Terpolymerization of CO 2 with Epoxides and Cyclic Organic Anhydrides or Cyclic Esters

: The synthesis of polymeric materials starting from CO 2 as a feedstock is an active task of research. In particular, the copolymerization of CO 2 with epoxides via ring-opening copolymerization (ROCOP) offers a simple, efﬁcient route to synthesize aliphatic polycarbonates (APC). In many cases, APC display poor physical and chemical properties, limiting their range of application. The terpolymerization of CO 2 with epoxides and organic anhydrides or cyclic esters offers the possibility, combining the ROCOP with ring-opening polymerization (ROP), to access a wide range of materials containing polycarbonate and polyester segments along the polymer 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. polyester segment with no incorporation of CO 2 , and only after the total consumption of PA the polycarbonate block was formed, also giving, in this case, a diblock polymer. The produced copolymers have a very narrow dispersity ( Ð = 1.19–1.22) and the molecular weight increases with the polymerization time ( M n up to 21.2 kg mol − 1 ).


Introduction
The pervasiveness of polymers in our daily life is a consolidated reality in recent decades. Indeed, the reason for the fortune of polymeric materials is due to both their unique physical and chemical properties and their cheapness compared to other structural materials. In the last few years, however, there is a growing body of evidence that the large success of these materials has determined a negative effect on terrestrial and marine ecosystems with the presence of microplastics that have become ubiquitous on our planet [1]. This situation has engendered the growing attention of the polymer industries and the scientific community to find biodegradable, more sustainable polymeric materials. Parallel to this trend, the use of CO 2 as a carbon feedstock has also gained momentum due to the rising interest in using such an inexpensive, non-toxic molecule as a starting material for the synthesis of polymers [2,3].
In particular, the alternating ring-opening copolymerization (ROCOP) of CO 2 with epoxides has offered a conceptually simple route for the synthesis of aliphatic polycarbonates (APC), which show a clear advantage in terms of biodegradability with respect to polyolefins [4][5][6]. APC, however, often display poor chemical and mechanical properties 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][8][9]. Aliphatic polyesters are another important class of biopolymers that can be conveniently obtained by the ring-opening polymerization (ROP) of cyclic esters [10][11][12][13] or by the ROCOP of epoxides with cyclic anhydrides [14][15][16][17]. Transition metal complexes generally catalyze all these polymerization processes through a coordination-insertion mechanism. Actually, taking a closer look at the polymerization mechanism of the ROCOP of epoxides with CO 2 or cyclic anhydrides depicted in Scheme 1, it is evident that the formation of the metal-alkoxo bond (a' in Scheme 1) is a key intermediate in the propagation process. In analogy, the ROP of cyclic esters proceeds via the formation of a metal-alkoxo bond (a in Scheme 1) that allows the ring-opening of the following monomer unit.
Given this mechanistic scenario, it is easy to imagine that it is possible to design a metal complex able to promote both types of polymerization, allowing us to obtain various block-copolymers. In principle, it is possible to obtain copolymers with polycarbonate and polyester segments and modulate the nature of the polycarbonate and polyester blocks, permitting the synthesis of new materials with tailored properties.
Notwithstanding the potential of such an approach, the efforts to develop efficient catalytic systems that cannot only incorporate CO 2 but also give rise to unprecedented new materials have raised good results only recently.
This review will cover the last advancements (since 2003) in the metal-catalyzed and metal-free terpolymerization of CO 2 with epoxides and cyclic esters or cyclic organic anhydrides for the obtaining of polycarbonate-polyester copolymers.

Terpolymerization of CO 2 with Epoxides and Cyclic Anhydrides
Polymeric materials containing ester and carbonate linkages have shown potential as biodegradable implants and, in addition, it is possible to adjust the degradation rate by regulating the length of the polyester and polycarbonate blocks. The first approach to synthesize poly(ester-block-carbonate)s in a one-pot reaction was the copolymerization, via ROP, of cyclic esters and cyclic carbonates promoted by stannous octanoate [18]. This conceptually simple approach has some limitations because six or seven-membered cyclic carbonates suitable for ROP must be synthesized through time-consuming multistep protocols [19,20].

Zinc Complexes
Only in 2006, Liu and coworkers reported on the terpolymerization of propylene oxide (PO) with CO 2 and maleic anhydride (MA) catalyzed by polymer-supported bimetallic catalyst (PBM) 1 of general formula P-Zn[Fe(CN) 6 ] a Cl 2-3a (H 2 O) b , with P being the polyether type chelating agent and a ≈ 0.5 and b ≈ 0.76 [21]. Notably, the catalyst was inactive in the PO/MA copolymerization whereas it gave the poly(propylenecarbonate) (PPC) from PO/CO 2 . In the terpolymerization experiments (pCO 2 = 4.0 MPa, T = 60 • C, t = 24 h), the polymer yield increased up to a 5:3 ratio between PO and MA, and a further increase in the MA content was detrimental for the polymerization activity. 1 H and 13 C NMR and IR spectroscopy revealed a random microstructure characterizing the resulting copolymers ( Figure 1). The DSC thermograms showed a single transition with T g values (29.1-56.1 • C) increasing by increasing the MA content in agreement with the microstructure revealed by NMR spectroscopy. Reproduced with modification and permission from ref. [22]. Copyright ( Sons. Notably, in spite of the polymerization feed simultaneously conta omers, the final polymer was a diblock copolymer with a polyester poly(cyclohexenecarbonte) (PCHC) block. Indeed, by following the p tion by in situ IR spectroscopy, the exclusive formation of the polye total consumption of DGA was evident. From the mechanistic point that the first step was the formation of zinc alkoxide by the ring-openi by the preferential and irreversible insertion of DGA until this mono consumed and only after the zinc alkoxide can allow the insertion of C of the polycarbonate block. The polymerization of succinic anhydri lower reactivity, and vinyl-CHO was also accomplished.
