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Insertion Copolymerization of Difunctional Polar Vinyl Monomers
with Ethylene
Shahaji R. Gaikwad,
†
Satej S. Deshmukh,
†
Rajesh G. Gonnade,
‡
P. R. Rajamohanan,
§
and Samir H. Chikkali*
,†,∥
†
Polyolefin Lab, Polymer Science and Engineering Division,
‡
Center for Material Characterization, and
§
Central NMR Facility,
CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India
∥
Academy of Scientific and Innovative Research, Anusandhan Bhawan, 2 RafiMarg, New Delhi 110001, India
*
SSupporting Information
ABSTRACT: A single-step synthesis, structural characterization and
application of a neutral, acetonitrile ligated, palladium−phosphinesulfonate
complex [{P∧O}PdMe(L)] (P∧O=κ2-P,O−Ar2PC6H4SO2Owith Ar = 2-
MeOC6H4;L=CH
3CN) (3) in coordination/insertion copolymerization of
ethylene with difunctional olefin is investigated. In a significant development,
complex 3was found to catalyze insertion copolymerization of industrially
relevant 1,1-disubstituted difunctional vinyl monomers for the first time.
Thus, insertion copolymerization of ethyl-2-cyanoacrylate (ECA or super
glue) and trifluoromethyl acrylic acid (TFMAA) with ethylene produced the corresponding copolymers with 6.5% ECA and 3%
TFMAA incorporation. Increasing the concentration of difunctional olefins led to higher incorporation but at the expense of
lower activities. These observations indicate that complex 3tolerates difunctional vinyl monomers and provides direct access to
difunctional polyolefins that have not been attempted before.
The seminal work of Karl Ziegler and Giulio Natta laid the
foundation of coordination/insertion polymerization of
olefins.
1
Tremendous progress has been made, and today the
world produces roughly 145 million tons of polyolefins
annually.
2
Polyethylene (PE) occupies the top position (in
terms of production) among the polyolefins, and various grades
of PE are commercially produced.
3
PE is inherently a long
chain of hydrophobic methylene repeat units without any
functional group on the backbone. This partly limits the
potential application of PE in adhesives, binders, paints,
printing ink, dying, etc. Incorporation of even a small amount
of functional groups in PE can significantly enhance these
material properties and can further broaden the PE application
window. However, due to the high oxophilicity of early
transition metal based Ziegler−Natta type catalysts, the
functional group on the polar vinyl monomer coordinates to
the metal (occupying the vacant site) that generally leads to
catalyst poisoning. This limitation has been partly addressed by
postfunctionalization of PE,
4
ADMET polymerization of
functionalized dienes,
5
or OMRP of polar vinyl monomers
with ethylene.
6
However, these strategies suffer from typical
issues associated with free radical polymerization. Therefore, it
has been a long cherished dream of organometallic chemists to
synthesize functionalized PE in a single step via insertion
(co)polymerization of ethylene with industrially relevant polar
vinyl monomers.
7,8
Despite the significant progress in olefin
polymerization, insertion (co)polymerization of functional
olefins remained inaccessible until recently.
9
Apart from a few random attempts, the insertion (co)-
polymerization of functional olefins mainly explored two ligand
classes, (a) α-diimine ligands
10
and (b) the phosphinesulfonate
ligand system.
11
The Brookhart system mainly copolymerizes
acrylates with high branching density due to extensive chain-
walking.
12
However, the phosphine−sulfonate system displays
broad functional group tolerance, and various polar vinyl
monomers such as acrylate,
13
acrylonitrile,
14
vinyl acetate,
15
vinyl ethers,
16
acrylic acid,
17
and vinyl chloride
18
could be
incorporated. The excellent performance of the phosphinesul-
fonate system further rose the scientific aspirations, and
subsequent investigations focused on structural fine-tuning of
the catalyst.
19
Three crucial positions as depicted in Figure 1
were identified, among which the effect of the donor group “D”
has been extensively studied (see Figure 1: C1).
20
The above investigations mainly focused on insertion
copolymerization of monosubstituted polar vinyl monomers,
wherein one functional group is introduced into the polymer
Received: August 11, 2015
Accepted: August 13, 2015
Published: August 17, 2015
Figure 1. Structural tuning of C1 at designated positions and insertion
copolymerization to (a) monofunctional polyolefin (FPO) and (b)
1,1-disubstituted difunctional polyolefin (dFPO).
