Development of solvent-free synthesis of hydrogen-bonded supramolecular polyurethanes

Article abstract of DOI:10.1039/c4sc03804e

In this work we describe the solvent free synthesis of supramolecular polyurethanes held together by heterocomplementary triple hydrogen bonding. We perform a systematic evaluation on the base catalyzed synthesis of small molecule ureas and carbamates fro

Full text of DOI:10.1039/c4sc03804e

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Development of solvent-free synthesis of
hydrogen-bonded supramolecular polyurethanes†
Cite this: Chem. Sci., 2015, 6, 2382
ab
Kelly. A. Houton,a George M. Burslemab and Andrew. J. Wilson
*
In this work we describe the solvent free synthesis of supramolecular polyurethanes held together by
heterocomplementary triple hydrogen bonding. We perform a systematic evaluation on the base
catalyzed synthesis of small molecule ureas and carbamates from a range of isocyanates, amines and
alcohols in solution and in the solid state using ball milling. These optimized procedures are then shown
to be applicable to the synthesis of supramolecular polyurethanes using solvent-free methods.
Received 9th December 2014
Accepted 2nd February 2015
DOI: 10.1039/c4sc03804e
www.rsc.org/chemicalscience
(which has found use in covalent synthesis)22 to assemble/
disassemble co-ordination polymers.23
Introduction
Our group previously developed a series of hydrogen
bonding motifs,24–28 with which to construct supramolecular
polymers29–31 and employed a ureidoimidazole/amidoisocy-
tosine triply hydrogen bonded diad26 for assembly of supra-
molecular polyurethanes31–33 (Fig. 1a). These were prepared
via the ‘prepolymer’ approach (Fig. 1b), where an amorphous
diol 1 is reacted with MDI 2 giving an isocyanate terminated
pre-polymer 3 which is then end capped with hydrogen
bonding units to form a macromonomer 5. Upon addition of
a supramolecular chain extender 6, phase separation occurs,
conferring elastomeric properties upon the material 7. The
amorphous diol allows deformation of the material whilst
the hydrogen bonding units contained within the crystalline
phase promote retention of the original material congura-
tion.34 Synthesis of the macromonomer using this method is
a two-step one-pot reaction requiring the temperature to be
A major goal in synthetic (polymer) chemistry is the devel-
opment of synthetic methods that limit environmental
impact through elimination of (harmful) solvents, use of
lower temperatures and accelerated reaction time e.g.
through catalysis.1 In terms of solvent, the use of supercrit-
ical CO2 has proven useful but is limited to suitably soluble
polymers.2 Similarly, ionic liquids may be used,3 although
extraction of the product polymer still needs consideration.
Mechanochemical organic synthesis, currently undergoing a
revival,4–6 has been used to promote Knoevenagel condensa-
tions,7 Aldol reactions8 and Michael additions9 amongst
many other synthetically important organic trans-
formations.10 Signicantly, mechanochemical synthesis has
also been applied to the synthesis of organic frameworks11,12
and crystalline materials.12,13 Supramolecular polymers have
generated enormous interest during the last 10–15 years14,15
as a consequence of the opportunities to study assembly
mechanisms16–19 at a fundamental level, and, due to the
stimuli-responsive properties they possess, application in a
wide array of settings.20,21 Moreover, supramolecular poly-
mers offer opportunities for the design of synthetic methods,
given that synthesis of the low molecular weight building
blocks may be more amenable to common strategies
employed to limit environmental impact. Although covalent
mechanochemical syntheses have been extensively studied,5
we found no literature precedent for supramolecular polymer
material synthesis, although note the use of ultrasound
maintained at 87 C for 17.5 h (Fig. 1b).31 Furthermore, the
ꢀ
dimethylacetamide (DMAc) solvent is high boiling, hygro-
scopic and teratogenic. Polyurethanes have established
themselves in many areas of our everyday life, and their
production is subject to constant growth.35,36 Solvent-free
syntheses are desirable due to the difficulty in obtaining a
solvent, which is suitable for solubilizing high percentage
hard block containing phase separated polymers;37 polymers
with high urea content are liable to gelation and premature
precipitation, so oen require high temperatures and highly
polar solvents. Our supramolecular polyurethane elastomer
synthesis therefore represented an ideal model to develop
milder reaction conditions and explore
a solvent free
synthetic route. Herein we describe such a study by rst
outlining a systematic study on urea and carbamate forma-
tion in both the solution and the solid state and then the
application of our ndings to the synthesis of a supramo-
lecular polyurethane.
