinski, L. M.;
Murtuza, S.; Kacker, S.; Sen, A. J . Am. Chem. Soc. 1998, 120, 1932.
(7) For similar cationic ethylene complexes, see: (a) Rix, F. C.;
Brookhart, M.; White, P. S. J . Am. Chem. Soc. 1996, 118, 4746. (b)
Hill, G. S.; Rendina, L. M.; Puddephatt, R. J . J . Chem. Soc., Dalton
Trans. 1996, 1809. (c) On addition of 2.5/1 ethylene/Pd at 193 K, the
NMR spectrum of the TMEDA complex exhibited only a single,
exchange-broadened olefin resonance at δ 4.65.
(5) (a) Axelson, D. E.; Levy, G. C.; Mandelkern, L. Macromolecules
1979, 12, 41. (b) Cheng, H. N.; Smith, D. A. Macromolecules 1986, 19,
2065. (c) Cheng, H. N.; Lee, G. H. J . Polym. Sci. Part B Polym. Phys.
1987, 25, 2355. (d) Hansen, E. W.; Blom, R.; Bade, O. Polymer 1997,
38, 4295. (e) McCord, E. F.; Shaw, W. H. J .; Hutchinson, R. A.
Macromolecules 1997, 30, 246. (f) Kolbert, A. C.; Didier, J . J . Polym.
Sci., Part B: Polym. Phys. 1997, 35, 1955. (g) Kolbert, A. C.; Didier, J .
G.; Xu, L. Macromolecules 1996, 29, 8591. (h) Randall, J . C.; Ruff, C.
J .; Kelchtermans, M. Recl. Trav. Chim. Pays-Bas 1991, 110, 543. (i)
Usami, T.; Shigeru, T. Macromolecules 1984, 17, 1756. (j) The Aldrich
Library of 13C FT NMR Spectra 1st ed.; Aldrich Chemical Co:
Milwaukee, WI, 1993.

2384 Organometallics, Vol. 17, No. 12, 1998
Communications
nistic insight into the nature of polymerization pro-
cesses, and a H NMR study of ethylene polymerization,
ca. δ 0.87 grew sufficiently intense that the 13C-coupled
resonances were obscured. On warming to 283 K,
ethylene was no longer present in solution.
1
utilizing 13C-enriched Pd(13Me2)(tmeda), was carried
out. The Pd-13Me (δ 0.32, br d, J HC 136 Hz) and B-13-
Me (δ 0.34, br d, J HC 118 Hz) resonances of [Pd(13Me)-
(C2H4)(tmeda)][B(13Me)(C6F5)3] were particularly no-
table, and on addition of ethylene at 193 K, careful
monitoring of the 1H NMR spectrum revealed the
appearance of a weak double triplet centered at δ 0.86
(J HC 126 Hz). This resonance is attributable to a 13C-
enriched methyl chain end resulting from the initial
migratory insertion of ethylene into the Pd-13Me bond
to give a Pd-CH2CH213CH3 group.9 As the temperature
was raised to 263 K, the reaction proceeded further and
the 13C-coupled resonance gained both in intensity and
in complexity, indicating the formation of a number of
new 13C-enriched enriched sites. Interestingly, similar
coupling of the broad CH2 resonance (∼δ 1.24) to 13C
was not observed in these early stages, and thus, any
enrichment of CH2 groups must be secondary in nature.
Eventually rather broad oligomer methyl resonances at
Surprisingly, while an in situ 13C{1H} NMR spectrum
at 298 K exhibited a very intense [B(13Me)(C6F5)3]-
quartet at δ 10.7, none of the resonances of the polymer
formed at this temperature (methyl at δ 11.2-22.9,
methylene at δ 27.2-40.0, methine at δ 38.0-39.5)
exhibited an even remotely comparable intensity. En-
riched oligomer was, therefore, prepared utilizing 13C-
enriched catalyst at 298 K under 10 atm of ethylene,
but again, we observed no resonances with enhanced
intensities indicative of specific 13C-enrichment in the
13C{1H} NMR spectrum. Indeed, the relative intensities
of all CH2 and CH3 resonances in the region δ 10-40
were similar to those observed in the 13C{1H} NMR
spectrum of the oligomer formed using unlabeled cata-
lyst (Figure 1). Thus, it would seem that the combina-
tion of product incorporation and a chain-walking
process at higher temperatures must result in random
redistribution of the original palladium-bonded methyl
group to a large number of sites in the oligomeric
product.
(8) (a) Zambelli, A.; Ammendola, P.; Grassi, A.; Longo, P.; Proto, A.
Macromolecules 1986, 19, 2703. (b) Zambelli, A.; Sacchi, M. C.;
Locatelli, P.; Zannoni, G. Macromolecules 1982, 15, 211. (c) Zambelli,
A.; Locatelli, P.; Sacchi, M. C.; Rigamonti, E. Macromolecules 1980,
13, 798. (d) Tritto, I.; Li, S. X.; Boggioni, L.; Sacchi, M. C.; Locatelli,
P.; O’Neil, A. Macromol. Chem. Phys. 1997, 198, 1347. (e) Longo, P.;
Grassi, A.; Pellecchia, C.; Zambelli, A. Macromolecules 1987, 20, 1015.
(9) The presence of a small amount of 13C-enriched methane (δ 0.17,
d, J HC 125 Hz), resulting from decomposition, was also observed, but
the intensity of this resonance remained constant during warming of
the sample to 263 K.
Ack n ow led gm en t. This research was made possible
by funding from the Natural Sciences and Engineering
Research Council of Canada (Research and Strategic
Grants Programs).
OM971060P