Incorporation of vinyl chloride in insertion polymerization

Article abstract of DOI:10.1002/anie.201209724

A palladium-catalyzed insertion copolymerization of vinyl chloride (VC) and ethylene gave chlorinated polyethylene with CH₃CHCl(CH ₂)_n units (see scheme; C blue, O red, Pd orange, S yellow). The CH₃CHCl end groups form by 2,1-insertion of VC into palladium hydride complexes, as revealed by detailed labeling studies. This first example of VC incorporation (up to 0.4 mol %) clearly shows that insertion (co)polymerization of VC is in principle feasible.

Full text of DOI:10.1002/anie.201209724

Angewandte
Chemie
DOI: 10.1002/anie.201209724
Copolymerization
Incorporation of Vinyl Chloride in Insertion Polymerization**
Hannes Leicht, Inigo Gçttker-Schnetmann, and Stefan Mecking*
Dedicated to Pierre Braunstein on the occasion of his 65th birthday
Catalytic polymerizations of ethylene and propylene are
practiced industrially on a large scale. Vinyl chloride (VC) is
the monomer produced on the largest scale after ethylene and
propylene.[1] However, the incorporation of VC into (co)poly-
mers by insertion polymerization has remained elusive, and
VC even inhibits polyethylene formation.[2] The origin of this
reactivity is believed to be very specific for VC. Unlike for
other vinyl monomers, such as acrylates,[3–5] acrylonitrile,[6]
and others,[7–12] inhibition of polymerization by coordination
Scheme 1. 2,1-Insertion of VC into a metal methyl bond, net-1,2-
insertion by chain walking, and b-Cl elimination.
of the functional groups of the free monomer and of repeat
units formed from its incorporation into the polymer chain is
not considered to be problematic for VC. Vinyl chloride is
a weak k-Cl donor (comparable, for example, to methylene
chloride). Extensive studies with early- and late-transition-
metal catalysts have revealed that a different more funda-
mental problem prohibits the catalytic polymerization of
vinyl chloride.[2a–d] Thermodynamically favorable b-chloride
elimination occurs subsequently to the incorporation of VC
into the growing chain to afford inactive metal chloride
complexes, and thus irreversibly deactivates the catalyst.
The ß-chloro substituted alkyl species that undergo this
detrimental reaction are formed by a net 1,2-insertion of VC
into the growing chain. For an insertion of VC into late-
no detectable chlorine content is formed by these catalysts.[2e]
We now report, that among complexes L1Pd to L4Pd studied
(Figure 1), complexes [L2PdCH3(dmso)], [{L2PdCH3}2],
[L2PdH(PtBu3)], and [L4PdCH3(dmso)] catalyze the forma-
tion of chlorinated copolymers from ethylene and VC owing
to a partial suppression of chain walking after monomer
insertion.
[2a,b,d]
À
transition-metal carbon bonds, DFT calculations
and
experimental evidence point to an initial insertion in a 2,1-
fashion. However, the propensity of late-transition-metal
polymerization catalysts for “chain-walking” results in a net
1,2-incorporation of VC (Scheme 1). This unfavorable out-
come has been observed for a-diimine palladium complexes,
bis(imino)pyridine iron and cobalt complexes, and for salicy-
laldiminato, and phosphine–enolato nickel complexes.[2c,d,13]
Owing to the low propensity of neutral phosphine–
sulfonato palladium complexes for chain walking,[14] we
decided to study their polymerization properties towards
vinyl chloride. Most recently, related cationic phosphine–
phosphine oxide palladium catalysts have been reported to
exhibit reduced activity for ethylene polymerization in the
presence of VC. However, an ethylene homopolymer that has
Figure 1. Catalyst precursors L1Pd to L4Pd and ORTEP plots of
[L2PdCH3(dmso)] and [{L2PdCH3}2] with hydrogen atoms omitted for
clarity and thermal ellipsoids set at 50% probability.[15]
[*] H. Leicht, Dr. I. Gçttker-Schnetmann, Prof. Dr. S. Mecking
Department of Chemistry, University of Konstanz
Universitꢀtstrasse 10, 78464 Konstanz (Germany)
E-mail: stefan.mecking@uni-konstanz.de
Prior to copolymerization experiments, the reactivity of
[L1PdCH3(dmso)] toward VC was assessed by monitoring
experiments by NMR spectroscopy (caution: VC is carcino-
genic and requires special safety measures, see the Supporting
Information). VC (4.3 equiv) insertion into [L1PdCH3-
(dmso)] occurs, albeit slowly, at 298 K in [D2]methylene
chloride solution. A chloroalkyl palladium complex is not
detected, but formation of propylene (by b-Cl elimination)
[**] Financial support by Solvay SA, Belgium is gratefully acknowledged.
