Mechanistic insights into polar monomer insertion polymerization from acrylamides

Article abstract of DOI:10.1021/ja207110u

Figure Persented: N-Isopropyl acrylamide (NIPAM), N,N-dimethyl acrylamide (DMAA), and 2-acetamidoethyl acrylate (AcAMEA) were copolymerized with ethylene employing [(P∧O)PdMe(DMSO)] (1-DMSO; P∧O = κ²-P,O- Ar₂PC₆H₄SO₂O with Ar = 2-MeOC ₆H₄) as a catalyst precursor. Inhibition studies with nonpolymerizable polar additives show that reversible κ-O-coordination of free amide retards polymerization significantly. Retardation of polymerization increases in the order ethyl acetate ? methyl ethyl sulfone < acetonitrile < N,N-dimethylacetamide ≈ N-methylacetamide ≈ propionic acid < dimethylsulfoxide. Pseudo-first-order rate constants for the insertion into 1-DMSO were determined to increase in the order DMAA < AcAMEA < NIPAM < methyl acrylate. Exposure of 1-DMSO to NIPAM resulted in the formation of consecutive insertion products [(P∧O)Pd(C₆H₁₁NO ₂)_nMe] (n ≤ 3), as determined by electrospray ionization mass spectrometry. The solid-state structure of the methanol adduct of the 2,1-insertion product of NIPAM into 1-DMSO, [(P∧O) Pd{η¹-CH(CONHiPr)CH₂CH₃} (κ¹-O-MeOD)] (2-MeOD), was determined by single crystal X-ray diffraction. Both 2,1- and 1,2-insertions of DMAA into the Pd-Me bond of a [(P∧O)PdMe] fragment occur to afford a ca. 4:1 mixture of chelates [(P∧O)Pd{κ²-C,O-C(CH₂CH₃)C(O)NMe ₂}] (3) and [(P∧O)Pd{κ²-C,O-CH ₂C(CH₃)C(O)NMe₂}] (4). The four-membered chelate of 3 is opened by coordination of 2,6-lutidine (3 + 2,6-lutidine ? 3-LUT) with ΔH° = -41.8(10.5) kJ and ΔS° = -115(37) J mol^-1 K^-1.

Full text of DOI:10.1021/ja207110u

Article
pubs.acs.org/JACS
Mechanistic Insights into Polar Monomer Insertion Polymerization
from Acrylamides
Tobias Friedberger, Philipp Wucher, and Stefan Mecking*
Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz, Universitatsstrasse 10, 78457 Konstanz,
̈
Germany
S
* Supporting Information
ABSTRACT: N-Isopropyl acrylamide (NIPAM), N,N-dimethyl acrylamide (DMAA), and 2-acetamidoethyl acrylate (AcAMEA)
were copolymerized with ethylene employing [(P^O)PdMe(DMSO)] (1-DMSO; P^O = κ2-P,O-Ar2PC6H4SO2O with Ar = 2-
MeOC6H4) as a catalyst precursor. Inhibition studies with nonpolymerizable polar additives show that reversible κ-O-
coordination of free amide retards polymerization significantly. Retardation of polymerization increases in the order ethyl acetate
≪ methyl ethyl sulfone < acetonitrile < N,N-dimethylacetamide ≈ N-methylacetamide ≈ propionic acid < dimethylsulfoxide.
Pseudo-first-order rate constants for the insertion into 1-DMSO were determined to increase in the order DMAA < AcAMEA <
NIPAM < methyl acrylate. Exposure of 1-DMSO to NIPAM resulted in the formation of consecutive insertion products
[(P^O)Pd(C6H11NO2)nMe] (n ≤ 3), as determined by electrospray ionization mass spectrometry. The solid-state structure of
the methanol adduct of the 2,1-insertion product of NIPAM into 1-DMSO, [(P^O)Pd{η1-CH(CONHiPr)CH2CH3}(κ1-O-
MeOD)] (2-MeOD), was determined by single crystal X-ray diffraction. Both 2,1- and 1,2-insertions of DMAA into the Pd−Me
bond of a [(P^O)PdMe] fragment occur to afford a ca. 4:1 mixture of chelates [(P^O)Pd{κ2-C,O−C(CH2CH3)C(O)NMe2}]
(3) and [(P^O)Pd{κ2-C,O−CH2C(CH3)C(O)NMe2}] (4). The four-membered chelate of 3 is opened by coordination of 2,6-
lutidine (3 + 2,6-lutidine ⇌ 3-LUT) with ΔH° = −41.8(10.5) kJ and ΔS° = −115(37) J mol−1 K−1.
polymerization with these catalysts, like vinyl acetate or vinyl
chloride. β-X elimination (X = acetate, chloride) is one
significant decomposition route.4,22 In contrast, with neutral
Pd(II) phosphinesulfonato complexes, linear copolymers of
ethylene with a broad scope of polar vinyl monomers including
acrylonitrile, vinyl acetate, and acrylic acid are formed.2h,5−7
These catalysts have been studied as in situ mixtures of PdII or
Pd0 sources and ligands8 and employing [(P^O)PdMe(L)] (L
= pyridine, 2,6-lutidine, PPh3, 1/2 Me2NCH2CH2NMe2,
dimethylsulfoxide) complexes as well-defined single-compo-
nent catalyst precursors.9 Recent experimental and theoretical
mechanistic studies on the key intermediates of the insertion
(co)polymerization of acrylates, κ2-C,O-coordinated Pd(II)-
alkyl species [(P^O)Pd{κ2-CH(C(O)OMe)CH2CH(C(O)-
OMe)CH2CH3}], resulting from two consecutive 2,1-insertions
of MA, showed that the chelating κ-O-coordination of the
INTRODUCTION
■
Catalytic insertion polymerization of ethylene and propylene is
one of the most well-studied chemical reactions. In terms of
applications, it is employed for the production of more than 70
million tons of polyolefins annually.1 An insertion (co)-
polymerization of electron-deficient polar-substituted vinyl
monomers like acrylates has remained a challenge, however.
