Synthesis and characterization of ferrocene end-capped poly(ε-caprolactone)s by a combination of ring-opening polymerization and “click” chemistry techniques

Ferrocene (Fc) end-capped linear and star-shaped poly(ε-caprolactone)s (PCLs) with different numbers of arms were synthesized by a combination of ring-opening polymerization (ROP) and “click” chemistry techniques in a four-step reaction procedure. In the first step, the polymer backbones were prepared via ROP of the ε-caprolactone (ε-CL) monomer in bulk by employing the compounds with different numbers of hydroxyl groups as the multisite initiators and stannous octoate (Sn(Oct)₂) as the coordination-insertion catalyst. The hydroxyl end-groups of the obtained PCLs were then converted into a bromides and azides consecutively. In the final step, the Fc moiety was attached to the termini of the PCLs using a Cu(I)-catalyzed 1,3-dipolar cycloaddition reaction between the azide end-groups of the PCLs and the acetylene group of ethynylferrocene under ambient conditions. In all cases, the FT-IR and ¹H NMR spectra indicated a successful and quantitative transformation of the desired end-functional groups. The electrochemical properties of the Fc end-capped PCLs were also investigated via cyclic voltammetry (CV).     

Structural characterization of poly(ε-caprolactone)-grafted cellulose acetate and butyrate by solid-state C NMR, dynamic mechanical, and dielectric relaxation analyses

Investigations were made into the molecular dynamics and intercomponent mixing state in solid films of two series of cellulosic graft copolymers, cellulose acetate-g-poly(ε-caprolactone) (CA-g-PCL) and cellulose butyrate-g-PCL (CB-g-PCL), both series being prepared over a wide range of compositions with CAs or CBs of acyl DS ≈ 2.1, 2.5, and 2.95. It was shown by T_1ρ^H measurements in solid-state ¹³C NMR spectroscopy that all the copolymer samples, except ones using CA of DS = 2.98, formed an amorphous monophase in which the trunk and graft components were mixed homogeneously at least in a scale of a few nanometers. However, those copolymer samples gave, more or less, a response of dynamic heterogeneity, when examined under mechanical oscillation. Through dielectric relaxation measurements, a clear comparison was made between the CA-g-PCL and CB-g-PCL series, regarding the cooperativeness in segmental motions of the trunk and graft chains, directly associated with the extent of the dynamic heterogeneity. The cooperativeness was generally higher in the CB-based copolymer series, probably due to working of the butyryl substituent as an internal compatibilizer.     

A novel shape memory polynorbornene functionalized with poly(?-caprolactone) side chain and cyano group through ring-opening metathesis polymerization

A novel functionalized norbornene-based copolymer with long poly(?-caprolactone) side chain and cyano group was synthesized in the combination of ring-opening polymerization and ring-opening metathesis polymerization, which was characterized by means of ¹H NMR, gel permeation chromatography, thermogravimetry, differential scanning calorimetry, wide-angle X-ray diffraction, and stress-strain measurements. The shape memory effect of the copolymer was evaluated by dynamical mechanical analysis, shape recovery ability, and shape recovery speed. Having the properties of the good shape fixity and the large shape recovery, the functionalized polynorbornene copolymer is expected to use as a potential shape memory material.     

Synthesis, characterization and photocatalytic activity of β-Ga2O3 nanostructures

Nanostructured β-Ga₂O₃ samples were prepared by a combination of the solvothermal processes and subsequent heat treatments. β-Ga₂O₃ samples with various morphologies were obtained by using different kinds of solvent, including water, isopropanol and ethylene glycol. One-dimensional β-Ga₂O₃ nanorods were obtained in water medium, while β-Ga₂O₃ spheres were prepared in alcohol. The possible mechanism related to the phase formation and morphology of the as-prepared materials was discussed. Photocatalytic performance of the synthesized β-Ga₂O₃ samples was also examined. Results revealed that β-Ga₂O₃ sample prepared with ethylene glycol showed the highest photocatalytic activity for the degradation of salicylic acid. This could be ascribed to the high surface area, abundant hydroxyl groups, and wide band gap of β-Ga₂O₃ sample synthesized in ethylene glycol.     

