Terpolymerization of CO2 with Epoxides and Cyclic Organic Anhydrides Or Cyclic Esters
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catalysts Review Terpolymerization of CO2 with Epoxides and Cyclic Organic Anhydrides or Cyclic Esters 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 polycarbonates (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 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. Keywords: CO2; epoxides; cyclic esters; cyclic anhydrides; ROCOP; ROP Citation: Lamparelli, D.H.; 1. Introduction Capacchione, C. Terpolymerization of The pervasiveness of polymers 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. Catalysts 2021, 11, 961. https://doi.org/10.3390/catal11080961 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, x FOR PEER REVIEW 2 of 20 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-copolymers. 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 ester and carbonate 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 carbonates 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