Free macroradical formation

Due to the high reactivity of cumene, the reaction of the peroxyl macroradical with cumene occurs more rapidly than the intramolecular reaction and the formed POOH is only from the single hydroperoxyl groups. Such POOH decomposes with free radical formation much more slowly than POOH produced in PP oxidation in the solution and solid state. 

Values of all constants calculated on the experimental database are given in Table 2.4. From Table 2.4, it follows that the value of U is close to the energy at which chemical connections break, resulting in the formation of free macroradicals [9], The temperature Tm corresponds to the initial temperature of rubber decomposition. The value of a structural-mechanical constant y corresponds to complex composite materials on the rubber and phenol-formaldehyde resin base. It should be noted that experimentally determined minimum time of destruction, x , is in 100 times exceeds the period of atoms fluctuation in a solid body. Such a sharp increase xm is obviously connected to a significant quantity of filler in the RubCon material (about 90%), which results in a complex manner of crack development at destruction [8],... 

Monomer formation occurs when free macroradicals decompose via a chain mechanism without hydrogen atom transfer. The half-life temperature for PS is 360 °C. The thermal degradation products consist mainly of monomer, dimer and trimer. 

Although general features of the crosslinking procedure are the same for all polyolefins, substantial differences exist regarding the efficiency and mechanism of the process depending on the chemical structure and physical state of the material. Since basically all common procedures leading to polyolefin crosslinking are based on a free radical formation and subsequent recombination of macroradicals, it is important to be... 

The possibility of mechanical activation of some reactions of PVC with micromolecular compounds based on the formation of free macroradicals suggested the trials of its grafting and block-copolymerization with other macromolecular compounds. 

The reasons that determine the properties modification by tear and fatigue can be the following 1) the appearance of free macroradicals under the deformation conditions 2) the activation of some reactions with medium components 3) chains reorientation and packing in supramolecular formations 4) the appearance of adsorption and superficial-active properties of the mechano-destruc-tion products and 5) the ordering and plastic properties modification of the contact surfaces by friction with the polymer [595]. 

Mechanochemical degradation creates free macroradicals in pairs, practically without any side reactions, and most potential applications of this technique are centered around the formation and subsequent reactions of these reactive species. Elongational flow-induced degradation breaks polymer chains exactly at their center [42, 46]. This remarkable propensity is being explored in the author s laboratory as a simple means of obtaining well-defined block copolymers. Polymerization and... 

The mechanical rupture of polymer chains leads to the formation of free valences on the ends of the chain fragments. According to Hen ein (Ji) there are three basic possibilities for the depolymerization reaction. In the so-called homolytic cleavage two free macroradicals are formed ... 

When the emulsion polymerization is conducted in the absence of an emulsifier, this process is termed emulsifier free or soapless emulsion polymerization [68-73]. In this case, the particle formation occurs by the precipitation of growing macroradicals within the continuous... 

Upon photolysis of polypropylene hydroperoxide (PP—OOH) a major absorption at 1726 and 1718 cm has been observed in the IR spectrum, which is attributed to the carbonyl groups. Sometimes the macroradical having free radical site reacts with a neighboring newly born hydroperoxide causing the formation of a macroalkoxy radical [116]. 

In this case, two kinds of free radicals are formed leading to the formation of homopolymer and graft copolymer. The latter is due to the formation of cellulosic macroradicals. 

Well before the advent of modern analytical instruments, it was demonstrated by chemical techniques that shear-induced polymer degradation occurred by homoly-tic bond scission. The presence of free radicals was detected photometrically after chemical reaction with a strong UV-absorbing radical scavenger like DPPH, or by analysis of the stable products formed from subsequent reactions of the generated radicals. The apparition of time-resolved ESR spectroscopy in the 1950s permitted identification of the structure of the macroradicals and elucidation of the kinetics and mechanisms of its formation and decay [15]. 

Currently this model is one of the most commonly used in the theory of free-radical copolymerization. The formation of a donor-acceptor complex Ma... iVlbetween monomers Ma and in some systems is responsible for a number of peculiarities absent in the case of the ideal model. Such peculiarities are due to the fact that besides the single monomer addition to a propagating radical, a possibility also exists of monomer addition in pairs as a complex. Here the role of kinetically independent elements is played by ultimate units Ma of growing chains as well as by free (M ) and complex-bound (M ) monomers, whose constants of the rate of addition to the macroradical with a-th ultimate unit will be... 

