Insertion polymerization of acrylate

Full text of DOI:10.1021/ja808017n

Published on Web 12/30/2008
Insertion Polymerization of Acrylate
Damien Guironnet, Philipp Roesle, Thomas Ru¨nzi, Inigo Go¨ttker-Schnetmann, and Stefan Mecking*
UniVersity of Konstanz, Chair of Chemical Materials Science, Department of Chemistry,
D-78457 Konstanz, Germany
Received October 10, 2008; E-mail: stefan.mecking@uni-konstanz.de
Catalytic insertion polymerization of ethylene and propylene is
employed for the production of more than 60 million tons of
polyolefins annually.1 An insertion polymerization of electron
deficient polar-substituted vinyl monomers has remained elusive,
however. For example, polyacrylates are not accessible by insertion
polymerization.2
the methyl and the phosphine ligands mutually cis to each other.
While 1a-dmso exhibits κ-O coordination of dmso to the Ni(II)
center, in 2b-dmso coordination to the less electrophilic Pd(II)
center occurs via the sulfur atom (Figure 1 and Supporting
Information, SI).
A major advance was the finding by Brookhart et al. that cationic
Pd(II) diimine complexes can copolymerize ethylene and 1-olefins
with acrylates.3 Due to “chain walking” of the catalyst, the highly
branched copolymers contain acrylate units preferentially at the end
of branches. Linear ethylene-acrylate copolymers are obtained with
neutral Pd(II) phosphinosulfonate complexes, which do not “chain
walk”.4 These catalyst are also compatible with a broad scope of
functional vinyl monomers.5,6 The polymers obtained contain
ethylene (or 1-olefin) as the major component (g75 mol %), and
no consecutive insertions of electron deficient vinyl monomer into
the polymer chain have been accounted for. An acrylate unit inserted
in the polymer chain can κ-O coordinate to the metal center via
the carbonyl group. Further chain growth requires opening of this
chelate by ethylene. This mechanism has been demonstrated for
the diimine complexes3 and is also suggested for the phosphino-
sulfonate complexes.4
Figure 1. ORTEP plot of 2b-(K-S)-dmso. Ellipsoids are shown with 50%
probability. Hydrogen atoms are omitted for clarity.
The relative binding strength of dmso to the Pd(II) center in
1
We report on multiple insertion of acrylate in homo- and
copolymerizations, employing appropriately labile-substituted cata-
lyst precursors.
comparison to N-donor ligands was assessed by H NMR spec-
troscopy at 25 °C for 2a-dmso. Upon addition of 1.5 equiv of
NMe2nBu, pyridine or lutidine, the Pd-Me resonances of 2a-
NMe2nBu (δ 0.03), 2a-pyr (δ 0.24), and 2a-lut (δ -0.04),
respectively, appeared along with free dmso (δ 2.54), while the
characteristic resonances of 2a-dmso disappeared completely (cf.
SI).Thiscorrespondstoequilibriumconstantsfor[(P∧O)PdMe(dmso)]
+ L a [(P∧O)PdMe(L)] + dmso of Keq g 102 (L ) NMe2nBu,
pyridine, or 2,6-lutidine).
In line with this labile binding of dmso, 1,2-dmso were found to
be highly active single component catalyst precursors for the polym-
erization of ethylene. 1a-dmso produces oligomers with an average
activity approaching 106 TO h-1 (cf. SI) which is among the highest
values reported for neutral Ni(II) catalysts to date.12 For the less
electrophilic Pd(II) analogues 2-dmso high activities are observed at
The aforementioned neutral catalysts have been studied as in
situ mixtures of metal sources and ligands,4,7 and employing well-
defined single-component catalyst precursors [(P∧O)PdMe(L)] (L
8-11
1
) pyridine, PPh3, /2 tmeda (tmeda ) Me2NCH2CH2NMe2)).
However, these N- and P-based ligands L are relatively strong
σ-donors, and it must be anticipated that they severely compete
with the monomer for coordination at the metal center, rendering
a large portion of the metal centers in a dormant inactive state.
This not only adds complexity to studies of the polymerization
reaction, but limits the range of acrylate incorporation as a certain
ethylene concentration is required to achieve any substantial
polymerization rate.
Table 1. Ethylene Polymerizationa
catalyst
entry precursor [bar]
p
average TOF [mol (C2H4) PE
mol (Pd)-1 h-1
Mnb
branchingc
]
yield [g] [103 g mol-1] Mw/Mnb [/1000 C]
1-1 2a-dmso
1-2 2a-dmso
1-3 2a-dmso 7.5
1-4 2a-dmso 10
1-5 2a-pyr
1-6 2a-pyr
1-7 2a-pyr 10
1-8 2b-dmso
2
5
4.1 × 104
1.0 × 105
1.0 × 105
1.0 × 105
2.2 × 104
3.0 × 104
4.0 × 104
1.1 × 105
1.99
4.92
4.94
5.04
1.05
1.49
1.95
1.59
6.3
12.6
18.2
13.1
11.2
16.6
19.5
11.3
2.3
2.2
1.9
1.9
2.1
2.2
1.9
1.9
8
3
1
1
4
2
1
<1
As a potentially more labile ligand, dimethylsulfoxide (dmso)
was studied. Complexes 1,2-dmso were obtained in high yield from
the respective phosphine sulfonic acid and [(tmeda)MMe2] in dmso
as a solvent. While tertiary amines bind more strongly to the Ni(II)
and Pd(II) centers than dmso (vide infra), the high boiling point of
dmso enables dynamic removal of tmeda under vacuum and finally
formation of the dmso complexes.