We enjoyed stimulating discussions with Vincent Bodart and
Thomas Hermant. Thomas Rꢁnzi is gratefully acknowledged for X-
Ray diffraction analyses of [L2PdCH3tmeda0.5] and [{L2PdCH3}2],
and for providing complex [L2PdCH3(dmso)].
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201209724.
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experiments resulted in the formation of a poly-
mer, which was analyzed by NMR spectroscopy
and gel permeation chromatography (GPC;
Table 1; for details see the Supporting Informa-
tion). During these polymerizations the color of
the reaction mixture changes from nearly color-
less to the pale yellow color that is indicative of
decomposed catalyst. Based on this observation
the catalyst half-life time under polymerization
conditions is estimated to approximately 20 min
(see the Supporting Information). Generic cata-
lyst decomposition pathways have been recently
identified.[14b] However, we believe that net 1,2-
insertion of VC followed by b-chloride elimina-
tion as observed in the experiments monitored
by NMR spectroscopy (see above) constitutes
a major decomposition route.
Scheme 2. Formation of propylene, iso-butene (1), and 4-methyl-pent-1-ene (2) by
reaction of VC with [L1PdCH3(dmso)]. [L1PdCH2CH(CH3)2], propylene, 1, and 2 were
all identified by NMR spectroscopy.
NMR analyses of the polymers obtained
after net 1,2-insertion of VC is evident after 5 min. The
propylene formed reacts faster than VC with the remaining
[L1PdCH3(dmso)] to form a palladium isobutyl complex,
indicate that CHCl units have not been incorporated in a mid-
chain fashion in detectable amounts.[16] However, signals
typical of (long-chain) 2-chloroalkanes were present in the 1H
and 13C NMR spectra, indicating up to 0.1% incorporation of
VC when complexes [L2PdCH3(dmso)], [{L2PdCH3}2], or
[L4PdCH3(dmso)] were used (Figure 2 and Table 1). Most
characteristic, the CH3CHClCH2-R proton resonates at
4.08 ppm as a virtual sextet with JHH = 6.3 Hz, which is
[L1PdCH2CH(CH3)2],
by
1,2-insertion
(Scheme 2).
[L1PdCH2CH(CH3)2] undergoes b-H elimination to afford
iso-butene (1), as identified by comparison to genuine
samples of 1. In addition, 4-methylpentene (2) forms either
by VC insertion into [L1PdCH2CH(CH3)2] followed by b-Cl
elimination, or by double insertion of propylene into a palla-
dium hydride followed by b-hydride elimination. Further-
more, acetaldehyde slowly forms by (catalyzed) hydrolysis of
VC owing to the presence of traces of water.
Given the known higher propensity for ethylene insertion
into [L1PdCH3(dmso)] than for propylene insertion, these
results suggest that ethylene–VC copolymerizations need to
be conducted at moderate (3–4 bar) ethylene pressure to
facilitate significant VC insertion. Except for the control
experiments in the absence of catalyst or ethylene, all
1
correlated to a 13C signal at 58.44 ppm through JCH. This
CHCl signal couples to a doublet (3JHH = 6.6 Hz) at 1.56 ppm
(CH3CHClCH2-R; 13C NMR: 25.16 ppm) and to a multiplet
at d = 1.78 ppm (CH3CHClCH2-R; 13C NMR: 40.54 ppm), as
deduced from 1H homodecoupling, 1D 1H TOCSY-,
1H,1H gCOSY, and 1H,13C gHSQC experiments. Furthermore,
these resonances match the respective resonances of 2-
chloroheptane, 2-chlorodecane, and 12-chlorotridecan-1-ol
(for details see the Supporting Information).[17]
Table 1: Ethylene–VC copolymerizations with catalyst precursors L1Pd to L4Pd.[a]
entry catalyst
precursor
T
cat.