Considerable progress in this area has been achieved by the
development of d8-metal (late transition metal) complexes.
Their less oxophilic nature, in comparison with their early
transition metal counterparts, renders them more tolerant
toward polar moieties.2 In the mid 1990s, cationic Pd(II) α-
diimine complexes were reported to catalyze the insertion
copolymerization of ethylene and 1-olefins with acrylates.
Because of the “chain walking” of the catalyst, the highly
branched copolymers contain acrylate units preferentially at the
end of branches. The mechanism of this branch formation is
well understood from extensive variable temperature NMR
studies.3 Such studies have also provided an understanding of
the problems associated with monomers not amenable to
Received: August 2, 2011
Published: December 22, 2011
© 2011 American Chemical Society
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a
Table 1. Copolymerization with Ethylene
b
b
cd
,
e
e
entry
comon.
T [°C] p [bar] comon. conc. [mol L−1] polymer yield [g] TOF C2H4 TOF comon. XAA
Mn [103 g mol−1] Mw/Mn
1-1
1-2
1-3
1-4
1-5
1-6
1-7
1-8
1-9
NIPAM
NIPAM
NIPAM
DMAA
DMAA
DMAA
DMAA
DMAA
DMAA
AcAMEA
80
80
80
80
95
95
93
90
90
95
20
20
20
20
20
10
5
0.2
0.5
1.0
0.2
0.2
0.2
0.2
1.0
2.2
0.2
0.365
0.110
0.044
1.085
1.402
0.568
0.260
0.051
0.042
0.160
619
183
72
3
1
1
3
5
3
2
1
1
4
1.7
5.8
3.4
2.4
9.6
6.8
4.3
3.1
1.2
1.5
0.7
2.0
2.0
1.7
2.0
2.1
2.2
2.1
1.5
1.6
1.4
2.4
3.1
0.6
0.7
1.2
1.7
3.6
3.5
6.9
1922
2482
1000
456
88
5
20
5
72
f
1-10
266
a
b
Reaction conditions: 20 μmol 1-DMSO; total volume toluene + comonomer, 50 mL; polymerization time, 1 h. TOF in mol(monomer
c
d
e
incorporated) mol(catalyst)−1 h−1. Molar incorporation in copolymer. From H NMR in C2D2Cl4 at 130 °C. From GPC at 160 °C in 1,2,4-
trichlorobenzene vs linear polyethylene. Micro size reactor with 5 mL total reaction volume of the liquid phase.
1
f
Scheme 1. Copolymerization of Amide Comonomers with Ethylene (U = Unsaturated and S = Saturated Chain End)
second last inserted acrylate unit significantly retards further
benzenesulfonate) (1-DMSO) was employed. Because of the
relatively weak binding of DMSO by comparison to other
stabilizing ligands, such as pyridine, polymerizations can be
performed with reasonable activities also at low ethylene
pressure and thus at high [comonomer]/[ethylene] ratios.9e At
a relatively high [DMAA]/[ethylene] ratio ([DMAA] = 1 mol
L−1 and p(ethylene) = 5 atm; entry 1-8), an incorporation of
the tertiary amide of 3.6 mol % was observed. As expected, at a
given ethylene pressure, the incorporation of amide in the
polymer increases with the concentration of acrylamide
monomer in the reaction mixture (entries 1-5 vs 1-9 and 1-7
vs 1-8). At the same time, productivity decreases significantly.
Likewise, at a given acrylamide concentration, the ethylene
content of the copolymers increases with ethylene concen-
tration, and notably productivity also increases (entries 1-5 to
1-7). These findings already indicate that amide functions
retard the polymerization reaction. The catalyst was stable over
the time of the reaction and showed single site behavior as
concluded from molecular weight distributions Mw/Mn around
2. A comparison of the copolymer compositions formed under
identical reaction conditions, and of the reaction conditions
under which copolymers of similar degree of incorporation of
amide are formed, shows that DMAA competes less efficiently
with ethylene incorporation than NIPAM.
chain growth.10,11
Materials containing amide moieties are of general interest
due to the exceptional hydrogen-bonding ability of amides,
which can be reflected in specific properties and structures.
However, reports of amide-containing (co)polymers obtained
by insertion polymerization are rare. The copolymerization of
acrylamides passivated with aluminum alkyls by α-diimine
Ni(II) complexes was reported to yield copolymers with low
acrylamide contents (<5 mol %).12 Claverie et al. communi-
cated the preparation of linear ethylene−N,N-isopropylacryla-
mide (NIPAM) and ethylene−N-vinyl-2-pyrrolidone copoly-
mers with Pd(II) phosphinesulfonato complexes [(P^O)PdMe-
(pyridine)] as catalyst precursors.6d,13 An exclusive in-chain
incorporation of NIPAM was observed, and no NIPAM-derived
chain ends were detected. Beyond these findings, no
comprehensive picture of the properties of amides in insertion
polymerizations and a mechanistic understanding exist. We
now give a full account of the reactivity of acrylamides in
insertion (co)polymerizations catalyzed by neutral phosphine-
sulfonato Pd(II) complexes and rationalizations on polar
monomer insertion polymerization in general from the insights
gained.