2
5
2
a Reaction conditions: 100 mL of toluene; 80 °C, 3.5 µmol of Pd(II)
(entry 1-8: 90 °C, 1 µmol); 30 min polymerization time. b Determined by
GPC at 160 °C vs linear PE. c By 13C NMR at 130 °C; only methyl
branches observed.
In the solid state, 1a-dmso and 2b-dmso possess a distorted
square planar coordination geometry around the metal center, with
9
422 J. AM. CHEM. SOC. 2009, 131, 422–423
10.1021/ja808017n CCC: $40.75
2009 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Ethylene-MA Copolymerizationa
d
XMA
entry
catalyst precursor
p [bar]
MA concn [mol L-1
]
polymer yield [g]
TOF C2H4
TOF MA
Mn [103 g mol-1
]
Mw/Mn
b
b
2-1
2-2
2-3
2-4
2-5
2-6
2-7
2a-pyr
5
5
5
5
5
0.6
0.6
1.2
2.5
5
0.82
2.57
1.36
0.74
0.36
0.66
0.72
1128
3545
1317
493
149
433
120
368
391
268
161
243
258
9.6%
9.4%
23%
35%
52%
36%
34%
2.4
2.5
4.3
3.1
1.8
2.8
3.0
1.8
2.3
2.0
1.8
1.6
1.8
1.8
b
b
2a-dmso
2a-dmso
2a-dmso
2a-dmso
2a-dmso
2a-dmso
c
c
c
c
c
c
c
c
10
15
5
7.5
c
c
496
a Reaction conditions: total volume toluene
+ MA, 50 mL; 95 °C; 20 µmol Pd(II); 1 h
reaction time. b From GPC at 160 °C in
d
1,2,4-trichlorobenzene vs linear PE. c From GPC at 40 °C in THF, vs polystyrene standards. From H NMR at 130 °C.
1
low ethylene pressure. Activities are independent of monomer con-
centration at p(C2H4) g 5 bar for 2a-dmso (Table 1). This saturation
behavior suggests that dmso does not compete effectively with ethylene
for binding to the metal center even at these low pressures. By contrast,
activities observed with 2a-pyr and 2a-lut are lower and saturation
kinetics require p(C2H4) > 10 bar (Table 1 and SI).8
Ethylene-methyl acrylate (MA) copolymerization with 2a-dmso also
proceeds with substantially higher activity than with 2a-pyr (at only
5 atm C2H4; Table 2). Under otherwise identical reaction conditions,
molecular weight and MA-content of the obtained materials is virtually
identical (entries 2-1 and 2-2). This confirms that the same
catalytically active species are operative in both cases, which however
are in an unfavorable equilibrium with dormant pyridine complexes
in the case of 2a-pyr.
The reactivity of 2a-dmso facilitates copolymerizations at high
[MA]:[C2H4] ratios. An acrylate incorporation of 52 mol % MA was
observed at p(C2H4) ) 5 bar, [MA] ) 5 mol L-1 (entry 2-5). 13C
NMR analysis (cf. SI) of copolymers with XMA > 30 mol-% reveal,
in addition to isolated acrylate repeat units,4 “alternating” acrylate-
ethylene-acrylate sequences and consecutive acrylate units in the
polymer chain.
At a given p(C2H4), acrylate incorporation increases with increasing
[MA] as expected. This is primarily due to a decrease in rate of ethylene
incorporation, while the overall rate of MA incorporation is similar
(entries 2-2 to 2-4).13 This implies that monomer insertion after an
acrylate insertion is rate determining.
toluene (4 mol L-1) with 80 µmol 2a-dmso at 95 °C for 4 h yields
0.70 g nonvolatile oligomers (TON ≈ 100). FAB-MS analysis of this
sample reveals formation of up to MA-heptamers obtained by MA
insertion into a Pd-H bond; a number average degree of polymeri-
zation DPn ) ca. 5 was estimated by NMR analysis. NMR analysis
(cf. SI) of the product mixture confirms that most chains were formed
by MA-insertion into a Pd-H bond and that 2,1-insertion is predomi-
nant over the 1,2-modus. Analysis of olefinic and aliphatic endgroups
confirms that chain transfer occurs exclusively by ꢀ-H elimination.
In summary, copolymers with unprecedented acrylate incorporations
have been prepared. The homooligomerization of acrylate reported has
all mechanistic features of an acrylate insertion polymerization, namely
multiple insertions before chain transfer occurs. The rate of acrylate
copolymerization with the catalyst system studied appears to be
retarded by an intrinsically slow monomer insertion into the alkyl
species resulting from acrylate insertion.
Acknowledgment. Financial support by the BMBF (project
03X5505) and the DFG (Me 1388/4) is gratefully acknowledged. S.M.
is indebted to the Fonds der Chemischen Industrie.
Supporting Information Available: Detailed experimental procedures
and analytical data. This material is available free of charge via the Internet
at http://pubs.acs.org.
References
In polymerizations at different p(C2H4) and with an approximately
constant ratio of [MA]:[ethylene], not only the copolymer composition
but also activities are found to be virtually independent of the reaction
conditions (entries 2-4, 2-6, and 2-7). This suggests that the rate
determining step is insertion of monomer into the palladium alkyl bond
of [(P∧O)Pd{CH(COOMe)CH2R}(monomer)]. Contrary to current
suggestions,4 opening of four-membered chelates formed by κ-O
coordination of an inserted acrylate unit appears not to be the major
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in CH2Cl2; SI). In combination with the above finding that under
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R-carbonyl substituted alkyl (Scheme 1).14
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Scheme 1. Rate Limiting Step of Copolymerization
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The reaction of 2a-dmso with MA alone at elevated temperature
resulted in multiple consecutive insertions. Reaction of 2 g MA in
JA808017N
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