Challenges in the Catalytic Synthesis of Cyclic and Polymeric Carbonates

This journal is c The Royal Society of Chemistry 2012 Catal. Sci. Technol., 2012, 2, 2169–2187 2169

Cite this: Catal. Sci. Technol., 2012, 2, 2169–2187

Challenges in the catalytic synthesis of cyclic and polymeric carbonates

from epoxides and CO2

Paolo P. Pescarmona* and Masoumeh Taherimehr

Received 30th May 2012, Accepted 26th June 2012

DOI: 10.1039/c2cy20365k

The addition of carbon dioxide to epoxides to produce either cyclic carbonates or polycarbonates

is an important reaction allowing the conversion of a renewable, inexpensive and non-toxic

feedstock such as CO2 into useful products with many potential applications. In this perspective

article, an overview of the type of catalysts used for this reaction, the mechanisms with which

they operate and the parameters influencing their activity and selectivity are presented and

discussed critically. In this context, the main challenges to be tackled in this vibrant area of

research are highlighted.

1. Introduction

The growing concerns about the environmental impact of

carbon dioxide emissions produced by fuel combustion and

other human activities prompted the scientific community to

research ways to store or, preferably, reuse this molecule.1

In this context, the chemical fixation of CO2 to produce useful

compounds has attracted increasing interest.2,3 Carbon dioxide is

an inexpensive, widely available, non-toxic and renewable carbon

source, but its use as feedstock for the production of valuable

chemicals faces the challenge of its high thermodynamic stability.4

This implies that the conversion of carbon dioxide may require

a high energy input, as in the case of the photocatalytic

reaction with water to generate oxygen and methanol or

methane.5 An alternative approach consists in the reaction

of CO2 with compounds with a relatively high free energy, thus

providing a thermodynamically favourable process.4 Among

the reactions employing this last strategy, the addition of

carbon dioxide to epoxides is a relevant topic of research since

it is an atom-efficient reaction that can generate useful classes

of compounds as cyclic carbonates and polycarbonates

(Scheme 1).4,6–9 This perspective article focuses on this reaction

Centre for Surface Chemistry and Catalysis, KU Leuven, KasteelparkArenberg 23, 3001 Heverlee, Belgium.E-mail: [email protected]

Paolo P. Pescarmona

Paolo Pescarmona was born in1973 in Torino, Italy. Afterreceiving his Master Degreein Chemistry from the Univer-sity of Torino, he moved to theNetherlands where he obtainedhis Ph.D. from the DelftUniversity of Technology in2003. Since 2005 he works atthe University of Leuven (KULeuven), Belgium, first aspost-doc researcher and since2008 as professor. Presently,his group consists of six Ph.D.students and two post-docresearchers. His research

interests are steadily growing and embrace many aspects ofcatalysis and of materials chemistry. A common thread in hisresearch is the attention towards sustainability, as exemplifiedby the topic of this perspective article. His curiosity is not limitedto chemistry and he is keen on travelling and learning aboutdifferent cultures around the world.

Masoumeh Taherimehr

Masoumeh Taherimehr wasborn in 1983 in Ahvaz, Iran.In 2005 she received theDegree in Chemistry from theChamran University of Ahvazand in 2007 she obtainedher Master Degree from theLorestan University, Iran. In2010 she started her Ph.D. atthe KU Leuven under the super-vision of Prof. Pescarmona.Her research interests includehomogeneous and hetero-geneous catalysis for thesynthesis of organic compoundsfrom renewable sources, with afocus on the conversion ofcarbon dioxide to valuableproducts.

CatalysisScience & Technology

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2170 Catal. Sci. Technol., 2012, 2, 2169–2187 This journal is c The Royal Society of Chemistry 2012

and on the catalytic systems used to promote it, with the

purpose of providing a systematic analysis of the topic and of

underlining the challenges that still need to be tackled. Both

cyclic carbonates and polycarbonates are valuable compounds with

many established and potential applications. Cyclic carbonates are

the thermodynamically favoured products of the reaction of

epoxides with carbon dioxide.4,6,8 Their application range

is broad, the most employed compounds being ethylene

carbonate and propylene carbonate.4,6,7 Cyclic carbonates

can be used as intermediates in the synthesis of fine or bulk

chemicals,1,6 as exemplified by the Asahi Kasei industrial

process for the production of polycarbonates from bisphenol

A.10 This multi-step process involves the use of ethylene

carbonate as reagent and represents a greener alternative to

the currently main industrial process, which is based on the

highly toxic phosgene as reagent. Other applications of cyclic

carbonates are as environmentally friendly polar aprotic

solvents, with low odour and toxicity,7,11,12 and as electrolyte

solvents for lithium-ion batteries.13 The second class of

organic carbonates that can be obtained by the reaction of

carbon dioxide with epoxides are the polycarbonates. At this

stage, it is important to note that the term polycarbonate is

used to refer to various types of polymers, which can present

rather different physicochemical properties from each other. A

polycarbonate is defined as a polymer containing carbonate

groups (–O�(CQO)–O–) in its backbone chain: this descrip-

tion includes both the commercial polycarbonates that are

synthesised using bisphenol A as building block, and the

polymers obtained by the alternating copolymerisation of

CO2 with an epoxide, which are discussed in this article. The

bisphenol-based polycarbonates are thermoplastic polymers

with excellent mechanical and physical properties, including

high strength, rigidity, toughness, durability, transparency to

light, heat resistance and good electrical insulation. Owing to these

properties, bisphenol-based polycarbonates can substitute glass or

metal in many uses and find widespread application as materials

for data storage (e.g. CDs and DVDs), for electronic components,

for lenses, as construction materials and as automotive and

aircraft components.6,10 On the other hand, polycarbonates

obtained by copolymerising carbon dioxide and epoxides

display less suitable properties including low rigidity (glass

transition temperature of 35–40 1C for poly(propylene

carbonate) compared to about 150 1C for bisphenol-based

polycarbonates) and moderate thermal stability.8,14 So far

these features limited the application of these polymers,14

despite their biodegradability and the fact that they can be

prepared in a green process from renewable carbon dioxide.

These observations lead us to the identification of a first

challenge (1): the properties of alternating copolymers

obtained from the coupling reaction of carbon dioxide and

epoxide should be improved in order to attain practical applica-

tions for these materials. This might be achieved by investigating

different types of epoxides as substrates for the copolymerisation

reaction with CO2; by optimising the catalysts in order to

increase the molecular weight and crystallinity of the polymers;

by cross-linking the polymer chains; by copolymerisation

involving two different epoxides or an epoxide and another

type of monomer; through the use of fillers during the synthesis to

form (nano)composites; through blending with other polymers.14,15

The improved polymeric materials could find a vast range of

applications as engineering thermoplastics, elastomers, packaging

materials, binders, adhesives and coatings.8,14

From the point of view of green chemistry and sustainability,

the relevance of the synthesis of organic carbonates through the

reaction of carbon dioxide with epoxides lies in the low cost,

availability and renewable character of CO2 as carbon source

and in the features and potential applications of the products:

cyclic carbonates can be used as green solvents and provide

an alternative to the use of the highly toxic phosgene as

reagent,7,10,11 while polycarbonates could substitute synthetic

polymers prepared from non-renewable petroleum feedstock.8

On the other hand, it should be noted that the benefits on the

mitigation of the impact of CO2 emissions through this reac-

tion are expected to be limited.16 Presently, onlyB0.5% of the

CO2 generated by human activities is reused by the industry,

and of this amount only a small fraction is employed for the

synthesis of organic carbonates (i.e. acyclic carbonates, cyclic

carbonates and polycarbonates).1 The processes for the synthesis

of organic carbonates have the potential to reach a larger scale,

particularly if the applicability of the products and the efficiency

of their synthesis will be improved (see next sections for a more

detailed discussion). However, the yearly need for these and

other compounds that could be produced by CO2 fixation is

likely to remain low compared to the yearly amount of CO2

generated worldwide by fuel combustion. The minimisation

of carbon dioxide emissions will require most probably a

combination of approaches including the use of greener energy

sources as alternative to fossil fuels,1 the improvement of the

efficiency of existing processes generating CO2 as a waste

(e.g. transportation, heating, industrial processes) and worldwide

policies to avoid an unsustainable growth of the human population.

In this perspective article, the different mechanisms leading

to the catalytic synthesis of either cyclic or polycarbonates

from the reaction of carbon dioxide with epoxides are reviewed

and rationalised, and the parameters governing the activity and

the selectivity of the catalysts are critically discussed. The definition

of this general picture provides the context to introduce the various

open challenges in this field of research. The target of this article is

to help understanding the catalytic systems used for this reaction

and to suggest relevant research directions, and not to provide

Scheme 1 Possible products of the reaction of CO2 with epoxides: cyclic carbonate (a), polycarbonate (b) and polycarbonate containing ether

linkages (c).

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a comprehensive overview of all homogeneous and heterogeneous

catalysts reported for the reaction of CO2 with epoxides, for which

purpose excellent reviews are available.4,6–9,14,17–19

2. Mechanism of the addition of carbon dioxide to

epoxides

The reaction of CO2 with epoxides can generate two types of

products: cyclic carbonates and polycarbonates (a and b in

Scheme 1). Moreover, the consecutive insertion of two epoxides

in the polymer chain can occur instead of the alternating

copolymerisation of carbon dioxide and epoxide.8,14 This leads

to the presence of ether linkages in the polycarbonate (c in

Scheme 1), or to the formation of polyethers in the extreme

case of polymerisation occurring exclusively between epoxide

molecules (m = 0 in structure c).

The addition of CO2 to epoxides is generally carried out in

the presence of a catalytic system that can initiate the reaction

by activating either the epoxide or carbon dioxide, or both at

the same time. The epoxide is typically activated by interaction of

the oxygen atom with a Lewis acid, followed by a nucleophilic

attack that causes the opening of the epoxide ring. CO2 is a

linear, apolar molecule, though the two CQO bonds are polar,

implying that the carbon atom has a partial positive charge and

the oxygen atoms have a partial negative charge. Therefore, the

carbon atom can act as an electrophile and the oxygen atoms as

nucleophiles: the activation of CO2 can thus occur both through

a nucleophilic and an electrophilic attack (vide infra). It follows

from these considerations that most catalytic systems that have

been investigated for this reaction contain Lewis acid sites for the

electrophilic activation of epoxide and/or carbon dioxide and

Lewis base sites acting as nucleophiles. The two sites can belong

to two different compounds – for example the metal of a complex

as Lewis acid and the anion of a salt as Lewis base – or be parts

of a single compound, as in a complex containing a cationic

metal centre and a labile anionic ligand.

Different mechanisms for the reaction of carbon dioxide with

epoxides have been proposed, and in many cases demonstrated

Fig. 1 Phenolate metal complexes.