Letter
pubs.acs.org/macroletters
© 2015 American Chemical Society 933 DOI: 10.1021/acsmacrolett.5b00562
ACS Macro Lett. 2015, 4, 933−937
chain per insertion. A paradigm shift could be achieved if two
functional groups can be introduced in a polymer chain per
insertion (of a functional olefin), resulting in a doubly polar
polymer. Although highly desirable, there are no reports on
insertion copolymerization of 1,1-disubstituted difunctional
olefins to date.
Herein we report synthesis of the Pd−phosphinesulfonate
acetonitrile complex (3) and its implications in insertion
copolymerization of ethylene with industrially relevant 1,1-
disubstituted difunctional vinyl monomers (see Figure 1;
dFPO-difunctional polyolefin) for the first time.
In our pursuit to realize this goal, we attempted synthesis of
acetonitrile complex 3(Scheme 1). Ligand 1was treated with
[(COD)PdMeCl], and within 10 min the acetone dimer 2
(78%) was obtained.
21
Treatment of AgBF4in the presence of
acetonitrile (10 equiv) afforded the anticipated complex 3
which was isolated in good yield (92%) (Scheme 1, P-I).
An alternative, one-step synthesis was also explored. Reaction
of 1with [(COD)PdMeCl] in acetonitrile directly produced
complex 3(Scheme 1, P-II) in excellent yield (91%). The
existence of palladium complex 3was unambiguously
ascertained from spectroscopic and analytical data.
22
31P
NMR of complex 3displayed a characteristic resonance at
21.1 ppm (SI, S1). In a typical proton NMR, the palladium-
bound methyl protons (Pd−CH3) appeared at 0.18 ppm,
whereas the corresponding methyl carbon (Pd−CH3) appeared
at −2.4 ppm in a 13C NMR spectrum.
23
The NMR findings
were further corroborated by electrospray ionization mass
spectrum (ESI-MS +ve mode) which displayed a pseudomo-
lecular ion peak at m/z= 544.97 [M-ACN + Na]+(SI S6). A
single-crystal X-ray structure of complex 3displayed slightly
distorted square planar geometry at palladium (Figure 2) which
is crystallized in the orthorhombic Pbca space group. The
phosphine and the methyl group are mutually cis to each other,
whereas the acetonitrile is situated trans to the phosphine.
24
To shed light on the relative binding strength of donor
solvents, 1 equiv of DMSO was added to complex 3, and the
changes were tracked using proton and phosphorus NMR.
Addition of 1 equiv of DMSO led to complete disappearance of
acharacteristicPd−Me (in 3) resonance at 0.18 ppm;
concomitantly a new signal at 0.39 ppm appeared in the 1H
NMR spectrum (SI S8 and Table S1). The new resonance (at
0.39) can be readily assigned to a previously reported DMSO-
coordinated Pd−Me complex.
13
These experiments indicate
that acetonitrile binding strength is lower or as good as DMSO
binding strength.
The performance of 3in ethylene-functional olefin
copolymerization was evaluated, and representative polymer-
ization experiments are summarized in Table 1. The Pd-
complex 3catalyzes copolymerization of acrylonitrile and
ethylene at 95 °C. After polymerization, the volatiles were
evaporated under reduced pressure to obtain solid material.
Insertion copolymerization of acrylonitrile and ethylene
produced the desired copolymer with 9.2% acrylonitrile
incorporation (Table 1, run 1−1). The performance of 3in
another industrially important monomer, methyl acrylate, was
tested. An incorporation of 9.6% was observed at 0.6 mol/L
concentration, and the polymer molecular weight was 3100 g/
mol (run 1−2). Thus, catalyst 3tolerated the cyano- as well as
acrylate functional groups and produced functional copolymers
with reasonable molecular weights. Ethyl-2-cyanoacrylate
(ECA) was chosen as a representative 1,1-disubstituted
difunctional olefin as both the cyano- and acrylate group are
installed within the same olefin. It should be noted that
cyanoacrylates are commonly sold under the trade name “Super
Glue or Krazy Glue”, and the polymers thereof are
commercially produced and find applications in various
adhesives. It is one of the largest adhesives produced
worldwide.
