Application of Lead-Free Metal Halide Perovskite Heterojunctions for the Carbohalogenation of C-C Multiple Bonds

A graphitic carbon nitride/lead-free double perovskite heterojunction (g-C3N4/Cs2AgBiCl6) has been adopted as a heterogeneous photocatalyst under visible light irradiation. The employed material enabled the atom transfer radical addition-type carbohalogenation of multiple C-C bonds, including (internal) alkenes and alkynes, with alkyl halides. The protocol showed a remarkable functional group tolerance, compatible with the late-stage functionalization of natural and pharmaceutical derivatives, and could be easily scaled up, delivering >1 g of the desired products.

Ferrocene Catalyzed Radical Cascade Cyclization of Aryl 1,6-Diynes: Access to Cyanoalkylsulfonyl Fluorenes

A ferrocene-catalyzed cyanoalkylsulfonylative radical cascade cyclization of aryl 1,6-diynes using cycloketone oxime esters and DABCO.(SO₂)₂ (DABSO) is reported. The reaction proceeds with notable chemo- and regioselectivity, without requiring additional oxidants or reductants. Interestingly, the methodology facilitates the formation of an unprecedented dicyanoalkylsulfonylated derivative in the case of electronically deficient aryl 1,6-diynes. Additionally, this transformation is compatible with substrates derived from drugs and bioactive molecules, highlighting its synthetic utility.

Shake, shear, and grind! - the evolution of mechanoredox polymerization methodology

In the last half decade, mechanoredox catalysis has enabled an entirely new genre of polymerization methodology. In this paradigm, mechanical force, such as ultrasonic cavitation bubble collapse or ball mill grinding, polarizes piezoelectric nanoparticles; the resultant piezopotential drives the redox processes necessary for free- and controlled-radical polymerizations. Since being introduced, evolution of these methods facilitates exploration of mechanistic underpinnings behind key electron-transfer events. Mechanical force has not only been identified as a "greener" alternative to more traditional reaction stimuli (e.g., heat, light) for the synthesis of commodity polymers, but also a potential technology to enable the production of novel thermoplastic and thermoset materials that are either challenging, or even impossible, to access using conventional solution-state approaches. In this Feature Article, significant contributions to such methods are highlighted within. Advances and ongoing challenges in both ultrasound and ball milling driven reactions for radical polymerization and crosslinking are identified and discussed.

Recent Advances in the Application of ATRP in the Synthesis of Drug Delivery Systems

Advances in atom transfer radical polymerization (ATRP) have enabled the precise design and preparation of nanostructured polymeric materials for a variety of biomedical applications. This paper briefly summarizes recent developments in the synthesis of bio-therapeutics for drug delivery based on linear and branched block copolymers and bioconjugates using ATRP, which have been tested in drug delivery systems (DDSs) over the past decade. An important trend is the rapid development of a number of smart DDSs that can release bioactive materials in response to certain external stimuli, either physical (e.g., light, ultrasound, or temperature) or chemical factors (e.g., changes in pH values and/or environmental redox potential). The use of ATRPs in the synthesis of polymeric bioconjugates containing drugs, proteins, and nucleic acids, as well as systems applied in combination therapies, has also received considerable attention.

Sustainable Wacker-Type Oxidations

The Wacker reaction is the oxidation of olefins to ketones and typically requires expensive and scarce palladium catalysts in the presence of an additional copper co-catalyst under harsh conditions (acidic media, high pressure of air/dioxygen, elevated temperatures). Such a transformation is relevant for industry, as shown by the synthesis of acetaldehyde from ethylene as well as for fine-chemicals, because of the versatility of a carbonyl group placed at specific positions. In this regard, many contributions have focused on controlling the chemo- and regioselectivity of the olefin oxidation by means of well-defined palladium catalysts under different sets of reaction conditions. However, the development of Wacker-type processes that avoid the use of palladium catalysts has just emerged in the last few years, thereby paving the way for the generation of more sustainable procedures, including milder reaction conditions and green chemistry technologies. In this Minireview, we discuss the development of new catalytic processes that utilize more benign catalysts and sustainable reaction conditions.