p(C2H4) VC nVC:ncat yield TON[b] inc. VC n[PE]/n-
Mn (NMR)
[gmolÀ1 [f]
Mn (GPC)
[gmolÀ1 [g]
] (Mw/
[K] [mmol] [bar]
[g]
[mg] C2H4 [mol%][c] [mCPE][d]
]
Mn)
1
2
3
4
5
6
7
8
[L1PdCH3(dmso)] 353 50
4
4
4
4
–
4
4
4
4
4
4
4
3
3.2
2.2 704
1.9 724
1.5 533
0.4 142
0.5 178
3.2 1089
1.2 409
1.8 640
2.8 996
2.3 818
123 88
100 85
179 142
177 140
–
n.a.[e]
3.3:1
4.6:1
9.8:1
n.a.[e]
5.0:1
2.4:1
5.2:1
n.a.[e]
3.4:1
n.a.[e]
3.2:1
3.6:1
1.8:1
4700
8000
8000
8400
–
4500
14400
7100
5900
13400
–
2600 (1.8)
4200 (2.4)
4400 (2.6)
4000 (1.6)
–
2600 (2.2)
8000 (1.5)
5500 (1.8)
6600 (1.6)
6000 (2.5)
–
[{L2PdCH3}2]
[{L2PdCH3}2]
[{L2PdCH3}2]
[{L2PdCH3}2]
353 42
343 45
313 45
313 45
0.084
0.066
0.029
–
0.106
0.049
0.063
–
0.048
–
0.082
0.096
0.399
–
–
[L2PdCH3(dmso)] 343 47
[L2PdCH3(dmso)] 323 47
[L2PdCH3(dmso)] 343 45
[L3PdCH3(dmso)] 303 45
[L4PdCH3(dmso)] 333 45
127 96
268 203
282 223
315 250
298 236
9
10
11
12
13
14
–
343
–
3.6
–
–
–
[L2Pd13CH3(dmso)] 343 47
2.6 885
2.6 885
1.7 1432
215 163
151 115
8100
6100
3900
5300 (1.8)
3000 (2.2)
n.d.[h]
[L2Pd13CH3(dmso)] 343 47
[L2PdH(PtBu3)]
358 19
15
28
[a] For 2 h in 100 mL toluene, see the Supporting Information. [b] Turnover number (TON): polymerized C2H4 per Pd present (mol/mol).
1
[c] Incorporated VC determined by integration of the CH3CHClCH2-R H NMR signal versus polyethylene (PE) backbone signals in C2D2Cl4 at 373–
1
383 K. [d] Ratio of PE chains/monochlorinated PE chains (mCPE) determined by H NMR spectroscopy from the ratio of olefinic group versus
CH3CHCl group signals. [e] Not applicable. [f] Determined by integration of the olefinic versus backbone 1H NMR signals. [g] Determined by high
temperature GPC in trichlorobenzene at 403 K versus linear PE standards. [h] Not determined.
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Figure 2. 1H NMR spectrum of ethylene–VC copolymer (entry 2,
Table 1) with characteristic CH3CHClCH2 end group signals.
Figure 3. 13C NMR spectrum of ethylene–VC copolymer (entry 13,
Table 1) obtained with [L2Pd13CH3(dmso)], indicating 1JCC-coupled S2
end groups, whereas the CH3CHCl end group has natural abundance
These CH3CHClCH2-R groups may be formed either by
1,2-insertion of VC into the palladium methyl bond of
[L2PdCH3(dmso)], [{L2PdCH3}2], or [L4PdCH3(dmso)] and
subsequent ethylene insertion (also cf. Scheme 3a), or by 2,1-
insertion into a palladium hydride complex and subsequent
ethylene insertion (also cf. Scheme 3b).[18] To determine
13C.
[L2PdD(PtBu3)] also enables ethylene–VC copolymerization
with incorporation of CH3CHClCH2 groups reaching
0.4 mol% (Table 1, entry 14), although the yield is lower
than for reactions using [L2PdCH3(dmso)], [{L2PdCH3}2], or
[L4PdCH3(dmso)].
