Urea anions: Simple, fast, and selective catalysts for ring-opening polymerizations

Article abstract of DOI:10.1021/jacs.6b11864

Aliphatic polyesters and polycarbonates are a class of biorenewable, biocompatible, and biodegradable materials. One of the most powerful methods for accessing these materials is the ring-opening polymerization (ROP) of cyclic monomers. Here we report that the deprotonation of ureas generates a class of versatile catalysts that are simultaneously fast and selective for the living ring-opening polymerization of several common monomers, including lactide, δ-valerolactone, ε-caprolactone, a cyclic carbonate, and a cyclic phosphoester. Spanning several orders of magnitude, the reactivities of several diaryl urea anions correlated to the electron-withdrawing substituents on the aryl rings. With the appropriate urea anions, the polymerizations reached high conversions (～90%) at room temperature within seconds (1-12 s), yielding polymers with narrow molecular weight distributions (Crossed D sign = 1.06 to 1.14). These versatile catalysts are simple to prepare, easy to use, and exhibit a range of activities that can be tuned for the optimal performance of a broad range of monomers. (Chemical Equation Presented).

Full text of DOI:10.1021/jacs.6b11864

Article
pubs.acs.org/JACS
Urea Anions: Simple, Fast, and Selective Catalysts for Ring-Opening
Polymerizations
Binhong Lin and Robert M. Waymouth*
Department of Chemistry, Stanford University, Stanford, California 94305, United States
S
* Supporting Information
ABSTRACT: Aliphatic polyesters and polycarbonates are a
class of biorenewable, biocompatible, and biodegradable
materials. One of the most powerful methods for accessing
these materials is the ring-opening polymerization (ROP) of
cyclic monomers. Here we report that the deprotonation of
ureas generates a class of versatile catalysts that are
simultaneously fast and selective for the living ring-opening
polymerization of several common monomers, including
lactide, δ-valerolactone, ε-caprolactone, a cyclic carbonate,
and a cyclic phosphoester. Spanning several orders of
magnitude, the reactivities of several diaryl urea anions
correlated to the electron-withdrawing substituents on the
aryl rings. With the appropriate urea anions, the polymerizations reached high conversions (∼90%) at room temperature within
seconds (1−12 s), yielding polymers with narrow molecular weight distributions (Đ = 1.06 to 1.14). These versatile catalysts are
simple to prepare, easy to use, and exhibit a range of activities that can be tuned for the optimal performance of a broad range of
monomers.
lysts typically exhibit modest activities. Recently, we reported a
new concept for generating bifunctional catalysts by reacting
alkoxides with thioureas (Scheme 1b);44 these catalysts retained
the high selectivity of the thiourea/amine catalyst systems for
enchainment over other competing reactions (epimerization,
transesterification and backbiting), but are more active. In this
contribution, we report a hyperactive class of selective catalysts
derived from urea anions, which are highly tunable and effective
for the selective polymerization of several common cyclic
monomers (Scheme 1c).
INTRODUCTION
■
The progress in new catalytic methods and polymerization
processes continues to drive innovation in polymer chem-
istry.1−8 The development of simple, user-friendly, and
controlled polymerization processes has spawned a renaissance
in polymer chemistry.9,10 The versatility of these methods,
coupled with the ability to generate well-defined, functional
materials has enabled scientists with a wide array of
backgrounds to exploit the unique properties of well-defined
macromolecules to address an extensive range of scientific and
technological problems not only in chemistry, but also in
medicine, biology, physics, and materials science.9,11,12
RESULTS AND DISCUSSION
■
A variety of enzymatic,13,14 metal-based,12,15 and organic
catalysts5,16−19 are known for ring-opening polymerization
reactions. Considerable efforts in recent years have provided
catalysts systems that are highly active20−29 or highly
selective,17,24,25,30−40 but rarely both. As the ring-opening
polymerization of lactones is a transesterification reaction,
competitive transesterification of the resulting polymer can lead
to broadening of the molecular weight distribution, depending
on the relative rates of propagation (ring-opening) and chain
transfer (transesterification, backbiting).41 Organic catalysts
based on H-bonding motifs,17,42,43 such as thiourea (TU, 1-
[3,5-bis(trifluoromethyl)phenyl]-3-cyclohexylthiourea)/DBU
(DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene) (Scheme
1a),24,31,32 are unusual as polymerization catalysts in that they
exhibit high selectivities for the ring-opening of the lactones
relative to transesterification of the polymer, leading to narrow
molecular weight distributions.16,24,31,44 However, these cata-
Neutral ureas are readily prepared from commercially available
isocyanates and amines with minimal workup (Scheme 2a); a
series of several representative ureas (Scheme 2b) were studied
in detail for the ring-opening polymerization of several
common monomers (Scheme 2c) in THF.
