Migration insertion polymerization (MIP) of cyclopentadienyldicarbonyldiphenylphosphinopropyliron (FpP): A new concept for main chain metal-containing polymers (MCPs)

Article abstract of DOI:10.1021/ja400755e

We report a conceptually new polymerization technique termed migration insertion polymerization (MIP) for main chain metal-containing polymer (MCP) synthesis. Cyclopentadienyldicarbonyldiphenylphosphinopropyliron (FpP) is synthesized and polymerized via M

Full text of DOI:10.1021/ja400755e

Communication
pubs.acs.org/JACS
Migration Insertion Polymerization (MIP) of
Cyclopentadienyldicarbonyldiphenylphosphinopropyliron (FpP): A
New Concept for Main Chain Metal-Containing Polymers (MCPs)
Xiaosong Wang,* Kai Cao,† Yibo Liu,† Brian Tsang, and Sean Liew
Department of Chemistry and Waterloo Institute of Nanotechnology, University of Waterloo, 200 University Avenue, Waterloo,
Canada N2L 3G1
S
* Supporting Information
2e−. As a result, this allows a neutral ligand to coordinate via the
empty orbital.19 We therefore envision that MIRs15 can be used
ABSTRACT: We report a conceptually new polymer-
ization technique termed migration insertion polymer-
ization (MIP) for main chain metal-containing polymer
(MCP) synthesis. Cyclopentadienyldicarbonyldiphenyl-
phosphinopropyliron (FpP) is synthesized and polymer-
ized via MIP, resulting in air stable poly(cyclopentadienyl-
carbonyldiphenylphosphinobutanoyliron) (PFpP) display-
ing narrow molecular weight distribution. The backbone of
PFpP contains asymmetric iron units connected by both
phosphine coordination and Fe-acyl bonds, which is
representative of a new type of polymer. Furthermore,
PFpP is tested to be soluble in a wide range of organic
solvents and shown to possess reactive Fp end groups.
PFpP amphiphiles have therefore been prepared via an end
group migration insertion reaction in the presence of
oligoethylene phosphine.
to link organometallic units together into a macromolecular
chain structure through rational design of organometallic
monomers. For example, A−B type monomers can be designed
with organometallic groups capable of MIR acting as (A) and
neutral phosphine ligands acting as (B). To explore the
possibility of this new polymerization concept, cyclopenta-
dienylironcarbonyl (Fp) chemistry15 is used for the synthesis of
monomers and MCPs.7c,16 It is well-known that the reaction of
alkyldicarbonylcyclopentadienyliron (FpR) with nucleophilic
ligands, e.g. phosphine (PR3), leads to air stable phosphine
coordinated acyl complexes as a result of MIR.17 We therefore
expect that molecules containing both Fp and phosphine
groups can act as a difunctional A−B type monomer used for
macromolecule construction via a migration insertion polymer-
ization (MIP) process.
In this communication, we report a proof-of-concept that
MIP occurred as designed through bulk polymerization of the
monomer, cyclopentadienyldicarbonyldiphenylphosphino-
propyliron (FpP) (Scheme 1). The resulting polymer, poly-
he development of synthetic chemistry for metal-
T
containing polymers (MCPs)1 has led to exciting progress
in living supramolecular chemistry2 and in applications for
modern technologies,3 e.g. solar cells,4 security ink,5 and self-
healing smart materials.6 To date, most organometallic
monomers are polymerized using well-developed techniques
such as ring-opening polymerization,7 condensation,8 and living
chain polymerization.9 During the polymerization, metal
coordination structures usually remain intact. Either organic
or inorganic10 reactive groups pending from metal ligands are
actually responsible for the growth of polymer chains.
Organometallic propagating reactions for macromolecule
synthesis are rarely explored but offer opportunities for
innovative chemistry and for creation of novel polymers. For
example, by taking advantage of the photolability of metal
coordination bonds, ring-opening photopolymerization can be
carried out under mild conditions.11,12 Soluble metal
coordination chain polymers have been made using “complex
as metal” strategies,13 or direct metal coordination chemistry.14
Metal carbon σ-bonds have been built into transition metal
acetylide polymer backbones using oxidative coupling reac-
tions.1,4
Scheme 1. Scheme for Synthesis and MIP of FpP
(cyclopentadienylcarbonyldiphenylphosphinobutanoyliron)
(PFpP), contains asymmetric iron units connected by
phosphine coordination and metal−acyl bonds representing a
new type of MCP. The polymer exhibits narrow molecular
weight distribution and is soluble in many organic solvents:
THF, DCM, DMF, etc. The Fp end groups of FpP remain
reactive and can be used for end group functionalization via an
MIR coupling reaction.
