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Polymer Degradation and Stability

journal homepage: www.elsevier.com/locate/polydegstab

Comments on the depolymerization of derived from epoxides and carbon dioxide: A mini review

∗ Donald J. Darensbourg

Department of Chemistry, Texas A&M University, College Station, TX, 77843, United States

ARTICLE INFO ABSTRACT

Keywords: There are in general few examples where can be effectively recycled to their . If this si- Depolymerization tuation exists, the of these monomers to polymeric materials which possess identical properties and Carbon dioxide applications of their virgin derived counterparts is possible. In this brief review we have examined the Epoxides degradation of polycarbonates derived from carbon dioxide and epoxides to provide either their thermo- Degradation dynamically more stable product, the corresponding cyclic five-membered carbonate, or in special instances Recyclable their return to starting monomers. A mechanistic understanding of these pathways allows for the synthesis of polymers that will favor the latter pathway.

1. Introduction carbonate chain end by epoxide and subsequent ring-opening by anionic carbonate [17]. The living process is gen- The depolymerization of polymeric materials to selectively produce erally quenched by the addition of acidic methanol resulting in a hy- their corresponding monomers is highly desirable, nevertheless gen- droxyl end group. The chain growth process is explicitly described in erally not likely. Recently, it has been demonstrated for various poly- Scheme 3, where the free polymer chain has been shown to rapidly carbonates produced via the coupling of CO2 and epoxides that this is undergo depolymerization via a backbiting process. The development of possible, thereby, showing high levels of reversible polymerization re- second-generation bifunctional (salen)MX catalysts (Fig. 1), where the actions under rather mild reaction conditions. Since the copolymer- salen ligand bears a covalently tethered onium salt which supplies the ization of CO2 and epoxides is emerging as a viable and potentially nucleophile for epoxide ring-opening, aids in retarding this backbiting significant utilization of carbon dioxide, it seems appropriate to analyze process [18,19]. In this instance, the dissociated anionic copolymer current achievements in depolymerization pathways of these polymers chain maintains close contact with the metal complex via electrostatic [1–14]. Thereby, this analysis might lead to the design of polymers interaction with the positive charge of the onium salt. having the ability to be converted into their starting monomers. Con- Backbiting of the metal-free anionic copolymer chain has been sequently, this will allow for recycling of these monomers to materials shown to undergo cyclic carbonate formation much faster than the possessing identical properties and uses of their virgin polymeric ma- metal-bound copolymer (Scheme 4). Based on computational studies, terials. the transition state for proceeding from the open polymer chain to the Our initial studies of the depolymerization of a variety of copoly- tetrahedral intermediate is generally rather small, however, going from mers produced from CO2 and epoxides revealed the process to proceed the intermediate to cyclic carbonate and the corresponding anionic via a backbiting pathway leading to the corresponding cyclic carbonate polymer chain represents the activation barrier for cyclic carbonate following deprotonation of a hydroxyl end group by base (Scheme 1a) formation (Scheme 5)[20].

[15]. Alternatively, under special circumstances, the fate of the boxed Under copolymerization reaction conditions, where the CO2 pressure intermediate in Scheme 1a can lead to reformation of the corresponding is significant, the backbiting process proceeding via either process, i.e., monomers (Scheme 1b) [16]. metal-bound or metal-free, would be expected to occur by way of a car- Prior to discussing the depolymerization pathways for copolymers bonate end group. However, in the case of the degradation of an isolated produced from CO2 and epoxides, it is useful to review the catalytic copolymer with a hydroxyl end group, following deprotonation, back- mechanism for the metal catalyzed formation of these polycarbonates. biting would follow the alkoxide pathway as illustrated in Scheme 5. Scheme 2 depicts the commonly accepted series of steps relevant to Scheme 2 represents an idealized presentation of the pathway for copolymer production, where, the rds is the displaced of the growing copolymer formation in the absence of trace water or other protic

∗ Corresponding author. E-mail address: [email protected]. https://doi.org/10.1016/j.polymdegradstab.2018.01.019 Received 18 December 2017; Accepted 18 January 2018 $YDLODEOHRQOLQH)HEUXDU\ ‹(OVHYLHU/WG$OOULJKWVUHVHUYHG D.J. Darensbourg 3RO\PHU'HJUDGDWLRQDQG6WDELOLW\  ²

