LIVING RADICAL POLYMERIZATION OF ACRYLATES INITIATED AND CONTROLLED BY ORGANOCOBALT PORPHYRIN COMPLEXES

(Tetramesitylporphyrinato)cobalt(II) ((TMP)Co-.) and the octabromo der ivative ((Br8TMP)Co-.) mediate an effective living radical polymerizat ion 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)C o-. is substantially faster than that for (TMP)Co-. because of the hig her concentration of radicals resulting from greater dissociation of t he dormant organocobalt complex. Unusually large molecular weight low polydispersity acrylate homopolymers and block copolymers have been ob tained by this method. Kinetic studies for the conversion of methyl ac rylate (MA) to poly(methyl acrylate) (PMA) initiated and controlled by (TMP)Co-PMA are fully compatable with a living radical process mediat ed by the metallo radical ((TMP)Co-.). Overall apparent activation par ameters for the polymerization process (Delta H(app)double dagger = 28 +/- 2 kcal mol(-1); Delta S(app)double dagger = 4 +/- 1 cal K-1 mol(- 1)) are interpreted as sums of the activation parameters for radical p ropagation (Delta H-p double dagger similar to 4 kcal mol(-1); Delta S -p double dagger similar to 25 cal K-1 mol(-1)) and thermodynamic valu es for homolytic dissociation of (TMP)Co-PMA (Delta H degrees similar to 24 kcal mol(-1); Delta S degrees similar to 29 cal K-1 mol(-1)).