Ring-Opening Polymerization of l-Lactide Efficiently Triggered by an Amido-Indole. X-ray Structure of a Complex between l-Lactide and the Hydrogen-Bonding Organocatalyst

Spotting Trends in Organocatalyzed and Other Organomediated (De)polymerizations and Polymer Functionalizations

Organocatalysis has evolved into an effective complement to metal- or enzyme-based catalysis in polymerization, polymer functionalization, and depolymerization. The ease of removal and greater sustainability of organocatalysts relative to transition-metal-based ones has spurred development in specialty applications, e.g., medical devices, drug delivery, optoelectronics. Despite this, the use of organocatalysis and other organomediated reactions in polymer chemistry is still rapidly developing, and we envisage their rapidly growing application in nascent areas such as controlled radical polymerization, additive manufacturing, and chemical recycling in the coming years. In this Review, we describe ten trending areas where we anticipate paradigm shifts resulting from novel organocatalysts and other transition-metal-free conditions. We highlight opportunities and challenges and detail how new discoveries could lead to previously inaccessible functional materials and a potentially circular plastics economy.

Polythioesters Prepared by Ring-Opening Polymerization of Cyclic Thioesters and Related Monomers

Polyhydroxyalkanoates (PHAs) are biodegradable and biocompatible polyesters with a wide range of applications; in particular, they currently stand as promising alternatives to conventional polyolefin-based "plastics". The introduction of sulfur atoms within the PHAs backbone can endow the resulting polythioesters (PTEs) with differentiated, sometimes enhanced thermal, optical and mechanical properties, thereby widening their versatility and use. Hence, PTEs have been gaining increasing attention over the past half-decade. This review highlights recent advances towards the synthesis of well-defined PTEs by ring-opening polymerization (ROP) of cyclic thioesters - namely thiolactones - as well as of S-carboxyanhydrides and thionolactones; it also covers the ring-opening copolymerization (ROCOP) of cyclic thioanhydrides or thiolactones with epoxides or episulfides. Most of the ROP reactions described are of anionic type, mediated by inorganic, organic or organometallic initiators/catalysts, along with a few enzymatic reactions as well. Emphasis is placed on the reactivity of the thio monomers, in relation to their ring-size ranging from 4- to 5-, 6- and 7-membered cycles, the nature of the catalyst/initiating systems implemented and their efficiency in terms of activity and control over the PTE molar mass, dispersity, topology, and microstructure.

Isoselective Polymerization of rac-Lactide by Aluminum Complexes of N-Heterocyclic Carbene-Phosphinidene Adducts

The N-heterocyclic carbene-phosphinidene adducts (NHC)PH were reacted with AlMe3 in toluene to afford the monoaluminum complexes [{(IDipp)PH}AlMe3 ] and [{(IMes)PH}AlMe3 ] (IDipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene, IMes=1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene). In contrast, the dialuminum complex [{(Me IMes)PH}(AlMe3 )2 ] was obtained for Me IMes=1,3-bis(2,4,6-trimethylphenyl)-4,5-dimethylimidazolin-2-ylidene. These complexes served as initiators for the efficient ring-opening polymerization of rac-lactide in toluene at 60 °C. High degrees of isoselectivity were found for the poly(rac-lactide) obtained in the presence of the monoaluminum complexes (Pm up to 0.92, Tm up to 191 °C), whereas almost atactic polymers were produced by the dialuminum complex. Detailed mechanistic studies reveal that the polymerization proceeds via a coordination-insertion mechanism with the carbene-phosphinidene ligands acting as stereodirecting groups.

Organo-catalyzed/initiated ring opening co-polymerization of cyclic anhydrides and epoxides: an emerging story

This review deals with recent organo-catalyzed/initiated developments of co-polymerization of cyclic anhydrides and epoxides to access polyesters.

Comparative bindings of lactones, lactide, and cyclic carbonates: experimental insights into the coordination step of polymerization

Comparative bindings of several renowned monomers were investigated experimentally using B(C₆F₅)₃ as a Lewis acid model for the coordination step in ring-opening polymerization. A complete series of the X-ray crystal structures of the B(C₆F₅)₃ adducts with the monomers was reported. The X-ray structural studies and spectroscopic data revealed a coordination strength in the order lactones > tetrahydrofuran > cyclic carbonates > lactide.

