Synthesis of poly(vinyl acetate)‐<i>b</i>‐polystyrene and poly(vinyl alcohol)‐<i>b</i>‐polystyrene copolymers by cobalt‐mediated radical polymerization

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.

Section: ]A C H T U N G T R E N N U N G (Pvac)mentioning

confidence: 99%

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

Kumar

Gnanou

Champouret

et al. 2009

“…[28] This technique, known as cobalt-mediated radical polymerization (CMRP), allows poly(vinyl acetate) (PVAc) and poly(vinyl alcohol) (PVOH, merely upon hydrolysis) to be synthesized with a predetermined molecular weight, and to be constitutive components of block copolymers, which are very difficult to obtain by other techniques. [29][30][31][32] Well-defined statistical copolymers [33,34] could also be prepared and fullerene was successfully grafted by PVAc and PVOH. [35] Until recently, CMRP was explained by a reversible termination mechanism based on the homolytic cleavage of the terminal CoÀC bond.…”

Section: Introductionmentioning

confidence: 99%

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

Debuigne

Champouret

Jérôme

et al. 2008

Self Cite

181

318

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.

“…Under these conditions, poly(acrylonitrile) with a predictable molar mass and molar mass distri-using a [PVAc-CoA C H T U N G T R E N N U N G (acac) 2 ] macroinitiator preformed by CMRP. Among the co-monomers used are styrene, [17,18] octene, [19] ethylene, [19] N-vinyl pyrrolidone, [20] and more recently acrylonitrile (AN). [21] In this last case, the controlled character of AN polymerization initiated at 0 8C from the PVAc macroinitiator was clearly established provided that dimethylformamide (DMF) was used as the solvent.…”

Section: Introductionmentioning

confidence: 99%

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

Debuigne

Michaux

Jérôme

et al. 2008

Self Cite

131

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.