To evaluate the polymerization behavior of urea anions as
catalysts for ring-opening polymerization, urea 1 and KOMe
were combined in THF and added to a THF solution of L-
lactide (LA) at room temperature. Under these conditions, 94%
of the monomer was converted in just 6 s to generate isotactic
poly(L-lactide) (Mn,GPC = 20.6 kDa, molecular weight
distribution Đ = Mw/Mn = 1.06; Table 1, entry 3). The high
tacticity of poly(L-lactide) samples prepared with this approach
1
was confirmed by H homonuclear decoupled NMR spectros-
Received: November 16, 2016
© XXXX American Chemical Society
A
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a
Scheme 1
a
(a) Catalysis with thiourea (TU)/DBU. (b) Catalysis with thiourea anions. (c) Catalysis with urea anions.
a
Scheme 2
a
(a) Synthesis of ureas (excluding 7b). (b) Thiourea (TU) and ureas used for ROP (in parentheses are the relative kobs for the polymerization of CL
with KOMe normalized with respect to urea 1). (c) Monomers polymerized. (d) Typical reaction conditions for ROP.
copy and differential scanning calorimetry (Tm = 171 °C;
Figures S1 and S2 in the Supporting Information). Analysis of a
lower molecular weight sample prepared under analogous
conditions by MALDI-TOF mass spectrometry shows a single
series of ions separated by m/z 144, revealing minimal
transesterification of the polymer backbone (Figure 1). In
contrast, the anionic polymerization of L-lactide with KOMe
alone, under comparable conditions, was almost 200 times
slower than in the presence of KOMe/urea 1, and yielded
poly(LA) with a broad molecular weight distribution (Đ = 2.22;
Table 1, entry 1). Moreover, the polymerization with KOMe
alone45,46 results in epimerization of lactide and a correspond-
ing decrease in tacticity and melting points for the resulting
polymer.44 The combination of urea and KOMe in THF
generates a catalyst/initiator solution that is homogeneous,
leading to simultaneous initiation of polymer chains and fast
kinetics, and is less basic, minimizing lactide epimerization. This
1
hypothesis is supported by H NMR studies, which reveal that
B
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a
Table 1. Ring-Opening Polymerization with KOMe in the Presence and Absence of Ureas/TU
b
c
entry
monomer
L-LA
base/initiator
KOMe
urea/TU
time (s)
conv (%)
kobs (min−1
)
Mn,GPC (kDa)
Đ
1
2
3
1200
90
86
89
94
89
88
58
90
85
91
5
0.14 0.05
1.05 0.05
26.8 1.5
23.0
24.5
20.6
20.0
39.5
2.22
1.07
1.06
1.10
2.23
KOMe
TU
1
KOMe
6
d
4
KH (0.1%)/PyOH
KOMe
2
5
25.0 1.6
5
6
7
δ-VL
ε-CL
360
630
9
0.265 0.109
KOMe
1
6
7
0.0825 1 × 10−4
23.7 0.1
KOMe
15.3
15.1
39.8
1.06
1.09
3.52
e
8
KOMe
1
>110
9
KOMe
90
1.16 0.60
0.00385 7 × 10−5
0.074 0.002
0.183 0.003
0.146 0.001
10.8 0.1
10
11
12
13
14
KOMe
1
4
5
5
7
4
6
720
1840
360
315
12
KOMe
90
67
58
89
86
90
19.0
1.09
KOMe
KH/PyOH
KOMe
17.9
17.7
7.8
1.14
1.14
1.05
f
15
TMC-Bn
iPP
KOMe
5
24.1 1.3
16
KOMe
10
24.9 0.1
a
Unless otherwise specified, [KOMe]:[urea] or TU]0:[monomer]0 = 1:3:100 and [monomer]0 = 1 M in THF at room temperature (reactions
b
c
d
quenched with benzoic acid). Conversion determined by NMR. Polystyrene standard calibrated Mn,GPC
1:10:3:1000, catalyst (urea anion) loading = 0.1 mol %. [KOMe]0:[7]0:[monomer]0 = 1:5:100. [TMC-Bn]0 = 0.5 M.