RESULTS AND DISCUSSION
By comparison to previous studies of ethylene−methyl
acrylate (MA) copolymerization, the activities and incorpo-
rations in the copolymerizations of amides (cf. Table 1) are
lower. Under otherwise similar conditions (5 atm ethylene,
[MA] = 0.6 mol L−1, 95 °C), 1-DMSO yielded 2.57 g of an
ethylene−MA copolymer with 9.4 mol % MA content.9e To
shed light on the origin of this different behavior,
■
Copolymerization Studies and Copolymer Micro-
structure. The copolymerization of ethylene with the tertiary
amide dimethylacrylamide (DMAA), and for comparison also
with NIPAM, was studied (Table 1 and Scheme 1). As a
catalyst precursor, the DMSO-coordinated complex [(P^O)-
PdMe(DMSO)] (P^O = 2-[di(2-methoxyphenyl)phosphino]-
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copolymerizations of 2-acetamidoethyl acrylate (AcAMEA)
(entry 1-10) were studied. Whereas the copolymer was
obtained in a comparably low yield of 0.15 g, the incorporation
rate of 6.9 mol % AcAMEA was about two times higher than in
the ethylene−acrylamide copolymerizations. As expected, these
incorporations are more similar to MA incorporation, and the
lower incorporation of AcAMEA vs MA can be rationalized by
the higher steric bulk of the former, which disfavors
coordination vs the ethylene comonomer. The lower yield
suggests that amide moieties in the monomer in general, also in
a remote position, hinder the polymerization reaction (vide
infra).
equiv) of a nonpolymerizable saturated amide, N-methyl
acetamide (MAcA) or N,N-dimethyl acetamide (DMAcA),
respectively, was added to an NMR tube charged with 1-
DMSO in methylene chloride-d2 at room temperature, fast
exchange of coordinated DMSO and amide was observed. The
corresponding highfield-shifted proton resonance of DMSO (δ
= 2.65 and 2.70 ppm, respectively) did not correspond to
bound (δ = 2.91 ppm) or free (δ = 2.54 ppm) DMSO (Figures
S8 and S9, Supporting Information). No further reaction with
the acetamides was observed, even after warming the samples
to 60 °C for 90 min. This stability of the catalyst precursor in
the presence of amides at elevated temperature is in agreement
with ethylene homopolymerization data in the presence of
DMAcA (Table S1, Supporting Information). Average activities
determined from the polymer yields in 15 and 60 min
polymerization experiments are similar; in detail, the activity is
lower by one-fifth in the one hour experiment. A similar
decrease in activity over time was also found in homopolyme-
rizations of ethylene in the absence of any polar additives.6g
Catalyst activities in ethylene homopolymerizations employing
1-DMSO decreased with increasing acetamide concentration in
the polymerization mixture (Table 2). Compared with a
High-temperature NMR spectroscopy (see the Supporting
Information) unambiguously confirmed the copolymer nature
of the materials. The composition of the end groups was of
particular interest as the reported ethylene−NIPAM and
−NVP copolymers did not show end groups derived from
polar monomer,6d which differs from copolymers of ethylene
and other electron-withdrawing substituted polar vinyl
monomers obtained with Pd(II)−phosphinesulfonato com-
plexes where usually the majority of end groups are based on
the polar vinyl monomer. The ethylene−NIPAM copolymers
obtained here with 1-DMSO (entries 1-1 to 1-3) possessed no
observable NIPAM-derived end groups, in accordance with
previous findings by Claverie et al.6d In the ethylene−DMAA
copolymer with 3.5 mol % incorporation (entry 1-9), DMAA-
based repeat units were found to be incorporated in-chain, in
saturated, and in unsaturated chain-ends in a ratio of 6:3:1. The
ratio of saturated ethylene- and DMAA-derived end groups was
found to be 1:2, which roughly reflects the concentration of the
monomers14 in the reaction mixture; that is, there is no strong
preference for one of the monomers regarding insertion into a
Pd−H bond. This differs from chain growth by insertion into
alkyls, in which ethylene is preferred over the polar monomer
(Scheme 1). Accordingly, no saturated end groups resulting
from an insertion of DMAA and AcAMEA into the Pd−Me
bond of 1-DMSO were observed. In the unsaturated end
groups, ethylene-derived groups prevail by ca. 4:1. Considering
that the incorporation ratio of ethylene vs DMAA in the
polymer backbone is ca. 30, this indicates that β-H elimination
occurs preferentially from the insertion product of the
acrylamide polar monomer.15 Even for a copolymer with a
low DMAA incorporation formed at 20 bar ethylene (entry 1-
4) the amide-derived end groups were observable (ethylene- vs
amide-derived 1:12). With regard to the regiochemistry of the
insertion, both DMAA- and AcAMEA-derived unsaturated end
groups observed originate from a 2,1-insertion product
exclusively.
Table 2. Homopolymerizations of Ethylene in the Presence
a
of Polar Additives
c
additive
conc−. 1
average
b
Mn
polymer
TOF
[103
M /
wc
entry
additive
[mol L ] yield [g] [103 h−1] g mol−1] Mn
2-1
4.357
3.683
0.366
0.169
0.088
2.326
0.442
0.238
0.141
3.308
1.722
0.062
0.926
0.678
0.291
1.506
1.354
0.376
0.310
124.3
105.0
10.4
4.8
17.0
17.0
10.8
9.8
2.0
1.9
2.1
2.0
1.8
1.9
2.4
1.8
2.2
2.1
n.d.
1.7
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
2-2
MAcA
MAcA
MAcA
MAcA
DMAcA
DMAcA
DMAcA
DMAcA
EA
0.002
0.05
0.10
0.20
0.004
0.05
0.10
0.20
0.05
0.10
0.05
0.05
0.10
0.30
0.05
0.10
0.05
0.10
2-3
2-4
2-5
2.5
10.7
18.2
9.7
2-6
66.3
12.6
6.8
2-7
2-8
15.6
6.7
2-9
4.0
2-10
2-11
2-12
2-13
2-14
2-15
2-16
2-17
2-18
2-19
94.3
49.2
1.8
12.1
n.d.
