Monofunctional metathesis polymers via sacrificial diblock copolymers

Article abstract of DOI:10.1002/anie.200602323

(Chemical Equation Presented) A small price to pay: The second block of a diblock copolymer is "sacrificed" in order to leave behind a monofunctionalized metathesis polymer with a hydroxy end group. By incorporation of a dioxepine unit into the copolymer, a breaking point is created between the block to be end-functionalized and the block to be sacrificed.

Full text of DOI:10.1002/anie.200602323

Angewandte
Chemie
Functionalized Polymers
DOI: 10.1002/anie.200602323
Monofunctional Metathesis Polymers via
Sacrificial Diblock Copolymers**
Stefan Hilf, Elena Berger-Nicoletti, Robert H. Grubbs,
and Andreas F. M. Kilbinger*
Ring-opening olefin metathesis polymerization (ROMP) is a
powerful tool for the synthesis of highly functionalized
[
1]
polymers. The first well-defined ROMP catalyst systems
[
2]
[3]
based on titanium, molybdenum, and tungsten exhibited
relatively low tolerance toward functional groups. This low
tolerance could be exploited in the end-functionalization of
metathesis polymers. The high oxophilicity of the metal
centers made end functionalization possible by addition of
[
4]
aldehydes to the polymerization mixture. Late-transition-
[
5]
metal catalysts based on ruthenium do not allow for olefin
metathesis functionalization with aldehydes. Nonetheless,
several pathways have been described in the literature for
the end functionalization of ruthenium-catalyzed metathesis
polymers.
The most common method of terminating a ruthenium
carbene at the chain end of a polymer is achieved by adding
[
6]
ethyl vinyl ether. A methylene group is transferred onto the
polymer while cleaving the catalyst off the polymer chain end
at the same time. It could be shown that the cleaved-off
Fischer-type carbene can undergo further olefin metathesis
[
7]
reactions under certain conditions. For most polymer
synthetic applications, however, this species can be regarded
as inactive. In the presence of vinyl sulfides it could be shown
that such “Fischer carbenes” react as chain-transfer agents to
[
8]
yield monofunctional polymers. The polymers that were
prepared in this way show broad polydispersity indices (PDI
between 1.3 and 3).
Substituted methyl vinyl ethers have also been used for
[
9]
the termination of living metathesis polymers. In this way, a
variety of functional groups could be transferred to the
polymer chain end. Gibson and co-workers reported an end-
capping reaction whereby the polymerization reaction mix-
ture was exposed to an oxygen atmosphere, which resulted in
[*] S. Hilf, Dr. E. Berger-Nicoletti, Dr. A. F. M. Kilbinger
Institut für Organische Chemie
Johannes Gutenberg-Universität Mainz
Duesbergweg 10–14, 55099 Mainz (Germany)
Fax: (+49)6131-39-26138
E-mail: akilbing@uni-mainz.de
Prof. Dr. R. H. Grubbs
Arnold and Mabel Beckman Laboratories of Chemical Synthesis
Division of Chemistry and Chemical Engineering
California Institute of Technology, Pasadena, CA 91125 (USA)
E-mail: rhg@caltech.edu
[**] A.F.M.K. thanks the Fonds der Chemischen Industrie (FCI) for
financial supportand Dr. Oren Scherman for helpful discussions.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Communications
[
10]
an aldehyde end group after a reaction time of 24 hours.
A
further method describes the stoichiometric addition of
one equivalent of functional monomer to the catalyst, thereby
[
11]
turning the catalyst into a monofunctional initiator.
The end-functionalization reactions described above
suffer from several drawbacks. They typically take several
hours to complete, during which the metathesis-active
catalytic species can undergo further polymerization or
secondary metathesis reactions. These end-capping methods
can therefore not be employed in the presence of residual
monomer without significant broadening of the molecular
weight distribution. Even in the absence of residual monomer,
long reaction times for the termination reaction can lead to
molecular weight broadening as a result of chain-transfer
reactions to the polymer (back-biting), a fact that is exploited
[
12]
in the equilibration of telechelics. True monofunctionality
is also not guaranteed rigorously by some of the above
examples. It is important to stress that the presence of exactly
one functional group at the chain end is essential for many
applications, such as the synthesis of block copolymers, the
synthesis of conjugates with biomacromolecules like proteins,
polysaccharides, or polynucleotides, or the functionalization
of surfaces and nanoparticles. Today, many of these applica-
tions employ monofunctional polymers that are prepared by
anionic polymerization.
