TBD catalysis with dimethyl carbonate: A fruitful and sustainable alliance

Article abstract of DOI:10.1039/c2gc35191a

This work presents the synthesis of unsymmetric and symmetric organic carbonates as well as the synthesis of polycarbonates in an efficient and sustainable approach. All reactions were carried out at atmospheric pressure at 80°C and the use of classic toxic and harmful chemicals, such as phosgene and carbon monoxide, was avoided. The key finding of this manuscript is that the use of 1,5,7-triazabicyclo[4.4.0]dec-5-ene, TBD, an organocatalyst, in combination with dimethyl carbonate (DMC), a non-toxic and renewable starting material, allows the synthesis of the mentioned unsymmetric carbonates in yields of up to 98% under optimized conditions. The structure of the alcohols used for this approach was found to influence the DMC-ROH ratio required to maximize the yield of the desired structure. Finally, the results obtained for the synthesis of low molecular weight building blocks could be transferred to the catalytic synthesis of high molecular weight polycarbonates. The Royal Society of Chemistry.

Full text of DOI:10.1039/c2gc35191a

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PAPER
TBD catalysis with dimethyl carbonate: a fruitful and sustainable alliance†‡
Hatice Mutlu, Johal Ruiz, Susanne C. Solleder and Michael A. R. Meier*
Received 8th February 2012, Accepted 26th March 2012
DOI: 10.1039/c2gc35191a
This work presents the synthesis of unsymmetric and symmetric organic carbonates as well as the
synthesis of polycarbonates in an efficient and sustainable approach. All reactions were carried out at
atmospheric pressure at 80 °C and the use of classic toxic and harmful chemicals, such as phosgene and
carbon monoxide, was avoided. The key finding of this manuscript is that the use of 1,5,7-triazabicyclo
[4.4.0]dec-5-ene, TBD, an organocatalyst, in combination with dimethyl carbonate (DMC), a non-toxic
and renewable starting material, allows the synthesis of the mentioned unsymmetric carbonates in yields
of up to 98% under optimized conditions. The structure of the alcohols used for this approach was found
to influence the DMC–ROH ratio required to maximize the yield of the desired structure. Finally, the
results obtained for the synthesis of low molecular weight building blocks could be transferred to the
catalytic synthesis of high molecular weight polycarbonates.
of course highly toxic. Furthermore, the selective synthesis of
unsymmetric organic carbonates was accomplished using alkyl
Introduction
From the standpoint of global sustainability, the development of
new synthetic methods and catalysts that afford improved selec-
tivity and energy efficiency in combination with a feedstock
change to renewable raw materials, emerged as a strategy for the
design of environmentally compatible commodity chemicals and
monomers.1 In this aspect, unsymmetric and symmetric organic
carbonates are important intermediates for the chemical industry.
They can, for instance, act as useful protecting groups of alco-
hols and phenols, since they are more stable than the correspond-
ing esters under basic conditions.2 Additionally, organic
carbonates have found employment as monomers for organic
glasses and as solvents, for instance, in the manufacture of
lithium batteries.3 Although, the remarkable importance of aryl
and alkyl carbonates in various fields as chemical intermediates
is well documented by the presence of a large number of
patents4 and articles3,5 in the literature, very few carbonates are
available commercially. The conventional methods for the prep-
aration of organic carbonates suffer from disadvantages and still
require the use of toxic reagents,5,6 such as phosgene, dimethyl
sulfate, pyridine and carbon monoxide. Most commonly, chloro-
formate esters obtained from phosgene and alcohols were pro-
posed as safer substituents.7
halides, either via the alkylation of metal carbonate with various
alkyl halides and sulfonates in ionic liquids,8 or by inorganic cat-
alyst (e.g., Cs2CO3) based coupling of alcohols, carbon dioxide
and alkyl halides.9 Thus, bearing in mind these drawbacks, it
seems advisable to develop more convenient and environmen-
tally benign catalytic systems for the synthesis of carbonated
compounds.10
Along this idea, the organic carbonate interchange reaction
can be proposed as the most pursued eco-friendly “carbonyla-
tion” route to produce unsymmetric as well as symmetric carbon-
ates in the presence of organic, metal-free catalyst and dimethyl
carbonate (DMC). Since DMC can be synthesized using CO2 as
building block,11 and features high biodegradability and low tox-
icity, it incorporates several of the fundamental aspects of green
chemistry.1 In particular, DMC has been proposed as a substitute
of dimethyl sulphate, methyl halides and phosgene and reacts
either as methoxycarbonylating or as a methylating agent,
