Termination by Impurities and Deliberately Added Transfer Agents

5-3b-2 Termination by Impurities and Deliberately Added Transfer Agents 

Most anionic (as well as cationic) polymerizations are carried out in an inert atmosphere with rigorously cleaned reagents and glassware since trace impurities lead to termination [Hadji-christidis et al., 2000]. Moisture absorbed on the surface of glassware is usually removed by 

The hydroxide ion is usually not sufficiently nucleophilic to reinitiate polymerization and the kinetic chain is broken. Water has an especially negative effect on polymerization, since it is an active chain-transfer agent. For example, C s is approximately 10 in the polymerization of styrene at 25°C with sodium naphthalene [Szwarc, 1960], and the presence of even small concentrations of water can greatly limit the polymer molecular weight and polymerization rate. The adventitious presence of other proton donors may not be as much of a problem. Ethanol has a transfer constant of about 10-3. Its presence in small amounts would not prevent the formation of high polymer because transfer would be slow, although the polymer would not be living. 

Living polymers are terminated by the deliberate addition of chain-transfer agent such as water or alcohol to the reaction system after all of the monomer has reacted. 

---