Cooperative catalysis of cyclic carbonate ring opening: Application towards non-isocyanate polyurethane materials

Article abstract of DOI:10.1002/ejoc.201500313

The reaction between cyclic carbonates and amines to produce hydroxyurethanes is an important alternative to current urethane chemistry. In order to address the issue of slow reaction rates, an efficient ring opening of cyclic carbonates with amines has been achieved utilizing cooperative catalysis. A new Lewis acid/Lewis base combination substantially decreases the reaction times for small molecule systems to reach complete conversion. Although triazabicyclodecene (TBD) has a substantial impact on the reaction rate, the addition of lithium triflate (LiOTf) as a co-catalyst allows for the fastest ring opening reported in the current literature. Cooperative catalysis is also applied to the synthesis of polymers containing hydroxyurethane linkages and is able to achieve rapid conversion of the bis-cyclic carbonate and diamine precursors when compared with the uncatalyzed reaction.

Full text of DOI:10.1002/ejoc.201500313

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201500313
Cooperative Catalysis of Cyclic Carbonate Ring Opening: Application Towards
Non-Isocyanate Polyurethane Materials
Vince M. Lombardo,[a][‡] Elizabeth A. Dhulst,[b][‡] Emily K. Leitsch,[b] Nathan Wilmot,[c]
William H. Heath,[c] Anthony P. Gies,[c] Matthew D. Miller,[c] John M. Torkelson,*[b] and
Karl A. Scheidt*[a]
Keywords: Cooperative catalysis / Polymers / Amines / Cyclic carbonates / Polyurethane
The reaction between cyclic carbonates and amines to pro-
duce hydroxyurethanes is an important alternative to current
urethane chemistry. In order to address the issue of slow re-
action rates, an efficient ring opening of cyclic carbonates
(TBD) has a substantial impact on the reaction rate, the
addition of lithium triflate (LiOTf) as a co-catalyst allows for
the fastest ring opening reported in the current literature.
Cooperative catalysis is also applied to the synthesis of poly-
with amines has been achieved utilizing cooperative cataly- mers containing hydroxyurethane linkages and is able to
sis. A new Lewis acid/Lewis base combination substantially
decreases the reaction times for small molecule systems to
reach complete conversion. Although triazabicyclodecene
achieve rapid conversion of the bis-cyclic carbonate and di-
amine precursors when compared with the uncatalyzed reac-
tion.
urethanes (NIPUs), in which the urethane linkage is formed
in the absence of isocyanate moieties. Groszos described the
first synthesis of a NIPU, which involved the reaction of
bis-cyclic carbonate with a diamine resulting in hydroxy
carbamate formation (Scheme 1), the latter of which con-
tains the urethane unit with an additional hydroxy group.[4]
As a consequence, the polymer resulting from such reac-
tions is sometimes referred to as polyhydroxyurethane[5]
(PHU) in addition to be being called NIPU.[6] Cyclic carb-
Introduction
Polyurethanes (PU) are highly versatile materials with a
broad array of applications ranging from flexible and rigid
foams to elastomers and adhesives.[1] PUs are commonly
formed from the step-growth reactions of multifunctional
isocyanates with multifunctional alcohols, with worldwide
production exceeding 12 billion kg per year in 2010.[2] The
synthesis of PUs differs from that of most other polymers
because of the near-room-temperature cure, which is the re-
sult of the rapid reaction between alcohol and isocyanate
groups at ambient conditions. However, overexposure to
isocyanates can be harmful to your health if not handled
correctly. Additionally, there are increasing regulatory pres-
sures by state and federal agencies. As a result, there is a
clear need to develop new chemical strategies that result in
PUs and related materials.[3]
The challenges associated with PU synthesis can be par-
tially addressed by the development of non-isocyanate poly-
[a] Department of Chemistry, Center for Molecular Innovation
and Drug Discovery, Northwestern University
2145 Sheridan Road, Evanston, IL 60208, USA
E-mail: scheidt@northwestern.edu
http://sites.northwestern.edu/scheidt/
[b] Department of Chemical and Biological Engineering,
Department of Materials Science and Engineering
Northwestern University,
2145 Sheridan Road, Evanston, IL 60208, USA
E-mail: j-torkelson@northwestern.edu
http://torkelson.northwestern.edu
[c] The Dow Chemical Company
2301 N. Brazosport Boulevard, Freeport, TX 77541, USA
[‡] These authors contributed equally to this work.
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201500313.
Scheme 1. Synthetic methods of traditional and non-isocyanate
polyurethanes.
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J. M. Torkelson, K. A. Scheidt et al.
