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

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).

Olefin Polymerization Activity and Crystal Structure of Alkyliron(Ⅲ) Porphyrin Complexes

“…1 In comparison, Wayland et al reported alkylcobalt(III) porphyrin polymerization study, in which acrylic ester polymer bound cobalt porphyrin species were observed in 1 H nmr and those showed the controlled/living polymerization. 12,14 The difference between two metal porphyrins might come from the metal-carbon bond energies. The bond energy of the cobalt-carbon is typically 10 kcal/mole greater than that for the iron-carbon.…”

“…[11][12][13][14][15] However, substantial differences might exist for the two metals, as the cobalt-carbon bond energy is typically 10 kcal/mole greater than that for the iron-carbon bond in any given porphyrin and alkyl group. Also, as demonstrated in this report, iron(II) porphyrin is less prone to effect β-hydrogen abstraction in the growing polymer radical.…”

In situ bidentate to tetradentate ligand exchange reaction in cobalt-mediated radical polymerization

“…the polymerization by affecting the stability of the cobaltAcarbon bond of the dormant species, and therefore the active/dormant species equilibrium [12]. Cobalt complexes bearing high electron donating ligands such as porphyrin derivatives [49,55], cobaloximes [56] or 1,3-bis(2-pyridilimino)isoindolate ligands [57] (Fig. 1) present high ability to control the radical polymerization of conjugated vinyl monomers such as acrylates.…”

“…In the case of vinyl esters and some vinyl amides, DFT modeling showed that an extra-stabilization of the CACo bond occurred via an intramolecular chelation of the cobalt atom with the carbonyl function of the last monomer unit and is responsible for the good control observed (Scheme 2) [51,64]. This intramolecular chelation is made possible by the flexible geometry of Co(acac) 2 [67,68] that is not possible for cobalt complexes bearing tetradentate ligands with a rigid square-pyramidal geometry (such as cobalt porphyrins) [55,69,70]. Co(acac) 2 is however less efficient for acrylates polymerization as the result of the low stability of the CACo bond of the dormant species [71].…”

Cobalt(I)‐mediated living radical polymerization of methyl methacrylate

“…This cobalt species has the same ligand coordinating the metal catalysts previously reported, such as RhCl(PPh 3 ) 3 , 9(b) NiBr 2 (PPh 3 ) 2 , 10(b) and RuCl 2 (PPh 3 ) 3 . 5(a) Although some cobalt complexes are well known as suitable compounds that act as initiators of LRP 14 or as catalysts of chaintransfer reactions in the radical polymerization of vinyl monomers, 15 a cobalt catalyst for living ATRP initiated by an organic halide has never been reported, except for cobaltocene.…”