Hybrid Photo-induced Copolymerization of Ring-Strained and Vinyl Monomers Utilizing Metal-Free Ring-Opening Metathesis Polymerization Conditions

Article abstract of DOI:10.1021/jacs.9b09025

We introduce the hybrid copolymerization of two disparate monomer classes (vinyl monomers and ring-strained cyclic olefins) via living photopolymerization. The living character of the polymerization technique (metal-free photo-ROMP) is demonstrated by consecutive chain-extensions. Further, we propose a mechanism for the copolymerization and analyze the copolymer structure in detail by high-resolution mass spectrometry.

Full text of DOI:10.1021/jacs.9b09025

Communication
Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
pubs.acs.org/JACS
Hybrid Photo-induced Copolymerization of Ring-Strained and Vinyl
Monomers Utilizing Metal-Free Ring-Opening Metathesis
Polymerization Conditions
†
,‡
†,‡
†,§
†
⊥
Tim Krappitz, Kristina Jovic, Florian Feist, Hendrik Frisch, Vincent P. Rigoglioso,
James P. Blinco,† Andrew J. Boydston,*
,⊥
and Christopher Barner-Kowollik*
,†,||
†School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology (QUT), 2 George Street,
Brisbane, QLD 4000, Australia
§
Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
|
|
Macromolecular Architectures, Institut fu
Engesserstraße 18, 76131 Karlsruhe, Germany
̈
r Technische Chemie und Polymerchemie, Karlsruhe Institute of Technology (KIT),
⊥
Department of Chemistry, University of WisconsinMadison, 1101 University Avenue, Madison, Wisconsin 53706, United States
S Supporting Information
*
merizations with different ring-strained monomers, the
majority constituted of norbornene derivatives and dicyclo-
pentadiene. Furthermore, copolymerization using a bifunc-
ABSTRACT: We introduce the hybrid copolymerization
of two disparate monomer classes (vinyl monomers and
ring-strained cyclic olefins) via living photopolymeriza-
tion. The living character of the polymerization technique
20
tional initiator was shown, affording block copolymers.
Our approach merges the rich variety of vinyl monomers in
a hybrid copolymerization with ring-strained monomers
utilizing MF-ROMP conditions and paves the way for new
copolymer structures not accessible by other means. The
unsaturated backbone and side chains of these copolymers are
synthetically readily accessible for functionalization.
(
metal-free photo-ROMP) is demonstrated by consec-
utive chain-extensions. Further, we propose a mechanism
for the copolymerization and analyze the copolymer
structure in detail by high-resolution mass spectrometry.
Photoinduced homo- and copolymerization of norbornene
with vinyl monomers under MF-ROMP conditions were
performed utilizing ethyl-1-propenyl ether as well as (2-
methoxyvinyl)cyclohexane as initiators and 2,4,6-tris(4-
opolymerization within the same class of monomers,
following the same polymerization mechanism, can be
C
conducted for almost every polymerization technique. Hybrid
copolymerization and interconversion between different
polymerization mechanisms is rarely reported and mostly
confined to block copolymer formation. Zhang et al. recently
established an interconverting polymerization system using
heat and light as triggers for the interconversion of anionic
ring-opening polymerization (AROP) and photoinduced
electron transfer reversible addition−fragmentation chain
transfer polymerization (PET-RAFT), affording multiblock
+
−
methoxyphenyl)pyrylium tetrafluoroborate (MeOTPP BF )
4
as the oxidizing photocatalyst.
The two structurally distinct monomer classes usually follow
different polymerization mechanisms, involving radical cations
or radicals as the reactive intermediates. Whereas in hybrid
copolymerization one reactive species is generated that can add
both ring-strained andvinyl monomers to the growing chain
(Scheme 1).
