Conversion curves acrylic acid

Poly(acrylic acid) is not soluble in its monomer and in the course of the bulk polymerization of acrylic acid the polymer separates as a fine powder. The conversion curves exhibit an initial auto-acceleration followed by a long pseudo-stationary process ( 3). This behaviour is very similar to that observed earlier in the bulk polymerization of acrylonitrile. The non-ideal kinetic relationships determined experimentally in the polymerization of these two monomers are summarized in Table I. It clearly appears that the kinetic features observed in both systems are strikingly similar. In addition, the poly(acrylic acid) formed in bulk over a fairly broad range of temperatures (20 to 76°C) exhibits a high degree of syndiotacticity and can be crystallized readily (3). 

Figures 1 and 2 show the corresponding conversTon curves in toluene and in methanol solutions respectively. In the latter case log-log coordinates are used to represent the data. The conversion curves are then linear and their slope B, which is the exponent of time in the relation per cent conversion = Kt, measures the extent of auto-acceleration. B is referred to as the "auto-accele-ration index". For pure acrylic acid B = 1.8 - 2.0 in non polar solvents 3 tends towards unity.

Figure 1. Conversion curves of the polymerization of acrylic acid in toluene solutions (4). Monomer concentrations (volume per cent) (1) 100% (2) 95% (3) 90% (4) 85% (5) 80% (6) 65% (7) 50% (8) 27%. The polymer precipitates as a fine powder at all concentrations. Initiation by gamma-rays at 20°C and...

Methacrylic acid also polymerizes in bulk under precipitating conditions. It forms molecular associations very similar to those of acrylic acid. However, the conversion curves were found to be linear under a variety of experimental conditions temperatures of 16.5 to 60°C and broad ranges of initiation rates and monomer concentration in numerous solvents (7). It was assumed that structures of type III do arise but owing to steric hindrance and to the rigidity of the poly(methacrylic acid) molecule the monomer cannot align to form a "pre-oriented" complex as in the case of acrylic acid and propagation is not favored. 

Figure 7. Log-log plots of the conversion curves of the copolymerization of acrylic acid with methacrylic acid (. Mol % acrylic acid in the mixture (1) 0% (2) 23.5% (3) 45.5% (6) 71% (8) 83.2% (9) 86% (11) 100%. The copolymer formed in a mixture of 75 mol % acrylic acid contains 50% acrylic acid and 50%...

From the results presented above it can be concluded that the auto-accelerated conversion curves observed in the polymerization of acrylic acid, methacrylic acid and acrylonitrile are not caused by non-steady conditions arising as a result of the occlusion of growing chains in the precipitated polymer. This occlusion which is responsible for the post-polymerization observed in these systems only contributes to a limited extent to the over-all rates. 

Another peculiar feature of the grafting of acrylic acid is the break observed in the conversion curves particularly at low dose rates. The interpretation of this effect proposed above does not account for the fact that no break is observed with other monomers except at much higher grafting ratios (2, 4). These striking differences in kinetics for systems which in principle should exhibit comparable behavior are presumably related to differences in diffusion rates and polymer-polymer and polymer-monomer compatibilities. Little is known at present on the factors which govern these effects and on their influence on the kinetics. 

The oxidation of propane to acrylic add is a prominent example of where the splitting of the reaction over more than one site is discussed. A literature compilation [72] of data suggests two sites and a competition between partial and total oxidation via direct and intermediate deep oxidation. Data from the same system compiled in Figure 1.5 support this view. In Figure 1.5(a) one observes, under kinetically relevant conditions of below 10% conversion, opposing trends for total and partial oxidation where not only the sign of the trend but also the shape of the curve indicate that different sites and different limiting factors operate in this reaction. The side product acetic acid is also not a direct precursor to total combustion. From a... 

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