Living Radical Polymerization of Acrylates Initiated and Controlled by Organocobalt Porphyrin Complexes

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Abstract

(Tetramesitylporphyrinato)cobalt(II) ((TMP)Co•) and the octabromo derivative ((Br8TMP)Co•) mediate an effective living radical polymerization of acrylate monomers through the formation of dormant organocobalt complexes ((por)Co−PA) with the growing acrylate polymer radical (•PA). Radical polymerization of methyl acrylate controlled by (Br8TMP)Co• is substantially faster than that for (TMP)Co• because of the higher concentration of radicals resulting from greater dissociation of the dormant organocobalt complex. Unusually large molecular weight low polydispersity acrylate homopolymers and block copolymers have been obtained by this method. Kinetic studies for the conversion of methyl acrylate (MA) to poly(methyl acrylate) (PMA) initiated and controlled by (TMP)Co−PMA are fully compatable with a living radical process mediated by the metallo radical ((TMP)Co•). Overall apparent activation parameters for the polymerization process (Δ = 28 ± 2 kcal mol-1; Δ = 4 ± 1 cal K-1 mol-1) are interpreted as sums of the activation parameters for radical propagation (Δ ∼ 4 kcal mol-1; Δ ∼ 25 cal K-1 mol-1) and thermodynamic values for homolytic dissociation of (TMP)Co−PMA (ΔH° ∼ 24 kcal mol-1; ΔS° ∼ 29 cal K-1 mol-1).

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