Biobased Materials and Biocatalysis - American Chemical Society

Chapter 9

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Combining Sustainable Polymerization Routes for the Preparation of Polyesters, Polycarbonates, and Copolymers Charles Romain and Charlotte K. Williams* 317 RCS1, Department of Chemistry, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom *E-mail: [email protected].

Ring-Opening Polymerizations (ROP) and Ring-Opening Copolymerizations (ROCOP) of ‘oxygenated’ cyclic monomers (lactones, cyclic carbonates, epoxides, anhydrides) are promising routes to afford well-defined polymers. It is notable that both ROP and ROCOP require monomers that could be bio-derived, in line with the aspirations of sustainable polymerization processes. In addition, these two different methodologies can be combined to extend the scope of ‘oxygenated’ synthetic polymers. We have recently reported that a single catalyst may be applied for both ROCOP and ROP, so as to readily produce via a ‘one-pot’ procedure various copolymers. We have also uncovered a novel chemoselective catalyst control pathway, whereby the nature of the metal chain end group controls the polymerization cycle, and thereby the composition of the copolymer.

Introduction Routes for Polyesters and Polycarbonates Polyesters (PE) and polycarbonates (PC) are important classes of polymers which may contain different repeat unit chemistry (including combinations of aromatic, semi-aromatic, saturated backbones) and therefore have various different applications (including areas such as packaging, fibres and specialist © 2015 American Chemical Society In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


medical applications). PE and PC are industrially produced on large scale (> 5Mt/year), mainly by polycondensation methods: the condensation of diols and diacids affords polyesters whereas the condensation of diols and phosgene yields polycarbonates (1). Despite their commercial success, from the point of view of new materials synthesis, such routes are generally poorly controlled, require harsh conditions, only allow access to limited types of polymer architectures and produce side-products.

Figure 1. Ring-opening polymerization routes for the preparation of polyesters and polycarbonates.

Alternatively, aliphatic PE and PC can be obtained by the ring opening polymerization (ROP) of cyclic esters (2–4) or cyclic carbonates (5, 6), respectively (Figure 1). Thus, for example, the ROP of ε-caprolactone (ε-CL) yields the corresponding polycaprolactone (PCL), whereas the ROP of trimethylene carbonate (TMC) yields the corresponding linear polytrimethylene carbonate PTMC. These routes are well-controlled and yield, under mild conditions, well-defined polymers (with predicatable values for Mn, PDI, end-groups, etc). A plethora of catalysts (2–6) have been reported for ROP, some of which are highly stereocontrolled, enabling control over the resulting polymer tacticity (7). However, ROP is limited in terms of the substrate scope, in particular there remain only a narrow range of commercially available lactones/cyclic carbonates. Most studies are carried out using lactide (LA), ε-caprolactone (ε-CL) or β-butyrolactone (β-BL) or with trimethylene carbonate (TMC). It is notable that β-malolactatonates, monomers which are difficult to homopolymerize using ROP, have recently been copolymerized with other cyclic esters and carbonates as a route to polyhydroxyalkanoates (PHA) (8, 9). Indeed, the synthesis of new cyclic esters (10, 11) and cyclic carbonates for use in ROP is an important field (12–14). 136 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


As described in Figure 1, PE and PC can also be obtained by the ring-opening copolymerizartion (ROCOP) of epoxides and anhydrides/carbon dioxide (15, 16). Thus, the perfectly alternating copolymerization of epoxides with cyclic anhydrides affords polyesters (17–19), whereas the perfectly alternating copolymerization of epoxides with carbon dioxide affords polycarbonates (20–26). These ROCOP reactions are controlled polymerizations, enabling predictable polymer chain lengths (Mn, PDI), end group functionalities and, in some cases, tacticities. The ROCOP route is also advantageous in that it combines two different monomers, thereby enabling the differentiation of polymer composition/structure, and therefore polymer thermal and mechanical properties, by changing either or both of the co-monomers (17, 27). Control over the ROCOP stereochemistry has very recently been exemplified by Coates and co-workers, in the first report of stereocomplex polyester prepared via ROCOP of propylene oxide and succinic anhydride (27).

