Character, autoacceleration

The autoaccelerated character of acrylic acid polymerization is strictly correlated with such a form of monomer organization. The fast zip-up propagation takes place along oriented double bonds. Template mechanism of polymerization in these systems was also confirmed by examination of the tacticity of the polymer obtained. 

Autoacceleration Indexes. The values of /3 differ widely for the different systems, and moreover, their variation with temperature has a different character depending on the monomer under consideration. 

With acrylic acid the situation is very different. For 0.1-mm. thick films /3 is independent of temperature, whereas for 0.05-mm. thick films /3 rises above 40°C. At low temperatures ft is unusually small. This effect may be ascribed to the use of a selective inhibitor to prevent homopolymerization. It was shown above that in grafting vinylpyridine the autoaccelerated character of the reaction at 20°C. vanishes in the presence of inhibitors. Moreover, with acrylic acid the autoacceleration index is lower if a more efficient inhibitor (CuCl2) is used, and f decreases as the amount of inhibitor increases (Figure 8). 

The ratio of the rates of initiation and termination is an important factor determining the character of a polymerization. When termination is more rapid than initiation, propagation is limited and a high polymer is not formed. Such a situation is sometimes created intentionally for study purposes. For practical use it is of no great importance. Real systems have two extremes either initiation is equally rapid as termination or termination does not occur in the respective medium. In the first case, polymerization is stationary in the other, it is living (proceeding without transfer). In between these limits we find systems where termination is slower than initiation. They all exhibit an autoaccelerated course. The polymerizations are usually stationary at first with termination limited by the effect of the generated polymer. 

The difTerence in the mechanism of autoacceleration, depending on the size of the test tubes, changed the character of the PMMA produced in the reaction system. As shown in Figure 3, both M and MJM increased in the smallest test tube after the onset of autoacceleration. This is a typical phenomenon for the Trommsdorff effect, since it is led by the retardation of the termination reaction. Increase of M of the polymer, however, was not observed in the larger test tubes. Thermal runaway led to rapid decomposition of the initiator to produce more radicals transiently. This process produced more polymers having a lower degree of polymerization, lowering M,. 

Hence, the stated above results have shown that the autoacceleration effect is not presented as the critical phenomenon and is due to the power dependence specific character, which can be obtained within the frameworks of irreversible aggregation models. Its manifestation extent is defined by the parameters Cg and Dj. (and also q ), i.e., by the parameters, which control diffusive processes in solutions. In the general case stmcture formation conceptions cannot be used for the autoacceleration effect e>q)lanatioa... 

The induction periods and S-shaped form of the kinetic curves of oxidation at low temperatures and low oxygen pressure, and the detection in the oxidation products of water, acetaldehyde, formaldehyde, and methanol, formed in the decomposition of peroxides and peroxide radicals - all this indicates an autoaccelerated character of the process of oxidation of polyamides with degenerate branches to hydroperoxides. 

The character of the kinetic curves cannot be explained from the mechanism presented by the reactions (Equation 3.72-74). According to these reactions, based on the direct interaction of hydroperoxides with NO, the rate of decomposition is maximal at the beginning of the process and decreases with a decrease in the concentration of ROOH. The shape of the kinetic curves is characteristic of autoaccelerated reactions with an induction period. 

Thus, irrespective of the particular mechanism involved in the PP hydroperoxide decomposition by NO at room temperature, this process has an autoaccelerated character and proceeds with the participation of free radicals. 

The kinetic curves are presented in Figs. 75 and 76. The ratios IPN filler were 80 20 and 60 40 by mass. As can be seen from Fig. 75, with increasing concentration of initiator autoacceleration of the reaction begins earlier the conversion degree at the onset of autoacceleration is 0.03-0.05. Introduction of the filler (20 mass %) has no effect on the character of the curves for PBMA at initiator concentrations 5.0 x 10 and 2.96 x 10 moM (Fig. 75, curves V and 2 ). At the same time a decrease in initiator concentration to 0.74 X 10 moM shows some differences on the conversion curve (curve 30- The onset of autoacceleration is delayed from 160 min with-... 

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