. [KH]0:[PyOH]0:[2]0:[monomer]0 =
e
f
Figure 1. MALDI-TOF of a poly(L-lactide) sample (88% conv., Mn = 12.6 kDa, Đ = 1.06) prepared with [KOMe]:[1]:[LA] = 1:3:50.
the treatment of urea 5 with KOMe generates a urea anion H-
bonded to MeOH (Figure S3 in Supporting Information).
The KOMe/urea 1 catalyst system, despite being the slowest
in the urea anion series (see relative rates in Scheme 2b), is 25
times more active than the recently reported44 thiourea anions
(KOMe/TU, Table 1, entry 2) for the polymerization of LA in
THF, while still maintaining the high degree of control. Faster
rates were also observed for the neutral ureas47 with the DBU
cocatalyst: both urea 4 and urea 1, when combined with DBU,
exhibited faster rates for the polymerization of δ-valerolactone
in THF than TU (Table S1 in the Supporting Information).
The ring-opening polymerizations with urea anions can be
carried out either by combining KOMe with urea and then
introducing monomer, or alternatively, by first reacting urea
with KH, and subsequently initiating the polymerization from a
primary alcohol (Figure S4 in the Supporting Information). For
example, polymerization of LA with a combination of KH (low
catalyst loading: 0.1 mol % vs LA), PyOH (1-pyrenebutanol),
and urea 2 reached 89% conversion in 5 s (Table 1, entry 4).
The high catalytic activity and selectivity extends to the
polymerization of other cyclic monomers, such as δ-
valerolactone (VL), ε-caprolactone (CL), a cyclic carbonate
(benzyl 5-methyl-2-oxo-1,3-dioxane-5-carboxylate, TMC-Bn)
and a cyclic phosphoester (2-isopropoxy-2-oxo-1,3,2-dioxa-
phospholane, iPP) (Scheme 2c and Table 1). The activity of
urea anions can be tuned to these monomers’ different
reactivities. For example, the polymerization of VL with
KOMe/7 reaches 85% conversion in 1 s to yield poly(VL)
(Đ = 1.09; Table 1, entry 8), and the polymerization of CL
reaches 89% conversion in only 12 s to generate poly(CL) (Đ =
1.14, Table 1, entry 14). The relative activities of the different
urea anions for the polymerization of CL reveal that the most
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Figure 2. For (a)−(c), [KH]0 = 0.01 M, [ROH]0 = 0.02 M, [5]0 = 0.03 M and [CL]0 = 1 M (DP50). (a) Kinetic Plot of ln([CL]0/[CL]t) vs time.
(b) Evolution of Mn vs conversion. (Mn determined GPC vs polystyrene in THF.) (c) Evolution of molecular weight distribution Đ vs conversion of
ε-caprolactone. (d) Block copolymer synthesis (chain extension): GPC traces of poly(VL43) (dashed line, 9.8k Da, Đ = 1.06) and its subsequent
poly(VL43)-b-poly(LA200) block copolymer (solid line, 57k Da, Đ = 1.10).
Figure 3. Rate Law: plots of kobs against (a) [ROH], (b) [KH]0, and (c) 1/[ureatotal]. Reaction conditions were [ROH] = 0.01 M, [KH]0 = 0.01 M,
[5]0 = 0.03 M, [CL]0 = 1 M, unless the reaction component was the variable.
active urea anion, derived from urea 7, is approximately 2,800
times more active than the one derived from urea 1 (Scheme
2b and Table 1, entry 10 and 14). These catalysts are more
active than other organic catalysts16,19,44,47,48 and are faster than
or competitive with even the most active metal catalysts,48−58
while maintaining the high levels of selectivity observed for the
thiourea/amine type catalysts.
To probe the polymerization behavior, we investigated the
ring-opening polymerization of CL with a mixture of KH/5/1-
pyrenebutanol in THF. This combination yields polymerization
rates that can be conveniently monitored, in contrast to the
polymerizations of LA, which are too fast to monitor easily
even with the slowest urea in the series. As shown in Figure 2,
the polymerization of CL with PyOH/KH/5 exhibits the
characteristic of a living polymerization, exhibiting first-order
kinetics in CL (Figure 2a) and a linear increase in molecular
weight (Mn) with conversion (Figure 2b), while the molecular
weight distribution remains low up to high conversion (Figure
2c).
