catalysts

Review Terpolymerization of CO2 with and Cyclic Organic Anhydrides or Cyclic

David Hermann Lamparelli and Carmine Capacchione *

Dipartimento di Chimica e Biologia “A. Zambelli”, Università degli Studi di Salerno, Via Giovanni Paolo II n◦132, 84184 Fisciano, Italy; [email protected] * Correspondence: [email protected]; Tel.: +39-089-9543

Abstract: The synthesis of polymeric materials starting from CO2 as a feedstock is an active task of research. In particular, the copolymerization of CO2 with epoxides via ring-opening copolymerization (ROCOP) offers a simple, efficient route to synthesize aliphatic (APC). In many cases, APC display poor physical and chemical properties, limiting their range of application. The

terpolymerization of CO2 with epoxides and organic anhydrides or cyclic esters offers the possibility, combining the ROCOP with ring-opening (ROP), to access a wide range of materials containing and polyester segments along the chain, showing enhanced properties with respect to the simple APC. This review will cover the last advancements in the field, evidencing the crucial role of the catalytic system in determining the microstructural features of the final polymer.

Keywords: CO2; epoxides; cyclic esters; cyclic anhydrides; ROCOP; ROP

 

Citation: Lamparelli, D.H.; 1. Introduction Capacchione, C. Terpolymerization of The pervasiveness of in our daily life is a consolidated reality in recent

CO2 with Epoxides and Cyclic decades. Indeed, the reason for the fortune of polymeric materials is due to both their Organic Anhydrides or Cyclic Esters. unique physical and chemical properties and their cheapness compared to other structural Catalysts 2021, 11, 961. https:// materials. In the last few years, however, there is a growing body of evidence that the large doi.org/10.3390/catal11080961 success of these materials has determined a negative effect on terrestrial and marine ecosys- tems with the presence of microplastics that have become ubiquitous on our planet [1]. Academic Editor: Shaofeng Liu This situation has engendered the growing attention of the polymer industries and the scientific community to find biodegradable, more sustainable polymeric materials. Parallel Received: 20 July 2021 to this trend, the use of CO2 as a carbon feedstock has also gained momentum due to the Accepted: 9 August 2021 rising interest in using such an inexpensive, non-toxic molecule as a starting material for Published: 11 August 2021 the synthesis of polymers [2,3]. In particular, the alternating ring-opening copolymerization (ROCOP) of CO2 with Publisher’s Note: MDPI stays neutral epoxides has offered a conceptually simple route for the synthesis of aliphatic polycarbon- with regard to jurisdictional claims in ates (APC), which show a clear advantage in terms of biodegradability with respect to published maps and institutional affil- polyolefins [4–6]. APC, however, often display poor chemical and mechanical properties iations. compared to aromatic polycarbonates, and the incorporation of epoxides with various structural features does not always result in an improvement in the final properties of APC [7–9]. Aliphatic polyesters are another important class of biopolymers that can be conveniently obtained by the ring-opening polymerization (ROP) of cyclic esters [10–13] Copyright: © 2021 by the authors. or by the ROCOP of epoxides with cyclic anhydrides [14–17]. Transition metal complexes Licensee MDPI, Basel, Switzerland. generally catalyze all these polymerization processes through a coordination–insertion This article is an open access article mechanism. Actually, taking a closer look at the polymerization mechanism of the ROCOP distributed under the terms and of epoxides with CO or cyclic anhydrides depicted in Scheme1, it is evident that the for- conditions of the Creative Commons 2 mation of the metal-alkoxo bond (a’ in Scheme1) is a key intermediate in the propagation Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ process. In analogy, the ROP of cyclic esters proceeds via the formation of a metal-alkoxo 4.0/). bond (a in Scheme1) that allows the ring-opening of the following monomer unit.

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Catalysts 2021, 11, 961 process. In analogy, the ROP of cyclic esters proceeds via the formation of a metal-alkoxo2 of 19 bond (a in Scheme 1) that allows the ring-opening of the following monomer unit.

SchemeScheme 1. Representative 1. Representative mechanisms mechanisms of ofthe the copolymerization copolymerization ofof epoxides,epoxides, CO CO2,2 and, and cyclic cyclic esters esters (I) and(I) and the ROCOPthe ROCOP of of CO2 orCO cyclic2 or cyclic anhydrides anhydrides with with epoxides epoxides (II ().II ).

GivenGiven this this mechanistic mechanistic scenario, scenario, itit isis easyeasy to to imagine imagine that that it is it possible is possible to design to design a a metalmetal complex complex able able to to promote both both types types of polymerization,of polymerization, allowing allowing us to obtain us to variousobtain vari- block-. In principle, it is possible to obtain copolymers with polycarbonate and ous block-copolymers. In principle, it is possible to obtain copolymers with polycarbonate polyester segments and modulate the nature of the polycarbonate and polyester blocks, andpermitting polyester the segments synthesis ofand new modulate materials the with na tailoredture of properties. the polycarbonate and polyester blocks, Notwithstandingpermitting the synthesis the potential of ne ofw such materials an approach, with tailored the efforts properties. to develop efficient catalyticNotwithstanding systems that cannotthe potential only incorporate of such COan 2approach,but also give the rise efforts to unprecedented to develop newefficient catalyticmaterials systems have raised that cannot good results only onlyincorporate recently. CO2 but also give rise to unprecedented new materialsThis review have will raised cover good the last results advancements only recently. (since 2003) in the metal-catalyzed and metal-freeThis review terpolymerization will cover the oflast CO advancements2 with epoxides (since and cyclic2003) estersin the ormetal-catalyzed cyclic organic and anhydrides for the obtaining of polycarbonate–polyester copolymers. metal-free terpolymerization of CO2 with epoxides and cyclic esters or cyclic organic an- hydrides for the obtaining of polycarbonate–polyester copolymers. 2. Terpolymerization of CO2 with Epoxides and Cyclic Anhydrides Polymeric materials containing and linkages have shown potential 2. Terpolymerizationas biodegradable implants of CO and,2 with in Epoxides addition, it and is possible Cyclic toAnhydrides adjust the degradation rate byPolymeric regulating materials the length containing of the polyester ester and and polycarbonate carbonate linkages blocks. Thehave first shown approach potential to as biodegradablesynthesize poly(ester-block-carbonate)s implants and, in addition, in it a one-potis possible reaction to adjust was the the copolymerization, degradation rate by regulatingvia ROP, the of cyclic length esters of the and polyester cyclic and polycarbonate promoted by blocks. stannous The octanoate first approach [18]. This to syn- thesizepotentally poly(ester-block-carbonate)s straightforwrd way has some in a limitations one-pot becausereaction six was or seven-memberedthe copolymerization, cyclic via carbonates suitable for ROP must be synthesized through time-consuming multistep ROP, of cyclic esters and cyclic carbonates promoted by stannous octanoate [18]. This con- protocols [19,20]. ceptually simple approach has some limitations because six or seven-membered cyclic carbonates2.1. Zinc Complexessuitable for ROP must be synthesized through time-consuming multistep pro- tocols [19,20].Only in 2006, Liu and coworkers reported on the terpolymerization of propylene oxide (PO) with CO2 and maleic anhydride (MA) catalyzed by polymer-supported bimetallic cat- 2.1.alyst Zinc (PBM) Complexes1 of general formula P-Zn[Fe(CN)6]aCl2-3a(H2O)b, with P being the polyether type chelating agent and a ≈ 0.5 and b ≈ 0.76 [21]. Notably, the catalyst was inactive in Only in 2006, Liu and coworkers reported on the terpolymerization of propylene ox- the PO/MA copolymerization whereas it gave the poly(propylenecarbonate) (PPC) from ide (PO) with CO2 and maleic anhydride (MA) catalyzed by polymer-supported◦ bimetallic PO/CO2. In the terpolymerization experiments (pCO2 = 4.0 MPa, T = 60 C, t = 24 h), the catalystpolymer (PBM) yield 1 increasedof general up formula to a 5:3 P-Zn[Fe(CN) ratio between6 PO]aCl and2-3a(H MA,2O)b and, with a further P being increase the polyether in typethe chelating MA content agent was and detrimental a ≈ 0.5 and for theb ≈ polymerization0.76 [21]. Notably, activity. the1 catalystH and 13 Cwas NMR inactive and IR in the PO/MAspectroscopy copolymerization revealed a random whereas microstructure it gave the characterizing poly(propylenecarbonate) the resulting copolymers (PPC) from ◦ PO/CO(Figure2. In1). the The terpolymerization DSC thermograms showed experiments a single ( transitionpCO2 = 4.0 with MPa,Tg valuesT = 60 (29.1–56.1 °C, t = 24 C)h), the polymerincreasing yield by increased increasing up the to MA a content5:3 ratio in be agreementtween PO with and the MA, microstructure and a further revealed increase by in theNMR MA content spectroscopy. was detrimental for the polymerization activity. 1H and 13C NMR and IR spectroscopy revealed a random microstructure characterizing the resulting copolymers (Figure 1). The DSC thermograms showed a single transition with Tg values (29.1–56.1 °C) increasing by increasing the MA content in agreement with the microstructure revealed by NMR spectroscopy.

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Figure 1. The structure of CO2/PO/MA terpolymer obtained by Liu using complex 1 [21]. Figure 1. The structure of CO /PO/MA terpolymer obtained by Liu using complex 1 [21]. Later on, in 2008,Figure the development1. The structure 2of anof efficientCO2/PO/MA homogeneous terpolymer catalytic obtained system by Liu by using complex 1 [21]. Coates based on β-diiminateLater on, (bdi) in 2008, zinc the complex development 2 (Figure of an efficient 2) allowed homogeneous the synthesis catalytic system of a by Coates based on β-diiminate (bdi) zinc complex2 2 (Figure2) allowed the synthesis of poly(ester-block-carbonate)Later by the on, terpolymerization in 2008, the development (pCO = 0.3–5.4 of MPa,an efficient T = 50 °C, homogeneous t = ◦ catalytic system by a poly(ester-block-carbonate) by the terpolymerization (pCO2 = 0.3–5.4 MPa, T = 50 C, 0.2–3 h) of cyclohexeneoxide (CHO) with diglycolic anhydride (DGA) and CO2 [22]. tCoates = 0.2–3 h) based of cyclohexeneoxide on β-diiminate (CHO) with (bdi) diglycolic zinc anhydride complex (DGA) 2 (Figure and CO2 [ 222)]. allowed the synthesis of a poly(ester-block-carbonate) by the terpolymerization (pCO2 = 0.3–5.4 MPa, T = 50 °C, t = 0.2–3 h) of cyclohexeneoxide (CHO) with diglycolic anhydride (DGA) and CO2 [22].

