Centres dormant

The general mechanism in atom transfer radical polymerisation is depicted in Scheme 8.11. The main difference to conventional radical polymerisation is in the presence of a metal complex. Free radicals are generated from reaction between the initiator (such as an organic halide) and the metal species which further controls the reaction by reversibly transforming the free radicals into a dormant species.1"6 However, it ought to be pointed out that in ATRP contrary to, for example, Ziegler-Natta-type catalysts, the polymerisation does not take place at the metal centre. 

In polar solvents the equilibrium position shifts towards the ions, and in non-polar solvents towards the macroesters. Propagation on macroesters is about one hundred times slower than on ions they may be regarded as dormant centres [144],... 

Valuable data on the properties of active centres are obtained from kinetic measurements. They reveal the simultaneous existence of several centre types. The stable centres are active during the whole course of polymerization in addition, some fraction of decaying centres is also present. Isotactic centres exhibit stereoregulating ability and are, moreover, extremely active. Centres may oscillate between active and inactive (dormant) forms and some centres selectively polymerize enantiomers from a racemate. External effects, caused by specific properties of centres, will be discussed in subsequent chapters. In addition, the centres on which dienes are polymerized will be treated in Chap. 5, Sect. 4. The structure of these centres is a function of the coordinated diene, and it is therefore better presented together with propagation. 

Mutual transition of a dormant form into a living form and vice versa must be spontaneous, without external intervention. The concentrations of living and dormant forms are usually connected by an equilibrium. During macromolecule propagation, the centre can oscillate between the active and inactive states generally, the two forms have different mean life times. 

Dormant centres should not be confused with the inactive form of chain ends to which active centres are sometimes transformed by termination and which can be reversed to living forms only by slow re-initiation. Centres whose reactivity has been lost by termination are dead. The existence of living, dormant and dead centres is manifested in the mechanism and kinetics of the whole polymerization. The consequences of their occurrence will be discussed in the appropriate parts of the subsequent text (see Chap. 5, Sect. 8.1). 

Equilibria between various forms of living centres were treated in Chap. 5, Sect. 8.1. Equilibria of similar character control the arrangement and reactivity of all ionic centres. When polymerization-inactive structures participate in the equilibria, the number of active centres is reduced by the equilibrium amount of inactive forms. This phenomenon is usually not considered as termination the unreactive particles are treated as dormant. In the course of polymerization, however, the physico-chemical parameters of the system change as a function of the monomer-polymer transformation. Changes in permittivity, viscosity and the amount of polymer can cause shifts in ionization and dissociation equilibria. The kinetic manifestations of such changes are identical with the occurrence of termination. 

Passive (or dormant) failures could result in degraded performance without giving a definitive indication to the aew. This could go undetected until discovery during maintenance checks, or until discovered too late at a time when functionality is required. An example of the latter would be a dormant failure in an auto-land system where accuracy of the ILS centre line may be compromised. 

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