1.1
EA
DMSO
AcN
26.4
19.4
8.3
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
AcN
AcN
MES
43.0
38.7
10.7
8.8
MES
PrA
PrA
a
Reaction conditions: 2.5 μmol 1-DMSO, 10 bar, 80 °C, 30 min, total
b
volume toluene + additive 50 mL. TOF in mol(PE) mol(catalyst)−1
The ethylene−AcAMEA copolymer with 6.9 mol %
incorporation predominantly possesses acrylate over ethylene-
derived end groups both in the saturated and unsaturated end
groups (80% acrylate-derived unsaturated end groups, 85%
saturated end groups). An analogous pattern of end groups was
found previously for ethylene−MA copolymers.9e This suggests
that the pendant amide moiety of AcAMEA has no strong
influence on the microstructure of the copolymers, as expected.
In summary, β-hydride elimination after an acrylamide insertion
occurs less readily than in the case of acrylates, which is
distinctively reflected in the end group pattern.
c
h−1. From GPC at 160 °C in 1,2,4-trichlorobenzene.
polymerization experiment in the absence of additives (entry 2-
1), a 50-fold (MAcA, entry 2-5) and 30-fold (DMAcA, entry 2-
9) lowered activity were observed with acetamide concen-
trations of 0.2 M. Note that this concentration corresponds to
the minimum comonomer concentration applied for the
copolymerizations (Table 1). For all acetamide concentrations,
MAcA showed a slightly stronger inhibition than DMAcA. This
can be related to a stronger coordination of secondary vs
tertiary amide, likely due to the steric bulk of the latter.16 This
difference in coordination strength may likewise contribute to
the higher ethylene pressures required for the NIPAM
copolymerizations in comparison with DMAA.
Inhibition of Polymerization by Amides. The limited
polymerization activities of 1-DMSO in the presence of amide-
containing monomer raise the question of the nature of
interactions with the metal centers. When a small excess (3−5
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In order to relate the inhibition by amides with other relevant
functional groups present in vinyl monomers (or in the labile
ligand of the catalyst precursors), ethylene homopolymeriza-
tions in the presence of 0.05 M and higher concentrations of
various saturated polar-substituted compounds were studied
(Table 2 and Figure 1). DMSO (entry 2-12) inhibits
incorporated monomer (vide infra) and slower insertion into
electron-withdrawing substituted Pd−alkyls formed after an
insertion of polar comonomer by comparison to the
unsubstituted alkyls from ethylene insertion affect copolymer-
ization rates, as observed previously for other monomers.10
The binding strength of the ligand on the fourth
coordination site of the phosphinesulfonato Pd(II) methyl
catalyst precursor is decisive for its properties in (co)-
polymerization reactions. To this end, the relative binding of
DMSO vs polar additives was investigated by 1H NMR
spectroscopy. The equilibrium (1-DMSO + L ⇄ 1-L +
DMSO) is fast on the NMR time scale at room temperature;
only one DMSO signal (average of 1-DMSO and free DMSO)
is observed upon addition of variable amounts of polar additives
to a ca. 40 mM CD2Cl2 solution of 1-DMSO (Supporting
Information Figure S11, Table S3). The estimated equilibrium
constants Keq(EA) < 10−2, Keq(MES) < 10−2, Keq(AcN) ≈ 1,
Keq(DMAcA) ≈ 0.4, Keq(PrA) < 0.1, Keq(MAcA) ≈ 0.6 are in
qualitative agreement with the results of ethylene homopoly-
merization in the presence of 0.05 mol L−1 of the same additive
(Figure 1).18
A more labile coordination in the catalyst precursor allows
for copolymerizations at lower ethylene pressures, and
therefore higher incorporations of polar comonomer can be
achieved. Motivated by the lower inhibition of ethylene
polymerization by MAcA and DMAcA versus DMSO, we
investigated the formation of a DMF-coordinated [(P^O)Pd-
(Me)] complex (Scheme 2). A procedure similar to the
synthesis of 1-DMSO starting from 1-TMEDA9e (TMEDA =
N,N,N′,N′-tetramethyl ethylene diamine), that is, repeated
evaporation with excess sulfoxide or amide, respectively, to
remove the diamine, was studied. This approach resulted in
mixtures of starting material and product (1-DMF) that could
not be separated by means of extraction or crystallization. The
mixture was soluble in methylene chloride, and two Pd−Me
resonances could be observed at room temperature. The 13C
carbonyl resonance of DMF was only slightly low-field-shifted
to 165.35 ppm (162.49 ppm for free DMF under identical
conditions), which indicates a rather weak coordination of
DMF. A suitable alternative route is therefore the generation of
fragment 1 in the absence of other coordinating reagents than
DMF. To this end, chloride abstraction from the recently
reported dimeric cation bridged anionic complex [{(1-Cl)-μ-
Na}2] with silver salts was utilized.6g Stirring of [{(1-Cl)-μ-
Na}2] with 5 equiv of DMF and 0.95 equiv of silver
tetrafluoroborate in methylene chloride for one hour in a
sealed tube yielded 1-DMF as an off-white powder after
filtration, removal of the solvent, and washing with diethyl
ether. The proton resonances of the coordinated DMF were
low-field-shifted compared to free DMF (3.00 and 2.91 ppm for
Me protons and 8.13 ppm for the carbonyl bound proton;
Figure S12, Supporting Information). However, 1-DMF is not
stable in methylene chloride in the absence of excess DMF and
Figure 1. Inhibition of ethylene polymerization by saturated
compounds with various polar groups (reaction conditions: 2.5
μmol 1-DMSO, 10 bar, 80 °C, 30 min, total volume of toluene and
polar compound 50 mL, 0.05 M of additive).
polymerization to a slightly larger extent than the amides
MAcA and DMAcA, which is in agreement with the
1
aforementioned H NMR study. Ethyl acetate (EA, entries 2-
10) exhibits a much lower effect on polymerization by
comparison to all other compounds studied. Thus, the much
higher polymerization rates in ethylene−acrylate copolymeriza-
tion vs copolymerization of unsaturated amides can be traced to
an inhibition of polymerization by coordination of the amide
groups of the free monomer. The inhibition of ethylene
polymerization observed for acetonitrile (AcN), methyl ethyl
sulfone (MES), and propionic acid (PrA) is in qualitative
agreement with previous studies of the corresponding vinyl
monomers.6b,f,h,17 In this overall scheme, amides inhibit
relatively strongly, similar to the carboxylic acid and stronger
than the nitrile and the sulfone. For the oxygen-containing
monomers, the impact on polymerization can be rationalized by
the electronegativity of the oxygen-bound fragments OX and
corresponding expected κ-O-binding strength to the electro-
philic metal center (with the exception of the carboxylic acid,
which does not follow this trend, possibly because of
coordination in its deprotonated form).