aSchool of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK. E-mail:
A.J.Wilson@leeds.ac.uk; Fax: +44 (0)113 3436565; Tel: +44 (0)113 3431409
bAstbury Centre for Structural Molecular Biology, University of Leeds, Woodhouse
Lane, Leeds LS2 9JT, UK
† Electronic supplementary information (ESI) available: Synthetic procedures,
characterisation, and additional data associated with optimisation and
substrate tolerance studies. See DOI: 10.1039/c4sc03804e
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Fig. 1 Supramolecular polyurethane (PU) synthesis and assembly (a) schematic depicting assembly of a supramolecular PU elastomer mediated
by triple hydrogen-bonding between amidoisocytosine and ureidoimidazole (b) synthesis of the supramolecular macromolecular components;
reaction of telechelic diol with MDI and then amine leading to the ureidoimidazole recognition motif provides a statistical mixture of chain
elongated telechelic (denoted m) and capped MDI (denoted n). Addition of n + 1 equivalents of ditopic diamidoisocytosine 6 results in matched
stoichiometry of the hydrogen-bonding units.
reaction), hence synthesis of PUs normally requires anhydrous
conditions.40 PU catalysis is well known in the literature,36,38,41,42
Results and discussion
Organometallic dibutyltin dilaurate is extensively used on an
industrial scale,43 but residual amounts of the compound affect
the lifetime of the product44 and the environmental impact of
using heavy metals is substantial. Similarly, hindered organic
bases have been shown to catalyze urethane formation e.g.
diazabicyclo[2.2.2]octane (DABCO), although this too exhibits
toxicity and its reduced usage is desirable.36,38 Finally, the melt
trans-urethane reaction has been used for PU synthesis
although higher reaction temperatures are required.45
The primary approach used to synthesize polyurethanes (PUs) is
the polyaddition of a diisocyanate and a telechelic diol, usually
in the presence of a catalyst, which generally fall into two
categories; organotins or amines38 (although acids have also
recently been employed).39 This simple reaction combined with
a wide range of commercially available isocyanate and alcohol
starting materials results in an extensive array of products.38
However, the reaction is not necessarily clean; four degradation
pathways have been identied, as well as self-cyclization to form
dimers and trimers.38 As temperature increases, increased
byproduct formation occurs; reversion to starting material,
olen formation and urethane rearrangement via trans-
carbamoylation all occur due to the weak C–NH bond.38 The
high nucleophilicity of water towards isocyanates can also result
in the formation of a urea with the release of CO2 (foaming
Optimization of solution phase synthesis
To have a reference point for our studies on solid-state synthesis
of urethanes, it was necessary to develop optimized solution
phase conditions. Burkus previously reported conditions for
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Table 1 Effect of catalyst on conversion of 2 to 9a–ba
Entry Catalyst pKa Temp/ꢀC Conversion to 9ab Conversion to 9bb
triethylamine (Et3N) catalyzed carbamate formation.46 We
applied these and several variants on the catalyst to a model
reaction (Scheme 1, Table 1 and Fig. S1–7†) between 1-propanol
8a and 4,4-methylene-bis-(phenylisocyanate) (MDI) 2. Using
triethylamine as catalyst, precipitation of the carbamate 9a was
evident aer 1 hour and afforded product in over 60% yield.
