The anionic ring-opening polymerizations (AROPs) of N-sulfonyl aziridines, in the presence of organic superbases including phosphazene (t-Bu-P4), Verkade’s base (P(i-PrNCH2CH2)3N, TiPP), DBU, MTBD, and N,N,N′,N′-tetramethylguanidine (TMG), using N-benzyl-p-toluenesulfonamide (BnN(H)Ts) as initiator were explored to produce metal-free poly(sulfonylaziridine)s. Among the superbases used, the catalytic activity was found directly proportional to their basicity. Remarkably, t-Bu-P4 and TiPP gave a living/controlled AROP of 2-methyl-N-tosylaziridine (TsMAz), where t-Bu-P4 performed better, affording the metal-free and well-defined poly(sulfonylaziridine)s with high molar masses (Mn(SEC) > 30 kg mol–1) and low dispersities (Đ < 1.10) in 3.5 h. For the less reactive monomers of 2-methyl-N-ethylsulfonyl aziridine (EsMAz) and 2-phenyl-N-tosylaziridine (TsPhAz), t-Bu-P4 showed the same excellent catalytic efficiency (30 equiv, conv. > 95%, 5 h). The organocatalyzed AROP allowed the use of lower catalyst (t-Bu-P4) loading than the amount of initiator (BnN(H)Ts), but the propagating polymer chains were as many as the number of equivalents of the introduced initiators, which could lower the loading of catalyst used to amounts as low as 0.05 mol %.

Amphiphilic star-shaped block copolymers with biodegradable and biocompatible sequences are attracting increasing attention in biomaterials. Herein, we report the synthesis, characterization, solution properties, and biocompatibility of amphiphilic three-armed star-shaped poly(sarcosine)-block-poly(ε-caprolactone) (s-PSar-b-PCL) diblock copolymers. Tris-(2-aminoethyl)amine initiated ring-opening polymerization (ROP) of sarcosine N-carboxyanhydrides in a controlled manner, yielding star-shaped poly(sarcosine)s (s-PSars) with predictable molecular weights (from 3.3 to 9.8 kg mol−1) and narrow dispersities (ĐM < 1.1). Well-defined amphiphilic s-PSar-b-PCL diblock copolymers were synthesized by ROP of ε-caprolactone using s-PSars as the macroinitiators in one pot. The obtained s-PSars and s-PSar-b-PCL diblock copolymers were characterized using 1H NMR, 13C NMR, MALDI-ToF MS, and size-exclusion chromatography (SEC). Intrinsic viscosities ([η]) of the s-PSar and s-PSar-b-PCL were estimated using SEC-MALS-VISC-DRI. Dynamic light scattering and transmission electron microscopy analysis showed that the s-PSar-b-PCL diblock copolymers self-assembled into spherical aggregates with average hydrodynamic diameters of 56–169 nm in aqueous solution. MTT assays (cell viability test) certified the biosafety (relative cell viability >80%) of the copolymers and their self-assembled nanostructures. Taken together, we (i) synthesized the novel s-PSar-b-PCL diblock copolymers, (ii) investigated the solution properties of the s-PSar-b-PCLs and s-PSars, and (iii) demonstrated the biocompatibility of s-PSar-b-PCLs.

Ring-opening polymerization (ROP) of lactones and cyclic carbonates catalyzed by (super)strong Brønsted acids offers a valuable approach to generate biodegradable aliphatic polyesters. However, these strong acids usually lead to backbiting and decarboxylation; thus a mild and effective acidic catalysis for these ROPs becomes necessary. Inspired by weak Brønsted acidic catalysis in squalene–hopene cyclases, we propose that ortho-amido group(s) on benzoic acids would increase the acidity of the carboxylic moiety by intramolecular H-bonding, and make carboxylic acid active in promoting the ROPs. A series of o-amido- and o,o′-bis(amido)-benzoic acids are evaluated as typical intramolecular H-bonding enhanced Brønsted acidic catalysts in the ROPs. Both o-amido- and o,o′-bis(amido)-benzoic acids exhibited good to excellent performances in the rate and control of ROPs of δ-valerolactone (VL), ε-caprolactone (CL), and trimethylene carbonate (TMC) at room temperature in solutions. An exceptional carboxylic acid, o,o′-bis(pivalamido)benzoic acid, showed efficient activation in solution and precise control with high conversions (91–96%), predicted molecular weights from 3.09 to 10.31 kg mol−1, and narrow dispersities (Đ 1.03–1.12) in ROPs of CL and TMC. Well-defined diblock copolymers consisting of PTMC, PVL and PCL segments were synthesized. The controlled/living characteristics of the ROPs were verified by chain extension experiments. 1H NMR, SEC, and MALDI-TOF MS analyses strongly indicated that the obtained polymers were exactly the designated ones. A cationic monomer activation mechanism was proposed and was supported by NMR titrations. The experimental results indicated that mild and tunable ortho-amido benzoic acid with intramolecular H-bonding is a competent organocatalyst in living polymerization.

