Synergetic Effect of Monomer Functional Group Coordination in Catalytic Insertion Polymerization

Article abstract of DOI:10.1021/jacs.7b03087

PhS- and PhNH-functionalized dienes are copolymerized efficiently with butadiene to stereoregular copolymers by [(mesitylene)Ni(allyl)][BAr^F₄] (Ni-1). Overall polymerization rates and comonomer incorporations depend strongly on the l

Full text of DOI:10.1021/jacs.7b03087

Communication
pubs.acs.org/JACS
Synergetic Effect of Monomer Functional Group Coordination in
Catalytic Insertion Polymerization
Hannes Leicht, Inigo Gottker-Schnetmann, and Stefan Mecking*
̈
Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz, 78464 Konstanz, Germany
S
* Supporting Information
have a beneficial impact on monomer incorporations and
ABSTRACT: PhS- and PhNH-functionalized dienes are
copolymerized efficiently with butadiene to stereoregular
copolymerization rates.
Recently, [(mesitylene)Ni(allyl)][BArF ] (Ni-1)18,19 was
4
copolymers by [(mesitylene)Ni(allyl)][BArF ] (Ni-1).
found to be capable of copolymerizing a variety of silicon-,
boron-, nitrogen-, oxygen-, and phosphorus-based polar diene-
monomers with 1,3-butadiene (BD) or isoprene (IP), providing
access to functionalized 1,4-cis-poly(butadiene) copolymers.9
Comonomer conversion is persistently high in these copoly-
merizations, even at low comonomer concentrations. Addi-
tionally, qualitative NMR experiments suggested monomer
reactivity ratios of r = kCoMo/kBD > 1.
Such favorable comonomer incorporation is remarkable, but
its origin is ill understood. To gain more insight into the
copolymerization behavior of polar functionalized dienes and
the contributions of the functional groups, we studied dienes
with different functional groups and different linker lengths
(Chart 1, for detailed synthetic procedures cf. SI). A possible
4
Overall polymerization rates and comonomer incorpo-
rations depend strongly on the linker length between the
diene moiety and functional group, in, e.g.,
PhS(CH2)xC(CH2)CHCH2 (PhS-x-BD, x =
3−7), in particular for certain linker lengths high
comonomer reactivity ratios stand out. This effect is
related to a favorable binding of the comonomer to the
active site comprising coordination of its functional group,
which significantly enhances comonomer incorporation in
the growing polymer chain.
ince their inception by Ziegler and Natta, catalytic insertion
Spolymerizations have advanced to one of the largest scale
synthetic reactions carried out today. This is due to the ability
to control materials properties via the polymer microstructures.
Within this established scenario, an incorporation of polar vinyl
monomers remains challenging. Over the past 2 decades, less
oxophilic late transition metal catalysts have been discovered
that enable the copolymerization of ethylene with a wide variety
of polar vinyl monomers.1−7 Concerning stereoselective
polymerizations, only recently Nozaki8 et al. have reported
advances toward functionalized poly(propylenes) and we9 and
Cui10 et al. have reported stereoregular functionalized poly-
(dienes) and poly(methoxystyrenes)11,12 from insertion co-
polymerization.
Chart 1. Dienes and Model Compounds Used in
(Co)polymerizations with Butadiene
Although an incorporation of polar and reactive groups in the
polymer is desirable in terms of its materials properties, during
polymerization it has adverse effects exclusively. Thus, in the
mechanistically well understood copolymerizations of ethylene
with polar vinyl monomers, polymerization rates and polymer
molecular weights are reduced by the presence of comonomer.
This effect has been traced to κ-X coordination of the
functional group of free comonomer or already incorporated
comonomer-derived repeat units. The functional group
reversibly blocks coordination sites for further chain growth.
Further, this adverse effect of the free monomers’ functional
groups is all the more relevant, as copolymerization parameters
disfavor comonomer incorporation, such that high comonomer
concentrations are required even if comonomer conversion
remains low.13−17
(beneficial) interaction of the functional group with the catalyst
should be reflected by a dependence on the linker length itself
and also on the coordination strength of the heteroatom.