Figure 2. Terpolymerization of CHO, DGA and CO with β-diiminate zinc complex 2 by Coates. Reproduced with Figure 2. Terpolymerization of CHO, DGA2 and CO2 with β-diiminate zinc complex 2 by Coates. modification and permission from ref. [22]. Copyright (2008) John Wiley and Sons. Reproduced with modification and permission from ref. [22]. Copyright (2008) John Wiley and Sons. Interestingly, in spite of the polymerization feed simultaneously containing all three monomers, the final polymer was a diblock with a polyester block followed Notably, in spiteby aof poly(cyclohexenecarbonte) the polymerization feed (PCHC) simultaneously block. Indeed, containing by following all the three polymerization mon- Figure 2. Terpolymerization of CHO, DGA and CO2 with β-diiminate zinc complex 2 by Coates. omers, the final polymerreaction bywas in situa diblock IR spectroscopy, copolymer the exclusive with a formationpolyester of block the polyester followed block by up toa the totalReproduced consumption with of DGA mo wasdification evident. Fromand thepermission mechanistic from point ofref. view, [22] it was. Copyright clear (2008) John Wiley and poly(cyclohexenecarbonte)that the first (PCHC) step was block. the formation Indeed, of zincby following alkoxide by thethe ring-opening polymerization of CHO reac- followed tion by in situ IR spectroscopy,bySons. the preferential the and exclusive irreversible formation insertion of of DGA the untilpolyester this monomer block wasup to completely the total consumption consumedof DGA was and only evident. after the From zinc alkoxidethe mechanistic can allow thepoint insertion of view, of CO it2 withwas theclear growth that the first step wasof the the polycarbonate Notably,formation inof block. zincspite Thealkoxide of polymerization the polymerizationby the ring-opening of succinic anhydridefeed of CHO simultaneously (SA), followed albeit with containing all three mon- lower reactivity, and vinyl-CHO was also accomplished. by the preferential andomers,In irreversible 2010, the Zhang final andinsertion polymer coworkers of DGA showedwas until a that diblock this the heterogeneous monomer copolymer was double completely with metal a cyanide po lyester block followed by a consumed and only after the zinc alkoxide can allow the insertion of CO2 with the growth complexpoly(cyclohexenecarbonte) (DMCC) 3 obtained by the reaction(PCHC) of K block.3Co(CN) Indeed,6 with ZnCl by2 promotes following the the polymerization reac- of the polycarbonateterpolymerizationtion block. by inThe situ polymerizati of IR CO 2spectroscopy,with CHOon of and succinic MA the [23 anhydridexclusive]. The proposede (SA), formation active albeit site with of for the this polyester block up to the lower reactivity, andcatalyst vinyl-CHO is a Zn atomwas inalso a tetrahedralaccomplished. structure with the Co atom playing a spectator role.total The consumption catalyst was highly of active DGA and was selective, evident. giving aFrom complete the conversion mechanistic of CHO point of view, it was clear In 2010, Zhang and coworkers showed◦ that the heterogeneous double metal cyanide (pthatCO2 =the 4.0 first MPa, Tstep = 90 wasC, t the = 5 h)formation and a complete of zinc selectivity alkoxide toward by the the polymeric ring-opening of CHO followed complex (DMCC) product.3 obtained The by resulting the reaction polymer of was K3 poly(ester-Co(CN)6block with-carbonate), ZnCl2 promotes but it also the contained ter- a polymerization of variableCOby2 thewith amount preferential CHO of and polyether MA and [23]. linkages irreversible The (2.9–11.7%) proposed insertion depending active site on of thefor DGAreaction this catalystuntil conditions. this monomer was completely In particular, the use of THF as a inhibits the formation of polyether linkages due is a Zn atom in a tetrahedralconsumed structure and only with after the the Co zincatom alkoxide playing a can spectator allow role. the insertionThe of CO2 with the growth to the coordination of the THF molecule to the Zn2+ center. The mechanism proposed for catalyst was highly active and selective, giving a complete conversion of CHO (pCO2 = 4.0 COof /CHO/MAthe polycarbonate terpolymerization block. catalyzed The by polymerizati a Zn–Co(III) DMCCon catalyst of succinic is depicted anhydrid in e (SA), albeit with MPa, T = 90 °C, t = 5 h)2 and a complete selectivity toward the polymeric product. Notably, Schemelower2. reactivity, and vinyl-CHO was also accomplished. the resulting polymer Unusually,was poly(ester- notwithstandingblock-carbonate), the heterogeneous but it naturealso contained of the catalyst, a variable the dispersity In 2010, Zhang and coworkers showed−1 that the heterogeneous double metal cyanide amount of polyetherwas linkages narrow ( Ð(2.9–11.7%)= 1.4–1.7) and depending the Mn was on up tothe 14.1 reaction kg mol conditions.. In partic- ular, the use of THFcomplex as a solvent (DMCC) inhibits 3 theobtained formation by of the polyether reaction linkages of K3 Co(CN)due to the6 with ZnCl2 promotes the ter- coordination of thepolymerization THF molecule toof theCO 2Zn with2+ center. CHO The and mechanism MA [23]. proposedThe proposed for active site for this catalyst CO2/CHO/MA terpolymerizationis a Zn atom cata in lyzeda tetrahedral by a Zn–Co(III) structure DMCC with catalyst the is Codepicted atom in playing a spectator role. The Scheme 2. catalyst was highly active and selective, giving a complete conversion of CHO (pCO2 = 4.0 MPa, T = 90 °C, t = 5 h) and a complete selectivity toward the polymeric product. Notably, the resulting polymer was poly(ester-block-carbonate), but it also contained a variable amount of polyether linkages (2.9–11.7%) depending on the reaction conditions. In partic- ular, the use of THF as a solvent inhibits the formation of polyether linkages due to the coordination of the THF molecule to the Zn2+ center. The mechanism proposed for CO2/CHO/MA terpolymerization catalyzed by a Zn–Co(III) DMCC catalyst is depicted in Scheme 2.

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Scheme 2. Proposed mechanism for CHO, MA and CO2 terpolymerization using Zn-Co(III) DMCC catalyst 3. Reproduced with modification and permission from ref. [23] . Copyright (2010) Elsevier.

SchemeSchemeNotably, 2. 2.Proposed Proposed notwithstanding mechanism mechanism for the CHO,for CHO,heterogeneous MA andMA COand2 terpolymerization COnature2 terpolymerization of the catalyst, using Zn-Co(III) using the dispersity Zn-Co(III) DMCC DMCC catalyst 3. Reproduced with modification and permission from ref. [23]. Copyright (2010) Elsevier. catalystwas narrow 3. Reproduced (Ð = 1.4–1.7) with and modification the Mn was andup to permission 14.1 kg mol from−1. ref. [23]. Copyright (2010) Elsevier. Zinc glutarate (ZnGA) 4 was also found to be a versatile catalyst for the terpolymer- Zinc glutarate (ZnGA) 4 was also found to be a versatile catalyst for the terpolymer- izationNotably, of CO2 with notwithstanding PO and various the cyclic heterogeneous anhydrides. Indeed, nature in of2014, the Meng catalyst, reported the ondispersity ization of CO2 with PO and various cyclic anhydrides. Indeed, in− 2014,1 Meng reported on wasthe synthesis narrow of(Ð PO/phthalic = 1.4–1.7) and anhydride the Mn (PA)/CO was up2 to copolymers 14.1 kg mol (pCO. 2 = 5.0 MPa, T = 75 ◦°C, the synthesis of PO/phthalic anhydride (PA)/CO2 copolymers (pCO2 = 5.0 MPa, T = 75 C, t = 15 h) using toluene as a solvent [24]. Notably, in this case the formation of polycar- t = 15Zinc h) using glutarate toluene (ZnGA) as a solvent 4 was [24 ].also Intruguingly, found to be in thisa versatile case the catalyst formation for of the polycar- terpolymer- bonateizationbonate is isof favoredfavored CO2 with overover PO thethe and polyesterpolyester various formation;formation; cyclic anhydrides consequently,consequently,. Indeed, thethe resultingresulting in 2014, terpolymersterpolymers Meng reported on (Mn up to 221 kg mol−−11, Ð = 2.1–3.9) consist of a polycarbonate chain randomly interrupted (theMn upsynthesis to 221 kg of mol PO/phthalic, Ð = 2.1–3.9) anhydride consist of (PA)/CO a polycarbonate2 copolymers chain randomly (pCO2 = 5.0 interrupted MPa, T = 75 °C, bytby = PA PA15 units h)units using (4.8–10.5%) (4.8–10.5%) toluene and andas a lowera a solventlower amount amount [24]. of polyetherNota of polyetherbly, linkagesin this linkages case (2.7–7.1%). the(2.7–7.1%). formation On one On hand, oneof polycar- hand, the lower reactivity of PA vs. CO2 was explained with the slower insertion of the thebonate lower is reactivity favored ofover PA vs.the COpolyester2 was explained formation; with consequently, the slower insertion the resulting of the PA terpolymers into PA into the Zr-alkoxo bond; on the other hand, the increase in the polymer yield and Mn the Zr-alkoxo bond; on− the1 other hand, the increase in the polymer yield and M observed (Mn up to 221 kg mol , Ð = 2.1–3.9) consist of a polycarbonate chain randomlyn interrupted observed when PA is present in the feed was explained with a faster insertion of PO into whenby PA PA units is present (4.8–10.5%) in the feed and was a lower explained amount with aof faster polyether insertion linkages of PO into(2.7–7.1%). the zinc On one benzoatethe zinc benzoate growing growing chain with chain respect with to respec the zinct to carbonatethe zinc carbonate chain (Scheme chain3 (Scheme). 3). hand, the lower reactivity of PA vs. CO2 was explained with the slower insertion of the PA into the Zr-alkoxo bond; on the other hand, the increase in the polymer yield and Mn observed when PA is present in the feed was explained with a faster insertion of PO into the zinc benzoate growing chain with respect to the zinc carbonate chain (Scheme 3).

Scheme 3. Mechanism for terpolymerization of PA, PO and CO2 with ZnGA 4 [24]. Scheme 3. Mechanism for terpolymerization of PA, PO and CO2 with ZnGA 4. Reproduced with modification and permission from ref. [24]. Copyright (2014) Royal Society of . These opposite effects determine an ideal value for the PA/PO ratio giving the maxi- mumThese activity opposite and highest effects molecular determine weight, an ideal and value this for ratio the in PA/PO the feed ratio was giving experimentally the maxi- mumfound activity to be 1:8. and However, highest molecular the observed weight, decrease and this in the ratio M inn could the feed also was be experimentallyexplained con-

foundsidering to bethe 1:8. presence However, of diacid the observed impurities decrease in the in anhydride the Mn could that also acts be as explained a chain-transfer consid- eringSchemeagent. the 3. presence Mechanism of diacid for terpolymerization impurities in the anhydrideof PA, PO and that CO acts2 with as achain-transfer ZnGA 4 [24]. agent. DSCDSC thermograms show show that that the the introduction introduction of ofanan aromatic aromatic ter-monomer ter-monomer in the in pol- the polymerymerThese sensibly sensibly opposite enhances enhances effects the the T determinegT withg with respect respect an to ideal to the the correspondingvalue corresponding for the PA/POpolycarbonate polycarbonate ratio giving withwith anan the maxi- increasemumincrease activity ofof 66 ◦°CC and incorporatingincorporating highest molecular 5.6%5.6% ofof PA.PA. weight, and this ratio in the feed was experimentally foundThe to same be 1:8. catalytic However, system the4 was observed also used decrease for the synthesis in the M ofn pseudo-interpenetrating could also be explained con- ◦ poly(propylenecarbonate)sidering the presence of by diacid the terpolymerization impurities in (thepCO anhydride2 = 5.4 MPa, that T = 70actsC, as t = a 36 chain-transfer h) of agent. DSC thermograms show that the introduction of an aromatic ter-monomer in the pol- ymer sensibly enhances the Tg with respect to the corresponding polycarbonate with an increase of 6 °C incorporating 5.6% of PA.

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Catalysts 2021, 11, 961 5 of 19 The same catalytic system 4 was also used for the synthesis of pseudo-interpenetrat- ing Thepoly(propylenecarbonate) same catalytic system 4 was by alsothe terpolymerizationused for the synthesis (p ofCO pseudo-interpenetrat-2 = 5.4 MPa, T = 70 °C, t = 36 2 ingh) poly(propylenecarbonate)of CO2 with PO and pyromellitic by the terpolymerization dianhydride ((PMDA)pCO = 5.4 up MPa, to 4%T = 70(in °C, this t = case, 36 Mn in- h) of CO2 with PO and pyromellitic dianhydride (PMDA) up to 4% (in this case, Mn in- COcreased2 with up PO to and 862 pyromellitic kg mol−1), dianhydride resulting in (PMDA) a noticeable up to 4%improvement (in this case, inM ntheincreased mechanical and creased up to 862− 1kg mol−1), resulting in a noticeable improvement in the mechanical and upthermal to 862 kgproperties mol ), resulting with respect in a noticeable to the corresponding improvement in polycarbonate the mechanical [25]. and thermal propertiesthermal properties with respect with to respect the corresponding to the corresponding polycarbonate polycarbonate [25]. [25]. More recently, Williams et al. described the synthesis of a new dinuclear zinc com- More recently, WilliamsWilliams etet al.al. describeddescribed the synthesis of a newnew dinuclear zinc com- plex 5 (Figure 3) that promotes the terpolymerization of CHO/PA/CO2 (pCO2 = 3.0 MPa, T plex 5 (Figure(Figure3 3)) that thatpromotes promotes the the terpolymerization terpolymerization of ofCHO/PA/CO CHO/PA/CO2 2(p(COpCO2 =2 =3.0 3.0 MPa, MPa, T T== 100 = 100 100 °C, °C,◦C, t = t 18= 18 h) h)h)[26]. [[26].26 ].

