Macroradicals, tertiary

The presence of sulphonic and carboxylic groups enables the iron ions to be in the vicinity of the cellulose backbone chain. In this case, the radicals formed can easily attack the cellulose chain leading to the formation of a cellulose macroradical. Grafting of methyl methacrylate on tertiary aminized cotton using the bi-sulphite-hydrogen peroxide redox system was also investigated [58]. 

In the course of the depolymerization primary and tertiary terminal macroradicals are formed. A difference in the reactivity these radicals is explained by assuming that the /3-scission depends on the rotational energy barrier the terminal C-C- bond in the primary and tertiary terminal radicals (52). 

The rate constant of a transfer reaction will therefore be the higher, the weaker C-H bond is attacked by a peroxyl radical. From this it is obvious that the maximum rate of oxidation of polyethylene will increase with increasing number of tertiary hydrogens in the polymer [13]. Since the process includes the interaction of a macroradical with a macromolecule which both are of restricted translational mobility, the maximum rate of oxidation does not depend on the low content of reactive allyl hydrogens in polyethylene. 

The thermal or photochemical homolysis of tertiary hydroperoxides leads to the formation of alkoxy macroradicals 3-scission of alkoxy macroradicals may occur. This leads to a, 3-unsaturated ketones on the butadiene component and induces the scission of the butadiene-SAN grafts. The macroradical so formed on the SAN macrophase is the precursor, after isomerization, of the oxidation of the styrenic component according to... 

Formation of butyl branches takes place by a back-biting mechanism via a six-membered ring, while transfer to polymer from macroradicals is a reasonable source to LCB with tertiary chlorine (7 9). We have suggested an alternative mechanism which also explains the formation of internal double bonds and LCB with tertiary hydrogen (7, 8). This mechanism is based on transfer to polymer from chlorine atoms produced in the mechanism for transfer to monomer ... 

On the other hand, crosslinking can be assigned to the recombination of macroradicals produced in reactions (35) and (39) with those formed by abstraction of a tertiary hydrogen from the main chain by the methyl radicals, viz. 

The first step of the oxidative reactions is a hydrogen abstraction on the polymeric backbone. Once formed, the primary methylene macroradicals (A) rearrange to yield the stable tertiary benzylic radicals (B) ... 

The bond strength between hydrogen and carbon in PO chains weakens in the series of groups methyl (-CH3), methylene (-CH2-), methine (-CH-) group (6). That is, hydrogen gets detached more easily from a tertiary atom of carbon. In view of this, it can be assumed that a higher rate of formation of macroradicals should be... 

The analysis of MFI values (Fig. 10.6b) shows that with a PP concentration up to 25 wt%, the blends have a low MFI value (the melt viscosity is high). Low additions of EPR, same as PE additions (Table 10.5), cause an increase in MFI values. Since propylene units belong to the molecular structure of EPR, it appears that at equal concentrations of PE and EPR in blends with PP, the concentration of hydrogen atoms, bonded to tertiary carbon atoms, is higher in the second case. That is why, specificities of I A-grafting reaction, related to the concentration and the reactivity of macroradicals, are more pronounced in PP/EPR than in PP/PE blends. 

As has been shown, PP oxidation occurs predominantly intramolecularly, the kinetic chain moves along the macromolecule. Macroradical RO2, formed by the oxidation of polypropylene, reacts with a hydrogen atom from the tertiary C atom located in the P-position relative to the peroxide radical of their molecules. As a result, intramolecular transfer of a macromolecule oxidized PP formed "blocks" of several adjacent OH-groups. 

Some H radicals, not involved in Reaction 5, diffuse through the polymer bulk and extract H atoms from other macromolecules, resulting in the formation of isolated macroradicals in the polymer bulk (Scheme 2, Reaction 6). The probability of extraction of the H atom decreases according to the following order allyl, tertiary, secondary, primary, with a reactivity ratio between secondary and tertiary of 1 versus 9 (Arnaud, Moisan, and Lemaire 1984). 

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]. 

Of special practical interest in spinning from solution of some polymers may become the macroradicals reactions with aromatic diamines. In this way, chemically modified polymers -containing free aromatic aminic-groups may be obtained. In their turn, they may be either diazotized or coupled with phenols or tertiary mixtures of amines, azoic structures being thus formed at the ends of the macromolecular chains. Depending on the chemical nature of the coupling agent employed, different colorations may results ... 

