Organometallic-mediated Radical Polymerization Research Articles

Visible light-induced radical polymerization of vinyl acetate mediated by organo-nickel N2O2 Schiff-base complexes

Four nickel(II) complexes with symmetrically substituted N2O2 tetradentate Schiff-base ligands, prepared from the 2:1 condensation of 3-tert-butyl-salicylaldehyde, and ethylenediamine (1), o-phenylenediamine (2), 1,2-cis,trans-cyclohexyldiamine (3), or 1,3-diaminepropane (4), were synthesized. These NiII Schiff-base complexes (1–4) were used as control agents for the polymerization of vinyl acetate (VAc) photoinitiated by diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) according to a photoinduced organometallic-mediated radical polymerization (photo-OMRP) mechanism. All complexes presented good control ability, although the polymerization mediated by 3 showed the best control over molecular weight (Mn matching Mn,th) and dispersity (Ð < 1.50). The livingness of the polymers has been confirmed by LED on and LED off experiments, the polymerization stopped immediately, and no monomer conversion was observed during the light-off period, indicating a negligible concentration of the active radical in the dark. Further, to demonstrate the living nature of this system, block copolymers of poly(vinyl acetate)-b-poly(methyl acrylate) were synthesized using the sequential visible-light-induced process. Kinetic results and computational investigations supported the activation/deactivation equilibrium exerted by complexes 1–4, which occurred on only one face of the complexes via reversible deactivation (RD) mechanism.

One-Pot Synthesis of Polyethylene-Based Block Copolymers via a Dual Polymerization Pathway.

Polyethylene-poly(methyl acrylate) multiblock copolymers were obtained using a catalyst system consisting of a pentamethylcyclopentadienyl cobalt complex (CoIII(η5-C5H5)P(OMe)3I2) and iso-butyl modified methylaluminoxane (MMAO). As the chain-growth mechanism, the self-switching between organometallic-mediated radical polymerization (OMRP) and coordination-insertion polymerization (CIP) is suggested. As a possible polymerization mechanism, we propose that (1) the methyl and iso-butyl groups transfer from Al to Co and the single methyl acrylate (MA) unit insertion into the Me-Co and H-Co allows for the in situ formation of Co-C(COOMe) bonds as an initiator for OMRP of MA, (2) the migratory insertion of ethylene into Co-C(COOMe) bonds leads to the formation of an alkyl-Co species as active species for CIP of ethylene, and (3) MA insertion to the alkyl-Co to regenerate a Co-C(COOMe) bond. The architectures of copolymers were confirmed by various nuclear magnetic resonance (NMR), thermogravimetric analysis (TGA)/differential scanning calorimetry (DSC), and size-exclusion chromatography (SEC) analyses.

Photoinduced organometallic mediating radical polymerization of acrylates mediated by CoII complexes of non-symmetrical tetradentate Schiff-base ligands

Three new cobalt(II) complexes with non-symmetrically-substituted N2O2 tetradentate Schiff-base ligands, prepared from the 1:1:1 condensation of 3-etoxy-salicylaldehyde, salicylaldehyde and ethylenediamine (H2Etsal − 1), o-phenylenediamine (H2Phsal − 2) or 1,2-cis,trans-cyclohexyldiamine (H2Cysal − 3), were synthesized. The complexes 1–3 were characterized by UV–Vis, FTIR, and MALDI-TOF spectroscopy, elemental analysis, molar conductance, cyclic voltammetry, and computational methods. The CoII-Schiff-base complexes (1–3) were used as controlling agents for the polymerization of n-butyl acrylate (BA) photoinitiated by diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO), according to an organometallic-mediated radical polymerization (OMRP) mechanism. Although, all complexes have presented good control ability, the polymerization mediated by complex 1 showed the best control over molecular weight (Mn matching with Mn,th) and dispersity (Ð < 1.50). Kinetic results and computational investigations support the activation/deactivation equilibrium, exerted by complexes 1–3, occurring by one face of the complexes via reversible termination (RT) mechanism.minim.