Synthesis and characterization of a novel graft copolymer with poly(n-octylallene-co-styrene) backbone and poly(?-caprolactone) side chain

A novel graft copolymer consisting of poly(n-octylallene-co-styrene) (PALST) as backbone and poly(?-caprolactone) (PCL) as side chains was synthesized with the combination of coordination copolymerization of n-octylallene and styrene and the ring-opening polymerization (ROP) of ?-caprolactone. Poly(n-octylallene-co-styrene) (PALST) backbone was prepared from the copolymerization of n-octylallene and styrene with high yield by using the coordination catalyst system composed of bis[N,N-(3,5-di-tert-butylsalicylidene)anilinato]titanium(IV) dichloride (Ti(Salen)₂Cl₂) and tri-isobutyl aluminum(Al(i-Bu)₃). The molar ratio of each segment in the copolymer, and the molecular weight of the copolymer as well as the microstructure of the copolymer could be adjusted by varying the feeding ratio of both styrene and n-octylallene. The hydroxyl functionalized copolymer PALST-OH was prepared by the reaction of mercaptoethanol with the pendant double bond of PALST in the presence of radical initiator azobisisbutyronitrile (AIBN). The target graft copolymer [poly(n-octylallene-co-styrene)-g-polycaprolactone] (PALST-g-PCL) was synthesized through a grafting-from strategy via the ring-opening polymerization using PALST-OH as macroinitiator and Sn(Oct)₂ as catalyst. Structures of resulting copolymer were characterized by means of gel permeation chromatography (GPC) with multi-angle laser light scattering (MALLS), ¹³C NMR, ¹H NMR, DSC, polarized optical microscope (POM) and contact angle measurements.     

Fluorinated ?-caprolactone: Synthesis and ring-opening polymerization of new α-fluoro-?-caprolactone monomer

Fluorinated polyester was synthesized from a novel α-fluoro-?-caprolactone monomer (α-FCL). The monomer was synthesized from commercially available cyclohexene oxide in three steps. Baeyer-Villiger reaction using 3-chloroperoxybenzoic acid of the corresponding 2-fluorocyclohexanone resulted in formation of two isomeric lactones α-and-?-fluoro-?-caprolactones in 1:1.2 molar ratio, respectively. Poly(α-fluoro-?-caprolactone) was synthesized by bulk ring-opening polymerization of α-fluoro-?-caprolactone monomer (α-FCL) catalyzed by stannous octanoate, Sn(Oct)₂ at 120 °C for 6 h. Relationships between reaction time, polymer yield, and molecular weight were established. The ring-opening polymerization of ?-fluoro-?-caprolactone (?-FCL) produce geminal fluorohydrin, which is not stable and it is subsequently dehydrofluorinated to give the corresponding aldehyde. Copolymerization of α-FCL with ?-caprolactone (?-CL) in various feed ratios was also investigated. Detail microstructure analyses of the copolymers were accomplished from ¹H, ¹³C and 2-D NMR data. Presence of fluorine atoms is expected to regulate the chemical and physical properties of the polyester.     

Oxidative degradation of poly (vinyl acetate) and poly (ε-caprolactone) and their mixtures in solution

The oxidative degradation of the poly (ε-caprolactone) (PCL), poly (vinyl acetate) (PVAC) and their mixtures in dichlorobenzene has been investigated at various temperatures (70-130°C) in the presence of benzoyl peroxide. The interaction between the polymers is quantified by monitoring the molecular weights of individual polymers using gel-permeation chromatography. The various physical mixtures employed in the present investigation are , and wt%/wt% PCL/PVAC. Experimental data indicated that the degradation is random without cross-linking and repolymerization. An optimum in degradation temperature (corresponding to maximum degradation rate) of 105°C was observed for the entire range of polymer compositions (0-100% PCL) investigated. This optimum temperature of degradation is characteristic mostly of the initiator and only to a small extent of the degrading polymer system. The experimental results of the mixtures indicated that the degradation rates of PVAC are significantly enhanced, while the degradation rates of PCL are decreased in the physical mixture. This can be attributed to the proton-accepting and proton-donating nature of PCL and PVAC, respectively. A radical mechanism for the oxidative degradation of pure polymers and their mixtures has been proposed and a model based on continuous distribution kinetics was developed considering the interaction of the polymers through hydrogen abstraction and the parameters were evaluated numerically. The activation energies for the peroxide attack for the PCL and PVAC are 10.5 and , respectively. The activation energies for the random chain scission of PCL and PVAC are 10.6 and , respectively.     