Compared to ethylene homopolymerization in the presence
of the saturated amide DMAcA (Table S2, Figure S10,
Supporting Information), in copolymerization of DMAA
(entries 1-5 to 1-7), activities are ca. 5-fold lowered. This
suggests that also coordination of functional groups of
Scheme 2. Synthesis and Decomposition of 1-DMF
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decomposed to the multinuclear species 1n, which precipitates
during the course of the NMR experiment. Oligomerization of
the [(P^O)PdMe] fragment via intermolecular coordination of
a second oxygen atom of the sulfonate group9a,e appears to be
favored over coordination of DMF.
Insertion of Acrylamides into 1-DMSO. Pseudo-first-
order rate constants for the insertion into the Pd−Me bond of
1-DMSO were determined by 1H NMR spectroscopy at 25 °C
(Figure 2, Table 3).19 Both acrylamides (NIPAM, DMAA)
Figure 3. ORTEP plot of the molecular structure of 2-MeOD drawn
with 50% probability ellipsoids. All hydrogen atoms and cocrystallized
solvent molecules are omitted for clarity. Selected bond length (Å) and
angles (deg): Pd(1)−C(21) 2.059(5), Pd(1)−O(7) 2.116(4), Pd(1)−
O(3) 2.132(4), Pd(1)−P(1) 2.201(1), O(7)−C(56) 1.445(6),
C(24)−O(6) 1.259(6), N(1)−C(24) 1.328(7), N(1)−C(25)
1.472(7), P(1)−Pd(1)−O(3) 94.9(1), O(7)−Pd(1)−C(21) 88.2(2),
C(21)−C(24)−N(1) 117.6(5), C(21)−C(24)−O(6) 122.6(5),
N(1)−C(24)−O(6) 119.8(5), C(25)−N(1)−C(24) 122.9(5),
C(25)−N(1)−C(24)−O(6) −2.0.
Figure 2. Pseudo-first-order plots of the time-dependent decrease of
■
the Pd−Me resonance of 1-DMSO at 25 °C for DMAA ( ), NIPAM
lized with two free molecules of methanol-d4 per asymmetric
unit. The Pd center shows a distorted square planar
coordination with methanol-d4 coordinated trans to the
phosphorus atom. While 2-MeOD is poorly soluble in
methanol, its solubility is strongly increased by the addition
of a small amount of DMSO to the methanol solution.
However, immediate β-hydride elimination was observed in the
presence of DMSO. After refluxing for one hour, over 90% of 2
was converted to N-isopropylcrotoneamide (Figure S13). In
the context of the absence of conjugated NIPAM-derived end
groups in the ethylene−NIPAM copolymers (vide supra), it is
noteworthy that β-H elimination after 2,1-insertion of NIPAM
can be observed in principle.
The insertion products of NIPAM into 1-DMSO were
directly observed by electrospray ionization (ESI) mass
spectrometry. After a reaction time of 22 h, the products of
consecutive NIPAM insertions into the Pd−Me bond of 1-
DMSO and into the Pd−hydride, formed by β-H elimination,
were clearly detected (Figure S15, Supporting Information).
For all signals, the found isotope patterns fully agree with
expected values.
●
▲
▼
( ), MA ( ), and AcAMEA ( ). [Pd] = 45 mM in CD2Cl2; 20 equiv
polar monomer.
Table 3. Pseudo-First-Order Rate Constants for the First
Insertion into the Pd−Me Bond of 1-DMSO at 25 °C
−1
monomer
kobs [10−5 s ]
MA
83.2(58)
20.0(7)
10.7(1)
7.76(9)
NIPAM
AcAMEA
DMAA
inserted considerably slower than MA. Note, however, that the
observed rate constants determined also include preinsertion
equilibrium competition of DMSO coordination and κ-O amide
binding vs π-coordination of the vinyl group. Accordingly, the
secondary amide moiety of the acrylate AcAMEA slows down
the insertion considerably by comparison to methyl acrylate
(Table 3).
Characterization of the reaction mixture obtained after
1
complete insertion of NIPAM into 1-DMSO by H NMR
spectroscopy was complicated because of multiple isopropyl
resonances resulting from several different insertion products.
