Controlled/living ring-opening polymerization of ?-caprolactone catalyzed by phosphoric acid

The bulk ring-opening polymerization (ROP) of ?-caprolactone (?-CL) by various phosphoric acids using phenylmethanol as the initiator was conducted. 1,1??-bi-2-Naphthol (BINOL)-based phosphoric acid was found to be an effective organocatalyst for ROP leading to polyesters at 90°C. The overall conversion to poly(?-caprolactone) was more than 96% and poly(?-caprolactone) with M _w of 8400 and polydispersity index of 1.13 was obtained. ¹H NMR spectra of oligomers demonstrated the quantitative incorporation of the protic initiator in the polymer chains and showed that transesterification reactions did not occur to a significant extent. The controlled polymerization was indicated by the linear relationships between the number-average molar mass and monomer conversion or monomer-to-initiator ratio. In addition, the present protocol provided an easy-to-handle, inexpensive and environmentally benign entry for the synthesis of biodegradable materials as well as polyesters for biomedical applications.     

Controlled/living ring-opening polymerization of ε-caprolactone catalyzed by phosphoric acid

The bulk ring-opening polymerization(ROP) of ε-caprolactone(ε-CL) by various phosphoric acids using phenylmethanol as the initiator was conducted.1,1’-bi-2-Naphthol(BINOL)-based phosphoric acid was found to be an effective organocatalyst for ROP leading to polyesters at 90℃.The overall conversion to poly(ε-caprolactone) was more than 96% and poly(ε-caprolactone) with M w of 8400 and polydispersity index of 1.13 was obtained.1 H NMR spectra of oligomers demonstrated the quantitative incorporation of the protic initiator in the polymer chains and showed that transesterification reactions did not occur to a significant extent.The controlled polymerization was indicated by the linear relationships between the number-average molar mass and monomer conversion or monomer-to-initiator ratio.In addition,the present protocol provided an easy-to-handle,inexpensive and environmentally benign entry for the synthesis of biodegradable materials as well as polyesters for biomedical applications.     

Stimuli-responsive polypeptide materials prepared by ring-opening polymerization of α-amino acid N-carboxyanhydrides

Design and synthesis of biodegradable stimuli-responsive polypeptides are important areas considering their promising applications in biomedical fields. This article summarizes the most recent progresses in the development of stimuli-responsive polypeptide materials prepared via ring-opening polymerization of α-amino acid N-carboxyanhydrides. We discuss the design, synthesis and structure-property correlation of emerging materials including thermo-responsive, redox-responsive, photo-responsive and biomolecule responsive polypeptides. Considering the unique structural features of amino acids, we try to emphasize that the thermo-responsive properties not only depend on the amino acid structure but also rely on the secondary structures of polypeptides. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Investigation of α-amino acid N-carboxyanhydrides by X-ray diffraction for controlled ring-opening polymerization

The need for a scalable synthesis of not sequence defined polypeptides as biomaterials is met by the ring-opening polymerization of α-amino acid N-carboxyanhydrides (NCAs). Even though this polymerization technique appears straight forward, it holds pitfalls in terms of reproducibility and overall control over the polymerization conditions, which depends, beside choice of solvent or initiator, significantly on reagent purity. In addition, the synthesis of monomers can lead to the formation of racemic amino acids. Thus, in this work, we describe the benefits of highly pure monomers in order to control nucleophilic ring-opening polymerization NCAs. Hereby, monomer purity is investigated by relating melting points of NCAs with single-crystal and powder X-ray diffraction crystallography data, which further proves retained stereo-information of NCAs.     

The role of pyridine derivatives in living carbocationic polymerization: Lewis base or nucleophile?

