Propagation living polymerization

The reaction of radicals with nitroxides is reversible. 09 This means that the highest temperature that the technique can reasonably be employed at is ca 80 °C for tertiary propagating species and ca 120 °C for secondary propagating species.22 These maximum temperatures are only guidelines. The stability of alkoxyamines is also dependent on solvent (polar solvents favor decomposition) and the structure of the trapped species. This chemistry has led to certain alkoxyamines being useful as initiators of living polymerization (Section 9.3.6). At elevated temperatures nitroxides are observed to add to monomer albeit slowly. 3IS 5" 523... 

The most important side reactions are disproportionation between the cobalt(ll) complex and the propagating species and/or -elimination of an alkcnc from the cobalt(III) intermediate. Both pathways appear unimportant in the case of acrylate ester polymerizations mediated by ConTMP but are of major importance with methacrylate esters and S. This chemistry, while precluding living polymerization, has led to the development of cobalt complexes for use in catalytic chain transfer (Section 6.2.5). 

Combining control over architecture with control over the stereochemistry of the propagation process remains a holy grail in the field of radical polymerization. Approaches to this end based on conventional polymerization were described in Chapter 8. The development of living polymerization processes has yet to substantially advance this cause. 

Another very important visible light-initiated reaction of alkyl aluminum porphyrins is their 1,4-addition to alkyl methacrylates to produce ester enolate species [Eq. (4)]. This enolate then acts as the active species in the subsequent polymerization of the acrylate monomer. For example, Al(TPP)Me acts as a photocatalyst to produce polymethylmethacrylate with a narrow molecular weight distribution in a living polymerization process [Eq. (4)]. Visible light is essential for both the initiation step (addition of methylmethacrylate to Al(TPP)Me) and the propagation... 

The presence of 1,1-diphenylethene (DPE) in the polymerization of a monomer results in a living polymerization [Raether et al., 2002 Wieland et al., 2002]. DPE adds reversibly to a propagating chain to form a dormant stable radical with no (or very low) reactivity to add monomer. This sets up an equilibrium between propagating chains (active species) and dormant DPE-terminal radicals. 

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