SHORT COMMUNICATION
onates are an attractive alternative to isocyanates because as base, the reaction time to full conversion was 75 min,
of their ease of synthesis from inexpensive glycidyls/bis-ox- more than a factor of 2 greater than that with the higher
iranes,[7] low toxicity, and biodegradability.[8] However, the dual catalyst concentration described above. This result led
aminolysis of five-membered cyclic carbonates has poor us to consider the use of a guanidine base, triazabicyclo-
ambient temperature reactivity and has been studied most decene (TBD), in place of DBU. Commonly used in organic
commonly in solvent at elevated temperatures, with reaction synthesis,[12] TBD, ca. 1 pKa unit more basic than DBU,[13]
times of many hours or days.[9] Although there have been has proven effective in the ring-opening polymerization of
many research efforts recently devoted to NIPU or lactones,[14] and has recently been reported to facilitate the
PHU,[5b,5d,9,10] there remains a significant need to develop aminolysis of cyclic carbonates at ambient temperature in
successful approaches to synthesize NIPU with rapid and dimethyl sulfoxide.[10] Interestingly, the use of TBD alone
effective ambient temperature reactions comparable to at 0.10 mol/mol amine (0.37 mol/L) in the cyclic carbonate
those used in conventional PU synthesis.
ring-opening reaction afforded full conversion in 30 min, a
better result than achieved by the dual catalysis system of
LiOTf/DBU at similar concentrations of each component
(see Table 1). Ultimately, the unique combination of LiOTf
and TBD, each at 0.10 mol/mol amine (0.36 mol/L), as a
dual catalysis system resulted in full conversion within
10 min of the reaction initiated at room temperature. To the
best of our knowledge, these conditions are the fastest
cyclic carbonate ring-openings reported in the litera-
ture.[9b,15] While previous research has reported that lithium
salts are able to accelerate the ring opening of five-mem-
bered cyclic carbonates,[9d,16] our study has not found that
LiOTf by itself (Table 1, entry 2) rapidly accelerates the
ring-opening reaction of cyclic carbonates with an amine
when compared with TBD. However, the comparison of re-
actions in the presence of TBD but without LiOTf (Table 1,
entry 6) and in the presence of TBD and LiOTf (Table 1,
entry 7) indicates that cooperative catalysis occurs when
both Lewis acid and Lewis base are present.
The profound impact on the ring opening of cyclic carb-
onates by the combination of TBD and LiOTf indicates
that the catalysts work cooperatively. TBD can act as a bi-
functional base or as a nucleophile to catalyze numerous
reactions as well as the aminolysis of esters and carb-
onates.[17] TBD has recently been found to open cyclic carb-
onates to form an activated TBD amide intermediate, which
is then displaced by an amine.[10] Lithium salts have been
shown to increase the IR stretch of cyclic carbonates (1785
to 1791 cm–1), which indicates an increase in the electro-
philicity of the carbonyl carbon.[9d] It is proposed in Fig-
ure 1 that lithium triflate increases the electrophilicity of the
cyclic carbonate 4 forming a Lewis acid activated cyclic
carbonate 5 which can then be attacked by TBD forming
tetrahedral intermediate 6, followed by collapse to the acti-
vated carbamate 7. Nucleophilic attack of the activated
carbamate 7 by an amine and subsequent deprotonation of
the amine by TBD generates a mixture of carbamate a and
b products, as well as releasing TBD for further cyclic carb-
onate ring opening.
Results and Discussion
Given our interest in the area of cooperative catalysis,[11]
we initiated a platform to investigate cyclic carbonate ring
openings using an oxophilic Lewis acid in conjunction with
an organic Lewis base to simultaneously activate the elec-
trophile (carbonate) and nucleophile (amine, see Table 1).
After initial screening of various Lewis acids and bases in
the model reaction of small-molecule cyclic carbonate 1 and
benzylamine 2, we determined that the cooperative use of
lithium salts with 1,8-diazabicycloundec-7-ene (DBU)
achieved fast ring opening in reactions initiated at room
temperature with high catalyst loading and in the absence
of solvent. The reactions are exothermic, which can result
in some temperature rise during a rapid reaction. In par-
ticular, with lithium triflate (LiOTf) at 0.20 mol/mol amine
(0.61 mol/L) and DBU as base at 0.40 mol/mol amine
(1.22 mol/L), a stoichiometrically balanced reaction of
benzyl glycerol carbonate and benzylamine in the absence
of solvent led to full conversion (Ͼ 99% reaction of func-
tional groups) in 35 min to a mixture of 3a and 3b (Table 1,
entry 4). This compares to reaction times of 1440 min in
the absence of both LiOTf and DBU (Table 1, entry 1), and
120 min in the presence of DBU alone (Table 1, entry 3).
