Isotopic Investigation of Hydrogen Transfer Related to Cobalt-Catalyzed Free-Radical Chain Transfer

Abstract

The process of cobalt porphyrin catalyzed chain transfer in methacrylate free radical polymerizations shows a deuterium kinetic isotope effect greater than 3, indicating that hydrogen atom transfer occurs in the rate-limiting step of the catalytic cycle. This step involves formation of a transient hydridocobalt(III) complex by a reversible hydrogen atom abstraction from an incoming radical; the other product is an olefin. In some cases, the resulting radical can be trapped as the alkylcobalt(III) product. The use of labeled cyclic olefins indicates that the formation of the alkylcobalt(III) species can be at least partially stereoselective in some cases, and when it is, the addition is cis. Deuterated vinylidene olefins such as methyl methacrylate-d 8 were found to exchange the hydrogen of a cobalt porphyrin hydride to deuteride under free radical conditions. This exchange reaction can be used for deuterium labeling experimentssthe transient cobalt deuterides are similarly trapped by addition of protioolefins. Thermal isomerization of the labeled product with time results in gradual loss of stereoselectivity, but observation of the initial stereoselectivity of this reaction indicates an underlying limit to the reaction selectivity. The ready availability of radical homolysis products will always limit the selectivity of the products. Similar experiments with acetylenes usually result in trans-vinyl products except when steric effects dominate. This shift from cis products with olefins to trans products for acetylenes is unexplained but may be attributed to radical intermediates or may indicate a role for a second cobalt complex in the transition state.

Control of Radical Polymerizations by Metalloradicals

“…When perdeuterated MMA is substituted for MMA using the conditions in Table I, the average polymer chain length increases by a factor of 4.4±0.4 (8). Both the deuterium isotope effect and the (TAP)Co 11 concentration dependence for PMMA chain length indicate that the rate determining step for terminating polymer radical chain growth is β-Η abstraction by (TAP)Co 11 .…”

Catalysis Involving the H• Transfer Reactions of First‐Row Transition Metals

“…By comparing the termination rate constant during a MMA -d 0 polymerization with that during a MMA -d 5 polymerization, the isotope effect k tr(H) / k tr(D) can be estimated as 2.93 at 60 ° C [62] . It can also be estimated as 3.5 (between 40 and 80 ° C) by comparing the effi ciency of a cobalt catalyst for chain transfer during MMA polymerization to the effi ciency of the same catalyst during the polymerization of MMA -d 8 [63] . While macrocyclic cobalt(II) complexes are effective catalysts for chain transfer (see above), the hydrides they form during the catalytic cycle are unobservable, presumably because they are so effi cient in transferring H • back to monomer (1.26) [72] .…”

Modeling the Synthesis of Butyl Methacrylate Macromonomer by Sequential ATRP‐CCTP

“…Models of radical CCTP of styrene and methacrylates can also be found in the literature, with a focus on methyl methacrylate and its copolymerization . In addition to initiation, propagation, and termination, the generally accepted mechanism for CCTP, used in this work, consists of two reactions—hydrogen abstraction and reinitiation.…”

“…In the first reaction (Equation I in Scheme 2 ), a growing radical (subscript i represents chain length) reacts with a Co(II) complex to produce a macromonomer, also of length i , and a Co(III)H intermediate. In the subsequent reinitiation reaction (Equation II), the Co(III)H intermediate reacts with a monomer to yield a monomeric radical () and regenerates the Co(II) complex . As a macromonomer of chain length of 1 is simply a monomer, a monomer renewal mechanism (Equation III) is required in order to correctly model the conversion profile …”