In 2010, Zhang and coworkers showed that the heterogeneous d complex (DMCC) 3 obtained by the reaction of K3Co(CN)6 with ZnC polymerization of CO2 with CHO and MA [23]. The proposed active is a Zn atom in a tetrahedral structure with the Co atom playing a catalyst was highly active and selective, giving a complete conversion MPa, T = 90 °C, t = 5 h) and a complete selectivity toward the polymer the resulting polymer was poly(ester-block-carbonate), but it also c amount of polyether linkages (2.9-11.7%) depending on the reaction c Later on, in 2008, the development of an efficient homogeneous catalytic system by Coates based on β-diiminate (bdi) zinc complex 2 ( Figure 2) allowed the synthesis of a poly(ester-block-carbonate) by the terpolymerization (pCO 2 = 0. 3-5.4 MPa, T = 50 • C, t = 0.2-3 h) of cyclohexeneoxide (CHO) with diglycolic anhydride (DGA) and CO 2 [22]. Later on, in 2008, the development of an efficient homogeneous catalytic system by Coates based on β-diiminate (bdi) zinc complex 2 ( Figure 2) 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]. Reproduced with modification and permission from ref. [22]. Copyright  Notably, in spite of the polymerization feed simultaneously containing all three monomers, the final polymer was a diblock copolymer with a polyester block followed by a poly(cyclohexenecarbonte) (PCHC) block. Indeed, by following the polymerization reaction by in situ IR spectroscopy, the exclusive formation of the polyester block up to the total consumption of DGA was evident. From the mechanistic point of view, it was clear that the first step was the formation of zinc alkoxide by the ring-opening of CHO followed by the preferential and irreversible insertion of DGA until this monomer was completely consumed and only after the zinc alkoxide can allow the insertion of CO2 with the growth of the polycarbonate block. The polymerization of succinic anhydride (SA), albeit with lower reactivity, and vinyl-CHO was also accomplished.
In 2010, Zhang and coworkers showed that the heterogeneous double metal cyanide complex (DMCC) 3 obtained by the reaction of K3Co(CN)6 with ZnCl2 promotes the terpolymerization of CO2 with CHO and MA [23]. The proposed active site for this catalyst is a Zn atom in a tetrahedral structure with the Co atom playing a spectator role. The 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 particular, the use of THF as a solvent inhibits the formation of polyether linkages due to the coordination of the THF molecule to the Zn 2+ center. The mechanism proposed for CO2/CHO/MA terpolymerization catalyzed by a Zn-Co(III) DMCC catalyst is depicted in Scheme 2.
Notably, in spite of the polymerization feed simultaneously containing all three monomers, the final polymer was a diblock copolymer with a polyester block followed by a poly(cyclohexenecarbonte) (PCHC) block. Indeed, by following the polymerization reaction by in situ IR spectroscopy, the exclusive formation of the polyester block up to the total consumption of DGA was evident. From the mechanistic point of view, it was clear that the first step was the formation of zinc alkoxide by the ring-opening of CHO followed by the preferential and irreversible insertion of DGA until this monomer was completely consumed and only after the zinc alkoxide can allow the insertion of CO 2 with the growth of the polycarbonate block. The polymerization of succinic anhydride (SA), albeit with lower reactivity, and vinyl-CHO was also accomplished.
In 2010, Zhang and coworkers showed that the heterogeneous double metal cyanide complex (DMCC) 3 obtained by the reaction of K 3 Co(CN) 6 with ZnCl 2 promotes the terpolymerization of CO 2 with CHO and MA [23]. The proposed active site for this catalyst is a Zn atom in a tetrahedral structure with the Co atom playing a spectator role. The catalyst was highly active and selective, giving a complete conversion of CHO (pCO 2 = 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 particular, the use of THF as a solvent inhibits the formation of polyether linkages due to the coordination of the THF molecule to the Zn 2+ center. The mechanism proposed for CO 2 /CHO/MA terpolymerization catalyzed by a Zn-Co(III) DMCC catalyst is depicted in Scheme 2.
Notably, notwithstanding the heterogeneous nature of the catalyst, the dispersity was narrow (Ð = 1.4-1.7) and the M n was up to 14.1 kg mol −1 .
These opposite effects determine an ideal value for the PA/PO ratio giving the maxi mum activity and highest molecular weight, and this ratio in the feed was experimentally found to be 1:8. However, the observed decrease in the Mn could also be explained con sidering the presence of diacid impurities in the anhydride that acts as a chain-transfe 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 Scheme 2. Proposed mechanism for CHO, MA and CO 2 terpolymerization using Zn-Co(III) DMCC catalyst 3. Reproduced with modification and permission from ref. [23]. Copyright (2010) Elsevier.
Zinc glutarate (ZnGA) 4 was also found to be a versatile catalyst for the terpolymerization of CO 2 with PO and various cyclic anhydrides. Indeed, in 2014, Meng reported on the synthesis of PO/phthalic anhydride (PA)/CO 2 copolymers (pCO 2 = 5.0 MPa, T = 75 • C, t = 15 h) using toluene as a solvent [24]. Notably, in this case the formation of polycarbonate is favored over the polyester formation; consequently, the resulting terpolymers (M n up to 221 kg mol −1 , Ð = 2.1-3.9) consist of a polycarbonate chain randomly interrupted by PA units (4.8-10.5%) and a lower amount of polyether linkages (2.7-7.1%). On one hand, the lower reactivity of PA vs. CO 2 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 M n 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). Notably, notwithstanding the heterogeneous nature of the catalyst, the dispersity was narrow (Ð = 1.4-1.7) and the Mn was up to 14.1 kg mol −1 .
Zinc glutarate (ZnGA) 4 was also found to be a versatile catalyst for the terpolymer ization of CO2 with PO and various cyclic anhydrides. Indeed, in 2014, Meng reported on 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 bonate is favored over the polyester formation; consequently, the resulting terpolymer (Mn up to 221 kg mol −1 , Ð = 2.1-3.9) consist of a polycarbonate chain randomly interrupted by PA units (4.8-10.5%) and a lower amount of polyether linkages (2.7-7.1%). On on hand, the lower reactivity of PA vs. CO2 was explained with the slower insertion of th PA into the Zr-alkoxo bond; on the other hand, the increase in the polymer yield and M observed when PA is present in the feed was explained with a faster insertion of PO int the zinc benzoate growing chain with respect to the zinc carbonate chain (Scheme 3). These opposite effects determine an ideal value for the PA/PO ratio giving the maxi mum activity and highest molecular weight, and this ratio in the feed was experimentally found to be 1:8. However, the observed decrease in the Mn could also be explained con sidering the presence of diacid impurities in the anhydride that acts as a chain-transfe 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. These opposite effects determine an ideal value for the PA/PO ratio giving the maximum activity and highest molecular weight, and this ratio in the feed was experimentally found to be 1:8. However, the observed decrease in the M n could also be explained considering the presence of diacid impurities in the anhydride that acts as a chain-transfer agent.