depending on the reaction conditions.10,12 The carbonate inter-
change reaction was investigated by many researchers,5 however,
since this reaction is an equilibrium reaction, sophisticated pro-
cedures, high temperatures (>100 °C) and rather complicated het-
erogeneous catalysts systems (including MCM-41-TBD,13 Mg/
La metal oxide,14 CsF/α-Al2O3,15 nano-crystalline MgO16 and
metal–organic frameworks17) have been applied to shift the equi-
librium towards the desired product. Furthermore, although
unsymmetric organic carbonates, compared to symmetric ones,
are more useful synthons, the synthetic routes are becoming even
more complex. On the other hand, organocatalysts are steadily
approaching the performance of organometallic catalysts and
enzymes. Therefore, in an effort to develop a sustainable and
selective unsymmetric organic carbonate synthesis, 1,5,7-triaza-
bicyclo[4.4.0]dec-5-ene (TBD) can be proposed as an alternative
However, the methods based on phosgene produce a high
quantity of chloride salts as side products and phosgene itself is
Karlsruhe Institute of Technology, Institute of Organic Chemistry, Fritz-
Haber-Weg-6, Building 30.42, 76131 Karlsruhe, Germany. E-mail:
m.a.r.meier@kit.edu; http://www.meier-michael.com;
Fax: (+49)-721-608-46800; Tel: (+49)-721-608-48326
†Electronic supplementary information (ESI) available: General pro-
cedures, characterization data of the products are provided. See DOI:
10.1039/c2gc35191a
‡This paper is dedicated in memoriam to Tommy Tóth.
1728 | Green Chem., 2012, 14, 1728–1735
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catalyst. TBD represents the prototype of a group of strong gua-
nidine bases,18 which have been previously utilized as organoca-
talysts in many transformations for the synthesis of fine
chemicals19 and the polymerizations of diverse monomers, either
via polycondensation20 or ring-opening polymerization.21 In
addition, along the advantages of ease of handling and mildness
of reaction conditions, the recently developed simple and techno-
logically convenient synthesis of TBD using inexpensive and
non-toxic chemicals,22 make it a catalyst of choice.
It is well know that carbonate linkages within a polymer chain
introduce a spectrum of properties such as reduced biodegrada-
tion time and enhanced mechanical performance.23 Moreover,
symmetric organic carbonates, especially long-chain ones, find
application in lubricant, cosmetic, plasticizer and fuel compo-
sitions.24 For all these reasons, the development of new phos-
gene- and metal-free synthetic routes to polycarbonates and low
molecular weight organic carbonates is of major interest. Thus,
we report here the reactivity of DMC in the presence of commer-
cially available TBD at low catalyst loadings (1.0 mol%), which
allowed us to set up a simple and mild approach to the synthesis
of, on one hand, a wide range of symmetric and unsymmetric
carbonates, and on the other hand polycarbonates from different
renewable diols. We also present the synthesis of castor oil and
citronellol derived α,ω-diene symmetric carbonates, which could
be further polymerized via acyclic diene metathesis (ADMET).25
methoxycarbonylating agent, and at higher temperatures acts pri-
marily as a methylating agent.10 Furthermore, high temperatures
favour the elimination of CO2 from the organic carbonate, thus
facilitating the formation of the corresponding ether. In this case,
we found that the best results in terms of yield and selectivity
were achieved by performing the reactions at 80 °C.
A number of different alcohols were chosen to evaluate the
scope of the reaction; the first experiments were carried out
using a simple primary alcohol (1-octanol) in order to optimize
the reaction conditions. In a typical experiment, an excess of
DMC was reacted with the alcohol in the presence of TBD. The
product distribution was monitored by GC and GC-MS analysis
and in some cases by NMR. The results obtained from these
experiments are summarized in Table 1. While investigating the
effect of the DMC/alcohol (DMC/ROH) ratio at 80 °C using a
catalyst loading of 1.0 mol% (to alcohol), it became clear that
the rate of the reaction was somewhat reduced if the DMC/
alcohol ratio was increased to 12 : 1 (Table 1, entry 4); in other
words, the use of a larger excess of DMC did not further
improve the yield of the reaction. It was thus necessary to
reach a compromise between the amounts of unsymmetric and
symmetric carbonate (undesirable product in this case) obtained
in the reaction. Thus, we chose a DMC/ROH ratio of 5 : 1 as the
one providing the best relation between yield of unsymmetric
carbonate and excess of DMC, and at the same time the lowest
amount of symmetric carbonate. Moreover, longer reaction times
did not improve the conversion, since the competitive formation
of the symmetric carbonates began to be more pronounced.