Electrochemical Investigation of Iron-Catalyzed Atom Transfer Radical Polymerization

Use of iron-based catalysts in atom transfer radical polymerization (ATRP) is very interesting because of the abundance of the metal and its biocompatibility. Although the mechanism of action is not well understood yet, iron halide salts are usually used as catalysts, often in the presence of nitrogen or phosphorous ligands (L). In this study, electrochemically mediated ATRP (eATRP) of methyl methacrylate (MMA) catalyzed by FeCl3, both in the absence and presence of additional ligands, was investigated in dimethylformamide. The electrochemical behavior of FeCl3 and FeCl3/L was deeply investigated showing the speciation of Fe(III) and Fe(II) and the role played by added ligands. It is shown that amine ligands form stable iron complexes, whereas phosphines act as reducing agents. eATRP of MMA catalyzed by FeCl3 was investigated in different conditions. In particular, the effects of temperature, catalyst concentration, catalyst-to-initiator ratio, halide ion excess and added ligands were investigated. In general, polymerization was moderately fast but difficult to control. Surprisingly, the best results were obtained with FeCl3 without any other ligand. Electrogenerated Fe(II) effectively activates the dormant chains but deactivation of the propagating radicals by Fe(III) species is less efficient, resulting in dispersity > 1.5, unless a high concentration of FeCl3 is used.

Photoredox Catalysis in Photocontrolled Cationic Polymerizations of Vinyl Ethers

ConspectusAdvances in photocontrolled polymerizations have expanded the scope of polymer architectures and structures that can be synthesized for various applications. The majority of these polymerizations have been developed for radical processes, which limits the diversity of monomers that can be used in macromolecular design. More recent developments of photocontrolled cationic polymerizations have taken a step toward addressing this limitation and have expanded the palette of monomers that can be used in stimuli-regulated polymerizations, enabling the synthesis of previously inaccessible polymeric structures. This Account will detail our group's studies on cationic polymerization processes where chain growth is regulated by light and highlight how these methods can be combined with other stimuli-controlled polymerizations to precisely dictate macromolecular structure.Photoinitiated cationic polymerizations are well-studied and important processes that have control over initiation. However, we wanted to develop systems where we had spatiotemporal control over both polymer initiation and chain growth. This additional command over the reaction provides the ability to manipulate the growing polymer with an external stimulus during a polymerization, which can be used to control structure. To achieve this goal, we set out to develop a method to photoreversibly generate a cation at a growing chain end that could participate in a controlled polymerization process. We took inspiration from previous work on cationic degenerate chain transfer polymerizations of vinyl ethers that used thiocarbonylthio chain transfer agents. These polymerizations were initiated by a strong acid and gave well-defined poly(vinyl ether)s. We posited that we could remove the acid initiator in these systems and reversibly oxidize the thiocarbonylthio chain ends in these reactions with a photocatalyst to give a photocontrolled cationic polymerization of vinyl ethers. This Account will focus on our journey to discover cationic photocontrolled polymerizations. We will summarize our initial developments and detail our mechanistic understanding of these reactions using both organic and inorganic based photocatalysts, and we will outline more recent efforts to expand cationic degenerate chain transfer polymerizations to other thioacetal initiators. Finally, we will detail how these photocontrolled cationic polymerizations can be used to switch monomer selectivity in situ using light to control polymer structure. At the end of the Account, we will discuss our vision for future potential applications of these photocontrolled cationic polymerizations in the synthesis of novel block copolymers and next generation cross-linked networks.

Effect of halogen and solvent on iron-catalyzed atom transfer radical polymerization

Efficient exchange of Br in iron-catalyzed ATRP in anisole provided well-controlled polymers with low dispersity as opposed to the Cl-based initiating system, which resulted in large dispersities due to the slower activation/deactivation with Cl.

Synthesizing a Hybrid Nanocomposite as an Affinity Adsorbent through Surface-Initiated Atom Transfer Radical Polymerization Catalyzed by Myoglobin

