Living polymerization systems equilibrium

The major approach to extending the lifetime of propagating species involves reversible conversion of the active centers to dormant species such as covalent esters or halides by using initiation systems with Lewis acids that supply an appropriate nucleophilic counterion. The equilibrium betweem dormant covalent species and active ion pairs and free ions is driven further toward the dormant species by the common ion effect—by adding a salt that supplies the same counterion as supplied by the Lewis acid. Free ions are absent in most systems most of the species present are dormant covalent species with much smaller amounts of active ion pairs. Further, the components of the reaction system are chosen so that there is a dynamic fast equilibrium between active and dormant species, as the rates of deactivation and activation are faster than the propagation and transfer rates. The overall result is a slower but more controlled reaction with the important features of living polymerization (Sec. 3-15). 

The approach of Dainton and Ivin [1] is general, simple, and formally quite correct. Practically it really yields only a limited amount and quality of information on the polymerizing systems. The mechanistic approach of Eisenberg and Tobolsky [3] is more specialized it only applies to living systems. However, it yields information not only on monomer-polymer equilibria but also on the equilibrium distribution of molecular mass. The work of Tobolsky was extended by Wheeler et al. who further refined equilibria calculations in homopolymerizations [4, 5] a general solution of equilibrium copolymerizations in living media was developed by Szwarc [6]. These latter developments are not based on formal thermodynamics. 

The concept of flash chemistry can be applied to polymer synthesis. Cationic polymerization can be conducted in a highly controlled manner by virtue of the inherent advantage of extremely fast micromixing and fast heat transfer. An excellent level of molecular weight control and molecular-weight distribution control can be attained without deceleration caused by equilibrium between active species and dormant species. The polymerization is complete within a second or so. The microflow system-controlled cationic polymerization seems to be close to ideal living polymerization within a short residence time. 

In conventional addition polymerizations the growing chains are formed by some initiation processes and destroyed by some virtually irreversible terminations. The conversion of monomer into polymer eventually could be quantitative, provided that the initiation continues throughout the process. In the absence of termination or chain transfer the growing polymers remain living and then the polymerizing system ultimately has to attain a state in which the living polymers are in equilibrium with their monomer. The equilibrium concentration of the monomer, Me, provides valuable information leading to the determination of the appropriate Kp. To clarify this point, let us consider the equilibria... 

Until recently it was claimed that in the cationic polymerization of THF the final THF-polyTHF mixture contains no cyclic oligomers u. Further examination reveals that cyclic oligomers are formed, albeit slowly, when the living system is kept at the living polymer-monomer equilibrium for longer reaction times 2- 3). This is illustrated in Fig. 3.7, showing the increase of concentration of cyclic tetramer (in arbitrary units) with time 3). Under these conditions the monomer-polymer equilibrium is reached in about 1 hr. 

Anionic and later cationic pol3Tnerization gave most of examples of living pol3rmerization systems until recently, when more sophisticated methods of manipulation with free-radical polymerization processes become available. These methods are based on the use of the compounds which reversibly react with propagating radical and convert it to the so-called dormant species . When the equilibrium between the active and dormant species is regulated by special catalysts based on a transition metal, this process is called atom transfer radical polymerization (ATRP). If this equilibrium is provided by stable radicals such as nitroxides, the process is called stable free-radical polymerization (SFRP). In the case when dormant species are formed via a chain transfer rather than reversible termination reactions, this process is referred to as reversible addition fragmentation chain transfer (RAFT) polymerization. All these techniques allow to produce macromolecules of desired architecture and molecular masses. 

As indicated in the above examples, for a specific monomer the rate of exchange as well as the position of the equilibrium and, to some extent, the zero-order monomer transfer constants depend on the nature of the counter anion, in addition to temperature and solvent polarity. Therefore, initiator/coinitiator systems that bring about controlled and living polymerization under a certain set of experimental conditions are largely determined by monomer reactivity. 

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