Mechanisms atom transfer radical addition

Atom radical transfer polymerisation (ATRP) has its roots in atom transfer radical addition (ATRA), which involves the formation of 1 1 adducts of alkyl halides and alkenes, and is also catalysed by transition metal complexes. ATRP is a modification of the Kharasch addition reaction (Kharasch et al. 1945) although there may be some differences (Minisci 1975). A general mechanism for ATRP is shown in Scheme 10.5. In ATRP the radicals or the active species are generated through a reversible redox process catalysed by a transition metal complex (Mtn-L/... 

Scheme 5.4 Addition mechanism of PSBronto Ceo through atom transfer radical addition.

Evidence based on product mixtures now suggests, at least in the cases of a-halocarbonyl and perhaloalkyl starting marterials, that these reactions are in fact atom transfer radical cyclizations (equation 166)324,325. In them, the palladium catalyst is proposed to have roles both as the radical initiator and as a trap for iodine, similar to the more commonly used hexabutylditin. Intramolecular allyl halide-alkyne cyclizations proceed with trans-addition to the triple bond this is evidence that a still different mechanism may be operating in these cases (equation 167)1,326. 

Figure 5.10. Mechanism of atom-transfer radical living polymerization. In this process, addition of atom-transfer agents results in initiating radicals that react with monomers. Rather than terminating polymer growth, halogenated end units are formed that are capable of propagating chain growth when additional monomer is added.

The fifty chapters submitted for publication in the ACS Symposium series could not fit into one volume and therefore we decided to split them into two volumes. In order to balance the size of each volume we did not divide the chapters into volumes related to mechanisms and materials but rather to those related to atom transfer radical polymerization (ATRP) and to other controlled/living radical polymerization methods reversible-addition fragmentation transfer (RAFT) and other degenerative transfer techniques, as well as stable free radical pol5mierizations (SFRP) including nitroxide mediated polymerization (NMP) and organometallic mediated radical polymerization (OMRP). 

ESR spectroscopy was successfully applied to quantify radical concentration in the polymerizations [4, 8-11], However, the direct detection method of ESR did not reveal information on many additional points that ate very significant in radical polymerization chemistry so far. For example, the length of propagating chain is not known, direct observation of the penultimate unit effect is almost impossible, and detailed mechanisms of radical reactions remain extremely difficult to examine. These problems have not yet been fully resolved but the development of controlled radical polymerization techniques, especially atom transfer radical polymerization (ATRP), enables us to resolve some of these problems. 

Novel rathenium complexes with carborane ligands were employed as efficient catalysts for controlled polymer synthesis via Atom Transfer Radical Polymerization (ATRP) mechanism. The ability of carborane ligands to stabihze high oxidation states of transition metals allows the proposed catalysts to be more active than their cyclopentadienyl counterparts. The proposed catalysts do not reqnire additives such as aluminium alkoxides. It was shown that introdnction of amine additives into the polymerization mixture leads to a dramatic increase of polymerization rate leaving polymerization controlled. The living nature of polymerization was proved via post-polymerization and synthesis of block copolymers. 

Controlled/ Living radical polymerization (CRP) of vinyl acetate (VAc) via nitroxide-mediated polymerization (NMP), organocobalt-mediated polymerization, iodine degenerative transfer polymerization (DT), reversible radical addition-fragmentation chain transfer polymerization (RAFT), and atom transfer radical polymerization (ATRP) is summarized and compared with the ATRP of VAc catalyzed by copper halide/2,2 6 ,2 -terpyridine. The new copper catalyst provides the first example of ATRP of VAc with clear mechanism and the facile synthesis of poly(vinyl acetate) and its block copolymers. 

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