Scheme 1. sources, where in this instance the thus formed is of the process monitored by in situ infrared spectroscopy, the depolymeriza- composition Nuc OH. In the presence of adventitious or added tion process exhibited an induction period of 10min reaction period water, the copolymer consists of a bimodal distribution of Nuc OH (Fig. 2). The induction period and rate of depolymerization were ob- and HO OH polymer chains due to rapid and reversible chain served to be highly dependent on the nature and concentration of the transfer processes, or exclusively HO OH depending on the quan- anion, suggestive of an equilibrium process between protonated and tity of added water and the nature of the initiating nucleophile. It deprotonated polymer chains (equation (2)). Indeed, in the presence of should be noted here that the hydroxyl end groups of these copolymers a stong base such as sodium bis(trimethylsilyl)amide (NaHMDS), these can serve as macro-initiators for ring-opening polymerization (ROP) depolymerization reactions exhibit no induction period. reactions with other cyclic monomers. For example, see equation (1) for In order to better define the mechanism of the depolymerization the formation of a block copolymer between polypropylene carbonate process, the molecular weight of the copolymer, along with its poly- and polylactide. These processes can be carried out in a two-step, one- dispersity were monitored as a function of time. As illustrated in Fig. 3, pot process, and clearly demonstrate from the lack of any cyclic car- where the azide concentration has been greatly reduced relative to that bonate production that initiation of lactide polymerization is faster than in Fig. 2 in order to slow down the rate of depolymerization, sequential backbiting. We have summarized our efforts in this general area for loss of cyclic carbonate was observed with little increase in molecular formation of a variety of block polymers employing this technique weight distribution which is indicative of a stepwise process as shown [21–23]. in Scheme 6.

(1)

The generally observed pathway for the depolymerization of poly- The energy of activation barrier (Ea) for the depolymerization of carbonates derived from CO2 and epoxides is formation of the ther- poly(styrene carbonate) to styrene carbonate for reactions carried out modynamically more stable product of CO2/epoxide coupling, the in the presence of azide anion was determined to be 46.7 ± 2.2 (kJ/ cyclic carbonate. Previously, we have examined several of these pro- mol). This is similar to the activation barrier noted for the formation of cesses by way of infrared spectroscopy and size exclusion chromato- styrene carbonate during the copolymerization of styrene oxide and graphy which I will describe in detail below. CO2 catalyzed by (salen)CoX complexes in the presence of initiating Our initial study involved the depolymerization of a purified sample anions (50.7 kJ/mol). On the other hand, depolymerization of a pure of well-characterized poly(styrene carbonate) which contained 99% poly(styrene carbonate) sample in the presence of n-Bu4NN3 and carbonate linkages [15]. Of importance, the copolymer's molecular (salen)CrCl displayed a much higher Ea value of 141.2 kJ/mol. These weight distribution was monomodal and narrow, i.e., deprotonation of observations taken together indicate that production of cyclic carbo- the hydroxyl end groups of the copolymer chain was carried out using nates during copolymerization reactions originates from the free an- anions similar to those employed in the polymerization reactions. For ionic copolymer chains as previously described. example, in the presence of [nBu4N][N3] in toluene at 70 °C for a The depolymerization of other aliphatic polycarbonates were shown

 D.J. Darensbourg 3RO\PHU'HJUDGDWLRQDQG6WDELOLW\  ²

afford cis-indene carbonate by a photochemical process (vide infra) [24]. An important deviation from the depolymerization pathway dis- cussed above was first discovered for the copolymer derived from cy- clopentene oxide and carbon dioxide. In this instance, the products of

the degradation were the epoxide and CO2 [25]. That is, poly(cyclo- pentene carbonate) undergoes conversion into its corresponding monomers, thereby providing an ideal method for recycling. The investigation of the depolymerization of poly(cyclopentene carbonate)was motivated by a high-level computational study [16]. This latter analysis predicted the process depicted in Scheme 1b had a lower energy barrier (13.3 kcal/

mol) than the Ea (19.9 kcal/mol) for the generally seen depolymeriza- tion pathway shown in Scheme 1a. By way of contrast, in the process illustrated in Scheme 1a the trans isomer of cyclopentene carbonate is expected, whereas the cis isomer is provided upon degradation. These observations are outlined in Scheme 7 in more detail. As seen in Scheme 7, trans-cyclopentene carbonate has to result from the backbiting attack of the deprotonated hydroxyl chain end on the carbonate carbon center. However, the trans-cyclopentene carbo- nate product was calculated to be 19.2 kcal/mol higher in enthalpy than its boat conformer. This is a consequence of the trans-conformation Scheme 2. being highly unstable due to the strain energy of two five-membered fused rings. Alternatively, cis-cyclopentene carbonate results from the

backbiting of a carbonate end group, hence, under reduced CO2 pres- sure this pathway is retarded relative to epoxide formation. See Figs. 4 and 5 for the product distribution from the degradation of poly(cyclo- pentene carbonate) as a function of pressure. Unlike the degradation of the aliphatic polycarbonates which un- dergo depolymerization at greatly inhibited rates in the presence of binary metal catalyst systems, poly(cyclopentene carbonate) degrades