In order to enhance the selectivity, different catalyst amounts
of TBD in the presence of constant molar ratio of DMC/ROH
(5 : 1) were investigated. For example, catalyst loadings greater
than 1.0 mol% led to rapid reaction; the reaction was complete
in 15 min at a catalyst loading of 5.0 mol% (entry 5, Table 1).
In this case the selectivity for the desired unsymmetric carbon-
ate was 91%; higher catalyst loadings only marginally improved
this result. On the other hand, decreasing the catalyst loading to
0.5 mol% did not result in comparably better selectivity
(compare entries 2 and 6 in Table 1). 1.0 mol% TBD loading
thus seemed to be a good compromise in terms of conversion
and selectivity. It is well known that the removal of methanol
strongly determines the efficiency of TBD-catalyzed transesterifi-
cation; consequently, a model reaction was carried out under
Results and discussion
Unsymmetric organic carbonate synthesis
As discussed in the introduction, most of the catalytic systems
for the synthesis of organic carbonates require the toxic and
hazardous reagent phosgene at some stage. In order to address
this problem, several groups used dimethyl or diethyl carbonate
as a substituent for phosgene.10 Moreover, organocatalysis may
provide a way to make current chemical processes more environ-
mentally benign due to the avoidance of toxic transition metal
catalysts. Understanding the evolution of organocatalysts reveals
TBD as a potent catalyst for enhancing the performance of
certain reactions. Thus we studied the direct condensation of an
alcohol and dimethyl carbonate using TBD in a homogenous
organocatalytic reaction (Scheme 1). No additional solvents were
required for this reaction.
First, it was important to set the proper reaction temperature,
since, as aforementioned, DMC exhibits a versatile and tuneable
chemical reactivity that depends on the experimental conditions,
especially on the reaction temperature. Although there is no clear
cut-off, it is known that in the presence of nucleophiles at the
Table 1 Carbonylation of 1-octanol under different reaction conditions
TBD
Entry DMC/ROH (mol%) Time
Conv.a (%) Sel.b (%)
1
2
3
4
3 : 1
5 : 1
10 : 1
12 : 1
5 : 1
5 : 1
5 : 1
5 : 1
1.0
1.0
1.0
1.0
5.0
0.5
1.0
1.0
1 h
1 h
1 h
1 h
96
>98
97
96
>98
83
95
97
98
91
93
89
92
reflux temperature (T
∼
90 °C), DMC acts as
a
5
15 min
1 h 30 min >98
15 min
30 min
6
7c
8d
>98
>98
a Conversions were calculated by GC analysis using tetradecane as
internal standard. b The selectivity towards the unsymmetric carbonate.
c Reaction performed under continuous flow of argon. d Reaction carried
out in the presence of molecular sieves 4 Å.
Scheme 1 Reaction of an alcohol and dimethyl carbonate in the pres-
ence of TBD.
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fixed reaction conditions (DMC/ROH = 5 : 1, 1.0 mol% TBD,
80 °C) applying a continuous stream of an inert gas or molecular
sieves, respectively (entry 6 and entry 7 in Table 1).
These results revealed that no clear increase in the rate of the
reaction can be achieved using such conditions. However, the
selectivity in both the continuous gas flow and addition of mol-
ecular sieves reactions decreased due to the formation of the
symmetric carbonate.
The same experimental conditions were adopted for the syn-
thesis of the unsymmetric carbonates presented in Table 2, in
order to evaluate potential differences in the reactivity of differ-
ent alcohols. Alkyl methyl carbonates (n = 3–5), especially butyl
methyl carbonate, are suitable as co-solvents in lithium-ion bat-
teries.26 Thus, a clean and quantitative alternative synthesis of
butyl methyl carbonate is a matter of interest. In accordance,
butyl methyl carbonate was synthesized in relatively short time
and with 91% selectivity.
