Anionic chain polymerization nontermination

The first theoretical papers on anionic chain transfer to monomer were published long ago [2]. Later on, the effect of chain transfer to monomer on the molecular characteristics of the polymers formed in nonterminating polymerization was considered in a number of publications [28-33]. Usually, chain transfer is described according to a scheme that is similar to the free radical polymerization [21]... 

The chain can be terminated by a chain transfer reaction with the solvent or by reaction with an impurity in the reaction mixture. If the solvent cannot donate a proton to terminate the chain and if all impurities that can react with a carbanion are rigorously excluded, chain propagation will continue until all the monomer has been consumed. At this point, the propagating site will still be active, so the polymerization reaction will continue if more monomer is added to the system. Such nonterminated chains are called living polymers because the chains remain active until they are killed. Living polymers usually result from anionic polymerization because the chains caimot be terminated by proton loss from the polymer, as they can in cationic polymerization, or by disproportionation or radical recombination, as they can in radical polymerization. 

Chain-growth polymers are made by chain reactions— by the addition of monomers to the end of a growing chain. These reactions take place by one of three mechanisms radical polymerization, cationic polymerization, or anionic polymerization. Each mechanism has an initiation step that starts the polymerization, propagation steps that allow the chain to grow at the propagating site, and termination steps that stop the growth of the chain. The choice of mechanism depends on the stmcture of the monomer and the initiator used to activate the monomer. In radical polymerization, the initiator is a radical in cationic polymerization, it is an electrophile and in cationic polymerization, it is a nucleophile. Nonterminated polymer chains are called living polymers. 

The processes with slow reinitiation are called inhibition. In this case, the kinetics and molar masses are described by equations quite different from those for chain transfer to solvent [20]. Nevertheless, our calculations demonstrated that in nonterminating polymerization, in contrast to free radical polymerization, Eqs. (3.7) and (3.8) derived originally for fast initiation remain also valid for slow reinitiation (kri/ kts 1). In fact, the main condition for the validity of Eqs. (3.7) and (3.8) is not the high rate of reinitiation compared to transfer but the quasisteady-state approximation with regard to S, which is valid when (Skn/kp > 1. On the other hand, it was shown in Ref [11] that slow reinitiation leads to the second-order kinetics with respect to monomer. This effect was not, however, observed in anionic polymerization of nonpolar monomers. 

---