Types Of Copolymers Research Articles

Poly(3-hexylthiophene) Grafting and Molecular Dilution: Study of a Class of Conjugated Graft Copolymers.

A type of graft copolymer based on polysiloxane and regioregular poly(3-hexylthiophene) (P3HT) has been synthesised and its properties have been studied alongside those of its parent conjugated polymer—regioregular P3HT. Electrochemical analysis has revealed more significant changes in conformation of the copolymer film than was observed for P3HT. UV-Vis-NIR spectroelectrochemical investigation provided evidence of improved doping reversibility of the copolymer, despite its marginally increased band gap, as also confirmed by electroconductometric analysis. Evidence has been shown, indicating that polaron mobilities in both P3HT and the copolymer are higher than those of bipolaronic charge carriers, even though both systems exhibit standard doping/dedoping patterns. The grafted copolymer was tested in bulk heterojunction solar cells. Preliminary studies show a great potential of these polymers for application in photovoltaics. Power conversion efficiency of up to 2.46% was achieved despite the dilution of the P3HT chains in the copolymer.

Influence of poly(ε-caprolactone) end-groups on the temperature-induced macroscopic gelation of Pluronic in aqueous media

Temperature-induced gelation of amphiphilic copolymers in aqueous media has attracted a great deal of interest in recent years. We have investigated phase behavior, gelation, rheology, and structure of modified versions of the water-soluble copolymer of type PEO-PPO-PEO (Pluronic, F127) with short (PCL(5)) or long (PCL(11)) poly(caprolactone) blocks at both ends. By visual inspection of the studied semidilute and concentrated polymer samples at various temperatures, gelation and phase separation could be mapped and the length of the PCL blocks had a strong impact on the observed features. By using oscillatory shear experiments, the gel points of the systems were determined and an interesting anomaly of the complex viscosity was observed at high frequencies where the moieties were kinetically arrested. This irregularity was strengthened for the PCL(11) copolymer and this was ascribed to bridging of the micelles due to the long PCL blocks. The mesoscopic structure of the systems was studied by using small angle neutron scattering. The position and amplitude of the observed correlation peak, which discloses vital intermicellar correlations in the scattering function depends on temperature, concentration, and the length of the PCL blocks. Frequently the scattering function could be portrayed by a spherical core-shell micelle model with hard sphere interaction between them. The estimated aggregation number (Nagg) was substantially affected by the PCL content of the polymer and a significant influence of polymer concentration and temperature on the value of Nagg was found.

Impact of trifluoromethyl groups on the control of surface and optical properties of poly(methyl methacrylate)

Poly(methyl methacrylate) (PMMA) is a widely used polymer, but its surface properties are often not ideal for coating applications. Here, a wide range of surface and optical properties of PMMA were achieved by controlling the copolymer type, poly(trifluoroethyl methacrylate) (PTFEMA)-X, where X is the feed ratio (mol %) of trifluoroethyl methacrylate (TFEMA), where TFEMA includes hydrophobic –CF3 moiety. Four different PTFEMA-X samples (i.e. PTFEMA-5, -10, -20, and -40) were prepared by changing the monomer ratio between MMA and TFEMA. The surface energies, surface compositions, transmittances, and refractive indices of the samples were compared to those of PMMA and PTFEMA. The results showed a linear relationship between these properties and TFEMA content, demonstrating a potential method for controlling the surface and optical properties of PMMA. This is interesting as the only difference between MMA and TFEMA is that TFEMA has –CF3 moiety.

Facile preparation of palladium catalyst supported on silica nanoparticles through CS2 plasma pretreatment: Application to the Heck reaction for vinyl monomers and π-conjugated polymer synthesis