Scheme 2 Possible intermediates of the reaction of epoxides with CO2, exemplified in the case of the reaction catalysed by a metal complex. Nu is

a nucleophile that can originate from the metal complex or from a co-catalyst. RO- is an alkoxide or an aryloxide that can act as a nucleophile. The

dotted arrows indicate the possible back-biting reactions that would lead to the formation of cyclic carbonate.

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by means of characterisation with various spectroscopic and

analytical techniques. The predominance of a specific reaction

path depends on the nature of the catalytic system, but also on

the substrate and on the reaction conditions. In all cases the

activation of the epoxide requires a nucleophile, which is

the common feature of all catalytic systems employed for the

reaction of carbon dioxide with epoxides. Regardless of the

nature of the first step, all mechanisms involve an intermediate

containing a carbonate precursor, which can either react

further through consecutive additions of other epoxide and

CO2 molecules (propagation) to generate a polycarbonate, or

undergo a back-biting reaction leading to ring closure with

formation of the cyclic carbonate (Scheme 2). If polycarbo-

nates are the products of the reaction, the nucleophilic species

that serves as the initiating group will also be the end group of

the polymer chain.

The most widely studied type of catalysts for the reaction

of carbon dioxide with epoxides are homogeneous metal

complexes, which can be employed alone if they contain a

ligand that can act as a nucleophile, or in combination with a

co-catalyst providing the nucleophilic species. A variety of

metal centres acting as Lewis acid sites has been studied for

these complexes: Al, Cr, Mn, Co, Fe, Cu, Zn, Cd, Mg, Li and

lanthanoids.8,14 Both monometallic and bimetallic complexes

have been employed as catalysts for this reaction. The most

common ligands used for the synthesis of these complexes are:

salen and related ligands, b-diiminates (BDI), phenolates and

porphyrins (Fig. 1, 2, 4 and 7).19 Non-metallic homogeneous

catalysts based on salts of organic cations have been reported

as well (see Section 2.3). Heterogeneous catalysts are also

known: the most widely applied is zinc glutarate, but hetero-

geneous systems based on the immobilisation of homogeneous

Fig. 2 Metal complexes with salen and related ligands.

Scheme 3 Reaction of CO2 with epoxides: monometallic pathway involving one nucleophile. The nucleophile (Nu) may originate from the metal

complex or from a co-catalyst.

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catalysts on a support are receiving increasing attention (see

Section 4).14,18,19

Most of the mechanisms proposed for the reaction of CO2

with epoxides have been derived from studies of homogeneous

catalytic systems, but heterogeneous catalysts are considered

to follow similar pathways.20 In order to facilitate the systematic

discussion of the various reaction mechanisms, we divided them

into three categories based on the number of metal atoms

involved in the catalytic cycle: monometallic pathways, bimetallic

pathways and non-metallic pathways.

2.1 Monometallic pathways

This type of mechanisms is exemplified by the wide group of

homogeneous catalytic systems based on metal complexes.

These metal complexes can include a ligand that can act as

nucleophile. Alternatively the metal complex, usually referred

to as the catalyst, is used in combination with an added

nucleophile generally denoted as the co-catalyst.

(a) A common monometallic mechanism for the addition of

CO2 to epoxide catalysed by a metal complex proceeds

through the steps summarised in Scheme 3. First, the epoxide

coordinates to a Lewis acidic metal centre and gets activated

towards attack by a nucleophile. The nucleophilic attack leads

to opening of the epoxide ring and formation of a metal-bound

alkoxide. This alkoxide can in turn act as a nucleophile leading to

CO2 insertion into the metal alkoxide bond, generating a metal

carbonate intermediate. This carbonate species can either evolve

towards the cyclic carbonate or propagate by further addition of

epoxide and CO2 with formation of a polycarbonate (vide supra).

The occurrence of this mechanism has been supported by an

isotope-labelling experiment.21

(b) A monometallic mechanism involving two nucleophiles

has been proposed for metal salen complexes (e.g. Cr-salen),

which are characterised by a planar configuration with a

tetradentate coordination of the salen ligand (Fig. 2). The

axial positions of the metal are available for coordination with

nucleophiles. Of the two nucleophiles involved in the mechanism,

one is part of the metal complex as axial ligand (Nua) and the

second one (Nub) is added to the system and coordinates to

the metal in the remaining trans axial position, thus generating a

six-coordinated intermediate (II in Scheme 4).9,14,22 The coordi-

nation of the second nucleophile serves to labilise the other

metal-nucleophile bond, favouring coordination and nucleophilic

attack of the epoxide (III). The formed alkoxide species (IV) acts

in turn as a nucleophile that attacks CO2 to form a metal

carbonate species. The presence of a nucleophile in the trans

position to this carbonate (V) has been reported to favour the

further nucleophilic attack and ring opening of a new epoxide

molecule (similarly to what occurs in step III). Therefore, the

alternating copolymerisation of epoxide and carbon dioxide

leading to the formation of polycarbonates is the preferential

path with this mechanism.23 In general, the nucleophiles can

be neutral or ionic species. Kinetic studies indicated that anionic

six-coordinated intermediates (II in Scheme 4) are more effective

both in the opening of the epoxide ring and in the CO2 insertion

compared to their neutral six-coordinated analogues.9 It is has

been proposed, though not unequivocally established, that both

Nua and Nub can serve as initiator groups leading to the growth

of two polymer chains from one metal centre.9,14,24

Scheme 4 Reaction of CO2 with epoxides: monometallic pathway involving two nucleophiles. Note that Nua and Nub may be the same or

different type of nucleophile.

Scheme 5 Reaction of CO2 with epoxides: bimetallic pathway involving two monometallic complexes.

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2.2 Bimetallic pathways

The addition of CO2 to epoxides can follow a bimetallic

mechanism if the catalyst contains two metal centres or if

two monometallic catalysts act in collaboration.

(a) A bimetallic pathway involving two separate metal

complexes has been inspired by a mechanism proposed for

the asymmetric ring opening of epoxides.25 This intermolecu-

lar mechanism is similar to the monometallic pathway (2.1.a),

with the difference that the nucleophile that attacks the

epoxide activated by the metal centre is coordinated to a

second metal centre (Scheme 5).8 The bimetallic initiation is

followed by a monometallic pathway leading to cyclisation

and formation of cyclic carbonate or to propagation of the

alternating polycarbonate. Kinetic studies by means of in situ

infrared spectroscopy supported this mechanism for a Cr-salen

complex.26 This pathway is expected to occur when low

epoxide/catalyst ratios are employed.19 It has been suggested

that this mechanism tends to be followed when a weaker Lewis

base is used as co-catalyst, while mechanism 2.1.b is more

likely to occur in the presence of a stronger Lewis base that is

able to weaken the metal-nucleophile bond and allow con-

certed opening of the epoxide ring.27 If this mechanism is

followed, the reaction rate has a second-order dependence on

the metal concentration, while a first-order dependence is

observed in the case of a monometallic pathway.

(b) Metal complexes containing two neighbouring metal

centres may activate epoxide and CO2 simultaneously

(Scheme 6). This promotes the intramolecular nucleophilic

attack of the alkoxide to the carbon atom of the activated

CO2 molecule. This mechanism has been proposed to explain

the improved catalytic activity of bimetallic [Al(salen)]2O

complexes (Fig. 8) compared to monometallic salen complexes.28

It has been suggested that in this mechanism the co-catalyst,

tetratbuylammonium bromide (Bu4NBr), plays a second role

besides the ring opening of the epoxide by the bromide. Tetra-

buylammonium bromide can decompose to tributylamine, which

can form a carbamate salt with CO2 (Bu3N+–CO2

�) and this salt

can coordinate more easily to the second Al-centre compared to

CO2 alone.29

(c) The first step of the catalytic addition of carbon dioxide

to epoxides is not always the activation of the epoxide. Some

metal complexes containing an alkoxide or aryloxide group

(–OR) are able to undergo CO2 insertion to form a bidentate

carbonate as the first step (Scheme 7).8,14 This is not unexpected,

because the –OR groups are analogous to the alkoxide inter-

mediates formed upon epoxide insertion in the monometallic

mechanisms described above, and such metal-bound alkoxides

are able to react with CO2 (see Section 2.1). The reaction

continues through the coordination of the epoxide followed by

ring opening and insertion into the metal carbonate bond, which

was proposed to be the rate-determining step of the process.30

This type of mechanism has been proposed and investigated in

detail for bulky b-diiminate zinc complexes containing two

adjacent metal centres bridged by alkoxide (or carboxylate)

groups (Fig. 4).8,30 b-Diiminate zinc complexes can exist both

in dimeric (bimetallic) or monomeric (monometallic) form. The

most likely pathway is bimetallic, as sketched in Scheme 7, but

a contribution from a monometallic mechanism cannot be

excluded.30 The prevalence of the monomeric or dimeric form

of the complex in solution strongly depends on the steric

hindrance of the substituents.8 Catalysts for which the first

step of the mechanism is CO2 insertion with formation of a

bidentate species generally display high selectivity towards the

copolymerisation reaction.8,9 In this context, it has been

suggested that the presence of two adjacent metal centres

Scheme 6 Reaction of CO2 with epoxides: bimetallic pathway with simultaneous activation of epoxide and CO2 on a complex with two adjacent

metal centres.

Scheme 7 Reaction of CO2 with epoxides: bimetallic pathway with complexes containing two adjacent metal centres and –OR groups.

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can promote the selectivity of the reaction towards the growth

of a polycarbonate chain by simultaneously coordinating two

species (see Scheme 7), thus favouring the interaction of the

growing chain with new incoming reagents.19 Nevertheless, the

presence of two neighbouring metal centres is not a sufficient

condition for selectively generating polycarbonates, as proven

by the high selectivity towards cyclic carbonate displayed by

Al-salen bimetallic complexes following mechanism 2.2.b.28

2.3 Non-metallic pathways

Salts of organic cations have been employed alone as catalysts

for the reaction of carbon dioxide with epoxides. Particularly,

ionic liquids have been studied either as homogeneous or

heterogeneous catalysts, in the latter case in the form of

supported species.18 Carbon dioxide can dissolve in consider-

able amounts in ionic liquids, implying that the ionic liquids

can be employed with the double role of solvents and catalysts.

In general, reactions with ionic liquids as catalysts without the

assistance of metal centres require relatively high temperature

(Z 80 1C) in order to achieve good product yields, thus

favouring the formation of the thermodynamically more

stable cyclic carbonate as the main product. The mechanism

followed by these non-metallic catalysts depends on the nature

of the anion and cation (X� Y+) constituting the salt. Two

possible pathways may lead to formation of the carbonate

product (Scheme 8):

(a) The most common mechanism involves a nucleophilic

attack of the epoxide by the anion, which causes opening of

the epoxide ring, followed by the insertion of carbon dioxide

and finally by ring closure to generate the cyclic carbonate.