Figure 3. Synthesis of dinuclear zinc complex 5 by Williams et al. Reproduced with modification Figure 3. Synthesis of dinuclear zinc complex 5 by Williams et al. Reproduced with modification and andFigure permission 3. Synthesis from ref of [26].dinuclear Copyrigh zinct (2015) complex American 5 by Williams Chemical etSociety. al. Reproduced with modification permissionand permission from ref from [26]. ref Copyright [26]. Copyrigh (2015) Americant (2015) ChemicalAmerican Society. Chemical Society. By monitoring the reaction after 2 h, it was evident that there was the exclusive for- By monitoring the reaction after 2 h, it was evident that there was the exclusive mation of polyether linkages and no formation of PCHC, and after 18 h the formation of formationBy monitoring of polyether linkagesthe reaction and no after formation 2 h, it ofwas PCHC, evident and afterthat 18there h the was formation the exclusive of for- −1 a poly(ester-block-carbonate) (Mn up to 7 kg mol −, 1Ð = 1.20) was evident and confirmed by amation poly(ester- of blockpolyether-carbonate) linkages (Mn upand to no 7 kg formatio mol , nÐ of= 1.20)PCHC, was and evident after and 18 confirmedh the formation of size-exclusion chromatography (SEC) analysis and−1 diffusion-ordered spectroscopy bya poly(ester- size-exclusionblock chromatography-carbonate) (Mn (SEC) up toanalysis 7 kg mol and, Ð diffusion-ordered = 1.20) was evident spectroscopy and confirmed by (DOSY)(DOSY)size-exclusion experiments.experiments. chromatography (SEC) analysis and diffusion-ordered spectroscopy Similar results were also obtained by Castro-Osma and coworkers by using dinuclear (DOSY)Similar experiments. results were also obtained by Castro-Osma and coworkers by using dinuclear zinc complexes 6–8 supported by heteroscorpionate ligands (Figure 4) [27]. zinc complexesSimilar results6–8 supported were also by obtained heteroscorpionate by Castro-Osma ligands (Figure and coworkers4)[27]. by using dinuclear zinc complexes 6–8 supported by heteroscorpionate ligands (Figure 4) [27].

Figure 4. SynthesisSynthesis of of bimetallic bimetallic zinc zinc acetate acetate catalysts catalysts 6−68− supported8 supported by heteroscorpionate by heteroscorpionate ligands lig- ands[27]. [27].

The complexescomplexes 66––88were were active, active, in in the the presence presence of of DMAP, DMAP, in in the the terpolymerization terpolymerization of Figure 4. Synthesis of bimetallic zinc acetate◦ catalysts 6−8 supported by heteroscorpionate ligands CHO/PA/COof CHO/PA/CO2 (2 p(COpCO2 2= =4.0 4.0MPa, MPa,T T= = 8080 °C,C, tt == 16 h), giving a poly(ester-block-carbonate)-carbonate) [27]. −1 material (Mnn up to 3.9 kg mol −1, ,ÐÐ == 1.41–1.64). 1.41–1.64). The complexes 6–8 were active, in the presence of DMAP, in the terpolymerization 2.2. Chromium and Cobalt Complexes of CHO/PA/COIn 2011, Duchateu2 (pCO and2 = coworkers 4.0 MPa, reported T = 80 °C, on thet = 16 terpolymerization h), giving a poly(ester- (pCO = 5.0block MPa-carbonate), In 2011, Duchateu and coworkers reported on the terpolymerization (pCO22 = 5.0 MPa, Tmaterial = 80 ◦C, ( tM =n 18up h) to of 3.9 CHO kg mol with−1,CO Ð = and1.41–1.64). various anhydrides (SA, cyclopropane-1,2- T = 80 °C, t = 18 h) of CHO with CO2 and2 various anhydrides (SA, cyclopropane-1,2-dicar- dicarboxylicboxylic acid acidanhydride anhydride (CPrA), (CPrA), cyclopentane-1,2-dicarboxylic cyclopentane-1,2-dicarboxylic acid acid anhydride anhydride (CPA) (CPA) oror PA)2.2. promoted promotedChromium by by and two two Cobalt chromium chromium Complexes complexes complexes (Figure (Figure5): tetraphenylporphyrinato 5): tetraphenylporphyrinato chromium chro- chloride 9 and salophen chromium chloride 10 (where salophen = N,N’-bis(3,5-di-tert- mium Inchloride 2011, Duchateu9 and salophen and coworkerschromium chloride reported 10 on (where the terpolymerization salophen = N,N’-bis(3,5-di- (pCO2 = 5.0 MPa, butylsalicylidenyl)-1,2-phenylenediamine) activated by DMAP (4-(N,N-dimethylamino) T = 80 °C, t = 18 h) of CHO with CO2 and various anhydrides (SA, cyclopropane-1,2-dicar- pyridine) [28]. boxylic acid anhydride (CPrA), cyclopentane-1,2-dicarboxylic acid anhydride (CPA) or PA) promoted by two chromium complexes (Figure 5): tetraphenylporphyrinato chro- mium chloride 9 and salophen chromium chloride 10 (where salophen = N,N’-bis(3,5-di-

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Catalysts 2021, 11, x FOR PEER REVIEW 6 of 20 tert-butylsalicylidenyl)-1,2-phenylenediamine) activated by DMAP (4-(N,N-dimethyla- mino) pyridine) [28].

Catalysts 2021, 11, 961 6 of 19 tert-butylsalicylidenyl)-1,2-phenylenediamine) activated by DMAP (4-(N,N-dimethyla- mino) pyridine) [28].

Figure 5. Catalysts 9 and 10 used by Duchateu in the terpolymerization of CO2, CHO and dicarbox- ylic acid anhydrides (SA, CPrA, CPA, PA) [28].

FigureFigureIn 5. 5. Catalystsanalogy Catalysts9 andto 9 and10theused 10 polymerization used by Duchateu by Duchateu in the process terpolymerization in the terpolymerization observed of COby2, CHOCoates of CO and 2in dicarboxylic, CHO the andcase dicarbox- of the acid(bdi)Znylic anhydridesacid anhydridescomplexes, (SA, CPrA, (SA, the CPA,CPrA, formation PA) CPA, [28]. PA)of the [28]. poly ester is favored over the formation of the polycarbonate, resulting in the formation of a poly(ester-block-carbonate). Notably, the au- In analogy to the polymerization process observed by Coates in the case of the (bdi)Zn thorsIn also analogy showed to that the DSCpolymerization of poly(ester- processblock-carbonate) observed is by inconclusive Coates in thein giving case ofinfor- the complexes, the formation of the polyester is favored over the formation of the polycarbon- ate,mation(bdi)Zn resulting about complexes, in the the formationblocky the formationmicrostructure of a poly(ester- of the of blockpoly the-carbonate). estercopolymer is favored The because authors over the the also polyester formation showed and of pol- the thatycarbonatepolycarbonate, DSC of poly(ester-phases resulting areblock completely in-carbonate) the formation miscible, is inconclusive of givinga poly(ester- ina single givingblock value information-carbonate). for the about T Notably,g. Furthermore, the the au- blockyinthors the also microstructurecase showedof complex that of the10 DSC, copolymerthe of authors poly(ester- because noticedblock the that-carbonate) polyester the presence and is polycarbonateinconclusive of CO2 in the in phases givingpolymeriza- infor- aretionmation completely feed about completely miscible,the blocky suppresses giving microstructure a single the formation value of the for of thecopolymer polyetherTg. Furthermore, because linkages. the in In thepolyester particular, case of and by pol- co- complexpolymerizingycarbonate10, the phases the authors equimolar are noticed completely amount that the miscible, presence of CHO giving of and CO 2CPrA ain single the in polymerization thevalue presence for the feed ofTg .CO Furthermore, com-2, the pure pletely suppresses the formation of polyether linkages. In particular, by copolymerizing polyesterin the case was of complex obtained, 10 while, the authors without noticed CO2 an that amount the presence of 15–30% of CO of 2polyether in the polymeriza- linkages the equimolar amount of CHO and CPrA in the presence of CO2, the pure polyester was wastion observed.feed completely For all suppresses terpolymerizations, the formation Mn (up of polyetherto 19.2 kg linkages.mol−1) showed In particular, a linear corre-by co- obtained, while without CO2 an amount of 15–30% of polyether linkages was observed. lationpolymerizing with conversion the equimolar and the amount Ð was of≤1.6, CHO indicating and−1 CPrA controlled in the presence behavior. of CO2, the pure For all terpolymerizations, Mn (up to 19.2 kg mol ) showed a linear correlation with conversionpolyesterSoon andafter,was theobtained, Darensbourg,Ð was ≤ while1.6, indicating using without a related controlled CO2 an(salan) amount behavior. CrCl of complex 15–30% 11 of activated polyether by linkages PPNN3 −1 was observed. For all terpolymerizations,2 Mn (up to 19.2 kg mol ) showedn a linear corre-−1 in theSoon terpolymerization after, Darensbourg, of using CHO/PA/CO a related (salan), observed CrCl complex similar 11resultsactivated (M byup PPNN to 183 kg mol , −1 inÐlation the= 1.07–1.13) terpolymerization with conversion [29]. In ofthis and CHO/PA/CO case, the theÐ was poly(ester-2, ≤ observed1.6, indicatingblock similar-carbonate) resultscontrolled (M showedn up behavior. to 18 two kg moldistinct, Tg val- Ðues= 1.07–1.13)(48Soon °C after,and [29 115 ].Darensbourg, In °C). this Intriguingly, case, theusing poly(ester- a therelated majorblock (salan) -carbonate)reactivity CrCl complexof showed the anhydride two11 activated distinct visT bygà vis PPNN CO23 ◦ ◦ valueswasin the explained (48terpolymerizationC and in 115 termsC). Intriguingly,of of a CHO/PA/COslower ring-openi the major2, observed reactivityng step similar of thethe anhydridemetal-carbonate results (M visn upà vis to intermediate CO 182 kg mol−1, waswithÐ = explained1.07–1.13) the in [29]. terms monomer In of this a slower case, instead the ring-opening poly(ester- of a faster stepblock ofinsertion the-carbonate) metal-carbonate of the showed anhydride intermediate two distinct in the metal-Tg val- with the epoxide monomer instead of a faster insertion of the anhydride in the metal-alkoxo alkoxoues (48 bond °C and (Scheme 115 °C). 4). Intriguingly, the major reactivity of the anhydride vis à vis CO2 bond (Scheme4). was explained in terms of a slower ring-opening step of the metal-carbonate intermediate with the epoxide monomer instead of a faster insertion of the anhydride in the metal- alkoxo bond (Scheme 4).

SchemeScheme 4. 4.CO CO2 insertion2 insertion vs. vs. cyclic cyclic anhydride anhydride insertion insertion into the into metal the alkoxidemetal alkoxide intermediate. intermediate. Repro- Repro- ducedduced with with modification modification and and permission permission from reffrom [29 ].ref Copyright [29]. Copyright (2012) American (2012) American Chemical Society. Chemical Soci- ety.