Table 1. Cooperative catalysis screening results.
Entry Lewis acid (mol-%)[a]
Base (mol-%)[a] Time [min][b]
1
2
3
4
5
6
7
None
LiOTf (10)
None
LiOTf (20)
LiOTf (10)
None
None
None
DBU (40)
DBU (40)
DBU (10)
TBD (10)
TBD (10)
1440
720
120
35
75
30
We have considered the effects of different substrates and
amines on reaction time to Ͼ 99% conversion of starting
material and the ratio of carbamate products a and b
(Table 2). The fastest reaction times were achieved with the
electron-withdrawing para-CF3 benzyl substituent, which is
expected due to electronic effects.[8] In addition, the phenyl-
LiOTf (10)
10
[a] Relative to mol amine. [b] Indicates time to achieve Ͼ 99% con-
version of 1 to 3a and 3b.
When the dual catalyst loading was decreased, 0.10 mol/ substituted carbonate offered similar reaction times with
mol amine (0.36 mol/L) of each of LiOTf as acid and DBU the advantage of being easily obtained from abundantly
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Cooperative Catalysis of Cyclic Carbonate Ring Opening
ness of model reactions between small-molecule cyclic carb-
onates and amines, we also sought to demonstrate coopera-
tive catalysis in the production of polymers by the reaction
of difunctional cyclic carbonate molecules and difunctional
amines. Conventional PU synthesis often involves a reac-
tion between a prepolymer consisting of a polyether diol of
molecular weight (MW) between 600 and 2000 g/mol and a
low MW diisocyanate.[18] As shown in Scheme 2, we have
chemically functionalized a traditional polyurethane pre-
polymer [Voranol 220-056N from The Dow Chemical Com-
pany, with nominal MW = 2000 g/mol, reacted in excess
2,4-toluene diisocyanate (TDI)] to be end-capped with
glycerol carbonate, resulting in a mixture of dicyclic carb-
onate 11 and low MW dicyclic carbonate. This solution was
mixed at stoichiometric balance with a low MW diamine,
1,3-cyclohexane bis(methylamine) 12, with the reaction
initiated at 23 °C (Scheme 2). Comparisons were made be-
tween reactions in the presence of 0.10 mol/mol amine
(0.133 mol/L) of each of TBD and LiOTf (premixed into
the dicyclic carbonate mixture before addition of diamine)
and in the absence of catalyst. The concentrations of TBD
and LiOTf used in the synthesis of this polymer are roughly
a factor of three lower than those present in the reactions
of the small-molecule monofunctional cyclic carbonate and
amine reactions described above. This reflects the facts that
the catalyst system has limited solubility in the polymer and
that large quantities of residual catalyst in the polymer can
have deleterious effects on its properties and substantially
increase cost.
Figure 1. Proposed cooperative catalytic cycles of TBD and LiOTf
(catalytic cycle shown produces isomer a).
available styrene oxide. Ratios of carbamate products a and
b remained relatively unchanged when modifying the carb-
onate backbone or when altering the amines. Interestingly,
a significant ratio difference was observed when utilizing di-
substituted carbonate, which not only offered rapid reaction
times but also a single carbamate product a.
Table 2. Products of the model cyclic carbonate reactions.
Carbonate
R3
Time[a] [min]
a/b
Yield [%]
R1 = CH2OBn
Bn
Cy
Hex
10
15
10
4:1
4:1
4:1
80
83
85
R2 = H
R1 = Ph
R2 = H
Bn
Cy
Hex
Ͻ 3
Ͻ 3
Ͻ 3
3:1
3:1
3:1
92
93
90
R1 = Me
R2 = Ph
Bn
Cy
Hex
3
3
3
1:0
1:0
1:0
85
85
84
R1 = 4-F3CC6H4
R2 = H
Bn
Cy
Hex
1
1
1
3.5:1 95
3.5:1 91
3.5:1 90
[a] Indicates time to achieve Ͼ 99% conversion of the cyclic carb-
onate to a and b.
Scheme 2. Polymerization reaction, utilizing cooperative catalysis,
between dicyclic carbonate capped prepolymer mixture (low MW
structure not shown) and low MW diamine to produce polymer
Having shown the utility of TBD/LiOTf cooperative
catalysis in substantially increasing the speed and effective- (secondary alcohol product not shown). [a] relative to mol amine.
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J. M. Torkelson, K. A. Scheidt et al.