Catalysis by silver nanoparticles (Ag-NPs) in organic transformations has received growing attention due to their unique reactivity and selectivity. Herein, we investigated a versatile one-step approach for synthesizing thermally stable AgNPs using catechol (1,2-benzenediol) without additional reducing and stabilizing agents in aqueous solution. In an alkaline environment, oxidation of catechol played a dual role in the reduction of silver ions (Ag+) and stabilization of the AgNPs. Nanoparticles with different size and morphology were obtained under different experimental conditions. X-ray diffraction (XRD) analysis suggests the formation of crystalline AgNPs of average size 13, 38 and 47 nm and face centered cubic structure as the reaction pH varied. As demonstrated in dynamic light scattering (DLS) and scanning electron microscopy (SEM) images, AgNPs with uniform size distribution (50 nm) were synthesized at pH 11. The nanoparticles are thermally stable with a steady loss of weight up to 800 °C as confirmed by thermogravimetric analysis (TGA). Comparing to AgNPs@pH5 and AgNPs@pH8, AgNPs synthesized at pH 11 have shown significant catalytic activity in the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) with 61% conversion at 20 °C. The results suggested that stable and monodisperse nanoparticles with tunable catalytic activity could be produced as the pH of the reaction was altered.

Multiple combinations of six squaramides (Sq) and eight amines as the co-catalysts were tried in ring-opening polymerizations (ROPs) of cyclic carbonates, lactones, and lactides. Sq and sparteine co-catalysts enabled ROPs of trimethylene carbonate (TMC) to poly(trimethylene carbonates) (PTMCs) with benzyl alcohol (BnOH) as the initiator. The polymerization proceeded in 3 to 16 h without decarboxylation to afford polycarbonates with precise molecular weights (Mn,NMR = 1.95 to 10.1 kg mol−1) and narrow polydispersity indices (Đ = 1.12–1.17). 1H NMR, SEC and MALDI-ToF MS measurements of the obtained PTMCs clearly indicated the quantitative incorporation of the initiator at the chain end. Kinetics and chain extension experiments demonstrated the controlled/living nature for the ROP of TMC using Sq and sparteine. NMR titration experiments confirmed that the polymerization proceeded in a H-bonding dual activation mechanism. In addition, 1,3-propanediol, pentaerythritol, propargyl alcohol, furfuryl alcohol and N-(2-hydroxyethyl)maleimide were used as functional initiators leading to production of α,ω-dihydroxy telechelic, star-shaped, and clickable end-functionalized polycarbonates. Homopolymers of valerolactone (VL), caprolactone (CL), and lactide (LA) and diblock copolymers PVL-b-PTMC and PCL-b-PTMC were successfully synthesized by using Sq and 1,8-diazabicyclo[5.4.0]undec-7-ene binary co-catalysts with BnOH as the initiator in dichloromethane at room temperature. Block copolymers PTMC-b-PVL, PTMC-b-PCL, and PTMC-b-PLA were successfully obtained by using the binary catalysts. Squaramides combined with amine co-catalysts are a generally applicable polymerization tool.

Tritylium cation-catalyzed interrupted Povarov reactions afforded cis-4-aminobenzodihydropyrans in excellent yields (90 %) within 10 min by low catalyst loadings (1 mol-%). A mechanism involving Lewis acidic catalysis by a carbocation was proposed and validated. Changing a one-pot batch version of the reaction into a two-stage convergent continuous flow procedure led to a 10-fold time reduction (to 1 min) with 88 % yield.

•Amine-initiated ROP of LA was in controlled/living manner.•Binary H-bonding organocatalysts achieved the ROP of LA under mild conditions.•Well-defined PLAs and PSar-b-PLAs with expected molecular weights were prepared.Alcohol-initiated ring-opening polymerization (ROP) of cyclic esters for synthesizing polyesters was well established, but amine-initiated ROP of cyclic esters was rare. A challenge is slow initiation and fast propagation in the amine-initiated ROPs. To address the difficulties, an ionic H-bond donor (iHBD) and H-bond acceptor (HBA) binary organocatalysis in amine-initiated ROP of lactide (LA) was developed. Guanidinium hexahydro-2H-pyrimido[1,2-a] pyrimidin-1-ium [(HppH2)+] and tertiary amine sparteine (SP), cooperatively played the role of iHBD/HBA binary catalysts, efficiently promoted ROP of LA from amine initiators toward polylactide (PLA) and polysarcosine-block-polylactide diblock copolymer (PSar-b-PLA). Benzyl amine and N-methylbenzylamine initiated ROPs of LA under mild conditions produced PLAs with predictable molecular weights (Mn,NMR = 2.9–17.1 kg mol−1) and narrow dispersities (ÐM,SEC = 1.13–1.23). Macroinitiator PSar copolymerized with LA under the iHBD/HBA catalysis, producing PSar-b-PLAs with controlled molecular weights (Mn,NMR = 5.7–14.8 kg mol−1) and low dispersities (ÐM,SEC = 1.16–1.21). Kinetics and chain extension experiments confirmed that the amine-initiated ROP of LA in presence of (HppH2)+BF4−/SP was in controlled/living manner. 1H NMR, 13C NMR, SEC, and MALDI-ToF MS analyses indicated that the obtained PLA was exclusively initiated from the corresponding amine with amide linkage. An iHBD/HBA binary activation mechanism was proposed.Download high-res image (134KB)Download full-size image