Conducive 3D porous mesh of poly(ε-caprolactone) made via emulsion electrospinning

Use of Poly(ε-caprolactone) (PCL) as 3D porous scaffold, fibers and matrices has proved importance of this polymer in applications for tissue engineering besides others. Here we present an approach to generate uneven surfaced meshes of PCL via emulsion electrospinning with minimal use of organic solvent. Poly(vinyl alcohol) (PVA) was used as template polymer providing stability and alignment to PCL phase during electrospinning of oil-in-water emulsion of PCL. The emulsion properties including particle size, inter-particle distance and viscosity depended on the concentrations of PCL and PVA. Higher PVA content led to formation of smaller oil phase particles resulting into higher viscosity of the emulsion while a higher PCL content led to the formation of larger oil phase particles and correspondingly lower viscosity of the emulsion. A correlation between particle size of emulsion and diameter of the fibers obtained after electrospinning was found. The composite meshes of PCL-PVA obtained via emulsion electrospinning were washed with water to generate uneven surface on the meshes which was found to be highly favorable for cell growth in comparison to a uniform mesh of PCL made via solution electrospinning.     

Isoconversional kinetic analysis of ring-opening polymerization of ε-caprolactone: Steric influence of titanium(IV) alkoxides as initiators

Four titanium(IV) alkoxides, namely: Ti(IV) n-propoxide (1), Ti(IV) n-butoxide (2), Ti(IV) tert-butoxide (3), and Ti(IV) 2-ethylhexoxide (4), have been used as initiators in the bulk ring-opening polymerization (ROP) of ε-caprolactone (ε-CL). The influence of the alkoxide group on the course of the ROP of ε-CL was investigated by means of ¹H-NMR and differential scanning calorimetry (DSC). The ¹H-NMR spectra confirmed that the ROP reaction of ε-CL proceeded via the widely accepted coordination-insertion mechanism for each of the four initiators. Isoconversional methods have been used to evaluate non-isothermal DSC data via the equations of Friedman, Kissinger-Akahira-Sunose (KAS) and Ozawa-Flynn-Wall (OFW). The kinetic studies showed that the polymerization rate for the four initiators (1-4) was in the order of 1 > 2 ≈ 4 > 3. The lowest activation energies (40–47, 42–44, and 49–52 kJ/mol for the Friedman, KAS and OFW methods respectively) were found in the polymerizations using Ti(IV) n-propoxide (1), while the highest activation energies (84–107, 77–87, and 80–91 kJ/mol for the Friedman, KAS and OFW methods respectively) were obtained using Ti(IV) tert-butoxide (3). Differences in the rates of polymerization and the activation energies amongst the four initiators appeared to be governed mainly by the different degrees of steric hindrance in the initiator structure. These results represent important findings regarding the steric influence of the alkoxide groups on the kinetics of the ROP of ε-CL initiated by titanium(IV) alkoxides.     

Structural characterization of the δ-clathrate forms of syndiotactic polystyrene with n-alkanes

The δ-clathrate forms of syndiotactic polystyrene (s-PS) containing a homologous series of guests characterized by an increasing molecular volume (n-alkanes ranging from n-pentane to n-dodecane) have been investigated. The present analysis allows concluding that the volume of the guest plays a crucial role in the choice of the type of δ-clathrate that can be obtained. As a matter of fact, longer n-alkanes, such as n-octane, n-nonane or n-decane, turned out to give rise triclinic δ-clathrates presenting a guest/monomeric-unit ratio of 1/8, while δ-clathrates containing shorter alkanes, like n-hexane or n-heptane, instead, present a monoclinic or a triclinic structure with a guest/monomeric-unit ratio of 1/4. These results confirm the validity of a simple criterion to predict the type of s-PS δ-clathrate with a certain guest on the basis of the experimental d₀₁₀, as described in Macromolecules 2010, 43, 8549-8558. Moreover, our analysis confirms previous solid state ¹³C NMR and Raman spectroscopy data, indicating that shorter n-alkanes molecules, like n-hexane, are hosted in the cavities with an all-trans conformation while longer alkanes, like n-nonane, present bent conformations. The effect of n-alkanes molecules on the crystallization of s-PS amorphous samples has been investigated too.     

Novel 5-benzazolyl-10,15,20-triphenylporphyrins and β,meso-benzoxazolyl-bridged porphyrin dyads: Synthesis, characterization and photophysical properties

Novel 5-benzazolyl-10,15,20-triphenylporphyrins and β,meso-benzoxazole-linked diporphyrins were synthesized through La(OTf)₃ catalyzed reaction of newly prepared 5-(3,4-diaminophenyl)-10,15,20-triphenylporphyrin or 5-(3-amino-4-hydroxyphenyl)-10,15,20-triphenylporphyrin with aromatic aldehydes in 1,2-dichlorobenzene. On metalation with zinc acetate, freebase β,meso-benzoxazole-linked diporphyrin was successfully converted to the Zn-Zn diporphyrin complex in good yield. The synthesized porphyrin analogues were characterized using electronic absorption, IR and ¹H NMR spectroscopy in addition to mass and elemental analyses. The fluorescence studies of 5-benzazolyl-10,15,20-triphenylporphyrins showed efficient intramolecular energy transfer from the pyrene and fluorene subunits to the porphyrin core. In addition, the fluorescence quenching observed in β,meso-benzoxazolyl-bridged porphyrin dyads was attributed to the possible nonplanarity of a component of the diporphyrins. The freebase-Ni diporphyrin complex underwent strong emission quenching in comparison to that of freebase diporphyrin and dizinc diporphyrin analogues.     