1
copolymers. Despite the few reports on hybrid copolymeriza-
A selection of the obtained polymers and the corresponding
characteristics are summarized in Table 1. Methyl methacrylate
and methyl acrylate were utilized as the main representative
vinyl monomers and norbornene as the ring strained
monomer. Styrenes as well as vinyl ethers are readily oxidized
and cannot be polymerized using the same reaction
tions, some innovative approaches, such as tandem catalysis,
2
−6
7−9
10
11
afford block,
brush,
graft comb or bridge-like
polymer structures. Buchmeiser and co-workers demonstrated
a sophisticated tandem copolymerization of cyclic olefins with
ethylene via a reversible interconversion of the catalyst to
catalyze either ring-opening metathesis polymerization
21
conditions. No quenching agents were required as the
polymerization ceases in the dark due to reduction of the active
(
ROMP) or vinyl-insertion polymerization (VIP) to construct
19
1
2−16
chain end and regeneration of the catalyst. Prior to the herein
presented in-depth analysis, mass spectrometric studies have
previously not been conducted on polymers obtained via MF-
ROMP. For the norbornene homopolymer (entries 1 and 2,
Table 1), solely consisting of the highly apolar norbornene
repeating unit and the respective end groups originating from
the lateral chain.
The interconversion of two addition
polymerizations, e.g. cationic- and radical polymerization of
vinyl monomers was recently reported by Fors and co-
1
7,18
workers.
Metal-free ROMP (MF-ROMP) was demonstrated by
Boydston and co-workers and follows a step-growth mecha-
nism where the ω-chain end can be reversibly activated or
deactivated by oxidizing the double bond or reducing the
Received: August 20, 2019
Published: October 8, 2019
1
9
cationic radical. Those authors presented random copoly-
©
XXXX American Chemical Society
A
DOI: 10.1021/jacs.9b09025
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Scheme 1. Hybrid Copolymerization of Ring-Opening
Cyclic Olefins with Vinyl Monomers via MF-ROMP
Conditions
1
the utilized initiator, proton nuclear magnetic resonance ( H
NMR) spectroscopy clearly shows the anticipated α- and ω-
chain ends as highlighted by Boydston and co-workers (Figure
Figure 1. 1H NMR spectrum of poly(norbornene-co-MMA),
synthesized under MF-ROMP conditions with a 50/50 mixture of
the two monomers in dichloromethane and (2-methoxyvinyl)-
cyclohexane as initiator.
1
9
S5).
A corresponding ionized species was not identified in the
recorded electrospray ionization (ESI) mass spectra, most
likely due to low ionization efficiencies, however, the correct
repeating unit spacing of 94.08 Da was observed (refer to
Figure S16). A possible explanation for the species in the mass
spectrum could be the [2+2]-cycloaddition of the charged ω-
chain end with a neutral initiator molecule, as is often observed
CH resonances of the five-membered ring at 2.78 and 2.43
ppm.
The polymer sample contained 90% norbornene and 10%
methyl methacrylate using (2-methoxyvinyl)cyclohexane as
initiator. It is important to note that the same ω-chain end
identified for a norbornene homopolymer was also identified
22
in radical cation chemistry. However, the corresponding
species would inhibit the polymerization and would be in
1
via H NMR (trans, 6.28 and 4.71 ppm; cis, 5.81 and 4.30
1
conflict with the H NMR spectrum, therefore, the presence of
ppm). Critically, size exclusion chromatography (SEC) showed
−
1
the observed end group species may result from prefer-
ential ionization. After successful homopolymerization using
MF-ROMP conditions, the possibility of orthogonal reac-
tivities between MF-ROMP and radical polymerizations of
vinyl monomers was investigated. Thus, a polymerization of
norbornene via MF-ROMP in the presence of methyl
methacrylate (MMA) was performed. The presence of MMA
did not inhibit the ROMP, nor was it orthogonal to MF-
a monomodal shape with M = 3700 g·mol and M = 6300 g·
n w
1
−
mol , underpinning the copolymerization between norbor-
nene and MMA. Experiments performed under the same
conditions, yet without norbornene, did not result in the
formation of poly(methyl methacrylate), indicating that
norbornene has to generate cationic radicals to incorporate
MMA units (entry 6, Table 1). The same observation applies
in the absence of initiator (entry 7, Table 1).
1
ROMP, as can be seen in the H NMR spectrum in Figure 1.