A hybrid bifunctional core-shell nanostructure was synthesized for the first time via surface-initiated atom transfer radical polymerization (SI-ATRP) using myoglobin as a biocatalyst (ATRPase) in an aqueous solution. N-Isopropyl acrylamide (NIPA) and N-(3-aminopropyl)methacrylamide (APMA) were applied to graft flexible polymer brushes onto initiator-functionalized silica nanoparticles. Two different approaches were implemented to form the core-shell nanocomposite: (a) random copolymerization, Si@p(NIPA-co-APMA) and (b) sequential block copolymerization, Si@pNIPA-b-pAPMA. These nanocomposites can be used as versatile intermediates, thereby leading to different types of materials for targeted applications. In this work, a phenylboronic acid ligand was immobilized on the side chain of the grafted brushes during a series of postmodification reactions to create a boronate affinity adsorbent. The ability to selectively bind glycoproteins (ovalbumin and glycated hemoglobin) via boronic acid was assessed at two different temperatures (20 and 40 °C), where Si@pNIPA-b-APMA_BA (163 mg OVA/g of particle) displayed an approximately 1.5-fold higher capacity than Si@p(NIPA-co-APMA)_BA (107 mg OVA/g of particle). In addition to selective binding to glycoproteins, the nanocomposites exhibited selective binding for myoglobin due to the molecular imprinting effect during the postmodification process, that is, 72 and 111 mg Mb/g for Si@p(NIPA-co-APMA)_BA and Si@pNIPA-b-pAPMA_BA, respectively.

Modern trends in controlled synthesis of functional polymers: fundamental aspects and practical applications

Major trends in controlled radical polymerization (CRP) or reversible-deactivation radical polymerization (RDRP), the most efficient method of synthesis of well-defined homo- and copolymers with specified parameters and properties, are critically analyzed. Recent advances associated with the three classical versions of CRP: nitroxide mediated polymerization, reversible addition-fragmentation chain transfer polymerization and atom transfer radical polymerization, are considered. Particular attention is paid to the prospects for the application of photoinitiation and photocatalysis in CRP. This approach, which has been intensively explored recently, brings synthetic methods of polymer chemistry closer to the light-induced processes of macromolecular synthesis occurring in living organisms. Examples are given of practical application of CRP techniques to obtain industrially valuable, high-tech polymeric products.

The bibliography includes 429 references.

Iron-Based Catalytically Active Complexes in Preparation of Functional Materials

Iron complexes are particularly interesting as catalyst systems over the other transition metals (including noble metals) due to iron’s high natural abundance and mediation in important biological processes, therefore making them non-toxic, cost-effective, and biocompatible. Both homogeneous and heterogeneous catalysis mediated by iron as a transition metal have found applications in many industries, including oxidation, C-C bond formation, hydrocarboxylation and dehydration, hydrogenation and reduction reactions of low molecular weight molecules. These processes provided substrates for industrial-scale use, e.g., switchable materials, sustainable and scalable energy storage technologies, drugs for the treatment of cancer, and high molecular weight polymer materials with a predetermined structure through controlled radical polymerization techniques. This review provides a detailed statement of the utilization of homogeneous and heterogeneous iron-based catalysts for the synthesis of both low and high molecular weight molecules with versatile use, focusing on receiving functional materials with high potential for industrial application.

Reaction-based highly selective and sensitive monomer/polymer probes with Schiff base groups for the detection of Hg2+ and Fe3+ ions

It is urgent and important to detect heavy metals in environments. In this work, novel reaction-based fluorescent probes were obtained by Schiff base reaction. The probes with Schiff base moiety (-C=N-) undergo irreversible hydrolysis in the presence of Hg²⁺ and Fe³⁺. They exhibit perfect high selectivity and sensitivity to Hg²⁺and Fe³⁺ ions. Upon the addition of Hg²⁺and Fe³⁺, fluorescence intensity of the probes increased notably. And the color of the probe changes from brown to bright green under UV light, which can realize "naked eye" detection. In addition, Schiff base group was introduced into polyurethane chain through condensation polymerization reaction. As expected, the fluorescent polyurethane probe (P2) maintained the detection performance of its original small molecules (BSD). Even more P2 showed a more sensitive detection effect than BSD, and the detection limits of P2 for Hg²⁺ and Fe³⁺ reach 0.19 μM and 0.21 μM, respectively. It indicates that Reaction-based probes could be a useful tool for the detection of Hg²⁺ and Fe³⁺.

