Cobalt‐Mediated Radical Polymerization of Acrylonitrile: Kinetics Investigations and DFT Calculations

Abstract

The successful controlled homopolymerization of acrylonitrile (AN) by cobalt-mediated radical polymerization (CMRP) is reported for the first time. As a rule, initiation of the polymerization was carried out starting from a conventional azo-initiator (V-70) in the presence of bis(acetylacetonato)cobalt(II) ([Co(acac)(2)]) but also by using organocobalt(III) adducts. Molar concentration ratios of the reactants, the temperature, and the solvent were tuned, and the effect of these parameters on the course of the polymerization is discussed in detail. The best level of control was observed when the AN polymerization was initiated by an organocobalt(III) adduct at 0 degrees C in dimethyl sulfoxide. Under these conditions, poly(acrylonitrile) with a predictable molar mass and molar mass distribution as low as 1.1 was prepared. A combination of kinetic data, X-ray analyses, and DFT calculations were used to rationalize the results and to draw conclusions on the key role played by the solvent molecules in the process. These important mechanistic insights also permit an explanation of the unexpected "solvent effect" that allows the preparation of well-defined poly(vinyl acetate)-b-poly(acrylonitrile) by CMRP.

Radical Polymerization of Vinyl Acetate with Bis(tetramethylheptadionato)cobalt(II): Coexistence of Three Different Mechanisms

“…[44] Therefore, calculations for the present study were only carried out with B3PW91*. The contribution of exact exchange is reduced in this functional, which therefore provides better estimates of bond energetics for processes involving first-row transition metals in which a spin-state change is involved.…”

“…Diblock copolymers of VAc with N-vinylpyrrolidone, [19] styrene, [20] and acrylonitrile [21,22] have also been obtained by using [CoA C H T U N G T R E N N U N G (acac) 2 ]. The fluorinated analogues [Co-A C H T U N G T R E N N U N G (acac-F 3 )] (2; acac-F 3 = 1,1,1-trifluoropenta-2,4-dionato) and [CoA C H T U N G T R E N N U N G (acac-F 6 ) 2 ] (3; acac-F 6 = 1,1,1,5,5,5-hexafluoropenta-2,4-dionato) were investigated by Matyjaszewski et al for VAc and related monomers.…”

“…[45,46] A few bis-acetylacetonato systems have already been reported in previous studies, but with the energetics given only at the B3LYP level [27] or at the B3PW91* level on the fixed B3LYP-optimized geometries. [44] For better consistency, all geometries were fully reoptimized here. Previous computational work also established the preferred stereochemistry of all systems (see Scheme 2), [25] so no further computational exploration of this issue was necessary in the present case.…”

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

“…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].…”

“…This strategy was used to prepare well-defined block copolymers by sequential OMRP of monomers of different reactivity (Scheme 3, route B). For example, the addition of DMF or DMSO to PVAcACo(acac) 2 chains allowed to properly initiate the acrylonitrile polymerization with the formation of welldefined PVAc-b-PAN block copolymers, which is not possible without the addition of the coordinating solvent [68]. It is worth noting that in these above-mentioned cases, these neutral ligands do not substitute the original ones but further coordinate the metal center (Scheme 3, route B).…”