Other Transfer and Termination Reactions

Various transfer agents (denoted by S or XA as in Chap. 3), present as solvent, impurity, or deliberately added to the reaction system, can terminate the growing polymer chain by 

alcohols, acids, anhydrides, and esters have varying chain-transfer properties [Mathie-son, 1963]. The presence of any of these transfer agents in sufficient concentrations results in Reaction 5-28 becoming the dominant mode of termination. Termination by these compounds involves transfer of HO, RO, or RCOO anion to the propagating carbocation. Aromatics, ethers, and alkyl halides are relatively weak chain-transfer agents. Transfer to aromatics occurs by alkylation of the aromatic ring. 

Compounds such as amines, triaryl or trialkylphosphines, and thiophene act as inhibitors or retarders by converting propagating chains to stable cations that are unreactive to propagation [Biswas and Kamannarayana, 1976], for example 

Phosphines have been advantageously used to convert propagating carbocations to highly stable phosphonium ions that can be studied with 31P NMR [Brzezinska et al., 1977]. 

The most nucleophilic of the reagents discussed, such as water, alcohol (often with KOH), ammonia, and amines, are often used in excess to quench a cationic polymerization. This is typically carried out after complete (or at least maximum) conversion has been reached in order to inactivate the coinitiator by the process described above. 

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When the stoppage of chain growth involves, in addition to chain transfer to monomer, other transfer and termination reactions [Eqs. (8.129), (8.131), and (8.132)] as well, the concentration of the propagating species remains unchanged, and the polymerization rate is still given by Eq. (8.134). However, the degree of polymerization is decreased by all these transfer and termination reactions, and is... 

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