Microwave-Assisted Polymerizations: Recent Status and Future Perspectives

Facile green synthesis of a novel NiO and its catalytic effect on methylene blue photocatalytic reduction and sodium borohydride hydrolysis

NiO nanoparticles were synthesized from pine cone extract by green synthesis method, which is a simple, cost-effective, environmentally friendly and sustainable method. The particle size of NiO nanoparticles was determined to be in the range of 10-25 nm by X-diffraction differential and transmission electron microscope analysis, and the bandgap energy of NiO nanoparticles was determined to be 2.66 eV. The catalytic effect of NiO nanoparticles in both microwave-assisted sodium borohydride hydrolysis and photocatalytic reduction of methylene blue was examined and it was determined that they had a high catalytic effect in both applications. It was determined that the hydrogen production rate in sodium borohydride hydrolysis was 1135 mL/g/min. The activation energy of sodium borohydride hydrolysis is 29.69 kJ/mol and 29.59 kJ/mol for the nth-order and Langmuir Hinshelwood kinetic models, respectively. In the photocatalytic reduction of methylene blue with NaBH4, it was determined that the reduction did not occur in the absence of a catalyst, but in the presence of the catalyst, the reduction occurred 98% in 3 min. It was determined that NiO nanoparticles were used five times in the photocatalytic reduction of methylene blue and the reduction efficiency for the fifth time was 93%. It was determined that the photocatalytic reduction of methylene blue was pseudo-first order and the rate constant was 1.63 s-1. It was determined that NiO nanoparticles synthesized by the environmentally friendly green synthesis method can be used as catalysts for two different applications.

Polymer Processing under Microwaves

Over the last decades, microwave heating has experienced a great development and reached various domains of application, especially in material processing. In the field of polymers, this unusual source of energy showed important advantages arising from the direct microwave/matter interaction. Indeed, microwave heating allows regio-, chemio-, and stereo-selectivity, faster chemical reactions, and higher yields even in solvent-free processes. Thus, this heating mode provides a good alternative to the conventional heating by reducing time and energy consumption, hence reducing the costs and ecological impact of polymer chemistry and processing. This review states some achievements in the use of microwaves as energy source during the synthesis and transformation of polymers. Both in-solution and free-solvent processes are described at different scales, with comparison between microwave and conventional heating.

Stereoregulation, molecular weight, and dispersity control of PMMA synthesized via free-radical polymerization supported by the external high electric field

We show the remarkable effect of using static (DC) and alternating (AC) electric fields to control the free-radical polymerization of methyl methacrylate (MMA). The magnitude and/or frequency of the applied electric field (up to 154 kV cm^-1) were found to control the molecular weight, dispersity, and stereochemistry of the produced polymers.

Elemental Sulfur Mediated Novel Multicomponent Redox Polycondensation for the Synthesis of Alternating Copolymers Based on 2,4-Thiophene/Arene Repeating Units

A sulfur-based self-condensation method is investigated as an efficient tool for the synthesis of polythiophene derivatives. The reaction proceeds through multicomponent redox polycondensation between readily available diketone compounds and elemental sulfur in the presence of a Brønsted acid/base pair. Six different diketone derivatives have been screened and the polymerization is generalized by the synthesis of so-far-unprecedented alternating copolymers based on 2,4-thiophene/arene repeating units. By exploiting microwave heating the synthetic procedure is optimized, particularly for alternating copolymers containing aryl and thiophene units, such that a copolymer can be synthesized in only 24 h compared to the conventional process taking 6 d, yielding polymers within the same apparent weight average molar mass (M_w ). All obtained copolymers are analyzed in detail using size exclusion chromatography (SEC), nuclear magnetic resonance (NMR), attenuated total reflectance infrared spectroscopy (ATR-IR), thermal gravimetric analysis and differential scanning calorimetry (DSC).

Quantitative evaluation of the non-thermal effect in microwave induced polymer curing

Microwave irradiation is one of the most effective strategies to accelerate the curing of resin. However, the mechanism is still unclear. The debates mainly focus on how to quantitatively evaluate the ‘non-thermal’ effect of the microwave. In this work, the non-thermal effect on DGEBA with amine (D230) and anhydride (MHHPA) hardeners respectively was evaluated via an isothermal microwave curing reactor. The ‘thermal effect’ caused by the microwave was peeled off accurately. Iso-conversional kinetic analysis was performed based on the reaction extent from the real time FTIR spectrum, and the apparent activation energy (E_a) was calculated for the quantitative evaluation of the microwave non-thermal effect. The influence caused by the polarized functional groups, reaction temperature and the transition state were explored, and the microwave non-thermal effect on the mechanical performance of the cured thermosets was evaluated. Results indicated that the microwave non-thermal effect was able to reduce the E_a and accelerate the curing speed of epoxy resin. At the same time, the final curing extent was increased when the non-thermal effect of microwave was induced leading to enhanced mechanical and thermal performance.

