Stabilizers reaction with macroradicals

Recent studies indicate that a-tocopherol can also directly react with alkyl macroradicals, this reaction being more likely due to the higher mobility of the alkyl macroradicals compared to that of peroxy radicals [70]. The alkyl macroradicals react with a-tocopherol, giving a polymeric saturated molecule and a stabilized phenoxy radical (Scheme 13, Part B). The phenoxy radical can in turn interact with another alkyl macroradical leading to the formation of a,(3-unsaturated ketones [69, 70]. This mechanism indicates a double efficiency of stabiUzation of vitamin E one vitamin E molecule can convert up to two alkyl macroradicals to umeactive species. 

Methyl methacrylate (Fig. 1-4) and methacrylonitrile (6-5) are allylic-type monomers that do yield high molecular weight polymers in free radical reactions. This is probably because the propagating radicals are conjugated with and stabilized to some extent by the ester and nitrile substituents. The macroradicals are... 

Combination vs Disproportionation. There are two modes of termination one is the direct coupling (combination) of two free macroradicals to give a dead polymer chain of chain length i +j, with the rate coefficient At,c- The other mode is the so-called disproportionation, where a hydrogen atom is transferred from one of the radical chain ends to another radical, yielding two stabilized polymer chains, of which one carries a double bond. This reaction is associated with the rate coefficient t,d. The process is illustrated in equation 44 using polyethylene macroradicals as the example. It is important to notice that—in the case of macroradicals derived from other monomers—in principle any -hydrogen may be abstracted. 

The main way for their stabilization should be, in this situation, the crossed recombination, with block-copolymers formed in most cases. If one of the polymers shows a high degradation rate in comparison with another polymer, macroradicals of this type will predominantly appear in the reaction medium their stabilization will partially involve chain transfer, in which the macromolecules of the non-degraded polymer take part, and in the chains of which radicalic active centers will be formed. 

Opposite to chemisorption, physical adsorption has no influence on the rate of initiator decomposition. On the contrary, depending on the surface natixre of filler, pol5nnerization of vinyl monomers, in the presence of peroxides and azocompounds, is accelerated by fine, dispersed silica. The activation of monomeric molecules occurs, due to the complex formation of functional groups of monomers with OH-groups frequently present on filler surface. Also, the orientation of monomer molecules on the surface and stabilization of macroradicals may take place, hindering the termination reactions by decreasing molecular mobility in the adsorption layer. 

In the post-irradiation oxidation process, the rate of oxidation decreases by more than one order of magnitude in the first 100 hours, although alkyl macroradicals are continuously formed along with the formation of hydroperoxides (Scheme 9, Reaction 23). The termination reaction of thermo-oxidative processes is generally described as a Russell reaction between two peroxy species. The relative inunobility and the stability of the peroxy radical makes the Russell bimolecular termination strongly disfavored in the solid state at room temperature [22, 23]. More likely. 

To stabilize the polyethylene, the addition of a reactive species is required. The additive (ADH) reacts with the polymeric macroradical (alkyl or peroxy), giving a kinetically and thermodynamically stable radical (AD ) and a saturated macromolecule, according to the following reaction ... 

As mentioned above, it is known that intramolecular chain transfer, in particular, 1,5-hydrogen shift, does also occur during the polymerization of monomers that yield very reactive macroradicals, such as acrylates and acrylic acid. This so-called backbiting reaction, by which a secondary radical (SPR) is transformed into a more stabilized tertiary (MCR) one, proceeds via a six-membered cyclic transition state with rate coefficient kbb (see Scheme 1.17). In principle, intramolecular chain transfer to a remote chain position and intermolecular chain transfer to another polymer molecule may also take place.These latter processes are, however, found to be not significant in butyl acrylate polymerization at low and moderate degrees of monomer conversion and temperature. ... 

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