Auto-acceleration, effect

The auto-acceleration effect appears most marked with polymers that are insoluble in their monomers. In these circumstances the radical end becomes entrapped in the polymer and termination reactions become very difficult. It has been suggested that, in thermodynamic terms, methyl methacrylate is a relatively poor solvent for poly(methyl methacrylate) because it causes radicals to coil while in solution. The termination reaction is then determined by the rate at which the radical ends come to the surface of the coil and hence become available for mutual termination. 

We ll come back to that in a minute. First let s look at a typical example of experimental data, such as that obtained for the polymerization of methyl methacrylate. Plots of conversion versus time for solutions of up to 40% in benzene follow the predicted relationship nicely. But, at higher concentrations and in the bulk (i.e., 100% methyl methacrylate), the Trommsdorff or auto-acceleration effect can clearly be seen (Figure 4-19). 

The explanation for this effect (known variously as the gel effect, Tromsdorff effect or auto-acceleration effect) is that the chain termination reaction slows down during conversion and, as can be seen by reference to equations (2.5) and (2.6), a decrease in the termination rate constant leads to an increase in both overall rate and molecular weight. The reason for the drop in termination rate is that as the reaction mixture becomes more viscous the radical ends of the polymer chains find increased difficulty in diffusing towards each other, leading to the important mutual termination reaction. Small monomer molecules on the other hand find little difficulty in diffusion at moderate conversion so that propagation reactions are relatively little affected, until the material becomes semi-soUd, when the propagation rate constant also decreases. It is of interest to note that the gel effect may be induced by the addition of already formed poly(methyl methacrylate) or even another polymer such as cellulose tripropionate because such additions increase the viscosity of the system. 

For a free radical polymerisation system, an unsaturated polyester resin, an auto-acceleration was observed close to the onset of vitrification. To model the curing kinetics for these systems, including the mobility-controlled regions, a specific diffusion control model will need to be incorporated in a mechanistic reaction model. The heat capacity and the mobility factor can still give information about how vitrification is occurring, and how it is related to the auto-acceleration effect. 

The practical consequences of an auto-acceleration effect on exotherm and conversion are illustrated by the exotherm curves in Figure 9.1. The urethane methacrylate/methyl methacrylate resin that shows auto-acceleration at all oligomer/monomer ratios is compared with an unsaturated polyester/styrene resin, which only shows an autoacceleration (or gel effect ) at high alkyd/styrene ratios. The urethane methacrylate oligomer copolymerised with styrene shows virtually the same exotherm behaviour as the unsaturated polyester for equivalent styrene/oligomer unsaturation ratios. 

Only at low [M]o( 10%)Rp is as expected. P increases during the occurrence of the gel effect (also called Trommsdorff-Norish or auto-acceleration effect. Figure 2.2). This is the evidence for the explanation as conversion increases the viscosity of the system increases (especially at high [M]o). The chains cannot diffuse fast enough and the apparent termination rate constant decreases faster and faster. In this way, both Rp and P increase ... 

Bulk Polymerization. This is the method of choice for the manufacture of poly(methyl methacrylate) sheets, rods, and tubes, and molding and extmsion compounds. In methyl methacrylate bulk polymerization, an auto acceleration is observed beginning at 20—50% conversion. At this point, there is also a corresponding increase in the molecular weight of the polymer formed. This acceleration, which continues up to high conversion, is known as the Trommsdorff effect, and is attributed to the increase in viscosity of the mixture to such an extent that the diffusion rate, and therefore the termination reaction of the growing radicals, is reduced. This reduced termination rate ultimately results in a polymerization rate that is limited only by the diffusion rate of the monomer. Detailed kinetic data on the bulk polymerization of methyl methacrylate can be found in Reference 42. 

Studies of the copolymerization of VDC with methyl acrylate (MA) over a composition range of 0—16 wt % showed that near the intermediate composition (8 wt %), the polymerization rates nearly followed normal solution polymerization kinetics (49). However, at the two extremes (0 and 16 wt % MA), copolymerization showed significant auto acceleration. The observations are important because they show the significant complexities in these copolymerizations. The auto acceleration for the homopolymerization, ie, 0 wt % MA, is probably the result of a surface polymerization phenomenon. On the other hand, the auto acceleration for the 16 wt % MA copolymerization could be the result of Trommsdorff and Norrish-Smith effects. 