Synthesis and characterization of amphiphilic PLA-(PαN3CL-g-PBA) copolymers by ring-opening polymerization and click reaction

The present study prepared novel amphiphilic block-graft PDLLA-b-(PαN₃CL-g-PBA) and PLLA-b-(PαN₃CL-g-PBA) functional polyesters, containing a hydrophilic poly(α-azido-ε-caprolactone-graft-alkyne) (PαN₃CL-g-alkyne) segment and a hydrophobic poly(dl-lactide) (PDLLA) or poly(l-lactide) (PLLA) segment, using ring-opening polymerization of α-chloro-ε-caprolactone (αClCL) with a hydroxyl-terminated macroinitiator of PDLLA or PLLA, substituting pendent chloride with sodium azide. The copolymers were subsequently used for grafting of 2-propynyl-terminal benzoate moieties by way of Cu(I)-catalyzed Huisgen's 1,3-dipolar cycloaddition, thus producing a “click” reaction. Differential scanning calorimetry (DSC) and ¹H NMR, FT-IR, and GPC examined the characteristics of the copolymers. The critical micelle concentration (CMC) ranged from 2.7 mg L^?1 to 24.6 mg L^?1 at 25 °C and the average micelle size ranged from 106 nm to 297 nm. The length of the hydrophilic segment and the configuration of the lactide both influenced micelle stability. The micelle of PLLA-b-(PαN₃CL-g-PBA) provided high drug entrapment efficiency and loading content. The results from in vitro cell viability assays indicated that PLA-b-(PαN₃CL-g-PBA) shows low cytotoxicity.     

Inserting polyoxomolybdate cluster into poly(?-caprolactone) to create a class of new heteropolymer: Synthesis and supramolecular structures

In the present work, we report our work concerning the synthesis of heteropolymers of poly(?-caprolactone)-polyoxomolybdate-poly(?-caprolactone) using ring-opening polymerization (ROP) of ?-caprolactone monomer initiated by two hydroxyl groups attached on the two sides of a polyoxomolybdate cluster which was organically modified using pentaerythritols. Since the hydroxyl groups on polyoxomolybdate macroinitiator are less reactive, ring-opening polymerization were finally carried out by using 4,4′-(dimethylamino) pyridine to activate ?-caprolactone monomer. The polyoxomolybdate-containing heteropolymers were confirmed in detail by ¹H NMR, FT-IR, GPC, TGA and XPS methods. Although the size of polyoxomolybdate cluster is extremely small compared to PCL chains, it was also found that POM played an important role in crystallization process. Self-assembly of such heteropolymers in mixed solvents were investigated and the analysis of formed spherical aggregates with multiple concaves on the surface was supported by TEM results and 3D electron tomography.     

Synthesis,densification and characterization of TaB2–SiC composites

The TaB₂–27.9 vol% SiC composite was synthesized by self-propagating high-temperature synthesis starting from mechanically activated Ta, B₄C and Si reactants. The obtained powders were spark plasma sintered at 1800 °C and 20 MPa for 30 min total time, thus obtaining a 96% dense product. The latter one was characterized in terms of microstructure, hardness, fracture toughness, and oxidation resistance. The obtained results, particularly the fracture toughness, are promising when compared to those related to analogous materials reported in the literature and fabricated with similar and different processing routes.     

Microstructural and electrical characterization of bulk YBa2Cu3O7−δ ceramics

The value of critical current density at 77 K in “zero” applied field (J_c) characterizing the superconducting state for YBa₂Cu₃O_7−δ ceramics is closely related to the microstructure.The interrelationships between the microstructural factors such as pore volume fraction, oxygen content, average grain size, are complex. However, these factors also influence the normal state resistivity measured at room temperature (ρ₃₀₀). We demonstrate how the current carrying cross section influences Jc and ρ₃₀₀ in a similar way. Data, reported for two classes of YBa₂Cu₃O_7−δ: small grain porous ceramics and larger-grain denser ceramics, reveal an approximate linear relation between ρ_300 K and J_c. Extrapolation of this relation to a fully dense small grain YBa₂Cu₃O_7−δ ceramic yields values of ρ₃₀₀ = 0.4 mΩ cm and J_c = 10³ A cm⁻².     

