Cyclopentadienyl Chromium β-Diketiminate Complexes: Initiators, Ligand Steric Effects, and Deactivation Processes in the Controlled Radical Polymerization of Vinyl Acetate ChampouretYohan MacLeodK. Cory BaischUlrich PatrickBrian O. SmithKevin M. PoliRinaldo 2010 The new compounds CpCr(nacnac<sup>Ar,Ar<sup>′</sup></sup>) with nacnac<sup>Ar,Ar<sup>′</sup></sup> = Ar−N<u>---</u>C(Me)<u>---</u>CHC(Me)<u>---</u>N−Ar′ (Ar = Ar′ = C<sub>6</sub>H<sub>2</sub>Me<sub>3</sub>-2,4,6 or mes, <b>2</b>; C<sub>6</sub>H<sub>3</sub>Et<sub>2</sub>-2,6 or dep, <b>3</b>; Ar = C<sub>6</sub>H<sub>3</sub>Me<sub>2</sub>-2,6 or xyl and Ar′ = C<sub>6</sub>H<sub>3</sub><i>i</i>Pr<sub>2</sub>-2,6 or dipp, <b>4</b>) have been synthesized and used in polymerization experiments in addition to the previously known analogues with Ar = Ar′ = xyl, <b>1</b>, or dipp, <b>5</b>. 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<sup>xyl,xyl</sup>)(CH<sub>2</sub>CMe<sub>3</sub>) (<b>8</b>) was synthesized from CpCr(nacnac<sup>xyl,xyl</sup>)(OTs) (<b>7</b>) and Mg(CH<sub>2</sub>CMe<sub>3</sub>)<sub>2</sub>(dioxane), while <b>7</b> was obtained from CpCr(nacnac<sup>xyl,xyl</sup>)Cl (<b>6</b>) and AgOTs. The polymerizations carried out with (<b>1</b>−<b>5</b>)/V-70/VAc at elevated temperatures yielded rapid deactivation, suggestive of irreversible radical trapping. On the other hand, room-temperature polymerizations carried out with <b>6</b>/VAc proceeded, albeit slowly, to greater conversions. A labilizing effect of the Ar/Ar′ steric bulk is suggested by QM/MM calculations of Cr<sup>III</sup>−C BDE for models of the OMRP dormant species with <b>1</b>,<b> 5</b>, and the parent system where Ar = Ar′ = Ph. Thermal deactivation of the nacnac<sup>xyl,xyl</sup> system has been evidenced, with formation of the acetate complex CpCr(nacnac<sup>xyl,xyl</sup>)(OAc), <b>9</b>, 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 <b>2</b>,<b> 3</b>,<b> 6</b>,<b> 7</b>,<b> 8</b>, and <b>9</b> have been structurally characterized by X-ray diffraction methods.