These conditions replace the use of DMAc with toluene and
reduce the temperature required for carbamate formation
relative to the conditions used for pre-polymer synthesis, which
is likely to reduce side reactions.42 Subsequent studies on the
effect of catalyst, catalyst pKa and reaction temperature (Fig. S1–
7†) revealed DABCO and Et3N to be superior catalysts whereas
1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) was found to retard
conversion to biscarbamate. In general, a moderate increase in
temperature did not improve the conversion under all condi-
tions (Fig. S2 and 3†). In each case the reactions produced
carbamate, the urea side product of the foaming reaction
(Fig. S8†) and unreacted starting material. Consistent with the
literature for DABCO,47 reduced foaming and improved yield
was evident for DABCO and NEt3, whereas TBD produced
additional side products (also consistent with the literature)41
(Fig. S1–3†). A clear correlation between catalyst pKa and
carbamate formation was observed; as pKa increases (a property
proposed to promote foaming),48 conversion decreases (Table
1). Time dependence of the reaction was studied qualitatively by
IR, observing the loss of isocyanate absorption at 2200 cmÀ1
with time together with appearance of an absorption at 1739
cmÀ1 for the carbamate C]O (Fig. S9†); the rate at which
starting material is consumed appears to increase with time
which is consistent with autocatalysis of the reaction.49
1
2
3
4
5
6
7
8
None
None
Et3N
Et3N
DABCO 8.9 RT
DABCO 8.9 40
—
—
10.8 RT
10.8 40
RT
40
29
29
75
74
91
97
67
66
84
88
84
89
88
92
63
62
TBD
TBD
22
22
RT
40
a
Conditions: reaction concentration 0.05 M, 10 mol% catalyst (entries
3–8) used, 1 equiv. of 2, 2 equiv. of propanol 8a or 8b diethylene
b
glycol monoethyl ether, toluene, 6 h. Conversion to 9 is based on
comparison of NMR signals of crude reaction mixture.
subset of experiments using diethyleneglycol-mono-ethyl ether
8b (see Fig. S12–21†) as a small molecule mimic of the PEG–
PPG–PEG telechelic to be used in supramolecular polymer
synthesis; similar results were obtained to those with propanol
8a (Table 1).
These conditions were then applied to the synthesis of a
polyurethane macromonomer 3a (NCO : OH ratio 2 : 1, Fig. 1b,
step I) with IR used to monitor reaction progress (Fig. S22†). The
1H NMR resonances in this polymeric system, affected our
ability to differentiate between urea and urethane NH and
aromatic protons, hence we could only determine conversion
from isocyanate. Here, catalyst effect on conversion was less
pronounced (although for TBD with the higher pKa, the reaction
was less clean), whilst temperature did not affect overall
conversion (Fig. S23 and 24†). For entries 5, 7 and 8 a product
that was insoluble in DMSO-d6 was obtained; gelation during
these reactions and subsequent insolubility could suggest a
pseudo Tromsdorff (gel) effect, which is common in polymeri-
zation procedures (Table2).52
To complete our optimization study, we performed a solvent
screen and probed the effect of reaction concentration using
Et3N as catalyst (see Fig. S10 and S11† for details). At higher
concentrations, gelation occurred aer 10 minutes indicating
formation of a cross-linked network, (potentially through lateral
hydrogen bonding of carbamate groups)50 preventing further
conversion of starting materials. The lowest concentration gave
the best result (100% conversion to product at 0.01 M). From the
solvent screen, toluene was identied as most suitable; this
might indicate a role for catalysis of urethane formation via
hydrogen bond activated complexes.51 Finally, we repeated a
Development of solvent free syntheses
With an efficient catalyst identied and several parameters
optimized in solution phase, we investigated solvent-free
synthesis using mechanochemistry. We found limited evidence
Table 2 Effect of catalyst on conversion of 2 to 3aa
Entry
Catalyst
pKa
Temp/ꢀC
Reaction conversionb
1
2
3
4
5
6
7
8
None
None
NEt3
—
—
RT
40
RT
40
RT
40
RT
40
90
95
88
87
10.8
10.8
8.9
8.9
22
NEt3
DABCO
DABCO
TBD
Not soluble
100
Not soluble
Not soluble
TBD
22
a
Conditions: reaction concentration 0.05 M, 10 mol% catalyst (entries
b
3–8) used, 2 equiv. of 2, 1 equiv. of polyol 1, toluene, 6 h. Reaction
of NCO is based on comparison of NMR signals of crude reaction
mixture.