Renewable Monomers for Polyesters and Polycarbonates It is notable that both ROP and ROCOP routes require monomers, namely cyclic esters, cyclic carbonates, epoxides and anhydrides (Figure 2), that can be bio-derived (4, 28), in line with the goals of sustainable polymerization processes (29).

Figure 2. Selected examples of commonly investigated monomers that could be derived from renewable resources.

The use of CO2 as a carbon feedstock for PC synthesis is also of high interest from a sustainable chemistry perspective as CO2 is abundant, cheap and non-toxic (24). A recent life cycle analysis study on CO2-derived polycarbonates shows that the incorporation of 20% wt. of CO2 reduces the greenhouse gas emissions by 11-19% and reduces fossil resource depletion by 13-16%, as compared to an equivalent petrochemical polymer (such as polyethers) (30). 137 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Considering the syntheses of cyclic monomers from biomass, the fermentation of carbohydrates enables the facile production of (di)acids such as lactic acid and succinic acid. These diacids can subsequently be converted to lactide and succinic anhydride, respectively (31). In addition, from a hydroxymethylfurfural (HMF) platform, it is possible to obtain monomers such as ε-CL or phthalic anhydride (Figure 3) (32, 33). For example, Lobo et al, recently reported the synthesis of phthalic anhydride resulting from the Diels-Alder reaction between furan and maleic anhydride, followed by the subsequent elimination of water (33). Naturally derived terpenes are a viable source for various different epoxides, including limonene oxide and pinene oxide (31, 34). In addition, in collaboration with Meier and co-workers we have reported the synthesis of unsatured cyclohexene oxide, starting from fatty acid derived 1,4-cyclohexanediene (Figure 3) (35). Thus, there is a considerable potential for both the ROP and ROCOP methodologies to apply bio-derived monomers in line with the tenets of new green polymerization processes (29).

Figure 3. Selected examples of the synthesis of cyclic esters, cyclic anhydrides and epoxides obtained from renewable resources.

Tandem Catalyses for Poly(ester-co-carbonates) Given the success of the ROCOP and ROP routes to polyesters and polycarbonates, research has focussed on combining these two different methodologies to extend the scope of synthetic polymers and enable the synthesis of block copolymers such as poly(ester-co-carbonates) (16, 36–41). Thus, by combing two ROP processes, Romain et al. reported a sequential ‘one-pot’ synthesis, using a single catalyst, to prepare poly(trimethylene carbonate-co-lactides) featuring a heterotactic PLA block (36). Similarly, Guillaume and Carpentier and co-workers have described various syntheses, characterizations and properties of poly(ester-co-carbonates) obtained via sequential ROP of cyclic carbonates and cyclic esters (including lactones and 138 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


β-malolactonates) (37, 38, 42). Furthermore, Darensbourg et al. reported a tandem catalysis approach which combines ROCOP and ROP (39). The use of a cobalt salen catalyst for the ROCOP of styrene oxide/CO2 enables the efficient production of polystyrene carbonate, then after the addition of water to quench the ROCOP reaction, the DBU catalysed ROP of lactide can occur yielding block copolymers (39). Our research group have also applied tandem catalyses to combine ROCOP and ROP. Using a dizinc catalyst for ROCOP enabled the selective production of polycarbonate polyols. These polycarbonates were subsequently applied as macro-initiators for the yttrium catalysed ROP of lactide, enabling the preparation of ABA type poly(lactide-co-cyclohexenecarbonate-lactides) (43). As part of on-going studies into the preparation of block copoly(estercarbonates), we have recently discovered that a single catalyst may be applied for both ROCOP and ROP, so as to produce various copolymers (40). We have also uncovered a novel chemoselective catalyst control, whereby the nature of the metal chain end group controls the polymerization cycle, and thereby the composition of the copolymer.

Di-Zinc and Magnesium Catalysts for ROCOP Epoxide/CO2 Copolymerizations Our group have previously reported a series of bimetallic homogeneous catalysts for epoxide/CO2 copolymerizations (44–49). In particular, the di-zinc (44–47) and di-magnesium (48) catalysts were of significant potential due to their ability to operate under mild conditions with high activities and selectivities. The catalysts are homogeneous bimetallic complexes, whereby the metals are coordinated by a macrocyclic ancillary ligand and carboxylate co-ligands. The molecular structures of the carboxylate derivatives show that the complexes are ‘bowl shaped’, with one κ2-acetate group bridging the two metal centres on the concave face and one κ1-acetate co-ligand coordinating on the convex face of the molecule (46, 47).

