Autoacceleration in free radical

Both the Trommsdorff effect and thennal autocatalysis can lead to the autoacceleration in free-radical polymerization. The results indicated that the autoacceleration observed in the smallest test tube was mostly due to the Trommsdorff effect, while both the Trommsdorff effect and thermal autocatalysis strongly affected the onset of autoacceleration in the larger polymerization system. When an exothermic reaction is performed in a larger system, more heat tends to be accumulated in a reactor, since a sur ce-to-volume ratio is decreased as the size of the system increases. There was a critical size in the inner diameter of the test tube at which the behavior of the autoacceleration of the polymerization changes. 

The addition of an inert solvent to a bulk polymerization mass minimizes many of the difficulties encountered in bulk systems. As shown in Fig. 13.1, it reduces the tendency toward autoacceleration in free-radical addition. The inert diluent adds its heat capacity without contributing to the evolution of heat, and it cuts the viscosity of the reaction mass at any ven conversion. In addition, the heat of polymerization may be conveniently and effidently removed by refluxing the solvent Thus, the danger of runaway reactions is minimized. 

In free-radical polymerization, the autoacceleration (or gel effect, Trommsdorff effect) has been known for a long time 161 168>. 

Frontal polymerization discovered in 1972 (5) could be realized in free-radical polymerization because of its nonlinear behavior. If the top of a mixture of monomer and initiator in a tube is attached to an external heat source, die initiators are locally decomposed to generate radicals. The polymerization locally initiated is autoaccelerated by the c(xnbinatithermal autocatalysis exclusively at the top of the reaction systmn. An interface between reacted and unreacted regions, called propagating front, is thus formed. Pojman et al. extensively studied the dynamics of frontal polymerization (d-P) and its applicatim in matoials syndiesis (I -I3). 

O Neil, G.A., Wisnudel, M.B., and Torkelson, J.M. (1996) A critical experimental examination of the gel effect in free radical polymerization do entanglements cause autoacceleration Macromolecules, 29, 7477-7490. 

As simple as this seems, some serious difficulties can be encountered, particularly in free-radical bulk polymerizations. One of them is illustrated in Figure 12.1 [1], which indicates the course of polymerization for methyl methacrylate by either bulk polymerization or solution polymerization using various concentrations of benzene, an inert solvent. The reactions were carefully maintained at constant temperature. At low polymer concentrations, the conversion versus, time curves are described by Equation 9.19. As polymer concentrations increase, however, a distinct acceleration of the rate of polymerization is observed which does not conform to the classical kinetic scheme. This phenomenon is known variously as autoacceleration, the gel effect, or the Tromsdorff effect. 

Autoacceleration in the polymerization of MA poses a serious problem [21-23]. Saini et al. [24] attempted to polymerize MA by using /3-PCPY as the initiator with a view to minimize the difficulties experienced due to this phenomenon. The findings led to the conclusion that -PCPY can be used to obtain 19.5% conversion of MA without gelation due to autoacceleration, which is nearly double the conversion obtained by using the conventional free radical initiator (AIBN) in the same experimental conditions. 

Autoacceleration, Glass and Zutty (S) and Burnett and Melville 9) reported an increase in the rate and average degree of polymerization with increasing solution viscosity, heterogeneous conditions and chain coiling for free radical, vinyl polymerizations. Autoacceleration is also called Trommsdorff. (10) effect. 

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 gel or Trommsdorff effect (11) is the striking autoacceleration of the vinyl polymerization reaction as the viscosity of the monomer-polymer solution increases. Chain termination involving the recombination of two free radicals becomes diffusion controlled and this results in a decrease in the rate of termination. The concentration of active free radicals therefore increases proportionally. To sum up the gel effect the rate of Vazo catalyst initiation increases with temperature the rate of propagation or polymerization increases with the viscosity and the rate of termination of the growing polymer chains decreases with the viscosity. This of course also results in an increase in the molecular weight of linear polymers, but this has no practical significance when crosslinking is part of the reaction. 

By using the free-volume theory, one can successfully monitor the whole course of free-radical polymerization. It is commonly accepted that the presence of autoacceleration and limiting conversion in the polymerization reaction are due to the diffusion-controlled kt and kp, respectively. The initiation efficiency, f, will behave in a way similar to kp. 

Polystyrene and poly(methyl methacrylate) polymerizations are typical of homogeneous bulk chain-growth reactions. The molecular weight distributions of the products made in these reactions are broader than predicted from consideration of classical, homogeneous phase free-radical polymerization kinetics because of autoacceleration (Section 6.13.2) and temperature rises at higher conversions. 

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. 

When we combine this observation with the autoaccelerating tendencies of the system, the chain-transfer reactions to both the monomer and the polymer on one of the several positions which leads to branched-chain formation, and the possible reactivation of dead polymer molecules by hydrogen abstraction with monomeric free radicals [78], the complexity of the kinetics of vinyl acetate polymerization may be appreciated. Similar factors may be involved not only in the polymerization of other vinyl esters, but also in the fiee-radical polymerization of other types of monomers. 

Chain polymerizations which do not terminate by bimolecular chain coupling generally result in polymers with the most probable molecular weight distribution of 2.0 free radical polymerizations which terminate by bimolecular chain coupling result in pdi = 1.5. However, chain transfer to polymer, autoacceleration, slow initiation, and slow exchange between active species of different reactivities result in much higher polydispersities. 

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