Copolymerization experiments with the entire range of PhS-
functionalized comonomers showed that Ni-1 is tolerant
toward the functional group, resulting in the first stereoselective
copolymerization of sulfur-functionalized dienes with BD. From
We now for the first time provide evidence how
Received: March 28, 2017
precoordination of the comonomers’ functional groups can
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.7b03087
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
1
monitoring of the copolymerization by H NMR spectroscopy
Table 1. Rate Constants Obtained from Kinetic NMR
(cf. SI for detailed data), a significant influence of the
comonomer on the copolymerizations can be observed. A
kinetic plot of the copolymerization of BD with PhS-3-BD
reveals a change of the first-order rate constant for butadiene
consumption from kBD = 0.16 × 10−3 s−1 to k′BD = 3.7 × 10−3
s−1 after 60 h reaction time (Figure 1 and Table 1). This
Experiments for (Co)polymerizations of BD with Different
Comonomers and Model Compounds Catalyzed by Ni-1
a
entry
CoMo/additive
kBD [10−3 s−1
]
kCoMo [10−3 s−1
]
rCoMo
b
1-1
PhS-3-BD
PhS-4-BD
PhS-5-BD
PhS-6-BD
PhS-7-BD
PhNH-3-BD
PhNH-6-BD
5-BD
0.16
4.4
0.74
21
4.6
4.9
1.6
1.7
1.6
7.90
1.3
2.2
1.2
c
1-2
c
1-3
5.6
9.1
c
1-4
9.0
15
c
1-5
4.7
7.6
d
1-6
0.086
32.6
0.683
40.6
d
1-7
e
i
i
1-8
n.a.
n.a.
e
i
i
1-9
IP
n.a.
n.a.
f
1-10
PhS-6
49
g
1-11
PhNH-3
PhS-4-Ene
PhS-6-Ene
PhS-8-Ene
40
h
1-12
28
h
1-13
83
h
1-14
78
j
j
1-15
−
>6000
a
b
Monomer reactivity ratio rCoMo = kCoMo/kBD
1:120:24, T = 40 °C. Ni:BD:CoMo = ca. 1:400:60, T = 40 °C.
Ni:BD:CoMo = ca. 1:400:75, T = 40 °C. Ni:BD:CoMo = ca.
1:200:30, T = −15 °C. Ni:BD:additive = ca. 1:400:60, T = 40 °C.
Ni:BD:additive = 1:120:28, T = 40 °C. Ni:BD:additive = ca.
1:400:75, T = 40 °C. Not applicable, as reactions were conducted at a
different temperature. kBD was calculated for a BD homopolymeriza-
.
Ni:BD:CoMo =
c
d
e
f
g
h
i
j
tion from the observation that BD conversion reaches >99.5% after 5
min at 40 °C.
This high preference for the incorporation of a sulfur
containing monomer was also observed in a copolymerization
with PhS-4-BD. The entire comonomer present is already
incorporated at a BD consumption of 50% (after only 3 h at 40
°C) resulting in a copolymer with a comonomer:BD ratio of
30:70. However, >95% overall diene consumption is now
reached after only 6 h opposed to 6 days for the
copolymerization of PhS-3-BD although the dienes:Ni ratio
is higher. In contrast, the preference for insertion of PhS-5-BD,
PhS-6-BD, or PhS-7-BD vs butadiene-insertion is not that
pronounced as evidenced by complete comonomer consump-
tions at 93% (PhS-5-BD), 94% (PhS-6-BD), and 93% (PhS-7-
BD) BD consumption.20,22
These observed effects are even more pronounced for
copolymerizations with comonomers bearing a supposedly even
stronger coordinating PhNH-group. Although the copolymer-
ization of PhNH-3-BD takes ca. 10 days to reach >68% diene
conversion, the copolymerization of PhNH-6-BD only takes ca.
2−3 h for the same conversion under otherwise identical
conditions.
Again, an obvious change of the rate constant for the
consumption of BD is visible, once PhNH-3-BD is completely
consumed (Figure 1).20 The rate constant in the absence of free
comonomer (k′BD = 3.95 × 10−3 s−1) is ca. 50 times increased
compared to the rate constant at the beginning of the
polymerization (kBD = 0.086 × 10−3 s−1).