For organometallic compounds containing cisoidal anionic
and neutral ligands, migration insertion reactions (MIRs) often
occur to combine two ligands into a new anionic ligand. After
MIR, there is no change in the oxidation state of the metal
center, but the overall electron count of the metal decreases by
Received: January 25, 2013
© XXXX American Chemical Society
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Journal of the American Chemical Society
Communication
The monomer (FpP) was prepared from a salt-elimination
reaction between 1-chlorodiphenylphosphinopropane and
potassium cyclopentadienyldicarbonyliron (FpK) at room
temperature. The IR spectrum of the resulting compound
reveals two absorption frequencies for CO at 1952 and 2004
cm−1 (Figure S1). This suggests that only terminal CO exists as
designed.24 In addition, the existence of phosphine groups is
Table 1. GPC Results for PFpP Prepared from Migration
Insertion Polymerization of FpP
a
b
d
entry time/h yield
Mw
Mn (GPC/NMR)
PDI
DP
8
1
2
3
4
5
69
3920
3320/3230
6640/6460
9870/9700
1.18
1.09
1.33
12
16
18
75
7240
16
24
30
85
13090
15500
e
f
1
confirmed by 31P NMR (Figure 1). H NMR analysis reveals a
>95
12100/−
1.28
a
b
Polymerization were performed in bulk at 70 °C. Polymers were
isolated by precipitation of THF solution in hexane. Yields are
obtained by gravimetric analysis. DP: degree of polymerization. End
d
e
f
group is invisible in NMR. Sample for GPC was taken right after the
polymerization without precipitation.
assumed to have equal reactivity. The unexpected narrow PDI
for the polymers produced from MIP can be rationalized by the
low reactivity of the phosphine end group of PFpP (Figure S4).
It has been reported that phosphine with bulk groups slow
down MIRs.17,22 In the case of polymerization, increased steric
effects are expected from the formation of macromolecular
chains. Therefore, small molecules are expected to react faster
than larger molecules leading to the observed narrow PDI. A
detailed investigation of this phenomenon is currently under-
way.
1
Figure 1. H and 31P NMR spectra of FpP in DMSO-d6.
peak at 1.46 ppm representing protons for −CH2−Fe (Figures
1, S2, and S3).19 This suggests that the carbon−iron bond is
produced from the substitution reaction between the chloride
and the Fp metalate. The integration ratio between the Cp (4.8
ppm) and phenyl protons (7.28−7.40 ppm) is 1:2. This
indicates that the monomer possesses equal amounts of Fp and
phosphine groups as designed. However, FpP undergoes
intramolecular cyclic reactions in diluted organic solvents (see
Supporting Information).20
DSC analysis (Figure 2) indicates that PFpP has a Tg of ca.
100 °C. This Tg is well above room temperature and
Since MIR can also occur in solid state,18,21 bulk polymer-
ization of FpP was performed at 70 °C to suppress
intramolecular cyclic reactions. During the polymerization,
the viscosity of the system gradually increased until glass-like
solid material was produced with the stir bar frozen inside the
product. To terminate the polymerization, the reaction flask
was first cooled to room temperature. Then THF was added to
dissolve the solid products. Afterward, the clear solution was
transferred dropwise into hexane obtaining pale yellow
precipitates. For the sample that polymerized longer than 18
h, the precipitates were recovered in an almost quantitative
yield. Also the supernatant of the mixture was colorless
indicating that most of the organometallic monomers
participated in the polymerization. The resulting precipitates
are readily soluble in many solvents such as THF, DMSO,
DCM, acetone, DMF, toluene, benzene, and chloroform but
are insoluble in water and nonpolar solvents such as hexane. All
of the polymer solutions (yellow) were left exposed to air,
which resulted in no obvious color change and precipitation
during the experimental period of 4 days.