The preparation of a recyclable palladium complex supported on coated silica nanoparticles (SiNP) and its effectiveness as a catalyst in the Heck reaction is described. Functionalized silica-based NP were synthesized in three steps. First, commercial SiNP were subjected to a CS2 cold plasma. Thus, a thin coating film was formed, along with the creation of a thiol functional surface. Subsequently, a thiol-ene reaction with 2-vinylpyridine induced by AIBN produced (Py)-CH2-CH2S covalently bonded to the SiNP, which after PdCl2(CH3CN)2, ligand exchange afforded the corresponding palladium complex catalyst (Pd-SiNP). Functionalized Pd-SiNP were characterized by SEM, ICP, and XPS spectroscopy. The SiNP were straightforwardly converted to an effective palladium catalyst applicable in a number of Heck cross-coupling reactions with iodobenzene (IBz) and a variety of vinyls. Indeed, such heterogeneous catalyst probed to be recyclable (up to 9 times). Nevertheless, leaching of metal was evidenced by TEM, HRTEM, XRD, and XPS. Furthermore, due to the outstanding results, we extended the use of the new heterogeneous catalyst to achieve the synthesis of a soluble phenylenevinylene type (PPV) copolymer from 1,4-bis(dodecyloxy)-2,5-diiodobenzene and p-divinylbenzene (DVB).

Memory capacitance behavior at single resistance state in memristor and multi-state characteristic

With the advent of the information age, big data put forward higher requirements for capacity of storage devices. Compared with the method of reducing the size of the device to enhance the integration level, the high density storage of increasing the memory state of the single device will be very beneficial to solving the problem. In this work, we propose an idea of two-parameter and multi-state memory device involved in both resistance and capacitance operation levels. At first, a new donor-acceptor (D-A)-type copolymer is designed and synthesized. Then, the memory device of Al/copolymer/ITO structure is fabricated, and the current-voltage (<i>I-V</i>) and capacitance-voltage (<i>C-V</i>) curves are measured by a KEITHLEY 4200 semiconductor characterization system. The device not only displays the obvious memory resistance characteristics, but also has the memory capacitance behavior in single resistance state, which results in two resistance states and four capacitance states, so that the device has the capability of two-parameter and multi-state memory. In addition, the device shows more capacitance states after the switching behavior has been modulated by the voltage amplitude, which provides an effective method to control the memory states. In order to study the conductive mechanism of the device, we test the relationship between resistance and temperature. It is found that the resistance decreases with the increase of temperature, indicating that the device has the obvious semiconductor properties. Furthermore, the fitting results of <i>I-V</i> data show that the mechanism of resistance switching is in good consistence with the classical trap-controlled space charge limited current theory. The capacitance switching in single resistance state is closely related to the polarization characteristic of D-A structure in the copolymer film. The polarization force microscopy phase image shows that the copolymer film has obvious polarization and depolarization characteristics under the external electric field. Based on the polarization characteristics of copolymer, the correlation between memory resistance and memory capacitance is established by introducing a polarization operator of molecules, and the matrix model describing the two-parameter and multi-state memory characteristics is given. The above results show that the multi-state memory characteristics will store more information than 2-bits mode in a single cell, which will provide a reference for improving the storage density of information.

Double acceptor block-based copolymers for efficient organic solar cells: side-chain and π-bridge engineered high open-circuit voltage and small driving force

Different from the traditional concept of donor–acceptor (D–A) structures, the newly reported acceptor1–acceptor2 (A1–A2) type copolymers has been regarded as an efficient approach to boost the open circuit voltage (Voc).

Study of the perpendicular self-assembly of a novel high-χ block copolymer without any neutral layer on a silicon substrate.

A novel type of high-χ block copolymer, polystyrene-block-polycarbonate (PS-b-PC), which contains an active –NH– group on the polymer backbone between the PS block and the PC block, has been successfully synthesized. Vertical micro-phase separation can be successfully achieved on Si substrates with neutral-layer-free materials with a pitch of 16.8 nm. Water contact angle experiments indicate that PS and PC have approximate surface energy values on Si substrates. A hydrogen bond mechanism has been proposed for the formation of a periodic and lamella-forming phase separation structure, with the domains oriented perpendicular to the substrate. A combination of both theory and experimental verification proves that the hydrogen bonding plays a dominant role as a real driving force to promote vertical micro-phase separation in the absence of a neutral layer. Subsequently, the study of a novel block copolymer on four different types of substrate without any neutral layer further confirms that the newly synthesized material enables greater flexibility and potential applications for the fabrication of various nanostructures and functional electronic devices in a simple, cost-effective and efficient way, which is of considerable importance to contemporary and emerging technology applications.

Mesoporous N-doped graphene prepared by a soft-template method with high performance in Li-S batteries.