Scheme 4. CO2 insertion vs. cyclic anhydride insertion into the metal alkoxide intermediate. Repro- duced with modification and permission from ref [29]. Copyright (2012) American Chemical Soci- ety.

Catalysts 2021, 11, x FOR PEER REVIEW 7 of 20

CatalystsCatalysts2021 2021, 11, 11, 961, x FOR PEER REVIEW 77 of of 19 20 Chromium(III) complex 12 (TPPCrCl, Figure 6) with a porphyrin ligand in combina- tion with PPNCl was successfully used by Chisolm and coworkers for the terpolymeriza- tion of CO2/PO/SA (pCO2 = 4.0–5.0 MPa, T = 25 °C, t = 3–18 h) [30]. It is worth noting that the Chromium(III)PPNCl/CrChromium(III) ratio complex complex is crucial12 12(TPPCrCl, (TPPCrCl,to avoid Figurethe Figure formation6 )6) with with a a porphyrinof porphyrin polyether ligand ligand linkages; in in combina- combina- indeed, when tiontion with with PPNCl PPNCl was was successfully successfully used used by by Chisolm Chisolm and and coworkers coworkers for for the the terpolymeriza-terpolymeriza- 0.5 equiv. of PPNCl have been used, the form◦ation of polyether linkages is favored (up to tiontion of of CO CO2/PO/SA/PO/SA (p (pCOCO2 = =4.0–5.0 4.0–5.0 MPa, MPa, T T= 25 = 25°C, C,t = t3–18 = 3–18 h) [30]. h) [30 It]. is It worth is worth noting noting that 42%) over2 the polyester 2and polycarbonate linkages, whereas with 1.0 equiv. of PPNCl the thatthe thePPNCl/Cr PPNCl/Cr ratio ratiois crucial is crucial to avoid to avoid the formation the formation of polyether of polyether linkages; linkages; indeed, indeed, when whenamount0.5 equiv. 0.5 equiv.of of polyether PPNCl of PPNCl have linkages havebeen beenused, is drastically used,the form theation formation reduced of polyether of (<2%). polyether linkages In analogy linkages is favored to is other favored (up chromium to (upsystems,42%) to over 42%) forthe over thispolyester the system polyester and the polycarbonate polyester and polycarbonate formatio linkages, linkages,n whereas is also whereasfasterwith 1.0 than equiv. with the 1.0 of polycarbonate PPNCl equiv. ofthe one, PPNClleadingamount the ofto amountpolyether copolymers of polyetherlinkages with is a linkages drastically tapered/dibloc is drastically reducedk microstructure. (<2%). reduced In analogy (<2%). The In to analogy other authors chromium to otherattributed the chromiumhighersystems, reactivity for systems, this system of for SA this the over system polyester CO the2 to formatio polyester the highern formationis also solubility faster is alsothan of fasterthe the anhydridepolycarbonate than the polycar- in one,the reaction bonatemedium.leading one, to copolymers leading tocopolymers with a tapered/dibloc with a tapered/diblockk microstructure. microstructure. The authors attributed The authors the attributedhigher reactivity the higher of SA reactivity over CO of2SA to overthe higher CO2 to solubility the higher of solubilitythe anhydride of the in anhydride the reaction in themedium. reaction medium.

Figure 6. The (porphyrin)Cr(III)Cl complex 12 employed by Chisolm and coworkers [30]. FigureFigure 6. 6.The The (porphyrin)Cr(III)Cl (porphyrin)Cr(III)Cl complex complex12 12employed employed by by Chisolm Chisolm and and coworkers coworkers [ 30[30].].

AAA major major major breakthrough breakthrough breakthrough in in this thisin field this field wasfield was the was th usee use th ofe theof use the single of single the component single component component Co(III) Co(III) complex com-Co(III) com- 13plex(Figure 1313 (Figure (Figure7) tethering 7) 7) tethering tethering four quaternary four four qu aternaryqu ammoniumaternary ammonium ammonium salts [31 salts]. [31].salts [31].

Figure 7. The (salen)Co(III) complex 13 with quaternary ammonium salts in a molecule [31]. FigureFigure 7. 7.The The (salen)Co(III) (salen)Co(III) complex complex13 with 13 with quaternary quaternary ammonium ammonium salts in asalts molecule in a molecule [31]. [31]. This complex displayed one of the highest activities in the CO2/PO copolymerization, This complex displayed one of the highest activities in the CO /PO copolymerization, −1 2 reachingThis TOF complex up to displayed16,000 −h1 . oneIn the of presencethe highest of CO activities2/PO/PA, in thisthe complexCO2/PO alsocopolymerization, shows reaching TOF up to 16,000 h . In the presence of CO2/PO/PA, this complex also shows high reactivity with a total conversion−1 of PO only after 3.0 h (pCO2 = 3.5 MPa, T = 80◦ °C) highreaching reactivity TOF with up ato total 16,000 conversion h . In ofthe PO presence only after of 3.0 CO h2 (/PO/PA,pCO2 = 3.5 this MPa, complex T = 80 alsoC) shows and a calculated TOF = 12,000 h−−1.1 The resulting copolymers have a gradient poly(1,2-pro- andhigh a calculatedreactivity TOFwith = a 12,000 total conversion h . The resulting of PO copolymersonly after 3.0 have h ( apCO gradient2 = 3.5 poly(1,2- MPa, T = 80 °C) pylene carbonate-co-phthalate)s microstructure since, due to the highest reactivity of PA propyleneand a calculated carbonate-co-phthalate)s TOF = 12,000 h−1 microstructure. The resulting since, copolymers due to the have highest a gradient reactivity poly(1,2-pro- of compared to CO2, the polymeric chains formed in the initial stages are richer in PA, but PApylene compared carbonate-co-phthalate)s to CO2, the polymeric chains microstructure formed in the since, initial due stages to the are highest richer in reactivity PA, but of PA thethe consumption consumption of of this this comonomer comonomer favors favors the the formation formation of of polycarbonate polycarbonate chains chains in in the the compared to CO2, the polymeric chains formed in the initial stages are richer in PA, but lastlast stages. stages. The The resulting resulting copolymers copolymers have have a a very very narrow narrow dispersity dispersity ( Ð(Đ= = 1.03–1.22) 1.03–1.22) and and thehigh consumption molecular weight of this (Mn comonomerup to 354 kg mol favors−1−1). As the previously formation observed, of polycarbonate the incorporation chains in the high molecular weight (Mn up to 354 kg mol ). As previously observed, the incorporation oflastof PA PA stages. in in the thepolymeric Thepolymeric resulting chain chain copolymers enhances enhances the the have thermal thermal a very properties properties narrow of of dispersity the the final final polymer polymer (Đ = 1.03–1.22) with with and −1 respecthighrespect molecular to to the the corresponding corresponding weight (M PPC.n PPC. up to 354 kg mol ). As previously observed, the incorporation of PAAA dinuclear dinuclearin the polymeric Cr(III)Cr(III) salensalen chain complex enhances 14 (Figure the th 88))ermal waswas reported properties by Lu Lu of and andthe coworkers coworkersfinal polymer to with torespectpromote, promote, to in the inthe the correspondingpresence presence of of2 equiv. 2 equiv.PPC. PPNCl, PPNCl, the theterpolymerization terpolymerization of CO of2 CO/CHO/PA2/CHO/PA (pCO2 ◦ (p=CO 1 MPa,2A= dinuclear 1 T MPa, = 80 °C, T = Cr(III)t 80 = 0.5–6C, tsalen =h) 0.5–6 [32]. complex In h) the [32 ].fi 14rst In the(Figure2 h, firstthe system 28) h, was the only systemreported produced only by produced Lu the and polyester coworkers the to promote, in the presence of 2 equiv. PPNCl, the terpolymerization of CO2/CHO/PA (pCO2 = 1 MPa, T = 80 °C, t = 0.5–6 h) [32]. In the first 2 h, the system only produced the polyester

Catalysts 2021, 11, x FOR PEER REVIEW 8 of 20

Catalysts 2021, 11, 961 8 of 19

segment with no incorporation of CO2, and only after the total consumption of PA the polyesterpolycarbonate segment withblock no was incorporation formed, of also CO2 ,giving, and only in after this the case, total consumptiona diblock polymer. of The pro- PAduced the polycarbonate copolymers block have was a very formed, narrow also giving, dispersity in this ( case,Ð = 1.19–1.22) a diblock polymer. and the The molecular weight produced copolymers have a very narrow dispersity (Ð = 1.19–1.22) and the molecular increases with the polymerization time (Mn up to 21.2 kg− mol1 −1). weight increases with the polymerization time (Mn up to 21.2 kg mol ).

FigureFigure 8. Dinuclear8. Dinuclear chromium chromium catalyst catalyst11 developed 11 developed by Lu et al. [by32]. Lu et al. [32]. 2.3. Metal-Free Catalysts 2.3.Since Metal-Free the discovery Catalysts by Feng and coworkers that triethyl borane (TEB) in combination with oniumSince halides the discovery or alkoxides by promotesFeng and the coworkers formation of th polycarbonatesat triethyl borane by coupling (TEB) in combination COwith2 with onium PO or halides CHO, the or efforts alkoxi to extenddes promotes the use of thisthe metal-free formation system of polycarbonates to the terpoly- by coupling merization of CO2 with epoxides and anhydrides resulted in the synthesis of terpolymers havingCO2 with various PO microstructural or CHO, the features efforts [33 to]. extend the use of this metal-free system to the ter- polymerizationIn 2020, Meng reported of CO2 the with quadripolymerization epoxides and anhydrides of CO2 with PA,resu POlted and in CHO the in synthesis the of terpol- presenceymers having of TEB and various PPNCl, microstructural resulting in the formation features of [33]. the copolymer (pCO2 = 1 MPa, T = 70 ◦C, t = 24–96 h) with good selectivity (94%) with respect to the cyclic product [34,35]. In 2020, Meng reported the quadripolymerization of CO2 with PA, PO and CHO in The microstructure of the resulting quadripolymer was clarified by 1H and 13C NMR show- the presence of TEB and PPNCl, resulting in the formation of the copolymer (pCO2 = 1 ing the presence of four main blocks, i.e., poly(PA-alt-CHO), poly(PA-alt-PO), poly()MPa, T =(PPC), 70 °C, and t = poly(cyclohexene24–96 h) with good carbonate) selectiv (PCHC),ity (94%) and a with very lowrespect amount to the of cyclic product polyether[34,35]. linkagesThe microstructure (<1%). In addition, of the in resulting this case the quadripolymer formation of the was polycarbonate clarified by 1H and 13C segmentsNMR showing only starts the after presence the complete of four PA conversion main blocks, and thusi.e., afterpoly(PA- the formationalt-CHO), of the poly(PA-alt-PO), polyester segments. The resulting polymers display narrow dispersity (Ð = 1.14–1.21) and poly(propylene carbonate) (PPC), and−1 poly(cyclohexene carbonate) (PCHC), and a very a high molecular weight (Mn up to 77 kg mol ). It is worth reporting that the Tg can be low amount of polyether linkages (<1%). In addition, in this case the formation◦ of the pol- easily tuned by regulating the feed ratio with a wide temperature range (Tg = 82–116 C). ycarbonateAfterward, segments Li and coworkers only starts reported after on the terpolymerizationcomplete PA conversion of CO2 with and PA and thus after the for- ◦ CHO,mation in the of presencethe polyester of TEB segments. and PPNCl (ThepCO resu2 = 0.1lting MPa, polymers T = 80 C, display t = 0.25–2 narrow h) [36]. dispersity (Ð = Additionally, in this case the polycarbonate block starts forming only after the complete 1.14–1.21) and a high molecular weight (Mn up to 77 kg mol−1). Notably, the Tg can be easily consumption of PA in the feed, resulting in a poly(ester-b-carbonate) copolymer with little tapering,tuned by as shownregulating by NMR the spectra. feed rati Theo same with catalytic a wide system temperature also allows range the synthesis (Tg = 82–116 of °C). poly(ester-Afterward,b-carbonate) Li withoutand coworkers tapering by reported sequential on monomer the terpolymerization addition. The resulting of CO2 with PA and −1 copolymersCHO, in possessthe presence narrow dispersityof TEB and (Ð = PPNCl 1.09–1.15) (p andCOM2 =n up0.1 to MPa, 23.5 kg T mol= 80 .°C, t = 0.25–2 h) [36]. Additionally,Lately, Feng in obtained this case similar the resultspolycarbonate by using TEB block in combinationstarts forming with only Bu4NN after3 the complete ◦ (pconsumptionCO2 = 0.1 MPa, of T =PA 60 inC, the t = 0.75–18feed, resulting h) for the terpolymerizationin a poly(ester-b of-carbonate) CO2 with PO copolymer and with little SA/PA. The PO/SA/CO terpolymerization clearly shows higher reactivity toward the tapering, as shown by2 NMR spectra. The same catalytic system also allows the synthesis oxoanion of SA over CO2, leading to the preferential formation of the polyester resulting inof tapered poly(ester- poly(ester-b-carbonate)b-carbonate) without [37]. Only tapering at a low concentrationby sequential of SAmonomer (SA:PO = addition. 1:20), The result- aing random copolymers poly(ester- possessco-carbonate) narrow copolymer dispersity was obtained(Ð = 1.09–1.15) with 51% and of polyester Mn up to and 23.5 kg mol−1. 49% of carbonate. The PO/SA/CO terpolymerization leads to random copolymers also Lately, Feng obtained similar2 results by using TEB in combination with Bu4NN3 at PA:PO = 20:200, and only with the presence of 40% of PA in the feed the resulting (pCO2 = 0.1 MPa, T = 60 °C, t = 0.75–18 h) for the terpolymerization of CO2 with PO and copolymer displays a blocky nature. The terpolymerization of CHO/PA/CO2 activating 2 ◦ TEBSA/PA. with PPNClThe PO/SA/CO (pCO2 = 0.1 MPa, terpolymerization T = 80 C, t = 17–18 clearly h) shows shows the analogous higher behavior reactivity toward the ofoxoanion PO preferentially of SA over producing CO2, copolymersleading to withthe preferential a random microstructure formation andof the blocky polyester resulting in tapered poly(ester-b-carbonate) [37]. Only at a low concentration of SA (SA:PO = 1:20), a random poly(ester-co-carbonate) copolymer was obtained with 51% of polyester and 49% of carbonate. Notably, the PO/SA/CO2 terpolymerization leads to random copoly- mers also at PA:PO = 20:200, and only with the presence of 40% of PA in the feed the resulting copolymer displays a blocky nature. The terpolymerization of CHO/PA/CO2 ac- tivating TEB with PPNCl (pCO2 = 0.1 MPa, T = 80 °C, t = 17–18 h) shows the analogous behavior of PO preferentially producing copolymers with a random microstructure and