Supramolecular catalysis by recognition-encoded oligomers: discovery of a synthetic imine polymerase

All key chemical transformations in biology are catalysed by linear oligomers. Catalytic properties could be programmed into synthetic oligomers in the same way as they are programmed into proteins, and an example of the discovery of emergent catalytic properties in a synthetic oligomer is reported. Dynamic combinatorial chemistry experiments designed to study the templating of a recognition-encoded oligomer by the complementary sequence have uncovered an unexpected imine polymerase activity. Libraries of equilibrating imines were formed by coupling diamine linkers with monomer building blocks composed of dialdehydes functionalised with either a trifluoromethyl phenol (D) or phosphine oxide (A) H-bond recognition unit. However, addition of the AAA trimer to a mixture of the phenol dialdehyde and the diamine linker did not template the formation of the DDD oligo-imine. Instead, AAA was found to be a catalyst, leading to rapid formation of long oligomers of D. AAA catalysed a number of different imine formation reactions, but a complementary phenol recognition group on the aldehyde reaction partner is an essential requirement. Competitive inhibition by an unreactive phenol confirmed the role of H-bonding in substrate recognition. AAA accelerates the rate of imine formation in toluene by a factor of 20. The kinetic parameters for this enzyme-like catalysis are estimated as 1 × 10^-3 s^-1 for k _cat and the dissociation constant for substrate binding is 300 μM. The corresponding DDD trimer was found to catalyse oligomerisation the phosphine oxide dialdehyde with the diamine linker, suggesting an important role for the backbone in catalysis. This unexpected imine polymerase activity in a duplex-forming synthetic oligomer suggests that there are many interesting processes to be discovered in the chemistry of synthetic recognition-encoded oligomers that will parallel those found in natural biopolymers.

Alkaline Earth Metal-Mediated Highly Iso-selective Ring-Opening Polymerization of rac-Lactide

Alkaline earth (Ae) metal complexes of the aminophosphine borane ligand are highly active and iso-selective catalysts for the ring-opening polymerization (ROP) of rac-lactide (LA). The polymerization reactions are well controlled and produce polylactides with molecular weights that are precise and narrowly distributed. Kinetic studies reveal that the ROP of rac-LA catalyzed by all Ae metal complexes had a first-order dependency on LA concentration as well as catalyst concentration. A plausible reaction mechanism for Ae metal complex-mediated ROP of rac-LA is discussed, based on controlled kinetic experiments and molecular chain mobility.

Fast and controlled ring-opening polymerization of δ-valerolactone catalyzed by benzoheterocyclic urea/MTBD catalysts

New benzoheterocyclic urea/MTBD catalysts are highly efficient and controllable in the ring-opening polymerization of δ-valerolactone under solvent-free conditions or in solution.

Squaramide and amine binary H-bond organocatalysis in polymerizations of cyclic carbonates, lactones, and lactides

Multiple combinations of six squaramides and eight amines as co-catalysts were success in ROPs of cyclic monomers by H-bond donor and acceptor binary catalysis that established a general protocol.

Synthesis and catalytic application of magnesium complexes bearing pendant indolyl ligands

Three novel indole-based ligand precursors [HIndPh(R), R = methoxy, HIndPh(OMe) (); thiomethoxy, HIndPh(SMe) (); and N,N'-dimethylamino, HIndPh(NMe2) ()] have been synthesized via Sonogashira and cyclization reactions with moderate to high yield. Reactions of these ligand precursors with 0.6 equivalent of Mg(n)Bu2 in THF afforded the magnesium bis-indolyl complexes , respectively. All the ligand precursors and related magnesium complexes have been characterized by NMR spectroscopy and elemental analyses. The molecular structures are reported for compounds and . Under optimized conditions, compound demonstrates efficient catalytic activities towards the ring opening polymerization of l-lactide and ε-caprolactone in the presence of BnOH.

Tripodal hydrogen bond donor binding with sulfonic acid enables ring-opening polymerization

The first Brønsted acidic catalysis platform workable in all of the three major types of cyclic ester monomers including lactides, cyclic carbonates, and lactones, is described in this paper.

Ionic hydrogen bond donor organocatalyst for fast living ring-opening polymerization

A positive charge enhanced H-bond donor combined with H-bond acceptor as a bifunctional organocatalyst enables fast living ring-opening polymerization of lactide.

Three is company: dual intramolecular hydrogen-bond enabled carboxylic acid active in ring-opening polymerization

Dual intramolecular H-bonding made weak Brønsted acid an active catalyst in cationic ring-opening polymerizations.

Internal Lewis pair enhanced H-bond donor: boronate-urea and tertiary amine co-catalysis in ring-opening polymerization

A new protocol of internal Lewis pair enhanced H-bond (LPHBD) catalysis for ring opening polymerisation was developed using boronate-urea (BU) as a representative LPHBD combined with tertiary amines.

A squaramide and tertiary amine: an excellent hydrogen-bonding pair organocatalyst for living polymerization

A hydrogen-bond motif based on a squaramide and sparteine efficiently promoted the ring-opening polymerization of l-lactide at ambient temperature.