Scheme 1 Synthetic conditions applied to model reactions.
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Table 3 Effect of catalyst on conversion of 2 to 9a–ba
for urea/carbamate synthesis53 using ball milling but note it has
been shown to effect acylation reactions, thioisocyante and
thiourea synthesis.54,55 MDI 2 and 1-propanol 8a and dieth-
yleneglycol-mono-ethyl ether 8b were used as model substrates
to explore whether carbamate 9a–b formation was possible
using mechanochemical methods. For this study, the ball mill
was set to a vibrational frequency of 20 Hz for three minute
intervals and the reaction monitored by IR. All three catalysts
were again tested for activity in the bulk state and compared to
the uncatalyzed reaction under the same reaction conditions
(Fig. S25–34†).
Entry
Catalyst
pKa
Conversion to 9ab
Conversion to 9bb
1
2
3
4
None
Et3N
DABCO
TBD
—
93
95
93
80
86
98
98
87
10.8
8.9
22
a
Conditions: 10 mol% catalyst (entries 2–4) used, 1 equiv. of 2, 2 equiv.
of propanol 8, vibrational frequency was 20 Hz, 18 min. b Conversion to
9 is based on comparison of NMR signals of crude reaction mixture.
For 1-propanol 8a aer three minutes, a decrease in isocya-
nate absorbance at 2200 cmÀ1 was indicative of consumption of
starting material and the reaction beginning to take place
(Fig. 2a). A change in frequency and sharpening of the stretch at
ꢁ3300 cmÀ1 was also observed consistent with loss of alcohol
(OH stretch) and formation of a carbamate (NH stretch). Aer
eighteen minutes of milling, there was no IR absorption for the
isocyanate or –OH stretch for three of the reactions, high-
lighting similar reactivity for the Et3N, DABCO and uncatalysed
Having shown milling to effectively promote the model
reaction without a catalyst, substrate tolerance to the reaction
conditions was assessed for urea 11 and carbamate 12 forma-
tion so as to ascertain the effectiveness of the method for small
molecule synthesis. An isocyanate and either an amine or
alcohol (Scheme 2) were tested under these conditions (Table 4).
In our hands – the method is tolerant to a broad array of
functionality (Fig. S35–45†). Isocyanates bearing electron with-
drawing substituents 11a–b generally increased reactivity
whereas ethyl substituent 11c reacts less well presumably due to
the inductive effect of the alkyl group.56 Poorer nucleophiles
work less well. Many of the ureas synthesized using this method
are heavily hindered and we were unable to obtain some of
these via traditional solution methodology. Solid amines,
isocyanates and alcohols in addition to liquid counterparts were
tested. All substrates were found to be amenable to these
conditions indicating that liquid assisted grinding (LAG)
effects57 are negligible in promoting the reaction. This is
important given that both the model reaction (MDI and 1-
propanol) and the supramolecular macromonomer synthesis
(see below) are both reactions that involve a liquid component.
It is also noteworthy that in a recently described acid catalyzed
carbamate synthesis which was performed under ball milling
conditions, the reaction took place with a liquid catalyst,39
hence the current results unequivocally illustrate the utility of
catalyst free, ball milling for solvent free urea and carbamate
synthesis.
reactions (entries 1–3, Table 3) and indicating
a less
pronounced role for the catalyst although TBD was again
detrimental to carbamate 9a formation. Similarly synthesis of
carbamate 9b occurred with good conversion as evidenced in
the crude NMR spectra, a resonance at 4.23 ppm expected for
the OCH2–CO–NH group being diagnostic (Fig. 2b).