Herein, we describe the monofunctionalization of the
chain end of ruthenium-catalyzed metathesis polymers, which
can be carried out in the presence of residual monomer, yields
narrow molecular weight distributions, and allows the pres-
ence of functional groups in the monomer structure. To prove
our synthetic concept, we chose exo-N-phenylnorbornene-
Scheme 1. Synthesis and cleavage of the block copolymers. a) PPh3,
dichloromethane, room temperature; b) ethyl vinyl ether; c) 6n HCl,
methanol, dichloromethane; d) trifluoroacetic anhydride; e) pyrene-
butyroyl chloride; f) trimethylsilyl chloride.
2
,3-dicarboximide (PNI) as the monomer as it can be
[
13]
polymerized in a living fashion.
PNI was initiated with
catalyst 1 (Scheme 1) and polymerized to only 60% con-
version. Polymerization to low monomer conversion allowed
us to evaluate the influence of residual monomer on the
polydispersity of the monofunctional polymer. An analytical
sample was taken and quenched with ethyl vinyl ether. Next, a
large excess of dioxepine monomer 4a or 4b was introduced
into the reaction mixture, thereby resulting in the formation
of a diblock copolymer, which was quenched with excess ethyl
vinyl ether.
Figure 1 shows gel permeation chromatography (GPC)
traces of the first block 3a and diblock 6a, which clearly show
that the living chain end of 3 was an effective initiator for the
second monomer 4a. As the first monomer had only been
consumed to about 60%, the second block was most likely a
statistical mixture of monomers 2 and 4a. Polymerization of 2
to higher conversions (60–90%) before addition of the second
monomer was also successful. Molecular weight control of the
first polymer block is therefore possible by either varying the
reaction time before adding the second monomer or by
varying the monomer/catalyst ratio. After isolation and
purification, the block copolymer was dissolved in a mixture
of methanol/dichloromethane/HCl to cleave the acetal groups
Figure 1. GPC traces (THF, calibration with polystyrene) of the first
polymer block of monomer 2 (polymer 3a, dotted line), the diblock
copolymer of monomers 2 and 4a (polymer 6a, dashed line), and
monofunctional polymer 7 (solid line).
GPC analysis revealed that the molecular weight of the
first block (3a, Figure 1) and monofunctional polymer (7,
Figure 1) are almost identical. Moreover, the polydispersity
indices for polymers 3a and 7 (Figure 1) are identical and very
narrow (PDI = 1.1). The presence of residual monomer 2,
which can lead to severe molecular weight broadening in
many of the previously reported end-capping procedures, was
[
14]
of the second block. As can be seen in Scheme 1, cleavage
of the acetal groups decomposes the second polymer block (to
form 7) but leaves half a monomer unit of 4a attached to the
first block at a CÀC double bond.
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Angewandte
Chemie
1
shown not to have any effect on the polydispersity of the
monofunctional polymer.
had occurred. In the H NMR spectrum of the trifluoroace-
tate-functionalized polymer 8 (Figure 2 top), the terminal
olefinic protons (H , d = 6.04 ppm) are separated from other
olefinic peaks in the spectrum and can also be used for end-
group analysis. Reaction of polymer 7 with trimethylsilyl
chloride gave the trimethylsilyl-functionalized polymer 10.
The H NMR spectrum revealed the methyl end groups at d =
0.17 ppm (see the Supporting Information).
Polymers 3a and 7 were analyzed by MALDI-TOF mass
spectrometry (7 from 6a, see Figure 3; 7 from 6b, see the
Supporting Information). The mass distribution of polymer 7
3
The same experiment was carried out with the dioxepine
monomer 4b. The GPC trace of diblock copolymer 6b was
also shifted towards higher molecular weights in comparison
with that of 3a, thus indicating good reinitiation from the first
to the second block (see the Supporting Information). The
monofunctional polymer 7 that was prepared by acidic
cleavage of the polyacetal block of 4b shows a virtually
identical molecular weight and polydispersity index (PDI =
1
1
.1) as the first block. It is important to stress at this point that
the propagation rate of the dioxepine monomer is of no
importance for successful end functionalization of the first
polymer block. As the second polymer block is eventually
“
sacrificed”, only the reinitiation of the first polymer block to
the first unit of the dioxepine monomer is crucial. This
incorporation of the first unit of the dioxepine monomer
represents a breaking point, the junction between the end-
functionalized and sacrificed polymer block.