Comparison of the initial rate constants of BD and
comonomer consumption reveals a monomer reactivity ratio
of rCoMo = 7.9. Increasing the linker length to six methylene
units in PhNH-6-BD results in a ca. 380-fold increased rate
constant for the BD incorporation, whereas the monomer
reactivity ratio drops from 7.9 to 1.3.
Figure 1. First-order kinetic plots of the copolymerizations of BD with
PhS-3-BD or PhNH-3-BD catalyzed by Ni-1 at 40 °C.
significant, ca. 20-fold, increase of polymerization activity
coincides with the complete consumption, i.e., incorporation
of the entire amount of PhS-3-BD present in this experi-
ment.20,21 When the initial (copolymerization) rate constants
kBD and kCoMo are compared, a monomer reactivity ratio of
k
CoMo/kBD = rCoMo = 4.6 in favor of PhS-3-BD is found.
Strikingly, BD consumption has a significantly increased rate
constant (kBD = 4.4 × 10−3 s−1) in the presence of PhS-4-BD
that is 1 order of magnitude greater than the initial rate
constant for PhS-3-BD, while still exhibiting a comparable
strong preference for the incorporation of the comonomer
(rCoMo = 4.9).
For linker lengths longer than four methylene units, the rate
constants are slightly higher with a maximum observed for PhS-
6-BD (kBD = 9.0 × 10−3 s−1).
However, the monomer reactivity ratios are all similar (rCoMo
= 1.6−1.7) and clearly lower when compared to those of PhS-
3-BD or PhS-4-BD.
To further illustrate these experiments and also for
comparison with other data (vide infra), incorporation of
PhS-3-BD is complete at only 16% BD consumption (after ca.
60 h at 40 °C) resulting in a copolymer with a comonomer:BD
ratio of 60:40. Under these conditions, it takes 5−6 days to
reach >95% overall diene consumption (and the copolymer
formed is expected to exhibit a gradient copolymer structure).
The influence of linker lengths on rate constants and
monomer reactivity ratio can also be found in a less
B
DOI: 10.1021/jacs.7b03087
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Journal of the American Chemical Society
Communication
Figure 2. Influence of κ-X assisted precoordination and different linker lengths on polymerization activity and comonomer insertion.
pronounced fashion for sterically more hindered, i.e., weaker
coordinating, Ph2N- and TMS2N- groups. A general increase of
rate constants (BD incorporation as well as comonomer
incorporation) is observed for copolymerizations in the
presence of weaker coordinating groups and reflected in
presence of hexyl thiophenol ether or N-propylaniline (PhS-6
and PhNH-3) were performed under similar conditions. The
reactions reach >95% BD consumption already after 60 min
(PhS-6, kBD = 49 × 10−3 s−1) and 90 min (PhNH-3, kBD = 40 ×
10−3 s−1). The rate constant of the polymerization in the
presence of PhNH-3 is ca. 460-fold increased compared to the
rate constant of the copolymerization with PhNH-3-BD, thus
ruling out κ-X coordination as the only cause for such dramatic
differences in overall polymerization rates and indicating a
simultaneous κ-X and η4-diene coordination instead.
The influence of an already incorporated comonomer unit in
the polymer chain on the polymerization was probed in
polymerizations of BD in the presence of the model
compounds PhS-4-Ene, PhS-6-Ene, and PhS-8-Ene. These 1-
olefins qualitatively resemble the polymer chain in the sense
that they can coordinate through their olefin functions as well
as polar groups.24 Kinetic evaluations of these polymerizations
show that the rate constants are significantly higher compared
to the rate constants of the copolymerization (Table 1, entries
1-12, 1-13, and 1-14). For example, a BD conversion >95% was
observed after 96 min in the presence of PhS-8-Ene, thus
excluding an extensive inhibition of the polymerization by an
already incorporated comonomer unit.