Synthesis and characterization of four- and six-arm star-shaped poly(ε-caprolactone)-b-poly(N-vinylcaprolactam): Micellar and core degradation studies

Star-shaped copolymers with four and six poly(ε-caprolactone)-block-poly(N-vinylcaprolactam) (S(PCL-b-PNVCL)) arms were successfully synthesized by combining ring opening polymerization (ROP) of ε-caprolactone (CL) and reversible addition-fragmentation chain transfer (RAFT) polymerization of N-vinylcaprolactam (NVCL). The resulting star copolymers were characterized using ¹H NMR, GPC and UV–vis. The numbers of arms in the star-shaped PCL-b-PNVCL block copolymers were demonstrated using degradation studies under acidic conditions, and the individual PNVCL chains were characterized by GPC and ¹H NMR. In aqueous solution, star-shaped PCL-b-PNVCL block copolymers self-assembled into large aggregates or micelles with sizes varying from 54 to 300 nm, depending on the molecular weight of the copolymer and the relative lengths of the hydrophobic and hydrophilic segments. Micelles were characterized by atomic force microscopy (AFM), dynamic light scattering (DLS) and scanning electron microscopy (SEM).     

Sintering,microstructure and conductivity of La2NiO4+δ ceramic

Microstructure and electrical conducting properties of La₂NiO_4+δ ceramic were investigated in the sintering temperature range 1200–1400 °C. The results demonstrate that the microstructure and electrical conducting properties of La₂NiO_4+δ ceramic are sensitive to sintering temperature. Compared with a progressive densification development with sintering temperature from 1200 to 1300 °C along with an insignificant change in grain size, there is an exaggerated grain growth in the specimens sintered at higher temperatures. Increasing sintering temperature from 1200 to 1300 °C resulted in an enhancement of electrical conducting properties. Further increase of sintering temperature exceeding 1300 °C reduced the electrical conducting properties. A close relation between the microstructure and electrical conducting properties was suggested for La₂NiO_4+δ ceramic. With respect to the electrical conducting properties, the preferred sintering temperature of La₂NiO_4+δ ceramic was ascertained to be 1300 °C. The specimen sintered at 1300 °C exhibits a generally uniform microstructure together with electrical conductivities of 76–95 S cm⁻¹ at 600–800 °C.     

Polymerization of β-cyclodextrin with 1,2,3,4-butanetetracarboxylic dianhydride: Synthesis,structural characterization,and bisphenol A adsorption capacity

Water-soluble and water-insoluble β-cyclodextrin polymers (β-CDPs) were prepared from β-CD using 1,2,3,4-butanetetracarboxylic dianhydride (BTCA) as a cross-linker. The structures of the synthesized water-soluble and water-insoluble β-CDPs were characterized in detail using FT-IR, solution- and solid-state NMR, and ¹H diffusion-ordered spectroscopic techniques. The water solubility of β-CDP was dependent on the reaction temperature as well as the initial feed ratio of β-CD and BTCA. The BTCA content and the ratio of BTCA molecules cross-linked and grafted to β-CD were also found to influence the water solubility of the polymers. Insoluble β-CDPs were effective in adsorbing bisphenol A (BPA) from aqueous media, with this absorption being highest in materials of low BTCA content, which corresponds to material of high β-CD content per unit mass of β-CDP.     

Effect of polymer topology on the curing process and mechanical characteristics of epoxy thermosets modified with linear or multiarm star poly(ε-caprolactone)

A well-defined multiarm star copolymer, hyperbranched poly(glycidol)-b-poly(ε-caprolactone), with an average of 100-110 arms per molecule and a molecular weight of arms of 1000 g/mol (s-PCL) and a linear PCL analog (l-PCL) were used as modifiers in the curing of diglycidylether of bisphenol A (DGEBA) using ytterbium triflate as cationic initiator. The effect of the polymer topology on the curing and gelation was studied by dynamic scanning calorimetry (DSC) and rheometry. The addition of s-PCL to the resin left the complex viscosity (η∗) practically unaltered. In contrast the addition of l-PCL incremented substantially the viscosity. The addition of star-shaped modifiers decreased the shrinkage after gelation in a higher extent than the linear analog. The homogeneity of pure DGEBA and modified thermosets was proved by dynamic thermomechanical analysis (DMTA) and electronic microscopy (SEM). The addition of star-like structures led to a higher impact energy fracture in comparison to pure DGEBA and l-PCL modified thermosets and to a lower effect on the microhardness than the linear analog.