The living cationic polymerization of isobutylene induced by the 2-chloro-2,4,4-trimethylpentane/TiCl₄/hexane:methyl chloride (60:40, v:v)/-80°C system was studied in the presence of pyridine derivatives. Protic initiation, substantial in the absence of these additives, was virtually eliminated in their presence, and polyisobutylenes with controlled molecular weight and narrow molecular weight distribution were obtained. With some additives, however, proton elimination occurs, resulting in the exclusive formation of the exo olefin. The rate of elimination is independent of monomer concentration, i.e., it occurs during and after the polymerization. Results suggest that the proton elimination is due to the presence of an uncomplexed base, especially when complex formation with TiCl₄ is hindered by steric compression, but its approach of the polymer cation is not fully blocked.     

Borane/silane frustrated Lewis pairs for polymerization of β-substituted Michael acceptors

Frustrated Lewis pairs (FLPs) of Lewis acid (LA) B(C₆F₅)₃ and Lewis base hydrosilane [SiH] have been utilized to promote controlled polymerization of a challenging β-substituted Michael acceptor, methyl crotonate (MC), devoid of chain transfer side reactions. Mechanistic studies show that chain initiation involves LA-catalyzed 1,4-hydrosilylation of MC with [SiH] via FLP-type activation, generating a silyl ketene acetal nucleophile that participates in chain propagation via classic LA activation of monomer and a bimolecular conjugate addition mechanism. The role of the LA is conflicting in the two different catalytic cycles: the FLP activation in chain initiation requires LA-substrate (monomer) dissociation (or weak interaction) while the classic LA activation in chain propagation demands LA-monomer association (or strong interaction).     

Synthesis of β-fluoroamines by Lewis base catalyzed hydrofluorination of aziridines

Lewis base catalysis promotes the in situ generation of amine-HF reagents from benzoyl fluoride and a non-nucleophilic alcohol. The hydrofluorination of aziridines to provide β-fluoroamines using this latent HF source is described. This protocol displays a broad scope with respect to aziridine substitution and N-protecting groups. Examples of regio- and diastereoselective ring opening to access medicinally relevant β-fluoroamine building blocks are presented.     

Lewis acid–base interactions enhance explosives sensing in silacycle polymers

The high sensitivity of silole- and silafluorene-containing polymers for detecting organic nitro, nitrate, and nitramine explosives cannot be solely attributed to favorable analyte–polymer hydrophobic interactions and amplified fluorescence quenching due to delocalization along the polymer chain. The Lewis acidity of silicon in conjugated poly(silafluorene-vinylene)s is shown to be important. This was established by examining the ²⁹Si NMR chemical shifts (Δ) for the model trimer fragment of the polymer CH₃–silafluorene–(trans-C₂H₂)–silafluorene–(trans-C₂H₂)–silafluorene–CH₃. The peripheral and central silicon resonances are up-field from a TMS reference at −9.50 and −18.9 ppm, respectively. Both resonances shift down-field in the presence of donor analytes and the observed shifts (0 to 1 ppm) correlate with the basicity of a variety of added Lewis bases, including TNT. The most basic analyte studied was acetonitrile and an association constant (K _a) of 0.12 M⁻¹ was calculated its binding to the peripheral silicon centers using the Scatchard method. Spin-lattice relaxation times (T ₁) of 5.86(3) and 4.83(4) s were measured for the methyl protons of acetonitrile in benzene-d ₆ at 20 °C in the absence and presence of the silafluorene trimer, respectively. The significant change in T ₁ values further supports a binding event between acetonitrile and the silafluorene trimer. These studies as well as significant changes and shifts observed in the characteristic UV–Vis absorption of the silafluorene group support an important role for the Lewis acid character of Si in polymer sensors that incorporate strained silacycles. The nitro groups of high explosives may act as weak Lewis-base donors to silacycles. This provides a donor–acceptor interaction that may be crucial for orienting the explosive analyte in the polymer film to provide an efficient pathway for inner-sphere electron transfer during the electron-transfer quenching process. Figure   Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Cationic ring-opening polymerization of 3,3-bis（chloromethyl）oxacyclobutane in ionic liquids

Cationic ring-opening polymerization of 3,3-bis(chloromethyl)oxacyclobutane catalyzed by BF_3·OEt_2 was carried out in ionic liquids [bmim]BF_4 and [bmim]PF_6.The influences of BCMO concentration and molar ratio of BCMO/BF_3·OEt_2 on the molecular weights and yield of PBCMO were investigated.The polymerization in ionic liquids proceed to high conversions,although molecular weights are limited,similar to polymerization in organic solvent such as CH_2Cl_2.Follow a viewpoint of green chemistry, we feel ionic liquid [bmim]BF_4 is superior to [bmim]PF_6.Extracting [bmim]PF_6 from the product using organic solvent as extractant limits its advantage as a green reaction media.     

