Polyurethanes (PUs) are a class of versatile materials with great potential for use in different applications, especially based on their structure–property relationships. Their specific mechanical, physical, biological, and chemical properties are attracting significant research attention to tailoring PUs for use in different applications. Enhancement of the properties and performance of PU-based materials may be achieved through changes to the production process or the raw materials used in their fabrication or via the use of advanced characterization techniques. Clearly, modification of the raw materials and production process through proper methods can produce PUs that are suitable for varied specific applications. The present study aims to shed light on the chemistry, types, and synthesis of different kinds of PUs. Some of the important research studies relating to PUs, including their synthesis method, characterization techniques, and research findings, are comprehensively discussed. Herein, recent advances in new types of PUs and their synthesis for various applications are also presented. Furthermore, information is provided on the environmental friendliness of the PUs, with a specific emphasis on their recyclability and recoverability.

https://ir.unikl.edu.my/jspui/bitstream/123456789/15237/1/Polyurethane%20types%2c%20synthesis%20and%20applications%20%e2%80%93%20a%20review.pdf

Polyurethane types, synthesis and applications – a review

Here we describe the application of dendrimer-encapsulated Pd nanoparticles to fluorous biphasic catalysis. 1 Complexation of Pd/dendrimer composites with perfluorinated carboxylic acids renders the resulting nanocomposites preferentially soluble in fluorinated hydrocarbons. These new catalysts show high activity and selectivity for biphasic hydrogenation of alkenes and conjugated dienes. Moreover, the catalysts can easily be recovered and used for multiple reactions. Reactions in biphasic fluorous/organic systems were suggested by Horvath and Rabai in 1994 2 to facilitate recovery and recycling of soluble catalysts. The general approach to biphasic catalysis is illustrated in Scheme 1. 3 The system consists of organic and fluorous layers. The catalyst is selectively soluble in the fluorous phase, while the reactants are preferentially soluble in the organic solvent. Stirring, sonicating, and/or heating of the mixture leads to formation of a fine emulsion and partial homogenization (with some solvents, complete homogenization is obtained at elevated temperatures), and the catalytic reaction proceeds at the interface between the two liquids. When the reaction is over, the liquid phases are separated, the product is isolated from the organic phase, and the catalyst-containing fluorous layer is recycled. Such easy separation and recycling are particularly attractive in terms of “green chemistry”, and a number of fluorous phase-soluble catalysts have been reported in the literature, including some based on metal complexes. 4,5 Preparation of fluorous phase-soluble metal nanoparticles, however, has not previously been reported. Catalytic properties of metal nanoparticles have been explored since the pioneering studies of Rampino and Nord in the early 1940s. 6 Over the past decade research in this area intensified, 7

/pdf/dendrimer-encapsulated-pd-nanoparticles-as-fluorous-phase-3j3klgyqid.pdf

Dendrimer-Encapsulated Pd Nanoparticles as Fluorous Phase-Soluble Catalysts

2.6. Microencapsulated Cu(acac)2 [MC Cu(acac)2] 606 2.7. Microencapsulated Palladium [MC Pd] 607 2.8. Microencapsulated Methathesis Catalyst 609 3. Polymer Incarcerated Catalysts 611 3.1. Polymer Incarcerated Palladium [PI Pd] 611 3.2. Polymer-Micelle Incarcerated (PMI) Palladium 617 3.3. Polymer Incarcerated Ruthenium Catalyst 619 3.4. Polymer-Micelle Incarcerated Scandium Trifluoromethanesulfonate [PMI Sc(OTf)3] and Ruthenium Catalyst [PMI Ru] 621

https://pubs.acs.org/doi/pdf/10.1021/cr900066n

'Microencapsulated' and related catalysts for organic chemistry and organic synthesis