DSC thermograms show that the introduction of an aromatic ter-monomer in the polymer sensibly enhances the T g with respect to the corresponding polycarbonate with an increase of 6 • C incorporating 5.6% of PA. The same catalytic system 4 was also used for the synthesis of pseudo-interpenetrating poly(propylenecarbonate) by the terpolymerization (pCO 2 = 5.4 MPa, T = 70 • C, t = 36 h) of The same catalytic system 4 was also used for the synthesis of pseudo-interpenetra ing poly(propylenecarbonate) by the terpolymerization (pCO2 = 5.4 MPa, T = 70 °C, t = 3 h) of CO2 with PO and pyromellitic dianhydride (PMDA) up to 4% (in this case, Mn in creased up to 862 kg mol −1 ), resulting in a noticeable improvement in the mechanical an thermal properties with respect to the corresponding polycarbonate [25].
More recently, Williams et al. described the synthesis of a new dinuclear zinc com plex 5 (Figure 3) that promotes the terpolymerization of CHO/PA/CO2 (pCO2 = 3.0 MPa, = 100 °C, t = 18 h) [26]. Similar results were also obtained by Castro-Osma and coworkers by using dinuclea zinc complexes 6-8 supported by heteroscorpionate ligands (Figure 4) [27].   By monitoring the reaction after 2 h, it was evident that there was the exclusive formation of polyether linkages and no formation of PCHC, and after 18 h the formation of a poly(ester-block-carbonate) (M n up to 7 kg mol −1 , Ð = 1.20) was evident and confirmed by size-exclusion chromatography (SEC) analysis and diffusion-ordered spectroscopy (DOSY) experiments.

Chromium and Cobalt Complexes
Similar results were also obtained by Castro-Osma and coworkers by using dinuclear zinc complexes 6-8 supported by heteroscorpionate ligands (Figure 4) [27]. The same catalytic system 4 was also used for the synthesis of pseudo-interpenetrating poly(propylenecarbonate) by the terpolymerization (pCO2 = 5.4 MPa, T = 70 °C, t = 36 h) of CO2 with PO and pyromellitic dianhydride (PMDA) up to 4% (in this case, Mn increased up to 862 kg mol −1 ), resulting in a noticeable improvement in the mechanical and thermal properties with respect to the corresponding polycarbonate [25].
More recently, Williams et al. described the synthesis of a new dinuclear zinc complex 5 (Figure 3) that promotes the terpolymerization of CHO/PA/CO2 (pCO2 = 3.0 MPa, T = 100 °C, t = 18 h) [26]. By monitoring the reaction after 2 h, it was evident that there was the exclusive formation of polyether linkages and no formation of PCHC, and after 18 h the formation of a poly(ester-block-carbonate) (Mn up to 7 kg mol −1 , Ð = 1.20) was evident and confirmed by size-exclusion chromatography (SEC) analysis and diffusion-ordered spectroscopy (DOSY) experiments.

Chromium and Cobalt Complexes
In 2011, Duchateu and coworkers reported on the terpolymerization (pCO 2 = 5.0 MPa, T = 80 • C, t = 18 h) of CHO with CO 2 and various anhydrides (SA, cyclopropane-1,2dicarboxylic acid anhydride (CPrA), cyclopentane-1,2-dicarboxylic acid anhydride (CPA) or PA) promoted by two chromium complexes ( Figure 5): tetraphenylporphyrinato chromium chloride 9 and salophen chromium chloride 10 (where salophen = N,N'-bis(3,5-di-tertbutylsalicylidenyl)-1,2-phenylenediamine) activated by DMAP (4-(N,N-dimethylamino) pyridine) [28]. Catalysts 2021, 11, x FOR PEER REVIEW 6 of 2 tert-butylsalicylidenyl)-1,2-phenylenediamine) activated by DMAP (4-(N,N-dimethyla mino) pyridine) [28]. In analogy to the polymerization process observed by Coates in the case of th (bdi)Zn complexes, the formation of the polyester is favored over the formation of th polycarbonate, resulting in the formation of a poly(ester-block-carbonate). Notably, the au thors also showed that DSC of poly(ester-block-carbonate) is inconclusive in giving infor mation about the blocky microstructure of the copolymer because the polyester and po ycarbonate phases are completely miscible, giving a single value for the Tg. Furthermore in the case of complex 10, the authors noticed that the presence of CO2 in the polymeriza tion feed completely suppresses the formation of polyether linkages. In particular, by co polymerizing the equimolar amount of CHO and CPrA in the presence of CO2, the pur polyester was obtained, while without CO2 an amount of 15-30% of polyether linkage was observed. For all terpolymerizations, Mn (up to 19.2 kg mol −1 ) showed a linear corre lation with conversion and the Ð was ≤1.6, indicating controlled behavior.
Soon after, Darensbourg, using a related (salan) CrCl complex 11 activated by PPNN in the terpolymerization of CHO/PA/CO2, observed similar results (Mn up to 18 kg mol − Ð = 1.07-1.13) [29]. In this case, the poly(ester-block-carbonate) showed two distinct Tg va ues (48 °C and 115 °C). Intriguingly, the major reactivity of the anhydride vis à vis CO was explained in terms of a slower ring-opening step of the metal-carbonate intermediat with the epoxide monomer instead of a faster insertion of the anhydride in the meta alkoxo bond (Scheme 4).  In analogy to the polymerization process observed by Coates in the case of the (bdi)Zn complexes, the formation of the polyester is favored over the formation of the polycarbonate, resulting in the formation of a poly(ester-block-carbonate). Notably, the authors also showed that DSC of poly(ester-block-carbonate) is inconclusive in giving information about the blocky microstructure of the copolymer because the polyester and polycarbonate phases are completely miscible, giving a single value for the T g . Furthermore, in the case of complex 10, the authors noticed that the presence of CO 2 in the polymerization feed completely suppresses the formation of polyether linkages. In particular, by copolymerizing the equimolar amount of CHO and CPrA in the presence of CO 2 , the pure polyester was obtained, while without CO 2 an amount of 15-30% of polyether linkages was observed. For all terpolymerizations, M n (up to 19.2 kg mol −1 ) showed a linear correlation with conversion and the Ð was ≤1.6, indicating controlled behavior.