Photocontrolled reversible-deactivation radical polymerization of butyl acrylate mediated by Salen-type CoII complexes

Salen-type Co II complexes were applied in the photo-OMRP of BA and MA to synthetize a copolymer (poly[MA]– co -poly[BA]) under 390 and 400–800 nm LEDs. • Co II complexes with Schiff-base ligands were prepared. • The Co II complexes were applied in the photo-OMRP of butyl acrylate. • The photosystems proposed showed a living character being able to mediate the copolymerization of poly[MA]- b -[BA]. • Experimental and theoretical data suggest a majority reversible termination pathway for these photo-OMRP reactions. In this study, we synthesized the complex [Co(BuSalen)] using the novel tetradentate BuSalen ligand and analyzed its ability as a controlling agent for the photo-induced organometallic-mediated radical polymerization (photo-OMRP) of butyl acrylate (BA) using diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) as photo-initiator. BA polymerization with [Co(BuSalen)] reached significant control levels using the [BA]/[Co II ]/TPO molar ratio of 600/1/1. The [Co(Salen)] and [Co(Bu 2 Salen)] complexes were also obtained following literature methods aiming a comparison in the mediating ability in photo-OMRP. [Co(BuSalen)] and [Co(Bu 2 Salen)] showed better controlling ability, as observed in the copolymerization study (Đ= 2.64, 1.50, and 1.26 for complexes [Co(Salen)], [Co(BuSalen)] and [Co(Bu 2 Salen)], respectively). Kinetics studies and computational investigations support a reversible termination (RT) mechanism and a tailorable Co II complex reactivity mainly altered by Schiff-base steric factors. Calculated thermodynamic parameters agree with the controlling degree delivered by these complexes, since the polymerization control increases with increasing the ΔG values for the formation of dormant species.

Cobalt-Mediated Switchable Catalysis for the One-Pot Synthesis of Cyclic Polymers.

A cobalt salen pentenoate complex [salen=(R,R)-N,N'-bis(3,5-di-tertbutylsalicylidene)-1,2-cyclohexanediamine] is rationally designed as the catalyst for the ring-opening copolymerization (ROCOP) of epoxides/anhydrides/CO2 . Via migratory insertion of carbon monoxide (CO) into the Co-O bonds, the ROCOP-active species α-alkene-ω-O-CoIII (salen) can be rapidly and quantitatively transformed into α-alkene-ω-O2 C-CoIII (salen) telechelic linear precursors. Upon dilution of reaction mixtures, the homolytic cleavage of Co-C bonds induced by visible light generates α-alkene acyl radicals that spontaneously undergo intramolecular radical addition to afford organocobalt-functionalized cyclic polyesters and CO2 -based polycarbonates with excellent regioselectivity. The cyclic products can either react with radical scavengers to generate metal-free cyclic polymers or serve as photo-initiators for organometallic-mediated radical polymerization (OMRP) to produce tadpole-shaped copolymers.

Cobalt‐Mediated Switchable Catalysis for the One‐Pot Synthesis of Cyclic Polymers

AbstractA cobalt salen pentenoate complex [salen=(R,R)‐N,N′‐bis(3,5‐di‐tertbutylsalicylidene)‐1,2‐cyclohexanediamine] is rationally designed as the catalyst for the ring‐opening copolymerization (ROCOP) of epoxides/anhydrides/CO2. Via migratory insertion of carbon monoxide (CO) into the Co−O bonds, the ROCOP‐active species α‐alkene‐ω‐O‐CoIII(salen) can be rapidly and quantitatively transformed into α‐alkene‐ω‐O2C‐CoIII(salen) telechelic linear precursors. Upon dilution of reaction mixtures, the homolytic cleavage of Co−C bonds induced by visible light generates α‐alkene acyl radicals that spontaneously undergo intramolecular radical addition to afford organocobalt‐functionalized cyclic polyesters and CO2‐based polycarbonates with excellent regioselectivity. The cyclic products can either react with radical scavengers to generate metal‐free cyclic polymers or serve as photo‐initiators for organometallic‐mediated radical polymerization (OMRP) to produce tadpole‐shaped copolymers.

Electrochemical Exploration of Active Cu-Based Atom Transfer Radical Polymerization Catalysis through Ligand Modification.

The intersection between Cu-catalyzed atom transfer radical polymerization (ATRP) and organometallic mediated radical polymerization (OMRP) has been recently shown to be a result of competition between the CuI and CuII complexes of polyamine ligands for the same organic free radical. The tetradentate ligands N,N'-bis-2'-pyridylmethyl-ethane-1,2-diamine (L1) and N,N'-dimethyl-N,N'-bis-2'-pyridylmethyl-ethane-1,2-diamine (L2) form stable Cu complexes which, depending on their oxidation state, can either liberate or complex organic radicals. Herein, we show that this process may be affected by subtle changes to the ligand system. Switching from a tertiary amine (L2) to a secondary amine (L1) retains ATRP and OMRP activity through a series of cyclic voltammetry measurements in the presence of the initiator bromoacetonitrile.

