N-Heterocyclic carbenes for metal-free polymerization catalysis: an update

Switchable Organocatalysis from N-heterocyclic Carbene-Carbodiimide Adducts with Tunable Release Temperature

N-Heterocyclic carbenes (NHCs) are powerful organocatalysts, but practical applications often require in situ generation from stable precursors that "mask" the NHC reactivity via reversible binding. Previously established "masks" are often simple small molecules, such that the NHC structure is used to control both catalytic activity and activation temperature, leading to undesirable tradeoffs. Herein, we show that NHC-carbodiimide (CDI) adducts can be masked precursors for switchable organocatalysis and that the CDI substituents can control the reaction profile without changing the NHC structure. Large electronic variations on the CDI (e.g., alkyl versus aryl) drastically change the catalytically active temperature, whereas smaller perturbations (e.g., different para-substituted phenyls) tune the catalyst release within a narrower window. This control was demonstrated for three classic NHC-catalyzed reactions, each influencing the NHC-CDI equilibrium in different ways. Our results introduce a new paradigm for controlling NHC organocatalysis as well as present practical considerations for designing appropriate masks for various reactions.

Nontoxic N-Heterocyclic Olefin Catalyst Systems for Well-Defined Polymerization of Biocompatible Aliphatic Polycarbonates

Herein, N-heterocyclic olefins (NHOs) are utilized as catalysts for the ring-opening polymerization (ROP) of functional aliphatic carbonates. This emerging class of catalysts provides high reactivity and rapid conversion. Aiming for the polymerization of monomers with high side chain functionality, six-membered carbonates derived from 2,2-bis(hydroxymethyl)propionic acid (bis-MPA) served as model compounds. Tuning the reactivity of NHO from predominant side chain transesterification at room temperature toward ring-opening at lowered temperatures (-40 °C) enables controlled ROP. These refined conditions give narrowly distributed polymers of the hydrophobic carbonate 5-methyl-5-benzyloxycarbonyl-1,3-dioxan-2-one (MTC-OBn) (Đ < 1.30) at (pseudo)first-order kinetic polymerization progression. End group definition of these polymers demonstrated by mass spectrometry underlines the absence of side reactions. For the active ester monomer 5-methyl-5-pentafluorophenyloxycarbonyl-1,3-dioxane-2-one (MTC-PFP) with elevated side chain reactivity, a cocatalysis system consisting of NHO and the Lewis acid magnesium iodide is required to retune the reactivity from side chains toward controlled ROP. Excellent definition of the products (Đ < 1.30) and mass spectrometry data demonstrate the feasibility of this cocatalyst approach, since MTC-PFP has thus far only been polymerized successfully using acidic catalysts with moderate control. The broad feasibility of our findings was further demonstrated by the synthesis of block copolymers for bioapplications and their successful nanoparticular assembly. High tolerability of NHO in vitro with concentrations ranging up to 400 μM (equivalent to 0.056 mg/mL) further emphasize the suitability as a catalyst for the synthesis of bioapplicable materials. The polycarbonate block copolymer mPEG₄₄-b-poly(MTC-OBn) enables physical entrapment of hydrophobic dyes in sub-20 nm micelles, whereas the active ester block copolymer mPEG₄₄-b-poly(MTC-PFP) is postfunctionalizable by covalent dye attachment. Both block copolymers thereby serve as platforms for physical or covalent modification of nanocarriers for drug delivery.

Diphenyl phosphate/ethyl diphenylphosphinite as an efficient organocatalytic system for ring-opening polymerization of ε-caprolactone and δ-valerolactone

A novel controlled/living ROP method of ε-CL and δ-VL using ethyl diphenylphosphinite/diphenyl phosphate (EDPP/DPP) organocatalytic system was revealed, which involves the activated monomer mechanism and the reversible chain end deactivation process.

Organocatalytic Synthesis of Poly(hydroxymethylfuroate) via Ring-Opening Polymerization of 5-Hydroxymethylfurfural-Based Cyclic Oligoesters

The synthesis of hydroxymethylfuroate macrocyclic oligoesters c(HMF)n promoted by an N-heterocyclic carbene (NHC) organocatalyst is herein presented together with the subsequent organocatalytic, entropically-driven ring-opening polymerization (ED-ROP) leading to the fully...

Exploring Oxidative NHC-Catalysis as Organocatalytic Polymerization Strategy towards Polyamide Oligomers

The polycondensation of diamines and dialdehydes promoted by an N-heterocyclic carbene (NHC) catalyst in the presence of a quinone oxidant and hexafluoro-2-propanol (HFIP) is herein presented for the synthesis of oligomeric polyamides (PAs), which are obtained with a number-average molecular weight (M_n ) in the range of 1.7-3.6 kg mol^-1 as determined by NMR analysis. In particular, the utilization of furanic dialdehyde monomers (2,5-diformylfuran, DFF; 5,5'-[oxybis(methylene)]bis[2-furaldehyde], OBFA) to access known and previously unreported biobased PAs is illustrated. The synthesis of higher molecular weight PAs (poly(decamethylene terephthalamide, PA10T, M_n = 62.8 kg mol^-1 ; poly(decamethylene 2,5-furandicarboxylamide, PA10F, M_n = 6.5 kg mol^-1 ) by a two-step polycondensation approach is also described. The thermal properties (TGA and DSC analyses) of the synthesized PAs are reported.