Milling methods were then used for synthesis of supramolec-
ular polyurethanes 7 starting from MDI 2 and diol terminated
PEG–PPG–PEG 1 (2000 g molÀ1) (Scheme 3, step I). We focused on
generating supramolecular polymers 7a–b with a 2 : 1 and 4 : 1
NCO : OH ratio as in our previous work.31 A key feature of this
reaction is its heterogeneity; the ditopic nature of each component
results in polyurethanes 3a/b (and thus 5a/b) comprising a statis-
tical mixture of capped and chain extended polyol 1 obtained
alongside MDI 2 capped at both termini with the target amine (for
Fig. 2 Solvent free carbamate syntheses (a) IR spectra showing the
effect of ball milling at three minute intervals on carbamate 9a
formation;
a
decrease in absorption at 2200 cmÀ1 indicates
consumption of starting material. (b) 1H NMR (500 MHz, DMSO-d6) of
the crude product 9b obtained from reaction of 2 and 8b.
Scheme 2 Solvent free carbamate/urea synthesis.
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Table 4 Substrate tolerance for solvent free carbamate/urea synthesisa
Conversionb (%)
R2 or R4
R3
R1
Product
46c
99
100
60c
100
79
100
100c
50c
75c
55c
Benzimidazole
CH(C2H6)
CH(C2H6)
Benzimidazole
CH(C2H6)
C3H5
H
p-CN benzene
p-NO2 benzene
p-C2H5 benzene
–adamantane
–adamantane
p-NO2 benzene
–adamantane
–adamantane
p-CN benzene
p-NO2 benzene
p-C2H5 benzene
11a
11b
11c
11d
11e
12a
12b
12c
12d
12e
12f
CH(C2H6)
CH(C2H6)
H
CH(C2H6)
C3H5
CH2 O-nitrobenzene
CH2 O-nitrobenzene
CH2 O-nitrobenzene
CH2 O-bromobenzene
a
Conditions: no catalyst, 1 equiv. of isocyanate 10, 1 equiv. of amine/alcohol (2 equiv. in the case of MDI), vibrational frequency was 20 Hz, 10 min.
Conversion to 11/12 is based on comparison of NMR signals of crude reaction mixture. c Donates a solid–solid reaction.
b
DABCO the most procient. This contrasts to carbamate
formation in the model system. TBD in this case was also an
effective catalyst, with a high crude conversion and purity,
similar to that obtained using Et3N as catalyst (as evidenced by
NMR) (Fig. S46†). Therefore further work is required to under-
stand the complexity of the catalyst's interaction with polyols.
The macromonomer product 3a was then reacted further
with 2-amino-5,6-dimethylbenzimidazole 4 to produce the
gummy, malleable supramolecular macromonomer 5a (Scheme
3, step II). IR analysis of the material both before and aer
reaction (Fig. 3a) illustrated the disappearance of remaining
isocyanate groups within 10 minutes. Similarly, NMR analyses
of the synthetic sequence 1 to 3a to 5a illustrated formation of a
carbamate as evidenced by the appearance of a resonance at 4.2
ppm expected for the OCH2–CO–NH group (Fig. 3b). Overall,
this approach leads to an improved method: 10 mol% NEt3,
DABCO or TBD (see Fig. S47†), 40 minutes at 25 Hz with no
solvent relative to our original method (DMAc, 17.5 h at 87 ꢀC).
We then applied our most suitable conditions to the
Scheme
3 Solvent free synthesis of a supramolecular macro-
synthesis and assembly of the supramolecular polyurethane
elastomers 7a–b in one pot using triethylamine as catalyst
(Fig. 3b for 7a and Fig. S48†); this was seen as advantageous
because triethylamine can be removed under vacuum at the end
of the procedure. The heterocomplementary triple hydrogen
bonding array diamidoisocytosine (DAC) 6 could be added and
the reaction subjected to further milling until a homogeneous
tacky powder was achieved (ꢁ20 min).
monomer by ball milling.
