Controlling Polymer Composition in Organocatalyzed Photoredox Radical Ring-Opening Polymerization of Vinylcyclopropanes

Article abstract of DOI:10.1021/jacs.9b07230

Although radical polymerizations are among the most prevalent methodologies for the synthesis of polymers with diverse compositions and properties, the intrinsic reactivity and selectivity of radical addition challenge the ability to impart control over the polymerization propagation and produce polymers with defined microstructure. Vinylcyclopropanes (VCPs) can be polymerized through radical ring-opening polymerization to produce polymers possessing linear (l) or cyclic (c) repeat units, providing the opportunity to control polymer structure and modify the polymer properties. Herein, we report the first organocatalyzed photoredox radical ring-opening polymerization of a variety of functionalized VCP monomers, where high monomer conversions and spatial and temporal control were achieved to produce poly(VCPs) with predictable molecular weight and low dispersity. Through manipulating polymerization concentration and temperature, tunable l or c content was realized, allowing further investigation of thermal and viscoelastic materials properties associated with these two distinct compositions. Unexpectedly, the photoredox catalysis enables a postpolymerization modification that converts l content into the c content. Combined experimental and computational studies suggested an intramolecular radical cyclization pathway, where cyclopentane and cyclohexane repeat units are likely formed.

Full text of DOI:10.1021/jacs.9b07230

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Article
Controlling Polymer Composition in Organocatalyzed Photoredox
Radical Ring-Opening Polymerization of Vinylcyclopropanes
Dian-Feng Chen, Bret Boyle, Blaine McCarthy, Chern-Hooi Lim, and Garret M. Miyake
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 29 Jul 2019
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Controlling Polymer Composition in Organocatalyzed Photoredox Radical Ring-
Opening Polymerization of Vinylcyclopropanes
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Dian-Feng Chen, Bret M. Boyle, Blaine G. McCarthy, Chern-Hooi Lim, and Garret M.
Miyake*
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Department of Chemistry, Colorado State University, Fort Collins, CO 80523, United
States
ABSTRACT
Although radical polymerizations are among the most prevalent methodologies for the
synthesis of polymers with diverse compositions and properties, the intrinsic reactivity and
selectivity of radical addition challenges the ability to impart control over the
polymerization propagation and produce polymers with defined microstructure.
Vinylcyclopropanes (VCPs) can be polymerized through radical ring-opening
polymerization to produce polymers possessing linear (l) or cyclic (c) repeat units
providing opportunity to control polymer structure and modify the polymer properties.
Herein, we report the first organocatalyzed photoredox radical ring-opening
polymerization of a variety of functionalized VCP monomers, where high monomer
conversions and spatial and temporal control were achieved to produce poly(VCPs) with
predictable molecular weight and low dispersity. Through manipulating polymerization
concentration and temperature, tunable l or c content was realized, allowing further
investigation of thermal and viscoelastic materials properties associated with these two
distinct compositions. Unexpectedly, the photoredox catalysis enables a post-
polymerization modification that converts l content into the c content. Combined
experimental and computational studies suggested an intramolecular radical cyclization
pathway, where cyclopentane and cyclohexane repeat units are likely formed.
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INTRODUCTION
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The merger of photoredox catalysis1 and controlled polymerizations2 have drawn
increasing attention as to provide spatial and temporal control over the chain growth and
produce polymers with predictable composition, molecular weight (MW), and low
dispersity (Ð) under mild conditions.3 A variety of methodologies which implement light
to affect diverse reaction mechanisms have been discovered, including photoinduced atom
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transfer radical polymerization (ATRP),4 photoinduced electron/energy transfer reversible
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addition-fragmentation chain transfer polymerization,
photocontrolled cationic
polymerization,6 metal-free ring-opening metathesis polymerization,7 and others.8 Our
interest in photoredox-controlled polymerization originated with the motivation to develop
an organocatalyzed ATRP (O-ATRP). Our group and others have employed highly
reducing organic photoredox catalysts (PCs), such as perylene,3e phenothiazines,3f N,N-
diaryl dihydrophenazines3i and N-aryl phenoxazines,3j,9 for O-ATRP. To further advance
this nascent polymerization methodology we are motivated to discover new reactivity of
these organic photosystems that are otherwise inaccessible.