Catalytic cross-coupling reactions of allylic and propargylic
compounds are used for the formation of carbon–carbon and
carbon–hetero atom bonds;27 for this reason we investigated the
usefulness of our approach for the synthesis of unsymmetric
allylic and propargylic carbonates. Initial attempts were per-
formed to synthesize the unsymmetric carbonates of primary
alcohol derivatives (Table 2, entries 5–7). After optimization, the
maximum conversion for the synthesis of allyl methyl carbonate
was 88% with a DMC/ROH molar ratio of 7.5 : 1 in 1 h, with
quite high selectivity (94%). Prolonging the reaction times
resulted in the formation of the symmetric product. Applying
high molar ratios of DMC/ROH (such as 10 : 1, 15 : 1 and 20 : 1)
did not result in higher conversions. However, these results (88%
conversion, 94% selectivity) are very useful for synthetic pro-
cedures and are by far less toxic than usually applied synthetic
routes (i.e., the use of chloroformates). Moreover, the tendency
of substituted allylic alcohols to afford better yields than the
Table 2 Synthesis of various unsymmetric carbonates via TBD catalysed transesterification of DMC
Entrya
Product
DMC/ROH
5 : 1
Time
30 min
30 min
2 h
Conv.b (%)
Sel.b (%)
Yieldc (%)
1
2
3
98
99
98
91
93
97
85
89
93
5 : 1
5 : 1
4
5 : 1
30 min
1 h
99
88
98
98
80
99
97
94
94
92
94
98
94
80
92
89
75
94
5
7.5 : 1
5 : 1
6
40 min
2 h
7
5 : 1
8d
9
7.5 : 1
5 : 1
6 h
5 h
10e
11
5 : 1
5 : 1
5 : 1
5 : 1
3 h
20 h
4 h
4 h
98
98
98
98
97
99
98
97
93
95
93
89
12
13
14
15
7.5 : 1
5 : 1
8 h
3 h
99
99
97
94
93
90
16f
17
7.5 : 1
5 : 1
48 h
86
99
99
96
82
95
20 min
a All reactions were carried out with 15 mmol of the corresponding alcohol and 0.15 mmol (1.0 mol%) of TBD unless stated otherwise. b Conversions
and selectivity were calculated for crude reaction mixtures via 1H NMR (300 MHz) and/or GC and GC-MS with tetradecane as internal standard.
c Isolated yields obtained after purification according to methods mentioned in the ESI.† d 5.0 mol% Catalyst was used instead of 1.0 mol% TBD.
e Molecular sieves were added. f 10.0 mol% Catalyst was used instead of 1.0 mol% TBD.
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allyl alcohol was remarkable (compare results in entries 5 and 6
in Table 2). It should also be noted that divinyl carbinol
(Table 2, entry 13), a prochiral and bis-allylic alcohol, provided
the desired product with relatively good conversion and selectiv-
ity within 4 h. Further, the carbonylation of propargyl alcohol
proved to be more difficult; only a higher molar ratio of DMC/
ROH (7.5 : 1) in combination with a higher catalyst loading
(5.0 mol% TBD) resulted in a satisfactory conversion (entry 8,
Table 2).
Besides this, another interesting example was the carbonyla-
tion of poly(ethylene glycol) methyl ether (mPEG–OH, Mn ∼
500 Da). The covalent modification of biological macromol-
ecules and surfaces for many pharmaceutical and biotechnical
applications is accomplished by the coupling of PEG to the
peptide or protein.28 Hence, an important aspect in this process
is the incorporation of various PEG functional groups, like anhy-
drides, chloroformates and carbonates, which can easily conju-
gate to the protein. Within this in mind, mPEG–OH was
successfully carbonated to the corresponding unsymmetric
organic carbonate (entry 9, Table 2) with quantitative conversion
and high selectivity.
no production of acid components upon in vivo implantation.
Moreover, the use of renewable diols is desirable. Therefore, one
of the chemical precursors for the aliphatic polycarbonates was
1,3-propanediol (D1), which can be obtained from renewable
resources through economical and sustainable processes such as
microbial fermentation.30 1,6-Hexanediol (D2) and the fatty acid
derived (E) icos-10-ene-1,20-diol (D3) were investigated as
alternative diols. Industrially, 1,6-hexanediol is prepared by the
hydrogenation of adipic acid and since the first pilot plant produ-
cing adipic acid via a fermentation process using non-food,
plant-based feedstock was recently set up, D2 can be considered
as (potentially) biomass derived as well.31
TBD catalyzed polycondensation between DMC and the
respective diol occurred in two steps: (a) hydroxyl and carbonate
end groups reacted with the elimination of alcohol (MeOH) to
yield the oligomers; (b) transesterification between two carbon-
ate end groups with elimination of DMC. A representative pro-
cedure for the synthesis of polymers is depicted in Scheme 2.
Based on the results obtained for the synthesis of the unsym-
metric carbonates, the initial polymerization attempts were per-
formed with 1.0 mol% TBD related to the diol molecule.