Upon concentration of the methylene chloride solution, a
precipitate formed, which could be isolated as a pale yellow
solid by filtration and washing with a small amount of
methylene chloride. Its low solubility in methanol allowed for
1H and 1H,1H−COSY NMR analysis, which clearly revealed the
2,1-insertion product of NIPAM into the Pd−Me bond (2)
(Figures S13, S14, Supporting Information). Layering a filtrated
methanolic solution of 2 with pentane at room temperature
afforded single crystals of the methanol adduct suitable for X-
ray diffraction (Figure 3). Both enantiomers with respect to the
absolute configuration of the methine carbon C(21) cocrystal-
By contrast, applying an analogous procedure to DMAA, no
consecutive insertions were observed by NMR or ESI mass
spectrometry (Figure S16, Supporting Information). A mixture
of the 2,1- and 1,2-insertion products was observed by NMR
after stirring 1-DMSO with 20 equiv of DMAA for 18 h at 25
°C. However, the isolated products were contaminated with
homopolymer of DMAA, formed by free-radical polymer-
ization, and small amounts of insertion products into Pd−
hydride species, formed after β-H elimination. Clean DMAA
insertion products in a ∼4:1 ratio of 2,1- and 1,2-insertion were
obtained by chloride abstraction from [{(1-Cl)-μ-Na}2] with
silver tetrafluoroborate (Scheme 3). The complexes were fully
characterized by one- and two-dimensional NMR techniques
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Scheme 3. Synthesis of Dimethyl Acrylamide Insertion Products 3,4
Figure 4. Aliphatic region of 1H NMR spectra of dimethyl acrylamide insertion products 3,4 (formed in a 4:1 ratio) in CD2Cl2 at 25 °C. S: Residual
Et2O resonances. Inset: Close up of 31P NMR spectra of 3,4 under identical conditions.
(Figure 4). The ethyl CH3 protons of 3 appear as a triplet at
0.78 ppm, whereas the methyl protons of 4 show a doublet
splitting at 1.32 ppm. Remarkably, the resonances of the
diastereotopic methylene protons 3 and 3′ in 4 differ by more
than 1 ppm. The 13C carbonyl resonances are significantly low-
field-shifted to 184.58 ppm for the 2,1-insertion product 3 and
187.30 ppm for the 1,2-insertion product 4, indicating a chelate
structure of the complexes, in which the amide group
coordinates to the fourth coordination site of the palladium
center.20 Also, the wavenumber of the carbonyl stretching band
of 3,4 is decreased from 1634 cm−1 in free DMAcA to 1584 and
1573 cm−1 in the insertion products (Figure S21, Supporting
Information). According to the Cambridge Structure Database,
κ-O-coordination has been observed exclusively in Pd(II) amide
complexes.21 Thus, a similar κ-O-coordination appears likely in
3,4. Noteworthy, the same low-field-shifted carbonyl reso-
nances indicating chelate formation were observed in the
(contaminated) insertion products starting from 1-DMSO. In
contrast, single insertion of MA results in the formation of the
2,1-insertion product [(P^O)Pd{CH(CH2CH3)C(O)OMe}-
(DMSO)] exclusively, in which DMSO coordinates to the Pd
center rather than the chelating coordination of the ester
moiety in the alkyl fragment. This different behavior relates to
the above conclusions on coordination strength of amide
moieties, as derived from the extent of inhibition of
polymerization (Figure 1).
behavior of the monomers and other relevant ligands to the
presumed catalyst resting state was investigated. Note that an
opening of a chelate by coordination of an incoming ligand will
be favored at low temperature because of the unfavorable
entropic contribution.
Coordination of ethylene (10 equiv in solution) to 3,4 was
1
not observed even at −80 °C by H NMR. However, 3,4 are
precursors to highly active ethylene polymerization catalysts
even at a low ethylene pressure of 2.5 bar (Table 4). Activation
Table 4. Ethylene Homopolymerizations with 3,4 as a
a
Catalyst Precursor
b
entry
p [bar]
polymer yield [g]
average TOF [104 h−1]
1
2
3
2.5
5.0
7.5
1.63
3.19
4.07
4.7
9.1
11.6
a
Reaction conditions: 2.5 μmol 3,4, 30 min, 80 °C, 50 mL of toluene.
b
TOF in mol(ethylene) mol(catalyst)−1 h−1.
for polymerization requires coordination and insertion of
ethylene, but after a few insertions have occurred, the active
species will no longer be prone to chelating coordination of the
amide end group because of an entropic disfavoring of large
ring chelates. Thus, even if chelate opening by ethylene binding
to 3,4 is unfavorable, the latter can be converted to the active
species eventually. Polymerization activities are little dependent
on ethylene concentration at pressures above 5 atm.
Accordingly, at 10 atm, activities are similar to polymerizations
with the labile-coordinated 1-DMSO as a catalyst precursor.
Reactivity of Dimethyl Acrylamide Insertion Products.
Further chain growth requires π-coordination of olefinic
monomer to the Pd center and hence opening of the κ-O-
coordinated chelate in 3,4. To this end, the coordination
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Scheme 4. Reactivity of 3,4 toward Triphenylphosphine
No coordination of DMAA or DMSO, respectively, was
observed in the presence of up to 20 equiv at room temperature
(Figures S22-1 to -3, Supporting Information). Both 13C
carbonyl resonances of 3,4 remained unaltered. However, small
amounts of N,N-dimethylcrotoneamide (DMCA), formed by β-
H elimination from the 2,1-insertion products, were found after
24 h. By comparison, 3,4 is virtually stable over this period of
time in methylene chloride solution in the absence of additional
reagents. The four-membered chelate of 3 was readily opened
in the presence of equimolar amounts of PPh3 and undergoes
rapid β-H elimination (Scheme 4). The two 31P resonances of
the opened chelate 3′-PPh3 show a doublet splitting with 2JPP,cis
= 39 Hz. No opened chelate with a trans coordination of the
phosphorus atoms (3-PPh3) was observed, assumingly because
of the trans-effect of the phosphines. The organic product of β-
H elimination (DMCA) as well as the resulting metal hydride
complex (5-PPh3) appear in 1H and 31P NMR spectra (Figures
S23 and S24, Supporting Information). By contrast, the five-
membered chelate 4 was stable under these conditions even
with an excess of PPh3.