SHORT COMMUNICATION
Major differences in fractional conversion of functional to increase in Mn from 2600 g/mol in the macromer to
groups, p (with 0 Յ p Յ 1), were observed in the presence oligomers with Mn of about 5000, 9000, and 13000 g/mol
and absence of the TBD/LiOTf catalyst. For example, after corresponding to 2, 3–4, and 5 prepolymer units incorpo-
2 min, p = 0.39 (39% conversion) with catalyst and 0.06 rated into oligomers (see the Supporting Information). The
without catalyst and after 60 min, p = 0.56 with catalyst material exhibited a Young’s modulus of 3.2 MPa and an
and 0.30 without catalyst. A possible contributor to the elongation at break of 360% at room temperature, the latter
extraordinary difference in fractional conversion after 2 min being consistent with traditional PU elastomer properties
is a temperature rise resulting from the exothermic nature and the former with traditional rubber properties reported
of the reaction, which would be greater in the faster in the literature.[19]
reacting system. Figure 2 demonstrates that both the
catalyzed and uncatalyzed reactions provide acceptable fits
Conclusions
to results expected for second-order reactions, i.e., first
order in cyclic carbonate concentration and first order in
amine concentration. Second-order reactions at stoichio-
metric balance should obey the following Equation (1):
A rapid opening of cyclic carbonates has been demon-
strated using a cooperative catalysis system involving TBD
and LiOTf. These new conditions are applicable to a variety
of carbonates and amines offering rapid reaction times in
all cases. This catalyst system has also been applied to poly-
mer synthesis and has demonstrated the ability to achieve
high conversion in the reactions between cyclic-carbonate-
capped prepolymers and diamines at ambient temperature.
These results have laid a foundation for further studies with
the goal of a step-growth polymerization of bis-cyclic carb-
onates and diamines utilizing our cooperative catalysis con-
ditions.
co/c = 1/(1 – p) = 1 + kcot
(1)
in which co is the initial functional group concentration,
c is the functional group concentration at reaction time t,
and k is the second-order reaction rate parameter. With
approximate linearity in co/c for both uncatalyzed and
catalyzed reactions, the catalyzed reaction exhibits a k value
that is about twice that of the uncatalyzed reaction. The
slower nature of the reaction observed with the synthesis of
polymer as compared with the reaction of small-molecule
model compounds results from the fact that both the func-
tional groups and catalyst loading are at much lower con-
centrations at the outset of the polymer synthesis as com-
pared with the reaction of the model small-molecule sys-
tem. Nonetheless, the achievement of substantial conver-
sion within minutes demonstrates the effectiveness of the
TBD/LiOTf system in catalyzing the synthesis of PHU at
ambient temperature by the reaction of bis-cyclic carbonate
with diamine.
Experimental Section
Typical Procedure: Epoxide (1.00 mmol) was added to tetra-butyl
ammonium iodide (TBAI, 0.100 mmol), and the reaction was
1
heated to 80 °C and stirred until completion as determined by H
NMR spectroscopy. Upon completion, the reaction mixture was
dissolved in dichloromethane, washed with water, dried with mag-
nesium sulfate, filtered, and concentrated in vacuo to afford the
carbonate. Lithium triflate (0.100 equiv.) and 1,5,7-triazabi-
cyclo[4.4.0]dec-5-ene (TBD, 0.100 equiv.) were added to a 2 dram
vial inside a glove box. The appropriate five-membered cyclic carb-
onate (1.00 equiv.) was then added and the mixture was stirred for
10 min. The appropriate amine (1.00 equiv.) was then added and
the mixture was stirred until completion. The reaction was moni-
tored by LC/MS until Ͼ 99% consumption of the starting carb-
onate was achieved. Upon completion, the reaction mixture was
dissolved in a hexanes/dichloromethane mixture and purified by
column chromatography.
Supporting Information (see footnote on the first page of this arti-
cle): Representative experimental procedures for small molecule
and polymeric reactions and NMR spectra of the products.
Acknowledgments
This research was supported by the University Partnership Initia-
tive between Northwestern University and The Dow Chemical
Company.
Figure 2. Carbonate conversion fit to integrated second-order rate
equations for reactions in the presence and absence of catalyst.
Slopes provide kco values for the reactions. Calculated k values:
kuncat = 4.0ϫ10–5 Lmol–1 s–1 and kcat = 8.1ϫ10–5 Lmol–1 s–1 (with
co,uncat = 1.4 m and co,cat = 1.3 m).
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Cooperative Catalysis of Cyclic Carbonate Ring Opening
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Received: March 6, 2015
Published Online: March 30, 2015
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