Termination reversible

Polymer chains in LRP can be classified into three major categories dormant, active (free radical) and dead (killed by any kind of irreversible termination reaction). The first two classes correspond to the so-called living chains and the degree of hvingness of a polymer can be quantified by the proportion of those living chains as shown below  

Proportion of living chains in a classical free-radical polymerisation [radicals] 

In a classical free-radical polymerisation the concentration of active radicals is very small compared to the concentration of dead polymer chains, which make up the most of the polymer. In contrast, in an LRP the concentration of dead chains is only a few per cent of the concentration of dormant chains. Usually, the concentration of dormant chains is in the range of 10 -10 moll and the concentration of radical in the range of 10 5-10 moll .  

If the criteria of fast activation-deactivation together with fast and efficient initiation are fulfilled, the number average degree of polymerisation should increase linearly with monomer conversion according to  

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Most radicals are transient species. They (e.%. 1-10) decay by self-reaction with rates at or close to the diffusion-controlled limit (Section 1.4). This situation also pertains in conventional radical polymerization. Certain radicals, however, have thermodynamic stability, kinetic stability (persistence) or both that is conferred by appropriate substitution. Some well-known examples of stable radicals are diphenylpicrylhydrazyl (DPPH), nitroxides such as 2,2,6,6-tetramethylpiperidin-A -oxyl (TEMPO), triphenylniethyl radical (13) and galvinoxyl (14). Some examples of carbon-centered radicals which are persistent but which do not have intrinsic thermodynamic stability are shown in Section 1.4.3.2. These radicals (DPPH, TEMPO, 13, 14) are comparatively stable in isolation as solids or in solution and either do not react or react very slowly with compounds usually thought of as substrates for radical reactions. They may, nonetheless, react with less stable radicals at close to diffusion controlled rates. In polymer synthesis these species find use as inhibitors (to stabilize monomers against polymerization or to quench radical reactions - Section 5,3.1) and as reversible termination agents (in living radical polymerization - Section 9.3). 

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... 

The initiator association number y depends upon temperature, solvent and concentration of the system and is reported to be between two and seven (2) The association of polystyryl anions results in species A. Ak which do not react with styrene monomer. The macromolecular association is analogous to a reversible termination by combination. 

Finally, the use of stable free radical polymerization techniques in supercritical C02 represents an exciting new topic of research. Work in this area by Odell and Hamer involves the use of reversibly terminating stable free radicals generated by systems such as benzoyl peroxide or AIBN and 2,2,6,6-tetramethyl-l-piperidinyloxy free radical (TEMPO) [94], In these experiments, styrene was polymerized at a temperature of 125 °C and a pressure of 240-275 bar C02. When the concentration of monomer was low (10% by volume) the low conversion of PS which was produced had a Mn of about 3000 g/mol and a narrow MWD (PDI < 1.3). NMR analysis showed that the precipitated PS chains are primarily TEMPO capped, and the polymer could be isolated and then subsequently extended by the addition of more styrene under an inert argon blanket. The authors also demonstrated that the chains could be extended... 

The topics analysed here include reversible termination and the formation of p-tolyl end-groups on polystyrenes made in toluene. For unknown reasons, most authors have very largely ignored this Friedel-Crafts alkylation, which in a polymer context is a transfer reaction. It was unfortunately termed molecular termination by Overberger and was explored by his and Smets groups. 

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. 

Living radical polymerization (LRP) with reversible termination generally proceeds as... 

ATRP and NMP control chain growth by reversible termination. RAFT living polymerizations control chain growth through reversible chain transfer [Bamer-Kowollik et al., 2001, 2003 Chiefari and Rizzardo, 2002 Cunningham, 2002 D Agosto et al., 2003 Goto et al., 2001 Kwak et al., 2002 Moad et al., 2002 Monteiro and de Brouwer, 2001 Stenzel et al.,... 

This is explained as quasi living PIB is composed primarily of dormant, i.e., reversibly terminated chains. Thus, most added reagents, particularly strong nucleophiles, quench the Lewis acid coinitiator... 

Nitroxide mediated polymerization (NMP) [56, 57]. This consists in a thermally reversible termination reaction by a homolytic cleavage of a C-ON bond of an alkoxyamine, giving rise to an initiating alkyl radical (active species) and a nitroxyl radical, which brings control to the reaction [58]. 

The use of this phenomenon to control carbon-carbon bond-forming reactions relies on R being rapidly converted into another transient radical which, in the case of a polymerisation, occurs by repetitive addition to a monomer double bond to give the propagating polymer radical, P Thus, the PRE prevents dead polymer (P—P ) formation and the dormant concentration of P—T remains effectively constant. It follows that the excess of T ensures that reversible termination and addition of P to monomer are dominant reactions allowing all polymer chains to grow practically simultaneously (Section 10.5.4). 

Sequencing chemistry Reversible terminator Pyrosequencing Ligation Proton detection... 

Lactate dehydrogenase (LDH) catalyses the reversible terminal reaction in glycolysis which results in the formation of lactic acid. It is present in a number of cestodes (Table 5.5), which is not surprising as most species excrete lactate (Table 5.4). Even S. solidus, which produces mainly acetate and propionate, has an active... 

The oxyborane radical 15 acts as stable counter radical and assures the reversible termination. Molecular weights up to 150000 with a polydispersity of 2.4 were obtained. Such a system was also satisfactorily used by Jiang et al. [98]... 

Although in this example the authors claimed no living character to the synthesis, Opresnik et al. [227,228] described a similar synthesis in which some living character is seen. They also used disulfides as reversible termination agents in the presence of styrene, MMA and ethyl acrylate (EA). The first step involves the synthesis of polymeric precursor 48 under UV cleavage ... 

Termination is formally an irreversible deactivation of growing species. That is, reversible termination is not a real termination process and would be more appropriately labeled reversible deactivation. If this reversible deactivation is sufficiently dynamic, the number of growing species remains constant throughout the polymerization and all chains have the same opportunity to grow, resulting in polymers with narrow molecular weight distributions. This will be discussed in detail in Chapter 4. 

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. 

Since both NMCRP and ATRP (Sect. 4.4.2) are based on reversible termination, the effects observed for these will be similar and are further discussed later. 

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. 

These same researchers [317] reported the anionic polymerization of n-butyl cyanoacrylate in macroemulsion and miniemulsion. Dodecylbenzenesulfonic acid (DBSA) was used as the surfactant. The DBSA slows the rate of interfacial anionic polymerization through reversible termination, preventing an undesirably high degree of polymerization. Polymerization in macroemulsion resulted in a much higher degree of polymerization, perhaps due to droplet polymerization where the interface is less significant. 

This same research group also reported [318] the cationic polymerization of p-methoxystyrene in miniemulsion. DBSA was used as both a protonic initiator and surfactant. A monomer conversion of 100% was achieved in eight hours at 60 °C. Molecular weights were low (approximately 1,000) and solids of up to 40% could be achieved with good colloidal stability. Polymerization takes place at the interface, initiated by the proton, and terminated by water. Molecular weight increased with conversion, suggesting either reversible termination or decreasing termination. 

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. 

In the following a few examples of reversible terminations through recombination of ion pairs are given ... 

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