This mechanism is very similar to the monometallic pathway

described in 2.1.a.

(b) If the anion is bulky (e.g. BF4�, PF6

�), the interaction

between the epoxide and the anion is very weak and the anion

rather interacts with CO2 to generate a new ionic species

[X�CO2]�, that is more basic than the original anion X�

and thus able to attack the epoxide to generate a carbonate

intermediate and finally the cyclic product.18 This pathway

was proposed on the basis of a mechanistic study by means of

in situ ATR infrared spectroscopy.31

The heterogenisation of ionic liquids has been employed as a

strategy to develop recyclable catalysts for the synthesis of

cyclic carbonate from CO2 and epoxide.32–36 Among these

Scheme 8 Reaction of CO2 with epoxides: non-metallic pathways with epoxide activation as the first step (a) or with CO2 activation as the first step (b).

Scheme 9 Synthesis of cyclic carbonate catalysed by an immobilised ionic liquid with a side chain terminating with an –OH group.

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supported ionic liquids, the imidazolium-based ionic liquids

containing an –OH or a –COOH group in the cation are

particularly interesting from a mechanistic point of view.34,35

It has been proposed that the –OH of the alcoholic or carboxylic

group can interact with the oxygen atom of the epoxide, thus

helping the nucleophilic attack of the epoxide by the anion

(Scheme 9). The role of the –OH is similar to that of the Lewis

acidic metal centre in the metallic mechanisms (vide supra).

3. Catalytic activity and selectivity in the synthesis

of cyclic carbonates and polycarbonates

As discussed in Section 1, cyclic carbonates and polycarbonates

are both relevant compounds. However, if we want to design a

catalyst that is able to produce any of them efficiently it is

crucial to understand and control the parameters determining

the selectivity of the reaction of CO2 and epoxides towards each

of these two families of carbonates (challenge 2). The formation

of polycarbonates is thermodynamically less favourable than

the synthesis of cyclic carbonates, but it generally requires lower

activation energy.9,37 This means that by selecting a suitable

catalytic system and by tuning the reaction conditions it should

be possible to selectively produce either one or the other type of

carbonate. In the case in which the polycarbonate is the desired

product it is also important to avoid the formation of polyether

linkages (see Section 2) and to achieve polymers with high

molecular weight and low polydispersity. These parameters can

play a relevant role in obtaining polycarbonates with suitable

properties for application (see challenge 1). The control of the

selectivity is not the only challenge connected to the design of

improved catalysts for this reaction. The catalytic activity can

still be improved compared to the already known, promising

catalysts, in order to reach higher turnover frequencies under

mild temperature and pressure conditions with different types of

substrates, including sterically hindered and internal epoxides

and oxetanes (challenge 3).

A rationalisation of the factors determining the activity and

selectivity of known catalysts for the reaction of CO2 with

epoxides can be very useful for designing novel catalysts with

improved performance. For this purpose, the influence of the

nature of the catalyst, of the co-catalyst (if present), of the

substrate and the role of the solvent and of the reaction

conditions will be analysed and discussed in this section for

selected catalysts reported in the literature.

3.1 Metal-containing catalysts

Metal complexes are the most widely studied class of metal-

containing catalysts for the reaction of carbon dioxide with

epoxides (see Section 2). The knowledge gained by studying the

parameters that influence the catalytic behaviour of these complexes

can be rather general and apply to other catalytic systems, including

heterogeneous ones. Both the metal and the ligands play a role

in defining the properties of metal complexes as homogeneous

catalysts for the reaction between CO2 and epoxides.

(a) The nature of themetal centre in metal complexes plays a

crucial role in their catalytic performance. The monometallic and

bimetallic pathways discussed in Section 2 are characterised by

alkoxide and carbonate intermediates containing a metal–oxygen

bond. The strength of this bond is an important factor in

determining the activity and selectivity of the catalyst. A too

strong metal–oxygen bond makes the intermediate inactive

both towards another insertion, which would lead to propaga-

tion and copolymer production, and towards ring closure,

which would yield a cyclic carbonate. On the other hand, a

weak metal–oxygen bond can be displaced by a nucleophile or

a solvent molecule, leading to lower activity but also favouring

the back-biting reaction and, thus, increasing the selectivity

towards the cyclic carbonate. It can be concluded that inter-

mediate bond strength is needed for a good catalytic perfor-

mance and that the properties of the catalyst can be tuned by

the choice of the metal centre.38 The relevance of the nature of

the metal centre can be recognised by comparing the catalytic

Table 1 Selected metal complexes as catalysts for the reaction of CO2 with epoxides

Catalyst EpoxideaCatalystloading/mol%

T/1C

p(CO2)/bar

Reactiontime/h TONb

TOFc/h�1

Major carbonate product[selectivity (%)]

Mn

(kg mol�1)PDId

(Mw/Mn) Ref.

1a CHO 5 60 1 6 10 2 Polycarbonate 19.2 1.56 391b CHO 0.67 60 1 2 43 22 Polycarbonate 12.6 1.29 372a CHO 0.1 80 1 24 219 9 Polycarbonate [96] 6.2 1.19 422b CHO 0.1 80 1 2 170 86 Polycarbonate [499] 5.1 1.26 433a [X = F, L = THF] CHO 0.1 80 55 48 364 8 Polycarbonate 42 6 564b/PPNCl PO 0.05 25 15 2 — 328 Polycarbonate [499] 25.4 1.14 455b [X = OCCCl3]/PPNCl PO 0.05 25 15 1.5 — 568 Polycarbonate [499] 28.1 1.14 455c/PPNCl PO 0.05 25 1.5 2 — 235 Polycarbonate [499] 15.5 1.18 456b/Bu4NI SO 2.5 45 10 3 39 13 Cyclic carbonate [499] — — 697a [X = NO3]/DMAP PO 0.1 25 15 3 — 86 Polycarbonate [499] 13.2 1.29 228/PPNCl CHO 0.1 70 34 2 — 230 Polycarbonate [499] 91 1.15 509 PO 0.05 25 14 12 1540 128 Polycarbonate [99] 23.9 1.14 5110 PO 0.001 70 20 1 16 000 16 000 Polycarbonate [499] 300 1.31 5411 [R = iPr, R0 = Me] CHO 0.1e 50 7 2 449 224 Polycarbonate [95] 19.1 1.07 5913d CHO 0.1 50 7 0.5 364 729 Polycarbonate 23.3 1.15 3014c CHO 0.009 80 12 10 2720 272 Polycarbonate 261 1.6 6115b/DMAP CHO 0.2 80 50 24 500 21 Polycarbonate [499] 14.5 1.13 6716/Bu4NBr SO 2.5 25 1 3 12 4 Cyclic carbonate [499] — — 68

a CHO= cyclohexene oxide; PO= propylene oxide; SO= styrene oxide; b TON=mol of epoxide converted per mol of metal; c TOF=mol of

epoxide converted per mol of metal per hour; d PDI = polydispersity index; Mn = number average molar mass; Mw = weight average molar

mass; e Based on the monomeric complex.

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behaviour of complexes with the same ligand scaffold but with

different metal centres.

� Replacing the two zinc atoms in the bimetallic phenolate

complex 1 in Fig. 1 with magnesium atoms increased the

catalytic activity if the reaction was carried out at low CO2

pressure (1 atm, see Table 1).14,37,39 It was hypothesised that the

harder Lewis acid nature and higher oxophilicity of Mg2+ in

comparison with Zn2+ might make the former more effective

for epoxides activation under these reaction conditions.37

� Compared with their organozinc counterparts, many

organoaluminium-based catalysts are less active and selective

in the synthesis of polycarbonates and tend to promote cyclic

carbonate production.40 An explanation for the preferential

formation of cyclic carbonates with aluminium salen complexes

has been provided by theoretical calculations.41 It was found

that the dissociation of the growing polymer chain from

aluminium salen complexes is more favourable than from

chromium salen complexes. The dissociation can be followed

by ring closure with formation of the cyclic carbonate, thus

accounting for the higher selectivity towards this product.

� Dizinc macrocyclic complex 2a in Fig. 1 displays high

activity in the copolymerisation of CO2 and cyclohexene oxide

at low CO2 pressures.42 This catalytic behaviour was attributed to

the coordinative flexibility of the ligand and to the vicinity of

the two metal centres, which can facilitate bidentate binding of the

growing carbonate chain (see mechanism 2.2.c), thus lowering the

activation energy for CO2 insertion.14 Replacing zinc with cobalt

(2b) in this type of bimetallic complexes caused a significant

increase of the catalytic activity (Table 1). Considering that the

rate determining step is likely to be the epoxide ring opening by the

nucleophilic carbonate chain, the higher activity of the cobalt

complex can be attributed to an increased nucleophilicity of the

carbonate propagating species in the attack of the epoxide ring

when the metal centre is Co(II) compared to Zn(II).43

� The electronic configuration of the metal centre can

determine the type of coordination of the metal complex. In

the case of metal salen catalysts (Fig. 2), it has been shown that

a six-coordinated metal complex with a distorted octahedral

geometry (see mechanism 2.1.b) is necessary for the alternating

copolymerisation to proceed efficiently.44 For example, six-

coordinated salen complexes with high catalytic activity are

formed with chromium(III) and cobalt(III), which have half-filled

t2g orbitals. On the other hand, Schiff base Mn(III) complexes

are preferentially five-coordinated. These Mn-complexes have

low tendency to bind the epoxide substrates and, therefore, are

ineffective in catalysing the reaction between CO2 and epoxides.

(b) The ligands in ametal complex can be strongly coordinated to

the metal centre or be labile. Both types play a role in determining

the electronic and steric properties of the metal complex. The latter

ones can also actively participate in the catalytic reaction. From

these considerations, it becomes clear that a careful selection of the

ligands is crucial for optimising the catalytic activity and selectivity

of metal complexes.

The importance of the strength of the metal–oxygen bond in

the alkoxide and carbonate intermediates was underlined in

Section 3.1.a. The strength of this bond is not only influenced

by the metal, but also by the nature of the ligand(s) coordinated

to it. Electron-donating ligands and electron-donating groups

on the ligands can lead to a decrease in the metal–oxygen bond

strength, thus favouring the back-biting reaction and increasing the

selectively towards the production of cyclic carbonate. Conversely,

electron-withdrawing groups will decrease the electron density

of the metal centre, thus increasing its Lewis acid character.

The role of the ligand is intimately related to that of the metal

centre, of the co-catalyst and of the reaction conditions.

