Controlled Radical Polymerization of Vinyl Acetate with Cyclopentadienyl Chromium β-Diketiminate Complexes: ATRP vs OMRP
Résumé
The ability of compounds CpCrII(nacnacAr,Ar) (Ar = Dipp, 1; Xyl, 2) to catalyze the atom transfer radical polymerization (ATRP) of vinyl acetate has been explored using methyl 2-chloropropionate as initiator. Polymerizations were also carried out under reverse ATRP conditions with thermal initiation by 2,2′-azobis(4-methoxy2,4-dimethylvaleronitrile) (V-70) in the presence of compounds CpCrIII(nacnacAr,Ar)Cl (Ar = Dipp, 3; Xyl, 4). All data suggest that these processes are in fact proceeding by organometallic-mediated radical polymerization (OMRP). Stoichiometric reactions between complex 2 and the chlorinated compounds CH3CH(Cl)COOCH3, ClCH2COPh, CH3CHClOOCCH3, ClCH2OOCtBu, and ClCH2tBu in the absence of monomer aid in the interpretation of the polymerization results. The first two substrates show rapid one-electron oxidative additions to generate 4 and O-bonded CrIII products, whereas the third and fourth substrates add much more slowly to generate 4 and C-bonded CrIII products; ClCH2tBu does not react. DFT calculations on the competitive radical trapping processes (activation barriers and relative stability of the products) confirm the kinetic and thermodynamic preference for metal−carbon bond formation to generate the dormant species of OMRP.
Domaines
Chimie de coordination
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