Dormant species

Details of the mechanism of living polymerization have been refined continually (42), and a dormant species has been shown to be cmcial for such polymerization. The dormant species is in equiUbrium with the active species and the ratio can determine MWD and hence the quaUty of the living process (43). 

The reversible chain transfer process (c) is different in that ideally radicals are neither destroyed nor formed in the activation-deactivation equilibrium. This is simply a process for equilibrating living and dormant species. Radicals to maintain the process must be generated by an added initiator. 

Certain monomers may be able to act as reversible deactivators by a reversible addition-fragmentation mechanism. The monomers are 1,1-disubstituted and generate radicals that are unable or extremely slow to propagate or undergo combination or disproportionation. For these polymerizations the dormant species is a radical and the persistent species is the 1,1 -disubstituted monomer. 

Like all controlled radical polymerization processes, ATRP relies on a rapid equilibration between a very small concentration of active radical sites and a much larger concentration of dormant species, in order to reduce the potential for bimolecular termination (Scheme 3). The radicals are generated via a reversible process catalyzed by a transition metal complex with a suitable redox manifold. An organic initiator (many initiators have been used but halides are the most common), homolytically transfers its halogen atom to the metal center, thereby raising its oxidation state. The radical species thus formed may then undergo addition to one or more vinyl monomer units before the halide is transferred back from the metal. The reader is directed to several comprehensive reviews of this field for more detailed information. 

From the direct observation of the polymerization system by ESR spectroscopy, the concentration of N was determined [231], whereas [P ] was calculated from the polymerization rate at each conversion because of the difficulty of the direct determination of low [P ] values. The [N ] value increased during the initial period of the poymerization and reached to 4-6X10 5 mol/L. [P ] was estimated to be 1 -2 X10 8 mol/L. The K value was estimated to be 2.1 X10 11 with the experimentally determined values and Eq. (65), being constant during polymerization. If kc is assumed to be 108-109 L/mol s,then P-N dissociates one per 50-500 s, 0.6-6 molecules of St react, and then P is combined with N within 30-300 ms, resulting in the dormant species P-N. 

Thermal initiation and ordinary bimolecular termination also occur during polymerization in addition to initiation by the dissociation of the adduct or the active polymer chain-end dissociation and reversible temination (formation of the dormant species). Therefore, the degree of the control of the molecular weight and the molecular weight distribution is determined by the ratio of the polymer chains produced under control and uncontrol. If the contribution of the thermal initiation and bimolecular termination is very small, the molecular weight distribution is close to the Poisson distribution, i.e., Mw/Mn=1 + 1/Pn, where Pn is the degree of polymerization. It was shown that when the number of... 

A further prejudice that inhibited the acceptance of the F-Cat mechanism was the belief that esters, admitted to exist, but called dormant species , were in equilibrium with conventional propagating carbenium ions. This belief persisted despite the fact that no... 

The dissociation of PnD.A —> PnD + A is equivalent to what some authors have called reversible termination , and the activatable ester PnD is what has been called a dormant species. 

In this equilibrium, the number of the dormant species is much higher than the number of active species, which provides the above-mentioned control. Low radical concentration depresses chain propagation much less than termination (radical combination or disproportionation). In recent years, complexes of several different metals have been used successfully for ATRP of various monomers, whereby Cu(l) [60] and Ru( 11)-complexes [61] proved to be the most promising ones. In Fig. 6.6 two examples of such complexes are shown [58]. 

While in most of the reports on SIP free radical polymerization is utihzed, the restricted synthetic possibihties and lack of control of the polymerization in terms of the achievable variation of the polymer brush architecture limited its use. The alternatives for the preparation of weU-defined brush systems were hving ionic polymerizations. Recently, controlled radical polymerization techniques has been developed and almost immediately apphed in SIP to prepare stracturally weU-de-fined brush systems. This includes living radical polymerization using nitroxide species such as 2,2,6,6-tetramethyl-4-piperidin-l-oxyl (TEMPO) [285], reversible addition fragmentation chain transfer (RAFT) polymerization mainly utilizing dithio-carbamates as iniferters (iniferter describes a molecule that functions as an initiator, chain transfer agent and terminator during polymerization) [286], as well as atom transfer radical polymerization (ATRP) were the free radical is formed by a reversible reduction-oxidation process of added metal complexes [287]. All techniques rely on the principle to drastically reduce the number of free radicals by the formation of a dormant species in equilibrium to an active free radical. By this the characteristic side reactions of free radicals are effectively suppressed. 

T. K. Wu We tried to determine the amount of dormant species in homopolymerizations. We can measure the ratio of "exocyclic" vs. "endocyclic" methylene carbon resonance, and if the ratio is not exactly 1 to 2 we can get an estimate of the concentration of dormant ion. 

Relatively new controlled radical polymerization (CRP) methods, which were discovered in the mid-1990s, focused on establishing a precise equilibrium between the active and dormant species. Three approaches, namely atom transfer radical... 

To this end, a very widely used approach is ROP initiated by polyols (at least triols) in the presence of tin(II) bis-(2-ethyUiexanoate) [155,156]. By implementing this technique, alcohols are dormant species and have to be activated by reaction with tin(II) bis-(2-ethylhexanoate) into tin alkoxides to initiate or to propagate the polymerization. The alcohols are thus not activated at the same time and no side-reactions between them are observed. Besides, it is more appropriate to initiate... 

When monomer conversion is complete, a second batch of a different monomer can be added to form a block copolymer. If the second batch of monomer is not added quickly and the reaction conditions not altered to preserve the dormant species, there will be a continuous deterioration of the reaction system s ability to form block copolymers because of bimolecular termination between propagating radicals since no other competitive reaction is possible in the absence of monomer. The equilibrium between dormant species and radicals will be pushed toward the propagating radicals and their subsequent irreversible bimolecular termination. 

A variety of monomers, including styrene, acrylonitrile, (meth) acrylates, (meth) acrylamides, 1,3-dienes, and 4-vinylpyridine, undergo ATRP. ATRP involves a multicomponent system of initiator, an activator catalyst (a transition metal in its lower oxidation state), a deactivator (the transition state metal in its higher oxidation state) either formed spontaneously or deliberately added, ligands, and solvent. Successful ATRP of a specific monomer requires matching the various components so that the dormant species concentration exceeds the propagating radical concentration by a factor of 106. 

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). 

In addition to the choice of Lewis acid, added common ion salt, and temperature, the fast equilibrium between active and dormant species can be fostered by including additional nucleophiles (separate from the nucleophilic counterion) in the reaction system and by variations in solvent polarity. Nucleophiles act by further driving of the dynamic equilibrium toward the covalent species and/or decreasing the reactivity of ion pairs. Nucleophilic counterions and added nucleophiles work best in nonpolar solvents such as toluene and hexane. Their action in polar solvents is weaker because the polar solvents interact with the nucleophiles and nucleophilic counterions, as well as the ion pairs. Polar solvents such as methylene... 

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