Free radical reactions crosslinking

The free radical reactions of photolysis can result in chain scission at the amide linkage of the nylon polymer or crosslinking between polymeric chains (15, 17). Degradation of nylon by photolysis is independent of oxygen, heat, moisture or additives. 

There are several interesting polymerization schemes intermediate between a sequential IPN and an SIN. For example, in in situ prepared sequential IPNs, both monomers are polymerized via free radical reaction [He et ai, 1993 Rouf et ai, 1994]. The two monomers must have quite different reactivities towards the free radicals. This situation arises with vinyl or acrylic double bonds and aUylic double bonds. The allylic double bonds react about 100 times slower than acrylic or methacrylic bonds. Often, two initiators are used, one reacting at a lower temperature, and the other at a higher temperature. In one of the systems studied, based on methyl methacrylate and diallyl carbonate of bisphenol-A (DACBA), first, crosslinked PMMA was formed at moderate temperatures. Then, by just increasing the temperature after completion of the first polymerization, the synthesis of the allylic network followed. 

In another study, the monoacrylate 35 was grafted onto PP in the presence of tris(acryIoyhnethyl)propane (36) as coagent.Again close to 100% grafting yield was obtained. This was so despite the fact that 35 was anticipated to be an inhibitor of free radical reactions (in fact, phenols are poor inhibitors of (meth)acrylate polymerization - Section 5.3.4). The tris-acrylatc 36 and related species have previously been used for producing crosslinked/branched xhe structure... 

Halogenation reactions of unsaturated polymers follow two simultaneous paths, ionic and free radical. Ionic mechanisms give soluble products from chlorination reactions of polybutadiene." The free-radical mechanisms, on the other hand, cause crosslinking, isomerization, and addition products. If the free-radical reactions are suppressed, soluble materials form. Natural rubber can be chlorinated in benzene with addition of as much as 30% by weight of chlorine without cycliza-tion. " Also, chlorination of polyalkenamers, both cis and trans, yields soluble polymers. X-rays show that the products are partly crystalline. The crystalline segments obtained from 1,4-trans-polyisoprene are diisotactic poly( 0 /rw-dichlorobutamer)s while those obtained from the 1,4-cis isomer are diisotactic polyOAfieo-l,2-dichlorobutamer)s. ... 

The chemical crosslinks or chemisorptions of the polymer on to the filler surface due to free radical reaction between carbon atoms in the filler and in the rubber. 

Peroxides dissociate into free radicals at a temperature specific to the selected peroxide. With the peroxide mixed into polyethylene a free radical reaction takes place which results in crosslinks between the molecular chains resulting in a network of molecules which increase the heat distortion temperature of the polymer. 

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