Hydrogen Bond Donor Catalyzed Cationic Polymerization of Vinyl Ethers

Article abstract of DOI:10.1002/anie.202013419

The synthesis of high-molecular-weight poly(vinyl ethers) under mild conditions is a significant challenge, since cationic polymerization reactions are highly sensitive to chain-transfer and termination events. We identified a novel and highly effective h

Full text of DOI:10.1002/anie.202013419

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Accepted Article
Title: Hydrogen Bond Donor-Catalyzed Cationic Polymerization of Vinyl
Ethers
Authors: Veronika Kottisch, Janis Jermaks, Joe-Yee Mak, Ryan
Woltornist, Tristan Lambert, and Brett P. Fors
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202013419
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10.1002/anie.202013419
Angewandte Chemie International Edition
COMMUNICATION
Hydrogen Bond Donor-Catalyzed Cationic Polymerization of Vinyl
Ethers
Veronika Kottisch, Janis Jermaks, Joe-Yee Mak, Ryan A. Woltornist, Tristan H. Lambert*, and Brett P.
Fors*
Department of Chemistry and Chemical Biology
Baker Lab, Cornell University
Ithaca, NY 14853
E-mail: brettfors@cornell.edu
E-mail: tristan.lambert@cornell.edu
Supporting information for this article is given via a link at the end of the document.
Abstract: The synthesis of high molecular weight poly(vinyl ethers)
under mild conditions is a significant challenge since cationic
polymerizations are highly sensitivity to chain transfer and
termination events. We identified a novel and highly effective
hydrogen bond donor (HBD)-organic acid pair that can facilitate
controlled cationic polymerization of vinyl ethers under ambient
conditions with excellent monomer compatibility. Poly(vinyl ethers) of
molar masses exceeding 50 kg/mol can be produced within one hour
without elaborate reagent purification. Modification of the HBD
structure allowed tuning of the polymerization rate while density
functional theory (DFT) calculations helped elucidate crucial
intermolecular interactions between the HBD, organic acid, and
polymer chain end.
We attribute the loss of control to higher amounts of elimination
at the chain end and subsequent chain transfer, likely mediated
by the PCCP anion (Figure 1a).
a) Intramolecular deprotonation gives chain transfer
OR
Pn
O
R
OMe
OH
MeO2C
MeO2C
MeO2C
Initiates new chains
H
OR OR
Pn
H
H
O
+
O
MeO
CO2Me
O
–
MeO
O
OMe
Terminated
polymer chain
O
OMe
OMe
b) This work: Attenuated basicity through hydrogen bonding
OR
Pn
O
R
H
H
H
O
• faster polymerization
• Mn > 50 kg/mol
• neat conditions
Living polymerizations are a powerful tool that enable
[1]
O
MeO
syntheses of materials with well-defined structural complexity.
O
OR
• tolerant to moisture
However, developing polymerization methods with living
characteristics is challenging and often requires specialized rea-
gents and reaction conditions. Specifically, living cationic
processes often require rigorously dried solvents, cold
temperatures, and additives to avoid deleterious termination and
–
MeO
OMe
H
O
O
B
OMe
OMe
Figure 1. a) Proposed mechanism of chain transfer. b) Hydrogen bond donors
(HBDs) can attenuate basicity and facilitate higher molecular weight polymers.
chain transfer events.[2–16] Therefore, designing
a robust
controlled cationic polymerization under mild conditions that can
produce high molecular weight polymers remains a challenge.
