Termination by Transfer Agents

Water is an especisJly effective chain terminating agent. For example, its Qr,s value is approximately 10 in the polymerization of styrene at 25°C with sodium naphthalene. So the presence of even trace quantities of water can significantly reduce the polymer molecular weight and polymerization rate. Water terminates propagating carbanions by proton transfer  

The hydroxide ion formed is not sufficiently nucleophilic to reinitiate polymerization and the active center is thus effectively destroyed. In contrast, the Ctr,s value for ethanol being very small ( 10 ), its presence in small amounts does not limit the molecular weight.  

Oxygen and carbon dioxide from the atmosphere add to propagating carbanions to form peroxy and carboxyl anions  

As these anions are not reactive enough to continue propagation, the chains are effectively terminated. By adding a proton donor subsequently to the polymerization system, the peroxy and carboxyl anions are converted to OH and COOH groups. A notable example of the application of the latter reaction is the preparation of carboxyl ion terminated polybutadiene by anionic polymerization of butadiene with bifunctional initiators, followed by termination with CO2  

Reaction with acid then yields carboxyl-terminated polybutadiene (CTPB). Ter- 

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