Similar behavior is observed for the polymerizations carried
out with the entire family of ureas (Figure S5 in the Supporting
Information), indicating that the rate of polymerization is
D
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Scheme 3. Proposed Mechanism for Urea Anion Catalyzed Ring-Opening Polymerization of ε-Caprolactone
considerably faster than other competitive reactions, such as the
catalytic transesterification of the polymer backbone.41 Further
evidence for its living behavior is demonstrated by the close
correspondence of the observed number-average molecular
weights (Mn) with the initial monomer/initiator ratio ([M]0/
[I]0) and conversion (Table S2 and Figure S7 in the Supporting
Information), as well as the synthesis of a well-defined
poly(VL)-b-poly(LA) block copolymer by sequential introduc-
tion of VL and LA to the initiator/catalyst generated from
KOMe/urea 1 (Figure 2d and Figure S8 in the Supporting
Information).
The kinetics for the polymerization of CL with a PyOH
initiator, KH and urea 5 reveal that the rate is first order in
monomer concentration [CL] (Figure 2a), first order in alcohol
concentration [ROH] (Figure 3a), first order in base
concentration [KH]0 (Figure 3b), and inverse first order in
the total concentration of ureas species [ureatotal] ([KH]0 ≤
[ureatotal] Figure 3c), consistent with the rate law:
initiator or polymer chain-end by H-bonding. The decrease in
rate observed with excess urea can be attributed to the
reversible formation of dimers (or oligomers)59,60 of the urea
anion with neutral urea, which are proposed to be inactive for
polymerization. The productive ring-opening step is proposed
to be facilitated by the bifunctional activation of the alcohol
through H-bonding to the anionic portion of the urea anion
and of the lactone through H-bonding to the N−H of the urea
anion in intermediate RC. Nucleophilic attack by the H-bond
activated alcohol leads to ring-opening and regeneration of an
alcohol/urea anion adduct.
The facile synthesis of a broad array of ureas enabled us to
carry out investigations on the rates of polymerization as a
function of urea substituents. Displayed in Figure 4 is the linear
[KH]0[ROH]
−d[CL]
dt
= kobs[CL] = kp
[CL]
[ureatotal
]
([KH]0 ≤ [ureatotal])
(1)
As shown in Figure 3a, the rate remains first order in [ROH]
even when [ROH] > [KH]0, suggesting that the alcohol can be
reversibly activated by the urea anion, and that the urea anion
function as a catalyst for ring-opening, even at concentrations
lower than those of the propagating alcohol chain-ends [ROH]
(also see Table 1, entry 4). Similarly, the rate remains first order
in [KH]0 when [KH]0 > [ROH] (Figure 3b), consistent with
the reversible activation of the propagating alcohol by the urea
anion. As shown in Figure 3c, an increase of total urea loading
leads to a decrease in kobs, suggesting that neutral urea inhibits
the catalytic activity of the urea anion. We note, however, that
for some of the more reactive urea anions, the additions of
excess neutral ureas improves the solubilities of the initiator/
urea anion mixtures. Therefore, a ratio of [ureatotal]:[urea
anion] = 3:1 was adopted to ensure good solubilities for the
entire family of ureas studied.
Figure 4. Plot of ln(kp) vs the number of CF3 groups on diaryl ureas.
Left to right: ureas containing zero (urea 6), one (urea 4), two (urea
3), three (urea 2), or four (urea 1) CF3 substituents. All kp shown are
the average from two reaction runs (see Table S3 in Supporting
Information for kobs). The relative reactivities (Kp) of the monomers
(LA:TMC-Bn:iPP:VL:CL) are 6200:390:38:24:1 (calculated using kp
in reactions with urea 1 and urea 2).
free energy relationship between the observed rate constants
(kp, eq 1) for polymerization and the number of CF3
substituents on the diaryl ureas, specifically those containing
zero (urea 6), one (urea 4), two (urea 3), three (urea 2), or
four (urea 1) CF3 substituents. As shown in Figure 4, there is a
clear negative linear correlation observed between ln(kp) and
the number of CF3 substituents on the diaryl ureas, and this
trend is consistent for all the monomers studied. Schreiner has
observed a linear decrease in pKa with the number of CF3
substituents attached to the aromatic rings of diaryl ureas and
On the basis of the kinetic data, the mechanism in Scheme 3
for the ring-opening polymerization of CL is proposed.