Soon after, Darensbourg, using a related (salan) CrCl complex 11 activated by PPNN 3 in the terpolymerization of CHO/PA/CO 2 , observed similar results (M n up to 18 kg mol −1 , Ð = 1.07-1.13) [29]. In this case, the poly(ester-block-carbonate) showed two distinct T g values (48 • C and 115 • C). Intriguingly, the major reactivity of the anhydride vis à vis CO 2 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).
tert-butylsalicylidenyl)-1,2-phenylenediamine) activated by DMAP (4-(N,N-dimethyla mino) pyridine) [28]. In analogy to the polymerization process observed by Coates in the case of th (bdi)Zn complexes, the formation of the polyester is favored over the formation of th polycarbonate, resulting in the formation of a poly(ester-block-carbonate). Notably, the au thors also showed that DSC of poly(ester-block-carbonate) is inconclusive in giving infor mation about the blocky microstructure of the copolymer because the polyester and pol ycarbonate phases are completely miscible, giving a single value for the Tg. Furthermore in the case of complex 10, the authors noticed that the presence of CO2 in the polymeriza tion feed completely suppresses the formation of polyether linkages. In particular, by co polymerizing the equimolar amount of CHO and CPrA in the presence of CO2, the pur polyester was obtained, while without CO2 an amount of 15-30% of polyether linkage was observed. For all terpolymerizations, Mn (up to 19.2 kg mol −1 ) showed a linear corre lation with conversion and the Ð was ≤1.6, indicating controlled behavior.
Soon after, Darensbourg, using a related (salan) CrCl complex 11 activated by PPNN in the terpolymerization of CHO/PA/CO2, observed similar results (Mn up to 18 kg mol − Ð = 1.07-1.13) [29]. In this case, the poly(ester-block-carbonate) showed two distinct Tg val ues (48 °C and 115 °C). Intriguingly, the major reactivity of the anhydride vis à vis CO was explained in terms of a slower ring-opening step of the metal-carbonate intermediat with the epoxide monomer instead of a faster insertion of the anhydride in the metal alkoxo bond (Scheme 4).  Chromium(III) complex 12 (TPPCrCl, Figure 6) with a porphyrin ligand in combination with PPNCl was successfully used by Chisolm and coworkers for the terpolymerization of CO 2 /PO/SA (pCO 2 = 4.0-5.0 MPa, T = 25 • C, t = 3-18 h) [30]. It is worth noting that the PPNCl/Cr ratio is crucial to avoid the formation of polyether linkages; indeed, when 0.5 equiv. of PPNCl have been used, the formation of polyether linkages is favored (up to 42%) over the polyester and polycarbonate linkages, whereas with 1.0 equiv. of PPNCl the amount of polyether linkages is drastically reduced (<2%). In analogy to other chromium systems, for this system the polyester formation is also faster than the polycarbonate one, leading to copolymers with a tapered/diblock microstructure. The authors attributed the higher reactivity of SA over CO 2 to the higher solubility of the anhydride in the reaction medium. Chromium(III) complex 12 (TPPCrCl, Figure 6) with a porphyrin ligand in combination with PPNCl was successfully used by Chisolm and coworkers for the terpolymerization of CO2/PO/SA (pCO2 = 4.0-5.0 MPa, T = 25 °C, t = 3-18 h) [30]. It is worth noting that the PPNCl/Cr ratio is crucial to avoid the formation of polyether linkages; indeed, when 0.5 equiv. of PPNCl have been used, the formation of polyether linkages is favored (up to 42%) over the polyester and polycarbonate linkages, whereas with 1.0 equiv. of PPNCl the amount of polyether linkages is drastically reduced (<2%). In analogy to other chromium systems, for this system the polyester formation is also faster than the polycarbonate one, leading to copolymers with a tapered/diblock microstructure. The authors attributed the higher reactivity of SA over CO2 to the higher solubility of the anhydride in the reaction medium.  This complex displayed one of the highest activities in the CO2/PO copolymerization, reaching TOF up to 16,000 h −1 . In the presence of CO2/PO/PA, this complex also shows high reactivity with a total conversion of PO only after 3.0 h (pCO2 = 3.5 MPa, T = 80 °C) and a calculated TOF = 12,000 h −1 . The resulting copolymers have a gradient poly(1,2-propylene carbonate-co-phthalate)s microstructure since, due to the highest reactivity of PA compared to CO2, the polymeric chains formed in the initial stages are richer in PA, but the consumption of this comonomer favors the formation of polycarbonate chains in the last stages. The resulting copolymers have a very narrow dispersity (Đ = 1.03-1.22) and high molecular weight (Mn up to 354 kg mol −1 ). As previously observed, the incorporation of PA in the polymeric chain enhances the thermal properties of the final polymer with respect to the corresponding PPC.
A dinuclear Cr(III) salen complex 14 ( Figure 8) was reported by Lu and coworkers 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 A major breakthrough in this field was the use of the single component Co(III) complex 13 ( Figure 7) tethering four quaternary ammonium salts [31].
Catalysts 2021, 11, x FOR PEER REVIEW 7 of 2 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 tha the PPNCl/Cr ratio is crucial to avoid the formation of polyether linkages; indeed, whe 0.5 equiv. of PPNCl have been used, the formation of polyether linkages is favored (up t 42%) over the polyester and polycarbonate linkages, whereas with 1.0 equiv. of PPNCl th amount of polyether linkages is drastically reduced (<2%). In analogy to other chromium systems, for this system the polyester formation is also faster than the polycarbonate on leading to copolymers with a tapered/diblock microstructure. The authors attributed th higher reactivity of SA over CO2 to the higher solubility of the anhydride in the reactio medium.  This complex displayed one of the highest activities in the CO2/PO copolymerization reaching TOF up to 16,000 h −1 . In the presence of CO2/PO/PA, this complex also show high reactivity with a total conversion of PO only after 3.0 h (pCO2 = 3.5 MPa, T = 80 °C and a calculated TOF = 12,000 h −1 . The resulting copolymers have a gradient poly(1,2-pro pylene carbonate-co-phthalate)s microstructure since, due to the highest reactivity of PA compared to CO2, the polymeric chains formed in the initial stages are richer in PA, bu the consumption of this comonomer favors the formation of polycarbonate chains in th last stages. The resulting copolymers have a very narrow dispersity (Đ = 1.03-1.22) an high molecular weight (Mn up to 354 kg mol −1 ). As previously observed, the incorporatio of PA in the polymeric chain enhances the thermal properties of the final polymer wit respect to the corresponding PPC.