3/5a i.e. NCO to OH ratio of 2 : 1, 50% of polyol can be excepted to
be capped at both ends with MDI 2 whereas for 3/5b i.e. NCO to OH
ratio of 4 : 1 75% becomes capped). DAC 6 is then added to match
the stoichiometry of the ureidoimidazole hydrogen-bonding
motifs present in the system resulting in hydrogen-bond-mediated
assembly of an elastomer in which the chain extended telechelic
forms the so block and the end capped MDI integrates into the
hard-block with the DAC 6 resulting in different materials prop-
erties for different NCO to OH ratios.31
Table 5 Effect of catalyst on conversion of 2 to 3aa
Entry
Catalyst
pKa
Crude conversion/%
Initial studies focused on identifying suitable conditions for
synthesis of 3a. The reactants were subjected to milling at 20 Hz
for three-minute intervals inside the ball mill. A test reaction
with these conditions indicated urethane formation to be
slower than carbamate formation. We therefore increased the
vibrational frequency of milling to 25 Hz for ve minute inter-
vals for a total time of thirty minutes (Table 5). Macromonomer
3a formation was found to depend upon the catalyst with
1
2
3
4
None
NEt3
DABCO
TBD
—
70
78
100
93
10.8
8.9
22
a
Conditions: 10 mol% catalyst (entries 2–4) used, 0.5 equiv. of 1,
vibrational frequency was 25 Hz, 30 min. #Conversion is based on
comparison of NMR signals of crude reaction mixture.
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Fig. 4 DSC traces of MDI and PEG–PPG–PEG based supramolecular
polymer (4 : 1 NCO : OH ratio) synthesised by traditional solution (red)
and solventless mechanochemistry in a ball mill (black).
Conclusions
A systematic study on catalyst, temperature, solvent and reaction
concentration has been performed to optimize the synthesis of
carbamate, urethane and urea functional groups. This was
compared against a solvent free method for this synthesis that
exploits ball milling and which we developed in parallel. The
method was found to be applicable to a wide range of substrates
for small molecule synthesis and sufficiently efficient to proceed
without catalyst. It should be noted that an increase in concen-
tration was found to be detrimental in many cases as a result of
gelation whereas no such problems were observed with ball
milling. Most signicantly we applied the method to the synthesis
of a previously described supramolecular polyurethane 7a–b.
When compared to the optimized solution procedure, synthesis
was shown to be faster and cleaner. Ultimately, the method could
nd use in development of synthetic processes with a reduced
environmental impact and could operate in tandem with phos-
gene-free routes to isocyanates.35 Whilst mechanochemical
synthesis of MOFs and crystalline frameworks is well estab-
lished,12,13 synthesis of so materials using this method is less
prevalent; this makes the observation that supramolecular
ordering and phase separation can be achieved during solvent free
synthesis all the more remarkable, hence the results reported here
may have more widespread applicability to other supramolecular
polymer syntheses.
Fig. 3 (a) IR spectrum showing reaction of the MDI end-capped PU
(left), which shows free isocyanate at 2200 cmÀ1. Reaction with 2-
amino-5,6-dimethylbenzimidazole shows that after ten minutes of
milling (right), this absorption has disappeared suggesting urea
formation with the amine. (b) 1H NMR spectra (300 MHz, DMSO-d6) of
1, 3a, 5a and 7a obtained by ball milling.
Acknowledgements
This work was supported by Huntsman Polyurethanes. We wish
to thank Alberto Saiani and his group (University of Man-
chester) for assistance with DSC experiments and Huntsman
Polyurethanes for useful discussions.
Supramolecular polymer 7a–b formation was assessed by
differential scanning calorimetry (DSC) (Fig. 4 and S49†) which
revealed dened glass transitions anticipated for an elꢀastomeric
material. For 7b a broad exotherm shown at ꢁ À61 C can be
attributed to the glass transition of the PEG based polymer
ꢀ
backbone. A further transition at ꢁ97 C can be attributed to
Notes and references
transitions in the hydrogen bond containing hard blocks of the
ꢀ
ꢀ
polymer. 7a exhibits these transitions at ꢁÀ61 C and 120 C
respectively. These properties match those observed for supra-
molecular polymers prepared using the previously reported
method shown in Fig. 1.31
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