Cobalt Mediated Polymerizations

The kinetic semilogarithmic plot for this polymerization was linear but didn t pass the origin (Figure 4). A significant induction time was observed in the polymerization of VAc by Co(acac)2. This induction time was attributed to the slow conversion of Co(ll) to Co(lII) complex by in situ generated radicals. " This group also conducted a successful suspension polymerization of VAc in water using Co(acac)2 and prepared PVAc-PS and PVA-PS block copolymers by combination of the cobalt-mediated polymerization and ATRP. ... 

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). 

Polymerization employing Co complexes as catalysts or else polymers incorporating functionality that includes Co ions represent aspects of polymerization reactions of interest here. Cobalt-mediated free-radical polymerization of acrylic monomers has been reviewed.55 Co11 porphyrins act as traps for dialkylcyanomethyl radicals.1098 Alkyl complexes of Co(TMesP)... 

Scheme 6.79 Hydrosilylation of ketones [164], Dotz benzannulation chemistry [165], cobalt-mediated synthesis of angular [4]phenylenes [166], and nickel-mediated coupling polymerizations [167],...

The mechanism of Co(acac)2-mediated polymerization of Vac is still an open question. On the basis of an early work by Wayland and coworkers on the controlled radical polymerization of acrylates by complexes of cobalt and porphyrins, Debuigne and coworkers proposed a mechanism based on the reversible addition of the growing radicals P to the cobalt complex, [Co(II)], and the establishment of an equilibrium between dormant species and the free radicals (equation 8). Maria and coworkers, however, proposed that the polymerization mechanism depends on the coordination number of cobalt . Whenever the dormant species contains a six-coordinated Co in the presence of strongly binding electron donors, such as pyridine, the association process shown in equation 8 would be effective. In contrast, a degenerative transfer mechanism would be favored in case of five-coordinated Co complexes (equation 9). 

Theoretical work has also been devoted to examine the influence of reactions that interrupt the often repeated cycles of radical formation (activation) and cross-termination to dormant species (deactivation). For the nitroxide- and the cobalt-mediated systems, such reactions are the formation of R(—H) and YH by a usual radical disproportionation, which competes with the coupling of R and Y or by a direct fragmentation of R—Y to the hydroxylamine or a hydridocobalt complex and the alkene.22-33a-35-47-51 57 Even rather small fractions of these processes limit the maximum conversion and stop the polymerization prematurely in nearly indistinguishable ways, because they lead to an exponential decay of the dormant species. Before the end of conversion this does not affect the linear dependence of An on conversion and causes only minor increases of the polydispersity.57 To some extent the deteriorating effect of these reactions can be compensated by the rate enhancement through an additional initiation.50... 

Certain organometallic compounds can be used to mediate controlled/ living radical polymerization due to their liable and reversibly-cleavable metal-carbon bond. For example, organocobalt complex, such as tetramesitylporphyrinato Co(II) complex [Co(TMP)], has been used to mediate the CRP of aciylates. The first successful cobalt-mediated CRP of VAc was reported by Debuigne and Jerome et. al. using cobalt(ll) acetylacetonate complex Co(acac)2 (Scheme 2) ... 

Metal complexes may also act as initiators in stable radical-mediated polymerization with the metal complex perfonning the role of the stable radical. There are reports of titanoceneX cobalt, chromium, iron and... 

The synthesis of a well defined poly(vinyl alcohol)-b-poly(aciylic acid) (PVA-PAA) DHBC has been recently reported in the literature [32] by a two step synthetic scheme. First the synthesis of a poly(acrylonitrile) (PAN) block was realized via cobalt-mediated radical polymerization, using a poly(vinyl acetate) (PVAc) macroinitiator, followed by hydrolysis of both blocks. The polymerization was performed in DMF, a very good solvent for PAN, and at low temperature, where block copolymers with low polydispersity were obtained. The polymerization procedure led to well defined macromolecules with relatively high molecular weights. The obtained copolymers were transformed to the desired DHBCs by hydrolysis, using large excess of potassium hydroxide in a water/ethanol mixture. The successful completion of the hydrolysis reaction was monitored by NMR and IR spectroscopy. An additional macroscopical indication of the DHBC formation was the aqueous solubility of the reaction product. 

DEB 05] Debuigne A., Caille J.-R., Jerome R., Highly efficient cobalt-mediated radical polymerization of vinyl acetate , Angewandte Chemie International Edition, vol. 44, pp. 1101-1104, 2005. 

DEC 98] Decker C., Highly efficient cobalt-mediated radical polymerization of... 

More monomers are mentioned in patent literature but details of the reaction conditions are discrete. One such example is the CCTP of a hydrophilic acrylic macromonomer. Sodium acrylate and ammonium acrylate homopolymers are mentioned and most likely to be examples of cobalt-mediated radical polymerization (CMRP) rather than of CCTP. ... 

Figure 9 Cobalt(ll) bis(p-diketonates) used in cobalt-mediated radical polymerization.

Figure 36 General strategy for polymer release and regeneration of the Co" complex in cobalt-mediated radical polymerization of VAc (the nature of the PVAc-Co" dormant species is more likely a ligand adduct, like IX, or a chelated species, like X, depending on conditions, see Figure 10 and related discussion in Section 3.11.5.5).

Besides the most common CRP methods used to control the radical polymerization of vinylic monomers, other techniques have been developed over the time, and have been tested in miniemulsion or emulsion polymerization as well. These methods are described in the corresponding chapters of this comprehensive. They are iodine transfer polymerization (ITP and the reverse method, RITP), organotelluriirm-mediated CRP (TeRP), and cobalt-mediated radical polymerization (CoMRP). 

In the presence of catalytic amounts of vitamin B12, it has been possible to couple 1,1-disubstituted alkenes with defined regiochemistry under mild conditions in environmentally benign solvent system such as ethanol/water (Table 1). The mechanistic studies led the authors to conclude that organic free radicals are formed in this reaction. In the current process, the rate of combination of two radicals is increased by removing the persistent Co(ll) radical with Ti(in) or Zn. This is in contrast to cobalt-catalyzed free-radical chain transfer and cobalt-mediated living radical polymerization in which the cobalt macrocycle s role is to reduce the steady-state concentration of free radicals (Equation (33)). ... 

Four-arm stars [poly(vinyl acetate) ]4C6o have been prepared by cobalt-mediated radical polymerization of vinyl acetate (VAc) followed by addition of the PVAc-Co(acetylacetonate)2 chains to the fullerene [30]. The photoactive water-soluble [poly(vinyl alcohol) hCgo was then obtained by hydrolysis of the PVAc arms. Because... 

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