Lithium sulfur (Li-S) batteries, which have high theoretical capacity, are promising candidates for energy storage systems; however, several obstacles, including insulating nature and volumetric expansion of sulfur as well as shuttle effects of polysulfides, impede the commercialization of these batteries. Herein, we utilized mesoporous N-doped graphene (NGM) as a sulfur host, which was synthesized using a type of triblock copolymer, Pluronic F127 (PF127), as a soft template. PF127 not only acted as a spacer to prevent the graphene layers from restacking during the hydrothermal reaction, but also resulted in the formation of high-density wrinkles on the surface of the graphene sheets after annealing. The crumpled NGM with a nitrogen doping content of 4.80 at% had the high specific area and total pore volume of up to 958.72 m2 g-1 and 2.39 cm3 g-1, respectively, providing space for the accommodation and uniform distribution of sulfur in the cathode. Therefore, the sulfur content was increased to 87.2 wt% in the graphene sulfur composite, achieving the discharge capacity of 492.2 mA h g-1TE at 1.08 A g-1TE with a low capacity decay rate of 0.12% per cycle after 400 cycles. Thus, this study provides an effective strategy for enhancing the specific surface area and pore volume of graphene to develop Li-S batteries with high performance.

Photophysical and Photovoltaic Characterization of Flourene-Anthracene-Benzothiadiazole Based Donor–Acceptor Type Copolymers for Bulk Heterojunction Polymer Solar Cells

Photophysical and Photovoltaic Characterization of Flourene-Anthracene-Benzothiadiazole Based Donor–Acceptor Type Copolymers for Bulk Heterojunction Polymer Solar Cells

Characterization and Synthesis of This Comb Type Copolymer with Styrene Using A Macromonomer Containing Polyethylmethacrylate

In this study, firstly 7-hydroxy-4-chloromethyl coumarin (CMHC) was synthesized from reaction of ethyl 4-chloroacetoasetate with resorcinol. Then, the poly(ethyl methacrylate) coumarin end grouped having hydroxyl (CEMA) was prepared by ethyl methacrylate with using (CMHC) as initiator by atom transfer radical polymerization (ATRP) method. A macromonomer (CMEMA) was synthesized from reaction of methacryloyl chloride and poly(EMA) ended coumarin having OH group (CEMA). Molecular brush of P(CMEMA-comb-%16St) was synthesized by reaction of macromonomer CMEMA and styrene by free radical polymerization method (SRP). The structures of the prepared macromonomer and molecular brush were characterized by FT-IR and 1H-NMR techniques. The thermal behavior of P(CMEMA-comb-%St) has been investigated by TGA, and the glass transition temperatures have been measured on DSC. The average molecular weights and polydispersity were determined by GPC. The dielectric behavior of P(CMEMA-comb-%16St) was investigated as a function of temperature and frequency. The intrinsic viscosity [] of P(CMEMA-comb-%16St) was determinated.

Impact of Precise Control over Microstructure in Thiophene–Selenophene Copolymers

Controlling the sequence of repeat units in a synthetic polymer has been a long-standing topic of interest in chemistry. As methods to regulate sequence become more sophisticated, it is critical to consider how controlling the arrangement of repeat units along the polymer backbone impacts properties. In this work, thiophene–selenophene copolymers (statistical and periodic) were compared to elucidate the impact of periodicity on electronic properties and structural organization in conjugated macromolecules. Polymers were synthesized using catalyst-transfer polycondensation (CTP) enabling control over molecular weight and dispersity. The study revealed that optical bandgaps and redox potentials of periodic and statistical copolymers varied with composition in a predictable manner, regardless of monomer ordering along the chain. While the bandgaps of the two types of copolymers were indistinguishable, X-ray scattering revealed differences in solid-state packing. Both types of copolymers exhibited well-define...

Fluorocarbon-Free Dual-Action Textile Finishes Based on Covalently Attached Thermoresponsive Block Copolymer Brush Coatings.