This assumption is implicitly present not only in the traditional theory of the free-radical copolymerization [41,43,44], but in its subsequent extensions based on more complicated models than the ideal one. The best known are two types of such models. To the first of them the models belong wherein the reactivity of the active center of a macroradical is controlled not only by the type of its ultimate unit but also by the types of penultimate [45] and even penpenultimate [46] monomeric units. The kinetic models of the second type describe systems in which the formation of complexes occurs between the components of a reaction system that results in the alteration of their reactivity [47-50]. Essentially, all the refinements of the theory of radical copolymerization connected with the models mentioned above are used to reduce exclusively to a more sophisticated account of the kinetics and mechanism of a macroradical propagation, leaving out of consideration accompanying physical factors. The most important among them is the phenomenon of preferential sorption of monomers to the active center of a growing polymer chain. A quantitative theory taking into consideration this physical factor was advanced in paper [51]. 

The regeneration of nitroxyl radical from the product of the reaction of nitroxyl radical with the alkyl macroradical was proved in the following experiments [51]. The nitroxyl radical and initiator (dicumyl peroxide) were introduced in a PP powder and this sample was heated to T= 387 K in an argon atmosphere. The concentration of nitroxyl radical was monitored by the EPR technique. The nitroxyl radical was consumed in PP with the rate of free radical generation by the initiator (see Figure 19.3). Dioxygen was introduced in the reactor after the nitroxyl radical was consumed. The generation of peroxyl radicals induced the formation of nitroxyl radicals from the adduct of the nitroxyl radical with the PP macroradical. 

The results suggests that the copolymer has a graft structure and that the mastication medium involves three kinds of domains. The first is the inner domain of poly(vinyl chloride) which is only slightly penetrated by monomer. Polymerization is initiated by macroradicals created in the PVC domain causing the formation of a true copolymer. Short radical segments arising from transfer reactions migrate into the third external domain which consists practically entirely of pure monomer and there initiate polymerization. The second domain is the surface of the resin particle which is swollen by monomer. The free radicals created by bond rupture appear in this second domain. 

The formation of polymers with terminal LM during chain termination in free-radical polymerization is based on the ability of anthracene and some of its derivatives to participate in homolytical reactions It was established that anthracene-containing compounds interact with macroradicals which are generated in free-radical... 

The contribution of the homolytical substitution to the total sum of reactions occuning in free-radical polymerization in the presence of anthracene of its derivatives depends on the nature of the monomer and the anthracene-containing compound and on the polymerization conditions. It has been diown that the macroradicals of the methacrylic esters do not interact with anthracene under usual conditions On the other hand, in the polymerization of styrene, substitution with the formation of terminal LM of type II (Scheme 1) proceeds only if one or both meso-positions of the anthracene ring are free ... 

Some chemicals retard or suppress free-radical polymerization by reacting with primary radicals or macroradicals to yield radicals that are very stable to further reaction or yield nonradical products. These materials could be retarders or inhibitors. Retarders slow down the formation of polymer but inhibitors completely eliminate it. Oxygen is one of the most commonly known inhibitors for vinyl polymerization and good practice requires the removal of air from the reactor vessels before the reaction is started. It combines with active radicals to form unreactive peroxy radicals. 

The formation of block copolymers can result from the addition of excess styrene monomer to SMA macroradicals (13). Maleic anhydride has also been reported to homopolymerize when initiated by gamma-radlation of free radical Initiators. The highest conversions were obtained employing acetic anhydride as the solvent, in a ratio of solvent to monomer of 75 25 (14,15). 

Macroradicals can be prepared by free-radical-initiated solution polymerization of monomers in poor solvents. Monomers with solubility parameters similar to those of the macroradicals may form block copolymers in solvents that are poor solvents for both the macroradical and the block. The ability of a block macroradical to add an additional block is governed by the solubility parameter of the initial chain in the macroradical, and not by the solubility parameter of the end block. For the formation of macroradicals, it is essential that the solubility parameters of the monomer and polymer differ by at least 1.8 hildebrand units. For the formation of block copolymers, it is essential that the difference in solubility parameters of the monomer and macroradical be less than 3.2 hildebrand units. 

---