A dinuclear Cr(III) salen complex 14 (Figure 8) was reported by Lu and coworkers t promote, in the presence of 2 equiv. PPNCl, the terpolymerization of CO2/CHO/PA (pCO = 1 MPa, T = 80 °C, t = 0.5-6 h) [32]. In the first 2 h, the system only produced the polyeste This complex displayed one of the highest activities in the CO 2 /PO copolymerization, reaching TOF up to 16,000 h −1 . In the presence of CO 2 /PO/PA, this complex also shows high reactivity with a total conversion of PO only after 3.0 h (pCO 2 = 3.5 MPa, T = 80 • C) and a calculated TOF = 12,000 h −1 . The resulting copolymers have a gradient poly(1,2propylene carbonate-co-phthalate)s microstructure since, due to the highest reactivity of PA compared to CO 2 , the polymeric chains formed in the initial stages are richer in PA, but the consumption of this comonomer favors the formation of polycarbonate chains in the last stages. The resulting copolymers have a very narrow dispersity (Ð = 1.03-1.22) and high molecular weight (M n up to 354 kg mol −1 ). As previously observed, the incorporation of PA in the polymeric chain enhances the thermal properties of the final polymer with respect to the corresponding PPC.
A dinuclear Cr(III) salen complex 14 (Figure 8) was reported by Lu and coworkers to promote, in the presence of 2 equiv. PPNCl, the terpolymerization of CO 2 /CHO/PA (pCO 2 = 1 MPa, T = 80 • C, t = 0.5-6 h) [32]. In the first 2 h, the system only produced the polyester segment with no incorporation of CO 2 , and only after the total consumption of PA the polycarbonate block was formed, also giving, in this case, a diblock polymer. The produced copolymers have a very narrow dispersity (Ð = 1.19-1.22) and the molecular weight increases with the polymerization time (M n up to 21.2 kg mol −1 ).
Catalysts 2021, 11, x FOR PEER REVIEW segment with no incorporation of CO2, and only after the total consumption o polycarbonate block was formed, also giving, in this case, a diblock polymer. duced copolymers have a very narrow dispersity (Ð = 1.19-1.22) and the molecula increases with the polymerization time (Mn up to 21.2 kg mol −1 ).

Metal-Free Catalysts
Since the discovery by Feng and coworkers that triethyl borane (TEB) in com with onium halides or alkoxides promotes the formation of polycarbonates by CO2 with PO or CHO, the efforts to extend the use of this metal-free system to polymerization of CO2 with epoxides and anhydrides resulted in the synthesis o ymers having various microstructural features [33].
In 2020, Meng reported the quadripolymerization of CO2 with PA, PO and the presence of TEB and PPNCl, resulting in the formation of the copolymer ( MPa, T = 70 °C, t = 24-96 h) with good selectivity (94%) with respect to the cyclic [34,35]. The microstructure of the resulting quadripolymer was clarified by 1 H NMR showing the presence of four main blocks, i.e., poly(PA-alt-CHO), poly(PA poly(propylene carbonate) (PPC), and poly(cyclohexene carbonate) (PCHC), an low amount of polyether linkages (<1%). In addition, in this case the formation o ycarbonate segments only starts after the complete PA conversion and thus afte mation of the polyester segments. The resulting polymers display narrow disper 1.14-1.21) and a high molecular weight (Mn up to 77 kg mol −1 ). Notably, the Tg can tuned by regulating the feed ratio with a wide temperature range (Tg = 82-116 °C Afterward, Li and coworkers reported on the terpolymerization of CO2 with CHO, in the presence of TEB and PPNCl (pCO2 = 0.1 MPa, T = 80 °C, t = 0.25-Additionally, in this case the polycarbonate block starts forming only after the c consumption of PA in the feed, resulting in a poly(ester-b-carbonate) copolymer w tapering, as shown by NMR spectra. The same catalytic system also allows the s of poly(ester-b-carbonate) without tapering by sequential monomer addition. Th ing copolymers possess narrow dispersity (Ð = 1.09-1.15) and Mn up to 23.5 kg m Lately, Feng obtained similar results by using TEB in combination with (pCO2 = 0.1 MPa, T = 60 °C, t = 0.75-18 h) for the terpolymerization of CO2 with SA/PA. The PO/SA/CO2 terpolymerization clearly shows higher reactivity tow oxoanion of SA over CO2, leading to the preferential formation of the polyester in tapered poly(ester-b-carbonate) [37]. Only at a low concentration of SA (SA:PO a random poly(ester-co-carbonate) copolymer was obtained with 51% of polye 49% of carbonate. Notably, the PO/SA/CO2 terpolymerization leads to random mers also at PA:PO = 20:200, and only with the presence of 40% of PA in the

Metal-Free Catalysts
Since the discovery by Feng and coworkers that triethyl borane (TEB) in combination with onium halides or alkoxides promotes the formation of polycarbonates by coupling CO 2 with PO or CHO, the efforts to extend the use of this metal-free system to the terpolymerization of CO 2 with epoxides and anhydrides resulted in the synthesis of terpolymers having various microstructural features [33].
In 2020, Meng reported the quadripolymerization of CO 2 with PA, PO and CHO in the presence of TEB and PPNCl, resulting in the formation of the copolymer (pCO 2 = 1 MPa, T = 70 • C, t = 24-96 h) with good selectivity (94%) with respect to the cyclic product [34,35]. The microstructure of the resulting quadripolymer was clarified by 1 H and 13 C NMR showing the presence of four main blocks, i.e., poly(PA-alt-CHO), poly(PA-alt-PO), poly(propylene carbonate) (PPC), and poly(cyclohexene carbonate) (PCHC), and a very low amount of polyether linkages (<1%). In addition, in this case the formation of the polycarbonate segments only starts after the complete PA conversion and thus after the formation of the polyester segments. The resulting polymers display narrow dispersity (Ð = 1.14-1.21) and a high molecular weight (M n up to 77 kg mol −1 ). Notably, the T g can be easily tuned by regulating the feed ratio with a wide temperature range (T g = 82-116 • C).