Auto-acceleration was observed in the homopolymerization of methacrylic acid solutions over limited concentration ranges in methanol and in water. Perhaps under such conditions swelling of the polymer favors monomer diffusion leading to a larger amount of pre-oriented structures III. Alternatively, a monomer-solvent complex may arise which favors a pre-oriented structure and thus, may be responsible for the onset of a matrix effect (9). 

The auto-acceleration observed under such conditions is reduced ( = 1.15) and could partially result from non-steady-conditions but also from a "matrix effect" operating on the surface of unswollen polymer particles. It should be noted in this respect that the post-polymerization which is induced by the growing chains occluded in the precipitated polymer exhibits an initial rate very much lower than the rate observed during irradiation (Curve 1 in Figure 91 which suggests that the contribution of the growth of occluded chains to the over-all rate is small. 

However, DMF is a solvent for polyacrylonitrile and the polymerization occurs in a homogeneous medium for solutions containing 30 per cent monomer or less. This reduces the value of these experiments as an argument to show the influence of a matrix effect. Indeed the fact that auto-acceleration disappears when DMF is added to acrylonitrile was considered as a proof for the fact that precipitation of the polymer was the cause of autoacceleration. 

Abstract. Auto-accelerated polymerization is known to occur in viscous reaction media ("gel-effect") and also when the polymer precipitates as it forms. It is generally assumed that the cause of auto-acceleration is the arising of non-steady-state kinetics created by a diffusion controlled termination step. Recent work has shown that the polymerization of acrylic acid in bulk and in solution proceeds under steady or auto-accelered conditions irrespective of the precipitation of the polymer. On the other hand, a close correlation is established between auto-acceleration and the type of H-bonded molecular association involving acrylic acid in the system. On the basis of numerous data it is concluded that auto-acceleration is determined by the formation of an oriented monomer-polymer association complex which favors an ultra-fast propagation process. Similar conclusions are derived for the polymerization of methacrylic acid and acrylonitrile based on studies of polymerization kinetics in bulk and in solution and on evidence of molecular associations. In the case of acrylonitrile a dipole-dipole complex involving the nitrile groups is assumed to be responsible for the observed auto-acceleration. 

Here, gt takes into account one interesting feature of the free radical polymerization kinetics the auto acceleration in the rate of reaction (better known as the Trommsdorff or gel effect). The gel effect is taken into account in the model (42) by including the experimental correlation developed in [18] for the termination rate ... 

In the associates, polymerization occurs according to the zip-propagation mechanism, and auto-acceleration takes place. In the range of temperatures from 10 to GO C, the system gradually changes from the system dominated by occlusion to the system where template effect determines the kinetic behavior. In this range of temperatures, a significant post-effect occurs. ... 

A seemingly similar process of polymerization - the radiation induced polymerization of methyl acrylate in n-hexane - shows also auto-acceleration and post-polymerization. Polymerization was carried out under precipitating conditions. Using the results of kinetics examination, the authors showed that all effects can be explained by the mechanism based on trapped radical reactions in particles of precipitated polymer. No influence of any template effect was detected in these systems. 

On exposure to moisture, the crystalline anhydrides undergo an auto-accelerated polymerisation with evolution of carbon dioxide. This solid-state reaction proceeds without any observable change in the form of the crystal, and its effect on the X-ray pattern has been briefly investigated by Miller, Fankuchen and Mark (5). The probable mechanism of this process will be discussed later. 

The active species in the accelerating effect of hydrogen chloride is presumed (25) to be the chloride ion which acts as a strong base. In a protic environment—e.g., in the presence of water or a hydroxyl-containing compound—the hydrogen chloride becomes a strong acid and the auto-catalyzed decomposition is prevented. 

A typical effect observed in the synthesis of linear polymers by a free-radical mechanism is the auto-acceleration process. At a particular conversion, when sufficient polymer has accumulated in the system for the viscosity to reach a certain level, the rate of the bimolecular termination reaction begins to fall because of diffusional restrictions to the encounter of two chain ends. However, the initiation and growth rates are hardly affected. 

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