Vinylcyclopropanes (VCPs) are an intriguing class of monomers in that they can be
polymerized through various pathways to yield drastically different polymeric structures,
exhibiting low volume shrinkage or even volume expansion—a unique property of
modeling and restorative materials. 10 In a radical pathway, the radical ring-opening
polymerization (rROP) proceeds to yield a polymer possessing a combination of
unsaturated linear (l) and saturated cyclic (c) repeat units.11 These c repeat units have been
proposed to be cyclobutanes generated through a 4-endo-trig radical cyclization (Figure
1A).12 To evaluate l/c selectivity, a linear factor SL is defined here as the total percentage
of l repeat units. Most rROP approaches have utilized traditional free radical
polymerization techniques, which achieve low selectivity between l or c content and
12
produce polymers with ill-defined MW characteristics (Figure 1B).
To address this
limitation, Cu-ATRP was attempted for the rROP (ATrROP) of 1,1-diethyoxycarbonyl-2-
vinylcyclopropane (EtVCP) to produce well-defined polymers possessing a high l content
(SL = 98%).13 However, presumable bidentate coordination from EtVCP or poly(EtVCP)
to the Cu(I) catalyst inhibits polymerization to reach high monomer conversions (<50%)14
even at elevated temperatures (Figure 1C). Leveraging our recent advances in O-ATRP,
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we envisaged that the use of an organic PC could eliminate this unfavorable coordination,15
enabling high monomer conversion and l/c selectivity while affording spatial and temporal
control over the chain growth.
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Figure 1. Profile of radical ring-opening polymerizations of 1,1-disubstituted vinylcyclopropanes.
Herein, we demonstrate a photoredox-controlled approach utilizing N,N-diaryl
dihydrophenazine PCs to address the challenges of polymerization efficiency and l/c
selectivity (Figure 1D). Given their strong excited-state reduction potentials these organic
PCs are able to activate the dormant alkyl bromide polymer chain (Figure 1E, A) to
generate a propagating radical B. After monomer additions, recapping of the homoallylic
radical intermediate D generated from the ring-opening of C by the [2PC•+][Br−] complex
results in the dormant polymer E. Previously proposed backbiting cyclization of D may
occur to form c repeat units. To mitigate side-reactivity, low loading of the PC may
minimize radical concentrations in solution and minimize bimolecular radical termination
pathways. As such, in our system a series of well-defined poly(VCPs) were obtained with
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controlled MWs, low Ð, and predominately l repeat units (SL up to 98%). Moreover,
through manipulation of polymerization conditions including temperature and
concentration, tunable l/c selectivity was achieved (SL = 1−98%), thus enabling
investigation of the polymer properties associated with such distinct chemical
compositions. Moreover, a photoredox polymer-chain modification converting l
composition into c composition suggests that the cyclic repeat units in poly(VCPs) are
likely a mixture of cyclopentanes, cyclohexanes, and previously proposed cyclobutanes.
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RESULTS AND DISCUSSION
Synthesis of poly(EtVCP) with high l content. Initial polymerization studies employed
EtVCP as the monomer, diethyl 2-bromo-2-methylmalonate (DBMM) as the initiator, and
N,N-dimethylacetamide (DMAc) as the solvent to investigate the catalyst performance on
the polymerizations using standard O-ATRP conditions of ([EtVCP]:[DBMM]:[PC] =
[1000]:[10]:[1] with irradiation by white LEDs and a cooling fan to maintain
polymerization temperature at ~28 °C). Three PCs, including N,N-diaryl
dihydrophenazines 1−2, and 3,7-di(4-biphenyl)-N-naphthylphenoxazine 3, were found to
able to drive the polymerization and produced poly(EtVCP) with high l content (SL = 97%,
Table 1, entries 1−3). Polymers synthesized using PCs 2 and 3 displayed superior control
over polymer molecular weight, with a number average molecular weight (Mn) close to the
theoretical values, as is indicated by initiator efficiencies (I* = Mn,theo/Mn,measured × 100%) close
to 100% (entries 2−3). Conversely, polymerization with PC 1 afforded lower Ð = 1.21
(entry 1). In a comparison between common ATRP alkyl bromide initiators (Table S1),
DBMM proved to be superior, with polymerization reaching the highest monomer
conversion and producing polymers with low Ð and high I*. A screen of solvents (entries
4−6) revealed that EtOAc allowed for the best control over the polymerization, achieving
98% monomer conversion in 8 hours, producing poly(EtVCP) with Ð = 1.15, high SL of
97%, and predictable MW as evidenced by an I*of 99% (entry 4). No monomer conversion
was observed in the absence of light, PC, or initiator, providing evidence of a photoredox-
controlled ATRP mechanism (Table S2).
