Stabilizing macro-radicals

The iV-phenyl-P-naphthalamine also donates its H-atom to stabilize the macro-radical... 

Unsaturated hydrocarbons can add to the deposit-forming cyclic structures with five or six C-atoms through Diels-Alder reactions. The successive dehydrogenations produce polycyclic aromatic structures. In the third reaction class, aromatic species add to macro-radicals of the surface. Note that these radicals are greatly stabilized by the aromatic resonance. Thus, the third reaction class is more important at lower temperatures, typically, in TLE, visbreaking and delayed coking units. 

The medium in which elastomer degradation on rollers occurs represents a key factor in the evolution of the process. In an active medium (oxygen), polymer structuring may be observed in either branched or crosslinked reaction products thus resulting, as due to the reactions of macro radicals stabilization preferentially through transfer to the polymer. 

Bulk and suspension polymerization follow the same principles when all the system components are soluble in the monomer phase (initiators, macro radical, chain transfer agents). The rate of polymerization is not influenced by particle size and stabilizer type. 

The mechanism of peroxide crosslinking is shotvn in Scheme 16.38. Upon heating, the peroxide decomposes into the primary alkoxy radicals, tvhich may further react to secondary radicals. These radicals abstract hydrogen atoms from the EPM chain. In the case of EPM, crosslinks are formed by combination of two macroradicals, whereas in the case of EPDM crosslinks are formed via combination, but also via addition of the macro-radical to the pendent unsaturation of a second EPDM chain. The formation of C-C bonds explains the higher thermal stability of peroxide-cured EPDM in comparison with sulfur-vulcanized EPDM with its labile S-S crosslinks. The reactivity for peroxide cure increases in the series ENB DCPD < VNB, because of the decreased steric hindrance at the unsaturation. The efficiency of peroxide curing is enhanced by the addition of co-agents, that is, chemicals with two or more unsaturated bonds, which are actually built... 

As a consequence, multisegment block copolymers may be formed. The higher the initial polymer molecular weight and the monomer tendency to termination by combination, the more complex is the block copolymer structure. If the macro radicals are stabilized by transfer to monomer, the tendency of the monomer to homopolymerize or to block copolymerize depends on the reactivity ratio of the monomer toward the two radicals. 

The formation of macro-radicals created by non-thermally stable structures is observed prior to statistical chain scission. The thermal behavior of PMMA at temperatures below major degradation depends on many factors that have to be considered, in particular during certain stabilizer-free polymerization processes with high levels of radical generation. 

As discussed earlier in this chapter, a major difficulty in the compatibilization of PE/ PP blends by free radical reaction in the melt is to avoid the side reactions, PE cross-linking and PP chain scission. To overcome this difficulty, the use of a co-reagent is necessary in the process for either stabilizing the macro-radicals formed on PE and PP molecules, or leading the reaction directly to the PE/PP interface. 

However, inconsistent results had been reported by Puglia et al., where they found the thermal degradation rate increased and the onset temperature of thermal degradation decreased for epoxy resin after the addition of CNTs. It was attributed to the high thermal conductivity and specific surface areas of CNTs which can accelerate the transfer of heat. This interpretation is not completely substantiated. The improved thermal stability of PMMA was also proved in thermal degradation experiment of CNT/PMMA." A study by Troitskii et al. indicated that C60 can combine with free radicals produced in the thermal degradation process of PMMA, and form relative steady macro-free radicals. 

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