The weaker σ-donor pyridine coordinates to 3 in an
observable equilibrium at room temperature, whereas 4
retained its chelated structure as expected from the
aforementioned studies. About 2 equiv of pyridine were
required to completely form the opened chelate 3-PYR. 3-
PYR is relatively stable toward β-H elimination in the presence
of excess pyridine. After 24 h, only trace amounts of 5-PYR and
DMCA were observed along with the isomerized complex 3′-
PYR (for numbering cf. Scheme 4). By comparison to pyridine,
2,6-lutidine is believed to possess a weaker coordination
strength due to steric hindrance. Both opened chelate 3-LUT
and 3 can be observed simultaneously in the presence of 2
equiv of 2,6-lutidine at temperatures between 273 and 298 K.
On the basis of the ratio of 3 and 3-LUT obtained from 31P
NMR peak areas, the equilibrium constants K(T) = [3-LUT]/
([3] + [2,6-lutidine]) were calculated. Values for the reaction
enthalpy ΔH° = −41.8(10.5) kJ mol−1 and entropy ΔS° =
−115(37) J mol−1 K−1 were obtained by a Van’t Hoff plot
(Figure S25, Supporting Information). Such a negative enthalpy
and unfavorable entropy are generally expected for chelate
opening reactions, and the values found are in the same range
as reported for similar chelate opening reactions with ester-
derived five- and six-membered chelates in cationic α-diimine
complexes.3b,22,23 Extrapolation of the thermodynamic data to
typical polymerization conditions with [2,6-lutidine] = 1 mol
L−1 yields K(358 K) ≈ 1.5. Considering the higher coordination
strength of 2,6-lutidine in comparison to DMAA and ethylene
(with typical concentrations of both monomers being on the
order of a mol L−1 in polymerizations), it can be concluded that
chelate species formed after an amide insertion will be relevant
species in copolymerizations and retard polymerization.
SUMMARY AND CONCLUSIONS
■
These combined comprehensive experimental studies of
polymerization and stoichiometric insertion reactions reveal
fundamental features of insertion polymerization of amides.
Other than the somewhat electron-poorer carbonyl group of
acrylates, free amides coordinate strongly to the catalyst
studied, and hereby the polar monomer itself reversibly retards
polymerization. However, the catalysts are stable in the
presence of amides, and no indication of irreversible
deactivation pathways specific to amide monomers was
observed. Chelating coordination of amide moieties of polar
monomer incorporated in the growing chain also contributes
significantly to lower rates of copolymerization vs ethylene
homopolymerization. In detail, these chelates appear to be
more stable than corresponding ester chelates, and the impact
on polymerization rates is slightly more pronounced than for
acrylate copolymerizations. Insertion occurs preferentially in a
2,1-fashion, as expected for electron-poor vinyl monomers
because of the strong prevalence of electronic factors in the
absence of specific bulky substitution patterns of the catalyst.9f
1,2-Insertion is also clearly observed (ca. 20%) and occurs to a
larger extent than found for acrylates.9e,10 This is in-line with
electronics determining the regioselectivity, which will be
higher for the electron-poorer olefin. Net insertion rates into a
Pd−Me bond of the secondary and tertiary amides studied are
ca. 4- to 8-fold lower by comparison to methyl acrylate. Note
that these data include the coordination preequilibrium of
displacement of DMSO and competitive κ-O amide binding vs
the required π-coordination of the vinyl group. Though
consecutive insertions of amide monomer in copolymerizations
are unlikely because of their limited incorporation, stoichio-
metric studies reveal that consecutive insertions of the
secondary amide N-isopropylacrylamide can occur. Chain
transfer occurs more rapidly from the insertion product of
the polar amide monomer, Pd-CH(CONR2)-CH2-R, by
comparison to the unsubstituted alkyls from ethylene chain
growth, as is evident from end groups observed in the polymers
studied in this work and is also supported by observation of β-
hydride elimination in stoichiometric studies.
Overall, the copolymerization is limited most decisively by
the combination of retardation of polymerization by reversible
blocking of the coordination site by the amide groups of free
monomer and a limited insertion rate. This results in an
intrinsic trade-off between the amide content of the copolymer
and copolymerization productivity. Other than retardation of
polymerization by formation of chelates by polar moieties
incorporated in the growing polymer chain, this can not be
counterbalanced by increasing the monomer concentrations. In
a general view of Pd(II)-catalyzed copolymerization of polar
vinyl monomers CH2CHX, this comprehensive experimental
study of amide copolymerization, together with the inhibition
studies reported and a recent detailed theoretical study of
acrylonitrile copolymerization17 and mechanistic study of
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Journal of the American Chemical Society
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acrylate polymerization,10 evolves the criteria for a privileged
monomer. While a retardation of polymerization by slower
insertion into the α-X substituted alkyl formed by incorporation
of polar monomer as well as by chelating κ-X coordination of
comonomer-derived repeat units appear to be general
phenomena of such copolymerizations, the weak coordination
of the ester group of acrylate monomer ultimately is responsible
for the accessability of ethylene copolymers with high
incorporations of the polar comonomer.
In the context of these considerations, it is notable that to
date no specific irreversible deactivation reactions of neutral
Pd(II) polymerization catalysts have been reported for any
polar vinyl comonomer, although they likely occur in some
cases.
Chem. Soc. 2009, 131, 14606−14607. (f) Bouilhac, C.; Runzi, T.;
̈
Mecking, S. Macromolecules 2010, 43, 3589−3590. (g) Runzi, T.;
̈
Guironnet, D.; Gottker-Schnetmann, I.; Mecking, S. J. Am. Chem. Soc.
̈
2010, 132, 16623−16630. (h) Runzi, T.; Frohlich, D.; Mecking, S. J.
̈
̈
Am. Chem. Soc. 2010, 132, 17690−17691. (i) Daigle, J.-C.; Piche, L.;
Claverie, J. P. Macromolecules 2011, 44, 1760−1762.
(7) Olefin−CO copolymerizations: (a) Drent, E.; van Dijk, R.; van
Ginkel, R.; van Oort, B.; Pugh, R. I. Chem. Commun. 2002, 964−965.