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Table 1. Optimization of the synthesis of poly(EtVCP) with high l content.a
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Conv.
(%)b
72
Mn
Ð
I*
(%)d
69
SL
(%)b
97
Entry
PC
Solvent
[M]/[I]/[PC]
(kDa)c (Mw/Mn)c
1
2
1
2
3
1
1
1
1
1
1
1
1
1
1
1
DMAc
DMAc
DMAc
EtOAc
CH2Cl2
PhCl
1000/10/1
1000/10/1
1000/10/1
1000/10/1
1000/10/1
1000/10/1
1000/10/2
1000/10/0.5
1000/10/0.1
1000/20/1
1000/5/1
22.6
16.8
15.5
21.1
29.0
22.3
20.4
19.8
20.1
11.8
41.7
11.6
39.7
79.5
1.21
1.63
1.53
1.15
1.20
1.17
1.24
1.17
1.29
1.12
1.21
1.10
1.25
1.43
70
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98
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99
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98
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99
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98
97
95
90
107
99
97
97
97
96
94
97
97
92
97
97
97
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90
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97
7
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
98
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106
99
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91
101
92
500/10/1
2000/10/1
5000/10/1
104
127
aPolymerizations performed using 1.0 mmol of EtVCP, DBMM as the initiator, 1.0 mL of solvent, and
irradiated by white LEDs at 28 °C for 8 h. bMeasured by crude 1H-NMR. SL = l/(l + c). cMeasured by
GPC. dInitiator efficiency (I*) = Mn,theo/Mn,measured × 100%, where Mn,theo = MW(initiator) + MW(EtVCP)
× conversion × ([EtVCP]/[initiator]).
Using PC 1, catalyst loadings could be decreased to 100 ppm while still maintaining
polymerization control [Mn = 20.1 kDa, Ð = 1.29, and I* = 99%, (entry 9)]. Further,
modulation of the monomer or initiator stoichiometry allowed for synthesis of poly(EtVCP)
with targeted MWs (from 11.8 to 79.5 kDa) and low to moderate dispersities (Ð = 1.10–
1.43) (entries 10–14). First-order kinetics were observed for monomer conversion in the
PC 1 catalyzed ATrROP of EtVCP at 28 °C (Figure 2A). Polymerization progress analysis
revealed a linear increase in polymer MW as a function of monomer conversion, while low
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dispersity (Ɖ = 1.17–1.22) remained during the entire course of polymerization (Figure
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2B).
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Figure 2. Synthesis of poly(EtVCP) with high l content: (A) Plots of the natural log of monomer
conversion as a function of time. (B) Plots of experimentally measured Mn and dispersity as a function
of monomer conversion. (C) Chain-extension polymerization from a poly(EtVCP) macroinitiator with
PhVCP and GPC traces before and after polymerization. (D) Plots of monomer conversion and
experimentally measured Mn as a function of time for a pulsed irradiation experiment.
To evaluate the control imparted by this photoredox organocatalyzed ATrROP (O-
ATrROP), a series of experiments probing chain-end group fidelity were performed to
analyze the reversible activation-deactivation equilibrium. First, matrix-assisted laser
desorption ionization time of flight mass spectrum (MALDI-TOF MS) was performed
(Figure S11). In accordance with the proposed reversible activation-deactivation
mechanism, the polymer possessed predominately Br chain-end groups as well as minor H
chain-end groups, likely due to termination events during the polymerization or loss of the
Br chain-end group during the MALDI-TOF MS analysis. The presence of the bromide
chain-end groups allows for application of the synthesized polymers to serve as
macroinitiators in subsequent polymerizations to access polymers with more complex
composition. Thus, poly(EtVCP) (Mn = 11.6 kDa, Ð = 1.10, SL = 97%) was synthesized,
isolated, and employed as a macroinitiator for rROP of a different monomer 1,1-
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diphenyloxycarbonyl-2-vinylcyclopropane (PhVCP) to synthesize a poly(EtVCP)-b-
poly(PhVCP) copolymer. The synthesis of this block copolymer was supported by a
combination of NMR characterization (Figure S13) and a shift in the retention time of the
gel-permeation chromatography (GPC) polymer trace due to an increase in polymer MW
(Mn = 27.8 kDa, Ð = 1.23) (Figure 2C). Moreover, the block copolymer synthesis achieved
a high I* of 97%, highlighting the Br chain-end group fidelity of the macroinitiator. We
also demonstrated temporal control, a key feature of photoredox-catalyzed processes, using
a pulsed-irradiation experiment, where the polymerization proceeded only under light
irradiation, paused during dark periods (as long as 12 h), and could be resumed with
continued irradiation, further supporting the light-driven, reversible activation-deactivation
mechanism of O-ATrROP (Figure 2D).