Afterward, Li and coworkers reported on the terpolymerization of CO 2 with PA and CHO, in the presence of TEB and PPNCl (pCO 2 = 0.1 MPa, T = 80 • C, t = 0.25-2 h) [36]. Additionally, in this case the polycarbonate block starts forming only after the complete consumption of PA in the feed, resulting in a poly(ester-b-carbonate) copolymer with little tapering, as shown by NMR spectra. The same catalytic system also allows the synthesis of poly(ester-b-carbonate) without tapering by sequential monomer addition. The resulting copolymers possess narrow dispersity (Ð = 1.09-1.15) and M n up to 23.5 kg mol −1 .
Lately, Feng obtained similar results by using TEB in combination with Bu 4 NN 3 (pCO 2 = 0.1 MPa, T = 60 • C, t = 0.75-18 h) for the terpolymerization of CO 2 with PO and SA/PA. The PO/SA/CO 2 terpolymerization clearly shows higher reactivity toward the oxoanion of SA over CO 2 , leading to the preferential formation of the 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/CO 2 terpolymerization leads to random copolymers 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/CO 2 activating TEB with PPNCl (pCO 2 = 0.1 MPa, T = 80 • C, t = 17-18 h) shows the analogous behavior of PO preferentially producing copolymers with a random microstructure and blocky 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 CO 2 . The T g 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/CO 2 copolymers and from 121 • C to 135.1 • C in the case of the CHO/PA/CO 2 copolymers.
In Table 1, the results obtained in the CO 2 /epoxide/cyclic anhydrides' terpolymerization discussed in this first part are summarized.

Terpolymerization of CO 2 with Epoxides and Cyclic Esters
The synthesis of polyester-co-polycarbonate was also attempted by the terpolymerization of CO 2 with epoxides and cyclic esters combining the ROCOP and ROP mechanisms [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][39][40].

Zinc Complexes
ZnGA 4, obtained by the reaction of zinc oxide and glutaric acid, was active in the terpolymerization of CO 2 /PO/CL (pCO 2 = 2.8 MPa, T = 60 • C, t = 40 h), resulting in high molecular weight polymers (M n up to 27.5 kg mol −1 ) 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 13 C 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 T g of the PPC block (T g = 5.4-17.7 • C) and the T m of the PCL block (T m = 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 CO 2 /PO (pCO 2 = 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 (Figure 9) [43]. In order to obtain a terpolymer with an appreciable amount of polycarbonate linkages, an excess of CHO in the feed was necessary (CHO:LA = 3:1, pCO2 = 4.0 MPa, T = 90 °C, t = 16 h), giving high molecular weight polymers (Mn = 11.3-41.6 kg mol −1 ) with narrow dispersity (Ð = 1.09-1.96). The copolymers obtained by using L-LA instead of rac-LA show crystallinity with a melting point around 167 °C. The authors also reported the terpolymerization by using the (bdi) Zn catalysts developed by Coates (see Figure 2), showing, in this case, even a major tendency to incorporate a higher amount of polycarbonate linkages (up to 80%).
A ternary system composed of Y(CCl3COO)3/ZnEt2/glycerin 23 was used by Xianhong and coworkers to synthesize (pCO2 = 4.0 MPa, T = 70 °C, t = 10 h) terpolymers CO2/PO/L-LA with a high molecular weight (Mn = 7.2-15.4 kg mol −1 ) and broad dispersity (Ð = 4.2-9.9), with the molecular weight increasing by decreasing the L-LA content in the feed [45]. It is worth noting that the presence of L-LA in the polymeric backbone even at a low content (2.4% mol) results in a considerable increase in the mechanical and thermal properties.
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 of CO2/CHO and the ROP of CL (pCO2 = 0.1 MPa, T = 80 °C, t = 2-21 h) only when activated by CHO, and intriguingly, a polymerization feed composed by a mixture of 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 introducing 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 of the resulting polymers was rather low (Mn up to 4.8 kg mol −1 ), with narrow dispersity (Ð = 1. 38-1.49). This ability to selectively polymerize only one kind of monomer from a mixture and the ability to oscillate between the ROCOP and ROP mechanisms led to the definition of "switch catalysis" [47]. In order to obtain a terpolymer with an appreciable amount of polycarbonate linkages, an excess of CHO in the feed was necessary (CHO:LA = 3:1, pCO 2 = 4.0 MPa, T = 90 • C, t = 16 h), giving high molecular weight polymers (M n = 11.3-41.6 kg mol −1 ) with narrow dispersity (Ð = 1.09-1.96). The copolymers obtained by using L-LA instead of rac-LA show crystallinity with a melting point around 167 • C. The authors also reported the terpolymerization by using the (bdi) Zn catalysts developed by Coates (see Figure 2), showing, in this case, even a major tendency to incorporate a higher amount of polycarbonate linkages (up to 80%).
A ternary system composed of Y(CCl 3 COO) 3 /ZnEt 2 /glycerin 23 was used by Xianhong and coworkers to synthesize (pCO 2 = 4.0 MPa, T = 70 • C, t = 10 h) terpolymers CO 2 /PO/L-LA with a high molecular weight (M n = 7.2-15.4 kg mol −1 ) and broad dispersity (Ð = 4.2-9.9), with the molecular weight increasing by decreasing the L-LA content in the feed [45]. It is worth noting that the presence of L-LA in the polymeric backbone even at a low content (2.4% mol) results in a considerable increase in the mechanical and thermal properties.
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 of CO 2 /CHO and the ROP of CL (pCO 2 = 0.1 MPa, T = 80 • C, t = 2-21 h) only when activated by CHO, and intriguingly, a polymerization feed composed by a mixture of CO 2 /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 introducing CO 2 after the consumption of CL in the presence of CHO or by reverse order completely removing CO 2 after the formation of the PCHC block. The molecular weight of the resulting polymers was rather low (M n up to 4.8 kg mol −1 ), with narrow dispersity (Ð = 1. 38-1.49). This ability to selectively polymerize only one kind of monomer from a mixture and the ability to oscillate between the ROCOP and ROP mechanisms led to the definition of "switch catalysis" [47].
