Dormant centers

NMRP) type of controllers, which is one case of CRP (see Chapter 4 for background on CRP and NMRP), Hernandez-Ortiz et al. [47] used a multidimensional type of approach. Instead of using subscripts for every monomer type (multicomponent copolymerization), as mentioned in Section 12.2.4, they used three subscripts to count the number of total monomer units, total number of active (free radical) centers, and total number of dormant centers, namely, they have dealt with a multifunctional type of model. 

Kissin et al. [72, 73, 76, 77] explain the reduction in activity by the formation of Ti-CH2-CH3 structures after the insertion of ethene into Ti-H bonds, and these structures are the low-activity (or dormant) centers in polymerization because of the P-hydrogen agostic interaction. 

Chain transfer in the presence of hydrogen reactivates such dormant centers ... 

Dormant centers (Cj), formed after propylene 2,l-inserti(Mi into titanium-polymer bonds... 

Dormant centers (Cj), do not attach propylene for steric reasmis, but interact with " C0 the same as the active centers Cp. In this case, the QR CO method can define the total number of the centers (Cp + Cj), containing titanium-polymer bmids. 

Dormant centers can be transformed to an active state by interaction with hydrogen as a result of reactions (20) and (21) ... 

Surface titanium hydrides (CCTi-H) are highly reactive compounds and can be partially deactivated in side reactions by interaction with components of the catalytic system. As a result of these side reactions, the number of active centers for polymerization in the presence of hydrogen is as a rule lower than the total number of the centers containing titanium-polymer bonds (Cp + Cd) during polymerization without hydrogen (Table 7). In the case of ethylene polymerization, dormant centers (Cd) are not formed. But, for polymerization in the presence of hydrogen,... 

The kinetics of this process is strongly affected by an association phenomenon. It has been known that the active center is the silanolate ion pair, which is in equUibrium with dormant ion pair complexes (99,100). The polymerization of cyclosiloxanes in the presence of potassium silanolate shows the kinetic order 0.5 with respect to the initiator, which suggests the principal role of dimer complexes (101). 

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. 

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). 

An additional point to be clarified in these copolymerizations relates to the question as to whether the self- and cross-aggregated active centers are reactive entities in their own right or merely serve as dormant reservoirs providing unassociated, reactive centers. As has been noted previously in this review a body of results has appeared which involves associated organolithiums as reactive species in either initiation or propagation. 

To obtain a behavior similar to case 2 for free-radical polymerizations, it is necessary to decrease the influence of the bimolecular termination reaction by decreasing the concentration of active centers. This is possible by producing a reversible combination between a growing chain (a polymer radical P ) with a stable radical N to form an adduct P-N, which behaves as a dormant species ... 

Mechanistic Aspects of Cationic Copolymerizations The relative reactivities of monomers can be estimated from copolymerization reactivity ratios using the same reference active center. However, because the position of the equilibria between active and dormant species depends on solvent, temperature, activator, and structure of the active species, the reactivity ratios obtained from carbocationic copolymerizations are not very reproducible [280]. In general, it is much more difficult to randomly copolymerize a variety of monomers by an ionic mechanism than by a radical. This is because of the very strong substituent effects on the stability of carbanions and carbenium ions, and therefore on the reactivities of monomers substituents have little effect on the reactivities of relatively nonpolar propagating radicals and their corresponding monomers. The theoretical fundamentals of random carbocationic copolymerizations are discussed in detail and the available data are critically evaluated in Ref. 280. This review and additional references [281,282] indicate that only a few of the over 600 reactivity ratios reported are reliable. 

CF3C(0)0-ZnCl2] , that is suitably nucleophilic to stabilize the growing carbocationic center by reversible formation of the dormant species. 

Propagation is the most important elementary reaction in which a macro-molecular chain is formed. Control in new carbocationic polymerizations, in which well-defined polymers are prepared, might be explained by new mechanisms of propagation and new types of active centers involved. However, as discussed briefly in Section IV.B.4, we believe that only two types of species with different degrees of ionization are involved sp3-hybridized dormant species and sp2-hybridized carbenium ions [Eq. (43)] ... 

When a dormant species or alkoxyamine dissociates homolytically, a carbon-centered radical and a stable nitroxide radical are formed (Scheme 2). This is a reversible process and the reversible reaction is very fast - close to diffusion-controlled rates. With increasing temperature, the dissociation rate will increase, which will increase the concentration of the polymeric radicals (P ). These will have a chance to add to monomer before being trapped again, which allows growth of the polymer chains. The nitroxide is an ideal candidate for this process since it only reacts with carbon-centered radicals, is stable and does not dimerize, and in general couples nonspecifically with all types of carbon-centered radicals (at close to diffusion-controlled rates). 

Polymerization can be started using an alkoxyamine as initiator such that, ideally, no reactions other than the reversible activation of dormant species and the addition of monomer to carbon-centered radicals take place. The alkoxyamine consists of a small radical species, capable of reacting with monomer, trapped by a nitroxide. Upon decomposition of the alkoxyamine in the presence of monomer, polymeric dormant species will form and grow in chain length over time. Otherwise, polymerization can be started using a conventional free-radical initiator and a nitroxide. The alkoxyamine will then be formed in situ when an initiator molecule decomposes, and, after adding a monomer unit or two, is trapped by a nitroxide. 

Since the nitroxide and the carbon-centered radical diffuse away from each other, termination by combination or disproportionation of two carbon-centered radicals cannot be excluded. This will lead to the formation of dead polymer chains and an excess of free nitroxide. The build-up of free nitroxide is referred to as the Persistent Radical Effect [207] and slows down the polymerization, since it will favor trapping (radical-radical coupling) over propagation. Besides termination, other side reactions play an important role in nitroxide-mediated CRP. One of the important side reactions is the decomposition of dormant chains [208], yielding polymer chains with an unsaturated end-group and a hydroxyamine, TH (Scheme 3, reaction 6). Another side reaction is thermal self-initiation [209], which is observed in styrene polymerizations at high temperatures. Here two styrene monomers can form a dimer, which, after reaction with another styrene monomer, results in the formation of two radicals (Scheme 3, reaction 7). This additional radical flux can compensate for the loss of radicals due to irreversible termination and allows the poly-... 

A conventional free-radical initiator is added (contrary to some other controlled free-radical polymerization techniques) that generates radicals, which can add either to the monomer or the S=C moiety of the RAFT agent (step 1). In most cases the addition of small carbon-centered radicals to the RAFT agent is rapid and is not rate determining. Therefore, step (1) involves polymeric radical addition to 1 to form an intermediate radical species 2 that will fragment back to the original polymeric radical species or fragment to a dormant species 3... 

The borderline between transfer und termination is not very sharp in the Uter-ature and we shall use the following kinetic distinctions for trand er it has no direct kinetic effect, growing species are fully restored, every act of transfer forms one dead macromolecule. We drall also disoiss separately the special case of temporary termination being a reversible termination in which an active center becomes temporarily converted into its inactive (dormant) or much less active, isomeric counterpart. Termination forms one dead maaomolecule and annihilates one active species. 

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