We present fluorocarbon-free block copolymer brushes as potential systems for dual-action, i.e., soil-repellent and soil-releasing textile finishes. Polymer brushes were prepared by employing specifically engineered triblock copolymers consisting of a hydrophobic, a hydrophilic, and either a central or a terminal anchor block bearing several anchoring groups for sustainable immobilization using the "grafting to" approach on both flat Si wafers and rough cotton fabrics. The switching characteristics of both types of block copolymer brushes were investigated by exposing the brushes to conditions and stimuli that are similar to those applied during laundering in a washing machine and drying in a laundry dryer, respectively. Contact angle measurements were performed to evaluate the polarity and wettability of the block copolymer brushes after treatment in hot water and in air, or in a vacuum at elevated temperatures simulating the washing and the drying procedure of a textile fabric. While the block copolymer brush with the terminal anchor showed only minor changes in terms of the wetting characteristics and the brush morphology upon the applied stimuli, the block copolymer brush with the central anchoring block exhibited a significant change from a hydrophilic (soil-releasing) to a hydrophobic (stain-repellent) surface. This switching behavior was reversible and could be achieved on both, flat Si wafers, and much more pronounced on rough cotton fabrics. Atomic force microscopy and angle-resolved X-ray photoelectron spectroscopy investigations further indicated a complete rearrangement of the brush morphology. Accordingly, we regard this type of block copolymer brushes as a system that fully meets the basic requirements for an application as a dual-action textile finish, which can be reversibly switched with respect to water repulsion.

Acceptor Unit Effects for Ambipolar Organic Field-Effect Transistors Based on TIPS-Benzodithiophene Copolymers

Two narrow band gap triisopropylsilyl substituted benzo[1,2-b:4,5-b] dithiophene (TIPS-BDT) derivatives, P1 (1.65 eV) and P2 (1.46 eV) are synthesized for ambipolar organic field-effect transistors and complementary inverters. Two electron acceptor units, heptadecanyl substituted thieno[3,4-c]pyrrole-4,6-dione (TPD) and ethylhexyl substituted diketopyrrolo[3,4-c]pyrrole (DPP) are incorporated to tune the structure and resulting properties of the donor-acceptor type copolymers. Structural modification based on the acceptor unit variation, resulted in comparable electrochemical, optical, microstructural, and charge transporting properties, as well as environmental and operational stability. TIPS-BDT copolymers with TPD acceptor units show comparatively superior performance, with field effect mobility ∼10-3 cm2V-1s-1 for both holes and electrons and inverter gain ∼18 with poly(methyl methacrylate) gate dielectric.

Multifunctional approaches utilizing polymeric micelles to circumvent multidrug resistant tumors.

The concerns impeding the success of chemotherapy in cancer is descending efficacy of drugs due to the development of multiple drug resistance (MDR). The current efforts employed to overcome MDR have failed or are limited to only preliminary in-vitro investigations. Nanotechnology is at the forefront of the drug delivery research, playing pivotal role in chemotherapy and diagnosis, thereby providing innovative approaches for the management of the disease with minimal side effects. Recently, polymeric micelles (PMs) have witnessed significant developments in cancer therapy. PMs are self-assembled colloidal particles, with a hydrophilic head and a long hydrophobic tail, which enhance the solubility, permeability and bioavailability of drugs, due to the unique features of reaching higher concentration in the biological system, above critical micellar concentration. One of the effective approaches to improve the efficacy of chemotherapy and overcome drug resistance would be to employ multifunctional approach (combination of stimuli-responsive, utilization of drug resistance modulators and combination therapy) using PMs as drug delivery systems. Actively targeted, stimuli-sensitive and multifunctional approaches involve using single and/or combination of approaches (pH-responsive, temperature regulated, reduction-sensitive, ultrasound etc.) to combat drug resistant. The review will describe PMs, types of copolymers used in PMs, preparation and characterization of PMs. A comprehensive list of PMs tested in clinical trials is discussed. Lastly, this review covers stimuli-sensitive and multifunctional approaches to overcome MDR in cancer utilizing PMs.

Poly[oligo(2-ethyl-2-oxazoline)acrylate]-Based Poly(ionic liquid) Random Copolymers with Coexistent and Tunable Lower Critical Solution Temperature- and Upper Critical Solution Temperature-Type Phase Transitions.

