Macroradical mobility radical

The predominant mode of polymerization is in the interior of the particles and this leads to a reduction of macroradical mobility, usually referred to as radical occlusion, and a marked autoacceleration of the polymerization rate. 

Norrish and Smith [29] and later Tromsdorff et al. [30] described a polymerization of methyl methacrylate, the rate of which increased from a certain conversion. The number of monomers of similar behaviour was extended by methyl acrylate [31 ], butyl acrylate [32] and other acrylates [33] and methacrylates [34], and vinyl acetate. The effect was explained by the reduction of the termination rate caused by hindered macroradical mobil-ity in viscous medium it was called the gel effect, or the Norrish-Tromsdorff effect. The gel effect is clearly manifested in radical polymerizations of weakly transferring monomers in bulk. It is significant also in the presence of a good solvent. The gel effect is suppressed by the presence of poor solvents++ and by... 

Recombination reactions between two different macroradicals are readily observable in the condensed state where molecular mobility is restricted and the concentration of radicals is high. Its role in flow-induced degradation is probably negligible at the polymer concentration normally used in these experiments (< 100 ppm), the rate of radical formation is extremely small and the radicals are immediately separated by the velocity gradient at the very moment of their formation. Thus there is no cage effect, which otherwise could enhance the recombination efficiency. 

It can be seen from equation (2) that when y 0 the model falls into the classical expression for the rate of conversion of free radical polymerization. Equation (la) shows that this will be the case whenever all macroradicals have the same high mobility (i.e., as n tends to infinity) or when both entangled and non-entangled radicals have the same termination rate constant (i.e. a is equal to unity). 

As the polymerization reaction proceeds, scosity of the system increases, retarding the translational and/ or segmental diffusion of propagating polymer radicals. Bimolecular termination reactions subsequently become diffusion controlled. A reduction in termination results in an increase in free radical population, thus providing more sites for monomer incorporation. The gel effect is assumed not to affect the propagation rate constant since a macroradical can continue to react with the smaller, more mobile monomer molecule. Thus, an increase in the overall rate of polymerization and average degree of polymerization results. 

Another unique attribute of polymerizations of multifunctional monomers is the dominance of reaction diffusion as a termination mechanism [134,136, 143-146]. Reaction diffusion involves the mobility of radicals by propagation through unreacted functional groups. This termination mechanism is physically different from translation and segmental diffusion termination mechanisms which involve the diffusion of polymer macroradicals and chain segments to bring radicals within a reaction zone before terminating. Whereas normal termination mechanisms are related to the diffusion coefficient of the polymer, reaction diffusion must be considered differently. In essence, reaction diffusion is... 

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. 

Bolshakov et al. [44] observed a large difference in the reactivities of active centres in MMA and MA polymerizations at low temperatures (100-130 K). Substitution of the a proton with a —CH3 group results in steric hindrance of the centre and lower reactivity of the macroradical. At higher temperatures, steric hindrance is less severe, due to increased methyl mobility, and the reactivities of the radicals... 

The rate of propagation is affected much less than the rate of termination. The propagation reaction involves the reaction of a large radical with a small monomer whose diffusion rate is not changed significantly, whereas the termination process involves two macroradicals whose ends have reduced mobility, because motion of their centers of mass has become restrained. The net result in this case is an increase in the effective kp/k J ratio in Eq. (6-29) and an increase in the rate of polymerization. 

Stepwise decay was also observed when PMMA was irradiated in the presence of ethyl mercaptan (EtSH) [245]. The initial decay rate of the radicals measured at 150°K is proportional to the concentration of EtSH, indicating that the decaying pairs are mixed pairs formed by a radical from PMMA and a radical from EtSH. In fact, radiolysis of pure PMMA results in the formation of pairs of macroradicals. Some are due to main-chain scission, others to hydrogen abstraction from the polymer by CH 3 or CH30 radicals produced by side-chain scission. At 150°K, in pure irradiated polymethylmethacrylate, the mobility of the macroradicals is limited and their rate of decay comparatively low. In the presence of ethyl... 

Telogens are well known as substances, some bonds of which dispose to homolytic cleavage on reacting with a radical. A growing macroradical clips off the mobile hydrogen atom from ct-carbon of an aromatic ethyl... 

Rate constants of macroradical decay in isotactic polypropylene chtinge in relation to the content of the crosslinks [74]. For the first stage of crosslink formation, the rate constant of radical decay decreases by about 1 order for higher conversions of ca osslinking the rate constant increases. The initial decrease of the rate constant seems to be associated with a reduced mobility of macromolecular segments, while the subsequent increase with a gradual reduction of polymer crystallinity. 

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. 

The fcatalytic role of interface layer in 3 -D polymerization is certainly connected with the decreasing chain termination rate. Both rate of radical trapping and also the kinetic order of the reaction are decreased from the second to the first. This can be explained by several possible causes for this effect. In accordance with Ref. [7] chain termination represents the diffusion-control reaction and its rate depends on the mobility of macroradicals. If this mobility is decreased, so that condition tytp 1 takes place (fj and fp are characteristic moving times of the active center of macroradicals as a result of... 

Polymerization in the polymer-monomeric phase is characterized by two main conditions, First, it proceeds under a gel-effect condition, at which, by virtue of sizeable loss of transmission and segmental mobility of macroradical control on the rate of chain termination passes to the rate of its propagation. This means, that the acts of chain propagation and its termination take place as two different outcomes of the interaction of the active radical Rj (propagation of chain) or a frozen one, the so-called self-burial, in accordance with the terminology of Refs. [20, 21], that is, an inactive radical Rz (monomolecular chain termination). This can be represented by scheme (6.4) ... 

The types of postpolymerization kinetic curves shown can be qualitatively explained using the conception of the three reactive zones proposed earlier. Up to the moment when the polymerizing system is in the liquid monomer-polymeric phase (MPPh), namely up to the moment of conversion = 0 5, only a weak post-effect is observed. MPPh is characterized by low concentration of free radicals with a short life time. A visible post-effect is observed in the autoacceleration stage Po > Py , that is, at the beginning of polymer-monomeric phase elimination and formation of the interface layer at the phase division boundary between MPPh and the polymer-monomeric phase PMPh, which are new reactive zones. In such reactive zones the translational and segmental mobilities of the macroradicals are sharply decreased and the life times are sharply increased. This explains the essential post-effect. 

A major milestone in the history of polymer science was the macromolecular hypothesis by Staudinger [1]. The molecular structure of polymers started to emerge and nowadays, almost 80 years later, a knowledge base of respectable size has been built by the contributions of thousands of researchers. Nevertheless, there are still many aspects of free-radical polymerizations that are not fully understood. The bimolecular free-radical termination reaction is one such example. The first scientific papers dealing in some detail with the kinetics of this reaction, can be traced back to the 40 s when the gel-effect was discovered [2-4]. From subsequent research it became apparent that this reaction has a very low activation energy and is diffusion controlled under almost all circumstances. A major consequence of this diffusion-controlled nature is that the termination rate coefficient kt) is governed by the mobility of macroradicals in solution and is thus dependent upon all parameters that can exert an effect on the mobility of these coils. Consequently, kt is a highly system-specific rate coefficient and benchmark values for this coefficient do not exist. 

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