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

Poly(vinyl acetate) by OMRP : Increasing the steric encumbrance of the β‐diketonate R substituents in vinyl acetate (VAc) polymerization mediator [Co{OC(R)CHC(R)O} 2 ] from Me to t Bu sufficiently weakens the Co III PVAc bond of the polymer chain to allow it to operate by both associative (degenerative transfer) and dissociative (organometallic radical polymerization, OMRP) mechanisms (see scheme). The Co III PVAc species also acts as a transfer agent in the absence of Lewis bases, whereas the Co II complex shows catalytic chain transfer (CCT) activity. magnified image 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 °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.