In an effort to address these challenges, we recently
developed a novel single-component cationic polymerization of
vinyl ethers that can be conducted under ambient conditions.[17]
This approach employed pentacarbomethoxycyclopentadiene
(PCCP-H) as a bench-stable, strong organic acid,[18–20] which
rapidly protonated vinyl ethers to initiate cationic polymerization,
thereby enabling the synthesis of poly(vinyl ethers) with precise
control over the number average molar masses (Mn) and narrow
dispersities (Đ). We proposed that the polymer chain end exists
in equilibrium between a tight ion pair and a covalent adduct
rendering the polymerization stable to small amounts of
nucleophilic impurities, thus avoiding the need for monomer
purification or air free techniques.[17,20,21]
We postulated that the proximity of a PCCP-ester to the
C–H of the penultimate carbon of the chain end enabled this
intramolecular deprotonation. Upon reformation of PCCP-H, a
new polymer chain can be initiated through protonation of an
additional monomer, resulting in lower than predicted Mn values
and broader Đs through this chain transfer process.[22] To
address this issue, we proposed that attenuation of the PCCP
anion basicity would inhibit deprotonation and allow access to
larger molecular weight polymers while maintaining the
insensitivity of these polymerizations to moisture and
nucleophilic impurities. This goal could be achieved through
either modulation of the PCCP structure[23,24] or through additives
that interact with the PCCP anion at the chain end. We chose to
pursue the second approach because of the modularity of this
strategy.
Despite the advantages of this method, targeting polymers
with a degree of polymerization (DP) greater than 100 led to
materials with lower than expected Mn values and broadened Đs.
1
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10.1002/anie.202013419
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COMMUNICATION
a) Polymerization Results
b) Hydrogen bond donors (HBDs) 1–6
OMe
MeO2C
HBDs 1–6
S
1
O
O
O
O
OH
Me
R
Ar
S
Ar
+
MeO2C
Ar
Ar
n
N
H
N
H
N
H
N
H
OiBu
CO2Me
neat, N2
OiBu OiBu
Ar
Ar
N
H
N
H
MeO2C
CF3
2
3
Ar =
PCCP-H
S
P
S
P
S
CF3
Ph
Ar
Ar
Ar
Ar
Ar
P
Temp
Mn,exp
Mn,theo
N
H
N
H
N
H
N
H
Ph
N
H
O
Entrya
HBD M:PCCP-H:HBD
Time
Đ
O
HN
4
O
5
(°C)
(kg/mol) (kg/mol)
Ph
Ar
6
1
-
300:1:0
300:1:0.5
300:1:0.5
300:1:1
300:1:1
300:1:1
300:1:1
300:1:1
500:1:1
1000:1:2
100:1:0.2
100:0:1
rt
rt
rt
rt
rt
0
rt
rt
0
0
rt
rt
23 h
23 h
23 h
1 h
14.5
11.5
13.5
24.5
24.9
25.8
27.9
27.0
29.1
26.4
23.7
27.3
38.5
57.0
9.9
1.38
1.46
1.56
1.14
2.20
1.16
1.34
1.39
1.18
1.14
1.06
n/a
2b
3b
4
1
2
3
4
4
5
6
4
4
4
4
c) Selected GPC traces of poly(IBVE)
1.0
0.8
0.6
0.4
0.2
0.0
5b
6
1 min 8.8
Entry 1, no HBD
Entry 4, HBD 3
Entry 6, HBD 4
Entry 7, HBD 5
Entry 8, HBD 6
1 h
29.8
7
3.5 h
23 h
1 h
17.3
14.5
39.8
65.7
8
9
10
11
12c
1 h
5 min 10.3
24 h n/a
n/a
aVinyl ether (100–1000 equiv) was added to PCCP-H (0.007 mmol, 1 equiv) and
HBDs 1–6 (0.0014–0.014 mmol, 0.16–0.33 mol%) under inert atmosphere at the
indicated temperature. The reaction was quenched with MeOH/NEt3. Unless
otherwise noted, Mn and Đ were determined by multi angle light scattering. bMn and
Đ were determined against polystyrene standards. The MWD of entry 5 was
bimodal. cwithout PCCP-H.
10
11
12
13 14
Retention Time (min)
15
16
Figure 2. a) Optimization of IBVE polymerization in the presence of HBDs. b) Structures of HBDs. c) Selected GPC traces of catalyzed polymerizations.