Gallium and indium complexes containing the bis(imino)phenoxide ligand: synthesis, structural characterization and polymerization studies

A series of gallium and indium complexes containing the bis(imino)phenolate ligand framework were synthesized and completely characterized with different spectroscopic techniques.

Selective O-acyl ring-opening of β-butyrolactone catalyzed by trifluoromethane sulfonic acid: application to the preparation of well-defined block copolymers

HOTf catalyzes ring-opening of β-butyrolactone selectively via O-acyl bond cleavage, enabling efficient copolymerization.

Ammonium betaines: efficient ionic nucleophilic catalysts for the ring-opening polymerization of l-lactide and cyclic carbonates

Ammonium betaines: the missing piece between hydrogen-bonding and nucleophilic-based ROP catalysts.

Organic Catalysis for Ring-Opening Polymerization

Organic catalysis in ring-opening polymerization (ROP) has become a powerful alternative to more traditional metal-based catalysts. The field has developed to a point at which there are not only excellent low cost and easy to use organocatalysts for day-to-day polymerizations, but the ability to precisely control the synthesis of advanced polymer architectures and ROP monomers that are extremely challenging to polymerize with other catalysts now exists. This viewpoint article will highlight the key advances in organocatalyst design with the aim of encouraging the wider application of organic catalysts in ROP.

Synthesis and characterization of multi-alkali-metal tetraphenolates and application in ring-opening polymerization of lactide

A series of alkali metal complexes supported by two bulky tetraphenols were synthesized and characterized. The reactions of α,α,α',α'-tetra(3,5-di-tert-butyl-2-hydroxyphenyl)-p-xylene(para-tetraphenol) with (n)BuLi, sodium, and KSi(NMe(2))(3) gave bimetallic complexes 1, 2, and 3, respectively. Treatments of the α,α,α',α'-tetra(3,5-di-tert-butyl-2-hydroxyphenyl)-m-xylene(meta-tetraphenol) with 2 or 4 equiv of (n)BuLi afforded complexes 4 or 5, while the reactions of meta-tetraphenol with sodium and KSi(NMe(2))(3) gave only trimetallic complexes 6 and 7 for the additional p-π interaction. Complexes 1-7 were all characterized by single-crystal X-ray diffraction techniques. In the presence of benzyl alcohol, all complexes are active catalysts for the ring-opening polymerization of L-lactide. Comparatively, bimetallic complexes 1, 2, and 3 are more efficient catalysts because of their symmetric structures, in which complex 3 presents as a rare highly active potassium catalyst for the ring-opening polymerization of lactide, leading to polymers with good molecular weight control and narrow molecular weight distributions.

Mechanism of living lactide polymerization by dinuclear indium catalysts and its impact on isoselectivity

A family of racemic and enantiopure indium complexes 1-11 bearing bulky chiral diaminoaryloxy ligands, H(NNO(R)), were synthesized and fully characterized. Investigation of both the mono- and the bis-alkoxy-bridged complexes [(NNO(R))InX](2)[μ-Y][μ-OEt] (5, R = (t)Bu, X = Y = Cl; 8, R = Me, X = I, Y = OEt) by variable temperature, 2D NOESY, and PGSE NMR spectroscopy confirmed dinuclear structures in solution analogous to those obtained by single-crystal X-ray crystallography. The dinuclear complexes in the family were highly active catalysts for the ring-opening polymerization (ROP) of lactide (LA) to form poly(lactic acid) (PLA) at room temperature. In particular, complex 5 showed living polymerization behavior over a large molecular weight range. A detailed investigation of catalyst stereoselectivity showed that, although (R,R/R,R)-5 is highly selective for l-LA, only atactic PLA is obtained in the polymerization of racemic LA. No such selectivity was observed for complex 8. Importantly, the selectivities obtained for the ROP of racemic LA with (R,R/R,R)-5 and (R,R/R,R)-8 are different and, along with kinetics investigations, suggest a dinuclear propagating species for these complexes.

Ring-opening polymerization of ε-caprolactone catalyzed by sulfonic acids: computational evidence for bifunctional activation

The mechanism of ring-opening of ε-caprolactone by methanol catalyzed by trifluoromethane and methane sulfonic acids has been studied computationally at the DFT level of theory. For both elementary steps, the sulfonic acid was predicted to behave as a bifunctional catalyst. The nucleophilic addition proceeds via activation of both the monomer and the alcohol. The ring-opening involves the cleavage of the endo C-O bond of the tetrahedral intermediate with concomitant proton transfer. In both cases, the sulfonic acid acts as a proton shuttle via its acidic hydrogen atom and basic oxygen atoms. The computed activation barriers are consistent with the relatively fast polymerizations observed experimentally at room temperature with both catalysts.