Hydrogen-bond organocatalysis using (thio)urea and base has achieved massive success. Intramolecular H-bond (IMHB) assisted Brønsted acid (BA) catalysis, especially in polymerizations, was not explored. Here we suggested an IMHB–BA model in ring-opening polymerization (ROP) with γ-resorcylic acid (RA) as a representative catalyst, promoting ROPs of δ-valerolactone (VL) and ε-caprolactone (CL). The exceptional carboxylic acid RA showed efficient activation and precise control with high conversions (93–98%), predicted molecular weights from 3090 to 13000 g mol−1, narrow dispersities (Đ 1.02–1.08), and nearly Poisson distributions (Đ ≤ 1.03) at higher molecular weights. A dual IMHB in RA was estimated using computational calculations, which predicted short H-bond lengths and near 180° bond angles, denoting strong H-bonding. A cationic monomer activation mechanism was proposed and supported by NMR titrations. The controlled/living nature of the ROPs was confirmed by kinetics and chain extension experiments. 1H NMR, SEC, and MALDI-ToF MS analyses strongly indicated that the obtained PVL and PCL were exactly the designated ones. Synthesis of well-defined PVL-b-PCL and clickable end-functionalized PVLs again verified that the catalytic ROPs proceeded in a controlled/living manner, and suggested that IMHB–BA catalysis was a generally applicable method.

The natural organocatalyst phytic acid catalyzed one-pot Biginelli reactions by coupling β-ketoesters, aldehydes, and (thio)ureas to afford 3,4-dihydropyrimidin-2(1H)-ones/thiones. This phytic acid catalysis featured good to excellent isolated yields, solvent-free conditions, a simple workup, environmental friendliness, and a short reaction time.

The ring-opening polymerization of ε-caprolactone (ε-CL) using benzyl alcohol (BnOH) as initiator and 2,4-dinitrobenzenesulfonic acid (DNBA) as catalyst in acetonitrile at room temperature with a [ε-CL]0/[BnOH]0/[DNBA]0 ratio of 40/1/1 has been investigated. The polymerization proceeded to obtain poly(ε-caprolactone) (PCL) with controlled molecular weights. In addition, 1H NMR, SEC, and MALDI-ToF MS measurements demonstrated the initiator residue at the polymer chain end. Furthermore, propargyl alcohol, 5-hexen-1-ol, 2-hydroxyethyl methacrylate, 1,3-propanediol, and pentaerythritol were used as functional initiators to successfully obtain end-functionalized and α,ω-dihydroxy telechelic polyesters. The block copolymerization of PCL and PVL or PTMC provided conditions to afford well-defined poly(ε-caprolactone)-block-poly(δ-valerolactone) (PCL-b-PVL) and poly(ε-caprolactone)-block-poly(trimethylene carbonate) (PCL-b-PTMC).

Amphiphilic star-shaped block copolymers with biodegradable and biocompatible sequences are attracting increasing attention in biomaterials. Herein, we report the synthesis, characterization, solution properties, and biocompatibility of amphiphilic three-armed star-shaped poly(sarcosine)-block-poly(ε-caprolactone) (s-PSar-b-PCL) diblock copolymers. Tris-(2-aminoethyl)amine initiated ring-opening polymerization (ROP) of sarcosine N-carboxyanhydrides in a controlled manner, yielding star-shaped poly(sarcosine)s (s-PSars) with predictable molecular weights (from 3.3 to 9.8 kg mol−1) and narrow dispersities (ĐM < 1.1). Well-defined amphiphilic s-PSar-b-PCL diblock copolymers were synthesized by ROP of ε-caprolactone using s-PSars as the macroinitiators in one pot. The obtained s-PSars and s-PSar-b-PCL diblock copolymers were characterized using 1H NMR, 13C NMR, MALDI-ToF MS, and size-exclusion chromatography (SEC). Intrinsic viscosities ([η]) of the s-PSar and s-PSar-b-PCL were estimated using SEC-MALS-VISC-DRI. Dynamic light scattering and transmission electron microscopy analysis showed that the s-PSar-b-PCL diblock copolymers self-assembled into spherical aggregates with average hydrodynamic diameters of 56–169 nm in aqueous solution. MTT assays (cell viability test) certified the biosafety (relative cell viability >80%) of the copolymers and their self-assembled nanostructures. Taken together, we (i) synthesized the novel s-PSar-b-PCL diblock copolymers, (ii) investigated the solution properties of the s-PSar-b-PCLs and s-PSars, and (iii) demonstrated the biocompatibility of s-PSar-b-PCLs.