Autoacceleration effect

Ylides can be used as initiators to get a polymethacrylate of Pn 571 at 14% conversion to minimize the autoacceleration effect. 

Figure 5 shows similar experimental rate data for the DEGDMA/DMPA/TED polymerization. As seen in the case of HEMA, TED addition decreases both the initial rate and the maximum rate of polymerization of DEGDMA. As described earlier, polymerization of DEGDMA results in a highly crosslinked polymer. The autoacceleration effect is characterisitc of highly crosslinked systems as the diffusional limitations reduce the carbon-carbon radical termination kinetic constant... 

From these experimental and modeling studies, the mechanism of the living free radical polymerizations initiated by a combination of TED and DMPA have been elucidated. The TED produces DTC radicals that preferentially cross-terminate with the propagating carbon radicals. By this cross-termination reaction, the carbon radical concentration is kept low (as was shown in figure 6) and the rate of polymerization is decreased, as is the autoacceleration effect. This suppression of the autoacceleration peak in HEM A polymerizations and, interestingly, in DEGDMA polymerization has been observed to increase as the TED concentrations are increased. This behavior has been predicted successfully by the model as well. 

The studies of Olaj et al. [13] also undoubtedly contribute to our understanding of the basis of the autoaccelerating effects. At the same time, however, they confirm the empirical state of our present knowledge of the gel effect. We propose models which are then tested by confrontation with reality. So far there exist several suitable models, but we lack a test for discarding or modifying the less correct ones. 

Because vinyl polmerizations arc exothermic, the increased polymer production associated with the autoacceleration effect can cause a temperature rise and faster initiator decomposition. Runaway reactions or explosions may result if the heat of reaction is not removed efficiently. 

We assume here that the concentrations of monomer and initiator remain sensibly constant during the polymerization, and that any dependence of termination rate constants on macroradical size and concentration or autoacceleration effects can be neglected. Tliis means that the molecular weight distributions to be derived can be expected to apply to low-conversion polymers. Commercial macromolecules, whose polymerizations are often finished at high conversions, may have distributions that differ from those calculated here. Section 6.14.2 discusses the size distributions of such polymers. 

Increasing the concentration of catalyst brings about a reduction in the time to the exothermic peak. The exothermic peak temperature increases as the percentage of catalyst is increased. This increase in temperature is due to the autoacceleration effect that occurs when the viscosity of the monomer-polymer solution increases very rapidly with polymer formation. The percentage of conversion is approximately constant, except for a drop at the 1.2% and 1.5% vazo... 

This propagation of the hydroperoxide is of great importance because it produces both the ketone and alcohol from the start of the reaction. As soon as Q=0 (cyclohexanone) is formed, an autoacceleration effect is observed, due to the reaction ROOH + Q = O RO + = O + H2O... 

The ionic properties of the polymer are not only conducive to initiation, i.e. formation of species depicted in Figure 8-c, but also to the propagation steps, i.e. the assumptions of charged, aigzag state by the monomer. This is supported by the observed autoacceleration effect of the polymerization processes both in bulk and in solution. 

If the initiator concentration used in a free-radical polymerization system is low and insufficient, leading to a large depletion or complete consumption of the initiator before maximum conversion of monomer to polymer is accomplished, it is quite likely to observe a limiting conversion poo which is less than the maximum possible conversion pc, as shown in Fig. 6.2. This is known as the dead-end effect and it occurs when the initiator concentration decreases to such a low value that the half-life of the kinetic chains approximates that of the initiator. However, if there is autoacceleration effect or gel effect (described later) leading to a sharp rise in rate of polymerization, viscosity of medium, and degree of polymerization, pure dead-end effect cannot be observed. 

Problem 6.11 Isoprene was polymerized in bulk at several temperatures using AIBN at an initial concentration of 0.0488 mol/L in dead-end polymerization experiments (Gobran et al., 1960). In every case, the conversion increased with time until a limiting value was obtained beyond which no further polymerization was observed. No autoacceleration effect was observed in this system. The data of fractional degree of conversion (p) with time, including the limiting value of conversion (poo), determined at each temperamre are shown... 

