Macroradical formation

The initiation efficiency depends on the relationship between the rate of macroradical formation and that of initiation. In Table 1 are compiled results on certain redox systems employed for the initiation of cellulose grafting either by direct oxidation of cellulose (or its derivative) or chain transfer from active low molecular weight radicals. Table 1 indicates that in systems where the matrix acts as a reducing agent (1st group), the initiation efficiency does not exceed 15%, i.e. only a minor portion of macroradicals formed at the first stage of oxidation initiates graft polymerization while the rest is oxidized to stable... 

The formation of PP macroradicals is an easy process initiated by more or less any radical initiator. It occurs spontaneously in oxidative processes. Alkyl radiccils (except for methyl) are usually not reactive enough to initiate an efficient macroradical formation in PP. Oxyl radicals, formed by a thermal decomposition of peroxides, are the most convenient species for crosslinking initiation. The transfer of the radical centre to PP is selective to a certain extent. At temperatures usual for peroxide decomposition, the ratio of the rate constants of the abstraction of hydrogen from primary, secondary, and tertiary carbon by the oxyl radical is approximately 1 3 10 [2]. 

Macroradical formation, accompanying permanently mechano-dispersion and mechanodestruction of semicrystalline polymers during vibratory milling, may be also evidenced by the utilization of... 

This chapter considers the investigation related to modification of unsaturated mb-bers by means of ozonation for improving their adhesion properties. Mechanism of macroradicals formation in the ozonation process as well as the influence of the contact time in the modification on adhesion properties of the mbbers has been studied. It has been shown that the best characteristics comparing with the initial ones can be obtained at 1 h ozonation. The gluing strength increases by 10-70% on average at that. 

In ozonation, a partial double bonds breakage in the robber macromolecules that leads to the macroradicals formation occurs (Fig. 17.1). Ozone molecules are attached to the point of the robber double bonds breakage with the formation of epoxy groups [4] ... 

Other evidence for macroradical formation is the coreaction that occurs when two polymers are jointly subjected to mechanical action or when the polymerization of monomers is initiated by the presence of a shear-degraded polymer (see Chapter V). 

Initiation - which leads to macroradicals formation, mainly via the random scissions of the chain ... 

Although melt degradation of polymers is a primary consequence of chain scission, peroxides play as important a role in macroradical formation, leading to further degradative reactions. Reagents... 

---