Reaction of a neutral urea with potassium alkoxide or KH
generates a urea anion that can reversibly activate the alcohol
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thioureas in DMSO.61,62 If this trend were to hold true for the
ureas in THF, this would indicate a correlation between the
rate of polymerization and the pKa of the neutral ureas; that is,
less acidic ureas (more basic urea anions) correlate to faster
rates.
Scheme 4. Proposed Bifunctional Mechanisms for the
Guanidine TBD and Urea Anions
This reactivity trend suggests that the nucleophilic activation
of the alcohol through H-bonding to the anionic urea plays a
key role in influencing the rate. More basic urea anions (derived
from less acidic ureas) are more effective at activating the H-
bonded alcohol toward nucleophilic attack, leading to faster
rates for the formation of the tetrahedral intermediate (INT1,
Scheme 3). Nevertheless, the role of the remaining N−H in the
urea anion appears important, as evidenced by the poor
selectivity exhibited by the methylated urea 7b in the
polymerization of CL with KOMe, resulting in a bimodal
molecular weight distribution for the polymer (Figure S9 in the
Supporting Information). 7b also exhibited reaction rate 7.6
times slower than its nonmethylated counterpart 7.
ization. An additional advantage of the approach reported here
is that the activities of the urea anions (or thiourea anions44)
can be tuned by the introduction of different substituents on
the ureas, whereas the reactivity of TBD is considerably more
difficult to modulate.
The high polymerization rates and selectivities for ring-
opening relative to transesterification are characteristic features
of these urea anion catalysts. To address the origin of the high
selectivity for monomer ring-opening relative to that for
polymer transesterification, the relative rates of transesterifica-
tion of δ-valerolactone and ethyl acetate (a model for the open
chain esters of the polymer chain) with 1-pyrenebutanol using
the anion of urea 1 were measured. Analysis of the initial rates
revealed that the ring-opening of VL (kobs = 9.8 min−1) was
significantly faster than transesterification of ethyl acetate (kobs
= 1.5 × 10−3 min−1). These relative rates provide clear kinetic
evidence that the urea anions exhibit high selectivities for ring-
opening of lactones relative to transesterification of open chain
esters. Nevertheless, at high conversion when the concentration
of the lactone monomer is very low,41 transesterification of the
polymer chains will occur, as evidenced by the broadening of
the molecular weight distributions at high conversions (Figures
S4−S6 in the Supporting Information).
The origin of this high selectivity is not fully understood. We
previously proposed24,44 that the selectivity for ring-opening of
thiourea/amine and thiourea anion catalysts could be partially
rationalized by the preferential binding of the s-cis lactones to
the thioureas or thiourea anions. However, due to solubility
limitations and high reactivities of the urea anions, we were
unable to measure the binding constants of lactones and ethyl
acetate to the urea anions. Further computational studies are in
progress, as we hypothesize that both the reversible binding of
carbonyls (Scheme 3, step a) as well as the formation of the key
tetrahedral intermediates (Scheme 3, step b, INT1) may have
lower barriers for lactones than open chain esters.
CONCLUSION
■
In summary, urea anions constitute a versatile class of readily
accessible bifunctional catalysts for ring-opening polymer-
izations. Depending on the substituents on the urea anions,
the activities of these catalysts span 3 orders of magnitude and
can be readily matched to the reactivity of different classes of
monomers. With the urea anions, polymerization of several
common monomers reach high conversion in just seconds with
excellent control. Kinetics studies reveal that the urea anions
function as catalysts, and can mediate polymerization at catalyst
concentrations lower than that of propagating chain ends. The
operational simplicity of the urea anions, coupled with their
high activities and selectivities, illustrate the potential of this
class of catalysts both for ring-opening polymerization and
other reactions that require the simultaneous activation of
nucleophiles and electrophiles.
METHODS
■
For general experimental procedures and characterizations, please see
the Supporting Information.
Polymerization of L-LA (Table 1, Entry 3). In a N2-filled
glovebox, 72 mg of L-LA (0.5 mmol) was dissolved in 0.25 mL of
THF in a 4 mL vial containing a micro stir bar. A stock solution
containing 1.0 mg of KOMe (0.015 mmol) and 21.8 mg of 1 (0.045
mmol) in 0.6 mL of THF was prepared in a separate vial. Then 0.2 mL
of the stock solution containing the initiator and the catalyst was
added to the L-LA solution. At 6 s, the reaction was quenched by the
addition of 0.3 mL of THF containing about 10 mg of benzoic acid.
