Chain initiation redox

Variable valence transition metal ions, such as Co VCo and Mn /Mn are able to catalyze hydrocarbon autoxidations by increasing the rate of chain initiation. Thus, redox reactions of the metal ions with alkyl hydroperoxides produce chain initiating alkoxy and alkylperoxy radicals (Fig. 6). Interestingly, aromatic percarboxylic acids, which are key intermediates in the oxidation of methylaromatics, were shown by Jones (ref. 10) to oxidize Mn and Co, to the corresponding p-oxodimer of Mn or Co , via a heterolytic mechanism (Fig. 6). 

Evans and co-workers carried out a detailed study of Fenton s reagents (FeS04 H202) as a redox initiator, where OH are known to be the chain initiating species. The mechanism is as under ... 

Similarly, a recent study141 of the homogeneous oxidation of cyclohexene by various low-valent phosphine complexes of Group VIII transition metals yielded no definite proof for initiation by oxygen activation. Results were consistent with reactions involving chain initiation via the usual redox reactions of the metal complexes with traces of hydroperoxides. Long induction periods were observed with peroxide-free hydrocarbons. 

A variety of other means, besides thermal decomposition of initiator, can be used to produce radicals for chain initiation, such as redox reactions, ultraviolet irradiation, high-energy irradiation, and thermal activation of monomers. The expression for Ri wUl be different in each case and inserting it into the same equation (6.23) will yield the corresponding expression for the rate of polymerization. 

Wolfram [90] has shown that polymer add-on increases with pH of the reducing solution, up to pH 7, where it appears to level, and THPC dissociation and sulfur-sulfur bond cleavage also increase with pH. The fact that polymer add-on is not higher at pH 9.2 than at pH 7.0 suggests interference of alkahnity in a subsequent reaction step. The most probable complications are in the chain initiation or propagation steps, given that Wolfram indicates that the cysteine-persulfate redox system is optimum in the pH region of 1.5 to 3.5. 

A commercial guargum supplied by Hindustan Chemicals and Gums, Bhiwani, India, has been used. To remove the protein, fat and fibrous impurities, a method given by Whitcomb et al. 3] has been adopted to purify the guargum. The various amounts of polyacrylamide chains have been grafted onto guargum main chain using redox initiators by solution polymerisation technique. 

Reactive acrylic adhesives generally consist of a solution of a toughening rubber (chlorosulphonated polyethylene) in a partly polymerized mixture of monomers this is mainly methylmethacrylate but ethane diol dimethacrylate is added as a cross-linking agent. The remaining monomer is polymerized in a free radical chain polymerization redox initiation involves an organic peroxide and a tertiary amine. Acrylic cements consist of a partly polymerized acrylic monomer containing an initiator. Cure is established by the thermal or UV decomposition of the initiator (see Radiation-cured adhesives). 

Reactions (22) and (23) require metal ions which can exist in two oxidation states with a suitable redox potential. Reaction (22) is usually much more rapid than reaction (23), and the metal is converted mostly in its most oxidized state, so that the rate of chain initiation depends on reaction (22). In the presence of metals, these reactions thus initiate additional autoxidation chains, which accelerate the rates of lipid oxidation. 

The presence of the organically modified MMT, typically with octadecylam-monium (ODA), yielded in an even more pronounced degradation due to the influence of the ammonium ion which becomes preponderant. It was assumed that may generate acidic sites in the clay layers and even the complex crystallographic structure of clay may result in some acidic sites after functionalization [116]. Associated to the catalytic effect of transition metal cations via the reversible photochemically initiated redox reactions, it induced the formation of free radicals and chain scission upon UV exposure. Therefore, the degradation of these nanocomposites is much faster than the ones with raw MMT. 

By plotting the redox potentials of the electron carriers (Table 14.1) the pathway of the water-derived electrons may be followed. This is referred to as the Z-scheme (Figure 14.4), first proposed by Hill and Bendall in 1960. Although alternative models have been proffered the Z-scheme remains fundamental to our understanding of photosynthetic electron transport. The interaction of a photon with the reaction centre of PS II initiates a chain of redox reactions. The first recognized electron acceptor of PS II is a bound molecule of metastable phaeophytin a (unchelated chlorophyll a) which instantly donates the electrons to one of two protein-bound plastoquinone (Figure 8.7c) molecules which has been identified as the primary... 

Emulsion Process. The emulsion polymerization process utilizes water as a continuous phase with the reactants suspended as microscopic particles. This low viscosity system allows facile mixing and heat transfer for control purposes. An emulsifier is generally employed to stabilize the water insoluble monomers and other reactants, and to prevent reactor fouling. With SAN the system is composed of water, monomers, chain-transfer agents for molecular weight control, emulsifiers, and initiators. Both batch and semibatch processes are employed. Copolymerization is normally carried out at 60 to 100°C to conversions of - 97%. Lower temperature polymerization can be achieved with redox-initiator systems (51). 

Suitable catalysts are /-butylphenylmethyl peracetate and phenylacetjdperoxide or redox catalyst systems consisting of an organic hydroperoxide and an oxidizable sulfoxy compound. One such redox initiator is cumene—hydroperoxide, sulfur dioxide, and a nucleophilic compound, such as water. Sulfoxy compounds are preferred because they incorporate dyeable end groups in the polymer by a chain-transfer mechanism. Common thermally activated initiators, such as BPO and AIBN, are too slow for use in this process. 

Another method for producing petoxycatboxyhc acids is by autoxidation of aldehydes (168). The reaction is a free-radical chain process, initiated by organic peroxides, uv irradiation, o2one, and various metal salts. It is terrninated by free-radical inhibitors (181,183). In certain cases, the petoxycatboxyhc acid forms an adduct with the aldehyde from which the petoxycatboxyhc acid can be hberated by heating or by acid hydrolysis. If the petoxycatboxyhc acid remains in contact with excess aldehyde, a redox disproportionation reaction occurs that forms a catboxyhc acid ... 

A typical recipe for batch emulsion polymerization is shown in Table 13. A reaction time of 7—8 h at 30°C is requited for 95—98% conversion. A latex is produced with an average particle diameter of 100—150 nm. Other modifying ingredients may be present, eg, other colloidal protective agents such as gelatin or carboxymethylcellulose, initiator activators such as redox types, chelates, plasticizers, stabilizers, and chain-transfer agents. 

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. 

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