25
Interestingly though, very little information exists
on the reactivity of such an industrially highly relevant 1,1-
disubstituted difunctional olefin in insertion (co)-
polymerization reaction.
26
Having known that the probability of functional group
coordination (acrylate or cyano) to the metal and subsequent
catalyst deactivation is increased by 2-fold in ethyl-2-
cyanoacrylate, we begin our exploration with a very low (0.03
mol/L) ECA concentration at 5 bar ethylene pressure (Table 1,
run 1−3). After polymerization, the reactor content was
transferred to a Schlenk flask; volatiles were stripped off; and
the solid mass was washed with excess chloroform to yield solid
polymeric material.
27,28
The proton NMR of the solid was
recorded in deuterated tetrachloroethane at 130 °C in a 10 mm
NMR tube, and the amount of incorporation was determined
(see Figure 3).
29
Typically, 1H NMR of the copolymer
displayed distinct signals at 4.47 (Hc), 2.87−2.30 (Ha), 1.49,
and 1.38 (Hb) ppm suggesting ECA incorporation. The proton
NMR findings were further corroborated by 13C NMR, which
Scheme 1. Synthesis of the Acetonitrile-Ligated Pd−
Phosphinesulfonate Acetonitrile Complex (3)
Figure 2. Molecular structure of complex 3. Solvent molecules and H
atoms are omitted for clarity, and thermal ellipsoids are drawn at the
50% probability level.
ACS Macro Letters Letter
DOI: 10.1021/acsmacrolett.5b00562
ACS Macro Lett. 2015, 4, 933−937
934
revealed a very characteristic signal at 113.7−117.3 ppm that
can be assigned to in-chain CN incorporation.
30
A representative HMBC spectrum (SI, S16D) of a low
molecular weight copolymer fraction revealed cross-peaks that
can be assigned to −CH2protons (diastereotopic in nature) of
ECA (2.20−2.80 ppm) and −CH2carbons originating from
ethylene repeat units (28−30 ppm) (SI sect. 5.2).
22
The 2D
C−H correlation spectra displayed cross peaks that established
the connectivity between “a”type protons to “a”carbons; “a”
protons to “b”type carbon; and vice versa (see Figures S16B−
D). In addition to these 2D-NMR experiments, MALDI-ToF-
MS (SI S17A−E) also indicated ECA incorporation, and
various copolymer fragments could be identified. This was
further confirmed by comparing the reported ET−ECA
copolymer produced by the radical polymerization method
30
as well as control experiments.
31
Thus, 1H NMR of a
copolymer obtained in run 1−3 revealed a very low
incorporation of 0.32%. Increasing the concentration of ECA
resulted in increased incorporation (run 1−3to1−5). On the
other hand, increasing ethylene pressure (from 1 to 10 bar) led
to decreased ECA incorporation (run 1−6, 1−4, and 1−7).
Thus, the highest incorporation of 6.5% could be achieved at
ambient ethylene pressure and 0.06 mol/L ECA concentration.
The rigorous washing protocol may wash out the low molecular
weight fraction and might lead to lower polydispersity in some
cases. The DMSO complex of type 3displayed similar reactivity
in the insertion copolymerization of 1,1-disubstituted functional
olefin, along with ca. 2.% incorporation (run 1−8). It is most
likely that ethylene insertion in the Pd−Me bond leads to chain
propagation, or the 2,1-/1,2-insertion of ECA in the Pd−Me
bond initiates the polymerization, as the three initiating groups
(IG-1, IG-2, and IG-3) could be detected by 13C NMR (SI
section 6). To further strengthen our hypothesis, we
investigated insertion copolymerization of another 1,1-disub-
stituted functional olefin(trifluoromethyl acrylic acid:
TFMAA). About 0.7% TFMAA incorporation was observed
at a very low TFMAA concentration (0.06 mol/L run 1−9).
However, increased incorporation of 3% could be achieved at
higher TFMAA concentration (3.0 mol/L run 1−10). A
characteristic proton resonance at 2.1 ppm indicated formation
of an ethylene−TFMAA copolymer (SI S48). Furthermore, the
absence of characteristic proton resonance at 3.4 ppm ruled out
the presence of a TFMAA homopolymer (SI S50).
