Propagation, diffusion-controlled

In the propagation step a monomer molecule adds to the free radical end of a growing chain and in so doing generates another radical. This process is fast by comparison with many chemical reactions, but it is not fast enough to be diffusion controlled under normal circumstances. However, as the polymerisation 

An interesting question is why does the reaction stop before 100% conversion of monomer to polymer The answer to this is found by carrying out the later stages of the reaction at higher temperatures. At progressively higher temperatures, the achievable final conversion increases. It finally becomes 100% at a temperature that corresponds to the glass transition temperature of the solid polymer  

If we work back from solid polymer by adding progressively more amounts of liquid monomer, we see that the glass transition temperature of the now plasticised polymer decreases with the amount of monomer added. Monomer cannot diffuse through polymer below the glass transition temperature, so the reaction stops when the amount of monomer decreases by the amount necessary to raise the transition temperature of the mixture above the reaction temperature. However, the reaction will continue to proceed if the temperature of the reaction is raised, and will go to 100% conversion if the final temperature is above the transition temperature of the soHd polymer. For this reason, many industrial processes carry out the polymerisation using a temperature profile that finishes with a high temperature to ensure that there is no unreacted monomer left in the final product. 

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Diffusion controlled propagation and fall in initiator efficiency as veil. 

Figure 6. Styrene emulsion polymerization—critical conversion for diffusion-controlled propagation as a function of temperature (---) X cr2 0. 740 + 1.846 X...

For health, safety, and environmental reasons, significant efforts have been made to reduce the residual monomer content in commercial latexes. Some of the techniques are already being apphed industrially, whereas others are still being developed. In general, the available methods for the reduction of residual monomer are based on two different concepts. The first of these aims at further conversion of the monomer by increasing the diffusion-controlled propagation rate of the polymerization reaction, while the second involves removal of the residual monomer. 

As a result, O Brien and Bowman developed a comprehensive photopolymerization model. It incorporates heat and mass transfer effects, diffusion-controlled propagation and termination, and temporal and spatial variation of species concentration, temperature, and hght intensity. This model is applied to systems with varying diermal and optical properties. The absorbance of the polymerizing system is varied by altering either the initiator concentration, sample thickness, or molar absorption coefficient of the initiator. Based on simulations they concluded that the choice of initiator and sample thickness limits the initiator concentration usable to achieve complete monomer... 

Marten and Hamielec [73 have proposed the following relationship for diffusion-controlled propagation... 

Beyond Interval II, the second maximal polymerization rate can be attributed to the gel effect. The bimolecular termination reaction becomes diffusion-controlled in the latex particles and the average number of free radicals per particle increases signihcantly in the latter stage of polymerization, thereby leading to an acceleration of the free radical polymerization. The rate of polymerization then decreases continuously toward the end of polymerization due to the depletion of monomer and/or the diffusion-controlled propagation reaction in the reaction loci. 

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