Macro-radical

When these polymers are subjected to light of A = 365 nm in bulk vinyl monomer, (MMA or styrene) grafted or extensive crosslinking polymers were produced. The photografting or photocrosslinking occurs through the macro-radicals photochemically generated on the backbone of the polymer ... 

Decreasing with chain transfer to monomer or transfer agents. (This process is not as limited by diffusion as macro-radical-macroradical termination reactions.)... 

Only active species (e.g., macro radicals or macro ions) can add further monomer molecules in the propagation process. 

Recently Kargin, Usmanov and Aikhodzhayev [7] prepared a grafted copolymer of nitrocellulose with vinyl chloride or vinylidene chloride. The method roughly consists in the formation of free macro-radicals of nitrocellulose (through y-irra-diation or — more effectively — ozonization of nitrocellulose). After that, the monomer of vinyl chloride or vinylidene chloride is reacted with the macro-radical in the presence of a redox catalyst. This eventually gives a copolymer of the following diagrammatic structure ... 

When the decomposition of the hydroperoxide is activated by adding ferrous ions, the reaction proceeds by a redox system giving only macro-radicals ... 

These macro-radicals react with atmospheric oxygen [44] to give oxidation products (Scheme lb). 

Scheme 1 a Electrons, ions, excited neutrals and photons that are present in a discharge react with the polymer surface to form radicals, b The macro-radicals formed on the polymer surface react with atmospheric oxygen to give oxidation products... 

Co+3, Mn+2 and Fe+2 have been found to be effective in producing free radical sites on the polymer backbone through the alcohol groups present on them [75]. In an alternative method, free radical initiators like BPO and AIBN are thermo-chemically activated to give rise to macro-radical sites on polymer backbone to initiate grafting of desired vinylic monomer. The efficiency of these initiators was found to be predominantly dependent on the nature of monomer while the course of reaction depended on the relative reactivity of monomer versus that of the macro-radical. 

The reactions shown in Scheme 4a,b are assumed to operate during the initiation processes involving redox systems. The resultant radicals (S, HO and S04 ) were assumed to interact with the polymer, producing macro-radicals (via H abstraction) and initiating grafting in the presence of monomer [76],... 

Nitroxide mediated polymerization (NMP) is another type of controlled radical polymerization technique used to synthesize polymer hybrids. It relies on the reversible trapping of growing macro-radicals by nitroxide to form dormant species in which the C-ON covalent bond is thermally cleavaged (Fig. 19). At a polymerization temperature, the equilibrium between dormant and active species is strongly shifted to the dormant side, which Emits the irreversible chain termination reaction. 

We have investigated in details several typical reactions of macro-radicals with various agents oxidation, hydrogen elimination and monomer addition 38). 

Polyfvinyl chloride) (PVC) is produced by mass, suspension, and emulsion processes. Mass polymerization is an exatiiple of a heterogeneous bulk system. PVC is virtually insoluble in vinyl chloride because the polymer is about 35% more dense than the monomer under normal polymerization conditions. Vinyl chloride, however, is quite soluble in polymer. The two phases in PVC polymerizations are pure monomer and monomer-swollen polymer. Polymerization proceeds in both phases, but it is very much faster in the polymer-rich phase because the mobility of macro radicals and mutual termination reactions are. severely restricted (cf. Section 6.13.2). 

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

Hence the Norrish II process leads to formation of an alkene [3(a)] or chain scission [3(b)], The fate of the pol3nneric macro-radical depends on the sequence distribution of the copolymer. Consider the macroradlcal ... 

The radicals suffer further reactions with formation of small molecules and other free (macro)radicals. The formation of two small molecules with six carbons (some through hydrogen transfer) are shown below as an example ... 

If two homopolymers are masticated together then terminal macro-radicals (-Mj ) and (-M2O are formed that can cross-terminate to give a block copolymer. In this case there will be a blend of two homopolymers as well, since formation of the cross-termination product will compete with recombination of the macro-radicals. There will also be incomplete chain scission of the homopolymers. This strategy has also been employed to produce compatibilization and enhanced interfacial adhesion of immiscible homopolymers since the block copolymer will be soluble in each phase and thus able to bridge the phase boundary. This and other topics concerning polymer blends are discussed in Section 1.3. 

By applying the given formulas at 450 K, the recombination constant of macro-radicals in molten polymethylene becomes... 

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 H/C ratio of the deposit gradually decreases and, at high temperatures, the hard aromatic C-C and C-H bonds can be broken also. Initially, Pch3 and Pch2 are the preferred positions for H-abstraction reactions with the formation of Rch2 and Rch macro-radicals. Less stable radicals are generated next. All the radicals are active either in H-abstraction or in substitutive additions otherwise, they recombine with each other. 

For revealing of influence of conformational state of macro-radicals on kinetics of radical polymerization of acrylate- and methacrylate-guanidines in water mediums with the help of viscosimetry method the values of macroscopic viscosities in solutions modeling reaction mixtures at low conversion degrees were measured and obtained data were compared with kinetic ones. 

Terminal macro-radicals produced in polyethylene by the Norrish type I reaction exhibit a high reactivity in hydrogen transfer reactions. Hydrogen abstraction from a neighbouring chain... 

The fact that the decrease in molecular weight in the photodegradation of polymethylvinylketone does not continue at the same rate throughout the irradiation has been ascribed to the occurrence of a competing reaction that opposes the main chain scission process. This is assumed to be the formation of new polymer—polymer linkages by mutual recombination of macro-radicals resulting from the addition of CHj or CH3—CO to unsaturated chain ends formed in the Norrish type II reaction [55]. Evidence for the presence of such macro-radicals is found in the production of graft copolymers when solutions of polymethylvinylketone in various monomers (acrylonitrile, methyl methacrylate, vinyl acetate) are irradiated [57]. 

However, there is no chemical evidence for such a process. Main chain scission in irradiated polymethylmethacrylate is probably due to the decomposition of the macro-radical produced in reaction (5) namely,... 

The nine-line ESR spectrum observed after irradiation at room temperature has been attributed to the propagating radical formed in reaction (7) [76]. However, according to other workers [77] this signal could also result from the addition of another radical to residual monomer molecules. It is of interest to note that the quantum yield of main chain scissions is five to ten times smaller than the quantum yield of side group splitting by reaction (5), whereas both processes occur with the same yield in the radiolysis of polymethylmethacrylate [78]. This indicates that only a fraction of the macro-radicals decompose according to reaction (7) at room temperature. 

Such a reaction has been shown to give 1-butene and carboxyl groups in equivalent yields (about 2 x 10-2) in the liquid-phase photolysis of undiluted di-n-butylterephthalate [116]. From Table 4 it can be seen that the quantum yields of chain scission and of carboxyl group formation are almost identical this suggests that reaction (22) is the main cause of chain scission in the photolysis of polyethylene terephthalate. It must also be pointed out that reactions (19) and (21) do not necessarily yield chain scission, since the probability of the macro-radicals escaping the cage is rather low in a rigid matrix. Indeed, the appearance of an absorption maximum near 775 cm-1 in the infrared spectrum of polyethylene terephthalate irradiated at 313 nm has been ascribed to... 

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