While improved conversion in MF-ROMP using non-
deoxygenated reaction mixtures was observed for the
The proton resonances of the methoxy group at 3.60 ppm, and
for the methyl group at 0.84 ppm, are suitable signals for the
identification of MMA units in the copolymer. The
norbornene repeating unit can be readily identified via the
resonances at 5.34 and 5.20 ppm, associated with the
backbone double bonds formed during ROMP, as well as the
23
homopolymerization of norbornene, the copolymerization
with vinyl monomers requires inert reaction conditions. The
incorporation of MMA in a copolymerization with norbornene
completely ceased in the presence of oxygen (entry 3, Table
1). Any attempted copolymerization of norbornene with vinyl
a
Table 1. Different (Co)polymers Obtained Utilizing MF-ROMP Conditions
d
Mtotal:I:PC [M] (mol·L−1) conversion M1 (%) conversion M2 (%) time (min) M (g·mol−1) M (g·mol−1)
entry M1 (%)
M2
D̵
0
n
w
b
b
b
,
c
1
2
3
4
5
6
7
8
9
1
100
100
50
50
50
0
/
/
108:1:0.05
102:1:0.05
100:1:0.05
100:1:0.03
100:1:0.03
100:1:0.03
101:1:0.03
112:1:0.03
97:1:0.03
2.3
2.0
2.6
2.3
2.0
2.3
2.3
2.2
1.9
1.9
44
35
36
14
7
/
/
n.d.
13
24
/
/
0
3
5
/
/
n.d.
5
100
90
90
60
60
60
60
60
60
60
17600
9900
/
3600
1700
/
25900
15500
/
6000
4700
/
1.5
1.6
/
1.7
2.7
/
MMA
MMA
MA
MMA
MMA
TFEMA
BA
50
45
51
51
/
/
/
2100
3400
2200
4400
2200
5600
2.1
2.2
2.6
0
IBA
97:1:0.03
3
a
All reactions were performed in dry dichloromethane at 445 nm LED @ 10 W (0.7 A, 8.85 V input) for the stated period of time. (2-
1
b
c
methoxyvinyl)cyclohexane was utilized as initiator. The conversions were determined using H NMR in CDCl . Non-deoxygenated. Ethyl-1-
propenyl ether. Abbreviations: methyl methacrylate (MMA); methyl acrylate (MA); 2,2,2-trifluoroethyl methacrylate (TFEMA); n-butyl acrylate
3
d
(
BA); isobutyl acrylate (IBA).
B
DOI: 10.1021/jacs.9b09025
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
monomers (Table 1 and Figure S15) exhibited a higher
conversion of norbornene compared to the vinyl monomer
conversion, likely as a result of the high ring-strain of
norbornene and the known fast polymerization kinetics in
MMA and norbornene. The copolymer exhibits a far better
ionization due to the rather polar MMA repeating units.
The singly charged region (z = 1) was used for further
analysis, whereas higher charged regions (e.g., z = 2) also
exhibit the characteristic copolymer pattern. The mass
spectrum contains repetitive signal patterns characteristic for
copolymers, such as isotopic patterns with an m/z spacing
corresponding to both monomers (norbornene, Δ = 94.08 Da;
MMA, Δ = 100.05 Da) and the spacing of Δ = 5.97 Da
between neighboring isotopic patterns, corresponding to the
difference in mass between norbornene and MMA repeating
units (Figure 2 inset). Comparison of the high-resolution mass
spectra of poly(norbornene-co-MMA) obtained using either
ethyl-1-propenyl ether or (2-methoxyvinyl)cyclohexane as
initiators provided insight into the end group functionalities
of the copolymers (Figures S17 and S18).
2
4
ROMP.
Unambiguous evidence for the hybrid copolymerization
between the two monomer classes was provided by SEC/ESI-
MS analysis of the polymer samples (Figure 2).
Matching sum formulas for the copolymer mass spectra were
found if the α-chain end remains intact, while the ω-chain end
is converted to an aldehyde under cleavage of the alkoxy group
during ionization. The degradation yielding an aldehyde can
also be observed for a norbornene homopolymer and the
initiator under MF-ROMP conditions in acid containing
solvents such as CDCl (Figures S6 and S7).