Therefore, the optimum features of the ligand can be extremely

different for different systems. Many studies demonstrated the

impact of changes in the structural and chemical features of the

ligand and in the nature of its substituents on the catalytic

activity and selectivity of metal complexes.

� Various studies on the effect of changes in the structure

and in the substituents of the ligand have been carried out with

metal salen complexes. In the case of cobalt salen complexes

(X=OOCCCl3, complexes 4a, 4b and 5b in Fig. 2), ligands with a

non-cyclic diimine backbone showed lower catalytic activity for

copolymerisation of propylene oxide and CO2 compared to their

counterparts with the cyclic diimine backbone (Table 1). Moreover,

it was found that electron-withdrawing substituents in the para-

position of the phenolate are detrimental to the copolymerisation

activity (compare 5b with X = OOCCCl3 and 5c in Table 1).45

� The substituents on the aromatic ring of the salen ligand

can also have a steric effect: in the reaction of styrene oxide

with CO2, a significant decrease in the activity and polymer

selectivity of a Co-salen complex was observed when changing

the groups in the ortho-position of the phenolate from tert-butyl

to very bulky groups (i.e. �CMe2Ph, �SiMe2tBu, see 5b with

X = DNP and 5d in Fig. 2).46

� Changing the cyclohexane moiety of the diimine back-

bone of a salen ligand to a phenyl group (salphen ligand, 6 in

Fig. 2) was reported to affect the selectivity of the chromium

complex in the reaction of CO2 with propylene oxide. The

Cr-salphen complex (6a) has a fully conjugated ligand structure and

is more selective towards poly(propylene carbonate) (if using no

more than one equivalent of co-catalyst, see Section 3.2.a). The

Cr-salen complex (5a with X=Cl) has a non-conjugated fragment

and is more selective towards cyclic propylene carbonate. This

result was explained by the more electron-donating nature of

the salen ligand, which weakens the metal–oxygen bond, thus

favouring the dissociation of the intermediate species and leading

to the cyclic product (see Section 3.1.a).47

� The complexity of the catalytic reaction of carbon dioxide

and epoxides is underlined by the different effects that a same

change in the ligand can have with different metals. Switching

from a salen to a salphen ligand had a major effect on the

catalytic activity of Zn complexes employed with Bu4NI as co-

catalyst but did not affect the selectivity, which was directed

towards cyclic carbonates with a variety of epoxides (5e and 6b

in Fig. 2 and Table 1).48 The higher activity of the Zn-salphen

complexes stems from the higher Lewis acidity of the zinc

centre, which is ascribed not only to the electronic effects of

the ligand but also to the rigid, planar geometry imposed by

the ligand scaffold. The effect of the substituent in the para-

position on the phenolate was also investigated (6b and 6c in

Fig. 2): bulky groups in these positions are needed to prevent

the dimerisation of the Zn-salphen complexes, which would

cause a dramatic drop in catalytic activity.49

� The nature of the ligand determines whether the complex

has a rigid or flexible structure. In different contexts, both

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features have been claimed to be advantageous. Chromium salan

complexes (7 in Fig. 2 and Table 1) contain nitrogen atoms with

sp3 hybridisation, which makes them more flexible than salen

complexes and decreases the electrophilicity of the metal centre.

These features were proposed to favour the dissociation and

re-association of the co-catalyst and of the growing polymer

chain, thus explaining the much higher activity in the reaction of

propylene oxide and CO2 with the salan complexes compared to

their salen counterparts (7a and 7b vs. 4c and 4d).22 Similarly, the

high activity of the chromium salalen complex 8 (where the

salalen ligand contains a nitrogen atom with sp2 hybridisation

and the other with sp3 hybridisation, see Fig. 2 and Table 1) in

the copolymerisation of cyclohexene oxide and CO2 under mild

conditions was ascribed to its coordinative flexibility, which can

allow bidentate binding of the growing polymer chain, thereby

decreasing the energy barrier for CO2 insertion.50 More in

general, it has been suggested that the ability of a metal complex

to afford bidentate coordination or coordination of two species

(i.e. epoxide and carbonate or CO2 and ring-opened epoxide) at

the same time, tends to favour the propagation reaction, making

the catalyst selective for polycarbonate synthesis.19

� Functionalisation of the ligand with tailored groups can

positively affect the catalytic performance of a metal complex.

The presence of a side arm bearing a protonated piperidinium

group on a salen ligand was proposed to prevent cyclic

carbonate formation by protonating the polymer chain in case

it dissociated from the metal centre, thus avoiding the back-

biting reaction that would lead to the cyclisation reaction (9 in

Fig. 3 and Table 1).51

Another approach based on the functionalisation of the

ligand aimed at keeping dissociated anionic polymer chains in

the proximity of the metal centre by binding the co-catalyst to

the ligand: cobalt salen complexes with bound quaternary

ammonium units (10 in Fig. 3) displayed improved activity and

selectivity for copolymerisation and generated polycarbonates

with very high molecular weight (Table 1).52–55

� Introducing halide substituents in the ortho-position of

the aromatic rings of the ligands of the bimetallic phenolate

zinc complex 3 (Fig. 1 and Table 1) caused an increase in the

activity of the catalyst in the copolymerisation of cyclohexene

oxide with CO2. The increase was proportional to the electro-

negativity of the halide (Br o Cl o F), which causes a

Fig. 3 Cobalt salen complexes with functionalised side arms.

Fig. 4 b-Diiminate zinc complexes.

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decrease in electron density at the metal centre and, thus, an

increase in the binding ability of the metal to the epoxide.56,57

This is in line with the rate-determining step for bimetallic zinc

complexes being the activation of the epoxide rather than the

insertion of CO2 (see mechanism 2.2.c). The same trend is

followed by the bimetallic zinc complexes 2 in Fig. 1, which

showed a decrease in catalytic activity upon variation of the

substituent in the para-position on the aromatic ring from an

alkyl group to an electron-donating methoxy group.58

� Modifying a ligand towards optimum catalytic perfor-

mance of the metal complex often requires a fine tuning. This

is exemplified by the b-diiminate zinc complexes (Fig. 4 and

Table 1), for which the substituents at both the N-aryl ring and

the diimine backbone can influence strongly the catalytic

behaviour of the complex.8,59 For the b-diiminate zinc acetate

complexes, the N-aryl groups should provide sufficient steric

bulk to prevent the formation of tightly-bound, inactive

dimeric species, but should also not be so bulky to generate

encumbered, inactive monomeric species. The most active

species were shown to have intermediate features, allowing

the formation of highly-active loosely-bound dimers (12 in

Fig. 4).30 The introduction of electron-withdrawing groups

(e.g. �CN, �CF3, 13e–g in Fig. 4) on the diimine backbone

leads to a relevant enhancement of the catalytic activity in the

copolymerisation reaction.60 However, the presence of more

electron-withdrawing groups is detrimental to the activity (13h

in Fig. 4), probably because the consequently increased Lewis

acidity of the metal centre results in too strong metal–oxygen

bonds (see Section 3.1.a).19

� Another example of the electronic and steric effect of the

substituents is provided by bimetallic zinc anilido-aldimine

complexes as catalysts for the copolymerisation of cyclohexene

oxide and CO2 (Fig. 5 and Table 1). Bulky groups on the

aromatic rings can have either a positive or negative effect on

the activity according to their position within the complex.

An increase in activity was also observed using electron-

withdrawing fluorine substituents, whose proposed double

role is to decrease the electron density at the metal centres,

increasing the ability to bind CO2 and epoxide, and to decrease

the basicity of the ligand and, thus, the sensitivity of the

complex to protic impurities.61,62

Two of the most widely studied types of ligands in metal

complexes employed as catalysts for the reaction of carbon

dioxide with epoxides are the porphyrin and the salen ligands

(see Section 2). Both provide complexes with a planar structure

and tetradentate coordination of the metal centre. Depending

on the nature of the metal, these complexes may accommodate

also ligands in the two axial positions. These axial ligands can

be neutral or anionic, they are generally labile and, therefore,

can be displaced during the reaction.

Generally, the rate-determining step of the coupling reaction

between epoxides and CO2 is the epoxide ring-opening by a

nucleophile. This can be an external nucleophile or a nucleo-

phile belonging to the catalyst. In the case of salen or porphyrin

complexes, the nucleophiles occupy the axial sites in the

complex (see mechanisms 2.1.b and 2.2.a). Nucleophile ligands

in axial position may have two roles: as a nucleophile that

dissociates from the metal centre to attack the epoxide substrate

and open its ring; or as a nucleophile that decreases the strength

of the bond between the metal and the nucleophile in the trans

axial position (see mechanisms in Section 2). The features of the

axial ligand can affect both the degree of epoxide conversion

and the selectivity of the reaction. To achieve good activity, the

axial ligand should not be too strongly bound to the metal, so

that is can dissociate from it, while still being a sufficiently good

nucleophile to attack the epoxide. If the axial ligand is able to

dissociate easily from the complex, it might also be a good

leaving group from the metal-bound carbonate intermediates,

thus facilitating the back-biting reaction and favouring cyclic

carbonate production (see Scheme 2). On the other hand,

a nucleophile with poor leaving ability can suppress the

formation of cyclic carbonates and increase the selectivity

towards polycarbonates. If a complex with a nucleophile in

the axial position is used in combination with a co-catalyst

providing a second nucleophilic species to the reaction, the

effect of the axial ligand is tightly related to the nature of

the co-catalyst (see mechanism 2.1.b). These considerations

indicate that a fine tuning is generally required for finding the

optimum axial ligand for the desired reaction (cycloaddition

or copolymerisation).

� For example, the selectivity between polymeric and

cyclic carbonate in the reaction of propylene oxide with CO2

catalysed by a cobalt salen complex in combination with

Bu4NBr as co-catalyst was dramatically increased (from 3 to

78%) by changing the axial ligand (X) of the complex from

acetate to 2,4-dinitrophenolate (DNP) (5b in Fig. 2).63 When the

same complex was used as catalyst with bis(triphenylphosphine)-

iminium chloride [(PPN)Cl] as co-catalyst, a high selectivity towards

polycarbonate was observed also with axial groups that gave low

selectivity with Bu4NBr.45 The reaction rate was only slightly

affected by changes in the axial group, apart from the case in

which this was the non-nucleophilic ClO4�, which caused a

complete loss of activity. In another example with the same type

of cobalt salen complex but without any co-catalyst, it was found

that changes in the axial ligand resulted in substantial differences in

the rate of reaction between propylene oxide and CO2. The activity

decreased in the order X = Br 4 OBzF5 B OAc 4 Cl 4 I,

with a complete selectivity towards the polycarbonate in all

cases.64 This trend was not maintained if the reaction was

carried out in the presence of (PPN)Cl as co-catalyst: in such a

Fig. 5 Bimetallic zinc anilido-aldimine complex.