The synthesis of a series of dual thermosensitive nonionic-ionic random copolymers with varying compositions by reversible addition-fragmentation chain transfer polymerization is described. These copolymers contain oligo(2-ethyl-2-oxazoline)acrylate (OEtOxA) and either triphenyl-4-vinylbenzylphosphonium chloride ([VBTP][Cl]) or 3- n-butyl-1-vinylimidazolium bromide ([VBuIm][Br]) ionic liquid (IL) units. The copolymers having low content of ionic poly(ionic liquid) (PIL) (P[VBTP][Cl]/P[VBuIm][Br]) segments show only lower critical solution temperature (LCST)-type phase transition with almost linear increase of their cloud points with increasing percentage of ionic PIL segments. Furthermore, LCST-type cloud points ( TcLs) are found very sensitive and tunable with respect to the nature and concentration of halide ions (X- = Cl-, Br-, and I-) and copolymer compositions. However, copolymers with high content of ionic PIL segments show both LCST-type followed by upper critical solution temperature (UCST)-type phase transitions in the presence of halide ions. Dual LCST- and UCST-type phase behaviors are prominent and repeatable for many heating/cooling cycles. Both types of cloud points are found to be sensitive to copolymer compositions, concentration, and nature and concentration of the halide ions. The phase behaviors of both types of copolymers with a very high ionic content (>90%) are exactly similar to that of P[VBTP][Cl] or P[VBuIm][Br] homopolymers showing only UCST-type phase transition in the presence of halide ions. The inherent biocompatibility of the P(OEtOxA) segment along with the interesting dual thermoresponsiveness makes these copolymers highly suitable candidates for biomedical applications including drug delivery.

Investigation of the Effects of Non-Conjugated Co-Grafts on the Spectroelectrochemical and Photovoltaic Properties of Novel Conjugated Graft Copolymers Based on Poly(3-hexylthiophene).

A new type of polysiloxane copolymers, with conjugated–regioregular poly(3-hexylthiophene) (P3HT) and non-conjugated-poly(ethylene glycol) (PEG)-grafts have been synthesised, and their properties have been studied alongside those of the parent conjugated polymer (P3HT). Spectroelectrochemical and conductometric analyses revealed an early rise of the conductance of the polymers. Once spectral changes begin taking place, the conductance is stable, implying a loss of mobility of charge carriers, even though standard doping/dedoping patterns are observed. Prototype bulk heterojunction solar cells have been fabricated, based on P3HT/[6,6]-Phenyl-C61-butyric acid methyl ester (PCBM), as well as by substituting P3HT for each of the copolymers. The prototype solar cells achieved PCEs of up to 2.11%. This is one of the highest reported power conversion efficiency (PCE) for devices based on P3HT with low average molecular weight Mn = 12 kDa. Strong correlation between the structure of the copolymer and its photovoltaic performance was found. Elongation of PEG copolymer chain and the use of methyl group instead of terminal hydroxyl groups significantly improved photovoltaic performance.

Analytical considerations for measuring the globule size distribution of cyclosporine ophthalmic emulsions

Measurement of particle size and size distribution of complex drug products exhibiting complex rheological behaviors can be challenging as these properties may be beyond the theoretical assumptions of the measurement technique. Herein cyclosporine (CsA) ophthalmic emulsion was selected as a model complex system, and an in-depth assessment of particle size was performed using five fundamentally different particle sizing techniques, including dynamic light scattering (DLS), laser diffraction (LD), nanoparticle tracking analysis (NTA), cryogenic transmission electron microscopy (Cryo-TEM) and 2-dimensional diffusion ordered spectroscopy nuclear magnetic resonance (2D DOSY-NMR). The effect of various viscosity modifying and stabilizing excipients in the emulsions was assessed using four types of CsA formulations, i.e., 1) no viscosity modifying excipients, 2) carbomer copolymer type A (CCA), 3) Carbopol 1342, or 4) hydroxypropyl methyl cellulose (HMPC). In general, the variability of reported particle size increased, and is not as accurate, for emulsions dispersed in a non-Newtonian fluid and at higher emulsion concentrations. This effect was reduced in part by diluting the samples to lower volume fraction and a more Newtonian regime. To address the concern that sample dilution prior to measurement may induce physical instability in the emulsions, NTA was used to monitor average size at dilutions of up to 1:50,000. The size was found to remain constant and independent of the presence or type of stabilizer used. Cryo-TEM further confirmed that dilution did not alter particle size or morphology. Of the five evaluated techniques, Cryo-TEM and 2D DOSY NMR did not require dilution for measurement. The overestimate in DLS size measurements for certain CsA formulations was attributed to complex dispersant rheological behavior, particle-particle interactions, multiple light scattering events, and/or scattering interference from the polymers, which can be overcome by either testing under dilutions or by selecting one of the techniques less impacted by the interference of polymer.