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Synthesis of poly(EtVCP) with high c content. With the synthesis of poly(EtVCP) with
high l content successfully established, we desired to also selectively target high c content.
In free radical polymerization of vinylcyclopropane monomers it has been shown that using
solvents (compared to bulk polymerization) or increasing the polymerization temperature
promotes intramolecular backbiting cyclization, leading to decreased l composition.16
Accordingly, we first investigated the concentration effect on this O-ATrROP of EtVCP at
room temperature. Indeed, polymerizations at lower concentration produced polymers with
more c content as indicated by decreased SL (Table 2, entries 1–6). It is noteworthy that
polymerizations at low concentrations (0.098–0.192 mol/L) still retained excellent control
over the chain-growth as indicated by low Ð (1.12–1.15) and high I* (80–107%) (entries
4–6). The SL dramatically decreased from 67% to 38% when the polymerization using a
concentration of 0.192 mol/L was performed at 60 °C (entry 4 vs 7). Using high power blue
LED (Figure S27b) instead of the white LEDs continued to promote the formation of c
content (SL = 18%, entry 8). This result illuminates the crucial role of light intensity in
photocontrolled polymerizations.17 PC 2 was proven to be superior to 1, producing
poly(EtVCP) with I* of 99%, Ð of 1.09 and SL of 16% (entry 8 vs 9). Lowering the PC
loading resulted in slightly higher Ð and I* (entries 10–12). Control experiments that
omitted either light, PC 2, or DBMM from the system led to no monomer conversion (Table
S6). Moreover, good control of MW as achieved by manipulating the stoichiometry of
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monomer to initiator, producing poly(EtVCP) with low Ð of 1.03–1.16 and SL of 13–16%
(entries 13–15).
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Table 2. Optimization and synthesis of poly(EtVCP) with high c content.a
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Ð
Conc.
[mol/L]
Conv.
(%)b
Mn
I*
(%)d (%)b
SL
T
(°C)
Entry PC
[M]/[I]/[PC]
(Mw/Mn)
(kDa)c
c
1
2
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
1.429
0.833
0.455
0.192
0.122
0.098
0.192
0.192
0.192
0.192
0.192
0.192
0.192
0.192
0.192
28
28
28
28
28
28
60
60
60
60
60
60
60
60
60
1000/10/1
1000/10/1
1000/10/1
1000/10/1
1000/10/1
1000/10/1
1000/10/1
1000/10/1
1000/10/1
1000/10/0.5
1000/10/0.2
1000/10/0.1
1000/5/1
98
98
98
95
90
88
95
98
97
95
91
83
98
97
91
21.8
21.1
26.1
25.3
17.5
18.6
25.3
23.8
21.1
19.7
19.1
16.4
41.0
82.8
105.4
1.21
1.15
1.14
1.12
1.13
1.15
1.07
1.15
1.09
1.09
1.15
1.21
1.16
1.03
1.03
97
99
97
97
80
67
60
55
38
18
16
19
15
17
14
13
16
3
81
4
80
5
107
102
81
6
7
8e
86
9 e
10e
11e
12e
13e
14e
15e
99
104
102
109
102
124
183
1000/2/1
1000/1/1
aPolymerizations performed using 1.0 mmol of EtVCP, DBMM as the initiator, in EtOAc and irradiated
b
1
c
d
with white LED for 12h. Measured by crude H-NMR. SL = l/(l + c). Measured by GPC. Initiator
efficiency (I*) = Mn(theo)/Mn(measured), where Mn (theo) = MW(initiator) + MW(EtVCP) × conversion ×
([EtVCP]/[initiator]). e34 W blue LED was used.
