Mechanistic Insights into the Cobalt‐Mediated Radical Polymerization (CMRP) of Vinyl Acetate with Cobalt(III) Adducts as Initiators

Abstract

Over the past few years, cobalt-mediated radical polymerization (CMRP) has proved efficient in controlling the radical polymerization of very reactive monomers, such as vinyl acetate (VAc). However, the reason for this success and the intimate mechanism remained basically speculative. Herein, two mechanisms are shown to coexist: the reversible termination of the growing poly(vinyl acetate) chains by the Co(acac)2 complex (acac: acetylacetonato), and a degenerative chain-transfer process. The importance of one contribution over the other strongly depends on the polymerization conditions, including complexation of cobalt by ligands, such as water and pyridine. This significant progress in the CMRP mechanism relies on the isolation and characterization of the very first cobalt adducts formed in the polymerization medium and their use as CMRP initiators. The structure proposed for these adducts was supported by DFT calculations. Beyond the control of the VAc polymerization, which is the best ever achieved by CMRP, extension to other monomers and substantial progress in macromolecular engineering are now realistic forecasts.

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

“…The main goal is to exploit this ligand exchange for the synthesis of well-defined PVAc-b-PnBA block copolymers. Typically, a PVAcACo(acac) 2 with low dispersity (M w /M n 6 1.07, Table S2) is prepared from the alkylACo(acac) 2 [51] before the addition of 1.2 equiv. of Salen or Salen* ligand in solution in CH 2 Cl 2 (Scheme 4).…”

“…The alkylAcobalt(III) adduct initiator ([Co(acac) 2 A ((CHOCOCH 3 )ACH 2 ) <4 -R 0 )]; R 0 being the primary radical generated by 2,2 0 -azo-bis(4-methoxy-2,4-dimethyl valeronitrile) (V-70, Wako) was prepared as described previously and stored as a CH 2 Cl 2 solution at À20°C under argon [51]. All polymerizations and synthesis were performed by classical Schlenk techniques under argon using dry and degassed monomers.…”

“…Cobalt mediated radical polymerization (CMRP) [45,46] is extensively studied especially for its high ability to control the polymerization of a large range of monomers including acrylic monomers [47][48][49][50] and vinyl esters [12,17,46,51,52]. Depending on the reaction conditions, the CMRP process follows a reversible termination (RT) mechanism (Eq.…”

“…Depending on the reaction conditions, the CMRP process follows a reversible termination (RT) mechanism (Eq. (2), Scheme 1) [51] or a degenerative transfer (DT) one (Eq. (3), Scheme 1) [53,54].…”

“…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].…”