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Efficient Production of Poly(Cyclohexene Carbonate)

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Efficient Production of Poly(Cyclohexene Carbonate) polymers Article Efficient Production of Poly(Cyclohexene Carbonate) via ROCOP of Cyclohexene Oxide and CO2 Mediated by NNO-Scorpionate Zinc Complexes Sonia Sobrino 1, Marta Navarro 2, Juan Fernández-Baeza 1, Luis F. Sánchez-Barba 2,* , Agustín Lara-Sánchez 1 , Andrés Garcés 2 , José A. Castro-Osma 1 and Ana M. Rodríguez 1 1 Centro de Innovación en Química Avanzada (ORFEO-CINQA), Departamento de Química Inorgánica, Orgánica y Bioquímica, Universidad de Castilla-La Mancha, Campus Universitario, 13071 Ciudad Real, Spain; [email protected] (S.S.); [email protected] (J.F.-B.); [email protected] (A.L.-S.); [email protected] (J.A.C.-O.); [email protected] (A.M.R.) 2 Departamento de Biología y Geología, Física y Química Inorgánica, Universidad Rey Juan Carlos, Móstoles, 28933 Madrid, Spain; [email protected] (M.N.); [email protected] (A.G.) * Correspondence: [email protected]; Tel.: +34-91-488-8504 Received: 4 September 2020; Accepted: 18 September 2020; Published: 21 September 2020 Abstract: New mono- and dinuclear chiral alkoxide/thioalkoxide NNO-scorpinate zinc complexes were easily synthesized in very high yields, and characterized by spectroscopic methods. X-ray diffraction analysis unambiguously confirmed the different nuclearity of the new complexes as well as the variety of coordination modes of the scorpionate ligands. Scorpionate zinc complexes 2, 4 and 6 were assessed as catalysts for polycarbonate production from epoxide and carbon dioxide with no need for a co-catalyst or activator under mild conditions. Interestingly, at 70 ◦C, 10 bar of CO2 pressure and 1 mol % of loading, the dinuclear thioaryloxide [Zn(bpzaepe)2{Zn(SAr)2}] (4) behaves as an efficient and selective one-component initiator for the synthesis of poly(cyclohexene carbonate) via ring-opening copolymerization of cyclohexene oxide (CHO) and CO2, affording polycarbonate materials with narrow dispersity values. Keywords: scorpionate zinc complexes; ring-opening copolymerization (ROCOP); CO2 fixation; poly(cyclohexene carbonate) production 1. Introduction Over the last decade the conversion of carbon dioxide (CO2) into commercially viable commodities has attracted great interest in the scientific community, since carbon dioxide represents a real alternative carbon feedstock for a sustainable chemical industry [1,2]. CO2 is an attractive C-1 renewable building block [3] given its abundance in nature, low cost, non-toxicity, lack of colour and redox activity. Many chemical transformations are possible for this unsaturated molecule; however, the selective production of cyclic carbonates or polycarbonates through the cycloaddition or the ring-opening copolymerization (ROCOP) of CO2 with epoxides, respectively, (see Scheme1) is gaining high attention as a 100% atom-economy route to convert waste CO 2 into valuable materials [4,5]. Polymers 2020, 12, 2148; doi:10.3390/polym12092148 www.mdpi.com/journal/polymers Polymers 2020, 12, 2148 2 of 19 Polymers 2020, 12, x FOR PEER REVIEW 2 of 19 Scheme 1. Synthesis of cyclic and polycarbonates. Scheme 1. Synthesis of cyclic and polycarbonates. TheThus, structures cyclic carbonates of the resulting present importantpolycarbonates, applications which as can electrolytes, include up engineering to 50% of plastics,carbon dioxide solvent, infuel the additives, polymer and backbone, precursors will to determine fine chemicals their [ 6future], whereas applications. polycarbonates For instance, incorporate non-isocyanate very smart polyurethanesphysical features, (NIPUs) such ascan durability, be prepared moldability, employing lightness, initially transparency low molar andmass impact hydroxyl-telechelic resistance [7], polycarbonatesin addition to their [10,11], biodegradability whereas higher and molar biocompatibility mass CO2-derived that make polycarbonates them highly attractivefind numerous in the applicationsbiomedical field as engineering [8,9]. polymers, packaging plastics, elastomers, adhesives and coatings [12]. Nevertheless,The structures given of the the resulting high thermodynamic polycarbonates, stability which and can kinetic include inertness up to 50% of ofthe carbon CO2 molecule dioxide [13],in the the polymer environmental backbone, and will econom determineical viability their future of the applications. ROCOP depends For instance,on the capability non-isocyanate of the catalyticpolyurethanes system (NIPUs) to avoid can high be temperatures prepared employing and pressures initially [14], low which molar should mass hydroxyl-telechelicbe able, in turn, to operate at ambient temperatures and pressures [15] as well as controlling rates and polymer polycarbonates [10,11], whereas higher molar mass