Enantioselective bromocycloetherification by Lewis base/chiral Brønsted acid cooperative catalysis

A binary catalyst system for the enantioselective bromocycloetherification of 5-arylpentenols is described. The combination of an achiral Lewis base and a chiral Br?nsted acid affords good enantioselectivities for the cyclization of Z configured 5-arylpentenols to form bromomethyltetrahydrofurans. The constitutional site selectivity is highly dependent upon the aromatic substituent and the configuration of the double bond.     

Lanthanum heterocyclic Schiff-base complex initiated ring-opening polymerization of ε-caprolactone

Lanthanum complex supported by the heterocyclic Schiff-base ligand of N-(2-pyridyl)-3,5-di-tert-butyl-salicylaldimine was prepared and employed for the ring-opening polymerization(ROP)ofε-caprolactone(ε-CL).The polymers obtained with this initiator showed a unimodal molecular weight distribution implied that only one active species was present.Mechanism study revealed that the polymerization proceeds via acyl-oxygen bond cleavage.     

Bimetallic amine-bridged bis(phenolate) lanthanide aryloxides and alkoxides: synthesis,characterization, and application in the ring-opening polymerization of rac-lactide and rac-β-butyrolactone

A series of neutral bimetallic lanthanide aryloxides p-C6H4[OLnL(THF)n]2 [Ln = Y(1), Yb(2), Sm(3)(n = 1) and La(4)(n = 2), L = Me2NCH2CH2N{CH2-(2-O–C6H2–tBu2-3,5)}2] and alkoxides p-C6H4CH2[OLnL(THF)]2 [Ln = Y(5), Yb(6)] supported by an amine-bridged bis(phenolate) ligand have been synthesized through one-pot reactions of Ln(C5H5)3(THF), LH2 with p-benzenediol and 1,4-benzenedimethanol, respectively. All complexes have been fully characterized by elemental analyses, single-crystal X-ray diffraction analysis, and IR and multi-nuclear NMR spectroscopy(in the cases of 1, 4 and 5). Study of their catalytic behavior revealed that, in general, all complexes are efficient initiators for the polymerization of rac-lactide(LA) and rac-β-butyrolactone(BBL), except for 3 and 4 in the case of BBL. The influence imposed by lanthanides of different ionic radii and initiating groups of different structures on the activity, controllability, and stereoselectivity of polymerization were systematically studied and compared. Highly heterotactic PLA(Pr up to 0.99) and syndiotactic PHB(Pr ≈ 0.81) with high molecular weight and narrow polydispersity formed and were automatically capped with hydroxyl functionality at both ends.     

Exploring electronic versus steric effects in stereoselective ring-opening polymerization of lactide and β-butyrolactone with amino-alkoxy-bis(phenolate)-yttrium complexes