Poly(ε-lysine) and its derivatives via ring-opening polymerization of biorenewable cyclic lysine

Minireview focused on poly(ε-lysine) and its derivatives via ring-opening polymerization of biorenewable cyclic lysine.

Structure and activity relationship studies of N-heterocyclic olefin and thiourea/urea catalytic systems: application in ring-opening polymerization of lactones

Structure–activity relationship studies of N-heterocyclic olefin and thiourea/urea catalytic systems were performed and applied to ROP of lactones.

Towards the Preparation of Stable Cyclic Amino(ylide)Carbenes

Cyclic amino(ylide)carbenes (CAYCs) are the ylide-substituted analogues of N-heterocyclic Carbenes (NHCs). Due to the stronger π donation of the ylide compared to an amino moiety they are stronger donors and thus are desirable ligands for catalysis. However, no stable CAYC has been reported until today. Here, we describe experimental and computational studies on the synthesis and stability of CAYCs based on pyrroles with trialkyl onium groups. Attempts to isolate two CAYCs with trialkyl phosphonium and sulfonium ylides resulted in the deprotonation of the alkyl groups instead of the formation of the desired CAYCs. In case of the PCy₃-substituted system, the corresponding ylide was isolated, while deprotonation of the SMe₂-functionalized compound led to the formation of ethene and the thioether. Detailed computational studies on various trialkyl onium groups showed that both the α- and β-deprotonated compounds were energetically favored over the free carbene. The most stable candidates were revealed to be α-hydrogen-free adamantyl-substituted onium groups, for which β-deprotonation is less favorable at the bridgehead position. Overall, the calculations showed that the isolation of CAYCs should be possible, but careful design is required to exclude decomposition pathways such as deprotonations at the onium group.

Oxidative NHC-Catalysis as Organocatalytic Platform for the Synthesis of Polyester Oligomers by Step-Growth Polymerization

The application of N-heterocyclic carbene (NHC) catalysis to the polycondensation of diols and dialdehydes under oxidative conditions is herein presented for the synthesis of polyesters using fossil-based (ethylene glycol, phthalaldehydes) and bio-based (furan derivatives, glycerol, isosorbide) monomers. The catalytic dimethyl triazolium/1,8-diazabicyclo[5.4.0]undec-7-ene couple and stoichiometric quinone oxidant afforded polyester oligomers with a number-average molecular weight (M_n ) in the range of 1.5-7.8 kg mol^-1 as determined by NMR analysis. The synthesis of a higher molecular weight polyester (polyethylene terephthalate, PET) by an NHC-promoted two-step procedure via oligoester intermediates is also illustrated together with the catalyst-controlled preparation of cross-linked or linear polyesters derived from the trifunctional glycerol. The thermal properties (TGA and DSC analyses) of the synthesized oligoesters are also reported.

Bulk Organocatalytic Synthetic Access to Statistical Copolyesters from l-Lactide and ε-Caprolactone Using Benzoic Acid

The development of synthetic strategies to produce statistical copolymers based on l-lactide (l-LA) and ε-caprolactone (CL), denoted as P(LA- stat-CL), remains highly challenging in polymer chemistry. This is due to the differing reactivity of the two monomers during their ring-opening copolymerization (ROcP). Yet, P(LA- stat-CL) materials are highly sought after as they combine the properties of both polylactide (PLA) and poly(ε-caprolactone) (PCL). Here, benzoic acid (BA), a naturally occurring, cheap, readily recyclable, and thermally stable weak acid, is shown to trigger the organocatalyzed ring-opening copolymerization (OROcP) of l-LA and CL under solvent-free conditions at 155 °C, in presence of various alcohols as initiators, with good control over molar masses and dispersities (1.11 < Đ < 1.35) of the resulting copolyesters. Various compositions can be achieved, and the formation of statistical compounds is shown through characterization by ¹H, ¹³C, and diffusion ordered spectroscopy NMR spectroscopies and by differential scanning calorimetry, as well as through the determination of reactivity ratios ( r_LA = 0.86, r_CL = 0.86), using the visualization of the sum of squared residuals space method. Furthermore, this BA-OROcP process can be exploited to access metal-free PLA- b-P(LA- stat-CL)- b-PLA triblock copolymers, using a diol as an initiator. Finally, residual traces of BA remaining in P(LA- stat-CL) copolymers (<0.125 mol %) do not show any cytotoxicity toward hepatocyte-like HepaRG cells, demonstrating the safety of this organic catalyst.

N-Heterocyclic Carbenes in Materials Chemistry

N-Heterocyclic carbenes (NHCs) have become one of the most widely studied class of ligands in molecular chemistry and have found applications in fields as varied as catalysis, the stabilization of reactive molecular fragments, and biochemistry. More recently, NHCs have found applications in materials chemistry and have allowed for the functionalization of surfaces, polymers, nanoparticles, and discrete, well-defined clusters. In this review, we provide an in-depth look at recent advances in the use of NHCs for the development of functional materials.