Let us consider the kinetics of DMDAACh initial polymerization in more details. The autoacceleration effect in radical polymerization (the so-called gel-effect) plays an important role in polymers synthesis processes and during years number was a topic of intensive studies [20,29-32]. At present it is assumed, that the autoacceleration realization is due to a stmctural changes in polymer solution [31, 32]. The detailed interpretation of this point of view is adduced in the indicated above works. However, it should be noted, that polymerization autoacceleration beginning is accompanied by a number of other important effects, which get much less attention. Let us indicate the some from them. The attention is paid to very high molecular weights MM of polymer, which are realized on polymerization initial section at low reaction rates, small conversion degrees (up to 5%) and small reaction durations (of order of 5 min) [32], Secondly, as it... 

The authors [42 4] continued to study autoacceleration effect at DMDAACh radical polymerization within the frameworks of fractal and scaling approaches. In paper [39], two hmiting asymptotic regimes for the description of dynamical scaling of the aggregates growth in bath with particles initial concentration c. were considered. For these regimes definition the authors [39] introduced the... 

In Fig. 17 the dependences in both regimes for C(,= 4.0 and 5.5 mol/l, calculated by the indicated mode, are adduced. Let us note first of all, that the nonlinearity is typical for all calculated dependences, i.e., the reaction rate increasing at t growth or autoacceleration effect, which is expressed more clearly in case of regime 2 and at higher values c . It is obvious, that the plots nonlinearity is due to the power dependences in the Eqs. (31), (38) and (41), and therefore there are no reasons to consider autoacceleration effect as a critical phenomenon. The dependences adduced in Fig. 18, find out similar tendencies. 

Besides, the proposed treatment allows to taken into account monomer type influence on autoacceleration effect by the monomer length 1 using. In this case definition looks like [39] ... 

At present the autoacceleration effect absence is supposed at certain synthesis conditions, namely, at C(, and 0 small values [1], The last parameter is not included in the Eqs. (31) and (38) at all, although the curves discover the autoacceleration effect (Fig. 18). As to monomers concentration c , even for small c the autoacceleration effect is expressed clearly enough (Fig. 19), although at these conditions it can be realized at large t. It is necessary to indicate, that the proposed in work [39] and used here technique does not take into account monomer exhausting in reaction course, that is displayed inevitably in practice and results to reaction rate essential reduction. In other words, the effects of autoacceleration and monomer exhausting act in opposite directions and at small c the possibility of the first effect by the second one suppression. 

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... 

In paper [48] the autoacceleration effect in transesterification model reaction kinetics in a number of metal oxides nanoparticles was studied within the framework of chemical reactions fractal kinetics [49-51]. As it is known [50], the average number of cluster S, visited by random walk by which methylbenzoate and heptanol-1 molecules are simulated depends on time t as follows ... 

The same applies to the Trommsdorff or autoacceleration effect frequently encountered in bulk polymerization processes. We will refer to some of these while discussing a number of aspects which are typically found with or are pronounced in... 

Poly(vinyl chloride) made by suspension polymerization (Chapter 8) is a polymer in which molecular weight control is effectively by chain transfer—to monomer in this case. The Mv,/Mn ratio is slightly higher than the expected value of 2 because the polymerization is performed at rising temperatures, rather than isothermally, as assumed in the ideal kinetics discussed above, and possibly because of autoacceleration effects. 

In a stirred autoclave process, the polymerization is usually carried out in a single-phase region to facilitate the heat removal and to avoid fouling and forming cross-linked polymers. The presence of a viscous polymer-rich phase can also be the cause for the thermal runaway reaction via the autoacceleration effect [11]. When phase separation occurs in the reactor, chain termination in highly viscous polymer-rich phase becomes severely diffusion controlled, resulting in a rapid increase in the propagation rate. 

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