By performing the ROCOP of CO 2 /CHO, it was also possible to obtain a polycarbonate polyol (pCO 2 = 0.1 MPa, T = 80 • C, t = 16-25 h) that, after removing CO 2 , can be used for the synthesis of ABA triblock copoly(caprolactone-b-cyclohexene carbonate-b-caprolactone) 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 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 (CO 2 /epoxide) by using various epoxides (CHO and VCHO), anhydrides (PA, NA), and DL (ε-decalactone). The resulting pentablock copolymers show a single T g (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 promotes, in the presence of 1,2-cyclohexane diol (CHD), the formation of a hydroxyltelechelic 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 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.
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 CO 2 [50]. In order to circumvent this problem, a dual catalytic system was used: (1) the dizinc complex 25 promotes, in the presence of 1,2-cyclohexane diol (CHD), the formation of a hydroxyltelechelic 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 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.
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 promotes, in the presence of 1,2-cyclohexane diol (CHD), the formation of a hydroxyltelechelic 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 M n spanning from 50.700 to 114.6 kg mol −1 and narrow dispersity (Ð = 1.38-1.49). The thermal and mechanical 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 ( Figure 10) with the same ligand framework promoted the formation of ABA triblock copolymers by using DL with high activity [52]. 50.700 to 114.6 kg mol −1 and narrow dispersity (Ð = 1.38-1.49). The thermal and mechanical 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 ( Figure 10) with the same ligand framework promoted the formation of ABA triblock copolymers by using DL with high activity [52]. In particular, by performing the ROP of DL a dihydroxyl telechelic PDL was obtained that, in the presence of CO2, undergoes the transformation into the ABA triblock copolymer PCHC-b-PDL-b-PCHC. The raw copolymers can incorporate a high amount of CO2 (up to 23%) and possess a high molecular weight (38.0-71.9 kg mol −1 ) with narrow dispersity (Ð = 1.07-1.16). These materials display a single Tg (from −44 to −50 °C), evidencing the amorphous nature of the blocks and their complete miscibility, and only the polymers with a higher content of PCHC (>50%) show a second transition at higher temperatures (81; 110 °C). These materials show promising thermal and mechanical properties compared to PCHC and the possibility to modulate them by regulating the length of the blocks in the final polymer, potentially giving a wide range of applications.
Rieger and coworkers were able, by using a (bdi)-zinc complex 28 (Scheme 7), to obtain copolymers by the terpolymerization of 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 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 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 the formation of a PCHC block, finally releasing the CO2 the "residual" BBL polymerizes, giving, at the end, an ABA triblock copolymer PCHC-b-PHB-b-PCHC. (C) By lowering the CO2 pressure to pCO2 = 0.3 MPa, the rates of the ROCOP and ROP processes are comparable and therefore a statistical copolymer was formed. In particular, by performing the ROP of DL a dihydroxyl telechelic PDL was obtained that, in the presence of CO 2 , undergoes the transformation into the ABA triblock copolymer PCHC-b-PDL-b-PCHC. The raw copolymers can incorporate a high amount of CO 2 (up to 23%) and possess a high molecular weight (38.0-71.9 kg mol −1 ) with narrow dispersity (Ð = 1.07-1.16). These materials display a single T g (from −44 to −50 • C), evidencing the amorphous nature of the blocks and their complete miscibility, and only the polymers with a higher content of PCHC (>50%) show a second transition at higher temperatures (81; 110 • C). These materials show promising thermal and mechanical properties compared to PCHC and the possibility to modulate them by regulating the length of the blocks in the final polymer, potentially giving a wide range of applications.
Rieger and coworkers were able, by using a (bdi)-zinc complex 28 (Scheme 7), to obtain copolymers by the terpolymerization of CO 2 /BBL/CHO [53]. In particular, also in this case the CO 2 acts as a switching agent: (A) at pCO 2 = 4.0 MPa, the polymerization proceeds with the exclusive production of PCHC and the formation of the poly(hydroxybutyrate) (PHB) only starts after releasing CO 2 pressure, leading finally to a diblock copolymer PCHC-b-PHB. (B) In the absence of CO 2 , obviously, the system evolves to the formation of PHB and before to the total consumption of BBL feeding CO 2 (pCO 2 = 4.0 MPa) with the formation of a PCHC block, finally releasing the CO 2 the "residual" BBL polymerizes, giving, at the end, an ABA triblock copolymer PCHC-b-PHB-b-PCHC. (C) By lowering the CO 2 pressure to pCO 2 = 0.3 MPa, the rates of the ROCOP and ROP processes are comparable and therefore a statistical copolymer was formed.
The copolymers' molecular weights, in the case of the block copolymers, are higher (M n = 77.0-166 kg mol −1 ) than those obtained in the case of statistical copolymers (M n = 34.0-69.0 kg mol −1 ), in both cases showing narrow dispersity (Ð = 1.2-1.8). PCHCb-PHB and PCHC-b-PHB-b-PCHC display two T g values relative to the PHB and PCHC blocks, respectively (T g1 = 1-2 • C and T g2 = 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 (T g = 36-91 • C) that increases by increasing the amount of carbonate linkages in the polymer chain. Similar results were obtained with cyclopenteneoxide (CPO), but in this case the polymerization at a higher pressure (pCO 2 = 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 CO 2 with a zero order dependence at high pressure (between pCO 2 = 0.5-1 MPa) and first-order at lower pressure (pCO 2 < 0.5 MPa), indicating that under the latter conditions the insertion of CO 2 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 CO 2 /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 CO 2 for the formation of the PLC block. Notably, the PHB-b-PLC copolymers possess a high molecular weight (M n up to 233 kg mol −1 ) and narrow dispersity (Ð = 1.23-1.39), showing two T g = 1-3/26-133 • C. Statistical copolymers were also obtained by adjusting the CO 2 pressure (pCO 2 = 0.9 MPa), resulting in low conversion (up to 22% LO and 26% BBL in 22 h) and low molecular weight polymers (M n = 9.0 kg mol −1 ). , 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 polymer 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 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 233 kg mol −1 ) 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 pressure (pCO2 = 0.9 MPa), resulting in low conversion (up to 22% LO and 26% BBL in 22 h) and low molecular weight polymers (Mn = 9.0 kg mol −1 ).

