Macroradicals migration

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 processes of macroradical decay have been studied in high vacuum and in oxygen. The main consequence of the mechanism of the process is that macroradicals are able to eliminate hydrogen atoms from the adjacent chains and thus transfer to them the radical state. The radical state is transmitted further on by a series of hydrogen elimination acts. Thus it migrates in the polymer until it collides with another radical state. This may result in the disappearance of both radicals by recombination or by disproportionation. 

But the experiments on macroradical decay in vacuum do not give proofs of the postulated mechanism of radical state migration. We find such evidence studying the influence of oxygen on the radical decay process. 

As an explanation of the process of macroradicai decay the mechanism of radical state migration by means of hydrogen atoms elimination from adjacent chains has been suggested and confirmed. The activation energies of various macroradicai reactions have been measured and found to be very high. This is due to a specifity of the kinetic process in solid phase and to the participation of the diffusion phenomenon. Comparative investigation of the reactivity of various macroradicals reveals a great importance of the steric factor, i. e. the entropy ot the transition state for the kinetics of macroradicai reactions. 

This sequence reactions ensures easier migration of radical sites each to other. An increase in the rate of colli ns of macroradicals in a relatively rigid matrix will reduce the probability of main drain scissions. Double C=C bonds will contribute to the increase of effidency not only by the addition reactions but also by a possibility of opting mao or icals whkh terminate by combination exclusively. 

Orthopedic UHMWPE has a molecular mass of 2 10" a.m.u. or higher. In this state, the polymer has a high viscosity, even in the molten state. Thus, macroradicals have very low mobility, either in the molten or in the solid state, while the H radical, which has a diameter smaller than 1 A, can migrate in the polymer mass, even in the crystalline phase, where distances between C atoms are in the order of 4 A. H radicals resulting from Reaction 3 are very mobile and they can extract other H atoms intermolecularly or intramolecularly producing hydrogen, following Scheme 2. 

Taking into account the high viscosity of PTFE, one may presume that, during its thermal degradation, the recombination of the macroradicals involves the monomer as a chain-transfer agent in a relay mechanism [22], which is due to migration of the radical centre in a chain process ... 

It has also been demonstrated [52] that interaction of NO with radiation-produced macroradicals XFV at 150-200 Cleads to their decay without the production of ARs. The absence of ARs in irradiated PTFE was explained by the hindrance of spatial migration of the free valence within the rigid matrix of the fluorinated polymer in which the nitroso compounds formed by recombination of XIV with NO cannot serve as spin traps. It has been noted [52] that PTFE samples irradiated in air and exposed to NO at room temperature exhibit an ESR spectrum tentatively assigned to aminoxyl macroradicals, but the conditions under which these radicals are formed have not been clearly defined. Furthermore, to reliably identify the radicals by their solid-phase spectra, one should assess isotropic values with the appropriate parameters of low-molecular perfluoroalkylaminoxyl radicals studied thoroughly in the liquid phase [53, 54]. 

The concerted thermolysis of PVC and PVAc in PVC/PVAc blends accoimts for the formation of hydrogen chloride and acetic acid [a.lll]. These compounds migrated over phase boundaries into adjacent phases of the immiscible blend and cross-catalysed the dehydrochlorination and deacetylation. The co-reactivity of PS in blends with PVC, indicating the formation and reactivity of PS macroradicals, was found to be an important factor during the thermal degradation of PVC/PS blends, while the thermolysis of neat PS starts at 180 °C and is very rapid at temperatures above 250 °C. Unsaturations and low-molecular-weight volatile products that are formed in subsequent decomposition reactions of PS may interact with HCl and other gaseous products of PVC decomposition [a.ll2]. 

According to [5], they approach zero. Generally speaking, one of the features of the solid-phase oxidation is high E value of macroradical recombination. For instance, E of the second degree recombination of peroxy radicals reaches 50 - 100 kJ/mol [11, pp. 64, 69]. However, it is also known that PO recombination mechanism is superimposed with migration of free valences. Intensification of molecular movements due to plasticization or transition to amorphous systems causes an abrupt decrease of to 10 - 20 kJ/mol [11, p. 64]. These values are typical of transfers in liquids [6, p. 81]. In the present case, oxidation proeeeds in the polymer melt, i.e. molecular movements are unfrozen. Moreover, macroradical... 

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