Having established that VC incorporation into the
copolymer proceeds after b-hydride elimination by insertion
into the resultant palladium hydride species, it is evident that
the materials obtained are mixtures of ethylene homopolymer
chains, initiated by palladium methyl complexes, and of
monochlorinated polyethylene (mCPE) chains, initiated by
palladium hydride complexes. The ratio of olefinic/CH3CHCl
groups enables a rough estimate of the portion of all polymer
chains that consist of chlorinated chains. For example, for
entry 2 in Table 1 this ratio indicates that approximately every
fourth chain is chlorinated and that therefore approximately
30% of all palladium hydride complexes that initiate chain
growth produce VC-containing chains.[19] These results are
summarized for all polymerizations in Table 1, column 11.
In conclusion, for the first time an insertion copolymer-
ization of VC with ethylene has yielded chlorine-containing
copolymers. NMR analysis of the polymers, labeling, and
stoichiometric insertion studies reveal that incorporation of
CHCl units proceeds by 2,1-insertion of VC into palladium
hydride species. After this 2,1-insertion of VC, ethylene
insertion resulting in monochlorinated polyethylene is com-
petitive to chain walking (which through the net 1,2-insertion
of VC would result in a detrimental b-chloride elimination).
Regardless of the limited incorporation of vinyl chloride,
this first isolation of chlorine-containing polymers in combi-
nation with a mechanistic understanding represents a signifi-
cant impetus to a long-standing challenge. Future studies will
focus on further suppression of chain walking, which results in
the problematic net 1,2-insertion of VC, and on facilitating in-
chain incorporation of VC into polymers.
Scheme 3. Experimentally excluded 1,2-insertion of VC in
[L2Pd13CH3(VC)] (a), and 2,1-insertion of VC into palladium hydride
species as a source of 12CH3CHClCH2-R units found in ethylene VC
copolymers (b) and of 2-deuterated VC in a scrambling experiment (c).
which of these insertion modes is occurring, 13C-labeled
[L2Pd13CH3(dmso)] was used as a catalyst precursor. Analysis
of the polymers formed (Table 1, entries 12 and 13) indicates
that the 13C-labeled methyl group is located in > 95% in the
unfunctionalized polymer end group as evidenced by the
natural abundance 13CH3CH2CH2 (S2) end group signal split
into a doublet with 1JCC = 34.8 Hz and the 13C-labeled
13CH3CH2CH2 (S1) signal in comparison to the CH3CHClCH2
1
1
signals (no JCH, no JCC, and no signal enhancements as
a result of the 13C label were detected; Figure 3 and the
Supporting Information). That is, the observed CH3CHCl
group in the polymer is formed by 2,1-VC insertion into
a palladium hydride species [L2PdH(VC)] (Scheme 3b) and
not by 1,2-insertion into [L2Pd13CH3(VC)] (Scheme 3a).
This result is fully corroborated by a reaction (NMR tube)
of palladium deuteride complex [L2PdD(PtBu3)] with VC:
this reaction results in scrambling of the deuterium label into
the 2-position of VC to yield 2-deuterated VC by 2,1-insertion
and b-hydride elimination, but not into the 1-position to yield
1-deuterated VC (Scheme 3c; for experimental details see the
Supporting Information). It is noteworthy, that the use of
Received: December 5, 2012
Keywords: homogeneous catalysis · insertion · palladium ·
.
polymerization · vinyl compounds
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see
the
Supporting
Information.
CCDC 913301
([L2PdCH3tmeda0.5]), 913303 ([L2PdCH3(dmso)]), 913299
([{L2PdCH3}2]), 913300 ([{L3Pd}2]), and 913302 ([L4PdCH3-
(dmso)]) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
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[16] Isolated CHCl units incorporated in-chain have characteristic
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determined by: E. Boz, A. J. Nemeth, I. Ghiviriga, K. Jeon, R. G.
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[19] This estimate accounts for formation of one ethylene homopo-
lymer chain by palladium methyl complexes [L2PdCH3(dmso)],
[{L2PdCH3}2], and [L4PdCH3(dmso)] prior to formation of
mCPE chains by palladium hydrides. Note that when starting
from [L2PdH(PtBu3)] (entry 14, Table 1) approximately 36% of
all palladium hydride species form mCPE chains.
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