Solvent-free conditions were chosen to avoid solvent waste,
enhance conversions, and reduce the formation of cyclic struc-
tures. Whereas conventional catalytic polycondensation reactions
of AA–BB type monomers require 1 : 1 feed ratio, the 1st step
(Scheme 2) was performed under atmospheric pressure at 80 °C
in bulk using a DMC/diol molar ratio of 2 : 1. In this way, we
prevented the loss of DMC that would shift the reaction stoichi-
ometry. Furthermore, in this type of polymerization, low DMC
feed ratios yield low molecular weight chains with hydroxyl
terminal groups.32 However, with sufficient excess of DMC,
chain growth would continue by the reactions occurring both
between hydroxyl and methyl carbonate as well as between two
terminal methyl carbonate groups (with release of DMC). On the
other hand, if the DMC/ROH ratio is quite high, i.e., using a 4 to
1 stoichiometry, the terminal groups of the polymer would
entirely be methyl carbonate moieties and thus the polymeriz-
ation rate would be quite slow. In summary, for the propagation
to continue, a substantial fraction of chain ends must be methyl
carbonate moieties.
The behaviour of a few representative and less reactive sec-
ondary alkyl and aryl alcohols was also studied (Table 2, entries
10–15).
All substrates were transformed to the corresponding unsym-
metric carbonates in comparably good selectivity to the pre-
viously reported methods. Also in the case of homoallylic,
secondary alcohol, 4-phenyl-1-buten-4-ol (entry 15, Table 2),
almost quantitative conversion was obtained. Of special note is
that the tert-butyl methyl carbonate (entry 16, Table 2), which is
useful as an octane enhancer for gasoline,5 was selectively
obtained in 86% conversion within 48 h. Although it was
necessary to use a molar ratio DMC/ROH of 7.5 : 1 and catalyst
loading of 10.0 mol%, the procedure is still advantageous in
comparison to the method employing inorganic base (cesium
carbonate) with phase transfer catalyst (PEG-2000) under
pressure at 125 °C, in which the final yield was just 43%.29
Additionally to all these monoalcohols, a potential platform mol-
ecule for fine synthesis, glycerol, was tested as well under the
specified conditions: DMC/ROH molar ratio 5 : 1 and 1.0 mol%
TBD (Table 2, entry 17). When the reaction was conducted for
20 min, the synthesis of glycerol carbonate with conversion of
glycerol of >99.9% and selectivity of 96% was observed.
To favour the formation of the oligomers, continuous argon
flow was applied within the first step. Online monitoring by
GPC and/or NMR was performed on the crude reaction samples.
From these results (Table 3), it was clearly observed that the
Polycarbonate synthesis via TBD mediated polycondensation
Having demonstrated the efficiency of carbonylation in the pres-
ence of TBD as organocatalyst for the synthesis of several
unsymmetric organic carbonates, we investigated the polymeriz-
ation activity of TBD in the presence of diols with different
chain length and DMC. Aliphatic polycarbonates are important
precursors for the preparation of novel polyurethanes and are
conventionally synthesized either by polycondensation of phos-
gene with diols, or by the transesterification of five-membered
cyclic carbonate with selected diols through heterogeneous or
lipase-based catalysis.23 In addition to the growing interest in the
synthesis of aliphatic polycarbonates for fiber and film forming
applications, aliphatic polycarbonates have attracted increasing
attention as degradable biomaterials in recent years because of
Scheme 2 One pot, two-step TBD catalysed preparation polycarbo-
nates using DMC and renewable diols.
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Table 3 Results for the synthesis of aliphatic carbonates via the
polycondesation reaction of DMC and three renewable diols in the
presence of 1.0 mol% TBD
Table 4 Optimized reaction conditions for the polymerization of D1
and D2 with different catalyst loadings
First stepa
Time/Temp.
Mnb(Da)/PDI
(Mw/Mn)
Second step
Time/Temp.
Mnb(Da)/PDI
(Mw/Mn)
First stepa
Second step
Time/Temp.
Time/Temp.
Diol/TBD
(mol%)
Entry
P1
Diol
D1
Mnb(Da)/PDI (Mw/Mn)
Mnb(Da)/PDI (Mw/Mn)
Entry
P4
1 h/80 °C
1200/1.97
2 h/80 °C
2350/2.40
3 h/90 °C
3500/2.16
1 h/90 °C
12 h/100 °C
7500/2.15
1 h/90 °C
12 h/100 °C
15 500/1.85
D2/1
1 h/80 °C
13 500/ 2.18
1 h/90 °C
1 h/100 °C
1 h/120 °C
1 h/140 °C
1 h/150 °C
16 200/2.15
3 h/90 °C
5850/1.97
1 h/90 °C
1 h/100 °C
1 h/120 °C
1 h/140 °C
1 h/150 °C
33 000/1.94
3 h/90 °C
5850/1.97
P2
D2
P3
D3
4 h/80 °C
3200/2.39
P5
P6
D1/0.5
D2/0.5
1 h/80 °C
3000/2.17
2 h/80 °C
5150/2.31
a First step performed at 80 °C in bulk under continuous flow of argon.
b Data obtained from GPC performed in THF relative to PMMA
calibration.