Well-Defined Fluorinated Copolymers: Current Status and Future Perspectives

Well-defined fluoropolymers exhibit unique properties such as excellent oil and water repellency, satisfactory thermal stability, a low refractive index, and low surface energy. The origin of these properties is attributed to the presence of a strong electronegative and low polarizable fluorine atom in the backbone of such polymers, which leads to a strong C–F bond (with a high bond dissociation energy of 485 kJ mol–1). Because of these features, these polymers have found applications as functional coatings, thermoplastics, biomedical items, separators, and binders for Li ion batteries, fuel cell membranes, piezoelectric devices, high-quality wires and cables, and so forth. Usually, fluoropolymers are synthesized by the conventional radical (co)polymerization of fluoroalkenes, which leads to the production of (co)polymers with an ill-defined end group, uncontrolled molar mass, and high dispersity values. In the last two decades, significant developments of various reversible deactivation radical polymerization (RDRP) techniques have helped the design of macromolecular architectures (including block, graft, star, and dendrimers) on demand. However, for relevant new applications, well-defined fluoropolymers with controlled macromolecular architectures (e.g., block copolymers as thermoplastic elastomers and electroactive polymers or graft copolymers for fuel cell membranes) are required. Several RDRP methods, developed in the last two decades, have paved the way for the synthesis of (co)polymers with well-defined molar mass, dispersity, chain end-functionality, and macromolecular architectures. Some of these RDRP techniques have been successfully employed for the synthesis of well-defined fluorinated copolymers. These techniques include iodine-transfer polymerization (ITP), reversible addition–fragmentation chain-transfer (RAFT) polymerization, organometallic-mediated radical polymerization (OMRP), and, to a lesser extent, nitroxide-mediated polymerization (NMP). Impressive control of the molar mass parameters of the fluoropolymers synthesized via these techniques also encouraged the researchers to combine these techniques with other postpolymerization strategies, leading to innovative novel polymeric materials. Thus, synthesized well-defined fluoropolymers exhibited a unique combination of properties (such as excellent weather resistance, high thermal/chemical/aging resistance, morphological versatility, and a low dielectric constant/flammability/refractive index). These led to the application of such developed materials in various high-technology applications such as high-performance elastomers, coatings for marine antifouling applications, fluorinated surfactants, fuel cell membranes, and gel polymer electrolytes for Li ion batteries. Newer advances in the field of polymer synthesis techniques are the need of the hour in order to synthesize more advanced fluorinated materials which may change the use of such polymers in engineering and biomedical fields in the current century. However, it should be kept in mind that success in this regard shall heavily depend on a deeper understanding of the polymerization process and structure–activity relationships.

Functional Polyethylenes by Organometallic-Mediated Radical Polymerization of Biobased Carbonates.

Partly or fully renewable (co)polymers are gaining interest in both academia and industry. Polyethylene is a widely used polymer, classically derived from fossil fuels, with a high versatility stemming from the introduction of comonomers altering the mechanical properties. The introduction of renewable functionalities into this polymer is highly attractive to obtain functional, tunable, and at least partially renewable polyethylenes. We herein report the introduction of biosourced cyclic carbonates into polyethylene using organometallic-mediated radical polymerization under mild conditions. Molecular weights of up to 14 600 g mol-1 with dispersities as low as 1.19 were obtained, and the cyclic carbonate content could be easily tuned by the ethylene pressure during the polymerization. As a proof of concept, the hydrolysis of the cyclic carbonates of a representative copolymer was explored, and it provided polyethylene-bearing vicinal diols, with a hydrolysis degree of 71%. Given the multitude of chemoselective modifications possible on cyclic carbonates as well as the fact that many allylic- and alkylidene-type cyclic carbonates are accessible from renewable resources, this work opens up an avenue for the design of functional and more sustainable polyethylenes.