lower overall reaction times. Rate constants range from kBD
=
57.5 × 10−3 s−1 to kBD = 80.5 × 10−3 s−1 for copolymerization
with Ph2N-x-BD (x = 3, 4, 6) and from kBD = 31.2 × 10−3 s−1
to kBD = 123.7 × 10−3 s−1 for the entire range of linker lengths
from TMS2N-3-BD to TMS2N-6-BD (cf. SI, Table S1). At the
same time, the preference for comonomer insertion is reduced
and monomer reactivity ratios between rCoMo = 0.8 and rCoMo
=
1.5 are observed.23
Different mechanistic scenarios can rationalize the remark-
ably favorable comonomer reactivity ratios at certain linker
lengths: (a) A favored incorporation is only caused by the
comonomer’s linker substituent and not by the presence of
Lewis basic heteroatoms, i.e., even a nonfunctionalized 2-
hydrocarbyl substituted 1,3-butadiene shows an enhanced
incorporation. (b) A Lewis acid Lewis base interaction, i.e., κ-
X coordination, between the catalytically active metal center
and the heteroatom on the comonomer (acting as a ligand) is
responsible for an enhanced comonomer incorporation. This
consideration may be extended to the formation of a chelate by
simultaneous κ-X and η4-diene coordination of one comonomer
unit. (c) A combination of mechanistic scenarios a and b is
responsible. Suitable nonpolymerizable model compounds were
employed to assess the different contributions of the functional
group (PhS-6, PhNH-3), the comonomer’s linker substituent
(5-BD), or postinsertion inhibition (PhS-x-Ene, x = 4, 6, 8)
separately.
As a conclusion from these observations, mimics of the
functional group (like PhS-6 or PhNH-3) slow down the
polymerization. That is, free monomer and functional groups of
formed polymer present in the reaction solution can interact
with the metal center, without the involvement of further
binding sites (like the same comonomer molecules’ double
bonds). However, this can not account for the different
behavior of monomers with different linker lengths.
The influence of scenario (a) was probed in copolymeriza-
tions of BD and IP or 5-BD. A benchmark homopolymeriza-
tion of BD and copolymerizations of BD with IP or 5-BD could
not be followed at 40 °C because they were too fast. Hence,
these reactions were conducted at a lower temperature of −15
°C. For IP and 5-BD, >95% BD consumption is reached
already after ca. 2−3 h at 258 K, with an observed slight
preference for the incorporation of the comonomer (monomer
reactivity ratios rIP = 1.2, r5‑BD = 2.2) whereas BD
homopolymerization had already reached 80% BD conversion
when the sample was inserted into the NMR probe, and >95%
BD consumption was reached after an additional ca. 20 min at
−15 °C. Thus, the substitution of the diene moiety at the 2-
position results in a decreased polymerization activity
compared to a BD homopolymerization as well as a slightly
preferred incorporation of the substituted diene. However, a
profoundly increased comonomer reactivity ratio as observed
for, e.g., PhS-3-BD, is not observed, therefore excluding
scenario (a) as the prime reason for enhanced comonomer
incorporation.
From copolymerizations of ethylene with polar vinyl
monomers, it is well understood that a κ-X coordination of
the functional group of the last or penultimate incorporated
comonomer-derived repeat unit is the most relevant reason for
decreased polymerization rates.
Depending on a possible η4-diene coordination, the
conformation, and therefore the reactivity of the resulting
species, this can have different implications: First, if insertion
does not occur through the κ-X coordinated species but other
pathways, the copolymerization is slowed down and the
comonomer incorporation is not affected (Figure 2, case 1).
Second, if insertion is viable, the scenario can account for κ-X
assisted, increased comonomer incorporation as observed
prominently for PhS-3-BD, PhS-4-BD, and PhNH-3-BD.
Importantly, here the linker length will affect the ground
state of the coordinated starting species (cf. Figure 2 cases 2
and 3), the transition state, and intermediate species involved in
the polymerization. This affects the comonomer insertion step
as illustrated by PhS-3-BD and PhS-4-BD. Both are
incorporated preferentially, but the copolymerization is much
faster in the case of the latter (Figure 2, case 2 vs 3). Although
geometries of responsible ground- or transition states are
Concerning scenario (b), to elucidate the effect of κ-X
coordination alone, control polymerizations of BD in the
C
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Journal of the American Chemical Society
Communication
(10) Yao, C.; Liu, N.; Long, S.; Wu, C.; Cui, D. Polym. Chem. 2016, 7,
1264.
(11) Liu, D.; Yao, C.; Wang, R.; Wang, M.; Wang, Z.; Wu, C.; Lin, F.;
Li, S.; Wan, X.; Cui, D. Angew. Chem., Int. Ed. 2015, 54, 5205.