Facile Synthesis of Ultrahigh Molecular Weight Poly(Methyl Methacrylate) by Organic Halides in the Presence of Palladium Nanoparticles

A facile and versatile approach for the synthesis of ultrahigh molecular weight poly(methyl methacrylate) (PMMA) at mild conditions was developed. Certain organic halides combined with a catalytical amount of palladium nanoparticles (Pd NPs) were found to be very effective in initiating polymerizations of methyl methacrylate (MMA), methyl acrylate, vinyl acetate and other vinyl monomers. An ultrahigh molecular weight PMMA with a number-average molecular weight of 4.65 × 10⁶ Da and a weight-average molecular weight of 8.08 × 10⁶ Da was synthesized at 70 °C using 2-bromoisobutyric acid ethyl ester (EBiB) as an initiator in the presence of catalytical amount (10.1 ppm) of Pd NPs. A kinetic investigation found that the orders of polymerization with respect to EBiB, Pd NP and MMA were 0.23, 0.50, and 0.58, respectively. Proton nuclear magnetic resonance (¹H NMR) combined with matrix-assisted laser desorption ionization time of flight mass spectroscopy (MALDI-TOF) and gel permeation chromatography (GPC) were used to prove that the macromolecular chain had an end-group of EBiB residue. The electron spin resonance (ESR), transmission electron microscope (TEM), and X-ray photoelectron spectroscopy (XPS) results reveal that the reaction of EBiB with Pd NPs caused a bromo atom (Br) transfer from EBiB to Pd NPs and resulted in the generation of EBiB residue radical to initiate the polymerization of MMA and the formation of Pd^IIBr₂ on the surface of Pd nanoparticles.

Development of Environmentally Friendly Atom Transfer Radical Polymerization

Atom transfer radical polymerization (ATRP) is one of the most successful techniques for the preparation of well-defined polymers with controllable molecular weights, narrow molecular weight distributions, specific macromolecular architectures, and precisely designed functionalities. ATRP usually involves transition-metal complex as catalyst. As the most commonly used copper complex catalyst is usually biologically toxic and environmentally unsafe, considerable interest has been focused on iron complex, enzyme, and metal-free catalysts owing to their low toxicity, inexpensive cost, commercial availability and environmental friendliness. This review aims to provide a comprehensive understanding of iron catalyst used in normal, reverse, AGET, ICAR, GAMA, and SARA ATRP, enzyme as well as metal-free catalyst mediated ATRP in the point of view of catalytic activity, initiation efficiency, and polymerization controllability. The principle of ATRP and the development of iron ligand are briefly discussed. The recent development of enzyme-mediated ATRP, the latest research progress on metal-free ATRP, and the application of metal-free ATRP in interdisciplinary areas are highlighted in sections. The prospects and challenges of these three ATRP techniques are also described in the review.

Tailoring Polymer Dispersity in Photoinduced Iron-Catalyzed ATRP

Although dispersity (Đ) plays an important role in controlling polymer properties, there are very few chemical methods that can sufficiently tune it. Here we report a simple, batch, and environmentally benign photoinduced iron-catalyzed ATRP methodology that enables the efficient control of Đ for both homopolymers and block copolymers. We show that by judiciously varying the concentration of the FeBr₃/TBABr catalyst, a range of dispersities can be obtained (1.18 < Đ < 1.80) while maintaining monomodal molecular weight distributions. High end-group fidelity was confirmed by MALDI-ToF-MS and was further supported by the efficient synthesis of in situ block copolymers where the dispersity of the second block could be controlled upon demand. Importantly, through the use of low ppm amounts of the catalyst, perfect temporal control could be attained during intermittent "on/off" cycles. This work considerably expands the chemical toolbox for tuning Đ of homo- and block copolymers.

Iron Catalysts in Atom Transfer Radical Polymerization

Catalysts are essential for mediating a controlled polymerization in atom transfer radical polymerization (ATRP). Copper-based catalysts are widely explored in ATRP and are highly efficient, leading to well-controlled polymerization of a variety of functional monomers. In addition to copper, iron-based complexes offer new opportunities in ATRP catalysis to develop environmentally friendly, less toxic, inexpensive, and abundant catalytic systems. Despite the high efficiency of iron catalysts in controlling polymerization of various monomers including methacrylates and styrene, ATRP of acrylate-based monomers by iron catalysts still remains a challenge. In this paper, we review the fundamentals and recent advances of iron-catalyzed ATRP focusing on development of ligands, catalyst design, and techniques used for iron catalysis in ATRP.