The non-thermal effect of microwave was evaluated quantitatively by the calculation of activation energy and pre-exponential factors with an isothermal microwave.

Collagen-Based Materials Modified by Phenolic Acids-A Review

Collagen-based biomaterials constitute one of the most widely studied types of materials for biomedical applications. Low thermal and mechanical parameters are the main disadvantages of such structures. Moreover, they present low stability in the case of degradation by collagenase. To improve the properties of collagen-based materials, different types of cross-linkers have been researched. In recent years, phenolic acids have been studied as collagen modifiers. Mainly, tannic acid has been tested for collagen modification as it interacts with a polymeric chain by strong hydrogen bonds. When compared to pure collagen, such complexes show both antimicrobial activity and improved physicochemical properties. Less research reporting on other phenolic acids has been published. This review is a summary of the present knowledge about phenolic acids (e.g., tannic, ferulic, gallic, and caffeic acid) application as collagen cross-linkers. The studies concerning collagen-based materials with phenolic acids are summarized and discussed.

A Review of Microwave Synthesis of Zinc Oxide Nanomaterials: Reactants, Process Parameters and Morphoslogies

Zinc oxide (ZnO) is a multifunctional material due to its exceptional physicochemical properties and broad usefulness. The special properties resulting from the reduction of the material size from the macro scale to the nano scale has made the application of ZnO nanomaterials (ZnO NMs) more popular in numerous consumer products. In recent years, particular attention has been drawn to the development of various methods of ZnO NMs synthesis, which above all meet the requirements of the green chemistry approach. The application of the microwave heating technology when obtaining ZnO NMs enables the development of new methods of syntheses, which are characterised by, among others, the possibility to control the properties, repeatability, reproducibility, short synthesis duration, low price, purity, and fulfilment of the eco-friendly approach criterion. The dynamic development of materials engineering is the reason why it is necessary to obtain ZnO NMs with strictly defined properties. The present review aims to discuss the state of the art regarding the microwave synthesis of undoped and doped ZnO NMs. The first part of the review presents the properties of ZnO and new applications of ZnO NMs. Subsequently, the properties of microwave heating are discussed and compared with conventional heating and areas of application are presented. The final part of the paper presents reactants, parameters of processes, and the morphology of products, with a division of the microwave synthesis of ZnO NMs into three primary groups, namely hydrothermal, solvothermal, and hybrid methods.

Role of External Field in Polymerization: Mechanism and Kinetics

The past decades have witnessed an increasing interest in developing advanced polymerization techniques subjected to external fields. Various physical modulations, such as temperature, light, electricity, magnetic field, ultrasound, and microwave irradiation, are noninvasive means, having superb but distinct abilities to regulate polymerizations in terms of process intensification and spatial and temporal controls. Gas as an emerging regulator plays a distinctive role in controlling polymerization and resembles a physical regulator in some cases. This review provides a systematic overview of seven types of external-field-regulated polymerizations, ranging from chain-growth to step-growth polymerization. A detailed account of the relevant mechanism and kinetics is provided to better understand the role of each external field in polymerization. In addition, given the crucial role of modeling and simulation in mechanisms and kinetics investigation, an overview of model construction and typical numerical methods used in this field as well as highlights of the interaction between experiment and simulation toward kinetics in the existing systems are given. At the end, limitations and future perspectives for this field are critically discussed. This state-of-the-art research progress not only provides the fundamental principles underlying external-field-regulated polymerizations but also stimulates new development of advanced polymerization methods.

Glycerol: An Optimal Hydrogen Source for Microwave-Promoted Cu-Catalyzed Transfer Hydrogenation of Nitrobenzene to Aniline

The search for sustainable alternatives for use in chemical synthesis and catalysis has found an ally in non-conventional energy sources and widely available green solvents. The use of glycerol, an abundant natural solvent, as an excellent “sacrificial” hydrogen source for the copper-catalyzed microwave (MW)-promoted transfer hydrogenation of nitrobenzene to aniline has been investigated in this work. Copper nanoparticles (CuNPs) were prepared in glycerol and the efficacy of the glycerol layer in mediating the interaction between the metal active sites has been examined using HRTEM analyses. Its high polarity, low vapor pressure, long relaxation time, and high acoustic impedance mean that excellent results were also obtained when the reaction media was subjected to ultrasound (US) and MW irradiation. US has been shown to play an important role in the process via its ability to enhance CuNPs dispersion, favor mechanical depassivation and increase catalytic active surface area, while MW irradiation shortened the reaction time from some hours to a few minutes. These synergistic combinations promoted the exhaustive reduction of nitrobenzene to aniline and facilitated the scale-up of the protocol for its optimized use in industrial MW reactors.

