Polymerization dormant state

Like kact, kjeact depends upon the nature of the transferable atom. For instance, the deactivation of 1-phenylethyl radicals (structurally similar to the propagating polystyrene radicals) with the bromide complex Cu°(dNbpy)2Br (kjeact = 2.5 x lO s ) is almost 6 times faster than for the analogous chloride complex Cu°(dNbpy)2Cl (k eact = 4.3x 10 M s ). It is consequently expected and indeed observed that polymerization control is better if alkyl bromides and copper bromide-containing catalysts are used in ATRP provided that no side reactions take place in the system. However, if the alkyl halide initiator or polymeric dormant state can easily participate in nucleophilic substitution reactions, the use of chloride-based initiator and/or catalyst is advantageous due to lower reactivity of alkyl chlorides in nucleophilic substitution. For instance, the ATRP of 4-vinylpyridine using... 

Controlled polymerization requires that the initiation rate is at least comparable to that of propagation. Initiation in controlled/living carbocationic systems is usually carried out using models of growing species in their dormant state (e.g., the adducts of a monomer with protonic acids). This enables a similar set of equilibria to be established between carbocations and dormant species for initiation and for propagation. For example, 1-phenylethyl halides have similar reactivity as the macromolecular dormant species in styrene polymerizations, and I-alkoxyethyl derivatives are as reactive as the macromolecular species in the polymerization of vinyl ethers [Eq. (38)] ... 

NMR spectroscopic studies of the in-situ polymerization of C-enriched ethylene in the presence of Cp2Zr( CH3)2 and MAO by Tritto et al. have obtained direct evidence of the formation in solution of monomeric Cp2ZrMe+MeMAO and dinuclear (Cp2-ZrMe)2(M-Me)+MeMAO species, as well as the het-erodinuclear Cp2Zr(M-Me)2AlMe2+MeMAO cationic species. - The last two dinuclear complexes are possible dormant states for the active sites in olefin polymerization. 

Busico et al. [50-52] proposed a microstmctural approach to propene polymerization. It is stated that the regioirregular 2,1-insertion slows chain propagation. The active center with a secondary growing chain enters into a dormant state because of higher steric hindrance for subsequent monomer insertion. However, this approach cannot be considered general because an order higher than 1 is also observed in ethene polymerization. 

In NMP, living macrospecies can be temporarily trapped by a nitroxide species X resulting in the formation of dormant macrospecies (RjX), which are the targeted polymer molecules for CRP, in contrast with the typical dead polymer product P in other chain-growth polymerizations. For a sufficiently fast deactivation (k eact, Scheme 10.2), this dormant state is favored and the contribution of dead polymer molecules is minimized. This favoritism is enhanced as X does not undergo self-termination. 

Deactivation. One of the factors that complicates the quantification of active-site concentration (135) is the fact that metallocene cations are subject to equilibria between catalytically active and inactive forms. In situations in which intramolecular coordination of an arene group can occur, this process competes with monomer coordination in styrene (136) and possibly olefin polymerization. Another dormant state invoked to explain catalyst decay is the dimeric structure [Cp2Zr(CH3)(/u.-CH3)Zr(CH3)Cp2]+ in which a methyl group bridges two metallocene fragments. This has been characterized by NMR for the reaction of Cp2Zr( CH3)2 with MAO and other cocatalysts (136). 

The equilibrinm between dormant chains (P-N) and active chains (P ) is designed and the temperature is adjusted so as to heavily favor the dormant state, which effectively reduces the radical (P ) concentration to suf ciently low levels that allow controlled polymerization. For example, the equilibrium constant K in Eq. (11.11) for the polystyrene (PSt)/TEMPO reversible reaction in the bulk polymerization of styrene at 125°C in the presence of a PS-TEMPO adduct... 

Mechanistically, LRP is distinguished from conventional radical polymerization by the reversible activation process. P-X is activated to the polymer radical P by thermal, photochemical, or chemical stimuli. In the presence of monomer M, P will propagate until it is deactivated back to P-X. For practically important systems, it usually holds that [P]/[P-X] < 10 , meaning that a living chain spends most of its polymerization time in the dormant state (here, living chain denotes the sum of the dormant and active chains). The activation-deactivation cycles will be repeated enough times to allow all... 

Radical sources are typically from thermal activation of chemical initiators and propagate with monomers to be converted to polymer radicals P , which can be deactivated by X to become dormant P . Here X acts as a mediator or a radical capping agent. This reversible activation/deactivation cycle is the key that minimizes radical termination and slows down propagation. The equilibrium is much in favor of the deactivation dirertion with very small ratios of [P j/(P j. Radical species spend most of the time in their dormant state. The time interval between activation and deactivation is in the range of milliseconds, compared to seconds in conventional free-radical polymerization. In such a short lifetime, radical termination and transfer reactions are effectively suppressed, though not... 

In this view, various CLRP methods have been developed since the early 80s, each of them based on a different mechanistic approach and having encountered more or less success over the years. Basically, whatever the involved mechanism, their joint, key feature is the establishment of a dynamic equilibrium between propagating radicals, [P ], and various dormant species (i.e., end-capped, thus unable to propagate) throughout the polymerization process in order to decrease the occurrence of irreversible termination reactions to an extremely low level. The so-obtained equilibrium (Figure 1) is triggered and governed by thermal, photochemical, and/or chemical stimuli. For the success of such an approach, a polymer chain should spend most of the polymerization time under its dormant state. 

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