The replacement of current petroleum-based plastics with sustainable alternatives is a crucial but formidable challenge for the modern society. Catalysis presents an enabling tool to facilitate the development of sustainable polymers. This review provides a system-level analysis of sustainable polymers and outlines key criteria with respect to the feedstocks the polymers are derived from, the manner in which the polymers are generated, and the end-of-use options. Specifically, we define sustainable polymers as a class of materials that are derived from renewable feedstocks and exhibit closed-loop life cycles. Among potential candidates, aliphatic polyesters and polycarbonates are promising materials due to their renewable resources and excellent biodegradability. The development of renewable monomers, the versatile synthetic routes to convert these monomers to polyesters and polycarbonate, and the different end-of-use options for these polymers are critically reviewed, with a focus on recent advances in c...

Catalysis as an Enabling Science for Sustainable Polymers

Catalysis is key to the effective and efficient transformation of readily available building blocks into high value functional molecules and materials. For many years research in this field has largely focussed on the invention of new catalysts and the optimization of their performance to achieve high conversions and/or selectivities. However, inspired by Nature, chemists are beginning to turn their attention to the development of catalysts whose activity in different chemical processes can be switched by an external stimulus. Potential applications include using the states of multiple switchable catalysts to control sequences of transformations, producing different products from a pool of building blocks according to the order and type of stimuli applied. Here we outline the state-of-art in artificial switchable catalysis, classifying systems according to the trigger used to achieve control over the catalytic activity and stereochemical or other structural outcomes of the reaction.

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Artificial switchable catalysts

Chemically recyclable polymers: a circular economy approach to sustainability

Polylactide (PLA) is an attractive polymeric material due to its origin from annually renewable resources and its biodegradability. The ring-opening polymerization (ROP) of lactide initiated by Lewis acidic and oxophilic metal-based catalysts constitutes the method of choice to access PLA in a controlled and stereoselective manner. The design and synthesis of ligand-supported metal complexes to act as effective ROP initiators of lactide monomers have been the subject of numerous investigations over the past decades. In view of their oxophilic nature, well-defined group 4 metal complexes supported by polydentate supporting ligands have appeared as active initiators for lactide ROP. This perspective summarizes various classes of structurally well-defined group 4 metal initiators developed for lactide ROP. It also provides observed trends regarding their catalytic performance. Whenever appropriate and possible, catalyst structure-ROP performance (i.e. activity, control and stereoselectivity) relationships are rationalized.

Structurally well-defined group 4 metal complexes as initiators for the ring-opening polymerization of lactide monomers

Aaron J. Teator,†,‡,∥ Dominika N. Lastovickova,†,‡,∥ and Christopher W. Bielawski*,‡,§ †Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712, United States ‡Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS) and Departments of Chemistry and Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Republic of Korea

Switchable Polymerization Catalysts

We developed a salt-free reduction of WCl6 using 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (MBTCD) in toluene to give a low-valent trinulcear tungsten complex involving W(II) and W(III) centers, while in the presence of redox active ligands such as α-diketone and α-diimine the same reduction produced W(IV) complexes with the corresponding redox-active ligands, (α-diketone)WCl4 and (α-diimine)WCl4. A W(VI) complex with two α-diketone ligands, (α-diketone)2WCl2, was found to be synthetically equivalent to low-valent W(IV) species that trapped azopyridine to give (α-diketone)WCl2(azopyridine).

Salt-free reducing reagent of bis(trimethylsilyl)cyclohexadiene mediates multielectron reduction of chloride complexes of W(VI) and W(IV).