Figure 4. The ROCOP processes for the synthesis of polycarbonates (a) and polyesters (b). 139 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Both the di-zinc and magnesium catalysts are active at only 1 atmosphere pressure of CO2 affording perfectly alternating polycarbonates (> 99% carbonate linkages) (46, 48). The catalysts are also highly selective for polymer formation (>96%), with little or no formation of cyclic carbonate by-products (using cyclohexene oxide as the epoxide). Kinetic investigations, using the di-zinc catalyst, showed that the rate law is first order with respect to epoxide concentration and first order with respect to catalyst concentration (46). There is no dependence of the polymerization rate on CO2 pressure, between 1 and 40 bar, suggesting a zero order dependence with respect to CO2. This rate law suggests that the CO2 insertion reaction is not the rate determining step. The study of the catalytic cycle using DFT suggests a polymer chain shuttling mechanism (Figure 5) (47). According to such a mechanism, during propagation, the zinc alkoxide bond attacks CO2 leading to the formation of a new zinc carbonate bond. Subsequently, this zinc carbonate species attacks a coordinated CHO molecule to regenerate a zinc alkoxide bond. The DFT calculations suggest that the rate limiting step is carbonate attack on bound epoxide, in line with the experimental kinetic investigation (Figure 5). Epoxide/Anhydride Copolymerizations The promising results obtained using bimetallic catalysts for epoxide/CO2 copolymerizations prompted the investigation of the same catalysts for epoxide/anhydride copolymerizations (Figure 4). Both the di-zinc and magnesium catalysts were investigated for the ROCOP of cyclohexene oxide and phthalic anhydride. Both catalysts showed good activity, at 100 °C in neat epoxide or in toluene, forming alternating polyesters without significant formation of ether linkages (50). The catalysts show moderate to good activities, with turn-over-frequencies up to 100 h-1 for the di-magnesium catalyst (which was four times more active than the di-zinc catalyst). Kinetic studies showed that the rate was zero order (independent) of anhydride concentration (50), in line with the related zero order dependence on CO2 for the related ROCOP of CO2/epoxides. It is proposed that during propagation, there is a fast insertion of the anhydride into the zinc-alkoxide bond, followed by the slow insertion of the epoxide into the zinc carboxylate bond (Figure 5).

ROP of Cyclic Esters Polyester versus Polycarbonate Formation The success of the bimetallic catalysts for ROCOP prompted the investigation of the same catalysts for the ROP of cyclic esters. Interstingly, the di-zinc catalyst, 1 (Figure 4), was unable to polymerize ε-CL, either in neat monomer or under a range of solvent conditions. The ROP reactions were unsuccessful even in the presence of excess alcohol, which may function as a chain transfer agent, or under a range of different temperatures. It was discovered, however, that the addition of a catalytic quantity of epoxide (10 mol. % cyclohexene oxide vs. ε- CL) yielded an active ROP catalyst system (Figure 6 and Table 1). The catalyst was 140 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


able to polymerize up to 200 equivalents of ε-caprolactone in less than 1 hour. Alternatively, the cyclohexene oxide can be used as the reaction solvent (where it is present in approximately nine times higher concentration than ε-caprolactone) resulting in the exclusive formation of polycaprolactone (i.e. there is no evidence at all for any formation of poly(cyclohexene oxide).

Figure 5. Chain-shuttling mechanisms proposed for the ROCOP processes.

Figure 6. Polyester or polycarbonate formation, under N2 (left) and CO2 gas (right) atmospheres.