Despite using diluted conditions, first-order kinetics (Figure 3A), a linear increase in
polymer MW, and low to moderate Ɖ with respect to monomer conversions (Figure 3B)
were observed although MALDI-TOF MS analysis of a poly(EtVCP) sample (Figure S31)
showed a relatively higher contribution of H chain-end groups. Temporal control was also
achieved using a pulsed irradiation experiment at 60 °C with high power blue LED (Figure
3C). The polymer MWs before and after a dark period were similar to each other, while Ð
slightly increased after a complete “on-off” cycle (Figure D). Chain-extension experiment
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using isolated poly(EtVCP) with high c content (22.9 kDa, Ð = 1.16, SL = 15%) as the
macroinitiator with EtVCP at 28 °C was then performed (Figure E). Although good control
was obtained (Ð = 1.17, I* = 112%), only 46% monomer conversion was observed after
12 h, which we attributed to less reactive variants of Br chain-end group presented in the
macroinitiator. At elevated temperature intramolecular cyclization is greatly promoted
during polymerization, leading to increased formation of alkyl radicals (e.g., F in Figure
1E), and subsequent deactivation to form new alkyl bromide chain-end groups (e.g., G in
Figure 1E), which are reasonably more difficult than bromomalonate to reduce by *PC
(Figure S36–S37 and Table S9).
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Figure 3. Synthesis of poly(EtVCP) with high c content: (A) Plots of the natural log of monomer
conversion as a function of time. (B) Plots of experimentally measured Mn and Ð as a function of
monomer conversion. Plots of conversion versus time (C) and plots of Mn and Ð as a function of
monomer conversion (D) for pulsed irradiation experiment. Filled markers were data directly after
irradiations, while open makers were data after the dark periods. (E) Chain-extension polymerization
from a poly(EtVCP) macroinitiator with EtVCP and GPC traces before and after polymerization.
Post-polymerization modification of high l polymer-chain. Radical addition, such as
thiol-ene “click” reaction,18 has been used to functionalize polymers containing internal
C-C double bonds.19 The reactive alkene groups periodically spaced along the backbone
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of poly(EtVCP) with high l content piqued our curiosity about the stability of this polymer-
chain under photoredox conditions and the possibility to cross-link the resulting polymers.
The investigation began with an isolated poly(EtVCP) (21.3 kDa, Ð = 1.19, SL = 95%), PC
2 (0.1 mol%), and white LEDs in EtOAc at 28 °C (Table 3, entries 1–2). Interestingly,
reactions at 0.833 and 0.192 mol/L yielded soluble polymeric products with slightly
decreased SL values (90% and 86%, respectively) after 12 h, suggesting that transformation
of the alkene groups in the polymer-chain indeed occurred, albeit not through significant
cross-linking. Temperature also impacted this transformation such that SL decreased from
95% to 80% after 12 h at 60 °C (entry 3). Surprisingly, replacing the light source with high
power blue LED drastically promoted the conversion of l content into c content (SL = 34%,
entry 4). The increase in polymer MW (Mn = 35.2 kDa) when using the blue LED is likely
due to radical-radical coupling terminations.
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Table 3. Post-polymerization modification of poly(EtVCP) with high l contenta
Conc.
[mol/L]
Mn
Ð
SL
(%)c
T
(°C)
Entry
(kDa)b (Mw/Mn)b
1
2
0.833
0.192
0.192
0.192
28
28
60
60
23.8
24.3
27.6
35.2
1.22
1.23
1.15
1.17
90
86
80
34
3
4e
aReactions performed using 1.0 mmol of poly(EtVCP) (21.3 kDa, Ð = 1.09, SL = 95%), in EtOAc and
b
irradiated with white LED for 12h. The polymer was recovered quantitatively. Measured by GPC.
cMeasured by crude 1H-NMR. e34 W blue LED was used.