P7
D1/5
1 h/80 °C
2800/1.91
time required for this reaction step was determined by the chain
length of the employed diol. For the diol with shortest chain,
D1, the transesterification reaction (1st step in Scheme 2) took
∼1 h, and for D3, this was 4 h. Once a sufficient amount of oli-
gomers was formed, in an effort to facilitate the polymerization
by removing the unreacted DMC and methanol produced by the
condensation reactions and to reach high molecular weight, the
reaction temperature was increased to 90 °C with reduced
pressure.
a First step performed at 80 °C in bulk under continuous flow of argon.
b Data obtained from GPC performed in THF relative to PMMA
calibration.
The crucial point in the second step was to determine the final
temperature until which the polymerization could be carried out.
We clearly observed that the Mn and the yield of polymers syn-
thesized with D1 decreased with increasing temperature. Per-
forming the reaction at 100 °C for 1 h under vacuum resulted in
markedly decrease in Mn from 3500 Da to 2600 Da. This could
be ascribed to the evaporation of oligomers of D1 and thermal
degradation of the final polymer under these conditions. There-
fore, 90 °C was the best polymerization temperature for D1. In
contrast, 90 °C was inefficient to yield high molecular weight
polymers with D2 and D3; thus, the reaction temperature was
increased to 100 °C under vacuum. After 12 hours at 100 °C
under continuous vacuum the final Mn value for P2 was still not
high, indicating that these reaction conditions are not efficient
for D2. On the other hand, for P3 in Table 3 under the prelimi-
nary polymerization conditions and consistent with chain growth
was the observation that, by using a 2 : 1 DMC/D3 feed, the PDI
value decreases from 2.39 to 1.85 throughout the course of the
second step. This trend could be attributed to the longer reaction
times at 100 °C, which permitted the low molar mass products to
diffuse to catalyst and form products of higher molecular weight,
thereby reducing the low molecular weight fraction and decreas-
ing the PDI.
Fig. 1 GPC chromatogram for the polymers of D2 with two different
catalyst loadings, respectively 0.5 and 1.0 mol% TBD.
polymers synthesized with D2 in the presence of 0.5 and
1.0 mol% TBD, respectively (compare P6 and P4, Table 4).
These results represented a significant improvement with regards
to previously reported result for the same polymer, where
11 kDa were obtained via the polymerization of diethyl carbon-
ate with 1,6-hexanediol catalyzed by immobilized Candida
antarctica Lipase B.23 On the other hand, increasing the catalyst
loading to 5.0 mol% resulted in a decrease of the Mn values both
for D1 and D2.
Furthermore, a closer look at the polymerization results via
NMR revealed an interesting fact regarding the obtained molecu-
lar structure of the final polymer synthesized from D1. It became
evident that in the presence of high TBD loadings, the terminal
methyl carbonates were cleaved, and as a consequence of this
the formation of terminal allyl carbonate group was observed.
This clearly provided an explanation for the decrease of the Mn
in the presence of 5.0 mol% of TBD. Fig. 2 shows, in its inset,
the 1H NMR spectrum of the precipitated polymers obtained
from D1 in the presence of different amounts of TBD, in which
In order to further improve the Mn values for the polymer syn-
thesized with D2, the temperature was hourly increased to
150 °C during the second polymerization stage, which resulted
in significantly improved molecular weight (Table 4).
In further experiments, we observed that the polymerizations
of D1 and D2 were also promoted by lower loadings of TBD
(Table 4). By minimizing the catalyst loading to 0.5 mol%, for-
mation of polymer with Mn of 33 kDa for D2 was accomplished
(Table 4, entry 2). Fig. 1 shows the GPC chromatograms of the
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the effect of catalyst loading on the formation of the aforemen-
tioned cleavage can be clearly observed. Indeed, this cleavage,
leads to a allyl carbonate function displaying a different reactiv-
ity that might be further exploited in another context. Evidently,
it should be noted that the polymers obtained from D1 in the
presence of TBD are classified as poly(trimethylene carbonate),
and the proposed method can substitute the traditional synthesis
by ring-opening polymerization of 1,3-dioxan-2-one, which on
the other hand is obtained by refluxing 1,3-propanediol in an
excess of diethyl carbonate.
