(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).
The reactivity and equilibrium thermodynamic studies of tetra-p-sulfonatophenyl porphyrin rhodium hydride ([(TSPP)Rh-D]-4) with CO, aldehydes and olefins that produce formyl, alpha-hydroxyalkyl and alkyl complexes have been explored in water and compared with the related reactions in non-aqueous media.
Cobalt(II) porphyrin complexes ((por)Co II •) are used to illustrate how metalloradicals (Μ•) can function to control radical polymerization through both chain transfer catalysis and living polymerization. Chain transfer catalysis (CTC) is best achieved when there are minimal steric demands. This allows β-hydrogen abstraction from oligomer radicals by Μ•, as illustrated by the radical polymerization of methyl methacrylate in the presence of tetraanisylporphyrinato cobalt(II). When β-Η abstraction from the oligomer radical is precluded by sterics, then a metalloradical mediated living radical polymerization (LRP) can occur. Radical polymerization initiated and mediated by organo-cobalt tetramesitylporphyrin complexes manifest high living character as shown by the linear increase in M n with conversion, formation of block copolymers and relativity low polydispersity homo and block copolymers. Kinetic studies provide rate and activation parameters for the living radical polymerization process.Bond homolysis of an organometallic complex (M-C(CH 3 )(R)X) in solution proceeds through the intermediacy of a caged radical pair (M e »C(CH 3 )(R)X) that can recombine, separate into freely diffusing radicals, or react by Μ· abstracting a β-Η from the organic radical to form a metal hydride (M-H) and an olefin ( 7).In the absence of events that irreversibly terminate radicals and metal hydride, the homolytic dissociation of an organo-metal complex can potentially provide a constant equilibrium source of both an organic radical and a metal hydride. The broad objectives of this program are to evaluate the kinetic and thermodynamic factors that
Tetra(2,4,6 triisopropropyl phenyl)porphyrinatorhodium(II) ((TTiPP)RhII·1) is a persistent metal-centered radical with the odd electron in the rhodium(II) dz2 orbital. (TTiPP)RhII· forms 1:1 complexes with CO and CH2CH2 where the porphyrin ligand steric properties inhibit further reactions of the one-electron activated substrates. 1H NMR paramagnetic shifts at a series of temperatures are used in evaluating the thermodynamics for CO complex formation with 1 to form [(TTiPP)RhII(CO)]·2 (ΔH° = -5.5 ± 0.5 kcal mol-1; ΔS° = -9 ± 1 cal K-1 mol-1). Related 1H NMR studies show that the bonding of 1 with ethene is less favorable than CO.Key words: rhodium porphyrin, carbon monoxide, ethene, thermodynamics, complex formation, metal-centered radical.
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