The resulted poly(EtVCP) from polymer-chain modification (Table 3, entry 4)
exhibited excellent solubility in common organic solvents (e.g., EtOAc, ether, toluene, and
dicholomethane). As such, we believe it is the intramolecular radical cyclization, other than
polymer crosslinking, that contributes to the decrease of SL. Due to difficult structural
analysis based on NMR spectra,12 we designed a model reaction to mimic the radical
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cyclization and probe the mechanism. Three-step allylation of diethyl malonate afforded a
compound 4 as a 6:1 E/Z mixture at C8 position, which was then placed under similar
polymer-chain modification conditions (Figure 4). After 12 h, about 90% of starting
material was recovered, and E/Z isomerization at the C3 position (4′) was observed (E/Z =
5.2:1). We attributed this C-C double bond isomerization to a reversible cyclization/fast
ring-opening process, involving two potential pathways forming either cyclopropane or
cyclobutane. Moreover, the cyclization product(s) 5 was also observed (in 5% isolated
yield), which was likely a mixture of several cyclic bromides formed from distinct tandem
cyclization pathways (two of the potential products are outlined in Figure 4; also see Figure
S48–S51).
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Figure 4. Results of model reaction of 4.
However, for each tandem cyclization pathway, the resulting product consists of up
to 8 diastereomers, which plagued the structural analysis of 5 and identification of
cyclization mechanism. As such, we turned to density functional theory (DFT) calculations
to predict cyclization pathways of a radical intermediate 6, which is generated from two
monomer additions of the initiator DBMM (Figure 5). Although 4-endo-trig and 3-exo-trig
cyclizations of 6 are both endergonic (by 12.9 kcal and 2.7 kcal/mol, respectively), the
latter is more favorable by 10.2 kcal/mol, which matches the classic Baldwin’s rule.20 The
subsequent 5-exo-trig and 6-endo-trig cyclization pathways are both thermodynamically
exergonic by more than 10 kcal/mol (∆G = −11.8 and −14.0 kcal/mol, respectively),
indicating cyclopentane and cyclohexane are likely the favorable repeat units of chain-
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modified poly(EtVCP). Moreover, cycloheptane may be accessible as well in this radical
cyclization (Figure S52).
7
8
9
20
CO2Et
CO2Et
EtO2C
EtO2C
Me
7
10
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12
13
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15
10
5
H
CO2Et
(12.9)
EtO2C
H
4-endo-trig
CO2Et
ΔG0 = 12.9
EtO2C
EtO2C
CO2Et
Me
8
CO2Et
CO2Et
(2.7)
0
3-exo-trig
ΔG0 = 2.7
(0.0)
5-exo-trig
ΔG0 = -14.5
CO2Et
EtO2C
EtO2C
EtO2C
EtO2C
CO2Et
CO2Et
CO2Et
-5
Me
Me
6-endo-trig
ΔG0 = -16.7
6
9
-10
-15
-20
(-11.8)
CO2Et
CO2Et
EtO2C CO2Et
CO2Et
CO2Et
EtO2C
(-14.0)
EtO2C
Me
CO2Et
CO2Et
10
Figure 5. DFT calculations to predict late-stage cyclization pathways.
Mechanism for the formation of c content. In monitoring the polymerization performed
at 60 °C (Figure 3A), we found that the SL decreased over time (Table S7). This implies
that considerable intramolecular cyclization is competing with propagation during
polymerization at high temperature (60 °C). The c content is either from immediate
backbiting (Figure 1E, from intermediate D to F) after the ring-opening process (Figure
1E, from intermediate C to D), or from tandem radical cyclization of an unsaturated linear
polymer-chain of random length. To gain further insights into the chemical structures of c
repeat units, comparison of the proton and carbon NMR spectra of three poly(EtVCP)
samples synthesized from distinct methods, benzoyl peroxide (BPO) initiated bulk radical
polymerization (Mn = 25.1 kDa, Ð = 3.8, SL = 30%), photoredox ATrROP of 0.192 mol/L
with while LEDs at 60 °C (Mn = 21.1 kDa, Ð = 1.23, SL = 40%), and polymer-chain
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modification (Mn = 35.2 kDa, Ð = 1.17, SL = 34%, Table 3, entry 4), was performed. These
spectra are extremely similar (Figure S41–S43), suggesting that the aforementioned radical
polymerizations (or modification) produce similar c composition, which are likely
cyclobutanes, cyclopentanes, and cyclohexanes.