Catalysts 2021, 11, 961 9 of 19

copolymers only with a high content of PA in the feed. Genuine poly(ester-b-carbonate)s can be obtained by sequential monomer addition both in the case of PA/PO and CHO/PA followed by feeding CO2. The Tg of the resulting copolymers can be tuned by regulating the PA content in the final copolymer with values ranging from 32.5 ◦C to 46.2 ◦C in ◦ ◦ the case of the PO/PA/CO2 copolymers and from 121 C to 135.1 C in the case of the CHO/PA/CO2 copolymers. In Table1, the results obtained in the CO 2/epoxide/cyclic anhydrides’ terpolymeriza- tion discussed in this first part are summarized.

Table 1. Summary of terpolymerization of CO2 with epoxides and cyclic organic anhydrides.

Polymerization Conditions Mn *(Ð) Polymer Catalyst Monomers −1 Ref. pCO2, T, t kg mol Microstructure ◦ 1 CO2, PO, MA 4.0 MPa, 60 C, 24 h - random [21] ◦ 2 CO2, CHO, DGA 0.3–5.4 MPa, 50 C, 0.2–3 h 37 (1.2–1.4) block [22] ◦ 3 CO2, CHO, MA 4.0 MPa, 90 C, 5 h 14.1 (1.4–1.7) block [23] ◦ 4 CO2, PO, PA 5.0 MPa, 75 C, 15 h 221 (2.1–3.9) random [24] ◦ 4 CO2, PO, PMDA 5.4 MPa, 70 C, 36 h 862 (2.0–3.8) random [25] ◦ 5 CO2, CHO, PA 3.0 MPa, 100 C, 18 h 7 (1.20) block [26] ◦ 6–8 CO2, CHO, PA 4.0 MPa, 80 C, 16 h 3.9 (1.41–1.64) block [27] ◦ 9–10 CO2, CHO, SA/CPrA/CPA/PA 5.0 MPa, 80 C, 18 h 19.2 (1.1–1.6) block [28] ◦ 11 CO2, CHO, PA 3.5 MPa, 80 C, 12 h 18 (1.07–1.13) block [29] ◦ 12 CO2, PO, SA 4–5.0 MPa, 25 C, 3–18 h - tapered/block [30] ◦ 13 CO2, PO, PA 3.5 MPa, 80 C, 3 h 354 (1.03–1.22) gradient [31] ◦ 14 CO2, CHO, PA 1 MPa, 80 C, 0.5–6 h 21.2 (1.19–1.22) block [32] Metal-free catalyst ◦ TEB + PPNCl CO2, PO, PA, CHO 1 MPa, 70 C, 24–96 h 77.7 (1.14–1.21) alternated [34,35] ◦ CO2, CHO, PA 0.1 MPa, 80 C, 0.25–2 h 23.5 (1.09–1.15) block [36] ◦ TEB + Bu4NN3 CO2, PO, SA/PA 0.1 MPa, 60 C, 0.75–18 h 17.3 (1.04–1.2) tapered/random [37] ◦ TEB + PPNCl CO2, CHO, SA/PA 0.1 MPa, 80 C, 16–17 h 22.7 (1.06–1.09) tapered/block [37] * The highest reported value.

3. Terpolymerization of CO2 with Epoxides and Cyclic Esters The synthesis of polyester-co-polycarbonate was also attempted by the terpolymer- ization of CO2 with epoxides and cyclic esters combining the ROCOP and ROP mech- anisms [38]. This approach gives access to microstructures not accessible via ROCOP with organic anhydrides and has the advantage of using largely available monomers ε-caprolactone (CL), DL-lactide (LA) and β-butyrolactone (BBL) [14,16,36,38–40].

3.1. Zinc Complexes ZnGA 4, obtained by the reaction of zinc oxide and glutaric acid, was active in ◦ the terpolymerization of CO2/PO/CL (pCO2 = 2.8 MPa, T = 60 C, t = 40 h), resulting −1 in high molecular weight polymers (Mn up to 27.5 kg mol ) with narrow dispersity (Ð = 1.50–2.97)[41]. The catalytic activity decreases by increasing the content of CL beyond the 50% in mol in the feed. Notably, the system was inactive in the polymerization of CL alone and the production of cyclic carbonate contaminant was not observed. The 13C NMR analysis reveals a diblock microstructure with CL units directly linked to PC units and CL units in homosequences. Accordingly, the DSC thermograms display two ◦ transitions: one relative to the Tg of the PPC block (Tg = 5.4–17.7 C) and the Tm of the ◦ PCL block (Tm = 51.0–57.2 C). These polymers show excellent enzymatic biodegradability catalyzed by various lipases. The same catalytic system using glycidol terminated -PCL as a macromonomer produced the corresponding grafted copolymers in the presence of ◦ CO2/PO (pCO2 = 1.0 MPa, T = 60 C, t = 6 h) [42]. In 2006, Doring reported the first example of the terpolymerization of CO2/CHO/LA by using zinc acetate complexes 15–22 with aminoimidoacrylate (AIA) ligands (Figure9) [43]. Catalysts 2021, 11, x FOR PEER REVIEW 10 of 20

the corresponding grafted copolymers in the presence of CO2/PO (pCO2 = 1.0 MPa, T = 60 °C, t = 6 h) [42]. Catalysts 2021, 11, 961 In 2006, Doring reported the first example of the terpolymerization of CO2/CHO/LA10 of 19 by using zinc acetate complexes 15–22 with aminoimidoacrylate (AIA) ligands (Figure 9) [43].

FigureFigure 9. 9.(AIA)Zn(OAc) (AIA)Zn(OAc) complexes complexes15 15––2222[ 43[43].].

InIn order order to to obtain obtain a terpolymera terpolymer with with an an appreciable appreciable amount amount of polycarbonate of polycarbonate linkages, link- ◦ anages, excess an excess of CHO of inCHO the in feed the was feed necessary was necessary (CHO:LA (CHO:LA = 3:1, =p CO3:1,2 p=CO 4.02 = MPa, 4.0 MPa, T =90 T = C,90 −1 t°C, = 16 t = h 16), givingh), giving high high molecular molecular weight weight polymers polymers (M n(M=n 11.3–41.6 = 11.3–41.6 kg kg mol mol−)1) with with narrow narrow dispersitydispersity ( Ð(Ð= = 1.09–1.96).1.09–1.96). TheThe copolymerscopolymersobtained obtained by byusing usingL L-LA-LA instead instead of ofrac rac-LA-LA show show ◦ crystallinitycrystallinity with with a meltinga melting point point around around 167 167C. The °C. authors The authors also reported also reported the terpolymer- the ter- izationpolymerization by using theby using (bdi) Znthe catalysts (bdi) Zn developedcatalysts developed by Coates by (see Coates Figure (see2), showing, Figure 2), in show- this case,ing, in even this a case, major even tendency a major to incorporatetendency toa incorporate higher amount a higher of polycarbonate amount of polycarbonate linkages (up tolinkages 80%). (up to 80%). TheThe polymer-supported polymer-supported bimetallic bimetallic catalyst catalyst (PBM) 1(PBM)of general 1 of formula general P-Zn[Fe(CN) formula 6]P-a ≈ ≈ ClZn[Fe(CN)2-3a(H2O)b6]a(aCl2-3a0.5(H2 andO)b (a b ≈ 0.76)0.5 and developed b ≈ 0.76)by developed Liu was alsoby Liu effective was also in the effective terpolymer- in the ◦ izationterpolymerization (pCO2 = 4.0 (p MPa,CO2 = T 4.0 = 50–90MPa, T C,= 50–90 t = 16 °C, h) t of = CO16 h)2/PO/CL, of CO2/PO/CL, giving giving materials materials also containingalso containing polyether polyether linkages linkages [44]. [44]. A ternary system composed of Y(CCl COO) /ZnEt /glycerin 23 was used by Xi- A ternary system composed of Y(CCl3 3COO)3 3/ZnEt2 2/glycerin 23 was used by anhong and coworkers to synthesize (pCO = 4.0 MPa, T = 70 ◦C, t = 10 h) terpolymers Xianhong and coworkers to synthesize (pCO2 2 = 4.0 MPa, T = 70 °C, t = 10 h) terpolymers −1 CO /PO/L-LA with a high molecular weight (Mn = 7.2–15.4 kg mol ) and broad disper- CO22/PO/L-LA with a high molecular weight (Mn = 7.2–15.4 kg mol−1) and broad dispersity sity(Ð = ( Ð4.2–9.9),= 4.2–9.9), with with the molecular the molecular weig weightht increasing increasing by decreasing by decreasing the L-LA the L content-LA content in the L infeed the [45]. feed It [ 45is ].worth It is worthnoting noting that the that presence the presence of L-LA of in-LA the polymeric in the polymeric backbone backbone even at even at a low content (2.4% mol) results in a considerable increase in the mechanical and a low content (2.4% mol) results in a considerable increase in the mechanical and thermal thermal properties. properties. A major advance came in 2014 when Williams and coworkers reported that the dizinc A major advance came in 2014 when Williams and coworkers reported that the dizinc complex 24 bearing a reduced Robson-type macrocyclic ligand promotes the ROCOP complex 24 bearing a reduced Robson-type macrocyclic ligand◦ promotes the ROCOP of of CO2/CHO and the ROP of CL (pCO2 = 0.1 MPa, T = 80 C, t = 2–21 h) only when CO2/CHO and the ROP of CL (pCO2 = 0.1 MPa, T = 80 °C, t = 2–21 h) only when activated activated by CHO, and intriguingly, a polymerization feed composed by a mixture of by CHO, and intriguingly, a polymerization feed composed by a mixture of CO2/CHO/CHO only leads to the exclusive formation of PCHC (Scheme5)[ 46]. Indeed, CO2/CHO/CHO only leads to the exclusive formation of PCHC (Scheme 5) [46]. Indeed, the synthesis of PCL-b-PCHC was only possible by sequential monomer addition by the synthesis of PCL-b-PCHC was only possible by sequential monomer addition by in- introducing CO2 after the consumption of CL in the presence of CHO or by reverse order troducing CO2 after the consumption of CL in the presence of CHO or by reverse order completely removing CO2 after the formation of the PCHC block. The molecular weight completely removing CO2 after the formation of the PCHC block.−1 The molecular weight of the resulting polymers was rather low (Mn up to 4.8 kg mol ), with narrow dispersity −1 (Ðof =the 1.38–1.49). resulting Thispolymers ability was to selectivelyrather low polymerize(Mn up to 4.8 only kg mol one kind), with of monomernarrow dispersity from a mixture(Ð = 1.38–1.49). and the abilityThis ability to oscillate to selectively between polymerize the ROCOP only and one ROP kind mechanisms of monomer led from to the a definitionmixture and of “switch the ability ” to oscillate [47]. between the ROCOP and ROP mechanisms led to the definition of “switch catalysis” [47]. By performing the ROCOP of CO2/CHO, it was also possible to obtain a polycarbonate ◦ polyol (pCO2 = 0.1 MPa, T = 80 C, t = 16–25 h) that, after removing CO2, can be used for the synthesis of ABA triblock copoly(caprolactone-b-cyclohexene carbonate-b-caprolactone) by adding CL. Remarkably, from the thermal behavior, it was also evident that the presence of the PCHC block disturbs or, at a higher percentage, suppresses the crystallinity of the PCL blocks, allowing the preparation of amorphous polymer films with good transparency [48]. The same catalytic system 24 was also used to obtain pentablock copolymers by alternating ROCOP (anhydrides/epoxide), ROP (lactone) and ROCOP (CO2/epoxide) by using various epoxides (CHO and VCHO), anhydrides (PA, NA), and DL (ε-decalactone). ◦ The resulting pentablock copolymers show a single Tg (from −35 to 20 C), low molecular weight (10–16 kg mol−1)[49] and Ð = 1.06–1.16. Catalysts 2021, 11, x FOR PEER REVIEW 11 of 20