A series of methoxy‐amino‐bis(phenol)s (ONOO)H₂ possessing on the phenol rings R¹ ortho substituents with variable steric and electronic properties (R¹=CMe₂Ph, 1 ; CMe₂tBu, 3 ; CMe₂(4‐CF₃C₆H₄), 5 ; CPh₃, 9 ; Cl, 10 ) has been synthesized and further reacted with [Y{N(SiHMe₂)₂}₃](THF)₂ to give cleanly the corresponding yttrium compounds [Y(ONOO){N(SiHMe₂)₂}(thf)_n] ( Y‐x ); the solid‐state structures of Y‐3 and Y‐10 have been determined. These amido complexes have been used as initiators for the ring‐opening polymerization (ROP) of rac‐lactide (LA) and rac‐β‐butyrolactone (BBL) to provide heterotactically enriched poly(lactic acid)s (PLAs) and syndiotactically enriched poly(3‐hydroxybutyrate)s (PHBs), respectively, by means of a chain‐end control mechanism. Most of these polymerizations proceeded in a controlled fashion, giving polymers with narrow polydispersities and experimental molecular weights in good agreement with calculated values. The nature of the R¹ ortho substituents has a profound impact on the rates and, more spectacularly, on the stereocontrol of the polymerizations. The heterotactic stereocontrol in the ROP of rac‐LA appears to be governed essentially by steric considerations; the larger the substituent, the higher the heterotacticity: R¹=Cl (P_r=0.56)?CMe₃ (P_r=0.80)?CMe₂Ph (P_r=0.90)2(4 CF₃‐Ph) (P_r=0.93–0.94)≤CMe₂tBu (P_r=0.94–0.95)≤CPh₃ (P_r=0.95–0.96). On the other hand, the syndiotactic stereocontrol in the polymerization of rac‐BBL follows a quite different trend: R¹=Cl (P_r=0.42–0.45)?CMe₂tBu (P_r=0.62–0.70)3 (P_r=0.80)≤CMe₂(4 CF₃? Ph) (P_r=0.82–0.84)2Ph (P_r=0.89)3 (P_r=0.94), which suggests the involvement of electronic interactions. DFT computations on model intermediates confirmed a stabilizing C? H ??? π interaction between a methylene C? H of the ring‐opened BBL unit and the π system of one of the ortho‐aryl substituents of the ONOO ligand; by contrast, for model intermediates in the ROP of LA, no such C? H ??? π interaction involving the methyl group of lactate was observed.     

A novel oxidative ring-opening reaction of isoxazolidines: syntheses of β-amino ketones and β-amino acid esters from secondary amines

A novel oxidative ring-opening reaction of isoxazolidines upon treatment with an electrophile and subsequently with a basegives β-amino ketones and β-amino acid esters highly efficiently.     

Photoinitiated polymerization of β-cyclodextrin/methyl methacrylate host/guest complex in the presence of water soluble photoinitiator, thioxanthone-catechol-O,O′-diacetic acid

β-Cyclodextrin (β-CD) was used to complex the monomer, methyl methacrylate (MMA), yielding a water-soluble host/guest complex. Photoinitiated polymerization of β-CD/MMA complex was achieved in the presence of thioxanthone-catechol-O,O′-diacetic acid (TX-Ct), a one component water soluble photoinitiator. Photodecarboxylation of TX-Ct in water seems to be an important reaction mechanism. Therefore, resulting alkyl radicals are able to initiate the polymerization of β-CD/MMA host/guest complex in water.     

Lewis acid mediated highly regioselective intramolecular cyclization for the synthesis of β-lapachone

A highly regioselective intramolecular cyclization of lapachol mediated by Lewis acids including NbCl₅, AlCl₃, and FeCl₃ was developed for synthesizing β-lapachone in excellent yields without any formation of the isomer α-lapachone. This procedure was efficient, mild, and easily scalable that avoided using highly hazardous concd H₂SO₄. In the case of ZrCl₄ the cyclization was found to give α-lapachone as the main product. A possible mechanism for the Lewis acid mediated cyclization was also discussed.     

Thermochemical study of the reactions of acid–base interaction in an aqueous solution of α-aminobutyric acid

The heat effects of the interaction between a solution of α-aminobutyric acid and solutions of HNO₃ and KОН are measured by means of calorimetry in different ranges of рН at 298.15 K and values of ionic strength of 0.25, 0.5, and 0.75 (KNO₃). The heat effects of the stepwise dissociation of the amino acid are determined. Standard thermodynamic characteristics (Δ_r H ⁰, Δ_r G ⁰, and Δ_r S ⁰) of the reactions of acid–base interaction in aqueous solutions of α-aminobutyric acid are calculated. The connection between the thermodynamic characteristics of the dissociation of the amino acid and the structure of this compound is considered.