Polylactide (PLA) is a biodegradable polymer suitable for various biomedical applications, and it can also be used as a packaging material. 2] A plethora of single-site polymerization catalysts have been developed to produce PLA with a defined molecular weight and tacticity by ring-opening polymerization (ROP) of lactide, a biomass-derived monomer. One tool to control the polymerization rate of lactide monomers is redox-switchable catalysis, as introduced by Gibson et al. in 2006. Here, control over the rate of polymerization of rac-lactide was achieved by redox processes at the ethynylferrocene-substituted salen-type ligand within a titanium complex. Although several examples of ligand-centered redoxswitchable catalysis have been reported, only a few examples of redox-switchable catalysis by reactions at the metal center are known. Recently, Matyjaszewski et al. reported the first application of a metal-centered redox process with a Cu/ Cu system to mediate the atom-transfer radical polymerization of methyl acrylate. Subsequently, Diaconescu et al. showed that cerium complexes with a tetradentate salen-type ligand can exert redox control over the polymerization of l-lactide by changing the oxidation state of the cerium center. We report here that ROP of meso-lactide can be controlled by switching between an anionic Ce complex and a neutral Ce complex with two (OSSO)-type bis(phenolate) ligands. Cerium complexes 3 a and 3 b with two (OSSO)-type bis(phenolate) ligands were readily prepared from the reaction of tetra(isopropoxy)cerium (1) with two equivalents of ligand precursor 2 a or 2 b and isolated as purple powders in 83 and 63 % yield, respectively (Scheme 1). Cerium complexes with only one (OSSO)-type ligand were not accessible even from reactions of 3 a with 1, which gave a mixture of 3 a and unreacted 1. The H NMR spectrum of 3 a features diagnostic broad signals consistent with a diamagnetic Ce complex. For the eight tert-butyl groups in 3 a, four signals are observed, and this is indicative of the high symmetry of complex 3 a. Crystals suitable for X-ray diffraction were grown from n-hexane. The molecular structure of 3 a shows the Ce center coordinated by four phenolate and four thioether moieties of two bis(phenolate) ligands in a distorted square antiprismatic geometry (Figure 1). Complex 3 a shows D2 symmetry in solution and in the solid state. The average Ce O bond length of 2.171 is within the literature range for eight-coordinated cerium(IV) complexes with phenolate ligands (Ce O 2.087–2.237 , Ce S 3.055– 3.058 ). 14] Cyclic voltammetry of 2 a in CH2Cl2 at 25 8C indicated no reduction of 2 a in the electrochemical window of the solvent (Figure 2). An oxidation wave was observed at EOx = 1.248 V (vs. ferrocene). For complex 3 a, a reduction wave at ERed = 1.513 V was observed, which can be attributed to the reduction of Ce to Ce, which is followed by an oxidation wave of the reduced species at EOx = 1.01 V; this is similar to the observation of Diaconescu et al. with a salen-type ligand. A second oxidation wave was observed at EOx = 0.67 V, and it can be attributed to ligand oxidation. Electrochemical oxidation of the phenolate moiety of bis(phenolate)Fe complexes to give a phenoxy radical was previously reported. 16] Prompted by the electrochemical reduction of Ce to Ce, we screened for a suitable chemical reduction agent. An NMR-scale experiment in [D8]THF showed that upon addition of one equivalent of cobaltocene to a purple solution of 3 a an orange paramagnetic compound was formed instantly. Reduction of complex 3 a by addition of one equivalent of cobaltocene proceeded smoothly to give the corresponding Ce complex 4 a in 76 % yield. Oxidation of reduced species 4 a by addition of one equivalent of tungsten hexachloride, iodine, Ag[BPh4], or [FeCp2][B(C6F5)4] (Scheme 2) to form 3 a was observed by H NMR spectroscopy. Single crystals of 4 a suitable for X-ray analysis were grown from a THF/n-pentane solution at 25 8C. The molecular structure of the anion of 4 a is isostructural to the molecular structure of neutral complex 3 a (Figure 1). The cobaltocenium cation is located close to two phenyl rings of the (OSSO)-type ligands. The average Ce O and Ce S distances in 4 a (2.313, 3.178 , respectively) are Scheme 1. Synthesis of 3 a and 3 b by protonolysis of 1.

Switching the Lactide Polymerization Activity of a Cerium Complex by Redox Reactions

A series of group 4 metal complexes Zr-(1)2, Zr-(2)2, Zr-(3)2, Zr-(4)2, Zr-(5)2, Hf-(1)2, and Hf-(4)2 containing two bridged bis(phenolate) ligands of the (OSSO)-type were prepared by the reaction of the corresponding bis(phenol) and group 4 metal precursor MX4 (X = OiPr, CH2Ph) and isolated as robust, colorless crystals. NMR spectra indicate D2 symmetry, in agreement with the solid state structure determined by single crystal X-ray diffraction study of the complexes Zr-(1)2, Hf-(1)2, Zr-(3)2, Zr-(4)2, and Zr-(5)2. The complexes with the 1,4-dithiabutanediyl bridged ligands exhibit a highly symmetric coordination around the metal center. The introduction of the rigid trans-1,2-cyclohexanediyl bridged ligands led to a distorted coordination around the metal center in Zr-(4)2 and Zr-(5)2 when the ortho substituent is tert-butyl and the para substituent is larger than methyl. The complexes Zr-(1)2, Zr-(2)2, Zr-(3)2, Zr-(4)2 as well as Hf-(1)2 and Hf-(4)2 initiated the ring-opening polymerization of meso-lact...

Andreas Sauer

Papers

Structurally well-defined group 4 metal complexes as initiators for the ring-opening polymerization of lactide monomers

Salt-free reducing reagent of bis(trimethylsilyl)cyclohexadiene mediates multielectron reduction of chloride complexes of W(VI) and W(IV).

Switching the Lactide Polymerization Activity of a Cerium Complex by Redox Reactions

Synthesis, characterization, and lactide polymerization activity of group 4 metal complexes containing two bis(phenolate) ligands.

Bis(phenolato)molybdenum complexes as catalyst precursors for the deoxydehydration of biomass-derived polyols