3
A proposed mechanism for the copolymerization of
norbornene and MMA is shown in Scheme 2. Similar to the
mechanism proposed for the previously reported procedure for
MF-ROMP, the copolymerization commences with the
Figure 2. High-resolution mass spectrum of poly(norbornene-co-
MMA) with spacings of the isotopic patterns corresponding to the
exact masses of the repeating unit and the associated mass-to-charge
difference between the repeat units. The mass spectra were analyzed
19
initiation of norbornene. A subsequent metathesis of MMA
with the active chain end, and release of an α-vinyl ether,
results in an intermediate that can initiate the radical
polymerization of MMA. The released α-vinyl ether does not
26
using pyMacroMS.
19
initiate new chains as previously observed.
This hyphenated technique enables separation of the
polymer by chain length (SEC) and consecutive online
recording of the mass spectrum by ESI-MS, facilitating the
separation of the polymer fractions by different charge states
aiding an unambiguous identification of the macromolecular
The nucleophilic addition of the prior released α-vinyl ether
to the radical-cation of the active chain end will lead to the
regeneration of the vinyl ether end group allowing for further
norbornene addition. This assumption is in agreement with a
recent publication of Huang and co-workers reporting the
25
species according to their sum formula. Contrary to the
apolar norbornene homopolymer, species with multiple
charges were observed indicating a copolymer consisting of
27
nucleophilic addition of a vinyl ether to a radical cation.
Importantly, the kinetics for a 50/50 mixture of MMA and
Scheme 2. Proposed Mechanism for the Hybrid Copolymerization of Norbornene (M1) and MMA (M2)
C
DOI: 10.1021/jacs.9b09025
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Figure 4. Chain extension of poly(norbornene-co-MMA) with
norbornene (black and red lines) or a mixture of norbornene and
Figure 3. Conversions of norbornene and MMA at different times
(
A) and relative progress of the conversions (B). SEC traces at the
corresponding times (C) showing a gradual increase in molecular
2,2,2-trifluoroethyl methacrylate (blue and purple dashed lines).
weight M (D), while maintaining a monomodal distribution.
p
content remains the same as prior to the chain extension
(
Figure S11).
To demonstrate the living character and the end group
norbornene reveal an increase of the conversion over time for
both monomers, with norbornene being the predominantly
incorporated monomer, especially during the early stages of
the polymerization (Figure 3A,B).
fidelity of the developed technique, poly(norbornene-co-
MMA) was chain extended with a mixture of 2,2,2-
trifluoroethyl methacrylate and norbornene to obtain a
terpolymer consisting of two blocks of different gradient
copolymersa microstructure unprecedented in literature
The conversion after 3 h reaches 15% of norbornene and 5%
of MMA in the case of the experiment shown in Figure 3. The
conversions and molecular weights obtained with the general
procedure for hybrid copolymerization utilizing MF-ROMP
conditions can be increased by sequential addition of the
pyrylium salt (Table S1). The initially added pyrylium salt
does not fully recover and thus the polymerization stops. Our
kinetic data support a gradient copolymerization rather than a
block copolymerization as MMA is incorporated into the
polymer from the beginning and the entire molecular weight
distributionincluding the oligomer regionshifts toward
higher molecular weights (Figure 3C,D). Furthermore, in the
case of a polymer blend, a bimodal distribution would be
expected at a certain stage of the polymerization, which was
(
Figure 4, dashed lines, and Figure S12). A shift of the full
−
1
−1
mass distribution from M = 7500 g·mol to 55 700 g·mol
p
was observed.
In conclusion, we introduce the hybrid copolymerization of
ring-strained monomers with vinyl monomers utilizing MF-
ROMP conditions. The final copolymers contain a high
proportion of the norbornene repeating unit. Monomers
suitable for MF-ROMP but exhibiting less ring-strain
compared to norbornene may enable a shift of the equilibrium
further to the side of the vinyl monomer. The resulting
copolymers are promising candidates for future applications, as
the backbone, as well as the side chains, can be readily altered.
For example, introducing a cleavable group into the lateral
chain via the ring-strained monomer will open a promising
avenue for materials with novel features, especially in the field
13
not observed. Nonetheless, analysis via C NMR spectroscopy
points toward a high degree of blockiness, as no distinct
resonances corresponding to the diad of norbornene adjacent
to MMA were identified (Figures S8−S10). Caused by the
relatively low percentage of incorporated vinyl monomer, the
fraction of interconverting points in the polymer was too low
28
of biodegradable polymers.