The reaction mixture was taken out of the glovebox, and the solvent
was removed. Analysis: 94% conversion by NMR, Mn (vs PS) = 20.6
1
kDa, Đ = 1.06. Poly(L-LA): H NMR (CDCl3): δ 5.15 (1H, q), 1.57
The bifunctional mechanism proposed in Scheme 3
represents a new motif44 for catalysis by ureas and thioureas,
which are typically considered as H-bond donors.17,31,42,43,47,61
While ureas and thioureas are widely utilized as H-bond
activators for a variety of organic substrates,42,43,63 they have
also been investigated extensively as anion receptors43,64−66 and
in some cases deprotonations of the urea/thiourea have been
observed,66,67 depending on their pKa. Our results suggest that
this latter behavior can be exploited to generate efficient ring-
opening polymerization catalysts in which the urea anions act as
bifunctional catalysts to activate both the propagating alcohol
and the monomer. This mechanism is reminiscent of that
proposed24,25,30,34,68 for the ring-opening polymerization by the
guanidine TBD (1,3,5-triazabicyclo[4.4.0]dec-5-ene, Scheme
4), another versatile organic catalyst for ring-opening polymer-
(3H, d).
Polymerization of VL (Table 1, Entry 7). In a N2-filled glovebox,
50 mg of VL (0.5 mmol) was dissolved in 0.25 mL of THF in a 4 mL
vial containing a micro stir bar. A stock solution containing 3.5 mg of
KOMe (0.05 mmol) and 31.8 mg of 6 (0.015 mmol) in 2 mL of THF
was prepared in a separate vial. Then 0.2 mL of the stock solution
containing the initiator and the catalyst was added to the VL solution.
An aliquot of the reaction was removed and at 9 s was added to
approximately 10 mg of benzoic acid to be quenched. The aliquot was
taken out of the glovebox, and the solvent was removed. Analysis: 90%
conversion by NMR, Mn (vs PS) = 15.3 kDa, Đ = 1.06. Poly(VL): 1H
NMR (CDCl3): δ 4.08 (2H, t), 2.34 (2H, t), 1.67 (4H, m).
Polymerization of CL (Table 1, Entry 14). In a N2-filled
glovebox, 57 mg of CL (0.5 mmol) was dissolved in 0.25 mL of THF
in a 4 mL vial containing a micro stir bar. A stock solution containing
1.8 mg of KOMe (0.025 mmol) and 16.4 mg of 7 (0.075 mmol) in 1
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mL of THF was prepared in a separate vial. Then 0.2 mL of the stock
solution containing the initiator and the catalyst was added to the CL
solution. An aliquot of the reaction was removed and at 12 s was
added to approximately 10 mg of benzoic acid to be quenched. The
aliquot was taken out of the glovebox, and the solvent was removed.
Analysis: 89% conversion by NMR, Mn (vs PS) = 17.9 kDa, Đ = 1.14.
Poly(CL): 1H NMR (CDCl3): δ 4.05 (2H, t), 2.30 (2H, t), 1.64 (4H,
m), 1.37 (2H, m).
ACKNOWLEDGMENTS
■
This work was supported by the National Science Foundation
(NSF-CHE 1607092).
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in a 4 mL vial containing a micro stir bar. A stock solution containing
1.8 mg of KOMe (0.025 mmol) and 15.9 mg of 6 (0.075 mmol) in 1
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solution. An aliquot of the reaction was removed and at 10 s was
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In a N2-filled glovebox, 50 mg of VL (0.5 mmol) was dissolved in 0.15
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.6b11864.
(24) Lohmeijer, B. G. G.; Pratt, R. C.; Leibfarth, F.; Logan, J. W.;
Long, D. A.; Dove, A. P.; Nederberg, F.; Choi, J.; Wade, C.;
Waymouth, R. M.; Hedrick, J. L. Macromolecules 2006, 39, 8574−
8583.
■
S
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Supplemental figures and information regarding materials
used, instrumentation, synthetic procedures, character-
ization data, and kinetic data (PDF)
AUTHOR INFORMATION
Corresponding Author
*waymouth@stanford.edu
■
(28) Olsen, P.; Odelius, K.; Keul, H.; Albertsson, A.-C. Macro-
molecules 2015, 48, 1703−1710.
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V. G., Jr.; Hillmyer, M. A.; Tolman, W. B. J. Am. Chem. Soc. 2003, 125,
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ORCID
Robert M. Waymouth: 0000-0001-9862-9509
Notes
The authors declare no competing financial interest.
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