Poly (Ethylene Oxide)-Based Block Copolymer Electrolytes Formed via Ligand-Free Iron-Mediated Atom Transfer Radical Polymerization

The Br-terminated poly (ethylene oxide) (PEO-Br) is used as a green and efficient macroinitiator in bulk Fe-catalyzed atom transfer radical polymerization (ATRP) without the addition of any organic ligands. The polymerization rate is able to be mediated by PEO-Br with various molecular weights, and the decrease in redox potential of FeBr₂ in cyclic voltammetry (CV) curves indicates that an increased coordination effect is deteriorated with the depressing reaction activity in the longer ethylene oxide (EO) chain in PEO-Br. In combination with the study of different catalysts and catalytic contents, the methyl metharylate (MMA) or poly (ethylene glycol) monomethacrylate (PEGMA) was successfully polymerized with PEO-Br as an initiator. This copolymer obtained from PEGMA polymerization can be further employed as a polymer matrix to form the polymer electrolyte (PE). The higher ionic conductivity of PE was obtained by using a high molecular weight of copolymer.

Iron-Catalysed Radical Polymerisation by Living Bacteria

The ability to harness cellular redox processes for abiotic synthesis might allow the preparation of engineered hybrid living systems. Towards this goal we describe a new bacteria-mediated iron-catalysed reversible deactivation radical polymerisation (RDRP), with a range of metal-chelating agents and monomers that can be used under ambient conditions with a bacterial redox initiation step to generate polymers. Cupriavidus metallidurans, Escherichia coli, and Clostridium sporogenes species were chosen for their redox enzyme systems and evaluated for their ability to induce polymer formation. Parameters including cell and catalyst concentration, initiator species, and monomer type were investigated. Water-soluble synthetic polymers were produced in the presence of the bacteria with full preservation of cell viability. This method provides a means by which bacterial redox systems can be exploited to generate "unnatural" polymers in the presence of "host" cells, thus setting up the possibility of making natural-synthetic hybrid structures and conjugates.

Ligand- and solvent-free ATRP of MMA with FeBr3 and inorganic salts

A new cost-effective and safe protocol for the bulk ATRP of MMA uses FeBr₃, EBrPA and an inorganic compound (carbonate, bicarbonate, phosphate, hydroxide, chloride, bromide) of an alkali metal cation.

Iron-catalyzed regioselective alkylation of 1,4-quinones and coumarins with functionalized alkyl bromides

A simple and efficient Fe-catalyzed regioselective alkylation of 1,4-quinones and coumarins, using functionalized alkyl bromides as alkylating reagents, has been developed.

Iodine-mediated photoATRP in aqueous media with oxygen tolerance

Iodine-mediated photoATRP was conducted in aqueous media enabling fast and well-controlled polymerizations under visible light without deoxygenation.

Iron-Catalyzed Atom Transfer Radical Polymerization of Semifluorinated Methacrylates

Fluorinated polymers are an important class of functional materials that exhibit unique properties such as high chemical resistance, thermal stability, and low surface energy. Atom transfer radical polymerization (ATRP) of semifluorinated monomers catalyzed by copper catalysts often requires development of special conditions to control the polymerization and prevent side reactions such as base-catalyzed transesterification between the fluoro-containing monomers and solvents. In this paper, photoinduced iron-catalyzed ATRP was applied to the polymerization of a variety of semifluorinated methacrylate monomers. Polymerizations were initiated by photochemical generation of the Fe catalyst activator under blue light irradiation, enabling temporal control over the growth of polymer chains, and were well-controlled in various solvents, including fluorinated and nonfluorinated solvents, without undergoing any side reactions. Moreover, in situ chain extension and block copolymerization experiments demonstrated the preservation of chain end functionality, enabling facile synthesis of well-controlled block copolymers.

γ-valerolactone (GVL) as a bio-based green solvent and ligand for iron-mediated AGET ATRP

In this paper, γ-valerolactone (GVL), a bio-based polar solvent, was applied as green solvent for iron(III)-catalyzed AGET ATRP without any external ligand. GVL is a fully degradable, non-toxic green solvent and has complex ability to iron halide complexes through –OCO- group. GVL as the solvent and the ligand for AGET ATRP of MMA in a controlled manner, as proved by kinetic study, the low PDI values and the increase in polymer molecular weight versus monomer conversion. Chain re-initiation experiments and 1HNMR characterization were conducted to further confirm the living feature.