To obtain proof for the presence of a hydroxy function-
ality at the polymer chain end, polymer 7 (from 6a) was
treated with pyrenebutyroyl chloride to give the correspond-
ing ester 9. A GPC experiment with UV detection at l =
3
40 nm (characteristic for pyrenebutyric acid derivatives)
showed a signal for the pyrene-functionalized polymer 9 while
hydroxyl-functionalized polymer 7 shows no absorption at
this wavelength (see the Supporting Information). This
finding shows that the pyrene group was covalently attached
1
to the polymer chain. The H NMR spectrum of polymer 7
reveals a singlet at d = 4.14 ppm, which was attributed to the
methylene group adjacent to the hydroxy functionality
Figure 3. MALDI-TOF mass spectra of 3a and 7 (from 6a) showing
isotopically resolved mass peaks; inset: complete mass distribution.
(
Figure 2 bottom). Addition of trifluoroacetic anhydride to
the NMR tube resulted in a shift of the peak to d = 4.82 ppm,
which is in agreement with the described assignment (Figure 2
is shifted by m/z 30.09 compared to that of polymer 3 (the
1
2
À1
top). Furthermore, the olefinic protons H and H of the
exact mass of CH O is 30.01 gmol ). This difference
2
1
styrene-like initiator group can be observed in the H NMR
corresponds to the mass difference between the ethyl vinyl
ether end-capped polymer 3a and the hydroxy-functionalized
polymer 7. The mass distribution for polymer 7 provides no
evidence for a second monomer distribution from residual
dioxepine monomer. However, a very small mass distribution
owing to unfunctionalized polymer 3 (which is structurally
identical to 3a) can be seen in the mass spectrum of 7
spectrum of 7 at d = 6.33 and 6.60 ppm. Comparing the
1
2
integrals of the olefinic protons H and H with the integral
4
from H reveals that end functionalization in excess of 97%
(Figure 3). The functionalization of 7 with trimethylsilyl
chloride to give the silyl-protected alcohol was also successful
and was confirmed by MALDI-TOF mass spectrometry (see
the Supporting Information).
In conclusion, we have presented a route to monofunc-
tionalized olefin metathesis polymers. This route allows the
preparation of narrowly distributed polymers with commer-
cially available ruthenium catalysts. Monomeric exo-N-
phenyl-norbornene-2,3-dicarboximide was polymerized to
the desired molecular weight and subsequently turned into
a diblock copolymer by adding a cyclic olefinic acetal as a
second monomer. The second polymer block was decom-
posed under acidic conditions, thereby leaving exactly one
hydroxyl group at the end of the initial polymer block. We
believe that this route to monofunctional ring-opening meta-
thesis polymers presents a viable and less laborious alter-
native to carbanionic polymerization. Such polymers with
1
Figure 2. H NMR spectroscopic end-group analysis of polymers 8
(
top) and 7 (bottom). The structures show the two end groups of the
polymer chain.
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Communications
monofunctionalized end groups and narrow polydispersity
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2
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À1
À1
À1
General procedure for cleaving of the second block: HCl (6m,
mL) and methanol (2 mL) were added to a solution of the block
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4
copolymer (1 g) in dichloromethane (10 mL). The mixture was stirred
for 12 h at room temperature and subsequently precipitated into
methanol. The solid was recovered, dissolved in chloroform, and
reprecipitated into methanol. The polymer was dried under vacuum
to give a white solid (850 mg, ca. 80%, depending on the block ratio).
Detailed experimental procedures are described in the Support-
ing Information.
Received: June 9, 2006
Revised: August 29, 2006
Published online: October 31, 2006
Keywords: block copolymers · dioxepine · metathesis · polymer
.
chemistry · ring-opening polymerization
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