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case, the rates were significantly higher with all axial ligands,

but the highest turnover was now observed with X = OBzF5

(pentafluorobenzoate), while the selectivity towards the polymer

slightly decreased with X = OAc or Br.45,64

3.2 Co-catalysts and non-metallic catalysts

(a) With a variety of metal complex catalysts for the coupling

reaction between epoxides and CO2, the catalytic performance

can be greatly improved by the addition of a Lewis base as

co-catalyst.14,45 If the metal complex does not contain a group

that can act as a nucleophile (e.g. in Zn-salphen, see Section 3.1.b),

the co-catalyst is actually essential to observe any conversion

in the reaction between epoxide and CO2.48 Similarly to the

role of the nucleophilic axial groups in salen complexes (see

Section 3.1.b), the co-catalyst can play two different roles: as a

nucleophile that attacks and opens the epoxide coordinated to

the metal centre; or as a nucleophile that coordinates to the

metal centre increasing its electron density and thus weakening

the bond with other nucleophilic species (e.g. the growing

polymer chain) and providing a favourable electronic environ-

ment for the insertion of epoxide and CO2 molecules (see

mechanisms in Section 2).

A specific balance between nucleophilicity (towards attack

of the epoxide), coordination ability (towards the metal

centre), and leaving ability (from the carbonate intermediate

species) of the co-catalyst is required for enhancing the activity

and for tuning the selectivity of each different metal complex.

The selectivity is often controlled by the leaving ability of the

nucleophile: a poor leaving group will favour the growth of the

polymer chain while a good leaving group will promote

the back-biting reaction with cyclic carbonate production.

Co-catalysts can be neutral (e.g. an organic base) or ionic

(e.g. an ammonium salt), see Fig. 6. With ionic co-catalysts,

both the anion and the cation may influence the reaction

between CO2 and epoxide.45 For a given anion, the nucleo-

philicity towards the reagents will be higher if its cation is

bulkier and thus exerts lower ion-pairing electrostatic attrac-

tion towards the anion.18

� The ratio between co-catalyst and catalyst has been

reported to play a decisive role in determining the selectivity

towards cyclic or polymeric carbonates. A higher ratio of

nucleophile to metal centres can favour the selectivity towards

cyclic carbonate over polycarbonate, because the excess of

nucleophile tends to displace the metal-bound carbonate

intermediate, thereby favouring the back-biting reaction leading

to the formation of the cyclic product.9,14,65 As an example, it

has been found that a co-catalyst/catalyst ratio higher than 2

tends to favour the selectivity towards the cyclic carbonate with

chromium salen complexes.41,47,66

� The importance of the use of a co-catalyst is exemplified

by the catalytic behaviour of tetraphenylporphyrin aluminium

complexes (15a in Fig. 7). These aluminium complexes without

co-catalyst promoted the homopolymerisation of propylene

oxide with very little incorporation of CO2 into the polymer

chain. On the other hand, in the presence of 4-(dimethylamino)-

pyridine (DMAP) as co-catalyst, the alternating copolymerisation

of propylene oxide and CO2 became the dominant reaction.

The proposed role of DMAP is two-fold: it promotes the

insertion of CO2 into the growing polymer chain and increases

the rate of ring opening of the epoxide by the carbonate

intermediate (mechanism 2.1.b).23

Similarly, the cobalt porphyrin complex 15b in Fig. 7 produced

polycarbonate selectively from the reaction of cyclohexene oxide

and CO2 in the presence of DMAP as co-catalyst in a 0.5–1 ratio

to the complex (Table 1). Under the same reactions conditions,

Fig. 6 Co-catalysts employed in the reaction of CO2 with epoxides.

Fig. 7 Tetraphenylporphyrin metal complexes.

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but without DMAP, poly(cyclohexene oxide) was the only

product. If the DMAP/complex ratio was increased to 10,

both polycarbonate and cyclic carbonate were produced, in

agreement with what is observed with other types of complexes

(vide supra).67

� An example of the effect of the leaving ability of the anion

of the co-catalyst on the selectivity is provided by studies

with the cobalt salen complex 5b (Fig. 2) as catalyst and a

tetrabutylammonium salt (Bu4NY) as co-catalyst in the reac-

tion of propylene oxide with CO2. Changing the anion (Y�) of

the co-catalyst to poorer leaving groups, that is from I� and

Br� to Cl�, F� or CH3COO�, suppressed the back-biting

reaction and thus increased the selectivity towards the poly-

carbonate. In the case of F� and CH3COO� this was accompanied

by a significant decrease in the reaction rate.45,63 The explanation of

this decrease in activity is not straightforward as it requires

considering the possible interactions of the anion with the ammo-

nium cation, with the metal centre and with the reagents.

The same cobalt salen complex 5b (Fig. 2) was employed to

study the role of the steric hindrance of the co-catalyst. It was

proposed that the ideal ionic co-catalyst for achieving high

activity and selectivity in the copolymerisation of carbon

dioxide and epoxides consists of an anion with poor leaving

ability (e.g.Cl�, see above) and of a bulky cation (e.g. bis(triphenyl-

phosphoranylide)ammonium, PPN+). For neutral co-catalysts,

sterically hindered strong organic Lewis bases (e.g. 7-methyl-1,5,7-

triazabicyclo[4.4.0]dec-5-ene (MTBD)) are preferred over other

strong but less bulky organic bases (e.g. N-methylimidazole),

which can strongly coordinate to the cobalt centre, thus making

it catalytically inactive.45

The positive features of bis(triphenylphosphoranylide)-

ammonium chloride as co-catalyst were also demonstrated

by a study of the copolymerisation of cyclohexene oxide with

CO2 with chromium salen complex 5a (X = Cl) in Fig. 2. The

reaction rate was significantly increased using (PPN)Cl instead

of N-methylimidazole as co-catalyst. The improvement was

ascribed to the combination of an anionic nucleophile with a

non-interacting cation as PPN+, in which the positive charge

is delocalised over the large cation. With this Cr-salen complex

and this co-catalyst, mechanism 2.1.b, which implies a shorter

initiation time and thus a higher reaction rate, is expected to be

favoured over mechanism 2.2.a.27

� A study of the effect of the anion of the co-catalyst used in

combination with a bimetallic aluminium salen complex (16 in

Fig. 8 and Table 1) was performed by carrying out the reaction

of styrene oxide with CO2 in the presence of different tetra-

butylammonium halides (Bu4NY, with Y=F, Cl, Br, I). In all

cases the reaction was completely selective towards the cyclic

carbonate, but the conversion of the epoxide varied between a

maximum of 62% with Bu4NBr to almost no conversion with

Bu4NF. The overall order of activity was Br 4 I 4 Cl 4 F.68

This somehow peculiar trend can be explained considering

that the catalytic activity of the halide is determined by a

balance between its nucleophilicity and leaving-group ability:

a good nucleophile would favour the initial opening of the

epoxide ring, but it would also be a worse leaving group in the

last step of the reaction, i.e. in the back-biting reaction leading

to the formation of cyclic carbonate. This balance is also

influenced by the nature of the Lewis acid site, if present,

and by the nature of the cation of the halide salt. Therefore,

different catalytic systems can show different order of activity

between the halides.18,68 For example, Zn-salphen complexes

(6b in Fig. 2) display higher activity in the synthesis of various

cyclic carbonates with Bu4NI as co-catalyst compared to

Bu4NBr.69 Even for the same catalyst, the order of activity

between halides can be different with different epoxides: with a

bimetallic iron amine triphenolate complex as catalyst, Bu4NI

led to higher rates of conversion of terminal epoxides compared

to Bu4NBr, while with sterically hindered epoxides Bu4NBr was

more active. This behaviour has been explained on the basis of

the smaller size of Br�, which can be advantageous in the

nucleophilic attack of sterically congested epoxides.70

Additionally, also the stability of the tetraalkylammonium

salt during the reaction might play a role in the catalytic

process (see Section 2.2.b).29

(b) Ionic or neutral nucleophilic compounds, similar to those

used as co-catalysts together with metal catalysts, can also be

employed alone as catalysts for the addition of carbon dioxide

to epoxides (see Section 2.3). Examples of catalysts of this type

include ionic liquids, ammonium, pyridinium and phospho-

nium salts, and strong organic bases.7,71 These non-metallic

compounds can be used as homogeneous catalysts or be

immobilised on a support to generate heterogeneous catalysts.18

These catalytic systems are intrinsically simpler compared to the

binary systems consisting of a metal complex and a co-catalyst.

This facilitates the correlation between their physicochemical

features and their catalytic properties, particularly in the case of

homogeneous systems. Still, more than one reaction mechanism

is possible with non-metallic catalysts (see Section 2.3) and the

reaction rate depends both on the nucleophilicity of the anion

and on the features of the cation. For example when the anion is

a halide, bulkier cations provide weaker electrostatic attraction

to the halide, thus increasing their nucleophilicity towards the

opening of the epoxide ring.18 This implies that the order of

activity between halides of quaternary ammonium salts can

change as a function of the nature of the ammonium cation (see

previous paragraph).68,72,73 The absence of a metallic Lewis

acid in these catalytic systems implies that generally higher

reaction temperatures are needed to achieve a good degree of

epoxide conversion, driving the reaction towards the thermo-

dynamically favoured cyclic carbonates.7Fig. 8 Bimetallic aluminium salen complex.

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3.3 Substrate

The rate of reaction and selectivity towards polymeric or cyclic

carbonate formation does not depend only on the features of

the catalytic system, but is also influenced by the nature of the

epoxide substrate. Both steric and electronic effects play a role.

� Steric congestion around the epoxide ring tends to cause a

decrease in conversion rate in the reaction with carbon dioxide.

This steric effect is more likely to hinder the nucleophilic attack of

the epoxide rather than its coordination to the Lewis acid metal

centre.48,69,70,74 This explains the lower conversion degree usually

observed with internal epoxides (e.g. cyclohexene oxide) or

a-disubstituted epoxides (e.g. 1,1-dimethyloxirane) compared to

those of terminal epoxides with a linear alkyl chain (e.g. propylene

oxide, 1,2-epoxyhexane).48,70

� Cyclohexene oxide has a higher tendency to form poly-

carbonate rather than cyclic carbonate compared to other epoxides

such as propylene oxide or styrene oxide. This preference of

cyclohexene oxide for polycarbonate formation is ascribed to the

strain in forming the five-membered cyclic carbonate ring imposed

by the adjacent cyclohexane ring.14,75 As a consequence, the

synthesis of poly(propylene carbonate) is often accompanied by

the formation of cyclic carbonate while the synthesis of

poly(cyclohexene carbonate) tends to proceed more selectively.