This chapter describes composite materials composed of dendrimers and metals or semiconductors. Three types of dendrimer/metal-ion composites are discussed: dendrimers containing structural metal ions, nonstructural exterior metal ions, and nonstructural interior metal ions. Nonstructural interior metal ions can be reduced to yield dendrimer-encapsulated metal and semiconductor nanoparticles. These materials are the principal focus of this chapter. Poly(amidoamine) (PAMAM) and poly(propylene imine) dendrimers, which are the two commercially available families of dendrimers, are in many cases monodisperse in size. Accordingly, they have a generation-dependent number of interior tertiary amines. These are able to complex a range of metal ions including Cu2+, Pd2+, and Pt2+. The maximum number of metal ions that can be sorbed within the dendrimer interior depends on the metal ion, the dendrimer type, and the dendrimer generation. For example, a generation six PAMAM dendrimer can contain up to 64 Cu2+ ions. Nonstructural interior ions can be chemically reduced to yield dendrimer-encapsulated metal nanoparticles. Because each dendrimer contains a specific number of ions, the resulting metal nanoparticles are in many cases of nearly monodisperse size. Nanoparticles within dendrimers are stabilized by the dendrimer framework; that is, the dendrimer first acts as a molecular template to prepare the metal nanoparticles and then as a stabilizer to prevent agglomeration. These composites are useful for a range of catalytic applications including hydrogenations and Heck chemistry. The unique properties of the interior dendrimer microenvironment can result in formation of products not observed in the absence of the dendrimer. Moreover the exterior dendrimer branches act as a selective gate that controls access to the interior nanoparticle, which results in selective catalysis. In addition to single-metal nanoparticles, it is also possible to prepare bimetallic nanoclusters and dendrimer-encapsulated semiconductor nanoparticles, such as CdS, using this same general approach.

Dendrimer-Encapsulated Metals and Semiconductors: Synthesis, Characterization, and Applications

Composite colloids of gold and polypyrrole were prepared using two different methods: 1, using pyrrole colloid, created by the oxidation of pyrrole by ferric chloride, to subsequently reduce chlorauric acid and, 2, oxidizing pyrrole monomer with chlorauric acid in a sodium dodecylbenzene sulfonate solution. In each case, the polypyrrole colloid consisted of irregularly shaped particles approximately 500 nm in diameter. The gold produced in each case was in the form of irregular spheres, approximately 407 nm in diameter in method 1 and 13 nm in method 2. X-ray photoelectron spectroscopy was used to determine the oxidation state of the species present. Transmission electron microscopy and light scattering data were used to determine the particle sizes of both gold and polypyrrole colloids. Energy dispersed spectrum X-ray analysis and electron diffraction were used to confirm the presence of metallic gold in the composite colloids. The second-order rate constant for the reaction of chlorauric acid with pyrrole in dilute solution was found to he 13 M -1 s -1 . Aqueous solutions of palladium, platinum, rhodium, cobalt, tin, silver, zinc, nickel, titanium, cadmium, mercury, arsenic, and selenium were also examined for their potential to act as oxidants to produce composite polypyrrole colloids. Palladium, platinum, and rhodium salts were suitable oxidants, producing polypyrrole in less than 12 h.

https://iopscience.iop.org/article/10.1149/1.1405802/pdf

Reaction of Pyrrole and Chlorauric Acid A New Route to Composite Colloids

Polymerization of acrylamide initiated by the redox system pinacol—Mn(III)

Polymerization of 2-hydroxyethyl methacrylate (HEMA) initiated by the redox system manganese triacetate (Mn(III)/cyanoacetic acid (CAA) was investigated kinetically in DMF and in HOAc/water (80:20 v/v). Monomer (M) conversion was followed gravimetrically. Polymerization was carried out at 31°C and at 41°C. The rate of polymerization R p in DMF is expressed by the equation, R p α [M] 1,4 [Mn(III)] 0,4 [CAA] 0,4 . In HOAc/water, the polymerization exhibited a strong dependence on the concentration of Mn(III). That is, at low concentrations of Mn(III), the rate variations are [M] 1,0 , [CAA] 0,5 and [Mn(III)] 0,5 , while at high Mn(III) concentration the polymerization rates are independent of [Mn(III)] and vary as a function of [M] 1,5 and [CAA] 0,5 . Enhanced oxidation of the primary radicals by Mn(III) at higher concentrations has been elicited to explain the observed orders. The degree of polymerization was found to increase with increase in concentrations of monomer and decrease in concentrations of CAA and Mn(III). The polymerization rates as well as the molecular weights of the polymers are higher in HOAc/water than in DMF. The enhanced polymerization rates have been attributed to the increased propagation and the reduced termination rates.