32
An 19F
NMR spectrum (Figure 4) of the oligomeric fraction (accessed
by suspending the copolymer in excess chloroform) revealed
characteristic signals at −65.1, −66.1, −66.7, and −68.4 ppm.
These chemical shifts are similar to those reported for
ethylene−trifluoropropene copolymers (the closest copolymer
known)
33
and can be readily assigned to ET−TFMAA
oligomers. The absence of a characteristic splitting pattern
and signal broadening hampered further analysis, and the
resonances can be tentatively assigned to structure A−D(SI
S51). The NMR findings were further supported by IR, which
revealed a characteristic (CO) band at 1710 cm−1(SI S49),
that can be ascribed to acidic CO and is in line with earlier
Table 1. Insertion Copolymerization of Ethylene with Functional Olefins Catalyzed by Complex 3
a
run FO (mol/L) C2H4(bar) % incor.
b
yield (g) Mn (103g/mol)
c
Mw/Mn
c
1−1 AN (1.2) 5 9.2
d
0.13 ND ND
1−2 MA (0.60) 5 9.6
d
0.39 3.1 1.3
1−3 ECA (0.03) 5 0.3 0.90 4.7 1.5
1−4 ECA (0.06) 5 2.1 0.79 8.2 1.4
1−5 ECA (0.12) 5 4.9 0.87 6.4 1.7
1−6 ECA (0.06) 1 6.5 0.20 5.8 1.6
1−7 ECA (0.06) 10 1.9 1.09 8.3 1.4
1−8
d
ECA (0.06) 1 2.01 0.21 4.9 1.2
1−9
e
TFMAA (0.06) 1 0.7 0.37 ND ND
1−10
e
TFMAA (3.0) 1 3.0 0.07 2.8 1.2
a
Reaction conditions: 3=20μmol in DCM, toluene = 50 mL (toluene + functional olefin); temperature = 95 °C, time = 1 h, AN = acrylonitrile,
MA = methyl acrylate, ECA = ethyl-2-cyanoacrylate, TFMAA = trifluoromethyl acrylic acid.
b
Incorporation was determined by high-temperature 1H
NMR in C2D2Cl4at 130 °C.
c
Determined by high-temperature GPC at 160 °C in trichlorobenzene against PS standard; ND = not determined.
d
DMSO complex of type 3was used as a catalyst.
e
Toluene = 5/3 mL, TFMAA incorporation was determined by high-temperature 1H NMR in
C2D2Cl4at 130 °C (for run 1−9) and C6D6+ TCB (10:90) mixture at 120 °C (for run 1−10).
Figure 3. 1H NMR of the ET−ECA copolymer in C2D2Cl4at 403 K
(Table 1, run 1−6).
ACS Macro Letters Letter
DOI: 10.1021/acsmacrolett.5b00562
ACS Macro Lett. 2015, 4, 933−937
935
reports. Furthermore, a typical −OH band was observed at
3368 cm−1, which, together with CO band, indicates the
existence of a carboxylic group in the copolymer. MALDI-ToF-
MS spectra of the copolymer also indicated TFMAA
incorporation and various copolymer fragments could be
identified (see Table S6 and SI S52A−C). Interestingly,
TFMAA does not homopolymerize under the current polymer-
ization conditions and thus rules out the possibility of
homopolymer contamination.
22
The reduced polymer yields
with increasing ECA/TFMAA concentration and low molecular
weights indicate reversible inhibition of polymerization, due to
the coordination of functional groups located on the 1,1-
disubstituted olefin to the metal. However, the fact that the
copolymers could be obtained suggests that the presence of 1,1-
disubstituted functional olefins does not necessarily lead to
detrimental catalyst decomposition.
In summary, a single-step synthetic protocol to access an
acetonitrile-ligated Pd−phosphinesulfonate complex (3) was
established. Our investigations demonstrate that insertion
copolymerization of ethylene with 1,1-disubstituted difunc-
tional olefin is possible. Complex 3tolerates the two functional
groups located within the same polar vinyl monomer and
successfully copolymerizes ECA and TFMAA with ethylene. A
combination of multinuclear NMR, MALDI-ToF-MS, and
control experiments suggest in-chain ECA incorporation. High-
temperature NMR investigations revealed an unprecedented
6.5% incorporation of ethyl-2-cyanoacrylate and 3% incorpo-
ration of TFMAA in a copolymer. These copolymers represent
a novel class of functional polyolefins with two functional
moieties (cyanoacrylate or fluoro-acrylic acid) incorporated at a
time. Investigations on the insertion copolymerization of other
industrially relevant 1,1-disubstituted polar vinyl monomers
and the mechanism are currently in progress.
