Disproportionation, termination

An extremely favorable consequence of both strategies is the presence of significant amounts of covalent, or inactive, chain ends. This substantially lowers the overall concentration of reactive chain ends which results in a decrease in the occurrence of unwanted side reactions such as termination, disproportionation, or combination. This enables the polymer chain to grow in a controlled fashion, exhibiting many of the attributes typically associated with a living polymerization. However, it should be pointed out that the occurrence of these side reactions is not eliminated and in the strictest sense, the polymerizations are not truly living. 

Batch Reactors. One of the classic works in this area is by Gee and Melville (21), based on the PSSA for chain reaction with termination. Realistic mechanisms of termination, disproportionation, and combination, are treated with a variety of initiation kinetics, and analytical solutions are obtained. Liu and Amundson (37) solved the simultaneous differential equations for batch and transient stirred tank reactors by using digital computer without the PSSA. The degree of polymerization was limited to 100 the kinetic constants used were not typical and led to radical lifetimes of hours and to the conclusion that the PSSA is not accurate in the early stages of polymerization. In 1962 Liu and Amundson used the generating function approach and obtained a complex iterated integral which was later termed inconvenient for computation (37). The example treated was monomer termination. 

As we have seen in previous sections, the radical chain polymerization involves several possible modes of chain termination — disproportionation, coupling, and various chain transfer reactions. These contribute to the complexity of molecular... 

Rp (Equation 3.10) and DP are both dependent on k /kt, with DP also a function of mode of termination (disproportionation versus combination) and chain transfer. Transfer can occur to monomer, solvent or any other species in the system. In some cases, chain transfer agents are added deliberately to Umit and control polymer DP . These agents are... 

In addition to this, termination via disproportionation is excluded from the kinetic scheme. Although for most monomers termination by combination is the dominant mode of termination, disproportionation reactions can generally not be ignored in MWD studies. The method outlined above will thus certainly not be suitable for monomers like MMA [75] but only for those monomers with a minimal amount of disproportionation. The attention will therefore be focused to the acrylates of which it is generally believed... 

T ratio of chain termination (disproportionation) and transfer to propaga-... 

Note 3 See also combination, cross-termination, disproportionation, and spontaneous termination. 

Efficiency (f). The precise determination off is difficult and often inaccurate. The method most often used consists in comparing the number of primary free radicals formed by decomposition of the initiator with the number of primary free radicals having generated a chain through the use of H- or " C-labeled initiators. Such labeled polymeric chains can be easily separated from the side products generated by the initiator decomposition by simple selective precipitation. An alternative method relies on the determination of M to count the chains. This second method requires both the knowledge of the mechanisms of termination (disproportionation and/or recombination) and the absence of transfer reactions. The values measured are very dependent on the reaction conditions. 

With a chain reaction polymerisation the kinetic chain length v is defined as the average number of monomeric units that is added per initiating species (in this case a free radical). The kinetic chain length is defined independently from the mode of termination (disproportionation or combination), therefore, to get to the actual average molecular mass a correction has to be made. 

---