(12) Liu, D.; Wang, M.; Wang, Z.; Wu, C.; Pan, Y.; Cui, D. Angew.
Chem., Int. Ed. 2017, 56, 2714.
worthwhile issues of interest, the presence and operation of a κ-
X assisted, enhanced incorporation is clearly evident.
In conclusion, we have observed severe effects of different
functional groups and linker lengths on polymerization rate
constants and monomer reactivity ratios in copolymerizations
of BD and polar functionalized dienes catalyzed by Ni-1.
Although certain linker lengths lead to an unprecedented
preference for the incorporation of the polar functionalized
diene, all obtained copolymers are stereoregular (cf. SI).
Depending on the linker length, the strong enhancement of
incorporation is accompanied by a severe or moderate decrease
of the polymerization rate vs BD homopolymerization. For
other “nonmatched” linker lengths, the incorporation of the
comonomer is only slightly favored while at the same time
polymerization rates are only moderately influenced. The
correct choice of a suitable linker length thus enables a facile
copolymerization of dienes functionalized with difficult polar
moieties including PhS- or even PhNH- groups. We suggest
that this remarkable behavior results from a κ-X assisted
precoordination of the functional group to the metal center
based on kinetic NMR experiments with various comonomers
and model compounds.
(13) Noda, S.; Nakamura, A.; Kochi, T.; Chung, L. W.; Morokuma,
K.; Nozaki, K. J. Am. Chem. Soc. 2009, 131, 14088.
(14) Guironnet, D.; Caporaso, L.; Neuwald, B.; Gottker-Schnetmann,
̈
I.; Cavallo, L.; Mecking, S. J. Am. Chem. Soc. 2010, 132, 4418.
(15) Nozaki, K.; Kusumoto, S.; Noda, S.; Kochi, T.; Chung, L. W.;
Morokuma, K. J. Am. Chem. Soc. 2010, 132, 16030.
(16) Friedberger, T.; Wucher, P.; Mecking, S. J. Am. Chem. Soc. 2012,
134, 1010.
(17) Schuster, N.; Runzi, T.; Mecking, S. Macromolecules 2016, 49,
̈
1172.
(18) O’Connor, A. R.; Urbin, S. A.; Moorhouse, R. A.; White, P. S.;
Brookhart, M. Organometallics 2009, 28, 2372.
(19) O’Connor, A. R.; White, P. S.; Brookhart, M. J. Am. Chem. Soc.
2007, 129, 4142.
(20) The virtually complete comonomer consumption was
1
determined by the absence of olefinic comonomer signals in the H
NMR spectra.
(21) Please note that this corresponds at the same time to a gradual
transition from a copolymerization of BD and PhS-3-BD to a mere
homopolymerization of BD, thus rationalizing the description of this
behavior by determination of two different rate constants for the two
distinct (co)polymerization regimes.
(22) Note that in these experiments the initial ratio of
BD:comonomer is ca. 5:1 and the BD:comonomer ratio increases
significantly during the polymerization. That is, the incorporation of
comonomer is still favored as also reflected by kCoMo/kBD = rCoMo > 1
for all comonomers described in this example.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.7b03087.
Kinetic plots, synthetic procedures, and selected NMR
spectra (PDF)
(23) Note for these copolymerizations that the polymerization
behavior deviates in some cases significantly from first-order behavior
for so far unknown reasons.
AUTHOR INFORMATION
Corresponding Author
*stefan.mecking@uni-konstanz.de
ORCID
Hannes Leicht: 0000-0002-4694-9399
Stefan Mecking: 0000-0002-6618-6659
Notes
■
(24) Note that in detail, the 1-olefin function will coordinate stronger
than a trisubstituted olefinic moiety of the polymer backbone. Also,
these model compounds rather resemble the coordination behavior of
an arbitrary, incorporated comonomer unit in the polymer backbone
than an ultimately incorporated comonomer unit as they can not
simulate a possible involved metal-comonomer bond. Notwithstand-
ing, the essential feature of a potential bidentate κ-S, η2-olefin
coordination is reflected.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Karen Burke, Maggie Vielhaber, and Stephan
Rodewald for fruitful discussions. Financial support by The
Goodyear Tire & Rubber Company is gratefully acknowledged.
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DOI: 10.1021/jacs.7b03087
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX