This	is	the	peer	reviewed	version	of	the	following	article Chem. Eur. J. 2015, 21 (16),
6115.,	which	has	been	published	in	final	form	at	[10.1002/chem.201406334].	This	
article	may	be	used	for	non-commercial	purposes	in	accordance	with	Wiley	Terms	
and	Conditions	for	Self-Archiving.	

Al(III) Catalyzed Formation of Poly(limonene)carbonate: DFT
Analysis of the Origin of Stereoregularity
Leticia Peña Carrodeguas,[a] Fernando Castro-Gómez,[a] Joan González-Fabra,[a] Carles Bo*[a,b] and
Arjan W. Kleij*[a,c]

[a]

[b]

[c]

L. Peña Carrodeguas, Dr. F. Castro-Gómez, J. González-Fabra,
Prof. Dr. C. Bo, Prof. Dr. A. W. Kleij
Institute of Chemical Research of Catalonia (ICIQ), Av. Països
Catalans 16, 43007 Tarragona, Spain
E-mail: akleij@iciq.es
Prof. Dr. C. Bo
Departament de Química Física i Inorgànica, Universitat Rovira i
Virgili, Marcel·lí Domingo s/n, 43007 Tarragona, Spain
Prof. Dr. A. W. Kleij
Catalan Institute of Research and Advanced Studies (ICREA), Pg.
Lluís Companys 23, 08010 Barcelona, Spain
Supporting information for this article is given via a link at the end of
the document.

Abstract: Amino-triphenolate derived Al(III) complexes combined with suitable nucleophiles have been investigated as binary catalysts for
the coupling of limonene oxide and carbon dioxide to afford alternating polycarbonates. These catalysts are able to produce stereo-regular,
perfectly alternating trans-polymers from cis-limonene oxide, whereas the pure trans isomer and cis/trans mixture give rise to lower degrees
of stereo-regularity. The best Al(III) catalyst shows the potential to mediate the conversion of both stereo-isomers of limonene oxide with high
conversion levels of up to 71% under neat conditions indicating a high robustness and atom-efficiency of this catalytic process. Computational
studies have revealed unique features of the binary catalyst system among which is the preferred nucleophilic attack on the quaternary
carbon centre in the limonene oxide substrate.

Introduction
Currently there is a high demand for chemical processes that enable the conversion of renewable feed stocks into value-added
chemicals as to increase the overall sustainability of our societies.[1] In this regard, the use of carbon dioxide as a carbon resource
has attracted much interest during the last decade and substantial progress has been made to use this synthon in organic
synthesis.[2] The co-polymerization of epoxides and CO2 is a successful and relevant example of using a renewable carbon feed
stock and converting it into a material of widespread commercial and academic interest.[3] However, chemical processes leading to
such CO2-based materials still depend on petroleum-based feed stocks despite the impressive advancements made in this area
using various types of epoxide monomers.[4,5]
The use of bio-renewable based monomers such as limo-nene oxide has received far less attention, and we are aware of only a
few reports dealing with the successful co-polymerization of this monomer with CO2.[5] (R)-Limonene is a naturally occurring terpene
that is available in large amounts, and the epoxidized form is commercially available at low cost as a mixture of cis and trans isomers
(Figure 1; A and B). Its structural resemblance to cyclohexene oxide (the most widely used epoxide in copolymerization reactions
with CO2) makes it thus an ideal target to provide a cost-effective, bio-based polycarbonate from a renewable feed stock. Though,
some important challenges remain to be solved: limonene oxide represents an internal, tri-substituted epoxide monomer and the
kinetic barrier for its activation is significantly higher than for terminal epoxides.[6] There are only relative few reports describing the
efficient conversion of internal epoxides into their respective organic carbonates,[7-11] and the development of (more) powerful
catalytic systems[7] is thus of vital importance to solve these synthetically more challenging preparations that involve such monomers.

Figure 1. Schematic structures of the amino-trisphenolate metal complexes 1-4 used in this work, and cis and trans (R)-limonene oxide.

We recently developed Fe(III)[8] and Al(III)[9,11] amino-triphenolate complexes 1-4 (Figure 1) that show high efficiency in the
formation of functional organic carbonates from both terminal and internal epoxides. Further to this, we also found that these
complexes have the intrinsic ability to switch between penta- and hexa-coordination,[11] a feature that may be useful in the creation of
flexible coordination behavior around the metal ion in the presence of sterically more demanding substrates.[12]
In this work we report on a rare case of a catalytic system able to mediate the efficient and stereo-selective formation of
poly(limonene)carbonate, a tri-substituted oxirane monomer. Further to this, high conversion of the monomer limonene oxide can be
attained (up to 71%) with the cis isomer reacting significantly faster than the trans analogue. The rationale behind these observed
reactivity patterns has been investigated in detail using density functional theory (DFT) providing unique insight into the operative
modus of the catalyst system. Catalysts as the one reported herein pave the way for the conversion of other naturally occurring,
renewable compounds into valuable chemicals including biopolymers that are derived from epoxide/CO2 couplings. The development
of such bio-based polymers may give useful alternatives for existing, environmentally less attractive and potentially toxic copolymers,
of which bis-phenol A (BPA) based ones are most prominent.

Results and Discussion
Screening studies
Our previous results using amino-triphenolate complexes as catalysts for poly(cyclohexene)carbonate synthesis[12] prompted us to
evaluate their catalytic efficiencies in the co-polymerization of a commercially available mixture of cis/trans (40:60 by GC) (R)limonene oxide (see Table 1). Various combinations of catalysts (1-4; Scheme 1) and co-catalysts (nucleophiles abbreviated as Cl,
Br and I) and their relative ratios were probed in a screening phase. At 42ºC complex 1 (entry 1) exhibited no catalytic activity,
whereas complex 2 (entry 2) gave a reason-able conversion of 20% furnishing a copolymer with high chemo-selectivity (CO2 linkages
>99%) and high trans incorporation (78%). In contrast to the Zn(BDI) catalyst reported by Coates,[5a] 2 shows lower activity but does
maintain activity throughout a long period of time (entries 3-8). The presence of the Al-complex is a requisite for the synthesis of
poly(limonene)carbonate as the use of co-catalyst alone (PPNCl = Cl; entry 9) does not lead to any observable conversion.
In general, Al-complex 2 proved to be the most efficient mediator of the copolymerization reaction and co-catalyst Cl (PPNCl) and
Br (PPNBr) the best co-catalysts with the former one leading to a higher percentage of trans units in the polymer product; (co)catalyst loadings required for efficient catalysis were 0.5-1.0 mol%. Longer reaction times provided higher conversion levels, but
increasing temperatures (entries 16 and 17) had no favorable effect on the activity/selectivity.[13] As the reaction proceeds with time
(entries 3-6) it can be noted that the amount of trans units in the polymer product stays stabile around 70%. This result suggests that
both trans- (A) and cis-(R)-limonene oxide (B) are converted by binary catalyst system 2/Cl. Interestingly, the 1H NMR spectra of the
isolated polymer samples display three separate peaks (δ = 5.05, 5.08 and 5.12 ppm, respectively: see Figure 2 and the Supporting
Information for more details) for the CH groups located near the carbonate linkages (colored blue in the scheme of Table 1). The
peaks at 5.05 and 5.12 ppm were previously assigned to different regio-related trans units in the poly(limonene)carbonate (Figure
above Table 1),5 whereas the signal at 5.08 ppm was tentatively assigned to the presence of cis units (Figure 2).
This screening study shows that aminotriphenolate complex 2 in combination with a suitable nucleophile additive (PPNCl) affords
active and robust catalyst systems for limonene oxide/CO2 couplings with high selectivity for the copolymer.

Table 1. Co-polymerization of (R)-limonene oxide and CO2 using complexes
[a]
1-4 and various nucleophilic PPN-based co-catalysts.

Entry

Cat
[mol%]

Co-cat
[mol%]

T
b
[ºC]

t
[h]

Conv
[c]
[%]

Trans
[d]
(%)

1

1 (0.5)

Cl (0.25)

42

18

0

-

2

2 (0.5)

Cl (0.25)

42

18

20

78

3

2 (1.0)

Cl (0.50)

42

4

4

77

4

2 (1.0)

Cl (0.50)

42

6

16

68

5

2 (1.0)

Cl (0.50)

42

24

50

72

6

2 (1.0)

Cl (0.50)

42

48

60

70

7

2 (1.0)

Cl (0.25)

42

48

53

74

8

2 (0.5)

Cl (0.25)

42

48

47

72

9

−

Cl (1.0)

42

48

0

−

10

2 (1.0)

Cl (0.5)

42

12

28

68

11

2 (1.0)

Br (0.50)

42

12

35

74

12

2 (1.0)

I (0.50)

42

12

3

97

13

3 (1.0)

Cl (0.50)

42

24

31

74

14

4 (0.5)

Cl (0.25)

42

18

5

85

15

2 (1.0)

Cl (1.0)

42

24

18

76

16

2 (1.0)

Cl (1.0)

60

24

18

67

17

2 (1.0)

Cl (1.0)

70

24

11

67

18

2 (1.0)

Br (1.0)

45

24

23

77

[a] Conditions: cis/trans (R)-limonene oxide (1.0 g, 6.57 mmol), metal complex
(quantity indicated), co-catalyst (quantity indicated), 10 bar initial CO2
pressure, 30 mL autoclave, neat. [b] Internal temperature inside the autoclave.
1
[c] Determined by H NMR of the crude reaction mixture with amount of
1
carbonate linkages >99%. [d] Total % trans in co-polymer determined by H
NMR using signal integration, see Supporting Information for more details.

Optimization of the Copolymerization Reaction.
In order to get a better understanding of the origin of the presence of trans and cis units in the isolated poly(limonene)carbonates, we
then used pure trans and cis (R)-limonene oxide and investigated their behavior separately in the copolymerization with CO2, and
compared the results with those obtained for the commercial mixture of cis/trans limonene oxide (Table 2).

1

Figure 2. (a) Amplification (4.30-5.30 ppm) of a typical H NMR spectrum of an isolated sample of poly(limonene)carbonate containing two types of trans units as
well as one cis unit. Conditions: catalyst 2 (1.0 mol%), PPNCl (0.5 mol%), 42ºC, 18 h, p(CO2) = 10 bar, mixture of A and B as substrate; (b) Comparative NMR
trace for a copolymer obtained from pure cis limonene oxide, for conditions see entry 8, Table 2. Note that the peaks at δ = 5.05, 5.08 and 5.13 ppm relate to the
blue-coloured H´s in the polymer figure. Further details are provided in the Supporting Information, Figures S4-S22.

The copolymerization of a commercial mixture of trans and cis limonene oxide (A/B) with CO2 in a conventional autoclave reactor
furnishes poly(limonene)carbonate of relatively low molecular weight (entries 1-3; Mn around 3.0 Kg·mol-1) and the total reaction time
seems to have little effect on the polymer properties (cf, entries 1-3 and 5-6), although some depolymerization of the formed
polycarbonate cannot be ruled out completely. The total amount of trans units in the formed polycarbonate was followed in time and
showed a stable progress in time amounting to about 70-75%.
Interestingly, when more strict anhydrous conditions were used (entries 4-6; Fischer-Porter reactor, see Supporting Information,
Figures S2 and S3) the molecular weight values increased up to 6.7 Kg·mol-1, indicating that chain transfer reactions by trace
amounts of water are likely responsible for this difference.[14] This assumption was supported by end-group inspection of the MALDITOF spectra recorded for these polymers (Supporting Information, Figures S23-S26). In all cases where the commercial mixture A/B
of limonene oxide was used as substrate, the stereo-regularity of the produced copolymer was similar. Then, pure trans (A) and cis
(B) limonene oxide were probed under similar polymerization conditions (entries 7-13) to assess whether the type of substrate has
any influence.
Table 2. Co-polymerization of pure trans (A), pure cis (B) or a mixture of
trans/cis (R)-limonene oxide (A/B) and CO2 using complex 2 and PPN-Cl (Cl).
[a]
S refers to the epoxide substrate.

Entry

S

Cl
[mol%]

t
(h)

Conv
[b]
[%]

Cis/Trans
[c]
[%]

n
[d,e]

M

Mw/Mn
[d]

1

A/B

0.5

12

37

25:75

2.9

1.33

2

A/B

0.5

24

47

24:74

3.1

1.38

3

A/B

0.5

48

60

30:70

2.4

1.49

4

[f]

A/B

0.25

48

53

24:76

6.7

1.55

5

[f]

A/B

0.5

24

31

25:75

5.2

1.42

6

[f]

A/B

0.5

87

54

26:74

5.5

1.47

7

A

0.5

60

59

27:73

3.1

1.28

8

B

0.5

24

67

7:93

7.0

1.32

9

[f]

B

0.5

24

71

2:98

5.9

1.40

10

[f]

B

0.25

48

53

9:91

5.8

1.43

11

[f]

B

0.5

8

27

4:96

4.8

1.35

12

[f,g]

B

0.5

24

54

5:95

9.1

1.48

13

[f,g]

B

0.5

24

49

4:96

10.6

1.43

14

[h]

A/B

3.0

34

42

>95

i

n.a.

n.a

15

h

A

3.0

64

29

>95

i

n.a.

n.a

16

h

i

n.a.

n.a

B

3.0

64

3

n.d.

[a] Conditions: cis/trans, cis or trans (R)-limonene oxide (1.0 g, 6.57 mmol),
Al-complex 2 (1 mol%), co-catalyst (quantity indicated in mol%), 10 bar initial
CO2 pressure, 30 mL autoclave, 42ºC, neat; S stands for the type of
1
substrate. [b] Determined by H NMR of the crude reaction mixture with
amount of carbonate linkages >99%. [c] Total % trans in co-polymer
1
determined by H NMR using signal integration, see Supporting Information
for more details. [d] Determined by GPC, see Supporting Information for
-1
details. [e] In Kg·mol . [f] Reaction carried out in a Fischer-Porter reactor at 5
bar/45ºC. [g] Different stirring technique applied (see Supporting Information).
[h] Only co-catalyst PPN-Cl used at 90ºC; only cyclic limonene carbonate
observed. N.a. = not applicable, n.d. = not determined.

Remarkably, whereas the use pure trans limonene oxide A (entry 7) results in the formation of a low molecular weight copolymer,
the use of the pure cis isomer B gives substantially higher molecular weight material (entry 8) and its conversion is significantly faster.
Moreover, the resultant copolymer based on pure B shows a higher degree of stereo-regularity (92% trans units, one type) compared
with the copolymer based on pure trans A (73% trans units). This implies that the coupling of pure cis limonene oxide is more stereoselective and suggests that nucleophilic attack of the chloride is surprisingly favored on the most substituted carbon center of the
oxirane unit in B. The use of more strict anhydrous conditions failed to give improved polymer properties when using pure B as
substrate (entries 9-11) despite varying both the co-catalyst concentration and reaction time. However, when a different stirring
technique (Figure S3)[15] was applied in the reactor (entries 12 and 13), improved molecular weights of the resultant copolymers could
be achieved up to an appreciable 10.6 Kg·mol-1 (Figure S30) while maintaining similar polydispersities (Mw/Mn values) around 1.4.
Finally, attempts to convert limonene oxide (A/B mixture of isomers or pure isomers, entries 14-16) in the absence of catalyst 2 were
done. The highest conversion was achieved with the commercial mixture of limonene oxide (at 90ºC), whereas the use of pure A
(29%) and pure B gave much lower conversion. In these latter reactions full selectivity for the cyclic limonene carbonate was noted
(Figures S21, S22 and S32), emphasizing the crucial role of catalyst 2 to form the copolymer under much milder conditions.
The properties of the obtained poly(limonene)carbonates were further assessed by TGA and DSC analysis.[16] The copolymers of
lower molecular weight (e.g., entry 9, Table 2, cis monomer B used; Mn = 5.9 Kg·mol-1, Mw/Mn = 1.40; Figure S33) shows an onset
temperature for decomposition around 180ºC (Ta = 241ºC) measured by TGA, whereas higher molecular weight samples (cf., entries
12 and 13; Figure S34) display lower midpoint temperatures Ta of 204 and 199ºC, respectively. As for glass transition behavior, the
Tg values among the polymer samples derived from a mixture of isomers A/B (entry 5, Table 2; Figure S35) or pure B (entry 9, Table
2; Figure S36) both isolated after 24 h using similar reactions conditions were rather similar (Tg = 78 and 81ºC, respectively). The
highest molecular weight sample from entry 13 showed the highest Tg value around 112ºC (Figure S38) comparable to what was
found previously by Coates et al. for the poly(limonene)carbonate derived from the trans substrate A.[5a]
Our results contrast in some aspects with those reported by Coates et al.[5a] in the sense that (1) lower activities are noted for the
binary catalyst 2/Cl, (2) in the presence of this latter binary catalyst system the cis-isomer of limonene oxide reacts significantly faster
than the trans one, whereas for the Coates system this isomer could not be converted, and (3) the use of the cis-isomer B results in a
stereo-regular, virtually all trans-regular copolymer whereas the Coates catalyst is selective towards stereo-regular
poly(limonene)carbonate based on the conversion of trans isomer A. In order to get more insight into the chemo- and stereo-selective
features of the catalytic process, detailed computational studies (DFT) were carried out to evaluate the difference in reactivity
observed for isomers A and B, and the origin for the selective conversion of pure cis limonene oxide into trans
poly(limonene)carbonate (vide infra).

Computational Studies.
Considering a bimetallic mechanism,[17] we further investigated the alternating copolymerization of (R)-limonene oxide and CO2 by
using density functional theory (DFT) methods at the B97D3/6-311G**/LANL2DZ level of theory. Both initiation and propagation
reactions comprising the Al-catalyst 2 were studied as described below giving important and unique insight in the chemo and stereoselectivities observed experimentally.
Initiation reaction. The ring opening of the trans (A) and cis (R)-limonene oxide (B) was first evaluated in the presence of chloride
as well as bromide nucleophiles. This step usually involves a concerted transition state (TS1 in Figure 3) which is characterized by
the breaking of the C −O epoxide bond and the simultaneous formation of a C −Cl/Br, leading to the formation of an alkoxide
intermediate (Int-1 in Figure 3).[6,11a] The nucleophilic attack can occur at the α carbon (most substituted carbon) or the β carbon
(least substituted carbon) atoms of the cis/trans limonene oxide, and therefore eight possible ways of epoxide ring opening should be
considered. The fourth and fifth columns of Table 3 collect the activation free-energies calculated for this step, once the epoxide is
activated by the Lewis acidic aluminum center in complex 2.
α/β

α/β

Table 3. NBO population analysis and activation energies for the epoxide
[a]
ring-opening step by nucleophilic attack of Cl− and Br−.
Substrate

Carbon

NBO population
[a]
analysis

TS ring-opening
-1 [b]
[kcal·mol ]
Cl

−

Br

−

α

0.27 (0.31)

2.8

8.9

β

0.10 (0.12)

4.7

10.2

α

0.27 (0.31)

0.7

6.1

β

0.11 (0.13)

3.8

9.1

[a] The first value corresponds to the charge obtained for isolated epoxide; whereas the second (in parenthesis), for the activated substrate due to coordination
with the Al-catalyst 2. [b] Ring-opening barriers calculated from the energy difference between the transition state TS and initial complex coordination. Cl and Br
were used as a nucleophilic reagent.
−

−

In general, barriers for the chloride attack are much lower than those obtained for the bromide attack. In all the cases, the α attack
is favoured over the β one, as supported by the NBO population analysis included also in Table 3. It is commonly thought that the
most substituted α carbon is less reactive than the least hindered because of the electronic effects induced by the methyl group. In
contrast, the α attack was found to be more feasible, with the cis conformation being the preferred way over the trans substrate by
2.1 kcal·mol-1. Alternatively, for the β attack, this energy difference is less marked (0.9 kcal·mol-1) although the cis isomer still remains
favored. Because of the better results obtained using the nucleophilic chloride, this species was selected as co-catalyst for the
alternating copolymerization, and thus decreasing the number of possible pathways to study. Figure 3 illustrates the free-energy
profiles for the Al-complex 2/chloride catalyzed initiation of the copolymerization of cis/trans-(R)-limonene oxide

Figure 3. Free-energy profiles for the initiation reaction of copolymerization between cis/trans-(R)-limonene oxide and CO2 catalyzed by the Al-complex 2/chloride
binary system, and considering nucleophilic attack on the α and β positions resulting in four different pathways. As solvent, 1-hexanol was evaluated at 25 ºC.
Barriers for the backbiting reactions appear in dashed lines. Note that only the pathway involving the trans substrate A (α attack) is visualized with accompanying
schematic structures.

and CO2, taking into account the nucleophilic attack by chloride on the α and β positions of both epoxides. The starting point is the
assembly of the isolated reactants, i.e., the Al-complex 2, chloride, cis/trans epoxide and CO2 for which the total free energy is set to
0.0 kcal·mol-1. Then, the catalytic cycle begins with the coordination of each epoxide to the Al(III) center of complex 2 (allowing for
epoxide activation) yielding two different complexes IC. This process is exergonic by 9.7 kcal·mol-1 for the cis-coordinated (trace in
green) complex and 11.0 kcal·mol-1 for the trans one (trace in blue), showing thus a (slight) preference towards formation of the latter
complex.
As indicated above, the ring-opening step leads to formation of the metal-alkoxide Int-1. It can be observed that the intermediates
obtained by nucleophilic attack on the α position are energetically more stable than those involving the β attack; the α carbon is more
electrophilic than the β one, as explained before by the NBO population analysis (Table 3). It is worth noting that the α carbon is a
stereogenic center; consequently the nucleophilic attack of chloride on this carbon center evolves with inversion of configuration.
Thus, the cis-coordinated epoxide evolves into the most stable trans-product Int-1 (trace in green) and the trans one is converted into
the cis metal-alkoxide intermediate (trace in orange). For the β attack, retention of configuration holds since the nucleophilic attack
does not involve a stereogenic center.
Following the epoxide ring opening by the nucleophilic chloride, CO2 is inserted into the Al−O alkoxide bond of Int−1 via transition
state TS2. The intermolecular CO2 insertion was located to be less energetically demanding for the β-trans and α-cis pathways, with
relative barriers of 1.7 and 2.9 kcal·mol-1, respectively. The highest barrier was calculated for the α-trans pathway, having an
activation energy of 24.6 kcal·mol-1. This step affords the hemi-carbonates Int-2, which are coordinated by two oxygen atoms to the
Al(III) center. These intermediates follow the same stability trends as the preceding TS2, and could suffer isomerization (through TS-

isom) to form the linear hemi-carbonate Int-2’. In this case, the β-trans and α-cis pathways still lead to the most stable intermediates.
The isomerization of Int-2

Figure 4. Free-energy profiles for the propagation of the bimetallic copolymerization between CO2 and cis/trans-(R)-limonene oxide mediated by the Al-complex
2/chloride binary system, and considering the nucleophilic attack by the carbonyl oxygen (O[2]) on the α carbon of the epoxides. As solvent, 1-hexanol was
º
evaluated at 25 C. The barriers for the isomerization and the backbiting reactions of the initiation process appear in grey dashed lines. Note that only the pathway
involving the trans substrate A (α attack) is visualized with accompanying schematic structures (i.e., formation of a cis-cis dimer).

is rate-determining for the α-trans, β-cis and α-cis profiles, with activation barriers (calculated from the alkoxide intermediate Int-1) of
29.5, 23.1 and 25.5 kcal·mol-1, respectively. In the case of the β-trans profile, this is valid if the reaction is evaluated only until
formation of the intermediate Int-2’ (rather than taking into account the subsequent backbiting reaction as further discussed below)
involving an activation barrier of 13.7 kcal·mol-1, which is calculated from the most stable complex IC.
Having reached this point, two possible routes can be followed by these intermediates; the backbiting of the linear hemicarbonate Int-2’ yielding the (undesired) cyclic carbonate, or consecutive addition of new epoxide and CO2 monomers allowing for
propagation towards an alternating copolymer. The backbiting reaction is shown in Figure 3 and goes through TS-CC. The latter
displays features of a classical SN2 type transition state similar to the epoxide ring opening with intramolecular ring-closing and
concomitant release of the chloride nucleophile. This step requires slightly lower barriers than that involved in the isomerization
reaction, with the β-trans profile being the only exception with relative barrier of 3.6 kcal·mol-1. The alternating propagation reaction is
separately discussed in the next section and will explain why there is a preference for polycarbonate formation from limonene oxide
and CO2 using Al complex 2/PPNCl as catalyst system.
Propagation reaction.
Once the linear hemi-carbonate Int-2’ is formed in the initiation process, several attack routes can be followed.[18] The carbonyl
oxygen of the four resulting Int-2’ species serve as nucleophiles for attacking two different epoxide conformations (cis or trans) on
two different carbon atoms (α or β). This situation generates sixteen possible profiles to investigate (see Scheme S1 in the
Supporting Information). In order to decrease the computational efforts, we decided to study the most feasible pathways, based on
the outcome from the initiation reaction. Thus, the number of pathways was reduced to four by considering the attack by the most

stable α-cis and β-trans hemi-carbonates on the α carbon (the most electrophilic atom) of the cis and trans epoxides. The free-energy
profiles for the alternating propagation step of cis/trans-(R)-limonene oxide and CO2, taking into account the previous considerations,
are illustrated in Figure 4.
The propagation process requires two aluminum centers (bimetallic mechanism).[17,19] It can be firstly observed the formation of a
very stable adduct between the intermediate Int-2’ and the complex having a new epoxide substrate coordinated to another Alcomplex 2 (IC-p). Natural bond orbital (NBO) population analysis on this complex shows small difference in the value of the charge
assigned to the oxygen atoms of the carbonate Int-2’. The oxygen atom labeled O[1] bound to the Al center in Figure 4 exhibits a
charge of -0.83; whereas for the carbonyl oxygen (O[2]), a value of -0.69 was obtained. Although the oxygen O[1] is slightly more
nucleophilic than the carbonyl oxygen O[2], reaction progress through O[1] leads to the higher barriers for subsequent steps of the
free-energy profile (see Figure S39 in the Supporting Information). In all cases, a substantial lower energy between the formed IC-p
and the transition state of the backbiting reaction (TS-CC; Figure 3 and grey traces in Figure 4) was found. Thus, for instance, the
resulting complexes between the α-cis Int-2’ and the coordinated cis and trans epoxides (i.e., α-cis-cis and α-cis-trans dimers in
Figure 4) were found to be more stable than their corresponding TS-CC by 14.8 and 22.0 kcal·mol-1, respectively. In the case of the
β-trans carbonate Int-2’ forming an initial adduct with each (coordinated) substrate attached to a second Al complex 2, this energy
difference becomes 15.7 kcal·mol-1 combining with the cis-epoxide and 20.3 kcal·mol-1 with the trans one that would result in dimers
α-trans-cis and α-trans-trans.
Following the reaction coordinate, the next step is the epoxide ring opening, which is undertaken by nucleophilic attack of the
carbonyl oxygen labeled O[2] (in Figure 4) on the most substituted carbon (α) of each epoxide isomer bound to one of the Al(III)
centers in IC-p. Similar as for the initiation process, in the propagation reaction the epoxide ring-opening is characterized by a
concerted transition state TS-p. The α-cis-cis profile shows the highest activation barrier being 20.9 kcal·mol-1 (having a relative
energy of 4.3 kcal·mol-1), calculated from the most stable intermediate of the initiation process Int-1 with a relative energy of −16.6
kcal·mol-1 (Figure 3). In contrast, the TS-p for the β-trans-cis pathway involves an activation barrier of only 6.9 kcal·mol-1 (estimated
from the adduct IC-p). In the case of the β-trans-trans and α-cis-trans profiles, these barriers were obtained in a similar way as
described for the previous pathways, and lead to values of 19.1 and 14.0 kcal·mol-1, respectively.
Once passing through the barrier for TS-p, the formation of the intermediate Int-p occurs which has both Al complexes still
coordinated. However, the strength of interaction between the oxygen from the alkoxide and the Al center is much stronger than that
observed for the oxygen O[1] of the coordinated carbonate and the Al(III) center from Int-2’. Hence, it is proposed that Int-p can
evolve into intermediate Int−p’ by releasing the Al complex from the carbonate and allowing for coordination of a new trans-(R)limonene oxide monomer. This reaction is endergonic by 2.6 kcal·mol-1 for the α-cis-trans profile. The remaining processes are
slightly exergonic, with a release of -1.5, -5.6 and -6.5 kcal·mol-1 in the case of the β-trans-trans, β-trans-cis and α-cis-cis pathways,
respectively. Interestingly, both the energetically most stable dimeric units Int-p’ resulting from the β-trans-cis and α-cis-cis profiles
will contain merely trans units in their backbone,[17] which is overall well in line with the experimental results. The current catalytic
process based on Al complex 2/PPNCl shows two main features: (1) a clear preference for the faster conversion of cis-limonene
oxide (B), and (2) the resulting copolymers contain a significant higher amount of trans versus cis units (up to 98:2, Table 2) where
the use of pure cis-limonene oxide will result in the formation of a nearly stereo-regular all-trans polycarbonate.

Conclusions
This work showcases the rare but efficient conversion of a tri-substituted oxirane (limonene oxide) and CO2 into a 100% bio-based
polycarbonate using an Al(III)amino-trisphenolate/PPNCl binary catalyst under mild reaction conditions (42ºC, 10 bar). The typical
features of this process involve a catalyst system based on an earth-abundant metal and modular, cheap and easily accessible
ligand systems. The catalyst is highly robust as testified by the high conversion levels that can be attained (>70%) of the limonene
oxide under neat reaction conditions. The poly(limonene)carbonates can be produced in a stereo-regular fashion (when pure cislimonene oxide is used) and its copolymer exhibits a Tg value of around 112ºC potentially useful in the context of finding bioalternatives for commercially produced polycarbonates. The DFT results show unique and important insight into the chemo- and
stereo-selectivity of the limonene oxide/CO2 coupling reaction mediated by Al complex 2/PPNCl. This information is highly valuable to
develop other types of bio-based poly(carbonates) and/or polyesters. These types of bio-based polymers are expected to grow
significantly in importance in the forthcoming development of new, sustainable alternative thermoplastics for the specialty polymer
industries. Taking into account the favorable characteristics of the presented Al(III) amino-triphenolate complex and their reactivity
and robustness, we are currently planning to widen the scope of accessible CO2-based polymers.

Experimental Section
General materials and methods

All solvents, reagents, and chemicals were purchased from commercial suppliers and used as received unless noted otherwise. Carbon dioxide was
purchased from PRAXAIR and used without further purification. The iron and aluminium(III) amino triphenolate complexes 1-4 were prepared following
previously reported protocols.[8,9,11] PPNI, PPNBr and PPNCl were prepared as previously described.[20] NMR spectra were recorded on a Bruker AV400 or AV-500 spectrometer and referenced to the residual NMR solvent signals. Mass spectrometric analyses were performed by the Research
Support Group at the ICIQ. Gel Permeation Chromatography (GPC) measurements were carried out externally at the Laboratoire de Chimie et
Procédés de Polymérisation (LCCP) in Lyon (France) and reported Mn values and polydispersities were determined against a series of discrete PS
standards. Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) were performed at ICIQ using a Mettler Toledo DSC822e
and TGA/SDTA851 machine, respectively.
Copolymerization reactions
Typical conditions: (R)-limonene oxide (1.0 g, 6.57 mmol), the Al catalyst (0.5-1.0 mol %) and co-catalyst (the respective chloride or bromide, 0.25-2.5
mol %) were placed in a 30 mL stainless steel reactor with a stirring bar. Three cycles of pressurization and depressurization with carbon dioxide were
applied (pCO2 = 5 Bar) before finally stabilizing the pressure at 10 bar. The reactor was then heated to the required temperature for the chosen time.
After that, the reactor was cooled down to room temperature before depressurizing. After this time, the conversion and selectivity was calculated using
1
H NMR analysis (CDCl3) of an aliquot of the reaction mixture. The starting material was removed from the reaction mixture in vacuo. The
poly(limonene)carbonates were further purified dissolving the reaction mixture in a small amount of dichloromethane followed by precipitation of the
products by addition of acidic methanol (HCl 1 M in methanol). After that, the polymer was filtered and dried in vacuo followed by analysis by,
depending on the sample, a combination of NMR, MALDI-TOF, GPC, TGA and DSC. Reactions under more rigorously dry conditions were conducted
in a Fischer-Porter reactor (see Supporting Information). Typically, (R)-limonene oxide (cis and trans mix-ture) and pure cis-(R)-limonene oxide were
each distilled from calcium hydride under reduced pressure following three cycles (30 min each) of vacuum-N2 purging, and stored in a glove box. The
catalyst and co-catalyst were dried by three cycles (30 min each) of vacuum-N2 purging in a silicon bath at 70ºC. The Fisher-Porter reactor was filled
inside the glove box and closed. After one cycle of pressurization and depressurization with carbon dioxide (pCO2 = 3 Bar) the reactor was pressurized
at 5 Bar and heated in a silicon bath at 45 ºC for the chosen time. The reaction mixture was dissolved in dichloromethane and the
poly(limonene)carbonate was precipitated with acidic methanol (HCl 1 M in methanol). Analysis was done as reported for the other poly(carbonate)
samples.
Computational details
All calculations in this study were carried out using the Gaussian 09 package.[21] The B97D3 functional was employed, which includes empirical
dispersion energy corrections as introduced by Grimme.[22] The standard 6-311G(d,p) basis set was used to describe the H, C, N and O atoms. The
relativistic effective core pseudo potential LANL2DZ was used for Al, Br and Cl atoms together with its associated basis set. Full geometry
optimizations were performed without constrains. The nature of the stationary points encountered was characterized either as minima or transition
states by means of harmonic vibrational frequencies analysis. Gibbs free energies were calculated at standard conditions (T = 298 K, p = 1 bar). In
order to introduce sol-vent effects, single point calculations were performed on the gas-phase optimized structures by using the polarizable continuum
model (PCM). The dielectric constant (ε) of the polarizable medium was set to the value reported for the simplest epoxide, ethylene oxide (ε = 12.42)[23]
as the reaction takes place in the limonene oxide rich phase. The 1-hexanol solvent was used for this purpose (ε = 12.51), as implemented in Gaussian.

Acknowledgements
We thank ICIQ, ICREA and the Spanish Ministerio de Economía y Competitividad (MINECO) through projects CTQ-2011-27385,
CTQ2011-29054-C02-02, the Severo Ochoa Excellence Accreditation 2014-2018 (SEV-2013-0319), and FPI fellowships to L.P.C.
and F. C. G. We also thank AGAUR for support through 2009-SGR-259. Dr. Noemí Cabello, Sofía Arnal and Vanessa Martínez are
acknowledged for the MALDI-TOF MS analyses.
Keywords: Al catalysis • DFT • limonene oxide • polycarbonates • stereoregularity
[1]

[2]

[3]

[4]

a) R. A. Sheldon, Green Chem. 2014, 16, 950-963; b) M. Poliakoff, P. T. Anastas, Nature 2001, 413, 257-257; c) M. Poliakoff, J. M. Fitzpatrick, T. R.
Farren, P. T. Anastas, Science 2002, 297, 807-810; d) R. Ciriminna, M. Lomeli-Rodriguez, P. Demma Carà, J. A. Lopez-Sanchez, M. Pagliaro, Chem.
Commun. 2014, 50, 15288-15296; e) M. Winkler, C. Romain, M. A. R. Meier, C. K. Williams, Green Chem. 2015, DOI: 10.1039/C4GC01353K.
For some recent reviews: a) M. Cokoja, C. Bruckmeier, B. Rieger, W. A. Herrmann, F. E. Kühn, Angew. Chem. Int. Ed. 2011, 50, 8510-8537; b) A.
Decortes, A. M. Castilla, A. W. Kleij, Angew. Chem. Int. Ed. 2010, 49, 9822-9837; c) N. Kielland, C. J. Whiteoak, A. W. Kleij, Adv. Synth. Catal. 2013, 355,
2115-2138; d) M. Mikkelsen, M. Jørgensen, F. C. Krebs, Energy Environ. Sci. 2010, 3, 43-81; e) M. Aresta, A. Dibenedetto, A. Angelini, Chem. Rev. 2014,
114, 1709-1742; f) Y. Tsuji, T. Fujihara, Chem. Commun. 2012, 48, 9956-9964; g) H. Maeda, Y. Miyazaki, T. Ema, Catal. Sci. Technol. 2014, 4, 1482-1497.
For reviews on this topic: a) M. R. Kember, A. Buchard, C. K. Williams, Chem. Commun. 2011, 47, 141-163; b) D. J. Darensbourg, Chem. Rev. 2007, 107,
2388-2410; c) K. Nozaki, Pure Appl. Chem. 2004, 76, 541-546; d) X.-B. Lu, D. J. Darensbourg, Chem. Soc. Rev. 2012, 41, 1462-1484; e) G. W. Coates, D.
R. Moore, Angew. Chem. Int. Ed. 2004, 43, 6618-6639; f) S. Klaus, M. W. Lehenmeier, C. E. Anderson, B. Rieger, Coord. Chem. Rev. 2011, 255, 14601479; g) X.-B. Lu, W.-M. Ren, G.-P. Wu, Acc. Chem. Res. 2012, 45, 1721-1735.
Selected original contributions: a) C. T. Cohen, T. Chu, G. W. Coates, J. Am. Chem. Soc. 2005, 127, 10869-10878; b) M. R. Kember, P. D. Knight, P. T. R.
Reung, C. K. Williams, Angew. Chem. Int. Ed. 2009, 48, 931-933; c) D. J. Darensbourg, S. J. Wilson, J. Am. Chem. Soc. 2011, 133, 18610-18613; d) K.
Nozaki, K. Nakano, T. Hiyama, J. Am. Chem. Soc. 1999, 121, 11008-11009; e) K. Nakano, T. Kamada, K. Nozaki, Angew. Chem. Int. Ed. 2006, 45, 7274-

[5]
[6]
[7]

[8]

[9]
[10]
[11]
[12]
[13]

[14]
[15]
[16]
[17]

[18]

[19]

[20]
[21]

[22]
[23]

7277; f) X.-B. Lu, L. Shi, Y.-M. Wang, R. Zhang, Y.-J. Zhang, X.-J. Peng, Z.-C. Zhang, B. Li, J. Am. Chem. Soc. 2006, 128, 1664-1674; g) G.-P. Wu, S.-H.
Wei, W.-M. Ren, X.-B. Lu, T.-Q. Xu, D. J. Darensbourg, J. Am. Chem. Soc. 2011, 133, 15191-15199; h) S. I. Vagin, R. Reichardt, S. Klaus, B. J. Rieger, J.
Am. Chem. Soc. 2010, 132, 14367-14369; i) E. K. Noh, S. J. Na, S. Sujith, S. W. Kim, B. Y. Lee, J. Am. Chem. Soc. 2007, 129, 8082-8083; j) Y. Liu, M.
Wang, W.-M. Ren, K.-K. He, Y.-C. Xu, J. Liu, X.-B. Lu, Macromolecules 2014, 47, 1269-1276; k) F. Jutz, A. Buchard, M. R. Kember, S. Bodil Fredriksen, C.
K. Williams, J. Am. Chem. Soc. 2011, 133, 17395-17405.
a) C. M. Byrne, S. D. Allen, E. B. Lobkovsky, G. W. Coates, J. Am. Chem. Soc. 2004, 126, 11404-11405; b) F. Auriemma, C. de Rosa, M. Rosaria Di
Caprio, R. di Girolamo, W. Chadwick Ellis, G. W. Coates, Angew. Chem. Int. Ed. 2014, DOI: 10.1002/anie.201410211.
F. Castro-Gómez, G. Salassa, A. W. Kleij, C. Bo, Chem. Eur. J. 2013, 19, 6289-6298.
a) C. Beattie, M. North, P. Villuendas, C. Young, J. Org. Chem. 2013, 78, 419-426; b) W. J. Kruper, D. V. Dellar, J. Org. Chem. 1996, 60, 725-727; c) M.
Bähr, A. Bitto, R. Mülhaupt, Green Chem. 2012, 14, 1447-1454; d) J. Wu, J. A. Kozak, F. Simeon, T. A. Hatton, T. F. Jamison, Chem. Sci. 2014, 5, 12271231; e) J. Qin, P. Wang, Q. Li, Y. Zhang, D. Yuan, Y. Yao, Chem. Commun. 2014, 50, 10952-10955; f) T. Ema, Y. Miyazaki, J. Shimonishi, C. Maeda, J.Y. Hasegawa, J. Am. Chem. Soc. 2014, 136, 15270–15279.
a) C. J. Whiteoak, E. Martin, E. C. Escudero-Adán, A. W. Kleij, Adv. Synth. Catal. 2013, 355, 2233-2239; b) C. J. Whiteoak, E. Martin, M. Martínez
Belmonte, J. Benet-Buchholz, A. W. Kleij, Adv. Synth. Catal. 2012, 354, 469-476; c) C. J. Whiteoak, B. Gjoka, E. Martin, M. Martínez Belmonte, E. C.
Escudero-Adán, C. Zonta, G. Licini, A. W. Kleij, Inorg. Chem. 2012, 51, 10639-10649.
C. J. Whiteoak, N. Kielland, V. Laserna, E. C. Escudero-Adán, E. Martin, A. W. Kleij, J. Am. Chem. Soc. 2013, 135, 1228-1231.
C. J. Whiteoak, A. H. Henseler, C. Ayats, A. W. Kleij, M. A. Pericàs, Green Chem. 2014, 16, 1552-1559.
a) C. J. Whiteoak, N. Kielland, V. Laserna, F. Castro-Gómez, E. Martin, E. C. Escudero-Adán, C. Bo, A. W. Kleij, Chem. Eur. J. 2014, 20, 2264-2275; b) V.
Laserna, G. Fiorani, C. J. Whiteoak, E. Martin, E. Escudero-Adán, A. W. Kleij, Angew. Chem. Int. Ed. 2014, 53, 10416-10419.
M. Taherimehr, S. M. Al-Amsyar, C. J. Whiteoak, A. W. Kleij, P. P. Pescarmona, Green Chem. 2013, 15, 3083-3090.
Please note that the lower conversion of the limonene oxide at higher temperatures is not likely an effect of catalyst decomposition. Previously we reported
that Al(amino-triphenolate) based catalyst systems can be easily operated at temperatures up to 90ºC without signs of decomposition and maintaining
excellent activity for organic carbonate formation, see ref. 9 and 11.
M. R. Kember, C. K. Williams, J. Am. Chem. Soc. 2012, 134, 15676-15679.
The influence of the stirring technique suggests that the increasing viscosity of the mixtures observed during the course of the copolymerization reaction (in
particular at high conversion levels) strongly affects the mass transfer properties.
The reported DSC data refer to the second heating run, see also the Supporting Information.
A monometallic mechanism was also investigated for the alternating copolymerization of CO2 and epoxides, affording higher free-energy barriers for the
propagation reaction. A manuscript including those results is in preparation. See also: K. Nakano, K. Nozaki, T. Hiyama, J. Am. Chem. Soc. 2003, 125,
5501-5510.
Note that the formulation cis-cis (for instance) for a dimeric carbonate unit refers to initial conversion of a cis-limonene oxide monomer followed by the
insertion (through an SN2 like pathway, TS-p in Figure 4) of a second cis-limonene oxide monomer to afford a trans-trans dimeric carbonate unit.
Alternatively, the conversion of two trans limonene oxide monomers in a similar way also leads to a trans-trans dimeric carbonate intermediate (cf., Int-p´
in Figure 4).
Williams et al. have recently described a bimetallic Zn catalyst that is active in CO2/CHO copolymerization, and computational efforts have also
demonstrated the necessity of two metal centres for efficient copolymerization, see: A. Buchard, F. Jutz, M. R. Kember, A. J. P. White, H. S. Rzepa, C. K.
Williams, Macromolecules 2012, 45, 6781-6795.
F. A. L’Epplatenier, A. Pugin, Helv. Chim. Acta 1975, 58, 917-923.
Gaussian, Revision D.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci,
G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R.
Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J.
Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi,
N. Rega, N. J. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R.
Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö.
Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, 2013, Gaussian, Inc., Wallingford CT.
a) S. Grimme, J. Comp. Chem. 2006, 27, 1787-1799; b) S. Grimme, S. Ehrlich, L. Goerigk, J. Comp. Chem. 2011, 32, 1456-1465.
CRC Handbook of Chemistry and Physics, D.R. Lide (ed), 84th Ed. CRC Press LLC, Florida 2003.

Entry for the Table of Contents:

FULL PAPER
The sporadic but efficient
copolymerization of (R)limonene oxide and CO2 into
a bio-renewable polycarbonate is reported. The process
is Al-catalysed showing
higher preference for the
conversion of the cis-substrate. The stereo-regular
features and the preference
for the formation of a virtually
all trans-copolymer have
been examined in detail using
DFT analysis.

Leticia Peña Carrodeguas,
Fernando Castro-Gómez, Joan
González-Fabra, Carles Bo* and
Arjan W. Kleij*
Page No. – Page No.

Al(III) Catalyzed Formation of
Poly(limonene)carbonate: DFT
Analysis of the Origin of
Stereoregularity