It was found that the macroradicals recorded by ESR spectroscopy usually have the unpaired electron located to a tertiary carbon atom. It means that the chemical bonds splitting preferentially occur to the chemical bonds containing a totally substituted carbon atom, i.e. at the a-methylstyrene structural units, Figure 3.303. 

A similar procedure was used to obtain spin-labelled TEE-HEP [49]. The presence of hexafluoropropylene (HFP) groups in this polymer leads to disturbance of the structural ordering typical of PTFE to more complex dynamics of their motion. After y-irradiation of powders and films of TFE-HFP copolymer in air, there are three types of stable peroxy macroradicals in the samples end radicals CF -CF O, secondary middle-chain radicals CF -CF(00 )-CF2, and tertiary middle-chain radicals CF2-C(CF3)(00 )-CF2. In contrast to PTFE, prolonged exposure (>100 hours) of these samples in a NO atmosphere at room temperature does not lead to the formation of aminoxyl macroradicals. However, two types of macroradicals are formed if TFE-HFP is heated with evacuation after the decay of radicals in a NO atmosphere. At 90 °C, the ESR spectrum demonstrates the presence of tertiary alkyl macroradicals CF2-C (CF3)-CF2 formed upon decay of the tertiary nitroso compounds [57]. On further increasing of the temperature up to 180 "C, the tertiary alkyl macroradicals... 

The mechanism of the conversion of free radicals in fluorinated polymers reflects the introduction of NO into the middle of the main chain without changing its orientation and the possible formation of CF radicals in the system under the action of light. The ARs appear in the temperature range where the tertiary nitroso compounds decay in the vacuum with the regeneration of tertiary alkyl macroradicals CF2-C (CFj)-CF2 As has been established [58], these tertiary macroradicals decay in an inert atmosphere. Decay of these radicals requires spatial transfer of free valence. The first step of radical decay is P-decomposition by the reaction [59] ... 

In the presence of NO formed upon decomposition of the tertiary nitroso compounds, the terminal radicals can be converted into terminal nitroso compounds reacting with the adjacent double bond and forming aminoxyl macroradicals ... 

This sequence of reactions explains why CF3-NO - CF,-CF, radicals are not formed during the photolysis of polymer samples irradiated in a vacuum and then exposed to NO. Under these conditions, there is no formation of the nitrates whose photolysis produces the NO, necessary for the conversion of tertiary alkyl radicals to the corresponding alkoxide macroradicals and then to CFj. Thus, aminoxyl macroradicals obtained in the TFE-HFP copolymer are optimal spin labels. Behaviour of CF,-C(CF3)(X)-CF,-NO -CF, radicals can reflect the motion of the segments located in the middle of polymer chain, whereas the ESR spectra of CF3-NO -CF,—CF, radicals contain information about the movements of terminal groups of macromolecules. 

The accumulation of ARs during the interaction of PI with a mixture of NO and oxygen obeys other kinetic regularities. The presence of leads to the appearance of a certain induction period in the process. The ESR signal of PI exposed to the NO + mixture is similar to that shown in Figure 6.1a. In the absence of NO, the iminoxyl macroradicals formed in both cases are stable for many months in an inert atmosphere and in air. The proposed scheme of possible reactions of the radical formation in PI involves four main stages [7] (i) generation of alkyl macroradicals by the reaction of NO with elastomers (ii) synthesis of tertiary macromolecular nitroso compounds (iii) acceptance of tertiary alkyl or allyl macroradicals and (iv) decay of ARs. 

The stabilizing effect of amines on radicals from polymers was extended to plastomers by Dubinskaya et al. [41]. The polymers used, polystyrene, poly-(methyl methacrylate), poly(vinyl acetate), and poly(a-methylstyrene), were put in solution with primary, secondary, and tertiary amines at concentrations from 2 to 5%. These solutions were then degraded in a vibratory mill at 80°K in vacuum and in air. It was found that the reactivity of amines with macroradicals in the solid state at low temperature decreases in the order secondary > primary > tertiary. The authors were also able to rank the reactivity of the several macroradicals studied with the same amine. The order of reactivity was peroxy poly(vinyl acetate) > polystyrene > poly(methyl methacrylate) = poly(a-methylstyrene). The last two polymers do not react with amines at low temperatures. They only react with secondary amines at room temperature. 

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