(b) Hearley, A. K.; Nowack, R. J.; Rieger, B. Organometallics 2005, 24,
2755−2763. (c) Kochi, T.; Nakamura, A.; Ida, H.; Nozaki, K. J. Am.
Chem. Soc. 2007, 129, 7770−7771. (d) Nakamura, A.; Munakata, K.;
Ito, S.; Kochi, T.; Chung, L. W.; Morokuma, K.; Nozaki, K. J. Am.
Chem. Soc. 2011, 133, 6761−6779.
(8) Drent, E.; van Dijk, R.; van Ginkel, R.; van Oort, B.; Pugh, R. I.
Chem. Commun. 2002, 744−745.
(9) (a) Vela, J.; Lief, G. R.; Shen, Z.; Jordan, R. F. Organometallics
2007, 26, 6624−6635. (b) Skupov, K. M.; Marella, P. R.; Simard, M.;
Yap, G. P. A; Allen, N.; Conner, D.; Goddall, B. L.; Claverie, J. P.
Macromol. Rapid Commun. 2007, 28, 2033−2038. (c) Borkar, S.;
Newsham, D. K.; Sen, A. Organometallics 2008, 27, 3331−3334.
(d) Noda, S.; Nakamura, A.; Kochi, T.; Chung, L. W.; Morokuma, K.;
Nozaki, K. J. Am. Chem. Soc. 2009, 131, 14088−14100. (e) Guironnet,
The aforementioned limitations being understood, they
should not obscure the fact that this insertion copolymerization
provides a useful access to amide-substituted polyethylenes,
which is not sensitive to the nature of the amide substitution
pattern.
ASSOCIATED CONTENT
■
D.; Roesle, P.; Runzi, T.; Gottker-Schnetmann, I.; Mecking, S. J. Am.
̈
̈
S
* Supporting Information
Chem. Soc. 2009, 131, 422−423. (f) Wucher, P.; Caporaso, L.; Roesle,
P.; Ragone, F.; Cavallo, L.; Mecking, S.; Gottker-Schnetmann, I. Proc.
̈
Supplemental tables and figures, general experimental proce-
dures, synthesis, additional NMR spectra, polymerization data,
and crystal structure of 2-MeOD. This material is available free
of charge via the Internet at http://pubs.acs.org.
Natl. Acad. Sci. U. S. A. 2011, 108, 8955−8959.
(10) Guironnet, D.; Caporaso, L.; Neuwald, B.; Gottker-Schnetmann,
̈
I.; Cavallo, L.; Mecking, S. J. Am. Chem. Soc. 2010, 132, 4418−4426.
(11) Related stoichiometric studies of acrylate insertion in Pd(II)
complexes: (a) Braunstein, P.; Frison, C.; Morise, X. Angew. Chem., Int.
Ed. 2000, 39, 2867−2870. (b) Agostinho, M.; Braunstein, P. Chem.
Commun. 2007, 58−60.
AUTHOR INFORMATION
■
Corresponding Author
stefan.mecking@uni-konstanz.de
(12) (a) Marques, M. M.; Fernandes, S.; Correia, S. G.; Ascenso, J.
R.; Caroco̧ , S.; Gomes, P. T.; Mano, J.; Pereira, S. G.; Nunes, T.; Dias,
A. R.; Rausch, M. D.; Chien, J. C. W. Macromol. Chem. Phys. 2000,
201, 2464−2468. (b) Fernandes, S.; Ascenso, J. R.; Gomes, P. T.;
Costa, S. I.; Silva, L. C.; Chien, J. C. W.; Marques, M. M. Polym. Int.
2005, 54, 249−255.
(13) Also cf. Conner, D. M.; Goodall, B. L.; McIntosh, L. H. U.S. Pat.
7524905 B2, 2009 for further polymerizations with these catalysts to
ethylene copolymers with 0.5 mol % N-vinylphthalimide incorpo-
ration.
(14) The concentrationof ethylene in toluene (85 °C) at 90 and 200
psi is 0.28 and0.68 mol L−1, respectively: Skupov, K. M.; Hobbs, J.;
Marella, P.; Conner, D.; Golisz, S.; Goodall, B. L.; Claverie, J. P.
Macromolecules 2009, 42, 6953−6963.
(15) Internal olefins were accounted for as ethylene-derived end
groups.
(16) The frequencies ofcarbonyl stretching bands of acetamide
(AcA), MAcA, and DMAcA decreasewith increasing degree of
substitution, indicating a higher negativepartial charge on the oxygen
atom: Gerrard, W.; Lappert, M. F.; Pyszora, H.; Wallis, J. W. J. Chem.
Soc. 1960, 2144−2151 .However, the overall coordination strength
appears to be dominatedby the steric bulk of the N-substituents, which
disfavorscoordination, as concluded from the inhibition studies.
(17) Nozaki, K.; Kusumoto, S.; Noda, S.; Kochi, T.; Chung, L. W.;
Morokuma, K. J. Am. Chem. Soc. 2010, 132, 16030−16042.
(18) That the trends differ somewhat in detail may be related to the
fact that the relative binding studies employ Pd−Me species, whereas
in polymerization, relative binding in the bulkier Pd−alkyls (alkyl =
growing chain) is relevant. Also, although the temperature dependence
of an equilibrium A + B ⇄ C + D should be expected to be limited,
the different temperatures of the NMR studies (25 °C) and the
polymerization studies (80 °C) can contribute.
ACKNOWLEDGMENTS
■
Financial support by the DFG (Me1388/10) is gratefully
acknowledged. The authors thank Lars Bolk for GPC, Boris
Neuwald for ESI-MS measurements, Anke Friemel for
recording VT 13C NMR spectra, and Thomas Runzi for
̈
performing various polymerizations in presence of additives.
Support by Inigo Gottker-Schnetmann in the determination of
̈
the crystal structure of 2-MeOD is acknowledged.
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