Specifically, we hypothesized that hydrogen bond donors
(HBDs) could diminish the basicity of the PCCP anion and
thereby inhibit chain transfer via interaction with one or more of
the PCCP carboxyl groups (Figure 1b). HBDs have found
extensive use as catalysts in small molecule synthesis over the
last decades due to their structural diversity, high efficiency, and
tunability.[25–30] However, utilizing HBDs in the cationic
polymerization of vinyl ethers has not yet been explored. In our
polymerization, two challenges would have to be addressed: 1)
the ideal HBD would bind well to the PCCP anion and moderate
its effective basicity, while 2) remaining close to the polymer
chain end to prevent termination by nucleophiles. With these
features in mind, we investigated the effect of an array of
structurally diverse HBDs 1–6 on the cationic polymerization of
isobutyl vinyl ether (IBVE) with PCCP-H (Figure 2).
We started our investigation by targeting a DP of 300 in
the presence of HBDs 1–6 (0.15-0.33 mol% relative to
monomer). The polymerization was terminated at high
conversion and analyzed by gel permeation chromatography
(GPC). The corresponding uncatalyzed reaction produced a
polymer with a Đ of 1.38 and an experimental Mn that was 10
kg/mol lower than the theoretical value (Figure 2a and 2c, entry
1). Upon the addition of Schreiner’s thiourea catalyst 1 or
squaramide 2[31,32] no improvement in molecular weight or Đ was
observed compared to the reaction without HBD (Figure 2a,
entries 2–3). Based on these results, we decided to look into
Indeed, upon the addition of 3, the polymerization reached high
conversion within only one hour to yield a polymer with a Mn of
24.5 kg/mol and narrow Đ of 1.14 (entry 4). An even more
dramatic rate increase was observed in the presence of 4,
leading to an exothermic and uncontrolled polymerization (entry
5). Upon cooling the reaction to 0 °C, we were able to
synthesize poly(IBVE) with a Mn of 29.8 kg/mol and Đ of 1.16.
Importantly, the experimental Mn was in excellent agreement
with the targeted value suggesting that little to no chain transfer
was occurring.
We attribute the success of 4 to its ability to donate three
H-bonds, which can bind to multiple methyl esters of the PCCP
anion. To probe this hypothesis, HBDs
5 and 6 were
synthesized, which can donate two and one hydrogen bonds,
respectively (entries 7 and 8).[35] Since aniline was simply
replaced by phenol in these structures, these HBDs should
closely resemble the geometric and electronic properties of 4.
Employing 5 and 6 under the optimized conditions resulted in
little to no polymer. When the polymerization was performed at
room temperature, poly(IBVE)s with Mn values of 17.3 and 14.5
kg/mol were obtained, respectively (entries
7
and 8).
Interestingly, the GPC traces of the uncatalyzed polymerization
and the reaction with 6 are almost identical (Figure 2c),
highlighting the need for at least two H-bonds on the thiophos-
phoramide skeleton to be
a competent catalyst. Having
established 4 as the optimal catalyst, we decided to target
different molar masses. Poly(IBVE) with molar masses up to 66
kg/mol and narrow Đ values were successfully synthesized
within one hour (entries 9 and 10).[36] These results highlight the
other classes of HBDs, specifically sulfamide
thiophosphoramide 4. HBD adopts twisted geometry
compared to 1 and 2, while 4 can donate a third H-bond.[27,33,34]
3
and
3
a
2
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10.1002/anie.202013419
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COMMUNICATION
dramatic improvement to our previous method. Additionally, the
polymerization can proceed at room temperature with lower
catalyst loading (entry 11). Lastly, no polymer was observed in
the absence of PCCP-H when employing 4 as the catalyst (entry
12). The molecular weight is solely controlled by the ratio of
PCCP-H to monomer.