Interestingly, using the same monomer mixture (ε-CL/CHO = 200/800) but adding CO2 led to exclusive formation of polycarbonate (polycyclohexene carbonate), resulting from ROCOP without any formation of polycaprolactone (Figure 6). Comparing the activity of the di-zinc catalyst for both polymerizations, under the same reaction conditions, shows that the ROCOP is significantly slower (TOF = 20 h-1) compared to the ROP of ε-CL (TOF ~ 100 h-1). Thus, the observed selectivity appears not to relate to the overall activity of the catalysts, but rather to particular selectivity occurring during the catalytic cycles. 141 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Table 1. Different Reaction Conditions for ROP and ROCOP Using 1 CHO (eq.)

ε-CL (eq.)

CO2 (bar)

Solvent

Time

Polymer

Mna (g/mol)

PDIa

-

200

-

Neat

16h

-

-

-

200

-

Toluene

2hb

PCL

21040

1.4

900

100

-

Neat

2hb

PCL

6020

1.2

900

100

1

Neat

15h

PCHC

1040

1.1

20

polystyrene standards,

b


Reaction conditions: 1 = 1 eq., 80 °C. Not optimized time.

a Determined by SEC vs.

Figure 7. The two polymerization cycles, enabling the production of polycarbonates (left) or polyesters (right). Chemoselectivity Such a reactivity can be rationalized by gaining insight into the mechanism. Thus, whereas catalyst 1 is unable to ring-open ε-CL in the absence of any additive, the catalyst reacts, and ring-opens, an equivalent of epoxide (CHO) to generate, in-situ, an alkoxide group as described in Figure 7. Under a N2 atmosphere, this latter alkoxide intermediate (i) is a viable initiator for the ROP 142 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


of ε-CL affording production of PCL. Under the same conditions, but where CO2 is present, it is proposed that rapid CO2 insertion into the in-situ generated Zn-alkoxide bond [intermediate (i)] occurs immediately leading to the formation of a zinc carbonate species (ii). The latter zinc carbonate intermediate (ii) is unable to initiate the ROP of ε-CL but is a viable intermediate and initiator for the ROCOP of cyclohexene oxide/carbon dioxide to afford polycarbonate production. Thus, it is proposed that the catalytic selectivity arises from the nature of the Zn-O bond. The zinc carbonate or carboxylate species can only initiate/propagate ROCOP, whereas the zinc alkoxide species can initiate/propagate both ROCOP and ROP. Thus, by controlling the nature of the Zn-O bond, a particular catalytic cycle can be selected. Combining ROP and ROCOP Routes With this mechanistic hypothesis and observed chemo-selectivity, it was possible to combine and control the ROP and ROCOP processes using a single catalyst as described in Figure 8 (40).

Figure 8. Combination of ROP and ROCOP processes, in one-pot with a single catalyst. The di-zinc catalyst afforded PCL quantatively, using a mixture of ε-CL/CHO (200/800). The subsequent addition of CO2 led to the in-situ formation of the zinc-carbonate intermediate (i) which initiated the ROCOP process and led to the subsequent formation of the polycarbonate block (Figure 9). The reverse order of monomer additions was also viable: when the dizinc catalyst was mixed with epoxide/CO2/lactone, the polycarbonate block was selectively formed. Then, after the removal of the CO2, using cycles of vacuum/nitrogen, the carbonate species (i) was transformed to the zinc alkoxide species (iii), by reaction with epoxide, and this species led to quantitative formation of a PCL block. 143 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 9. Illustrates the ‘switch’ between ROP and ROCOP processes.

Conclusions Di-zinc and magnesium catalysts were good initiators for various ringopening copolymerizations, using either epoxides/CO2 or epoxides/anhydrides and affording the corresponding polycarbonates or polyesters. The di-zinc catalyst, in the presence of epoxide, was also an active initiator (catalyst) for the ring-opening polymerization of ε-caprolactone. In addition, the di-zinc catalyst was able to selectively form block copoly(ester-carbonates) using mixtures of up to three different monomers and by combining the ROCOP and ROP processes. Finally, in the presence of an exogenous switch reagent, it is possible to ‘switch’ from ROCOP to ROP, or vice versa, to selectively afford various block copolymers. The scope of this methodology will be extended to include other cyclic monomers and it is envisaged it could lead to the production of new copolymers of controllable composition and properties.

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146 In Green Polymer Chemistry: Biobased Materials and Biocatalysis; Smith, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Biobased Materials and Biocatalysis - American Chemical Society

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