Irreversible recombination with counterion

As discussed already, termination of ring-opening polymerization may proceed by (a) irreversible recombination with counterion and (b) irreversible chain transfer to polymer. Other sources of termination are also possible, depending on the system (c) reaction with other components of the system, solvent or impurities and (d) different reactions of more reactive species existing in equilibrium with stable onium species. 

Termination by Irreversible Recombination with Counterion Irreversible collapse of ionic species via recombination with counterion is the most trivia] potential source of termination in cationic ring-opening polymerization. The relations discussed in Section II.B.6.C may also be applied to the present discussion. The termination will occur if counterion is nucleophilic enough (as compared with monomer) to compete successfully with monomer in Sn2 nucleophilic substitution and monomer is not nucleophilic enough to displace the counterion from the covalent termini ... 

As discussed in Section II.C, in any system in cationic ring-opening polymerization, a reaction of active species with polymer repeating unit may lead to chain transfer to polymer or termination (if the resulting branched or cyclic onium ions are not active), whereas recombination with counterion leads to termination in the case of irreversible reaction. The later reaction may be avoided by the proper choice of counterion. As the onium ions are generally inherently stable there is no other termination reaction, provided that impurities that may act as terminating agents are eliminated. 

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