Having demonstrated the effectiveness of 4 on the
polymerization of IBVE, the mechanism of action of the HBDs
was investigated more closely. We started by monitoring IBVE-
polymerization with a target Mn of 10 kg/mol in the presence of
0.1–0.2 mol% of 3–6 (Figure 3). The natural log plot of
conversion over time depicts
a
50-fold in-crease in
With optimal conditions in hand, we decided to explore the
robustness and monomer scope of the HBD-catalyzed
polymerization. In our previous study, PCCP-mediated
polymerizations had been successful under ambient conditions
without prior vinyl ether purification. Therefore, we performed the
polymerization (DP 300) under ambient atmosphere at 0 °C with
monomer that had only been filtered through a plug of basic
alumina. After one hour, the polymerization had reached high
conversion and the resulting polymer had comparable Mn and Đ
polymerization rate when 0.1 mol% of 4 was added to the
reaction. Employing HBD 5 (0.2 mol%) afforded a polymerization
10 times faster than the uncatalyzed polymerization, whereas
HBD 6 did not lead to an observable change in polymerization
rate. Interestingly, the same amount of sulfamide 3 led to 30-fold
rate increase, with only two donatable N–H bonds. These
findings again suggest that at least two hydrogen bonds are
necessary to effectively bind to the PCCP anion. Additionally,
the conformation of the HBD, HBD-strength, and presumably
as the samples produced under inert and dry conditions (Table 1, additional intermolecular interactions at the chain end all play a
entry 1 and Figure 2a, entry 6).[37] Moreover, a variety of vinyl
ethers could be polymerized under ambient atmosphere and
could achieve higher molecular weights (Table 1, entries 2–3).
n-Butyl vinyl ether (NBVE), ethyl vinyl ether (EVE), and
cyclohexyl vinyl ether (CyVE) were all successfully polymerized
with targeted DPs of 300. However, we did observe a slight loss
in control for CyVE. Lower polymerization temperatures and the
addition of a solvent were required for effective polymerization.
Presumably, the increase in steric bulk disrupts the HBD-PCCP
complex at the chain end, leading to more chain transfer.
crucial role (vide infra). This point is demonstrated by the
efficacy of 3 compared to 5. Of note, all polymerizations
maintain excellent control over Mn and Đ of the final polymer
(Figure 3, inset and Figures S15–S19).
0.1–0.2 mol% of 3–6
1 equiv PCCP-H
Me
R
n
rt, neat, N2
OiBu OiBu
OiBu
100 equiv
Table 1. Vinyl ether polymerization catalyzed by HBD 4 under ambient
Mn
Đ
atmosphere.
2.5
2.0
1.5
1.0
0.5
8.2
10.3
6.9
5.2
5.0
1.05
1.14
1.10
1.10
1.04
3
4
5
6
Entrya
Monomer
M:PCCP-H:4
Đ
Mn,exp
(kg/
mol)
Mn,theo
(kg/
mol)
none
1
2
3
4
IBVE
NBVE
EVE
300:1:1
300:1:1
300:1:1
300:1:0.5
27.8
22.8
13.3
31.2
26.4
24.5
17.9
35.2
1.16
1.08
1.15
1.37
0.0
0
20
40
60
80
100
120
140
CyVE
Time (min)
aVinyl ether (2.1 mmol, 300 equiv, filtered through a plug of
basic alumina) was added to PCCP (0.007 mmol, 1 equiv)
and HBD 4 (0.007–0.014 mmol, 0.16–0.33 mol%) under
ambient atmosphere at the indicated temperature. The
reaction was quenched after 1 h unless otherwise noted
with MeOH/NEt3. b2:1 CyVE:toluene (vol) at –10 °C.
Figure 3. Effect of HBD-catalyzed cationic polymerization of IBVE on
polymerization rate.
To further evaluate the role of the HBD in the
polymerization, we performed a 1H NMR titration of 4 with
PCCP–/n-Bu4N+ in CD2Cl2. Incrementally adding PCCP–/n-Bu4N+
(guest) to a solution of 4 (host) showed a downfield shift in the
N–H resonances of 4. Fitting the change in chemical shift to a
1:1 host-guest complex binding model revealed essentially
quantitative binding of the PCCP anion to 4 (Keq = 3512 M–1).[38]
In contrast, a much weaker binding constant was determined for
the complexation of PCCP anion with 5 (Keq = 47.3 M–1, see SI
for details), corroborating its diminished efficacy as an additive in
the polymerization.
