Effect of head-to-head addition in vinyl acetate controlled radical polymerization: why is Co(acac)2-mediated polymerization so much better?

in Macromolecules (in press)

The controlled polymerization of vinyl acetate has been recently achieved by several techniques, but PVAc with targeted Mn and low dispersity up to very high monomer conversions and high degrees of ... [more ▼]

The controlled polymerization of vinyl acetate has been recently achieved by several techniques, but PVAc with targeted Mn and low dispersity up to very high monomer conversions and high degrees of polymerization was only obtained with Co(acac)2 as controlling agent in the so-called CMRP, a type of organometallic mediated radical polymerization (OMRP). Other techniques (including ATRP, ITP, TERP, and RAFT/MADIX) have shown a more or less pronounced slowdown in the polymerization kinetics, which was attributed to the higher strength of the C−X bond between the radical PVAc chain and the trapping agent (X) in the dormant species and to a consequent slower reactivation after a less frequent head-to-head monomer addition. The reason for the CMRP exception is clarified by the present contribution. First, a detailed investigation by 1H, 13C and multiplicity-edited HSQC and DEPT-135 NMR of the PVAc obtained by CMRP, in comparison with a regular polymer made by free radical polymerization under the same conditions, has revealed that Co(acac)2 does not significantly alter the fraction of head-to-head sequences in the polymer backbone and that there is no accumulation of Co(acac)2-capped chains with a head-to-head ω end. Hence, both dormant chains (following the head-to-head and the head-to-tail monomer additions) must be reactivated at similar rates. A DFT study shows that this is possible because the dormant chains are stabilized not only by the C−Co σ bond but also by formation of a chelate ring through coordination of the ω monomer carbonyl group. The head-to-head dormant chain contains an inherently stronger C−Co bond but forms a weaker 6-membered chelate ring, whereas the weaker C−Co bond in the head-to-tail dormant chain is compensated by a stronger 5-membered chelate ring. Combination of the two effects leads to similar activation enthalpies, as verified by DFT calculations using a variety of local, gradient-corrected, hybrid and “ad hoc” functionals (BPW91, B3PW91, BPW91*, M06 and M06L). While the BDE(C−X) of model H-VAc−X molecules [X = Cl, I, MeTe, EtOC(S)S and Co(acac)2] are functional dependent, the BDE difference between head-to-head and head-to-tail dormant chain models is almost functional insensitive, with values of 5−9 kcal/mol for the ATRP, ITP and TERP models, 3−6 for the RAFT/MADIX model, and around zero for CMRP. [less ▲]

Organometallic-mediated radical polymerization of vinyl amides: Effect of metal coordination

Poster (2012, September 04)

Synthesis of novel poly(N-vinyl amide)s containing copolymers by cobalt-mediated radical polymerization

Poster (2012, May 10)

Poly(N-vinyl amide)s are found in many applications due to their valued properties including water solubility, biocompatibility, metal-coordination ability, etc. Although N-vinyl amides are easily ... [more ▼]

Poly(N-vinyl amide)s are found in many applications due to their valued properties including water solubility, biocompatibility, metal-coordination ability, etc. Although N-vinyl amides are easily polymerized via radical pathways, their growing radicals are quite reactive due to the lack of stabilizing group, rendering the synthesis of well-defined poly(N-vinyl amide)s challenging. Thus, we explored the organometallic-mediated radical polymerization (OMRP) of a series of N-vinyl amides using bis(acetylacetonato)cobalt(II) as controlling agent in order to develop a platform for the precision synthesis of poly(N-vinyl amide)s. [less ▲]

Effective cobalt-mediated radical coupling (CMRC) of poly(vinylacetate) and poly(N-vinylpyrrolidone) (co)polymer precursors

in Macromolecules (2010), 43(6), 2801-2813

Cobalt-mediated radical coupling (CMRC) is successfully applied to poly(vinyl acetate) (PVAc) and poly(N-vinylpyrrolidone) (PNVP) precursors for the first time. The coupling process is based on addition ... [more ▼]

