Formation of primary radicals

Radicals are formed by a collision of two monomer molecules of sufficient potential or kinetic energy, by decomposition of an excited molecule after the absorption of a light quantum, by initiator decomposition, and in many other ways. These are called the primary radicals because generally they exhibit a different reactivity and behaviour from the radical active centres generated from them. 

---

Reactions (1), (2) and (3) correspond respectively to the decomposition of the initiator with the formation of primary radicals, the addition of a first monomer to these primary radicals and the growth of the polymer chain by successive additions of monomers to the monomer type radicals. 

This behavior can be explained as follows. Vinyl chloride is moderately soluble in water (8) thus, the initiator radicals react with VC molecules dissolved in water, and polymerization begins. If there are no polymer particles present, a particle is supposed to be formed for each primary radical. If there are a few particles present, some of the primary or growing radicals may be captured by polymer particles. If the capture rate is equal to that of formation of primary radicals, no new particles form. 

Table 11.1 shows the enthalpies of reaction and activation barriers for fhe formation of primary radicals and transition state structures. Binding energies for the formation of pre- and post-reacfive complexes are listed in Table 11.2. The minimum energy structures (closed- and open-shell systems) are illustrated in...

XXXVIa is photolyzed in a benzene or heptane solution to a radical, which is characterized by the doublet spectrum125, 1341 and which arises in heptane on irradiation by 400—480 nm light already at -65 °C 12 . The intensity of the ESR signal increased with rising temperature. The optimum conditions for measurement were achieved at 10 °C. Further increase of temperature led to the decrease of intensity of the primarily formed radical and a new spectrum was recorded. It is not certain if this secondary spectrum belongs to only one radical of if it results from superimposition position of more radicals. The fast decay of radical intermediates occurred at above 50 °C or after the irradiation had been interrupted. The structure and scheme of the formation of primary radical C was proposed by134 14 . 

Plasma chemical reactions are characterized by a fast reaction time involving free radicals, ions and excited molecules. Since the formation of primary radicals and the subsequent radical reactions proceed very fast within usually 10 sec, which is much shorter than the gas residence time in a NTP reactor, the overall effictetKy is not influenced by gas residence time but is determined by the energy input to the NTP reactor. It is therefore important to know the energy efficiency not only for the commercial use but also for the optimization of the system. One of the important scaling parameters is the specific input energy (SIE Pdis Qr) which is defined as the ratio of energy input (Pdis) to the unit gas flow rate (Qr). 

According to the obtained data one may suggest that the process of oxidative coupling of isomeric ANSA and An leads to the formation of the copolymer products of different stmcture. In the case of 1,8-AAS4 and aniline process may include formation of primary radical cations ANSA and aniline, their pair-coupling, the subsequent oxidation of the resulting product in the radical-cation which is capable to further prolongation of the chain ... 

The initiation process constitutes the first reaction step in free radical polymerization, leading to the generation of (primary) radicals. The kinetics of the initiation process, ie its rate and effectiveness, are of fundamental importance in both theoretical studies and commercial applications. Commercial procedures mainly rely on the formation of primary radicals via thermal decomposition processes using azo- and peroxy-type compounds. Investigative kinetic studies are— to a large extent—carried out using photoinitiators, which decompose upon irradiation with UV or visible light. The main reason for this choice is the possibility to define exact start and end times of the initiation and subsequently the polymerization process. 

Because G(X) = G(H2) and this relationship is unaffected by irradiation conditions, it can be suggested that the intermolecular links and molecular hydrogen appear by other mechanism than the formation of primary radicals. Radicals can be oxidized or participate to the crosslinking of pristine polystyrene. 

Homolytic breakage leads to the formation of primary radicals (-CH2 in polyethylene) and then of secondary radicals by abstraction of hydrogen (-CH2-CH-CH2- in polyethylene), or reaction with oxygen (R-0-0). Owing to the high reactivity of the primary radicals, it is often difficult to assess the number originally formed, because several secondary reactions may occur after the generation of one primary radical. 

The literature describes the mechanism of photo-oxidative degradation in isotactic and atactic polypropylene well (Figure 5.25). However, the initiation step - the formation of primary radicals - has not been precisely explained. Photo-oxidative degradation in polypropylene, like thermal-oxidative degradation, also leads to the formation of hydrogen peroxides, ketones, esters, and acids. Figure 5.26 [653]. Simultaneously, molecular weight decreases. Table 5.2. 

The first microscopic process, the formation of primary radicals by the decomposition of initiator governs the rate of initiation and starts the growth of the macromolecule. The rate of initiation is known to be a function of the initiator efficiency (f), the initiator decomposition constant (k ) and the initiator concentration ([I]) and can be expressed by the following equation... 

It is suggested that the initiation of emulsion polymerization of unsaturated monomers is a two-step process. It starts in the aqueous phase by the primary free radicals derived from the water-soluble initiator or secondary derived from the emulsifier molecules. The second step occurs in the monomer-swollen micelles or particles and is caused by the water-soluble or water-insoluble oligomeric radicals which have entered [68]. In the homogeneous one-phase reaction system, both the formation of primary radicals and the growth of the polymeric chain take place in one phase. 

The first step includes the formation of primary radicals and their transformation to the surface active oUgomeric radicals through the addition of monomer units to the growing radical. 

Although exact and positive evidence for main chain scissions induced primarily by mechanical forces has not been obtained for most polymers, formation of primary radicals of the chain-scissioned type, which are listed in both Table 1 and Table 2, could be considered as experimental evidence supporting the mechanically induced main chain scissions, which are analogous reactions to those proved for both PP and PMMA either in the solid phase or in the liquid phase. Therefore, it seems reasonable to assume that the primary reactions induced by mechanical forces applied to polymers are main chain scissions on the basis of both the detected species of mechanoradicals and the considerations described above. 

The formation of primary radicals governs the rate of initiation and particle population. Because radical generation occurs in the aqueous phase, whereas radical termination occurs in the polymer particles, the polymerization rate and molecular weight can be increased at the same time. In vinyl chloride emulsion polymerization, the emulsifier greatly affects the polymerization kinetics and the physicochemical and colloidal properties of the polymer. The average polymer particle size is of the order 0.1-0.3 p,m, which is the size of primary particle nuclei in bulk and suspension polymerizations. The following is a summary of the typical kinetic features of batch vinyl chloride emulsion polymerization [61] ... 

---