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Characterizations of poly(EtVCP) with variable l or c content. To elucidate the
differences that the degree of l or c content has on the polymer’s behavior, the thermal and
viscoelastic properties were investigated for four polymer samples of comparable MW (21-
23 kDa) with SL values ranging from 95 % to 15 % (Figure 6). Polymer I, with an SL value
of 95 %, has a glass transition temperature (Tg) of 32.6 °C, as shown by differential
scanning calorimetry (DSC) (Figure 6B). As the SL value decreases from 70 % in polymer
II, to 35 % in polymer III, to 15 % in polymer IV, the Tg of the polymer samples also
decrease from 31.2 °C to 30.8 °C to 28.5 °C, respectively. Therefore, as the c content
increases, the side-chain free volume seems to increase. Further, the melting peaks
associated with all four of the polymer samples become less apparent with increasing c
content.
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Figure 6. Comparison of 1H-NMR spectra (A), DSC curves (B), viscosity flow curves (C),
creep tests (D) and 5% step-strain stress relaxation plots (E) of four poly(EtVCP) samples:
(I) 21.3 kDa, Ð = 1.19, SL = 95%; (II) 21.1 kDa, Ð = 1.17, S SL = 70%; (III) 22.3 kDa, Ð
= 1.18, SL = 35%; (IV) 22.9 kDa, Ð = 1.17, SL = 15%.
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To investigate the effect that l content of the polymer has on the viscoelastic properties,
a series of tests were completed on each polymer sample in the melt at isofrictional
conditions (Tg + 30 °C) which included a flow test with a strain rate sweep from 0.001 to
0.3 1/s, a creep test with a constant 100 Pa stress applied over 5 minutes, and a 5% step-
strain stress relaxation experiment over 5 minutes (Figure 6C–6E). In each test performed,
it was shown that the lower the SL value, the more elastic the polymer behavior. For
instance, polymer I has the lowest viscosity value (5.58 x 103 Pa*s at 0.001 1/s), is the most
deformed during the creep tests (8.47 x 102 %), and relaxes the fastest after a 5 % step-
strain (~ 5 seconds until a modulus value of 45 Pa was reached). In stark contrast, polymer
IV has the highest viscosity value (2.51 x106 Pa*s at 0.001 1/s), is the least deformed during
the creep tests (0.929 %), and relaxes the slowest after a 5 % step-strain (300 seconds until
a modulus value of 350 Pa was reached).
Table 4. Monomer Scope for Organocatalyzed rROP of Vinylcyclopropanesa
Conv.
Mn
Ð
I*
SL
Tg
Tm
Td
Entry Monomer Condition
(%)b (kDa)c (Mw/Mn)c (%)d (%)b (°C) (°C) (°C)
1
2
EtVCP
EtVCP
PrVCP
PrVCP
BuVCP
BuVCP
tBuVCP
A
B
A
B
A
B
A
98
97
98
99
98
99
98
21.1
21.1
25.7
29.3
27.2
31.9
29.3
1.15
1.09
1.12
1.11
1.15
1.10
1.19
99
99
93
83
98
85
91
97
16
97
10
93
6
32
30
9
94
-
359
360
357
362
354
364
3
93
-
4
6
5
−20 47
6
7e
−16
-
98
78
169 227
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8e
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tBuVCP
BnVCP
BnVCP
PhVCP
PhVCP
EEVCP
EEVCP
ClEVCP
ClEVCP
LMVCP
LMVCP
B
A
B
A
B
A
B
A
B
A
B
99
97
95
97
98
91
99
93
99
95
99
30.3
32.6
28.1
30.9
46.8
29.0
35.8
26.9
29.3
42.3
52.5
1.18
1.16
1.18
1.24
1.06
1.21
1.03
1.23
1.13
1.21
1.20
90
101
115
98
5
87
30
-
84
-
226
356
356
92
30
95
40
97
30
92
20
75
1
10
11e
12e
13
14
15
16
17
18
29
81
183 369
180 380
65
94
10
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12
13
14
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21
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25
26
27
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41
42
43
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45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
95
−27
−31
47
-
-
356
351
85
98
179 342
349
139 313
312
97
41
-
98
74
83
68
-
aCondition A: [EtVCP]: [DBMM]: [1] = 1000 :10 : 1, in 1.0 mL of EtOAc (0.833 mol/L), and with
homemade white LED beaker at 28 °C for 8h. Condition B: [EtVCP]: [DBMM] : [1] = 1000 :10 : 1, in
5.0 mL of EtOAc (0.192 mol/L), and with 34 W blue LED at 60 °C for 12h. bMeasured by crude 1H-
NMR. SL = l/(l + c). cMeasured by GPC. dInitiator efficiency (I*) = Mn(theo)/Mn(measured), where Mn (theo)
MW(initiator) + MW(EtVCP) × conversion × ([EtVCP]/[initiator]). ePhCl was used as the solvent.
