Cyclopentadienyl chromium β-diketiminate complexes : initiators, ligand steric effects, and deactivation processes in the controlled radical polymerization of vinyl acetate

Abstract

The new compounds CpCr(nacnacAr,Ar′) with nacnacAr,Ar′ = Ar−N---C(Me)---CHC(Me)---N−Ar′ (Ar = Ar′ = C6H2Me3-2,4,6 or mes, 2; C6H3Et2-2,6 or dep, 3; Ar = C6H3Me2-2,6 or xyl and Ar′ = C6H3iPr2-2,6 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(nacnacxyl,xyl)(CH2CMe3) (8) was synthesized from CpCr(nacnacxyl,xyl)(OTs) (7) and Mg(CH2CMe3)2(dioxane), while 7 was obtained from CpCr(nacnacxyl,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 CrIII−C BDE for models of the OMRP dormant species with 1, 5, and the parent system where Ar = Ar′ = Ph. Thermal deactivation of the nacnacxyl,xyl system has been evidenced, with formation of the acetate complex CpCr(nacnacxyl,xyl)(OAc), 9, as confirmed by an UV−vis study and by independent synthesis. This product is proposed to form by β-acetate transfer from the growing radical chain, triggered by a head−head monomer insertion. Compounds 2, 3, 6, 7, 8, and 9 have been structurally characterized by X-ray diffraction methods.