AIBN initiator illustrating the three conversion regimes

Most in depth studies of termination deal only with the low conversion regime. Logic dictates that simple center of mass diffusion and overall chain movement by reptation or many other mechanisms will be chain length dependent. At any instant, the overall rate coefficient for termination can be expressed as a weighted average of individual chain length dependent rate coefficients (eq. 20)  

Mahabadi and O Driscolh considered that segmental motion and center of mass diffusion should be the dominant mechanisms at low conversion. I hey analyzed data for various polymerizations and proposed that should be dependent on chain length such that the overall rate constant obeys the expression  

Various expressions have been proposed for estimating how the overall rate coefficient kt and the individual rate coefficients vary with the chain lengths of the reacting species,simple relationships of the following fomis are the most often applied  

Flowever, it has been pointed out that the value of A to in the expressions eqs. 25-23 should not be confused with the small radical fe, , rather, the value of represents the termination rate constant of a single unit chain if the implied diffusion mechanism was the rate-determining process. 

Recent work has allowed values of A, and a for bulk polymerization in dilute solution to be estimated. This work suggests values of At =A, 1x10 M and a 0.15-0.25 for both MMA and Some values of A, and a for S and 

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Figure 5.3 Conversion-time profile for bulk MMA polymerization at 50 °C with AIBN initiator illustrating the three conversion regimes. Data are taken from...

Figure 16 Conversion time profile for bulk MMA polymerization at 50 °C with AIBN initiator illustrating the three conversion regimes. Data are taken from Balke, S. T. Hamielec, A. E. J. Appl. Polym. Sci. 1973,17, 905-949. ...

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