CO2-derived polycarbonates find numerous molecularapplications weight as engineering and composition polymers, [16]. packaging In this plastics, context, elastomers, very active adhesives and selective and coatings metal-based [12]. catalysts have been successfully developed for the ROCOP of carbon dioxide and epoxides, Nevertheless, given the high thermodynamic stability and kinetic inertness of the CO2 frequentlymolecule [ 13in], the the presence environmental of a nucleophile and economical as co-cat viabilityalyst, with of thezinc ROCOP [17–25], depends chromium on [26,27], the capability cobalt [28,29],of the catalytic iron [30,31], system rare to earth avoid metals high temperatures[32,33] and aluminum and pressures [34,35] [ 14as], dominating which should metals be able,in this in field, turn, althoughto operate non-metal at ambient and temperatures organocatalyst and systems pressures have [b15een] as also well recently as controlling reported [36]. rates Similarly, and polymer very efficientmolecular bimetallic weight and system compositions have been [16]. also In this reported context, for very the activeselective and copolymerization selective metal-based of epoxides catalysts andhave carbon been successfully dioxide, in which developed the epoxide for the ROCOPis activated of carbon by one dioxide metal, while and epoxides, the attacking frequently nucleophile in the ispresence provided of aby nucleophile the second as centre co-catalyst, [37,38]. with zinc [17–25], chromium [26,27], cobalt [28,29], iron [30,31], rare earthConsidering metals [the32,33 current] and aluminumpotential large-scale [34,35] as dominating production metalsof aliphatic in this polycarbonates field, although by non-metal several companiesand organocatalyst [39–41], the systems employment have been of alsobiocompatible recently reported metals [such36]. Similarly,as zinc [42,43] very is effi highlycient bimetallicdesirable tosystems avoid havepotential been health also reported issues related for the to selective the toxici copolymerizationty of several metal-based of epoxides residues and carbon in the isolated dioxide, copolymersin which the [44,45]. epoxideIn isparticular, activated very by one active metal, zinc-based while the catalysts attacking in the nucleophile absence of is providedco-catalyst by have the beensecond described centre [ 37[17–25],38]. for polycarbonate production, some of them including alkoxide, amide, alkyl and acetateConsidering ligands the as currentnucleophile potential in a coordination-insertion large-scale production mechanism of aliphatic polycarbonates(see Chart 1). by several companiesOn the [ 39other–41], hand, the employment a key point of in biocompatible controlling both metals reactivity such as and zinc product [42,43] is selectivity highly desirable of the catalystto avoid is potential the nature health of the issues ligand related framework to the toxicity around ofthe several Lewis metal-based acidic metal residuescentre. In in this the context, isolated ourcopolymers research [44group,45]. has In particular, extensively very studied active the zinc-based coordination catalysts chemistry in the absenceof novel ofheteroscorpionate co-catalyst have ligandsbeen described [46], and [17 a– 25wide] for variety polycarbonate of applications production, have some been ofreported them including in homogenous alkoxide, catalysis. amide, alkyl For instance,and acetate we ligands have designed as nucleophile versatile in NNO-scorpionate a coordination-insertion alkyl and mechanism acetate zinc (see complexes Chart1). that behaved as single-componentOn the other hand, initiators a key point for the in controllingring-opening both co- reactivity and polymerization and product selectivityof cyclic esters of the [47–50], catalyst andis the for nature the ring-opening of the ligand frameworkcopolymerization around of the cyclohexene Lewis acidic oxide metal with centre. CO In2 [51], this respectively, context, our research as well asgroup dinuclear has extensively NNO-scorpionate studied thealkyl coordination zinc analogues chemistry that displayed of novel heteroscorpionateexcellent performances ligands in [46the], cycloadditionand a wide variety of epoxide of applications with carbon havedioxide been under reported mild and in homogenoussolvent-free conditions catalysis. [52]. For Now, instance, we takewe have on the designed challenge versatile of designing NNO-scorpionate more efficient alkyl cooperative and acetate homodinuclear zinc complexes NNO-scorpionate that behaved zinc as catalystssingle-component containing initiators thioalkoxide for theauxiliary ring-opening ligands to co- enhance and polymerization catalytic activity of for cyclic CO2 esters fixation [47 –into50], the selective production of polycarbonates under milder conditions. and for the
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