Redox polymerization of 2‐hydroxyethyl methacrylate, 2. Kinetics, mechanism and solvent effect using manganese triacetate/cyanoacetic acid as the redox system

Oxidation of ferrous orthophenanthroline (FeP) by peroxydiphosphate (PP) in aqueous medium at pH 1 was followed spectrophotometrically. Kinetic analysis has shown that oxidation occurs via the formation of an intermediate complex between FeP and PP. Equilibrium and rate constants were calculated. Influence of surfactants on the oxidation of FeP by PP was also investigated. The equilibrium constant for complex formation was found to be higher in the presence of surfactants. The enhanced complex formation has been attributed to the ionic interactions between the charged surfactant and the ionic species in the reaction medium. Polymerization of HEMA initiated by the redox system, FeP/PP, was carried out in aqueous medium, under the conditions of excess reductant over oxidant and excess oxidant over reductant. The polymerization followed different mechanisms under these conditions ; with excess oxidant, the growing polymer radicals underwent oxidative termination, while with excess reductant, primary radical termination was preferred. The effect of surfactants on the aqueous polymerization of HEMA using the redox system FeP/PP was also investigated. In addition to the decrease in rate, the polymerization followed a different mechanism in the presence of surfactants, the growing radicals terminated by mutual interaction.

Redox polymerization of 2‐hydroxyethyl methacrylate (HEMA). I. Influence of surfactants on polymerization kinetics

Polymerisation of acrylonitrile and methyl methacrylate initiated by the system cyclohexanone/Mn(III) has been investigated at 30–35°C in perchloric acid and sulphuric acid media The kinetics are consistent with the formation of an intermediate complex between the ketone and the oxidant whose decomposition leads to the initiating radical In perchloric acid the termination is effected by the oxidant whereas in sulphuric acid primary radicals terminate growing chains Rate and equilibrium constants have been determined and their significance discussed



Die durch das System Cyclohexanon/Mn(III) angeregte Polymerisation von Acrylnitril und Methylmethacrylat wurde bei 30–35°C in perchlorsaurem und schwefelsaurem Medium untersucht Die Kinetik last sich mit der Annahme der Bildung eines Komplexes zwischen dem Keton und dem Oxidans als Zwischenstufe vereinbaren, dessen Zerfall die startwirksamen Radikale liefert In Perchlorsaure erfolgt der Kettenabbruch durch das Oxidans, wahrend in Schwefelsaure die Primar-Radikale fur den Abbruch der wachsenden Ketten verantwortlich sind Geschwindigkeits- und Gleichgewichtskonstanten wurden bestimmt, und ihre Zuverlassigkeit wurde diskutiert

Redox polymerisation, 2. Kinetics of the reaction initiated by the system cyclohexanone/Mn(III)

The polymerization of acrylonitrile initiated by the redox system 2,2′-thiodiethanol/Ce4+ in dilute sulfuric acid was investigated in the temperature range of 15–25°C. Oxidation of the substrate in the absence of the monomer has also been studied. The reaction involves the formation of an intermediate complex between the metal ion and the protonated species of the reductant, whose decomposition gives rise to the initiating free radicals. Multual interaction of the growing macroradicals accounts for the termination of polymerization. A suitable kinetic scheme has been proposed and rate and equilibrium constants evaluated.

V. Mahadevan

Papers

Polymerization of acrylamide initiated by the redox system pinacol—Mn(III)

Redox polymerization of 2‐hydroxyethyl methacrylate, 2. Kinetics, mechanism and solvent effect using manganese triacetate/cyanoacetic acid as the redox system

Redox polymerization of 2‐hydroxyethyl methacrylate (HEMA). I. Influence of surfactants on polymerization kinetics

Redox polymerisation, 2. Kinetics of the reaction initiated by the system cyclohexanone/Mn(III)

Polymerization of acrylonitrile initiated by the redox system 2,2′‐thiodiethanol/Ce4+ in aqueous sulfuric acid