ASSOCIATED CONTENT
■
* Supporting Information
1
3
S
to be detected via C NMR.
In order to gain further information on the copolymerization
mechanism, we attempted a chain extension of a copolymer
consisting of MMA and norbornene. After purification of the
copolymer the chain growth was successfully re-initiated by
addition of a fresh norbornene feed and catalyst (Figure 4),
proving the living character of the hybrid copolymerization.
Critically, the complete molar mass distribution is shifted
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.9b09025.
Experimental details; analytical data including NMR,
SEC, MS, and UV/vis (PDF)
AUTHOR INFORMATION
Corresponding Authors
*christopher.barnerkowollik@qut.edu.au
aboydston@wisc.edu
■
−
1
toward higher masses, starting from M = 4800 g·mol and
p
−
1
shifting to M = 58 600 g·mol after chain extension. The
possibility of a chain extension further supports the hybrid
copolymerization proposed in Scheme 2. The H NMR spectra
p
*
1
ORCID
James P. Blinco: 0000-0003-0092-2040
Andrew J. Boydston: 0000-0001-7192-4903
Christopher Barner-Kowollik: 0000-0002-6745-0570
before and after chain extension also support hybrid
copolymerization, showing an increase in the content of
norbornene of the chain extended polymer, while the MMA
D
DOI: 10.1021/jacs.9b09025
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Author Contributions
Communication
(11) Song, W.; Han, H.; Wu, J.; Xie, M. A Bridge-like Polymer
Synthesized by Tandem Metathesis Cyclopolymerization and Acyclic
Diene Metathesis Polymerization. Polym. Chem. 2015, 6 (7), 1118−
‡
T.K. and K.J. contributed equally.
Notes
1
(
126.
The authors declare no competing financial interest.
12) Zou, Y.; Wang, D.; Wurst, K.; Kuhnel, C.; Reinhardt, I.;
Decker, U.; Gurram, V.; Camadanli, S.; Buchmeiser, M. R. Group 4
Dimethylsilylenebisamido Complexes Bearing the 6-[2-
Diethylboryl)phenyl]pyrid-2-yl Motif: Synthesis and Use in Tandem
Ring-Opening Metathesis/Vinyl-Insertion Copolymerization of Cyclic
Olefins with Ethylene. Chem. - Eur. J. 2011, 17 (49), 13832−13846.
13) Buchmeiser, M. R.; Camadanli, S.; Wang, D.; Zou, Y.; Decker,
U.; Kuhnel, C.; Reinhardt, I. A Catalyst for the Simultaneous Ring-
̈
Opening Metathesis Polymerization/Vinyl Insertion Polymerization.
̈
ACKNOWLEDGMENTS
■
(
C.B.-K. is grateful for an Australian Research Council (ARC)
Laureate Fellowship enabling his photochemical research
program as well as continued key support from the Queensland
University of Technology (QUT). T.K. gratefully acknowl-
edges funding by the Leopoldina Fellowship Programme,
German National Academy of Sciences Leopoldina (LPDS
(
Angew. Chem., Int. Ed. 2011, 50 (15), 3566−3571.
(
14) Wang, M.; Wang, D.; Widmann, L.; Frey, W.; Buchmeiser, M.
2
017 05). F.F. gratefully acknowledges the ARC for a Ph.D.
R. Tandem Vinyl Insertion-/Ring-Opening Metathesis Copolymeriza-
tion with ansa-6-[2-(Dimesitylboryl)-phenyl]pyrid-2-ylamido Zirco-
nium Complexes: Role of Trialkylaluminum and MAO. Polym. Chem.
Fellowship. C.B.-K. and J.B. gratefully acknowledge support for
this project by Evonik Industries and thank Dr. Thomas
Richter (Evonik) for helpful discussions and comments. This
work was further enabled by use of the Central Analytical
Research Facility (CARF) hosted by the Institute for Future
Environments (IFE) at QUT. The authors thank Dr. Aaron
Micallef for help with the NMR measurements.
2
(
016, 7 (10), 1987−1998.
15) Buchmeiser, M. R. Tandem Ring-Opening Metathesis − Vinyl
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R. Group 4 Metal Complexes Bearing the Aminoborane Motif: Origin
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