Reversible-termination mechanism

Main Methods Operating via a Reversible Termination Mechanism... 

The proposed mechanism of the polymerization includes that both RAFT and reversible termination mechanisms are operative in the system (Diagram 3.1). Their activity of the RAFT mechanism was confirmed first by carrying out polymerization of St in the presence of PMDTC and TMDPO at 2 < 365 nm where TMDPO is active but PMDTC is not active. In... 

The proof of the concept was provided as no polymer was formed when no UV radiation was applied neither was any obtained without the use of the photoinitiator BAPO, indicating the stability of the RAFT agent and absence of photoiniferter process. In the absence of the RAFT agent MESA, an uncontrolled process was observed which results in the formation of a polymer of PDI = 3.35. However, the authors noted that loss of control was observed on approaching higher conversions and attributed this to instability of the xanthate-polymer chain adduct toward UV radiation and beginning of reversible termination mechanism. 

A living radical polymerization mechanism was proposed for the polymerization of MMA23 -240 and VAc241 initiated by certain aluminum complexes in the presence of nilroxides. It was originally thought that a carbon-aluminum bond was formed in a reversible termination step. However, a more recent study found the results difficult to reproduce and the mechanism to be complex.242... 

Assuming that Reaction 5 is not reversible and that it is the ratedetermining step in the proposed termination mechanism, the rate of oxidation predicted on application of the steady-state theory is given by Equation A. 

Phosphonoformate is a pyrophosphate analog and inhibits both DNA polymerases and reverse transcriptase. However, toxicity may prevent longterm treatment of AIDS patients. Amantadine has a narrow antiviral specificity. It specifically inhibits initiation of the replication of influenza virus RNA of type A (but not of type B). Active only against retroviruses, 3 -azidothymidine is a reverse transcriptase inhibitor, which acts by a chain termination mechanism. It was synthesized in the early 1960s but only recently has been used in treatment of AIDS victims. More recently a series of 2, 3 -dideoxynucleosides, such as dideoxyinosine, have also been used.d Acyclic phosphonates, such as phosphonylmethoxypropyladenine, avoid the need for metabolic phosphorylation of the drug.6... 

In processes based on reversible termination, like NMCRP and ATRP (Sect. 4.4.2), a species is added which minimizes bimolecular termination by reversible coupling. In NMCRP this species is a nitroxide. The mechanism of nitroxide-mediated CRP is based on the reversible activation of dormant polymer chains (Pn-T) as shown in Scheme 1. This additional reaction step in the free-radical polymerization provides the living character and controls the molecular weight distribution. 

To induce this reversible termination, ATRP employs a transition metal complex with sufficient redox potential to deactivate propagating radicals. A halide atom, typically Cl or Br, is transferred reversibly (hence the name atom transfer ) to the metal complex. In the process the metal alternates between a lower and higher oxidation state. A general mechanism is shown in Scheme 5. 

As mentioned above, in both NMP and ATRP the exchange between the active and the dormant states is based on a reversible (although different) termination mechanism. Therefore, the exchange directly affects the radical concentration. In LRP by degenerative transfer, instead, this exchange is carried out by direct transfer of the w-end group between an active and a dormant chain. When an iodine atom is used as end group, the reaction can be expressed as follows ... 

Yet another possible mechanism that can cause an induction period is reversible coupling (or other reaction with itself) of the most plentiful free radical to yield a product of low stability. Initially, none of that product is present. In the early stages of the reaction, its formation can be the dominant and highly effective termination, keeping the reaction at a low rate. With time, however, the metastable product builds up and approaches equilibrium with the free radicals from which it is formed. Picture the product as a reservoir into which the reaction drains free radicals until it is filled to capacity. By default, another and slower termination mechanism then takes over, and the reaction speeds up accordingly. 

Living polymerization under a constant source of y-radiation in the presence of thiocarbonylthio compounds was first reported in 2001 by Pan and co-workers [15, 16], Scheme 4 shows the mechanism used by these authors to account for the living behaviour observed under a constant source of y-radiation. Under this scheme, y-radiation induces sequential homolytic cleavage of the carbon sulfur bond in the dithioester, yielding a stable thiocarbonylthiyl radical. The other half of the thiocarbonylthio compound (R1) initiates polymerization, and the resulting chains are then reversibly terminated by the stable radical. Pan and co-workers base this mechanism on the fact that the thiyl group of the Z-C(S)-S- is always attached to the head of the monomer. However, this explanation cannot differentiate between the two mechanisms, as polymers generated via RAFT will share the exact same structure. 

Few things are known in the control of photopolymerization reactions. In a light-induced reaction, a photoiniferter can be used. Both the initiation and the reversible termination are photoinduced. The mechanism of a classical living radical photopolymerization process is recalled in e21. 

There are four principal mechanisms that have been put forward to achieve living free-radical polymerization (1) Polymerization with reversible termination by coupling, the best example in this class being the alkoxyamine-initiated or nitroxide-mediated polymerization, as first described by Solomon et al. (1985) (2) polymerization with reversible termination by hgand transfer to a metal complex (usually abbreviated as ATRP),(Wang and Matyjaszewski, 1995) (3) polymerization with reversible chain transfer (also termed degenerative chain transfer)-, and (4) reversible addition/ffagmentation chain transfer (RAFT). 

Most experimental results indicate that GTP is a classic anionic polymerization operating by reversible termination (dissociative mechanism) [55, 56], and/or degenerative transfer [57, 58]. For example, termination occurs once monomer is consumed by backbiting at the pen-penultimate carbonyl to generate cyclic /3-ketoester endgroups as in classic anionic polymerizations (Eq. 11) [59]. Acids with pA a< 18 also terminate group transfer polymerizations, whereas... 

Narrow polydispersity, in principle, may be obtained in a free-radical polymerization process, if the process proceeds by a living mechanism, with no premature termination, and if all the propagating chains are initiated at about the same time, similar to what occurs in an anionic polymerization process (Georges et al., 1993). A variety of living radical polymerization systems have been developed in recent years. These are based on either reversible termination or reversible transfer of chain radical for which four principal mechanisms have been put forward (1) Polymerization with reversible termination (deactivation) of growing chains by coupling, the best example in this... 

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