=
Monomer scope. Subsequent exploration of the monomer scope revealed a variety of
vinylcyclopropanes with diverse substituents being amenable to this organocatalyzed
photoredox protocol (Table 4). Generally, polymerizations of 0.833 mol/L at 28 °C
achieved high conversions (91–98%), near 100% I*, low Ð (1.12–1.24), and predominantly
l contents (SL = 92–98%). Low concentration (0.192 mol/L) polymerizations at 60 °C with
high power blue LED were also high yielding (95–99% conversions), producing
poly(vinylcyclopropanes) with low Ð (1.06–1.20), moderate to excellent I* (65–115%),
and cyclic repeat units as the major composition (SL = 5–40%). The glass transition
temperature (Tg) of the resulting poly(vinylcyclopropanes) ranged from -31 to 94 °C, where
additional methylene groups to the side chain might increase the free volume of the
polymer, thereby significantly decreasing Tg. All of the obtained polymers exhibited
decomposition temperatures (Td) over 340 °C, except poly(tBuVCP), which started to
decompose at 227 °C (entries 7–8). Interestingly, the chemical compositions (l or c content)
do not significantly impact the Tg and Td, while most polymers exhibited melting points
(Tm) ranging from 47 to 183 °C at high l content, and became amorphous at high c content.
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Regardless of polymer compositions, no Tm was observed for poly(EEVCP) (entries 13–
5
6
14).
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CONCLUSION
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In conclusion, we have established a general photoredox strategy for the radical ring-
opening polymerizations of a series of functionalized vinylcyclopropane monomers,
producing poly(vinylcyclopropanes) with predictable MW and low Ð. The use of N,N-
diaryl dihydrophenazines as the photocatalyst enables high monomer conversions (> 90%)
under mild conditions. By manipulating polymerization concentration and temperature,
unprecedented regulation on the linear and cyclic compositions of obtained polymers was
achieved, which allowed investigations of composition-associated thermal and viscoelastic
properties. Significantly, we have discovered a novel polymer-chain modification that
converts l composition into c composition. The combination of a model reaction and DFT
computations disclose several tandem radical cyclization pathways to produce
cyclopentane and cyclohexane repeat units in the modified polymer. This post-
polymerization modification also suggests a novel mechanism, besides previously
proposed backbiting cyclization, to form the c composition during polymerization.
Combined, these findings provide valuable insights to guide the future design and
fabrication of poly(vinylcyclopropane)-based materials.
ASSOCIATED CONTENT
Supporting Information.
The Supporting Information is available free of charge on the ACS Publications website.
AUTHOR INFORMATION
Corresponding Author
*garret.miyake@colostate.edu
ORCID
Dian-Feng Chen: 0000-0003-1821-9783
Chern-Hooi Lim: 0000-0003-1823-6305
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Garret M. Miyake: 0000-0003-2451-7090
5
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Notes
7
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The authors declare no competing financial interest.
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ACKNOWLEDGMENT
This work was supported by Colorado State University, the Sloan Research Foundation,
and the National Institute of General Medical Sciences of the National Institutes of Health
under Award Number R35GM119702. The content is solely the responsibility of the
authors and does not necessarily represent the official views of the National Institutes of
Health. We acknowledge the use of computational resources provided by the XSEDE -
Comet supercomputer (NSF ACI-1053575). C.-H.L. is grateful for an NIH F32
Postdoctoral Fellowship (F32GM122392). B.G.M. is grateful for support from an NSF
GRFP. D.-F. C. thanks Bonnie L. Buss for technical assistance and helpful discussions.
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Controlling Polymer Composition in Organocatalyzed Photoredox Radical Ring-
Opening Polymerization of Vinylcyclopropanes
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Dian-Feng Chen, Bret M. Boyle, Blaine G. McCarthy, Chern-Hooi, Lim, and Garret M.
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Miyake*
Department of Chemistry, Colorado State University, Fort Collins, CO 80523, United
States
RO2C CO2R
PC
Visible
PC
Light
and/or
Visible
Light
n
n = 1, 2, 3
Mild Conditions; > 90% Conversions; Controlled MW; Ð = 1.03-1.24; Tunable Composition
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