Mechanistically Guided Predictive Models for Ligand and Initiator Effects in Copper-Catalyzed Atom Transfer Radical Polymerization (Cu-ATRP)

Copper-catalyzed atom transfer radical polymerization (Cu-ATRP) is one of the most widely used controlled radical polymerization techniques. Notwithstanding the extensive mechanistic studies in the literature, the transition states of the activation/deactivation of the growing polymer chain, a key equilibrium in Cu-ATRP, have not been investigated computationally. Therefore, the understanding of the origin of ligand and initiator effects on the rates of activation/deactivation is still limited. Here, we present the first computational analysis of Cu-ATRP activation transition states to reveal factors that affect the rates of activation and deactivation. The Br atom transfer between the polymer chain and the Cu catalyst occurs through an unusual bent geometry that involves pronounced interactions between the polymer chain end and the ancillary ligand on the Cu catalyst. Therefore, the rates of activation/deactivation are determined by both the electronic properties of the Cu catalyst and the ligand-initiator steric repulsions. In addition, our calculations revealed the important role of ligand backbone flexibility on the activation. These theoretical analyses led to the identification of three chemically meaningful descriptors, namely HOMO energy of the catalyst ( EHOMO), percent buried volume ( Vbur%), and distortion energy of the catalyst (Δ Edist), to describe the electronic, steric, and flexibility effects on reactivity, respectively. A robust and simple predictive model for ligand effect on reactivity is thereby established by correlating these three descriptors with experimental activation rate constants using multivariate linear regression. Validation using a structurally diverse set of ligands revealed the average error is less than ±2 kcal/mol compared to the experimentally derived activation energies. The same approach was also applied to develop a predictive model for reactivity of different alkyl halide initiators using R-X bond dissociation energy (BDE) and Cu-X halogenophilicity as descriptors.

Radical-Type Reactivity and Catalysis by Single-Electron Transfer to or from Redox-Active Ligands

Controlled ligand-based redox-activity and chemical non-innocence are rapidly gaining importance for selective (catalytic) processes. This Concept aims to provide an overview of the progress regarding ligand-to-substrate single-electron transfer as a relatively new mode of operation to exploit ligand-centered reactivity and catalysis based thereon.

Bromoalkyl ATRP initiator activation by inorganic salts: experiments and computations

The bromoalkyl ATRP initiator EBrPA is activated by many alkali, alkaline-earth and ammonium salts, leading to MMA polymerization, but only the iodides yield a controlled process because of a degenerative transfer mechanism contribution.

Iron-catalysed atom transfer radical polyaddition for the synthesis and modification of novel aliphatic polyesters displaying lower critical solution temperature and pH-dependent release behaviors

Novel aliphatic polyesters were synthesized and quantitatively modified by click reactions to obtain amphiphilic polymer brushes for nano-carrier applications.

Regioselective Synthesis of γ-Lactones by Iron-Catalyzed Radical Annulation of Alkenes with α-Halocarboxylic Acids and Their Derivatives

An abundant and low toxicity iron catalyst has enabled regioselective annulation of alkenes with α-halocarboxylic acids and their derivatives. The reaction proceeds smoothly without any additional ligands, bases, and additives to afford a variety of γ-lactones in good yields. A proposed reaction pathway through radical annulation is supported by some mechanistic studies, involving radical clock and isotope labeling experiments. The present method was applied to the practical iron-powder-promoted synthesis of γ-lactones.

Ligand-free iron-based electrochemically mediated atom transfer radical polymerization of methyl methacrylate

This article reports the iron-catalyzed ligand-free eATRP.

Normal, ICAR and photomediated butadiene-ATRP with iron complexes

FeX₂ or FeX₃ (X = Cl ≫ Br) alone or with P ≫ X > O > N > C ligands and bromoester initiators enable the successful ATRP of butadiene in toluene at 110 °C.

Addressing the role of triphenylphosphine in copper catalyzed ATRP

A new Atom Transfer Radical Polymerization (ATRP) process with triphenylphosphine (PPh₃) and [Cu^IIMe₆TREN]²⁺ as the catalyst system is reported.

Externally controlled atom transfer radical polymerization

ATRP can be externally controlled by electrical current, light, mechanical forces and various chemical reducing agents. The mechanistic aspects and preparation of polymers with complex functional architectures and their applications are critically reviewed.