Reversible termination polymerization

Scheme 4. Schematic mechanistic representation of reversible termination polymerization [14]...

Diagram 3.7. The proposed mechanism of reversible termination polymerization... 

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... 

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... 

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.,... 

Some cationic ring-opening polymerizations take place without termination and are reversible. Oxirane and oxetane polymerizations are seldom reversible, but polymerizations of larger-sized rings such as tetrahydrofuran are often reversible. The description of reversible ROP is presented below [Afshar-Taromi et al., 1978 Beste and Hall, 1964 Kobayashi et al., 1974 Szwarc, 1979]. It is also applicable to other reversible polymerizations such as those of alkene and carbonyl monomers. The propagation-depropagation equilibrium can be expressed by... 

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]. 

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. 

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. 

In summary, microflow systems are quite effective for molecular-weight distribution control of very fast, highly exothermic free-radical polymerizations. The superior heat transfer ability of the microflow system in comparison with conventional macrobatch systems seems to be responsible for the high molecular-weight distribution controllability. It should be noted that the controllability is much lower than is achieved by conventional living free-radical polymerization, because residence time control does not work for controlling radical intermediates. The lifetime of a radical intermediate is usually much shorter than the residence time in a microflow system. It is also noteworthy that the more rapid and exothermic the polymerization is, the more effective the microflow system is. These facts speak well for the potentiality of microflow systems in the control of highly exothermic free-radical polymerization without deceleration by reversible termination. 

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. 

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