Cyclopentadienyl Chromium,β-Diketiminate Complexes: Initiators, Ligand Steric Effects, and Deactivation Processes in the Controlled Radical Polymerization of Vinyl Acetate

The new compounds CpCr (nacnac(Ar.Ar')) with nacnac(Ar.Ar)' = Ar-N-C(Me)-CH(Me)-=N-Ar' (Ar = Ar' = C6H2Me3-2,4,6 or mes, 2; C6H3Et-(2),6 or dep, 3; Ar = C6H3Me,-2,6 or xyl and Ar' = C(6)H(3)iPr(2)-2,6,6 or dipp, 4) have been synthesized and used in polymerization experiments in addition to the previously known analogues with Ar = Ar' = xyl, 1, or dipp, 5. The compounds were used as moderators for the polymerization of vinyl acetate (VAc) initiated by V-70, according to an OMRP mechanism. The alkylchromium(III) thermal initiator CpCr(nacnac(xyl,xyl))(CH2CMe3) (8) was synthesized from CpCr(nacnac(xyl,xyl))(OTs) (7) and Mg(CH2CMe3)(2)(dioxime), while 7 was obtained from CpCr(nacnac(xyl),(xyl))Cl (6) and AgOTs. The polymerizations carried out with (1-5)/V-70/VAc at elevated temperatures yielded rapid deactivation, suggestive of irreversible radical trapping. On the other hand, room-temperature polymerizations carried out with 6/VAc proceeded, albeit slowly, to greater conversions. A labilizing effect of the Ar/Ar' steric bulk is suggested by QM/MM calculations of Cr-III-C BDE for models of the OMRP dormant species with 1, 5, and the parent system where Ar = Ar' = Ph. Thermal deactivation of the nacnac(xyl,xyl) has been evidenced, with formation of the acetate complex CpCr(nacnac(xyl,xyl))(OAc), 9. its confirmed by an UV-vis study and by independent synthesis. This product is proposed to form by beta-acetate transfer from the growing radical chain, triggered by a head-head monomer insertion. Compounds 2, 3, 6, 7, 81 and 9 have been structurally characterized by X-ray diffraction methods.