Reaching High Stereoselectivity and Activity in Organocatalyzed Ring-Opening Polymerization of Racemic Lactide by the Combined Use of a Chiral (Thio)Urea and a N-Heterocyclic Carbene

Stereochemical control during polymerization is a key strategy of polymer chemistry to achieve semicrystalline engineered plastics. The stereoselective ring-opening polymerization (ROP) of racemic lactide (rac-LA), which can lead to highly isotactic polylactide (PLA), is one of the emblematic examples in this area. Surprisingly, stereoselective ROP of rac-LA employing chiral organocatalysts has been under-leveraged. Here we show that a commercially available chiral thiourea (TU1), or its urea homologue (U1), can be used in conjunction with an appropriately selected N-heterocyclic carbene (NHC) to trigger the stereoselective ROP of rac-LA at room temperature in toluene. Both a high organic catalysis activity (>90% monomer conversion in 5-9 h) and a high stereoselectivity (probability of formation of meso dyads, P_m, in the range 0.82-0.93) can be achieved by thus pairing a NHC and a chiral amino(thio)urea. The less sterically hindered and the more basic NHC, that is, a NHC bearing tert-butyl substituents (NHC_tBu), provides the highest stereoselectivity when employed in conjunction with the chiral TU1 or U1. This asymmetric organic catalysis strategy, as applied here in polymerization chemistry, further expands the field of possibilities to achieve bioplastics with adapted thermomechanical properties.

Thermo-induced inversion of water-in-water emulsion stability by bis-hydrophilic microgels

HYPOTHESIS

Stabilization of water-in-water (W/W) emulsions resulting from the separation of polymeric phases such as dextran (DEX) and poly(ethyleneoxide) (PEO) is highly challenging, because of the very low interfacial tensions between the two phases and because of the interface thickness extending over several nanometers. In the present work, we present a new type of stabilizers, based on bis-hydrophilic, thermoresponsive microgels, incorporating in the same structure poly(N-isopropylacrylamide) (pNIPAM) chains having an affinity for the PEO phase and dextran moieties. We hypothesize that these particles allow better control of the stability of the W/W emulsions.

EXPERIMENTS

The microgels were synthesized by copolymerizing the NIPAM monomer with a multifunctional methacrylated dextran. They were characterized by dynamic light scattering, zeta potential measurements and nuclear magnetic resonance as a function of temperature. Microgels with different compositions were tested as stabilizers of droplets of the PEO phase dispersed in the DEX phase (P/D) or vice-versa (D/P), at different concentrations and temperatures.

FINDINGS

Only microgels with the highest DEX content revealed excellent stabilizing properties for the emulsions by adsorbing at the droplet surface, thus demonstrating the fundamental role of bis-hydrophilicity. At room temperature, both pNIPAM and DEX chains were swollen by water and stabilized better D/P emulsions. However, above the volume phase transition temperature (VPTT ≈ 32 °C) of pNIPAM the microgels shrunk and stabilized better P/D emulsions. At all temperatures, excess microgels partitioned more to the PEO phase. The change in structure and interparticle interaction induced by heating can be exploited to control the W/W emulsion stability.

A chiral thiourea and a phosphazene for fast and stereoselective organocatalytic ring-opening-polymerization of racemic lactide

Control of stereoregularity is inherent to precision polymerization chemistry for the development of functional materials. A prototypal example of this strategy is the ring-opening polymerization (ROP) of racemic lactide (rac-LA), a bio-sourced monomer. Despite significant advances in organocatalysis, stereoselective ROP of rac-LA employing chiral organocatalysts remains unexplored. Here we tackle that challenge by resorting to Takemoto's catalyst, a chiral aminothiourea, in the presence of a phosphazene base. This chiral binary organocatalytic system allows for fast, chemo- and stereoselective ROP of rac-LA at room temperature, yielding highly isotactic, semi-crystalline and metal-free polylactide, with a melting temperature as high as 187 °C.

Unprecedented coupling of natural rubber and ELP: synthesis, characterization and self-assembly properties

Developing new biomaterials is an active research area owing to their applications in regenerative medicine, tissue engineering and drug delivery.

C–C couplings in water by micellar catalysis at low loadings from a recyclable polymer-supported Pd(ii)–NHC nanocatalyst

A robust and recyclable polymer-supported Pd(ii)–NHC₂ nanocatalyst enables highly efficient micellar catalysis of the Suzuki–Miyaura reaction at low loadings with a broad substrate scope.

Isoselective Ring-Opening Polymerization of rac-Lactide from Chiral Takemoto's Organocatalysts: Elucidation of Stereocontrol

Despite significant advances in organocatalysis, stereoselective polymerization reactions utilizing chiral organocatalysts have received very little attention, and much about the underlying mechanisms remains unknown. Here, we report that both commercially available (R,R)- and (S,S)-enantiomers of chiral thiourea-amine Takemoto's organocatalysts promote efficient control and high isoselectivity at room temperature of the ring-opening polymerization (ROP) of racemic lactide by kinetic resolution, yielding highly isotactic, semicrystalline and metal-free polylactide (PLA). Kinetic investigations and combined analyses of the resulting PLAs have allowed the stereocontrol mechanism, which eventually involves both enantiomorphic site control and chain-end control, to be determined. Moreover, epimerization of rac-LA to meso-LA is identified as being responsible for the introduction of some stereoerrors during the ROP process.

