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

Abstract

The complex [Co(II)(tmhd)(2)] (4; tmhd = 2,2,6,6-tetramethylhepta-3,5-dionato) has been investigated as a mediator for controlled radical polymerization of vinyl acetate (VAc) and compared with the analogue [Co(II)(acac)(2)] (1; acac = acetylacetonato). A relatively well controlled process occurs, after an induction time, with 2,2'-azobis(4-methoxyl-2,4-dimethylvaleronitrile) (V-70) as radical initiator at 30 degrees C. However, whereas the polymerization essentially stops after about six initiator half-lives in the presence of 1, it continues with a first-order rate law in the presence of 4. The successful simulation of the kinetic data shows that 4 operates simultaneously by associative (degenerative transfer, DT) and dissociative (organometallic radical polymerization, OMRP) mechanisms. The occurrence of OMRP was confirmed by an independent polymerization experiment starting from an isolated and purified [Co(tmhd)(2)](PVAc) macroinitiator. The polymer molecular weight evolves linearly with conversion in accordance with the expected values for one chain per Co atom when DT is the predominant mechanism and also during the pure OMRP process; however, observation of stagnating molecular weights at long reaction times with concomitant breakdown of the first-order rate law for monomer consumption indicates a competitive chain-transfer process catalyzed by an increasing amount of Co(II). In the presence of external donors L (water, pyridine, triethylamine) the DT pathway is blocked and the OMRP pathway is accelerated, and polymerization with complex 4 is then about five times slower than with complex 1. The reversal of relative effective OMRP rate constants k(eff) (4>1 in the absence of external donors, 4<1 in their presence) is rationalized through competitive steric effects on Co(III)-C and Co(II)-L bond strengths. These propositions are supported by (1)H NMR studies and by DFT calculations.

Living Radical Polymerization by the RAFT Process – A Third Update

“…[144] The relative importance of these mechanisms depend on the specific complex, the reaction conditions, and the monomers. A recent study by Kumar et al [145] indicates that both the DT and OMRP mechanisms operate simultaneously in the case of VAc polymerization mediated by Co(tmhd) 2 (9) (Chart 2).…”

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

“…(2), Scheme 1) [51] or a degenerative transfer (DT) one (Eq. (3), Scheme 1) [53,54]. The latter is observed when the amount of radicals exceeds the amount of cobalt complex.…”

Organometallic‐Mediated Radical Polymerization: Developing Well‐Defined Complexes for Reversible Transition Metal‐Alkyl Bond Homolysis

“…Replacing the CH 3 groups in the acac ligand with tert-butyl groups (3) permits V-70 initiated polymerization of VOAc by both DT and RT mechanisms, the latter becoming effective due to increased steric bulk of the ligands. [20] Compounds bearing more electrondonating ketoaminato ligands (4 and 5) also permit an RT mechanism, but at a rate slower than that with 2, due to the electronic stabilization of the Co III state; increased steric bulk near the metal centre (5) again increases the observed RT rate (see Figure 2). [17] A further opportunity to control polymerization mechanism is offered by the addition of donor ligands.…”

“…Nevertheless, the greatest successes in OMRP have resulted from the use of cobalt reagents, yielding systems capable of forming polymers and block copolymers of acrylates, vinyl acetate, and acrylonitrile (AN) in an effectively living fashion. [12][13][14][15][16][17][18][19][20] Although we present only a brief descriptive survey of cobalt-based CRP, a recent review summarizes the detailed mechanistic considerations at play. [11] Early reports of Co OMRP demonstrated the living polymerization of acrylates by organocobalt porphyrins [7] and cobaloximes.…”