To test the chain end fidelity while achieving higher
molecular weights, the polymerizations of IBVE were quenched
with excess benzyl alcohol and 5-hexyn-1-ol. Chain-end analysis
by 1H NMR of the purified polymer showed close agreement
NMR
GPC
between the Mn
and Mn
(Scheme 1 and Figure S20–23),
highlighting that good chain-end fidelity is maintained at high
conversion and high molar masses.
Scheme 1. HBD-Catalyzed Polymerization Maintains Good Chain-End
Fidelity (Mn is reported in kg/mol).
Lastly, we utilized DFT-calculations to model the
interaction between 4, PCCP anion and the oxocarbenium ion
chain end. Interestingly, the geometry-optimized transition
structure revealed several crucial interactions that facilitate the
polymerization. As depicted in Figure 4, 4 sits slightly off-center
above the PCCP anion and forms two hydrogen bonds with
neighboring methyl-ester carbonyls (Figure 4 top, dashed lines).
Me
O
Ph
HO
Ph
n
OiBu OiBu
NEt3, DCM, rt
MnNMR = 29.4
MnGPC = 30.1
PCCP-H
4
Me
n
OiBu
neat
0 °C, N2
OiBu OiBu OiBu
HO
4
Me
O
3
n
4
OiBu OiBu
NEt3, DCM, rt
MnNMR = 31.8
MnGPC = 32.9
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COMMUNICATION
The third N–H bond coordinates to the π-system of the PCCP
anion. The growing polymer chain is located in a pocket
between HBD and PCCP anion. The growing polymer chain
protrudes from the binding pocket while monomer is shuttled in,
forming a key C–H to O interaction between the vinyl α-C–H and
an additional PCCP-carbonyl.16,19 This model also rationalizes
the robustness of the polymerization to external nucleophiles in
that the cleft formed between the HBD and the PCCP anion
could protect the chain end from exogenous nucleophiles while
Acknowledgements
This work was supported by the NSF under Award CHE-
1752140 (B.P.F.) and NIH (R35GM127135) (T.H.L). We thank
Dr. Erin E. Stache and Dr. Elizabeth A. McLoughlin for their help
with the preparation of this manuscript. B.P.F. thanks 3M for a
Non-Tenured Faculty Award and the Alfred P. Sloan Foundation
for a Sloan Research Fellowship.
a)
Keywords: cationic polymerization • hydrogen bond donor
Me
catalysis • ambient conditions
Me
+
O
S
Me
[1]
[2]
[3]
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[5]
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In conclusion, we have demonstrated the unique ability of
thiophosphoramide 4 to catalyze the PCCP-mediated cationic
polymerization of vinyl ethers. This system allows for rapid
synthesis of poly vinyl ethers with Mn’s up to 60 kg/mol. The
number of hydrogen bonds plays a crucial role in this mode of
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4
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COMMUNICATION
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[36] Under the current conditions, polymers with molecular weights above
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[37] The addition of 1 equiv of water leads to slower polymerization, while
maintaining good control. Higher amounts of water (10-40) do not
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5
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COMMUNICATION
Entry for the Table of Contents
Insert graphic for Table of Contents here.
Me
Me
O
S
P
Me
HBD
H
H
Ar
Ar
Ar
H
R’
n
H
N
H
N
N
Me
Me
O
OR
OR OR
poly(vinyl ethers)
H
H
H
MeO
O
O
O
–
• unique catalysis • neat conditions
• tolerant to moisture
• faster polymerization
MeO
O
OMe
OMe
• Mn > 60 kg/mol
O
OMe
We found that a unique combination of hydrogen bond donor (HBD) and acid catalysis enables rapid vinyl ether polymerization under
ambient atmosphere. This controlled polymerization can furnish polymers with molecular weights above 60 kg/mol. Importantly,
several key intermolecular interactions between the organic acid and HBD are crucial for successful polymerization.
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