Mediated polymerization

The first steps towards living radical polymerization were laken by Otsu and colleagues283 in 1982. In 1985, this was taken one step further with the development by Solomon et al.l0 of nitroxide-mediated polymerization (NMP). This work was first reported in the patent literature30 and in conference papers but was not widely recognized until 1993 when Georges et aL, applied the method in... 

The identification of both phenylethyl and 1-phenyl-1,2,3,4-lelrahydronaphthalenyl end groups in polymerizations of styrene retarded by FeCl3/DMP provides the most compelling evidence for the Mayo mechanism.316 The 1-phenyl-1.2,3,4-tetrahydronaphthalenvl end group is also seen amongst other products in the TEMPO mediated polymerization of styrene,317318 However, the mechanism of formation of radicals 96 in this case involves reaction of the nitroxide with the Diels-AIder dimer (Scheme 3.63). The mechanism of nitroxide mediated polymerization is discussed further in Section 9.3.6. 

Further discussion on the effects of the reaction media and Lewis acids on lacticily appears in Section 7.2. Attempts to control laciicily by template polymerization and by enzyme mediated polymerization are described in Section 7.3. Devising effective means for achieving stereochemical control over propagation in radical polymerization remains an important challenge in the field. 

A number of recent papers have explored enzyme-mediated polymerization. Monomers polymerized include MMA, S, AM and derivatives. The area has been reviewed by Singh and Kaplan222 and Gross et al. n... 

Most polymerizations in this section can be categorized as stable (Tree) radical-mediated polymerizations (sometimes abbreviated as SFRMP). In the following discussion systems have been classed according to the type of stable radical involved, which usually correlates with the type of bond homolyzed in the activation process. Those described include systems where the stable radical is a sulfur-ccntered radical (Section 9.3.2), a selenium-centered radical (Section 9.3.3), a carbon-centered radical (Sections 9.3.4 and 9.3.5), an oxygen-centered radical (Sections 9.3.6, 9.3.7), or a nitrogcn-ccntcrcd radical (Section 9.3.8). Wc also consider polymerization mediated by cobalt complexes (Section 9.3.9) and certain monomers (Section 9.3.5). 

Stable carbon-centered radicals, in particular, substituted diphenylmethyl and triphenylmethyl radicals, couple reversibly with propagating radicals (Scheme 9.11). With, the carbon-centered radical-mediated polymerization systems described to dale, the propagating radical should be tertiary (e.g. methacrylate ester) to give reasonable rates of activation. 

Triphenylinethyl terminated polymers (41) are formed in polymerizations conducted in the presence of triphenylmethyl thiol (40).9 5 Transfer constants for 40 are similar to other thiols (17.8 for S, 0.7 for MM A, compare Section 6.2.2.1). When the polymers (41) are heated in the presence of added monomer it is presumed that the S-CPh bond is cleaved and triphenylmethyl-mediated polymerization according to Scheme 9.11 can then ensue to yield chain extended or block polymers (42). 

The literature on Nitroxide-Mediated Polymerization (NMP) through 2001 was reviewed by Hawker el al. vu 7 More recently the subject has been reviewed by Sluder and Schulte10 and Solomon.109 NMP is also discussed by Fischer110 and Goto and Fukuda" in their reviews of the kinetics of living radical polymerization and is mentioned in most reviews on living radical polymerization. A simplified mechanism of NMP is shown in Scheme 9.17. 

Catala and coworkers167JuiS made the discovery that the rate of TEMPO-mediated polymerization of S is independent of the concentration of the alkoxyamine. This initially surprising result was soon confirmed by others.23 69 Gretza and Matyjaszewski169 showed that the rate of NMP is controlled by the rate of thermal initiation. With faster decomposing alkoxyamines (those based on the open-chain nitroxides) at lower polymerization temperatures, the rate of thermal initiation is lower such that the rate of polymerization becomes dependent on the alkoxyamine concentration, Irrespective of whether the alkoxyamine initiator is preformed or formed in situ, low dispersities require that the alkoxyamine initiator should have a short lifetime. The rate of initiation should be as fast as or faster than propagation under the polymerization conditions and lifetimes of the alkoxyamine initiators should be as short as or shorter than individual polymeric alkoxyamines. 

Other Oxygen-Centered Radical-Mediated Polymerizations... 

Metal complexes may also act as initiators in stable radical-mediated polymerization with the metal complex performing the role of the stable radical. 

In contrast to the situation with copper-based catalysis, most studies on ruthenium-based catalysts have made use of preformed metal complexes. The first reports of ruthenium-mediated polymerization by Sawamoto and coworkers appeared in I995.26 In the early work, the square pyramidal ruthenium (II) halide 146 was used in combination with a cocatalyst (usually aluminum isopropoxide). 

The Chemistry of Radical Polymerization Table 9.19 Initiators for Telluride-Mediated Polymerization ... 

SFRMP stable free radical mediated polymerization... 

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