■ASSOCIATED CONTENT
*
SSupporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsmacro-
lett.5b00562.
Synthetic protocols, text, figures, tables, method to
determine percentage functional olefin incorporation,
spectroscopic and analytical data, CIF files, and crystallo-
graphic data/processing method for 3(CCDC 1012218)
(PDF)
■AUTHOR INFORMATION
Corresponding Author
*E-mail: s.chikkali@ncl.res.in. Phone: +91 20 25903145.
Author Contributions
All authors have given approval to the final version of the
manuscript.
Notes
The authors declare no competing financial interest.
■ACKNOWLEDGMENTS
Financial support from DST-India (Ramanujan Fellowship:
SR/S2/RJN-11/2012 and SR/S1/IC-60/2012) is gratefully
acknowledged. We thank Prof. Stefan Mecking for proof
reading and useful discussion; Mrs. D. Dhoble is acknowledged
for HT-GPC analysis. SRG and SSD would like to thank CSIR
and UGC, respectively, for the junior research fellowship.
CSIR-National Chemical Laboratory and SPIRIT (DCPC) is
gratefully acknowledged for additional support. The Authors
acknowledge NMR Center for Advanced Research
(NMRCAR) at CSIR-NCL funded by CSIR, New Delhi,
through 12th FYP project, CSC 0405.
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ACS Macro Letters Letter
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ACS Macro Lett. 2015, 4, 933−937
936
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disubstituted difunctional olefins. A scifinder search with “Insertion
polymerization of 1,1-disubstituted functional olefin”as key word
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(27) Ethyl-2-cyanoacrylate is known to polymerize in the presence of
traces of air. Hence, polymerization was carried out under inert gas
conditions, and after polymerization, workup was done under positive
argon flow; for moisture-induced homopolymerization of ethyl-2-
cyanoacrylate see: (a) Mankidy, P. J.; Rajagopalan, R.; Foley, H. C.
Chem. Commun. 2006, 1139. (b) Weiss, C. K.; Ziener, U.; Landfester,
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(28) To avoid radical polymerization of ethyl-2-cyanoacrylate, radical
scavenger BHT was added to the polymerization reactor. See
Supporting Information.
(29) The functional olefin incorporation was determined by 1H
NMR; a detailed calculation method is presented in the Supporting
Information (SI, section 5).
(30) Poly(ethylene−ECA) copolymers were prepared by Eisenbach
et al. using radical initiators and were fully characterized by 13C NMR.
See: (a) Sperlich, B.; Eisenbach, C. D. Acta Polym. 1996,47, 280.
(b) Eisenbach, C. D.; Lieberth, W.; Sperlich, B. Angew. Makromol.
Chem. 1994,223, 81.
(31) Apart from NMR, various control experiments ruled out the
possibility of existence of an ECA homopolymer in the ET−ECA
copolymer. See SI section 7 for control experiments.
(32) First radical homopolymerization of TFMAA was reported in
2013 by Patil et al. The chemical shift for -CH2- protons next to the
quaternary carbon was reported to be 3.4 ppm. See: Patil, Y.; Hori, H.;
Tanaka, H.; Sakamoto, T.; Ameduri, B. Chem. Commun. 2013,49,
6662.
(33) Ethylene−trifluoropropene copolymerization has been recently
reported by Lanzinger et al. 19F NMR of the copolymer displayed a
resonance at −71.4 ppm, which is assigned to in-chain −CF3groups.
Given the similarity between this copolymer and the herein
investigated ET−TFMAA oligomers, the observed chemical shift can
be assigned to groups A−D. See: Lanzinger, D.; Giuman, M. M.;
Anselment, T. M. J.; Rieger, B. ACS Macro Lett. 2014,3, 931.
ACS Macro Letters Letter
DOI: 10.1021/acsmacrolett.5b00562
ACS Macro Lett. 2015, 4, 933−937
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