Core-Shell Double Gyroid Structure Formed by Linear ABC Terpolymer Thin Films

The synthesis and self-assembly in thin-film configuration of linear ABC triblock terpolymer chains consisting of polystyrene (PS), poly(2-vinylpyridine) (P2VP), and polyisoprene (PI) are described. For that purpose, a hydroxyl-terminated PS-b-P2VP (45 kg mol^-1 ) building block and a carboxyl-terminated PI (9 kg mol^-1 ) are first separately prepared by anionic polymerization, and then are coupled via a Steglich esterification reaction. This quantitative and metal-free catalyst synthesis route reveals to be very interesting since functionalization and purification steps are straightforward, and well-defined terpolymers are produced. A solvent vapor annealing (SVA) process is used to promote the self-assembly of frustrated PS-b-P2VP-b-PI chains into a thin-film core-shell double gyroid (Q²³⁰ , space group: Ia3¯d) structure. As terraces are formed within PS-b-P2VP-b-PI thin films during the SVA process under a CHCl₃ vapor, different plane orientations of the Q²³⁰ structure ((211), (110), (111), and (100)) are observed at the polymer-air interface depending on the film thickness.

Pd(ii)–NHC coordination-driven formation of water-soluble catalytically active single chain nanoparticles

Intramolecular coordination of a linear copolymer precursor yields single chain nanoparticles (SCNP's) consisting of Pd(ii)-NHC₂ crosslinks and showing a beneficial SCNP effect when used for the Suzuki coupling in water.

Intramolecular Quaternization as Folding Strategy for the Synthesis of Catalytically Active Imidazolium-Based Single Chain Nanoparticles

A novel synthetic strategy to single chain nanoparticles (SCNP) based on a previously unexplored intramolecular reaction as a means to autonomously fold a parent copolymer precursor is reported. The latter is a statistical styrenic-type copolymer synthesized by RAFT polymerization and is composed of four different comonomer units, including styrene, grafted poly(ethylene oxide) chains, and antagonist benzimidazol- and chlorobenzyl-based units. The two latter functions are reacted together by a quaternization reaction to spark the folding process, creating imidazolium-based cross-link points. Formation of SCNP with a hydrodynamic diameter <10 nm is established by a combination of different characterization techniques. A subsequent metathesis step for exchanging chloride anions by acetate ones of imidazolium moieties enables a use of SCNP for the organocatalyzed benzoin condensation, thanks to the ability of imidazolium acetate to generate SCNP-supported N-heterocyclic carbenes as organocatalysts.

Activated lipidic cyclic carbonates for non-isocyanate polyurethane synthesis

Activated 5-membered cyclic carbonates were prepared from glycerol and fatty acid derivatives.

Quantitative Side-Chain Modifications of Methionine-Containing Elastin-Like Polypeptides as a Versatile Tool to Tune Their Properties

Tuning the lower critical solution temperature (LCST) of temperature-responsive recombinant elastin-like polypeptides has usually been achieved by designing different protein sequences, in terms of amino acid composition and length, implying tedious molecular cloning steps. In the present work, we have explored the chemoselective alkylation of methionine as an easy means to modify elastin repeat side chains and easily modulate the LCST of the polypeptides. Such a versatile synthetic method shall practically be exploited to modulate any properties of recombinant polymers.

From the N-Heterocyclic Carbene-Catalyzed Conjugate Addition of Alcohols to the Controlled Polymerization of (Meth)acrylates

Among various N-heterocyclic carbenes (NHCs) tested, only 1,3-bis(tert-butyl)imidazol-2-ylidene (NHC(tBu) ) proved to selectively promote the catalytic conjugate addition of alcohols onto (meth)acrylate substrates. This rather rare example of NHC-catalyzed 1,4-addition of alcohols was investigated as a simple means to trigger the polymerization of both methyl methacrylate and methyl acrylate (MMA and MA, respectively). Well-defined α-alkoxy poly(methyl (meth)acrylate) (PM(M)A) chains, the molar masses of which could be controlled by the initial [(meth)acrylate]0/[ROH]0 molar ratio, were ultimately obtained in N,N-dimethylformamide at 25 °C. A hydroxyl-terminated poly(ethylene oxide) (PEO-OH) macro-initiator was also employed to directly access PEO-b-PMMA amphiphilic block copolymers. Investigations into the reaction mechanism by DFT calculations revealed the occurrence of two competitive concerted pathways, involving either the activation of the alcohol or that of the monomer by NHC(tBu) .

Carbocationic polymerization of isoprene using cumyl initiators: progress in understanding side reactions

The cationic polymerization of isoprene using cumyl chloride/B(C₆F₅)₃ and cumyl ether/TiCl₄ systems was investigated in dichloromethane or in dichloromethane/methylcyclohexane mixtures varying the polymerization conditions.

Poly(2-oxazoline)-based nanogels as biocompatible pseudopolypeptide nanoparticles

Hydrophilic nanogels based on partially hydrolyzed poly(2-ethyl-2-oxazoline) were synthesized in dilute aqueous media in the presence of 1,6-hexanediol diglycidyl ether as a cross-linker. Nanogel formation was monitored by DLS and HSQC NMR spectroscopy, and the final nano-objects were characterized by DLS, TEM, AFM, and NanoSight analyses. Nanogels with a hydrodynamic radius of 78 nm exhibiting a slight positive surface charge were obtained. MTS assays (cell metabolic activity test) evidenced that nanogels were nontoxic in the investigated concentration range (i.e., 0.1 to 400 μg/mL) and that no specific interaction with bovine serum albumin was observed.

Fatty acid-based (bis) 6-membered cyclic carbonates as efficient isocyanate free poly(hydroxyurethane) precursors

Highly reactive (bis) 6-membered cyclic carbonates (b6CC) were prepared from fatty acid derivatives. Isocyanate free poly(hydroxyurethane)s were then synthesized and characterized.