An improved synthesis of poly(amidoamine)s for complexation with self-amplifying RNA and effective transfection

Aza-Michael addition to synthesise poly(amidoamines) was optimised to minimise appearance of bimodal molecular weight distributions caused by a radical-branching side-reaction. This significantly improved cellular delivery of a model self-amplifying RNA vaccine.

Non-thermal microwave effects in radical polymerization of bio-based terpenoid (meth)acrylates

Non-thermal microwave effects are operative for terpenoid acrylates but not for methacrylates, provided that a minimum irradiation power is applied.

Enabling technologies in polymer synthesis: accessing a new design space for advanced polymer materials

This review discusses how developments in laboratory technologies can push the boundaries of what is achievable using existing polymer synthesis techniques.

Enhancement of the Mechanical Properties of Polyimide Film by Microwave Irradiation

Polyimide films have conventionally been prepared by thermal imidization of poly(amic acid)s (PAAs). Here we report that the improvement of tensile strength while increasing (or maintaining) film flexibility of polyimide films was accomplished by simple microwave (MW) irradiation of the PAAs. This improvement in mechanical properties can be attributed to the increase in molecular weight of the polyimides by MW irradiation. Our results show that the mechanical properties of polyimide films can be improved by MW irradiation, which is a green approach that requires relatively low MW power, very short irradiation time, and no incorporation of any additional inorganic substance.

Microwave assisted hydrothermal preparation of rice straw hydrochars for adsorption of organics and heavy metals

A series of rice straw hydrochars were produced through a microwave-assisted hydrothermal treatment method, characterized and used for the adsorption of three organics and two heavy metals from aqueous solutions. The hydrochars have carbon contents from 37.44% to 43.31%, are rich in oxygen containing functional groups, and the equilibrium of hydrothermal carbonization reactions could be reached rapidly in microwave environment. The hydrochars can effectively adsorb the model pollutants, the maximum adsorption capacities of Congo red, berberine hydrochloride and 2-naphthol at 298 K and initial concentration of 0.5 mg/mL were 222.1, 174.0 and 48.7 mg/g, respectively, and those of Zn²⁺ and Cu²⁺ were 112.8 and 144.9 mg/g, respectively. Adsorption thermodynamic parameters were calculated. These results suggest that microwave-assisted hydrothermal treatment is an effective method for the rapid production of hydrochars, and rice straw hydrochars are promising adsorbents for the removal of water pollutants such as organics and heavy metals.

Microwave-Assisted Rapid Preparation of Mesoporous Phenolic Resin Nanospheres toward Highly Efficient Solid Acid Catalysts

A novel microwave-assisted polymerization and self-assembly protocol was developed to prepare ordered mesoporous phenolic resin (MPRN) with nanospherical morphology for the first time. This unique strategy dramatically saved the synthesis time about 2 days with an energy-efficient way. Owing to its abundant phenyl groups in the framework, it was easily transformed to benzenesulfonic acid-functionalized MPRN (SO₃H-MPRN) by simple sulfonation treatment. The obtained SO₃H-MPRN sample still possessed a large surface area, two-dimensional hexagonal mesoporous structure, and uniform spherical shape. Importantly, because of its intrinsic organic framework, the pore surface of SO₃H-MPRN was hydrophobic. Accordingly, it exhibited the excellent catalytic activity and selectivity in aqueous formaldehyde-participated Prins reaction and water-medium Fischer-indole reaction. On the basis of material characterizations and the control experiments, this remarkable catalytic performance could be ascribed to the synergetic effect derived from its short mesoporous channel and hydrophobic pore surface, which resulted in the decreased reactant diffusion limitation and the reduced water competitive adsorption. Also, it was stable in water because of the periodically arranged acid species in the resin framework and thus was easily recycled and used repetitively for at least five times.

The effect of linker length on ConA and DC-SIGN binding of S-glucosyl functionalized poly(2-oxazoline)s

A series of poly(2-oxazoline) based glycopolymers with different linkers were prepared via thiol–ene click reaction and cationic ring opening reaction. The binding of these polymers to lectins were studied.

Specific microwave effect on Sn- and Ti-MFI zeolite synthesis

A specific microwave effect, demonstrated by the differences on Sn- and Ti-MFI zeolite preparation and characterization, provides a new direction for investigating microwave effects.