Moreover, decarboxylation is a side reaction which is known
to commonly occur during the synthesis of polycarbonates at
high reaction temperature.33 It is noteworthy to mention that we
did not observe the corresponding signals in our NMR spectra,
indicating that TBD does not cause such side reactions (at least
within our temperature limits).
under solventless conditions by mixing (at room temperature)
DMC and the corresponding alcohol in ratio of 2.1 : 1, along
with a catalytic amount (1.0% mol) of the guanidine base TBD
and subsequent heating to 80 °C.
The synthetic results of the reactions are presented in Table 5.
Generally speaking, this reaction gave very good conversions
with all tested alcohols allowing us to obtain the corresponding
symmetric organic carbonates as the only product.
No product arising from the methylation of the alcohol was
ever observed under these reaction conditions. However, as was
anticipated, in the case of benzyl alcohol the reaction was slower
and afforded the desired product only after 45 h. Moreover, the
carbonate interchange reaction of citronellol with DMC led to a
new terpenoid, which could be further investigated as a potential
monomer for olefin metathesis polymerization. The non-conven-
tional step-growth polymerization, acyclic diene metathesis
(ADMET) polymerization, can be considered as a versatile route
for the access of a wide range functionalized hydrocarbon poly-
mers with carbon–carbon double bonds25 since the catalysts
employed are tolerant towards many functional groups and the
reaction conditions are quite mild. Moreover, we established
ADMET as an efficient and successful method for the transform-
ation of rapeseed and fatty acid-derived monomers to a number
of biomass based biodegradable and non-biodegradable poly-
mers.20,34 Thus, ADMET can be considered as a method for the
synthesis of strictly linear unsaturated aliphatic polymers with
precise control of backbone functionalities. For this reason, the
α,ω-diene monomer M1 (see Table 5), obtained by the diaddi-
tion of 10-undecen-1-ol, was used for polycarbonate synthesis in
a step-growth fashion via ADMET polymerization. In order to
study the molecular weight variations of the resulting polymers,
the first experiments (Table 6) were performed in bulk at 80 °C
under continuous vacuum for 1 h using three different metathesis
catalysts: Grubbs catalyst 2nd generation (C1), Hoveyda-Grubbs
catalyst 2nd generation (C2) and Zhan 1B catalyst (C3)
Symmetric organic carbonate synthesis and subsequent
ADMET studies of representative monomers
As an extension, we turned our attention to the utilization of this
approach for the synthesis of symmetric organic carbonates
which possess terminal double bonds and can thus be employed
as monomers in olefin metathesis polymerization. 10-Undecen-
1-ol and citronellol are two biomass derived alcohols and thus
the final symmetric organic carbonates can be considered as
renewable building blocks for the synthesis of aliphatic biobased
linear polycarbonates. In addition, to assess the scope of the
symmetric organic carbonate synthesis, the reactivity of allyl
alcohol, benzyl alcohol and the chiral, homoallylic, secondary
alcohol, 4-phenyl-1-buten-4-ol, were investigated. It is worth to
highlight the simplicity of the reaction, which was performed
Table 5 Selected results for the synthesis of symmetric organic
carbonates at 80 °C
Conv.b
(%)
Sel.b
(%)
Yield.c
(%)
Entrya Product
Time
16 h
14 h
1
2
99
99
99
99
95
95
3
4
12 h
45 h
99
99
99
99
94
93
5
8 h
99
99
95
a All reactions were carried out with molar ratio DMC/ROH of 1 : 2.1
and 15.0 mmol of the corresponding alcohol in the presence of 1.0 mol
% of TBD under bulk with continuous argon stream unless stated
otherwise. b Conversions were calculated for crude reaction mixtures via
1H NMR (300 MHz) and/or GC/GC-MS with tetradecane as internal
standard. c Yields obtained after purification methods mentioned in the
ESI.†
Fig. 2 1H NMR spectra of polymers synthesized with D1 in the pres-
ence of 0.1, 0.5, 1.0 and 5.0 mol% TBD, thus revealing the effect of
TBD on the possible side reaction: cleavage of terminal methyl
carbonate.
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Table 6 Selected results for ADMET of M1 at 80 °C with three
Table 7 Selected results for ADMET of M2 with C3 at 90 °C
different catalysts
Entry
C3 (mol%)/BQ (mol%)
Mna(Da)/PDI (Mw/Mn)
Entry
Cat. (mol%)
Mna(Da)/PDI (Mw/Mn)
P11
P12
P13
P14
1.0/2.0
2.5/5.0
5.0/10.0
5.0/20.0
640/1.19
P8
P9
P10
C1 (0.2)
C2 (0.2)
C3 (0.2)
9500/1.60
13 500/1.73
27 500/1.92
1600/1.75
4100/1.86
7900/1.81
a Data obtained for crude reaction mixtures from GPC performed in
THF relative to PMMA calibration.
a Data obtained for crude reaction mixtures from GPC performed in
THF relative to PMMA calibration.