� The nature of groups on the epoxide can affect its

tendency to produce preferentially either the cyclic or the

polymeric carbonate. For example, epoxides with electron-

withdrawing groups, such as styrene oxide, have a higher

tendency to produce cyclic carbonate compared to aliphatic

terminal or internal epoxides.76 This behaviour has been

explained on the basis of the most likely position at which

the epoxide ring is attacked by a nucleophile, leading to ring

opening (Fig. 9). For terminal epoxides such as propylene oxide, the

nucleophilic attack is considered to occur predominantly at the less

sterically hindered carbon atom of the epoxide ring (b-carbon),9

both due to its higher accessibility and to the electron-donating

effect of the alkyl group. On the other hand, for epoxides such

as styrene oxide the nucleophilic ring-opening occurs mainly at

the a-carbon as a consequence of the electron-withdrawing

inductive effect of the phenyl group. This can lead, after CO2

insertion, to an intermediate in which the electron-withdrawing

nature of the aromatic ring can favour the back-biting reaction

with formation of cyclic styrene carbonate (Scheme 10).46,76

Similarly, the electron-withdrawing nature of the chloromethyl

group of epichlorohydrin tends to drive the reaction towards the

cyclic carbonate product.77 In the case of propylene oxide, even if

the nucleophilic attack at the b-carbon of the epoxide ring is

favoured, the attack at the a-carbon can occur. This is demon-

strated by the generally observed presence of head-to-head and

tail-to-tail connections in the poly(propylene carbonate) chain,

whereas if the nucleophilic attack occurred always at the same

position the polymer would contain exclusively head-to-tail

connections (Fig. 10).23,45 A high percentage of head-to-tail

connections (regioregularity) is desirable as it can increase the

crystallinity and improve properties of the polymer including

thermal resistance, toughness and stiffness (see challenge 1).14

The nature of the catalytic system can strongly affect the

regioregularity of the obtained polycarbonate.63,64 Particularly,

it has been suggested that a chiral environment around the metal

centre can promote the regioselectivity of the reaction.46

� The nature of the epoxide substrate has a strong influence

on the properties of the alternating copolymer obtained upon

reaction with CO2. As mentioned in challenge 1, the physico-

chemical properties of these polycarbonates should be enhanced

in order to grant their applicability. This could be achieved by

increasing the rigidity of the polymer backbone through the

choice of suitable epoxides. Poly(propylene carbonate) has

good mechanical properties but rather low glass transition

temperature (B40 1C), while poly(cyclohexene carbonate) has

higher glass transition temperature (115 1C) but is brittle.

Recently, indene oxide has been introduced as substrate for

the synthesis of a promising polycarbonate, displaying the

highest glass transition temperature (134 1C) reported so far

for alternating copolymers of epoxides and carbon dioxide.14,78

This poly(indene carbonate) is thermally stable up to 249 1C,

which is in the same range as the thermal decomposition

temperature of poly(propylene carbonate) and lower than that

of poly(cyclohexene carbonate) (B300 1C).8,14

� Oxetanes represent another class of compounds that has

been recently studied as substrates for the formation of cyclic

and polymeric carbonates upon reaction with CO2.70,79,80

Compared to epoxides, oxetanes are characterised by lower ring

strain, which implies that the ring opening step is more challenging.

On the other hand, binding of the oxetanes to the Lewis acidic

metal centre can be favoured by their higher basicity compared to

epoxides.80 Anothermain difference between epoxides and oxetanes

is that for the former the five-membered cyclic carbonates are

the thermodynamically favoured products, while for the latter

the polycarbonate is thermodynamically more stable than the

six-membered cyclic carbonate.

3.4 Solvent and reaction conditions

(a) In order to achieve optimum performance of the catalytic

system, the contact between all components taking part in the

reaction should be maximised (challenge 4).69 In the case of a

Fig. 9 Most likely positions for a nucleophilic attack of the ring in

different types of epoxides.

Scheme 10 Possible back-biting reactions in the reaction of styrene

oxide with CO2.

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reaction with a binary homogeneous catalytic system, e.g. a

metal complex and an ammonium salt, this implies trying to

get a homogeneous solution containing CO2, the epoxide, the

catalyst and the co-catalyst. This can be a challenging task, as

these components can cover a rather broad range of polarity.

To complicate the situation, the solubility of CO2 in liquids

depends on its density,81 which increases with increasing

pressure and with decreasing temperature, while the solubility

of the catalyst and co-catalyst in an organic solution generally

increases with temperature. This means that it is difficult to

predict the effect of a change of temperature on the overall

solubility between the components. Moreover, a high degree

of dissolution of CO2 into the organic solution can cause a

decrease in the solubility of the catalytic species and possibly

induce their precipitation.82 In some cases, the catalyst and/or

co-catalyst are not soluble in the epoxide substrate in the

concentration chosen for the catalytic test, and a solvent is

added to the reaction mixture to achieve full dissolution of the

catalytic species. The presence of a solvent does not necessarily

affect the solubility of CO2 into the reaction mixture, but

would always cause a dilution of the solution, thus decreasing

the contact between the reaction components. This can

negatively affect the reaction rate, as demonstrated by the

much higher activity of a Zn-salphen/Bu4NI catalytic system

in a solvent-free, CO2-rich environment compared to that in

CH2Cl2 or other solvents.49,69 This means that if the catalytic

species dissolve in the epoxide at the temperature at which the

reaction will be carried out, the use of any additional solvent

should be generally avoided.

Another example of the importance of the contact between

the reaction components was provided by the synthesis of cyclic

carbonates carried out in imidazolium-based ionic liquids with

varying length of the alkyl chain of the cation (C2 to C8):

choosing ionic liquids bearing a longer alkyl chain and working

under conditions in which carbon dioxide is supercritical led to

enhanced catalytic activity as a consequence of the higher

solubility of epoxides and CO2 in the ionic liquid phase.83

If the desired product of the reaction of CO2 with epoxides

is the polycarbonate, the presence of alcohols or water even in

small amounts should be avoided, as these polar molecules can

react with the growing polymer chain to yield a hydroxyl-

terminated polycarbonate and a metal-alkoxide or metal-

hydroxide species from which a new polymer chain can grow.

Therefore, these chain transfer reactions limit the number of

repeating units and, thus, the molecular weight of the polymer

products.14 Moreover, it has been reported that the interaction

of water with the metal centre of the catalyst can cause

substantial loss in activity and polymer selectivity.45

(b) CO2 can be in its gas, liquid or supercritical state (i.e. above

its critical point, 31.1 1C, 73.8 bar) under the reaction conditions.

Performing the catalytic test in supercritical CO2 rather than

with CO2 in the gas phase can be advantageous for maintaining

a good contact between the components participating in the

reaction, particularly if the epoxide substrate has a relatively

low boiling point and would tend to shift to the gas phase at low

CO2 pressure.69

Carbon dioxide dissolves into epoxides but in general the

two species do not form a single phase. On the basis of the

temperature and of the CO2 pressure, the amount of CO2

dissolved can vary significantly, and in some conditions the

volume of the phase containing the epoxide can expand

sensibly upon CO2 dissolution. Carrying out the reaction in

a reactor with a viewing window can provide useful informa-

tion on the phase behaviour of the system.69

The effect of CO2 pressure on the catalytic activity and selectivity

was studied using cobalt and chromium salen complexes (Fig. 2) as

catalysts for the copolymerisation of CO2 and epoxides.47,63,84 In

general, it was found that the activity increases with CO2 pressure

until a maximum, after which the insertion of CO2 is not anymore

the rate-limiting step and further increase of the pressure causes

dilution of catalyst and substrate, thus decreasing the reaction rate.

On the other hand, the selectivity towards the cyclic carbonate side-

product with the Cr-salen catalyst was lower at higher CO2

pressure, possibly because high CO2 concentrations favour the

insertion of this molecule over the back-biting reaction leading to

cyclic carbonate.84 A similar trend of activity as a function of CO2

pressure, with a maximum value at an intermediate pressure and a

dilution effect causing a decrease of activity with increasing CO2

pressure, has been observed with heterogeneous catalysts based on

supported ionic liquids.32,33,36

(c) Since cyclic carbonates are thermodynamically more

stable than polycarbonates, carrying out the reaction of CO2

and epoxides at elevated temperatures can increase the selectivity

towards the formation of cyclic carbonates. On the other hand,

decreasing the reaction temperature will decrease the reaction

rate but can enhance the selectivity towards the polycarbonates:

since the activation energy for the synthesis of polycarbonates

is usually lower than that for the thermodynamically more

favoured cyclic carbonates, a lower reaction temperature can

allow a kinetic control of the reaction, thus favouring the

polycarbonates. This strategy has been employed to steer the

reaction towards polycarbonates with epoxides that would

otherwise tend to give higher selectivity towards the cyclic

carbonate, as those bearing an electron-withdrawing group

(see Section 3.3).76,77

4. Homogeneous vs. heterogeneous catalysts

The next generation of catalysts for the reaction of carbon

dioxide with epoxides should not only display enhanced

Fig. 10 Possible epoxide/CO2 connections in polycarbonates.

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activity and selectivity towards the desired carbonate product

(cyclic or polymeric), but should also fulfil the requirements

for a green, sustainable process. Therefore, research efforts

should be centred on catalytic systems that do not contain

potentially toxic species (such as, e.g., chromium or 4-(dimethyl-

amino)pyridine), are active in the absence of solvent (see

Section 3.4) and can be easily separated from the reaction

products and reused in consecutive runs without loss of

activity (challenge 5). The last feature is essential in order to

develop catalysts that could find industrial application in the

production of cyclic and polymeric carbonates. On the basis of

these considerations, a shift in the focus of the research from

homogeneous towards heterogeneous catalysis is expected. So

far, heterogeneous catalysts for the reaction of carbon dioxide

with epoxides have been studied less extensively compared to

homogeneous systems. The factors determining the activity

and selectivity of homogeneous catalysts (see Section 3) can

apply to their heterogeneous counterparts, but the design

flexibility that is typical of metal complexes is restricted in

the case of heterogeneous catalysts by the requirement of a

material that can be readily recovered and reused. The most

known and widely employed heterogeneous catalyst for the

copolymerisation of CO2 and epoxides is zinc glutarate,

[Zn(O2C(CH2)3CO2)]n.8,14 This zinc dicarboxylate catalyses

the production of poly(propylene carbonate) with high

molecular weight and with only minor contamination from

ether linkages or cyclic carbonate.14 However, its turnover

frequency is significantly inferior compared to that of the

most active metal complex catalysts.19 The catalytic reaction

catalysed by zinc glutarate has been proposed to follow the

bimetallic mechanism described in Section 2.2.c.20 It has been

suggested that the distance between two neighbouring zinc

centres is crucial in determining the activity of this family of

heterogeneous catalysts. Double metal cyanides (e.g.

