Autoacceleration of polymerization

Local Autoacceleration of Polymerization to Form a Propagating Front... 

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. 

Some typical examples of this autoacceleration are (Figure 5) Norrish and Smith ( 2) polymerized methyl methacrylate in bulk and in the presence of various precipitants and measured the polymerization rates dilatometrically. They determined that autoacceleration of the precipitation polymerizations was larger than that observed for the Trommsdorf effect in bulk polymerization. 

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. 

Autoacceleration in the rate of polymerization occurs also with other monomers. It is far more marked with methyl acrylate or acrylic... 

In quest of an explanation for this phenomenon, one is led to conclude either that the combination of constants occurring in the rate equation (12) must undergo a large increase when autoacceleration occurs or that a totally different mechanism of polymerization must take over. We should obviously prefer the former alternative if it will lead to a satisfactory explanation of the facts. An increase in kdj seems unlikely autoacceleration is not a function of the initiator. This leaves us with the ratio which will be required to increase by... 

The susceptibility of the polymerization of a given monomer to autoacceleration seems to depend primarily on the size of the polymer molecules produced. The high propagation and low termination constants for methyl acrylate as compared to those for other common monomers lead to an unusually large average degree of polymerization (>10 ), and this fact alone seems to account for the incidence of the decrease in A f at very low conversions in this case. 

The model captures the features seen in the experiment very well. The initial rate of polymerization and the conversion at the onset of autoacceleration are nearly identical with the experimentally generated values as is the rate of autodeceleration. The conversion and value of maximum rate are within 5%. The difference in maximum rate can be ascribed to the high sensitivity of autoacceleration on the At parameter a small change in can greatly influence the rate of polymerization during autoacceleration. The conversion at which the maximum rate occurs is dependent upon fcp, i.e., the free volume (and conversion) at which autodeceleration sets in. The simulated rate also shows a tail around 80% conversion, which can be ascribed to the DSC not capturing polymerization at high conversion and low rate. 

It can be observed that the initial rate of polymerization decreases and the autoacceleration peak is suppressed as the TED concentration is increased. The TED molecules generate dithiocarbamyl (DTC) radicals upon initiation. As a result, termination may occur by carbon-carbon combination which leads to a dead polymer and by carbon-DTC radical reaction which produces a reinitiatable ( living ) polymer. The cross-termination of carbon-DTC radicals occurs early in the reaction (with the carbon-carbon radical termination), and this feature is observed by the suppression of the initial rate of polymerization. As the conversion increases, the viscosity of the system poses mass transfer limitations to the bimolecular termination of carbon radicals. As has been observed in Figure 3, this effect results in a decrease in the ktCC. However, as the DTC radicals are small and mobile, the crosstermination does not become diffusion limited, i.e., the kinetic constant for termination of carbon-DTC radicals, ktCS, does not decrease. Therefore, the crosstermination becomes the dominant reaction pathway. This leads to a suppression of the autoacceleration peak as the carbon-DTC radical termination limits the carbon radical concentration to a low value, thus limiting the rate of polymerization. This observation is in accordance with results of previous studies (10) with XDT and TED, where it was found that when there was an excess of DTC radicals, the carbon radical concentration was lower and the cross-termination reaction was the dominant termination pathway. 

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 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. 

Because of the strong dependence of composite properties on this final conversion, it is imperative that models of polymerizing systems be used to predict the dependence of the rate of polymerization and, hence, conversion on reaction conditions. The complexities of modeling such systems with autoacceleration, autodeceleration, and reaction diffusion all coupled with volume relaxation are enormous. However, several preliminary models for these systems have been developed [177,125,126,134-138]. These models are nearly all based on the coupled cycles illustrated in Fig. 5. 

The conversion-time curves appear to be very similar to the shape typical of emulsion polymerization, i.e., an S-shaped curve is attributed to the autoacceleration caused by the gel effect (Smith-Ewart 3 kinetics, n>>l). The rate of polymerization-conversion dependence is described by a curve with two rate maxima. The decrease in the rate after passing through the first maximum is ascribed to the decrease of the monomer concentration in particles. Particle nucleation ends between 40 and 60% conversion, beyond the second rate maximum. This is explained by the presence of coemulsifier which stabilizes the monomer droplets against diffusive degradation. 

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. 

With both styrene and vinylpyridine, the autoacceleration index decreases as the reaction temperature rises. This effect can be considered normal behavior of polymerizing systems in which the gel effect is operative. As the temperature rises, the termination step, which involves the interaction of two polymeric chains in a highly viscous medium, increases in rate, and the over-all reaction tends to become normal. Ultimately, the stationary-state conditions may eventually apply. 

Rabagliati et al. (14) studied the polymerization of styrene in a three phase system containing an anionic-nonionic surfactant mixture and brine. Both AIBN and potassium persulfate initiators were used. The system was reported to be microemulsion continuous and even multicontinuous. (14). No autoacceleration was observed and the authors concluded that the polymerization exhibits an inverse dependence of the degree of polymerization on initiator concentration, similar to bulk solution polymerization. 

Although no systematic study is available for the rates of initiation, it seems that, on the basis of the preliminary comparison of the H-NMR spectra with the kinetics of polymerization (proceeding with an autoacceleration period), the first addition is fast and practically irreversible (ku > k fl) ... 

This high tendency of poly(ethylene oxide) to solvate cations and from the polymeric shell around the counterion leads to autoacceleration in polymerization (polymerization faster on solvated ion-pairs), and increase of conductivity with monomer conversion. Moreover, polymerization is not sensitive to the "external" solvating agents, e.g. crown ethers. 

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