N-Heterocyclic olefins and thioureas as an efficient cooperative catalyst system for ring-opening polymerization of δ-valerolactone

An organocatalytic ring-opening polymerization of δ-valerolactone has been developed.

Polarized olefins as enabling (co)catalysts for the polymerization of γ-butyrolactone

N-Heterocyclic olefins (NHOs) can homopolymerize GBL via anionic or zwitterionic pathways, whereby polymerization mode and polymer topology depend on the chemical structure of the NHO and the presence of LiCl as cocatalyst.

Photoinduced ring-opening polymerisation of l-lactide via a photocaged superbase

The phototriggered ring-opening polymerisation of l-lactide is demonstrated for the first time using a photocaged tetramethylguanidine.

Reinvestigation of the mechanism of polymerization of β-butyrolactone from 1,5,7-triazabicyclo[4.4.0]dec-5-ene

The questionable mechanism initially proposed to explain how 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) allows us to ring-open β-lactones, such as β-butyrolactone (BL), is reinvestigated here.

Polycondensation of methacrylates: auto-tandem organocatalysis using N-heterocyclic carbenes

N-Heterocyclic carbenes catalyzed the self-polycondensation of hydroxy-functionalized methacrylates and the direct polycondensation of n-butyl methacrylate with diols to produce new unsaturated polyesters.

Highly Polarized Alkenes as Organocatalysts for the Polymerization of Lactones and Trimethylene Carbonate

In this work, the activity of N-heterocyclic olefins (NHOs), a newly emerging class of organopolymerization catalyst, is investigated to affect the metal-free polymerization of lactones and trimethylene carbonate (TMC). A decisive structure-activity relationship is revealed. While catalysts of the simplest type bearing an exocyclic ═CH₂ moiety polymerize l-lactide (l-LA) and δ-valerolactone (δ-VL) in a non-living and non-quantitative manner, the introduction of methyl substituents on the exocyclic carbon radically changes this behavior. 2-Isopropylidene-1,3,4,5-tetramethylimidazoline is found to be highly active for a range of monomers such as l-LA, δ-VL, ε-caprolactone (ε-CL), and TMC, with quantitative conversion occurring within seconds with catalyst loadings of just 0.2 mol %. The high activity of this NHO further enables the ring-opening polymerization (ROP) of the macrolactone ω-pentadecalactone (PDL). However, this broad applicability is offset by a lack of control over the polymerizations, including side reactions as a consequence of its strong basicity. To overcome this, a saturated, imidazolinium-derived analogue was synthesized and subsequently demonstrated to possess a harnessed reactivity which enables it to polymerize both l-LA and TMC in a controlled manner (Đ_M < 1.2). NMR spectroscopic and MALDI-ToF MS experiments highlight the differences in polymerization pathways for 2-methylene-1,3,4,5-tetramethylimidazoline, in which the exocyclic carbon is not substituted, in contrast to 2-isopropylidene-1,3,4,5-tetramethylimidazoline, with the former operating via its nucleophilicity and the latter acting as a base with enolizable δ-VL.

Synthesis of degradable poly(ε-caprolactone)-based graft copolymers via a “grafting-from” approach

The controlled ring-opening polymerisation (ROP) of α-bromo-ε-caprolactone (αBrCL), a derivative of ε-caprolactone (εCL), and its copolymerisation with εCL is reported.

Polymerization of Ethylene Oxide, Propylene Oxide, and Other Alkylene Oxides: Synthesis, Novel Polymer Architectures, and Bioconjugation

The review summarizes current trends and developments in the polymerization of alkylene oxides in the last two decades since 1995, with a particular focus on the most important epoxide monomers ethylene oxide (EO), propylene oxide (PO), and butylene oxide (BO). Classical synthetic pathways, i.e., anionic polymerization, coordination polymerization, and cationic polymerization of epoxides (oxiranes), are briefly reviewed. The main focus of the review lies on more recent and in some cases metal-free methods for epoxide polymerization, i.e., the activated monomer strategy, the use of organocatalysts, such as N-heterocyclic carbenes (NHCs) and N-heterocyclic olefins (NHOs) as well as phosphazene bases. In addition, the commercially relevant double-metal cyanide (DMC) catalyst systems are discussed. Besides the synthetic progress, new types of multifunctional linear PEG (mf-PEG) and PPO structures accessible by copolymerization of EO or PO with functional epoxide comonomers are presented as well as complex branched, hyperbranched, and dendrimer like polyethers. Amphiphilic block copolymers based on PEO and PPO (Poloxamers and Pluronics) and advances in the area of PEGylation as the most important bioconjugation strategy are also summarized. With the ever growing toolbox for epoxide polymerization, a "polyether universe" may be envisaged that in its structural diversity parallels the immense variety of structural options available for polymers based on vinyl monomers with a purely carbon-based backbone.