Average Molecular Weight in Radical Polymerization

The average molecular weight in radical polymerization can be found from the kinetic model, Eq. (5.3.1), as follows. The kinetic chain length, v, is defined as the average number of monomer molecules reacting with a polymer chain radical dining the latter s entire lifetime. This will be the ratio between the rate of 

From the QSSA, the rate of initiation (r,) should be equal to the rate of termination (r = If transfer reactions are neglected, 

2v when termination is by combination only = V when termination is by disproportionation only av when there is mixed termination, 1 a 2 

As the initiator concentration is increased, the rate of polymerization increases [Eq. (5.3.14)], but e (and therefore q ) decreases [Eq. (5.4.3)]. Therefore, control of the initiator concentration is one way of influencing the molecular weight of the polymer. 

Another method of controlling the molecular weight of the polymer is by use of a transfer agent. In the transfer reaction, the total number of chain radicals in the reaction mass is not affected by Eq. (5.3.14). It therefore follows that the presence of a transfer agent does not affect r. However, the kinetic chain length. 

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