Free-radical mechanism, cross-linking

EPR spectroscopy is also frequently used in polymer science. Applications involve the detection of free radicals in cross-linking, high-energy irradiation, photochemical degradation and oxidation, and mechanical fracture of polymer chains. 

In contrast, antioxidants can have an opposite effect when peroxide curing. Because peroxide cross-linking involves a free-radical mechanism, and antioxidants are designed to scavenge free radicals, it is obvious that peroxide efficiency can be compromised by the addition of antioxidants. Thus the decomposition products of the ppds were acting as accelerators (29). 

The furfuryl esters of acrylic and methacrylic acid polymerize via a free-radical mechanism without apparent retardation problems arising from the presence of the furan ring. Early reports on these systems described hard insoluble polymers formed in bulk polymerizations and the cross-linking ability of as little as 2% of furfuryl acrylate in the solution polymerization of methylacrylate121. ... 

Fluoroelastomers based on polyvinylidene fluoride (VDF) can be cross-linked by ionic mechanism. However, if the polymer has been prepared in the presence of a cure site monomer (CSM) it can be cross-linked (cured) by a free radical mechanism. 

More recently, MPO-mediated oxidation of tyrosine to dityrosine (o o -dityrosine, or 3,3 -diiyrosine) focused attention as a marker reaction of neutrophile-dependent oxidative damage of proteins and peptides (G11, H14, S3). The reaction occurs both with free tyrosine as well as with tyrosyl residues incorporated into polypeptide structures. The mechanism of dityrosine formation utilizes a relatively long-lived phenoxyl radical that cross-links to dimeric and polymeric structures by formation of carbon-carbon bonds between the aromatic moieties of phenolic tyrosine residues (H14) (Fig. 9). 

Cross-Linking by Free Radical Mechanism (Peroxide Cure)... 

Initiators are used to initiate the curing reaction at elevated temperatures. Cross-linking or polymerization occurs by a free radical mechanism in which the double bond of the polyester chain reacts with the vinyl monomer that is usually styrene, and this reaction provides a three-dimensional network that converts the viscous resin to a hard thermoset solid. The initiators added decompose at elevated temperatures thus providing free radicals to initiate the cross-linking. Peroxyesters and peroxyketals are the most common classes of peroxides used as initiators. 

Cross-linking is carried out in iilu after a column is coated with one of the polymers listed in Table 27-3. One way of cross-linking is to incorporate a peroxide into the original liquid. When the film is heated, reaction between the methyl groups in the polymer chains is initialed by a free radical mechanism. ITie polymer molecules are then cross-linked through carbon-lo-carbon bonds. The resulting films are less... 

Cross-linking of the outer casing is achieved by grafting a vinyl silane to polyethylene by means of a peroxide as part of the cable extrusion process. The chemistry is shown in Scheme 2.1. When the cable is subsequently soaked in water, the alkoxy silanes hydrolyse and crosslink within the matrix. It is an unresolved question whether polymers that have been modified by a free radical mechanism will survive for the expected lifetime of underground cables (ca. 75 years). 

Free radical mechanisms also serve to explain the photo-cross-linking of various polymers, such as that of polyethylene accomplished with the aid of lightabsorbing additives such as benzophenone, quinone, benzoin, acetophenone, or their derivatives. When electronically excited by light absorption, these additives either directly abstract hydrogen from the polymer or decompose into free radicals capable of abstracting hydrogen, as shown in Schemes 7.10 and 7.11. 

Additional photoproducts, commonly generated via free radical mechanisms, have been identified. These include single-strand breaks, cross-links between the strands of the same double helix and between different DNA strands and adjacent protein molecules, and the so-called photohydrates (see Chart 8.5). 

Two types of thermoset polyimides are currently prepared commercially. They are based on low molecular weight bis imides such as bis maleimides or bis-5-norbomene-2,3-dicarboximides. Due to unsaturations, the materials cross-link by free-radical mechanism into tight networks. Michael type additions of primary and secondary amines to the bis maleimides are often used to chain-extend them before cross-linking. This reduces the cross-linking density and the brittleness [115]. The materials are designated by the term PMR, for polymerizable monomeric reactants. 

Halogenation reactions of unsaturated polymers follow two simultaneous paths, ionic and free radical. Ionic mechanisms give soluble products from chlorination reactions of polybutadiene [42]. The free-radical mechanisms, on the other hand, cause cross-linking, isomerization, and addition products. If the free-radical reactions are suppressed, soluble materials form. Natural rubber can be chlorinated in benzene, however, with addition of as much as 30% by weight of chlorine without cyclization [39, 40]. Also, chlorination of polyalkenamers both cis and trans yields soluble polymers. X-rays show that the products are partly crystalline [43, 44]. The crystalline segments obtained from... 

In direct fluorination of powdered high-density polyethylene with the gas, diluted with helium or nitrogen, the accompanying exotherm causes partial fusion. In addition, there is some destruction of the crystalline regions [137]. On the other hand, fluorination of single crystals of polyethylene can result in fluorine atoms being placed on the carbon skeleton without disruption of the crystal structure. The extent of cross-linking, however, is hard to assess [138]. The reaction has all the characteristics of free-radical mechanism [139] ... 

Delayed-Action Accelerated Vulcanization. If cross-link formation is by a free radical mechanism, delayed action could be the result of a quenching action by the monomeric polysulfldes formed by reactions between accelerator and sulfur. If the polymeric poly-thiyl radicals (cross-link precursors) are rapidly quenched by an exchange reaction before they are able to form cross-links, cross-link formation would be impeded until substantial depletion of the monomeric polysulfldes. This is illustrated in Scheme 4. 

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