High performance polyimides with good solubility and optical transparency formed by the introduction of alkyl and naphthalene groups into diamine monomers

Highly soluble and optically transparent polyimides with excellent thermal stability were prepared by the introduction of alkyl and naphthalene groups into a single diamine monomer under microwave-assisted polymerization.

A rapid microwave-induced synthesis of polyamides–containing amino acid moieties

5-(2-(9,10-Dihydro-9,10-ethanoanthracene-11,12-dicarboximido)propanoylamino)isophthalic acid as a novel aromatic dicarboxylic acid monomer was prepared in four steps. In the first step, cis-9,10-dihydro-9,10-ethanoanthracene-11,12-dicarboxylic acid anhydride was reacted with l-alanine in acetic acid solution and the resulting imide acid was obtained in high yield. In the second step, treatment of this imide acid with excess thionyl chloride gave acid chloride in good yield. In the last step, this acid chloride was reacted with 5-aminoisophthalic acid to provide novel bulky chiral aromatic dicarboxylic acid. The direct polycondensation reactions of this diacid with several aromatic diamines were carried out under microwave irradiation. In order to compare this technique with classical heating, the polymerization reactions were also performed under solution polycondensation conditions. The polymerization reactions occurred rapidly under microwave conditions and produced a series of novel optically active polyamides (PAs)–containing pendent 9,10-dihydro-9,10-ethanoanthracene-11,12-dicarboximido)propanoylamino group, with good yields and moderate inherent viscosities of 0.27–0.47 dL g−1. The resulting new PAs showed good solubility and are readily soluble in organic solvents. PAs were thermally stable, with 10% weight loss recorded at 411°C and 450°C under nitrogen atmosphere and char yields at 800°C higher than 44% and glass transition temperature above 163°C.

Microwave-assisted synthesis of block copolymer nanoparticles via RAFT with polymerization-induced self-assembly in methanol

A comparative study of microwave-assisted (MA) and conductive heating in RAFT dispersion polymerization formulations in MeOH that result in polymerization-induced self-assembly is detailed.

Crosslinked Poly(2-oxazoline)s as "Green" Materials for Electronic Applications

Poly(2-nonyl-2-oxazoline)₈₀-stat-poly(2-dec-9'-enyl-2-oxazoline)₂₀ and poly(2-dec-9'-enyl-2-oxazoline)₁₀₀ can be synthesized from the cationic ring-opening polymerization of monomers that can be derived from fatty acids from renewable resources. These (co)poly(2-oxazoline)s can be crosslinked with di- and trifunctional mercapto compounds using the UV-induced thiol-ene reaction. The complex permittivity of the corresponding networks increases with the temperature and decreases with the network density. In a frequency range from 10^-2 to 10⁶ Hz and at temperatures ranging from -20 to 40 °C, the changes of the real part of the complex permittivity as well as the loss factor can be explained by interfacial polarization within the material. At a temperature of 20 °C and a frequency of 50 Hz, the permittivity of the crosslinked (co)poly(2-oxazoline)s covers a range from 4.29 to 4.97, and the loss factors are in the range from 0.030 to 0.093. The electrical conductivities of these polymer networks span a range from 5 × 10^-12 to 8 × 10^-9 S/m, classifying these materials as medium insulators. Notably, the values for the permittivity, loss factor and conductivity of these copoly(2-oxazoline)s are in the same range as for polyamides, and, hence, these copoly(2-oxazoline)-based networks may be referred to as "green" alternatives for polyamides as insulators in electronic applications.

Highly efficient microwave-assisted synthesis of photoactive polyamides bearing 5-(3-acetoxynaphthoylamino)benzamide moieties

A novel aromatic dicarboxylic acid monomer, 5-[4-(3-acetoxynaphthoylamino)benzoylamino]isophthalic acid, was prepared in five steps. The direct polycondensation of this dicarboxylic acid with several aromatic diamines was performed under microwave irradiation and conventional heating in the presence of triphenyl phosphite/pyridine/ N-methyl-2-pyrrolidone/calcium chloride. The polymerization reactions were effectively preceded, and the resulting novel photoactive polyamides (PAs) were obtained in high yields and moderate inherent viscosities in the range of 0.32–0.58 dL g−1. Thermogravimetric analysis results showed that polymers were thermally stable, 10% weight loss temperatures were in excess of 430°C and 488°C, and char yields at 800°C under nitrogen atmosphere were higher than 62%. The resulting polymers exhibited ultraviolet–visible absorption bands at 268 and 312 nm in N,N-dimethylformamide (DMF) solution. Their photoluminescence in DMF solution demonstrated fluorescence emission bands maxima around 367 and 429 nm for all of the PAs.