Zn3[M(CN)6]2 with M= Fe or Co) are another class of widely

investigated heterogeneous catalysts for the copolymerisation

of CO2 and epoxides.14 However, these catalysts can present

relevant drawbacks, such as low CO2 incorporation in

the polymer chain and the requirement for harsh reaction

conditions. Metal oxides, zeolites and clays have also been

studied as heterogeneous catalysts for the reaction of CO2 with

epoxides, but all these systems suffer from one or more serious

limitations such as the requirement for a homogeneous base as

co-catalyst, low activities, need for harsh reaction conditions

in terms of temperature and CO2 pressure, or leaching of the

active species.85–87 A totally different strategy for preparing a

heterogeneous catalyst for the coupling reaction of CO2

and epoxides consists in immobilising active homogeneous

catalysts on the surface of a support.7,18,85 Typical materials

that can be used as supports are polymers or silica-based

materials, including mesoporous ordered materials with high

specific surface area such as MCM-41 and SBA-15.32,36,88,89

The immobilisation of the catalytically active species should be

performed through robust covalent grafting to the support, in order

to avoid possible leaching of active species during the reaction.88,89

A general drawback of supported catalysts compared to their

homogeneous counterparts is the lower activity due to diffusion

limitations of reagents and products between the solution and the

catalyst surface.90 This effect can be minimised by using supports

with sufficiently large pores, and by tuning the loading of the

supported species. On the other hand, in some cases the support can

play a catalytic role and the activity of the heterogeneous catalyst

can be higher than that of its homogeneous counterpart. For

example, the silica-supported phosphonium salt SiO2-[PrBu3P]Br

is more active than the homogeneous salt catalyst in the synthesis of

cyclic carbonate from CO2 and propylene oxide.91 This result was

ascribed to the presence of surface silanol groups in proximity of the

immobilised salt (Scheme 11). These silanols can act as electrophiles

and interact with the oxygen atom of the epoxide, increasing its

reactivity towards the ring opening by the Br� nucleophile with a

similar mechanism to that sketched in Scheme 9 (see Section 2.3).

Among supported catalysts, relatively few examples of immo-

bilisation of organometallic complexes have been reported,

while supported non-metallic catalysts, and particularly ionic

liquids, are receiving growing attention.7,18,85

Future research should aim at designing and preparing

heterogeneous catalysts with improved activity and a good

control of the product selectivity under milder reaction conditions

compared to the high temperatures and CO2 pressures

generally employed with the heterogeneous systems known

so far. This will most probably imply developing bifunctional

heterogeneous catalysts, containing both Lewis acidic metal

centres and nucleophilic species. A promising example in

this direction was provided recently by bimetallic Al-salen

complexes with quaternary ammonium bromides covalently

attached to the salen ligand, supported on amorphous silica or

MCM-41 (Fig. 11).7,92 These materials were tested as hetero-

geneous catalysts for the reaction of different epoxides with

CO2 both in batch and continuous flow reactors. The catalysts

showed good activity, but tended to deactivate upon reuse in the

batch mode, or after prolonged use in the flow mode. The

deactivation was attributed to degradation through dequaternisa-

tion of the ammonium salt (see Section 2.2.b) and the catalysts

could be reactivated by treating them with benzyl chloride to

regenerate the quaternary ammonium species.

In the perspective of an industrial application, it is also

important to reduce the costs connected to catalysts production.

This issue could be particularly relevant when developing

heterogeneous catalysts based on metal complexes immobilised

on a support, as the synthesis of many of the most commonly

Scheme 11 Proposed role of silanols in the reaction of CO2 with propylene oxide.

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employed complexes (see Sections 2 and 3) is very expensive.93

Reduction of the costs could be achieved by using ligands that

require less expensive synthesis procedures, while still leading

to catalysts with a good performance.

5. Direct synthesis of cyclic carbonates and

polycarbonates using alkenes as substrates

The epoxides used as substrates in the reaction with carbon

dioxide are typically prepared by the catalytic oxidation of

alkenes.94,95 The development of a process for the synthesis

of cyclic carbonates and polycarbonates using alkenes as

substrates would be attractive both from the point of view

of sustainability and of industrial application (challenge 6).

Performing the reaction with this single-step route would be

intrinsically safer and involve lower costs, as it would not

require the purification and handling of epoxides, which

are much more toxic than alkenes and carbonates. This

route would require a multifunctional catalyst that is able to

promote the oxidation of alkenes to epoxides and the following

reaction of the latter with CO2 to yield the carbonates

(Scheme 12). Alternatively, separate, non-interfering catalysts

could be employed. The process could be carried out as a one-

pot reaction or by two consecutive steps, with the addition of

CO2 being performed once the epoxidation is complete. On the

basis of these considerations, it can be foreseen that develop-

ing an efficient process for the direct synthesis of organic

carbonates from alkenes will involve challenges both

in catalyst design and process technology. Few studies of

catalysts for the direct synthesis of carbonates from alkenes

have been reported, typically combining known catalysts for

the two separate reactions (oxidation of alkenes and addition

of CO2 to epoxides).7,96 However, these systems are still

far from optimal and future research efforts should aim at

achieving higher activity with truly heterogeneous catalysts,

while minimising the formation of side-products of the

epoxidation reaction, controlling the selectivity towards

the desired carbonate, and using H2O2 or O2 as oxidant for

the conversion of the alkene to the epoxide.

6. Conclusions

The use of carbon dioxide as a feedstock for the production of

useful chemicals is a research topic that is receiving growing

attention both in academia and industry. In this context,

the reaction of CO2 with epoxides to produce either cyclic

carbonates or polycarbonates shows a large potential for

application. In this article, the effect of the various components

of the catalytic system and of the reaction conditions on the

reaction rate and selectivity were systematically discussed. This

knowledge is important in the perspective of tackling the various

open challenges that were outlined throughout the article:

(1) Development of polycarbonates that have suitable

properties for applications.

(2) Control of the selectivity of the reaction towards the

desired carbonate (cyclic or polycarbonate).

(3) Improvement of the catalytic activity in order to reach

high turnover frequencies under mild conditions for a broad

range of substrates including sterically hindered and internal

epoxides and oxetanes.

(4) Optimisation of the reaction conditions in order to

maximise the contact between reagents and catalysts.

(5) Development of heterogeneous catalysts that can be

easily separated and reused without loss of activity while being

competitive with the best homogeneous catalysts.

(6) Direct synthesis of cyclic carbonates and polycarbonates

from alkenes.

Achieving these targets will involve research efforts both

from scientific and technological points of view.

The design of novel, enhanced catalysts for the reaction of

CO2 with epoxides is a complex task: many parameters

determine the performance of the catalysts, and these factors

are often strongly interwoven. Therefore, changing the same

parameter can have an opposite effect on different catalytic

systems. This implies that the optimisation of the features of a

given catalytic system will generally require a fine tuning of its

different components (e.g. metal, ligands, co-catalyst). From

these considerations it becomes clear that rational design of

novel catalytic systems for the reaction of CO2 with epoxides

should be supported by a rich experimental activity. For this

purpose, the use of High-Throughput techniques can be very

beneficial as they enable carrying out a large set of catalytic

tests in parallel.97 This approach allows the rapid screening of

libraries of catalysts, and at the same time grants a high degree

of reliability in the comparison of different samples, as all

tests can take place simultaneously under exactly the same

conditions of temperature and pressure.36 Experimental work

can be nicely complemented by theoretical calculations,20,41,98

which can provide useful insight into the interaction between

the catalytic species and the reagents, thus helping the ratio-

nalisation of the experimental results and the design of new

catalysts and processes.

References

1 M. Mikkelsen, M. Jørgensen and F. C. Krebs, Energy Environ.Sci., 2010, 3, 43.

2 M. Aresta and A. Dibenedetto, Dalton Trans., 2007, 2975.3 E. A. Quadrelli, G. Centi, J.-L. Duplan and S. Perathoner,ChemSusChem, 2011, 4, 1194.

Fig. 11 Supported bimetallic salen complex.

Scheme 12 Direct synthesis of cyclic carbonates and polycarbonates

from alkenes.

Publ

ishe

d on

29

June

201

2. D

ownl

oade

d on

18/

02/2

015

14:0

6:27




4 T. Sakakura, J. C. Choi and H. Yasuda, Chem. Rev., 2007,107, 2365.

5 M. Anpo, H. Yamashita, Y. Ichihashi, Y. Fujii and M. Honda,J. Phys. Chem. B, 1997, 101, 2632.

6 T. Sakakura and K. Kohno, Chem. Commun., 2009, 1312.7 M. North, R. Pasquale and C. Young, Green Chem., 2010, 12, 1514.8 G. W. Coates and D. R. Moore, Angew. Chem., Int. Ed., 2004,43, 6618.

9 D. J. Darensbourg, Chem. Rev., 2007, 107, 2388.10 S. Fukuoka, M. Kawamura, K. Komiya, M. Tojo, H. Hachiya,

K. Hasegawa, M. Aminaka, H. Okamoto, I. Fukawa andS. Konno, Green Chem., 2003, 5, 497.

11 B. Schaffner, F. Schaffner, S. P. Verevkin and A. Borner, Chem.Rev., 2010, 110, 4554.

12 P. Lenden, P. M. Ylioja, C. Gonzalez-Rodrıguez, D. A. Entwistleand M. C. Willis, Green Chem., 2011, 13, 1980.

13 T. Ogasawara, A. Debart, M. Holzapfel, P. Novak and P. G. Bruce,J. Am. Chem. Soc., 2006, 128, 1390.

14 M. R. Kember, A. Buchard and C. K. Williams, Chem. Commun.,2011, 47, 141.

15 G. A. Luinstra, Polym. Rev., 2008, 48, 192.16 R. Zevenhoven, S. Eloneva and S. Teir, Catal. Today, 2006,

115, 73.17 A. Decortes, A. M. Castilla and A. W. Kleij, Angew. Chem., Int.