Cobalt Complexes
Salen cobalt complexes are highly active catalysts in the ROCOP of CO2 with epoxides and therefore are, in principle, viable candidates for the terpolymerization of CO2 Scheme 7. Different reaction pathway of 28 toward ROP of BBL, copolymerization of CPO with CO 2 , and terpolymerization of CHO, CO 2 and BBL. Reproduced with modification and permission from ref [53]. Copyright (2017) American Chemical Society.

Cobalt Complexes
Salen cobalt complexes are highly active catalysts in the ROCOP of CO 2 with epoxides and therefore are, in principle, viable candidates for the terpolymerization of CO 2 with epoxides and lactones. Unfortunately, these complexes are inactive in the ROP of cyclic esters, and consequently, the implementation of an active catalytic system for obtaining polycarbonate-b-polyester copolymers requires the use of multi-component systems able to synthesize the desired polymeric product.
An elegant strategy was developed by Darensbourg and Lu that used a combination of the bifunctional Co(III) salen complex 29 and DBU (1,8-diazabicyclo[5.4.0]undec-7ene) (Figure 11) for the terpolymerization (pCO 2 = 1.5 MPa, T = 25 • C, t = 2-6 h) of CO 2 /SO/LA [55].  Indeed, the cobalt complex 29 produced with high activity poly(styrene carbonate (PSC) from CO2 and 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-term nated PSC. Consequently, the polycarbonate formed, having the hydroxy chain-end, ca act as a macroinitiator for the ROP of LA catalyzed by DBU. As a matter of fact, afte Indeed, the cobalt complex 29 produced with high activity poly(styrene carbonate) (PSC) from CO 2 and SO, after the complete consumption of SO, a given amount of H 2 O, 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 H 2 O with respect to the cobalt catalyst and the removal of CO 2 , the addition of LA and DBU results in the production of the AB copolymer PSC-b-PLA with molecular weight M n up to 17.2 kg mol −1 and narrow dispersity (Ð = 1.04-1.12). The copolymers obtained from rac-LA display a single T g at lower values with respect to PSC (60-72 • C), and by polymerizing D-LA, the resulting diblock copolymers also display a T m = 133-137 • C depending on the length of the PLA block.
The same authors also used this strategy to synthesize ABA block copolymers from CO 2 /PO/LA. They used the system composed of the salen Co(III) complex 29 activated by PPNY (Y = CF 3 COO -) and DBU [56]. In this case, the obtaining of PLA-b-PPC-b-PLA was possible because of the addition of an excess of H 2 O (5-20 equiv. with respect to CO) to stop the CO 2 /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 rather low, M n 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 T g 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 T m = 110-128 • C depending on the length of the PLA blocks.
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].
Indeed, the Co(II) complexes (30a-32a) are active in the ROP of LA, and the Co(I complexes (30b-32b), in combination with PPNCl, are active in the ROCOP of CO2 w various epoxides (PO, CHO, SO). The terpolymerization was possible for the chain tran fer between the two metal centers (Scheme 8). Structures of salenCo II complexes (30a-32a) and salenCo III complexes (30b-32b) [57].
Indeed, the Co(II) complexes (30a-32a) are active in the ROP of LA, and the Co(III) complexes (30b-32b), in combination with PPNCl, are active in the ROCOP of CO 2 with various epoxides (PO, CHO, SO). The terpolymerization was possible for the chain transfer between the two metal centers (Scheme 8).
Indeed, the Co(II) complexes (30a-32a) are active in the ROP of LA, and the Co(III) 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 between the two metal centers (Scheme 8). By using 30a and 30b and PPNCl in an equimolar amount, the terpolymerization of CO 2 /LA/PO gives terpolymers, as revealed by 1 H and 13 C NMR analysis, possessing a multiblock microstructure with M n up to 13.6 kg mol −1 and narrow dispersity (Ð = 1. 19-1.47). Notably, the dispersity broadens in the absence of PPNCl (Ð = 3.15) and with two equivalents of PPNCl (Ð = 2.28), confirming the crucial role of the onium salt in the chain-transfer between the metal centers.
More recently, the same authors further developed this ternary system by changing the Co(II) and Co(III) complexes (30a and 30b), obtaining a more active system or combining a salen Co(III) complex with ZnGA and PPNCl [58].
More recently, the same authors further developed this ternary system by changing the Co(II) and Co(III) complexes (30a and 30b), obtaining a more active system or combining a salen Co(III) complex with ZnGA and PPNCl [58].
Finally, in the presence of an enantiopure chiral salenCo(III) complex 33 ( Figure 13) in combination with PPN-DNP (PPN = bis(triphenylphosphine)iminium, DNP = 2,4-dinitrophenoxide), Lu and coworkers also succeeded in producing CO2/CHO/BBL terpolymers with isotactic -PCHC blocks [59]. More in detail, when an equimolar amount of CHO and BBL is present in the feed, the terpolymerization proceeds smoothly with the good conversion of both monomers (pCO2 = 2 MPa, T = 40 °C, t = 2-4 h). The resulting copolymers display, in the 1 H NMR spectra, the signals relative to the carbonate-ester linkages, indicating a multiblock structure. 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 More in detail, when an equimolar amount of CHO and BBL is present in the feed, the terpolymerization proceeds smoothly with the good conversion of both monomers (pCO 2 = 2 MPa, T = 40 • C, t = 2-4 h). The resulting copolymers display, in the 1 H NMR spectra, the signals relative to the carbonate-ester linkages, indicating a multiblock structure. Molecular weights are rather low, M n = 3.3-14.6 kg mol −1 , with narrow dispersity (Ð = 1.19-1.44), and display thermal behavior with a T m = 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 CO 2 with epoxides and cyclic esters discussed in this second part are summarized.

Conclusions
The possibility to terpolymerize CO 2 with epoxides and other cyclic monomers (cyclic esters, organic anhydrides) offers not only a simple way to obtain a wide range of materials with unprecedented properties, but also the possibility to have such material in a completely sustainable way, combining CO 2 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 CO 2 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 CO 2 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.