Homolytic cleavage transition metal complexes

One of the most important components in the metal-catalyzed living radical polymerization is the transition-metal complex. As a catalyst, the complex induces reversible activation (homolytic cleavage) of... 

The ATP process developed by Sawamoto and coworkers [226], uses an initiating system consisting of carbon tetrachloride, dichlorotri(triphenyl-phosphine)-ruthenium (II) and methylaluminum bis(2,6-di-fe/"f-butylphenoxide) to polymerize methyl methacrylate [226]. The polymerization involves reversible and homolytic cleavages of carbon-halogen terminal groups assisted by transition metal complexes [226]. 

Another class of redox-initiating systems involves transition metal complexes which either alone provide radicals by homolytic cleavage or react with organic halides. The latter system applied under reversible conditions forms the basis of ATRP, one of the most powerful controlled/living radical polymerization systems vide infra). 

The penultimate unit effect may play a very important role in ATRR The rate constants of activation of monomeric and dimeric alkyl bromides with a CuBr-bpy (bpy=2,2 -bipyridine) complex as activator were determined. The ATRP relies on the reversible activation of a dormant alkyl halide through halogen abstraction by a transition metal complex to form a radical that participates in the classical free-radical polymerization figure (Fig. 2) prior to deactivation. In this equiUbrium, the alkyl radical (Pm ) is formed in an activated process, with a rate constant kact> by the homolytic cleavage of an alkyl halogen bond (Pm-Z) catalyzed by a transition metal complex in its lower oxidation state (Cu ). The relative values of fcact of the alkyl bromides were determined for CuBr/bpy catalyst systems in acetonitrile at 35°C. These systems followed the order EBriB (30) MBrP (3)>iBBrP (1) for monomeric initia-tors and MMA-MMA-Br (100) MA-MMA-Br (20) > MMA-MA-Br (5) > MA-MA-Br (1) for dimeric initiators. ... 

When polymerization occurs at a transition metal-carbon bond, this type of complexing weakens the metal-carbon bond and facilitates homolytic cleavage (202) with migration of the polymer radical to one... 

Although alkoxide hgands are often stable and fairly unreactive ancillary ligands, particularly when bonded to early transition metals, the C-0 bond in an alkoxide ligand can undergo special types of reactions when a related oxo complex is accessible [114]. Two mechanisms of both homolytic and heterolytic fashions for C-O bond cleavage in alkoxide hgands have been discussed. 

Peroxides of transition metals are themselves active intermediates in heterolytic and homolytic liquid-phase catalytic oxidation reactions of alkenes, aromatic hydrocarbons and alkanes. Heterolytic oxidations are characterized by a requirement for a free coordination volume near the transition metal atom. Homolytic oxidations proceed via M-O bond cleavage in peroxo complexes. 

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