much more rapidly in the presence of (salen)CrCl/n-Bu4NN3 than fol- lowing deprotonation of the hydroxyl end group by strong base. As indicated in Fig. 6, under these conditions the product is mainly the epoxide monomer, cyclopentene oxide. As was the case in the absence of metal catalyst, the production of cyclopentene oxide was decreased

and cyclic carbonate was increased in the presence of added CO2. Scheme 3. It is noteworthy here to point out that poly(indene carbonate) (A), which also would provide the trans isomer of indene carbonate which possesses two fused five-membered rings upon backbiting of the de- protonated hydroxyl end group of the copolymer chain, affords a small quantity of indene oxide during the depolymerization process [24]. Similar to the degradation of poly(cyclopentene carbonate), the amount of indene oxide produced was greatly enhanced in the case of the metal assisted depolymerization reaction.

The recent report by Liu, Lu and coworkers was inspired by the

Fig. 1. Cobalt complex used for epoxide/CO2 copolymerization. unprecedented observations we described for the depolymerization pathway exhibited by the completely recyclable poly(cyclopentene to similarly undergo endwise scission reactions to generate the corre- carbonate) [26]. In their study, the polycarbonate derived from 1- fi sponding cyclic carbonates [15]. For example, the energy of activation benzyloxycarbonyl-3,4-epoxy pyrrolidine (BEP), a ve-membered barriers were observed to increase in the order: poly(styrene carbonate) cyclic ether, and CO2 in the presence of a dinuclear Cr(III)-salen catalyst was shown to be quantitatively recyclable to its epoxide monomer and < poly(CO2-alt-epichlorohydrin) (76.2 kJ/mol) ≤ poly(propylene car- CO under rather mild reaction conditions (Scheme 8). bonate) (80.5 kJ/mol). This trend and magnitude in Ea values are in 2 good agreement with those observed during the formation of cyclic A highly noteworthy feature of this study was that it was demon- ffi carbonates afforded in the copolymerization processes. Hence, it can be strated to be possible to achieve a very e cient reversible process assumed that these processes are occurring via similar pathways, i.e. an which was governed by the temperature changes. That is, the poly- unzipping of the polycarbonate from its anionic chain end by a back- carbonate derived from BEP/CO2 with greater than 99% carbonate biting route. By way of contrast, poly(indene carbonate) was shown to linkages could be readily recycled several times with no loss in epoxide

 D.J. Darensbourg 3RO\PHU'HJUDGDWLRQDQG6WDELOLW\  ²

Scheme 4.

Scheme 5.

monomer or copolymer during a continuous process. Because BEP can be produced from furfural, a renewable resource, this process is pro- posed to be truly sustainable. Nevertheless, the synthesis of 1-benzy- loxycarbonyl-3,4-epoxy pyrrolidine (BEP) requires numerous steps of reasonable yields as reported by these researchers. A most promising report appeared recently in the literature, where a readily available epoxide obtainable from a renewable source, limo- nene oxide, was found to undergo depolymerization in the presence of a base. The observations described by Sablong, Koning and coworkers are depicted in Scheme 9 [27]. This study is particularly noteworthy be- cause it contains a commonly affordable bio-based epoxide monomer, and a monomer which does not involve a five-membered ring system. In the previously discussed cases, the epoxide monomers are af- forded upon polycarbonate depolymerization due to the unfavorability of two trans five-membered rings. However, in the instance of poly(li- monene carbonate) depolymerization, an alternative explanation is ff Fig. 2. ReactIR profile of styrene carbonate formation in the depolymerization of poly o ered. It should be noted that Auriemma and coworkers have shown

(styrene carbonate) using nBu4NN3. computationally that the isopropenyl and methyl groups preferentially occupy the equatorial positions of the chain conformation of the

Fig. 3. The reaction profile for cyclic styrene carbonate formation initiated by azide, along with the molecular weight data for the remaining copolymer chain.

 D.J. Darensbourg 3RO\PHU'HJUDGDWLRQDQG6WDELOLW\  ²

Scheme 6.

Scheme 7. Proposed reaction pathways for base initiated depolymerization of poly(cyclopentene carbonate).

Fig. 4. Depolymerization of poly(cyclopentene carbonate) in d-toluene at 110 °C sub- sequent to deprotonation with NaHMDS. The 1H NMR resonance from d-toluene was used as an internal standard. No other products were found during the degradation process. Fig. 6. Copolymer and product distribution during the depolymerization of poly(cyclo-

Extended time periods omitted from plots. pentene carbonate) in the presence of (salen)CrCl/n-Bu4NN3 in d-toluene at 110 °C.