Catalysts 2021, 11, 961 11 of 19 Catalysts 2021, 11, x FOR PEER REVIEW 11 of 20

Scheme 5. The switch catalysis mechanism, ROCOP and ROP promoted by 24. Reproduced with modification and permission from ref [46]. Copyright (2014) John Wiley and Sons.

By performing the ROCOP of CO2/CHO, it was also possible to obtain a polycar- bonate polyol (pCO2 = 0.1 MPa, T = 80 °C, t = 16–25 h) that, after removing CO2, can be used for the synthesis of ABA triblock copoly(caprolactone-b-cyclohexene carbonate-b-ca- prolactone) by adding CL. Notably, from the thermal behavior, it was also evident that the presence of the PCHC block disturbs or, at a higher percentage, suppresses the crys- tallinity of the PCL blocks, allowing the preparation of amorphous polymer films with good transparency [48]. The same catalytic system 24 was also used to obtain pentablock copolymers by al- ternating ROCOP (anhydrides/epoxide), ROP (lactone) and ROCOP (CO2/epoxide) by us- ing various epoxides (CHO and VCHO), anhydrides (PA, NA), and DL (ε-decalactone). Scheme 5. The switch catalysis mechanism, ROCOP and ROP promoted by 24. Reproduced with SchemeThe resulting 5. The switchpentablock catalysis copolymers mechanism, show ROCOP a single and TROPg (from promoted −35 to by 20 24 °C),. Reproduced low molecular with modificationmodificationweight (10–16 andand kg permissionpermission mol−1) [49] from from and ref ref Ð [ 46[46] =]. 1.06–1.16. Copyright. Copyright (2014) (2014) John John Wiley Wiley and and Sons. Sons. A more sophisticated technique was necessary to synthesize ABA block copolymers A more sophisticated technique was necessary to synthesize ABA block copolymers havingBy poly(limonene-carbonate)performing the ROCOP of (PLC) CO2/CHO, blocks it because was also of possiblethe incapability to obtain of athe polycar- dizinc having poly(limonene-carbonate)2 (PLC) blocks because of the incapability of the2 dizinc bonatecomplex polyol to catalyze (pCO the= 0.1 polymerization MPa, T = 80 °C, of limonenet = 16–25 oxideh) that, (LO) after with removing CO2 [50]. CO In ,order can be to complex to catalyze the polymerization of limonene oxide (LO) with CO [50]. In order usedcircumvent for the synthesisthis problem, of ABA a dual triblock catalytic copoly(caprolactone- system was used:b 1)-cyclohexene the dizinc 2complex carbonate- 25b pro--ca- toprolactone) circumvent by this adding problem, CL. Notably, a dual catalytic from the system thermal was behavior, used: (1) it the was dizinc also complexevident that25 promotes,motes, in inthe the presence presence of of 1,2-cyclohexane 1,2-cyclohexane diol diol (CHD), (CHD), the formation ofof aa hydroxyl-hydroxyl- thetelechelic presence PDL of by the the PCHC ROP blockof DL. disturbs This macroinitiator or, at a higher was percentage, then used, after suppresses the modification the crys- telechelictallinity of PDL the by PCL the blocks, ROP of allowing DL. This macroinitiatorthe preparation was of thenamorphous used, after polymer the modification films with ofof the the end end groups groups for for the the synthesis synthesis of of the the PLC PLC blocks, blocks, by by using using a a second second catalytic catalytic system system good transparency [48]. basedbased onon thethe AlAl aminotriphenolate aminotriphenolate complexcomplex26 26developed developedby byKleij Kleij[ 51[51],], as as shown shown in in The same catalytic system 24 was also used to obtain pentablock copolymers by al- SchemeScheme6 .6. ternating ROCOP (anhydrides/epoxide), ROP (lactone) and ROCOP (CO2/epoxide) by us- ing various epoxides (CHO and VCHO), anhydrides (PA, NA), and DL (ε-decalactone). The resulting pentablock copolymers show a single Tg (from −35 to 20 °C), low molecular weight (10–16 kg mol−1) [49] and Ð = 1.06–1.16. A more sophisticated technique was necessary to synthesize ABA block copolymers having poly(limonene-carbonate) (PLC) blocks because of the incapability of the dizinc complex to catalyze the polymerization of limonene oxide (LO) with CO2 [50]. In order to circumvent this problem, a dual catalytic system was used: 1) the dizinc complex 25 pro- motes, in the presence of 1,2-cyclohexane diol (CHD), the formation of a hydroxyl- telechelic PDL by the ROP of DL. This macroinitiator was then used, after the modification of the end groups for the synthesis of the PLC blocks, by using a second catalytic system based on the Al aminotriphenolate complex 26 developed by Kleij [51], as shown in Scheme 6.

Scheme 6. Block polymer synthesis using a dual catalytic system. Reproduced with modification and permission from ref. [50]. Copyright (2020) Royal Society of Chemistry.

The resulting biopolymers PLC-b-PDL-b-PLC have molar masses Mn spanning from 50.700 to 114.6 kg mol−1 and narrow dispersity (Ð = 1.38–1.49). The thermal and me- chanical properties are superior compared to PLC, and these terpolymers show good chemical recyclability through depolymerization with the same dizinc catalyst affording the starting monomers. Lately, a heterodinuclear Zn/Mg catalyst 27 (Figure10) with the same ligand frame- work promoted the formation of ABA triblock copolymers by using DL with high activ- ity [52].

Catalysts 2021, 11, x FOR PEER REVIEW 12 of 20

Scheme 6. Block polymer synthesis using a dual catalytic system. Reproduced with modification and permission from ref. [50]. Copyright (2020) Royal Society of Chemistry.

The resulting biopolymers PLC-b-PDL-b-PLC have molar masses Mn spanning from 50.700 to 114.6 kg mol−1 and narrow dispersity (Ð = 1.38–1.49). The thermal and mechani- cal properties are superior compared to PLC, and these terpolymers show good chemical recyclability through depolymerization with the same dizinc catalyst affording the start- ing monomers. Catalysts 2021, 11, 961 Lately, a heterodinuclear Zn/Mg catalyst 27 (Figure 10) with the same ligand12 frame- of 19 work promoted the formation of ABA triblock copolymers by using DL with high activity [52].

FigureFigure 10. 10.Heterodinuclear Heterodinuclear Zn/Mg Zn/Mg catalyst catalyst27 27[ 52[52].].

InIn particular, particular, by by performing performing the the ROP ROP of of DL DL a a dihydroxyl dihydroxyl telechelic telechelic PDL PDL was was obtained obtained that,that, in in the the presence presence of of CO CO2, undergoes2, undergoes the the transformation transformation into into the ABAthe ABA triblock triblock copolymer copoly- PCHC-mer PCHC-b-PDL-bb-PDL--PCHC.b-PCHC. The raw The copolymers raw copolymers can incorporate can incorporate a high amounta high amount of CO2 (up of CO to 2 −1 23%)(up to and 23%) possess and possess a high moleculara high molecular weight weight (38.0–71.9 (38.0–71.9 kg mol kg mol) with−1) with narrow narrow dispersity disper- − − ◦ (Ðsity= 1.07–1.16).(Ð = 1.07–1.16). These These materials materials display display a single a singleTg (from Tg (from44 to −4450 to C),−50 evidencing°C), evidencing the amorphousthe amorphous nature nature of the of blocks the blocks and their and completetheir complete miscibility, miscibility, and only and the only polymers the polymers with awith higher a higher content content of PCHC of PCHC (>50%) (>50%) show show a second a second transition transition at higher at higher temperatures temperatures (81; ◦ 110(81;C). 110 These °C). These materials materials showpromising show promis thermaling thermal and mechanical and mechanical properties properties compared com- to PCHCpared andto PCHC the possibility and the possibility to modulate to modulate them by regulatingthem by regulating the length the of length the blocks of the in blocks the finalin the polymer, final polymer, potentially potentially giving a giving wide rangea wide of range applications. of applications. RiegerRieger and and coworkers coworkers were were able, able, by by using using a (bdi)-zinc a (bdi)-zinc complex complex28 (Scheme 28 (Scheme7), to 7), obtain to ob- copolymers by the terpolymerization of CO /BBL/CHO [53]. In particular, also in this tain copolymers by the terpolymerization of2 CO2/BBL/CHO [53]. In particular, also in this case the CO2 acts as a switching agent: (A) at pCO2 = 4.0 MPa, the polymerization proceeds case the CO2 acts as a switching agent: (A) at pCO2 = 4.0 MPa, the polymerization proceeds with the exclusive production of PCHC and the formation of the poly(hydroxybutyrate) with the exclusive production of PCHC and the formation of the poly(hydroxybutyrate) (PHB) only starts after releasing CO2 pressure, leading finally to a diblock copolymer (PHB) only starts after releasing CO2 pressure, leading finally to a diblock copolymer PCHC-b-PHB. (B) In the absence of CO2, obviously, the system evolves to the formation PCHC-b-PHB. (B) In the absence of CO2, obviously, the system evolves to the formation of PHB and before to the total consumption of BBL feeding CO2 (pCO2 = 4.0 MPa) with of PHB and before to the total consumption of BBL feeding CO2 (pCO2 = 4.0 MPa) with the the formation of a PCHC block, finally releasing the CO2 the “residual” BBL polymerizes, formation of a PCHC block, finally releasing the CO2 the “residual” BBL polymerizes, giv- giving, at the end, an ABA triblock copolymer PCHC-b-PHB-b-PCHC. (C) By lowering ing, at the end, an ABA triblock copolymer PCHC-b-PHB-b-PCHC. (C) By lowering the the CO2 pressure to pCO2 = 0.3 MPa, the rates of the ROCOP and ROP processes are CO2 pressure to pCO2 = 0.3 MPa, the rates of the ROCOP and ROP processes are compa- comparable and therefore a statistical copolymer was formed. rable and therefore a statistical copolymer was formed. The copolymers’ molecular weights, in the case of the block copolymers, are higher −1 (Mn = 77.0–166 kg mol ) than those obtained in the case of statistical copolymers −1 (Mn = 34.0–69.0 kg mol ), in both cases showing narrow dispersity (Ð = 1.2–1.8). PCHC- b-PHB and PCHC-b-PHB-b-PCHC display two Tg values relative to the PHB and PCHC ◦ ◦ blocks, respectively (Tg1 = 1–2 C and Tg2 = 116–118 C), as a consequence of phase separation between the polycarbonate and polyester blocks, also confirmed by atom force microscopy (AFM). Conversely, the random copolymers display a single transition ◦ (Tg = 36–91 C) that increases by increasing the amount of carbonate linkages in the poly- mer chain. Similar results were obtained with cyclopenteneoxide (CPO), but in this case the polymerization at a higher pressure (pCO2 = 4.0–5.0 MPa) results in the formation of a gradient copolymer rather than a diblock copolymer. The kinetic study evidenced a change in the reaction order with respect to CO2 with a zero order dependence at high pressure (between pCO2 = 0.5–1 MPa) and first-order at lower pressure (pCO2 < 0.5 MPa), indicating that under the latter conditions the insertion of CO2 became the rate-limiting step [54]. As expected, the incorporation of polyester segments in both the statistical and block copolymers leads to an improvement in the mechanical properties compared to the brittle PCHC with a decrease in the Young modulus and tensile strength and an increase in the elongation at break for polymers with a high molecular weight (>100 kg mol−1). Efforts to terpolymerize CO2/BBL/LO (limonene oxide) evidenced that due to the low Catalysts 2021, 11, 961 13 of 19

