Redox systems initiators

Reactions take place in the presence of redox systems, initiators, and suitable pH medium. The nucleophilic sulphur atom from cysteines may allow a broad range of chemical modifications, including redox reactions [50] like thiol/disulfide exchange, oxygen transfer redox couples, and thiol/thlyl radical transfer reactions, all of which occur during steady-state cellular conditions [51],... 

The Cu /potassium disulfide [199] and Cu /metabisulfide [200] redox systems have been used in the polymerization of acrylonitrile and acrylamide, respectively. The cupric sulfate-hydrazine redox system in which hydrazyl radicals are responsible for the initiation was studied in the absence and presence of molecular oxygen. The Cu /hydrazine hydrate [201-204] and Cu /2-aminoethanol [205] systems were used for the polymerization of vinyl monomers. Misra [206] demonstrated that the polymerization of acrylamide could be initiated by the Cu /metabisulfide redox system. Initiating systems of cupric(n) ions in conjunction with dimethyl aniline [207] and a-amylase [208] were also reported. Cupric-ion-based redox reactions were successfully aj lied [209] to graft vinyl monomers onto wool and Nylon-6. 

Copolymerization is effected by suspension or emulsion techniques under such conditions that tetrafluoroethylene, but not ethylene, may homopolymerize. Bulk polymerization is not commercially feasible, because of heat-transfer limitations and explosion hazard of the comonomer mixture. Polymerizations typically take place below 100°C and 5 MPa (50 atm). Initiators include peroxides, redox systems (10), free-radical sources (11), and ionizing radiation (12). 

The most common water-soluble initiators are ammonium persulfate, potassium persulfate, and hydrogen peroxide. These can be made to decompose by high temperature or through redox reactions. The latter method offers versatility in choosing the temperature of polymerization with —50 to 70°C possible. A typical redox system combines a persulfate with ferrous ion ... 

Emulsion Polymerization. In this method, polymerization is initiated by a water-soluble catalyst, eg, a persulfate or a redox system, within the micelles formed by an emulsifying agent (11). The choice of the emulsifier is important because acrylates are readily hydrolyzed under basic conditions (11). As a consequence, the commonly used salts of fatty acids (soaps) are preferably substituted by salts of long-chain sulfonic acids, since they operate well under neutral and acid conditions (12). After polymerization is complete the excess monomer is steam-stripped, and the polymer is coagulated with a salt solution the cmmbs are washed, dried, and finally baled. 

Generally the oxidant is compounded in one part of the adhesive, and the reductant in the other. Redox initiation and cure occur when the two sides of the adhesive are mixed. There also exist the one-part aerobic adhesives, which use atmospheric oxygen as the oxidant. The chemistry of the specific redox systems commonly used in adhesives will be discussed later. The rates of initiation and propagation are given by the following equations ([9] p. 221). 

Polymerization in aqueous solution of acrylamide can also be fulfilled in thin layers (up to 20 mm) applied on a steel plate or a traveling steel band. Polymerization is initiated by persulfates, redox system, UV or y radiation. Polymerization proceeds in isothermal conditions as the heat of polymerization is dissipated in the environment and, additionally, absorbed by the steel carrier. Nonadhesion of the polymer to the carrier is ensured by the addition of glycerol to isopropyl alcohol or by precoating the steel band with a film based on fluor-containing polymers. This makes polymerization possible at a high concentration of the monomer (20-45%) and in a wider process temperature range. This film of polyacrylamide is removed from the band, crushed, dried, and packed. 