The macromolecular nature of the products was shown by
GPC with either THF or DMF eluents. There were no changes
in GPC traces for the same samples characterized immediately
and 2 days later. This confirmed that the polymers are
sufficiently stable in solution and can be used for further
studies. GPC results for the polymers are summarized in Table
1. As shown in Table 1, by increasing the time, the molecular
weight (Mn) gradually increased. This result suggests MIP
occurs. Interestingly, all polymers exhibit narrow molecular
weight distributions with polydispersities in the range between
1.09 and 1.33. These values are much narrower compared to
the expected PDI (ca. 2) for polymers produced from a
conventional step growth polymerization, in which the
monomer functional groups and polymer end groups are
Figure 2. DSC and TGA curves for PFpP.
demonstrates a thermoplastic characteristic of the polymer.
TGA analysis (Figure 2) indicates that the polymer loses ca.
60% of its original weight at 180 °C. The second stage of
weight loss starts at 432 °C. Finally, the polymer loses ca. 77%
of its original weight at 622 °C leaving ca. 23% char yield.
The molecular structure of PFpP was established by IR and
NMR spectroscopies. The IR spectrum reveals two absorption
frequencies at 1910 and 1600 cm−1 (Figure S1). This indicates
the presence of both terminal CO and inserted carbonyl groups
in the polymers as a result of an MIR.24 This is further verified
by 13C NMR analysis which shows resonance peaks at 219 and
272 ppm (Figure S5) corresponding to terminal CO and acyl
groups respectively.24 The 31P NMR reveals a new signal
appearing at 73 ppm that represents the coordination of
phosphines to iron centers.24
The 1H NMR analysis (Figure 3) of PFpP also shows
evidence of MIR. The Cp ring of PFpP at 4.3 ppm is shifted
upfield compared to the Cp ring of FpP (Figure 3). This upfield
shift provides evidence of phosphine assisted MIR occurring.17
The small peak displayed at 4.7 ppm is assigned to the Cp ring
of the Fp end group of PFpP. The peaks at 2.8 and 2.3 ppm are
assigned to two diastereotopic protons from the C(O)CH2 acyl
groups adjacent to the Fe chiral center.17 This assignment is
confirmed by 13C−1H HMQC 2D NMR (Figure S6). The
signal at 2−2.2 ppm is assigned to the central CH2 protons, and
B
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Journal of the American Chemical Society
Communication
units linked by both phosphine coordination and metal−acyl
bonds. The polymer also displays narrow molecular weight
distributions and is soluble in many organic solvents. The
reactivity of the Fp end group has been used for the synthesis of
oligoethylene functionalized PFpP. As MIR is a widely used and
well-studied organometallic reaction, the polymerization
concept of MIP opens up new possibilities to incorporate
various metal elements into a novel type of macromolecule
chain for functional material exploration.
Figure 3. 1H NMR spectrum for PFpP (Mn = 6640, Mw/Mn = 1.09) in
DMSO-d6.
ASSOCIATED CONTENT
■
S
* Supporting Information
1
1
Experimental details, IR, H NMR, 13C NMR, and H−1H
the signal at 0.9−1.3 ppm is assigned to the protons adjacent to
P(Ph)2.24 These signals have an integration ratio of 1:1. The
integration ratio of the phenyl protons at 7.1−7.8 ppm relative
to the Cp ring at 4.3 ppm is 2:1, which is in agreement with the
targeted molecular structure of PFpP.
1
COSY and H−13C HMQC 2D NMR for the monomers and
1
polymers; H NMR, TEM image, and DLS result for PFpP
amphiphiles and their assemblies. Experiments testing PFpP
end group reactivity and monomer solution behavior. This
material is available free of charge via the Internet at http://
pubs.acs.org.
1
Figure 4 shows 31P and H NMR spectra of the polymers
with an Mn of 3320 g/mol. Three peaks at 73, 72, and −14 ppm
AUTHOR INFORMATION
Corresponding Author
Xiaosong.wang@uwaterloo.ca
■
Author Contributions
†These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The Natural Sciences and Engineering Research Council of
Canada (NSERC) and the University of Waterloo are
acknowledged for financial support. We thank Prof. Arthur
Carty for reading and correcting the manuscript.
1
Figure 4. 31P and H NMR spectra of PFpP (Mn = 3320, Mw/Mn =
1.18) in DMSO-d6.
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