ceiling temperature (60 ◦C) of the polylimonenecarbonate (PLC), the only way to obtain block copolymers is to first obtain the PHB block via the ROP of BBL and then feed CO2 for the formation of the PLC block. Actually, the PHB-b-PLC copolymers possess a high −1 molecular weight (Mn up to 233 kg mol ) and narrow dispersity (Ð = 1.23–1.39), showing ◦ two Tg = 1–3/26–133 C. Statistical copolymers were also obtained by adjusting the CO2 Catalysts 2021, 11, x FOR PEER REVIEWpressure (pCO2 = 0.9 MPa), resulting in low conversion (up to 22% LO and 26% BBL13 of in20 −1 22 h) and low molecular weight polymers (Mn = 9.0 kg mol ).

Scheme 7. DifferentDifferent reaction reaction pathway pathway of of 2828 towardtoward ROP ROP of of BBL, BBL, copolymerization copolymerization of ofCPO CPO with with CO CO2, and2, and terpolymeriza- terpolymer- izationtion of ofCHO, CHO, CO CO2 and2 and BBL. BBL. Reproduced Reproduced with with modifi modificationcation and and permission permission from from ref ref [53] [53].. Copyright (2017) AmericanAmerican Chemical Society.Society. 3.2. Cobalt Complexes The copolymers’ molecular weights, in the case of the block copolymers, are higher Salen cobalt complexes are highly active catalysts in the ROCOP of CO2 with epoxides (Mn = 77.0–166 kg mol−1) than those obtained in the case of statistical copolymers (Mn = and therefore are, in principle, viable candidates for the terpolymerization of CO2 with Catalysts 2021, 11, x FOR PEER REVIEW34.0–69.0 kg mol−1), in both cases showing narrow dispersity (Ð = 1.2–1.8). PCHC-b-PHB 14 of 20 epoxides and lactones. Unfortunately, these complexes are inactive in the ROP of cyclic and PCHC-b-PHB-b-PCHC display two Tg values relative to the PHB and PCHC blocks, esters, and consequently, the implementation of an active catalytic system for obtaining respectively (Tg1 = 1–2 °C and Tg2 = 116–118 °C), as a consequence of phase separation be- polycarbonate-b-polyester copolymers requires the use of multi-component systems able tween the polycarbonate and polyester blocks, also confirmed by atom force microscopy to synthesize the desired polymeric product. (AFM).An Conversely, elegant strategy the random was copoly developedmers display by Darensbourg a single transition and Lu (T gthat = 36–91 used °C) a thatcombination An elegant strategy was developed by Darensbourg and Lu that used a combination increasesof the bifunctional by increasing Co(III) the amount salen complex of carbon 29ate and linkages DBU in(1,8-diazabicyclo[5.4.0]undec-7-ene) the polymer chain. Similar of the bifunctional Co(III) salen complex 29 and DBU (1,8-diazabicyclo[5.4.0]undec-7- results(Figure were 11) obtained for the terpolymerizationwith cyclopenteneoxide (pCO (CPO),2 = 1.5 butMPa, in thisT = case25 °C, the◦ t polymerization= 2–6 h) of CO 2/SO/LA ene) (Figure 11) for the terpolymerization (pCO2 = 1.5 MPa, T = 25 C, t = 2–6 h) of at[55]. a higher pressure (pCO2 = 4.0–5.0 MPa) results in the formation of a gradient copolymer CO2/SO/LA [55]. rather than a diblock copolymer. The kinetic study evidenced a change in the reaction order with respect to CO2 with a zero order dependence at high pressure (between pCO2 = 0.5–1 MPa) and first-order at lower pressure (pCO2 < 0.5 MPa), indicating that under the latter conditions the insertion of CO2 became the rate-limiting step [54]. As expected, the incorporation of polyester segments in both the statistical and block copolymers leads to an improvement in the mechanical properties compared to the brittle PCHC with a de- crease in the Young modulus and tensile strength and an increase in the elongation at break for polymers with a high molecular weight (>100 kg mol−1). Efforts to terpolymerize CO2/BBL/LO (limonene oxide) evidenced that due to the low ceiling temperature (60 °C) of the polylimonenecarbonate (PLC), the only way to obtain block copolymers is to first obtain the PHB block via the ROP of BBL and then feed CO2 for the formation of the PLC

block. Notably, the PHB-b-PLC copolymers possess a high molecular weight (Mn up to 233FigureFigure kg 11.mol 11.Complex− Complex1) and narrow29 and29 and organocatalyst dispersity organocatalyst (Ð DBU = 1.23–1.39), DBU [55]. [55]. showing two Tg = 1–3/26–133 °C. Sta- tistical copolymers were also obtained by adjusting the CO2 pressure (pCO2 = 0.9 MPa), resultingIndeed,Indeed, in low the the conversion cobalt cobalt complex complex(up to29 22%produced 29 LO produced and 26% with BBL highwith in activity 22high h) andactivi poly(styrene lowty molecular poly(styrene carbonate) weight carbonate) (PSC) from CO2 and SO, after−1 the complete consumption of SO, a given amount of H2O, polymers(PSC) from (Mn CO= 9.02 andkg mol SO,). after the complete consumption of SO, a given amount of H2O, which was added to the reaction mixture, resulting in the production of hydroxy-termi- 3.2.nated Cobalt PSC. Complexes Consequently, the polycarbonate formed, having the hydroxy chain-end, can act Salenas a macroinitiatorcobalt complexes for are the highly ROP active of LA catalysts catalyzed in the by ROCOP DBU. ofAs CO a 2matter with epox- of fact, after ides and therefore are, in principle, viable candidates for the terpolymerization of CO2 adding two equivalent of H2O with respect to the cobalt catalyst and the removal of CO2, with epoxides and lactones. Unfortunately, these complexes are inactive in the ROP of the addition of LA and DBU results in the production of the AB copolymer PSC-b-PLA cyclic esters, and consequently, the implementation of an active catalytic system for ob- −1 tainingwith molecular polycarbonate- weightb-polyester Mn up tocopolymers 17.2 kg mol requires and the narrow use of dispersity multi-component (Ð = 1.04–1.12). sys- The temscopolymers able to synthesize obtained the from desired rac-LA polymeric display product. a single Tg at lower values with respect to PSC (60–72 °C), and by polymerizing D-LA, the resulting diblock copolymers also display a Tm = 133–137 °C depending on the length of the PLA block. The same authors also used this strategy to synthesize ABA block copolymers from CO2/PO/LA. They used the system composed of the salen Co(III) complex 29 activated by PPNY (Y = CF3COO–) and DBU [56]. In this case, the obtaining of PLA-b-PPC-b-PLA was possible because of the addition of an excess of H2O (5–20 equiv. with respect to CO) to stop the CO2/PO copolymerization of a PPC with two α,ω hydroxy groups. Indeed, the presence of a hydroxy group on both chain-ends allows the growth of two PLA blocks, giving the desired PLA-b-PPC-b-PLA triblock copolymers. The molecular weight was ra- ther low, Mn up to 20.1 kg mol−1, with narrow dispersity (Ð = 1.02–1.04). Additionally, in this case, on one hand the copolymers obtained from rac-LA displayed a single Tg at higher values with respect to PPC (43–44 °C), and on the other hand, by polymerizing D-LA, the resulting ABA copolymers also displayed a Tm = 110–128 °C depending on the length of the PLA blocks. Later on, Pang and coworkers developed a ternary system composed by the dinu- clear Co(II) and the Co(III) complexes with salen ligands (Figure 12) and PPNCl [57].

Catalysts 2021, 11, 961 14 of 19

which was added to the reaction mixture, resulting in the production of hydroxy-terminated PSC. Consequently, the polycarbonate formed, having the hydroxy chain-end, can act as a macroinitiator for the ROP of LA catalyzed by DBU. As a matter of fact, after adding two equivalent of H2O with respect to the cobalt catalyst and the removal of CO2, the addition of LA and DBU results in the production of the AB copolymer PSC-b-PLA with −1 molecular weight Mn up to 17.2 kg mol and narrow dispersity (Ð = 1.04–1.12). The copolymers obtained from rac-LA display a single Tg at lower values with respect to PSC (60–72 ◦C), and by polymerizing D-LA, the resulting diblock copolymers also display a ◦ Tm = 133–137 C depending on the length of the PLA block. The same authors also used this strategy to synthesize ABA block copolymers from CO2/PO/LA. They used the system composed of the salen Co(III) complex 29 activated – by PPNY (Y = CF3COO ) and DBU [56]. In this case, the obtaining of PLA-b-PPC-b-PLA was possible because of the addition of an excess of H2O (5–20 equiv. with respect to CO) to stop the CO2/PO copolymerization of a PPC with two α,ω hydroxy groups. Indeed, the presence of a hydroxy group on both chain-ends allows the growth of two PLA blocks, giving the desired PLA-b-PPC-b-PLA triblock copolymers. The molecular weight was −1 rather low, Mn up to 20.1 kg mol , with narrow dispersity (Ð = 1.02–1.04). Additionally, in this case, on one hand the copolymers obtained from rac-LA displayed a single Tg at higher values with respect to PPC (43–44 ◦C), and on the other hand, by polymerizing ◦ D-LA, the resulting ABA copolymers also displayed a Tm = 110–128 C depending on the Catalysts 2021, 11, x FOR PEER REVIEWlength of the PLA blocks. 15 of 20 Later on, Pang and coworkers developed a ternary system composed by the dinuclear Co(II) and the Co(III) complexes with salen ligands (Figure 12) and PPNCl [57].