Organic peroxide-aromatic tertiary amine system is a well-known organic redox system 1]. The typical examples are benzoyl peroxide(BPO)-N,N-dimethylani-line(DMA) and BPO-DMT(N,N-dimethyl-p-toluidine) systems. The binary initiation system has been used in vinyl polymerization in dental acrylic resins and composite resins [2] and in bone cement [3]. Many papers have reported the initiation reaction of these systems for several decades, but the initiation mechanism is still not unified and in controversy [4,5]. Another kind of organic redox system consists of organic hydroperoxide and an aromatic tertiary amine system such as cumene hydroperoxide(CHP)-DMT is used in anaerobic adhesives [6]. Much less attention has been paid to this redox system and its initiation mechanism. A water-soluble peroxide such as persulfate and amine systems have been used in industrial aqueous solution and emulsion polymerization [7-10], yet the initiation mechanism has not been proposed in detail until recently [5]. In order to clarify the structural effect of peroxides and amines including functional monomers containing an amino group, a polymerizable amine, on the redox-initiated polymerization of vinyl monomers and its initiation mechanism, a series of studies have been carried out in our laboratory. 

In addition, we found that all of the functional monomers having amino group would act as an amine component and with LPO form a redox system to initiate the polymerization of functional monomer itself with the rate of polymerization as ... 

Random copolymer—Distribution of the monomer units does not follow any definite sequence. These are produced in bulk, aqueous, suspension, or emulsion using free radical initiators of the peroxide type or redox systems. 

Conventional radical initiators, various redox systems, /3-diketonates of transition metals alone and in conjunc-... 

In conjunction with Ag, CO , and acid, peroxydiphosphate forms an efficient redox system for polymerization of vinyl monomers. H2P208 is assumed to be an active species of peroxydiphosphate. The initiating species are OH and HP04 and the termination is considered to be exclusively by mutual method. The following mechanism has been proposed for the redox reaction [46]. 

Metal chelates afford a better initiating system as compared to other redox systems since the reactions can be carried out at low temperatures, thus avoiding wastage reactions due to chain transfer. Homopolymer formation is also minimum in these systems. It was observed by Misra et al. [66,67] that the maximum percentage of grafting occurs at a temperature much below the decomposition temperature of the various metal chelates indicating that the chelate instead of undergoing spontaneous decomposition receives some assistance either from the solvent or monomer or from both for the facile decomposition at lower temperature. The solvent or monomer assisted decomposition can be described as ... 

In aqueous solutions the persulphate ion is known as a strong oxidizing agent, either alone or with activators. Thus, it has been extensively used as the initiator of vinyl polymerization [43-47]. However, only later, Kulkarni et al. [48] reported the graft copolymerization of AN onto cellulose using the Na2S203/K2S20s redox system. 

To sum up, the optimum conditions for methylmethacrylate grafting onto pulp by the ceric ion redox system can be summarized as follows the grafting is done at 30°C for a 1-h reaction time, using liquor ratio 40 1, acid concentration 1%, initiator concentration 0.1%, and monomer 1 mL/g pulp. 

The initial radicals formed from the Ce(IV) ion redox system can initiate a monomer to polymerize and form an end group of the resulting polymer. When the reductant exhibits a carbonyl group, the amide group can be conveniently detected by the FT-IR spectrum of the polymer, such as polyacrylonitrile (PAN). The FT-... 

Based on the ESR studies and the end group analysis, the initiation mechanism of Ce(IV) ion redox systems is proposed as ... 

The synthetic methods of macromolecules having an active pendant group include (1) the transformation reactions of polymer and copolymers, and (2) polymerization and copolymerization of functional monomers having active pendant groups. The macromolecules, either in the shape of film or microbeads, can be used as the substrate. As we have mentioned previously, the rate of polymerization initiated with the Ce(IV) ion redox system is much faster than that initiated by Ce(l V) ion alone, as expressed in / r 1. Therefore, the graft... 

Two types of redox systems (Fig. 7) are used for batteries [14]. The standard potential (E°) of MnO 2 should be a good representation of the total energy of the oxide. For two-phase systems such as Pb02, Ag20, HgO, etc., the initial potential (E ) and middle potential (Em ) are equal to E°, from which we can calculate AF (-nFE°). For MnOz, a one-phase system, as shown in Fig. 7(A), the E (initial potential) cannot be used as E°. Ko-zawa proposed the middle potential (Em) of the S-shaped curve to be used as the E°... 

The initiation of graft copolymerization using this redox system, seems to proceed in accordance with the following scheme ... 

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