Fig. 3 GPC data for the precipitated polymer P14 (Table 7).
afforded the metathesis of 1,l-disubstituted alkenes but only
through cross-metathesis with internal olefins, which were no
more than disubstituted. Having in hand the symmetric organic
carbonate monomer M2 (Table 5, entry 3), synthesized with the
aforementioned method from citronellol and DMC in the pres-
ence of TBD, and triggered by the remarkable development of
well-defined transition-metal catalysts, especially on the basis of
highly stable Ru-complexes, which readily polymerize many
strained and hindered alkenes, we performed some test exper-
iments. Hence, considering the enhanced difficulty for the chal-
lenging polymerization of M2, the highly active Zhan catalyst
(C3 in Scheme 3) was our catalyst of choice. The initial screen-
ing showed that high temperature was necessary to achieve high
conversion. Consequently, the reaction was directly attempted
under solvent free conditions at 90 °C with 1.0, 2.5 and 5.0 mol
% C3 and the respective BQ amount (2.0 equivalents to the cata-
lyst) was added prior the catalyst addition. As already reported,
vacuum or continuous gas flow is a requirement for a successful
release of ethylene during the olefin metathesis polymerization
reactions. However, in case of the ADMET polymerization of
M2, tetramethylethylene is the condensate. Therefore, to further
accelerate the release of this compound (boiling point = 73 °C),
continuous vacuum was employed. Our results indicated that 1.0
and 2.5 mol% C3 were inefficient to polymerize (compare
results in Table 7) M2 and that the used amounts of BQ were
not enough to prevent the isomerisation occurring during the
ADMET reactions of this diene (observed by GC-MS and
NMR). This phenomenon is discussed in details in the exper-
imental part. On the other hand, a catalyst loading of 5.0 mol%
C3, along with 20.0 mol% of BQ, was efficient enough to yield
Scheme 3 The used metathesis catalysts and the GPC chromatogram
of M1 derived P10 in the presence of C3.
(Scheme 3). The precise control of the backbone functionality
can be interrupted with the possible side reaction: olefin isomeri-
sation of the terminal double bonds. Thus, benzoquinones
(especially 1,4-benzoquinone, BQ), which are very effective
additives for the prevention of the olefin isomerization35 were
added to the reaction mixture prior to the catalyst addition. The
analytic data of the polymers synthesized is summarized in
Table 6 and selected GPC traces are depicted in Scheme 3.
Monomer conversion was quantitative as determined by the
complete disappearance of the monomer signal in the GPC
traces of the reaction mixtures. The results clearly indicated that
M1 can be successfully polymerized in the presence of 0.2 mol
% C3, reaching Mn values of 27 kDa.
Nowadays of great interest is the applicability of terpenes as
renewable raw materials for the synthesis of new chemical inter-
mediates via different chemical transormations.36 Apart from
this, a considerable number of terpene derivatives have been
tested as polymer precursors, using different polymerization
mechanisms.37 Wagener and Konzelman investigated the reactiv-
ity of 1,1-disubstituted and trisubstituted olefins towards
ADMET in the presence of the highly active Lewis acid free
Schrock alkylidenes type catalyst.38 However, all attempts to
polymerize failed; the tungsten-based catalyst was unable to
promote metathesis chemistry with any of the mentioned substi-
tuted olefins. On the other hand, the molybdenum-based catalyst
1734 | Green Chem., 2012, 14, 1728–1735
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Table 7). The GPC chromatogram of the precipitated polymer is
represented in Fig. 3. The GPC monitoring of the reaction
revealed that the present reaction conditions provided a polymer
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Conclusion
The herein reported method, which is very easy to implement
and incorporates many features of green chemistry, such as clean
synthesis and the use of less toxic reactants, permits the synthesis
of unsymmetric carbonates from the parent alcohols under
solvent-free conditions with good selectivity. The influence of
several reaction parameters such as amount of DMC, catalyst
loading and reaction time on the reaction efficiency are dis-
cussed. Furthermore, the process opens new ways for a flexible
utilisation of renewable resources for non-food value-added pro-
ducts, since we have shown the transformation of a variety of
renewable alcohols into polycarbonates via ADMET polymeriz-
ation and via classic polycondensation.
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