FigureFigure 12. 12.Structures Structures of salenCo of salenCoII complexesII complexes (30a–32a (30a) and–32a salenCo) and IIIsalenCocomplexesIII complexes (30b–32b )[(30b57].–32b) [57].

Indeed,Indeed, the the Co(II) Co(II) complexes complexes (30a– 32a(30a)– are32a active) are inactive the ROP in the of LA,ROP and of theLA, Co(III) and the Co(III) complexes (30b–32b), in combination with PPNCl, are active in the ROCOP of CO2 with complexes (30b–32b), in combination with PPNCl, are active in the ROCOP of CO2 with various epoxides (PO, CHO, SO). The terpolymerization was possible for the chain transfer various epoxides (PO, CHO, SO). The terpolymerization was possible for the chain trans- between the two metal centers (Scheme8). fer between the two metal centers (Scheme 8).

Scheme 8. Copolymerization of LLA (L-Lactide), PO and CO2: the proposed cycle. Reproduced with modification and permission from ref [57]. Copyright (2018) Royal Society of Chemistry.

Catalysts 2021, 11, x FOR PEER REVIEW 15 of 20

Figure 12. Structures of salenCoII complexes (30a–32a) and salenCoIII complexes (30b–32b) [57].

Indeed, the Co(II) complexes (30a–32a) are active in the ROP of LA, and the Co(III) Catalysts 2021, 11, 961 complexes (30b–32b), in combination with PPNCl, are active in the ROCOP of CO152 with of 19 various epoxides (PO, CHO, SO). The terpolymerization was possible for the chain trans- fer between the two metal centers (Scheme 8).

Catalysts 2021, 11, x FOR PEER REVIEW 16 of 20

Scheme 8. Copolymerization of LLA (L-Lactide), PO and CO2: the proposed cycle. Reproduced with Scheme 8. Copolymerization of LLA (L-Lactide), PO and CO2: the proposed cycle. Reproduced with modificationmodification andand permissionpermission fromfrom refref [ 57[57]]. Copyright. Copyright (2018) (2018) Royal Royal Society Society of of Chemistry. Chemistry.

ByBy usingusing 30a30a andand 30b30b and PPNCl in in an an equimolar equimolar amount, amount, the the terpolymerization terpolymerization of CO2/LA/PO gives terpolymers, as revealed by 1H 1and 13C 13NMR analysis, possessing a of CO2/LA/PO gives terpolymers, as revealed by H and C NMR analysis, possess- multiblock microstructure with Mn up to 13.6 kg mol−1 and narrow−1 dispersity (Ð = 1.19– ing a multiblock microstructure with Mn up to 13.6 kg mol and narrow dispersity (Ð1.47).= 1.19–1.47 Notably,). Notably,the dispersity the dispersity broadens broadens in the absence in the of absence PPNCl of (Ð PPNCl = 3.15) (Ð and= 3.15) with and two withequivalents two equivalents of PPNCl of (Ð PPNCl = 2.28), (Ð confirming= 2.28), confirming the crucial the role crucial of the role onium of the salt onium in the salt chain- in thetransfer chain-transfer between betweenthe metal the centers. metal centers. MoreMore recently, recently, the the same same authors authors further further developed developed this this ternary ternary system system by changing by changing the Co(II)the Co(II) and Co(III)and Co(III) complexes complexes (30a and (30a30b and),obtaining 30b), obtaining a more a active more system active orsystem combining or com- a salenbining Co(III) a salen complex Co(III) withcomple ZnGAx with and ZnGA PPNCl and [58 PPNCl]. [58]. Finally,Finally, in in the the presence presence of of an an enantiopure enantiopure chiral chiral salenCo(III) salenCo(III) complexcomplex33 33( (FigureFigure 13 13)) inin combinationcombination with PPN-DNP PPN-DNP (PPN (PPN = =bis(tr bis(triphenylphosphine)iminium,iphenylphosphine)iminium, DNP DNP = 2,4-dini- = 2,4- dinitrophenoxide),trophenoxide), Lu Luand and coworkers coworkers also also succeeded succeeded in inproducing producing CO CO2/CHO/BBL2/CHO/BBL terpoly- ter- polymersmers with with isotactic isotactic -PCHC -PCHC blocks blocks [59]. [59 ].

FigureFigure 13. 13.Enantiopure Enantiopure chiral chiral dinuclear dinuclear (S,S,S,S)- (S,S,S,S)-3333complex complex [ 59[59].].

MoreMore inin detail,detail, whenwhen anan equimolarequimolar amountamount ofof CHOCHO andand BBLBBL isis presentpresent inin thethe feed,feed, thethe terpolymerizationterpolymerization proceedsproceeds smoothlysmoothly withwith thethe good good conversion conversion of of both both monomers monomers (pCO2 = 2 MPa, T = 40 °C, t = 2–4 h). The resulting copolymers display, in the 1H NMR spectra, the signals relative to the carbonate-ester linkages, indicating a multiblock struc- ture. Molecular weights are rather low, Mn = 3.3–14.6 kg mol−1, with narrow dispersity (Ð = 1.19–1.44), and display thermal behavior with a Tm = 204–220 °C, evidencing the presence of stereoregular crystalline blocks along the polymer chain. In Table 2, the main data relating to the terpolymerizations of CO2 with epoxides and cyclic esters discussed in this second part are summarized.

Table 2. Summary of terpolymerization of CO2 with epoxides and cyclic esters.

Polymerization Conditions Mn * (Ð) Polymer Catalyst Monomers Ref. pCO2, T, t kg mol−1 Microstructure 1 CO2, PO, CL 4.0 MPa, 50–90 °C, 16 h - random [44] 4 CO2, PO, CL 2.8 MPa, 60 °C, 40 h 27.5 (1.50–2.97) block [41] 4 CO2, PO, CL 1.0 MPa, 60 °C, 6 h 10.8 (1.3–1.6) grafted [42] 15–22 CO2, CHO, LA 4.0 MPa, 90 °C, 16 h 41.6 (1.09–1.96) alternated/random [43] 23 CO2, PO, L-LA 4.0 MPa, 70 °C, 10 h 15.4 (4.2–9.9) tapered/random [45] 24 CO2, CHO, CL 0.1 MPa, 80 °C, 2–21 h 4.8 (1.38–1.49) block [46] 24 CO2, CHO, CL 0.1 MPa, 80 °C, 16–25 h 13.8 (1.29–1.49) block [48] CO2, CHO, VCHO, 24 0.1 MPa, 100 °C 16 (1.06–1.16) block [49] PA/NA, CL 25–26 CO2, CHD, DL 2 MPa, 40–100 °C, 2–24 h 114 (1.38–1.49) block [50] 27 CO2, CHO, DL 2 MPa, 80 °C, 21 h 71.9 (1.07–1.16) block [52] 28 CO2, BBL,CHO/CPO 0.3–4 MPa, 60 °C, 0.1–7 h 166 (1.2–1.8) random/block [53] 28 CO2, BBL, LO 0.9–4 MPa, 40–60 °C, 8–22 h 233 (1.23–1.39) random/block [54] 29 CO2, SO, LA 1.5 MPa, 25 °C, 2–6 h 17.2 (1.04–1.12) block [55]

Catalysts 2021, 11, 961 16 of 19

◦ 1 (pCO2 = 2 MPa, T = 40 C, t = 2–4 h). The resulting copolymers display, in the H NMR spectra, the signals relative to the carbonate-ester linkages, indicating a multiblock struc- −1 ture. Molecular weights are rather low, Mn = 3.3–14.6 kg mol , with narrow dispersity ◦ (Ð = 1.19–1.44), and display thermal behavior with a Tm = 204–220 C, evidencing the presence of stereoregular crystalline blocks along the polymer chain. In Table2, the main data relating to the terpolymerizations of CO 2 with epoxides and cyclic esters discussed in this second part are summarized.

Table 2. Summary of terpolymerization of CO2 with epoxides and cyclic esters.

Polymerization Conditions Mn *(Ð) Polymer Catalyst Monomers −1 Ref. pCO2, T, t kg mol Microstructure ◦ 1 CO2, PO, CL 4.0 MPa, 50–90 C, 16 h - random [44] ◦ 4 CO2, PO, CL 2.8 MPa, 60 C, 40 h 27.5 (1.50–2.97) block [41] ◦ 4 CO2, PO, CL 1.0 MPa, 60 C, 6 h 10.8 (1.3–1.6) grafted [42] ◦ 15–22 CO2, CHO, LA 4.0 MPa, 90 C, 16 h 41.6 (1.09–1.96) alternated/random [43] ◦ 23 CO2, PO, L-LA 4.0 MPa, 70 C, 10 h 15.4 (4.2–9.9) tapered/random [45] ◦ 24 CO2, CHO, CL 0.1 MPa, 80 C, 2–21 h 4.8 (1.38–1.49) block [46] ◦ 24 CO2, CHO, CL 0.1 MPa, 80 C, 16–25 h 13.8 (1.29–1.49) block [48] ◦ 24 CO2, CHO, VCHO, PA/NA, CL 0.1 MPa, 100 C 16 (1.06–1.16) block [49] ◦ 25–26 CO2, CHD, DL 2 MPa, 40–100 C, 2–24 h 114 (1.38–1.49) block [50] ◦ 27 CO2, CHO, DL 2 MPa, 80 C, 21 h 71.9 (1.07–1.16) block [52] ◦ 28 CO2, BBL, CHO/CPO 0.3–4 MPa, 60 C, 0.1–7 h 166 (1.2–1.8) random/block [53] ◦ 28 CO2, BBL, LO 0.9–4 MPa, 40–60 C, 8–22 h 233 (1.23–1.39) random/block [54] ◦ 29 CO2, SO, LA 1.5 MPa, 25 C, 2–6 h 17.2 (1.04–1.12) block [55] ◦ 29 CO2, PO, LA 1.5 MPa, 25 C, 1–4 h 20.1 (1.02–1.04) block [56] ◦ 30–32 CO2, PO/CHO/SO, LA 2 MPa, 60 C, 4–48 h 13.6 (1.19–3.15) block [57] ◦ 33 CO2, CHO, BBL 2.0 MPa, 40 C, 2–4 h 14.6 (1.19–1.44) block [58] * The highest reported value.

4. Conclusions

The possibility to terpolymerize CO2 with epoxides and other cyclic monomers (cyclic esters, organic anhydrides) offers not only a simple way to obtain a wide range of mate- rials with unprecedented properties, but also the possibility to have such material in a completely sustainable way, combining CO2 with monomers originating from biomasses. The last decade has witnessed tremendous efforts in the development of efficient catalytic systems able to combine the ROP of cyclic esters and the ROCOP of CO2 or cyclic organic anhydrides with epoxides, allowing us to obtain polymers with various microstructural features spanning from statistical, to AB, ABA, and even more complex architectures. Notwithstanding these endeavors, however, fine control of the microstructure and the molecular weight is still a major challenge in the field. Furthermore, the number of metal centers active in the terpolymerization of CO2 with epoxides and cyclic esters of anhydrides is still limited, offering active catalysts only in the case of Zn, Cr and Co, and, only in the case of the terpolymerization with cyclic anhydrides, in the presence of metal-free borane-based catalysts. Therefore, this review is not only an overview on the progress in the field, but also shows that there is a large space for further developments. More precisely, higher control over the polymer microstructure, an extension to a wider range of monomers and the development of new catalytic systems based on other metal centers to improve the activity and the control of the polymerization process will be highly desirable targets in future developments.

Author Contributions: Conceptualization, C.C.; data curation, C.C. and D.H.L.; writing—original draft preparation, C.C.; writing—review and editing, C.C. and D.H.L. Both authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Data Availability Statement: Not applicable. Catalysts 2021, 11, 961 17 of 19

Conflicts of Interest: The authors declare no conflict of interest.

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