Cobalt-mediated radical coupling (CMRC) is successfully applied to poly(vinyl acetate) (PVAc) and poly(N-vinylpyrrolidone) (PNVP) precursors for the first time. The coupling process is based on addition of isoprene onto polymer chains preformed by controlled radical polymerization with cobalt complexes (CMRP). The extents of coupling were high (>90%) to moderate (75-80%) for PVAc and PNVP precursors, respectively. Effects of the length of the polymer precursors and conditions used in the polymerization step on the coupling efficiency are discussed. Mass spectrometry (MS) and nuclear magnetic resonance (NMR) analyses conducted on the coupling products demonstrate the preferential insertion of two isoprene units in the final polymers. The CMRC mechanistic proposal, supported by DFT calculations, is based on this microstructure feature. Finally, illustration of the macromolecular engineering potential of this technique is given by the preparation of symmetrical PVAc-b-PNVP-b-PVAc triblock copolymers starting from the corresponding PVAc-b-PNVP-[Co] diblock copolymer. [less ▲]

Cobalt-mediated radical polymerization (CMRP) and coupling reaction (CMRC): mechanistic advances ans synthetic opportunities

Poster (2009, December 14)

Key role of metal-coordination in cobalt-mediated radical polymerization of vinyl acetate

in Matyjaszewski, Krzysztof (Ed.) Controlled/living radical polymerization: progress in RAFT, DT, NMP & OMRP (2009)

Cobalt mediated radical polymerization (CMRP) of vinyl acetate (VAc) follows a reversible termination mechanism when initiated from a preformed alkyl-cobalt(III) complex. In these particular conditions ... [more ▼]

Cobalt mediated radical polymerization (CMRP) of vinyl acetate (VAc) follows a reversible termination mechanism when initiated from a preformed alkyl-cobalt(III) complex. In these particular conditions, CMRP functions as a stable free radical process and fine tuning of the Co-C bond strength becomes crucial. Increase of temperature and addition of molecules, such as water, dimethylformamide and dimethylsulfoxide, able to coordinate the cobalt complex appeared as efficient strategies to weaken the Co-C bond and thus to speed up the polymerization while maintaining a very good control of the VAc polymerization. The key role of metal-coordination was investigated by kinetic measurements combined with DFT calculations. [less ▲]

The influence of cobalt-coordination on cobalt-mediated radical polymerization of vinyl monomers

Poster (2009, March 19)

Nowadays, polymers are a part of everyday life. Researchers encouraged by growing need in high performance polymers develop new synthesis tools to manage the molecular architecture and thus the polymer ... [more ▼]

Nowadays, polymers are a part of everyday life. Researchers encouraged by growing need in high performance polymers develop new synthesis tools to manage the molecular architecture and thus the polymer properties. In this context, CRP (Controlled Radical Polymerization) techniques have been developed to obtain well-defined architectures and to control polymer parameters. Among these systems is Cobalt-Mediated Radical Polymerization (CMRP), which is based on the reversible deactivation of the growing radical chains with a cobalt complex, the cobalt (II) bis-acetylacetonate. The interest of this system is not only due to its ability to control the polymerization of very reactive monomers such as vinyl acetate (VAc) and N-vinylpyrrolidone (NVP), but also its peculiar mechanism which exhibits two pathways depending on the polymerization conditions; a reversible termination process and a degenerative chain transfer mechanism. Furthermore, it has been showed that the Co-C strength and thus the polymerization are strongly influenced by the use of some additives, such as water, dimethylformamide, dimethylsulfoxide and pyridine, which coordinate the cobalt free site. In this presentation we report the use of a preformed alkyl-cobalt(III) adduct as initiator for the polymerization of acrylonitrile (AN) and the use of these ligands in the CMRP system to synthesize well-defined poly(vinyl acetate)-b-poly(acrylonitrile) block copolymers. As a conclusion, cobalt-coordination appears today as a unique opportunity to adjust the Co-C bond strength and to push back the bounds of possibilities in terms of macromolecular engineering assisted by CMRP. [less ▲]

Cobalt-mediated radical polymerization of acrylonitrile: Kinetics investigations and DFT calculations

in Chemistry : A European Journal (2008), 14(25), 7623-7637

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 ... [more ▼]

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 °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. [less ▲]

Synthetic and mechanistic insights in cobalt mediated radical polymerization (CMRP) of vinyl acetate and acrylonitrile

Conference (2008, July 03)

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

in Chemistry : A European Journal (2008), 14(13), 4046-4059

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 ... [more ▼]

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. [less ▲]