Scheme 8. Fig. 5. Depolymerization of poly(cyclopentene carbonate) in d-toluene at 110 °C sub- sequent to deprotonation with NaHMDS under a reduced pressure.

 D.J. Darensbourg 3RO\PHU'HJUDGDWLRQDQG6WDELOLW\  ²

Scheme 9.

Fig. 7. (a) Copolymerization of trans-LMO with CO2. (b) Depolymerization of PLC to generate trans-LMO.

cyclohexane ring [28]. Hence, in either cis- or trans-limonene oxide with two trans five-membered fused rings. Of importance as well, li- (LMO), there is a preference for axial attack on the epoxide during the monene oxide is shown to degrade to its monomers. This is particularly ring-opening step leading to a diaxial disposition of the zinc alkoxide noteworthy since it represents an easily obtained polycarbonate derived and growing polymer chain. This is depicted in Fig. 7a for the trans- from limonene, a renewable resource [14]. LMO monomer. Similarly, the hydroxyl end group and the copolymer chain occupy diaxial positions, and upon its deprotonation is unable to Declarations of interest attack the carbon center of the carbonate chain (Fig. 7b). Instead, attack of the tertiary alkoxide chain end takes place on the adjacent carbon None. atom bonded to the carbonate group resulting formation of trans-LMO. This mechanistic interpretation is supported by the stability towards Acknowledgment depolymerization of the acetate end-capped poly(limonene carbonate), as well as the ease of depolymerization of poly(1-methyl cyclohexene We gratefully acknowledge the financial support of the National carbonate) to its corresponding cyclic carbonate under the same reac- Science Foundation (CHE-1665258) and the Robert A. Welch tion conditions. Foundation (A-0923). Following decarboxylation of the anionic copolymer chain, re- peating this backbiting pathway ultimately leads to complete formation References of trans-LMO and carbon dioxide. This mechanistic route is so far un- ique to poly(limonene carbonate), and provides a path for recycling a [1] D.J. Darensbourg, M.W. Holtcamp, Catalytic activity of zinc(II) phenoxides which possess readily accessible coordination sites. Copolymerization and terpolymer- copolymer derived from a biobased epoxide and CO2 affording a truly ization of epoxides and carbon dioxide, Macromolecules 28 (1995) 7577–7579. sustainable polymeric material. It has further been shown by Rieger and [2] G.W. Coates, D.R. Moore, Discrete metal-based catalysts for the copolymerization of coworkers via in situ infrared spectroscopic monitoring that at certain CO2 and epoxides: discovery, reactivity, optimization, and mechanism, Angew. limonene oxide concentrations, polymerization/depolymerization exist Chem. Int. Ed. 43 (2004) 6618–6639. [3] H. Sugimoto, S. Inoue, Copolymerization of carbon dioxide and epoxide, J. Polym. in equilibrium at 60 °C [29]. Sci. Polym. Chem. 42 (2004) 5561–5573. [4] D.J. Darensbourg, R.M. Mackiewicz, A.L. Phelps, D.R. Billodeaux, The copolymer- 2. Conclusions ization of CO2/epoxides catalyzed by metal salen complexes, Acc. Chem. Res 37 (2004) 836–844. [5] M.H. Chisholm, Z. Zhou, New generation polymers: the role of metal alkoxides as Analysis of the mechanistic pathways for the degradation of poly- catalysts in the production of polyoxygenates, J. Mater. Chem. 14 (2004) carbonates derived from epoxides and carbon dioxide has provided the 3081–3092. background for designing copolymers based on the nature of the ep- [6] D.J. Darensbourg, Making from carbon dioxide. Salen metal complexes as catalysts for the production of polycarbonates from epoxides and CO2, Chem. Rev. oxide monomers. For example, inspired by computational studies, it has 107 (2007) 2388–2410. been demonstrated that copolymers synthesized from epoxides derived [7] S. Klaus, M.W. Lehenmeier, C.W. Anderson, B. Rieger, Recent advances in CO2/ — from five-membered olefins, such as cyclopentene, are reluctant to epoxide copolymerization new strategies and cooperative mechanisms, Coord. Chem. Rev. 255 (2011) 1460–1479. degrade to their cyclic carbonate counterparts. Noting that these [8] M.R. Kember, A. Buchard, C.K. Williams, Catalysts for CO2/epoxide copolymer- polycarbonates degrade to their thermodynamically more stable cy- isation, Chem. Commun. 47 (2011) 141–163. cloaddition products, trans-cyclic carbonate, in the above instance the [9] X.-B. Lu, D.J. Darensbourg, Cobalt catalysts for the coupling of CO2 and epoxides to provide polycarbonates and cyclic carbonates, Chem. Soc. Rev. 41 (2012) expected cyclic carbonate is unstable due to the strain energy associated

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