Ed., 2010, 49, 9822.18 F. Jutz, J.-M. Andanson and A. Baiker, Chem. Rev., 2011,

111, 322.19 S. Klaus, M. W. Lehenmeier, C. E. Anderson and B. Rieger,

Coord. Chem. Rev., 2011, 255, 1460.20 S. Klaus, M. W. Lehenmeier, E. Herdtweck, P. Deglmann,

A. K. Ott and B. Rieger, J. Am. Chem. Soc., 2011, 133, 13151.21 Y.-M. Shen, W.-L. Duan and M. Shi, J. Org. Chem., 2003,

68, 1559.22 D.-Y. Rao, B. Li, R. Zhang, H. Wang and X.-B. Lu, Inorg. Chem.,

2009, 48, 2830.23 M. H. Chisholm and Z. Zhou, J. Am. Chem. Soc., 2004,

126, 11030.24 D. J. Darensbourg, Inorg. Chem., 2010, 49, 10765.25 E. N. Jacobsen, Acc. Chem. Res., 2000, 33, 421.26 D. J. Darensbourg and J. C. Yarbrough, J. Am. Chem. Soc., 2002,

124, 6335.27 D. J. Darensbourg, R. M. Mackiewicz, J. L. Rodgers and

A. L. Phelps, Inorg. Chem., 2004, 43, 1831.28 J. Melendez, M. North and R. Pasquale, Eur. J. Inorg. Chem.,

2007, 3323.29 M. North and R. Pasquale, Angew. Chem., Int. Ed., 2009, 48, 2946.30 D. R. Moore, M. Cheng, E. B. Lobkovsky and G. W. Coates,

J. Am. Chem. Soc., 2003, 125, 11911.31 T. Seki, J.-D. Grunwaldt and A. Baiker, J. Phys. Chem. B, 2009,

113, 114.32 Y. Xie, Z. Zhang, T. Jiang, J. He, B. Han, T. Wu and K. Ding,

Angew. Chem., Int. Ed., 2007, 46, 7255.33 J.-Q. Wang, X.-D. Yue, F. Cai and L.-N. He, Catal. Commun.,

2007, 8, 167.34 J. Sun, W. Cheng, W. Fan, Y. Wang, Z. Meng and S. Zhang,

Catal. Today, 2009, 148, 361.35 L. Han, H.-J. Choi, S.-J. Choi, B. Liu and D.-W. Park, Green

Chem., 2011, 13, 1023.36 C. Aprile, F. Giacalone, L. Liotta, J. A. Martens, P. P. Pescarmona

and M. Gruttadauria, ChemSusChem, 2011, 4, 1830.37 Y. L. Xiao, Z. Wang and K. L. Ding, Macromolecules, 2006,

39, 128.38 R. Srivastava, T. Bennur and D. Srinivas, J. Mol. Catal. A: Chem.,

2005, 226, 199.39 Y. Xiao, Z. Wang and K. Ding, Chem.–Eur. J., 2005, 11, 3668.40 H. Sugimoto and S. Inoue, Pure Appl. Chem., 2006, 78, 1823.41 G. A. Luinstra, G. R. Haas, F. Molnar, V. Bernhart, R. Eberhardt

and B. Rieger, Chem.–Eur. J., 2005, 11, 6298.42 M. R. Kember, p. D. Knight, P. T. R. Reung and C. K. Williams,

Angew. Chem., Int. Ed., 2009, 48, 931.43 M. R. Kember, A. J. P. White and C. K. Williams, Macromole-

cules, 2010, 43, 2291.44 D. J. Darensbourg and E. B. Frantz, Inorg. Chem., 2007, 46, 5967.45 X.-B. Lu, L. Shi, Y.-M. Wang, R. Zhang, Y.-J. Zhang, X.-J. Peng,

Z.-C. Zhang and B. Li, J. Am. Chem. Soc., 2006, 128, 1664.

46 G.-P. Wu, S.-H. Wei, W.-M. Ren, X.-B. Lu, B. Li, Y.-P. Zu andD. J. Darensbourg, Energy Environ. Sci., 2011, 4, 5084.

47 R. Eberhardt, M. Allmendinger and B. Rieger, Macromol. RapidCommun., 2003, 24, 194.

48 A. Decortes, M. Martınez Belmonte, J. Benet-Buchholz andA. W. Kleij, Chem. Commun., 2010, 46, 4580.

49 A. Decortes and A. W. Kleij, ChemCatChem, 2011, 3, 831.50 K. Nakano, M. Nakamura and K. Nozaki, Macromolecules, 2009,

42, 6972.51 K. Nakano, T. Kamada and K. Nozaki, Angew. Chem., Int. Ed.,

2006, 45, 7274.52 E. K. Noh, S. J. Na, S. Sujith, S. W. Kim and B. Y. Lee, J. Am.

Chem. Soc., 2007, 129, 8082.53 S. Sujith, J. K. Min, J. E. Seong, S. J. Na and B. Y. Lee, Angew.

Chem., Int. Ed., 2008, 47, 7306.54 S. J. Na, S. Sujith, A. Cyriac, B. E. Kim, J. Yoo, Y. K. Kang,

S. J. Han, C. Lee and B. Y. Lee, Inorg. Chem., 2009, 48, 10455.55 J. Yoo, S. J. Na, H. C. Park, A. Cyriac and B. Y. Lee, Dalton

Trans., 2010, 39, 2622.56 D. J. Darensbourg, J. R. Wildeson, J. C. Yarbrough and

J. H. Reibenspies, J. Am. Chem. Soc., 2000, 122, 12487.57 C. Koning, J. Wildeson, R. Parton, B. Plum, P. Steeman and

D. J. Darensbourg, Polymer, 2001, 42, 3995.58 M. R. Kember, A. J. P. White and C. K. Williams, Inorg. Chem.,

2009, 48, 9535.59 M. Cheng, D. R. Moore, J. J. Reczek, B. M. Chamberlain,

E. B. Lobkovsky and G. W. Coates, J. Am. Chem. Soc., 2001,123, 8738.

60 D. R. Moore, M. Cheng, E. B. Lobkovsky and G. W. Coates,Angew. Chem., Int. Ed., 2002, 41, 2599.

61 B. Y. Lee, H. Y. Kwon, S. Y. Lee, S. J. Na, S. I. Han, H. S. Yun,H. Lee and Y. W. Park, J. Am. Chem. Soc., 2005, 127, 3031.

62 T. Bok, H. Yun and B. Y. Lee, Inorg. Chem., 2006, 45, 4228.63 X.-B. Lu and Y. Wang, Angew. Chem., Int. Ed., 2004, 43, 3574.64 C. T. Cohen, T. Chu and G. W. Coates, J. Am. Chem. Soc., 2005,

127, 10869.65 A. Buchard, M. R. Kember, K. G. Sandeman and C. K. Williams,

Chem. Commun., 2011, 47, 212.66 D. J. Darensbourg, P. Bottarelli and J. R. Andreatta,Macromolecules,

2007, 40, 7727.67 H. Sugimoto and K. Kuroda, Macromolecules, 2008, 41, 312.68 W. Clegg, R. Harrington, M. North and R. Pasquale, Chem.–Eur. J.,

2010, 16, 6828.69 M. Taherimehr, A. Decortes, S. M. Al-Amsyar,

W. Lueangchaichaweng, C. J. Whiteoak, E. C. Escudero-Adan,A. W. Kleij and P. P. Pescarmona, Catal. Sci. Technol., 2012, DOI:10.1039/c2cy20171b.

70 C. J. Whiteoak, E.Martin, M.Martınez Belmonte, J. Benet-Buchholzand A. W. Kleij, Adv. Synth. Catal., 2012, 354, 469.

71 V. Calo, A. Nacci, A. Monopoli and A. Fanizzi, Org. Lett., 2002,4, 2561.

72 J.-Q. Wang, D.-L. Kong, J.-Y. Chen, F. Cai and L.-N. He, J. Mol.Catal. A: Chem., 2006, 249, 143.

73 Y. Zhao, J.-S. Tian, X.-H. Qi, Z.-N. Han, Y.-Y. Zhuang andL.-N. He, J. Mol. Catal. A: Chem., 2007, 271, 284.

74 D. J. Darensbourg, R. R. Poland and A. L. Strickland, J. Polym.Sci., Part A: Polym. Chem., 2012, 50, 127.

75 D. J. Darensbourg and A. L. Phelps, Inorg. Chem., 2005, 44, 4622.76 G. P. Wu, S. H. Wei, X. B. Lu, W. M. Ren and D. Darensbourg,

Macromolecules, 2010, 43, 9202.77 G.-P. Wu, S.-H. Wei, W.-M. Ren, X.-B. Lu, T.-Q. Xu and

D. J. Darensbourg, J. Am. Chem. Soc., 2011, 133, 15191.78 D. J. Darensbourg and S. J. Wilson, J. Am. Chem. Soc., 2011,

133, 18610.79 D. J. Darensbourg and A. I. Moncada,Macromolecules, 2009, 42, 4063.80 D. J. Darensbourg, A. Horn and A. I. Moncada, Macromolecules,

2010, 12, 1376.81 P. G. Jessop, T. Ikariya and R. Noyori, Chem. Rev., 1999, 99, 475.82 P. G. Jessop and B. Subramaniam, Chem. Rev., 2007, 107, 2666.83 H. Kawanami, A. Sasaki, K. Matsui and Y. Ikushima, Chem.

Commun., 2003, 896.84 D. J. Darensbourg, R. M. Mackiewicz and D. R. Billodeaux,

Organometallics, 2005, 24, 144.85 W.-L. Dai, S.-L. Luo, S.-F. Yin and C.-T. Au, Appl. Catal., A,

2009, 366, 2.

Publ

ishe

d on

29

June

201

2. D

ownl

oade

d on

18/

02/2

015

14:0

6:27




86 M. Tu and R. J. Davis, J. Catal., 2001, 199, 85.87 H. Yasuda, L. N. He, T. Takahashi and T. Sakakura, Appl. Catal.,

A, 2006, 298, 177.88 C. Baleizao, B. Gigante, M. J. Sabater, H. Garcıa and A. Corma,

Appl. Catal., A, 2002, 228, 279.89 M. Alvaro, C. Baleizao, D. Das, E. Carbonell and H. Garcıa,

J. Catal., 2004, 228, 254.90 K. Yu and C. W. Jones, Organometallics, 2003, 22, 2571.91 T. Takahashi, T. Watahiki, S. Kitazume, H. Yasuda and

T. Sakakura, Chem. Commun., 2006, 1664.92 M. North, P. Villuendas and C. Young,Chem.–Eur. J., 2009, 15, 11454.

93 M. North and C. Young, Catal. Sci. Technol., 2011, 1, 93.94 D. E. De Vos, B. F. Sels and P. A. Jacobs, Adv. Synth. Catal., 2003,

345, 457.95 P. P. Pescarmona, K. P. F. Janssen and P. A. Jacobs, Chem.–Eur. J.,

2007, 13, 6562.96 J. Sun, L. Liang, J. Sun, Y. Jiang, K. Lin, X. Xu and R. Wang,

Catal. Surv. Asia, 2010, 15, 49–54.97 P. P. Pescarmona, J. C. van der Waal, I. E. Maxwell and

T. Maschmeyer, Catal. Lett., 1999, 63, 1.98 Y. Ren, C.-H. Guo, J.-F. Jia and H.-S. Wu, J. Phys. Chem. A,

2011, 115, 2258.

Publ

ishe

d on

29

June

201

2. D

ownl

oade

d on

18/

02/2

015

14:0

6:27



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