Understanding the role of MAM molecular weight in the production of PMMA/MAM nanocellular polymers

Nanostructured polymer blends with CO2-philic domains can be used to produce nanocellular materials with controlled nucleation. It is well known that this nanostructuration can be induced by the addition of a block copolymer poly(methyl methacrylate)-poly(butyl acrylate)-poly(methyl methacrylate) (MAM) to a poly(methyl methacrylate) (PMMA) matrix. However, the effect of the block copolymer molecular weight on the production of nanocellular materials is still unknown. In this work, this effect is analysed by using three types of MAM triblock copolymers with different molecular weights, and a fixed blend ratio of 90 wt% PMMA and 10 wt% of MAM. Blends were produced by extrusion. As a result of the extrusion process, a non-equilibrium nanostructuration takes place in the blends, and the micelle density increases as MAM molecular weight increases. Micelle formation is proposed to occur as result of two mechanisms: dispersion, controlled by the extrusion parameters and the relative viscosities of the polymers, and self-assembly of MAM molecules in the dispersed domains. On the other hand, in the nanocellular materials produced with these blends, cell size decreases from 200 to 120 nm as MAM molecular weight increases. Cell growth is suggested to be controlled by the intermicelle distance and limited by the cell wall thickness. Furthermore, a theoretical explanation of the mechanisms underlying the limited expansion of PMMA/MAM systems is proposed and discussed.

Synthesis and characterization of a donor-acceptor type copolymer containing ester-functionalized benzo[c][1,2,5]thiadiazole as an accepting building block for organic photovoltaics

The benzo[c][1,2,5]thiadiazole (BT) based copolymer, poly[(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene)-alt-(diethyl 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole-5,6-dicarboxylate)] (PBDTBT-COOR), was synthesized by Stille polycondensation. The BT unit with carboxylate ester groups was designed and synthesized as an electron-withdrawing building block for donor-acceptor type copolymers with the aim of achieving good solubility and low-lying highest occupied molecular orbital energy levels of the synthesized polymer. Consequently, the synthesized PBDTBT-COOR copolymer showed excellent solubility in common organic solvents and a relatively deep HOMO energy level. An inverted organic photovoltaic device was fabricated using PBDTBT-COOR and [6,6]-phenyl C71 butyric acid methyl ester as the electron donor and the acceptor, respectively, with the device structure of indium tin oxide/ZnO/active layer/MoO3/Ag. Under the illumination condition of AM 1.5 G, i.e., 100 mW/cm2, the maximum power conversion efficiency of the device was 3.15% with an open-circuit voltage of 0.87 V, short circuit current of 9.68 mA/cm2, and a fill factor of 37.6%.

Liquid phase oxidation of cyclohexane over mesoporous cobalt silicates molecular sieves synthesized in strong acidic media by assembly of preformed CoS-1 precursors with triblock copolymer

Cobalt–incorporated mesoporous silica materials [MCoS(n), n = Si/Co = 20; 60] have been successfully prepared in strong acidic media by assembly of preformed CoS-1 precursors with triblock copolymer of the Pluronic type (P123) by a two steps procedure. They were characterized by various techniques including XRD, BET, FT-IR spectroscopy and diffuse reflectance UV–Vis (DRUV–Vis). The results show that the calcined MCoS materials contain Co(II) ions in tetrahedral environments under low and high Co content and the mesoporous walls contain the MFI structure building units. Liquid phase oxidation of cyclohexane using TBHP as oxidant was investigated on MCoS(n). The major products detected are cyclohexanone and cyclohexanol in all cases. The MCoS(n) catalysts are more active when water free TBHP was used and the conversion of cyclohexane increases when cyclohexane to TBHP molar ratio increases from 1:1 to 1:2, while the cyclohexanol selectivity decreases, which is ascribed to a further oxidation of cyclohexanol to cyclohexanone. The catalytic activity of MCoS(n) materials is enhanced as the cobalt content increases. It is worth noting that MCoS(20)is more active than Co-SBA15(20). The most active catalyst MCoS(20) acts as truly heterogeneous catalyst and was very stable after four runs.