Cobalt complexes of an OSNSO-tetrapodal pentadentate ligand: Synthesis, structures and reactivity

The novel pentadentate tetrapodal proligand 2,6-bis[(2-hydroxyphenyl)sulfanylmethyl]pyridine (1·H2) and its cobalt(II) complex [Co(1)] (2) were synthesized and characterized by several analytical (EA, ESI-MS) and spectroscopic methods (NMR or EPR, FT-IR), including X-ray crystallography for 1·H2. Cyclic voltammetry studies showed that 2 undergoes a reversible metal-based oxidation (CoII/CoIII). Complex 2 was designed to be applied to organometallic mediated radical polymerization (OMRP), however it exhibited an extremely poor solubility in non-coordinating solvents and several vinyl monomers (styrene, vinyl acetate and tert-butyl acrylate), which hampers its potential as moderator. Complex 2 has a high affinity towards Lewis bases, such as pyridine, leading to the clean formation of the mono-pyridine adduct 2·py, as confirmed by X-ray crystallography. In 2·py, ligand 1 is pentacoordinated to the CoII center, with the two thioether-phenolate (S,O) moieties oriented anti to each other, and the only free coordination site of the octahedron is completed by the additional pyridine, trans to the central pyridine linker of 1. The equilibrium between 2 and 2·py could be studied by 1H NMR. Complex 2 could be cleanly and quantitatively oxidized to its diamagnetic iodo cobalt(III) analog [Co(1)I] (3), by simple reaction with iodine. The latter could then be subjected to a halide abstraction reaction, mediated by K[B(C6F5)], affording the cationic complex [Co(1)][B(C6F5)], 4.

Cobalt(II) complexes of α-diimine derived from cycloalkylamines as controlling agents for organometallic mediated radical polymerization of vinyl acetate

A series of cobalt(II)-α-diimine complexes derived from cycloalkylamines (cycloalkyl = cyclopentyl (1a), cyclohexyl (1b), cycloheptyl (1c), and cyclooctyl (1d)) were synthesized: [CoCl2(Pent-DAB)] (2a), [CoCl2(Hex-DAB)] (2b), [CoCl2(Hept-DAB)] (2c), and [CoCl2(Oct-DAB)] (2d). The α-diimines 1a-d were characterized by FTIR, UV–Vis, and 1H- and 13C NMR spectroscopy, while their respective cobalt(II) complexes (2a-d) were characterized by elemental analysis, FTIR, UV–Vis, EPR, and computational studies. The ability of complexes 2a-d to mediate the vinyl acetate (VAc) polymerization initiated by AIBN, according to a cobalt-mediated radical polymerization (CMRP) mechanism, was explored. A mechanistic investigation of radical trapping based on the combination of theoretical and experimental results for CMRP using 2a-d was conducted. This study shows that the activation/deactivation equilibrium exerted by 2a-d occurs by only one of the faces of the complexes via reversible-termination mechanism. The molecular weight increased in direct proportion to the monomer conversion in the early stages of polymerization for complexes 2a and 2b, while uncontrolled polymerization was observed for 2c and 2d. The molecular weights indicate that the steric effect operated by the cycloalkyl substituents plays an important role in the polymerization control, and that a greater steric hindrance disfavors the dormant state for CMRP equilibrium.

Adsorption process of Co(acac)2 catalyst on the surface of mesoporous silica gel particles: an effective method to make a new supported catalyst for the controlled radical polymerization of vinyl acetate

The supported catalysts, specifically those containing cobalt(II) acetylacetonate (Co(acac)2), are among the important materials for the organometallic-mediated radical polymerization of vinyl acetate (VAc) monomers. In this study, we report an applicable and efficient technique to produce new supported Co(acac)2 catalysts through adsorption of Co(acac)2 on the surface of mesoporous silica gel particles. For this purpose, adsorption process of Co(acac)2 on these particles was studied from different perspectives such as mechanism, kinetics and thermodynamic. Adsorption experiments were conducted under various conditions, including various contact times, initial Co(acac)2 concentrations, and temperatures. At the end, a number of adsorption models with the highest compatibility with the experimental data were implemented. The polymerization of VAc was carried out using a new supported catalyst. This catalyst was highly efficient for either controlling the radical polymerization and synthesis of poly vinyl acetate with controlled molecular weight and molecular weight distribution.

Functional Polyethylene (PE) and PE-Based Block Copolymers by Organometallic-Mediated Radical Polymerization

The synthesis of ethylene-based block copolymers (BCPs) by using a single radical polymerization technique is attractive but remains to be a great challenge. The main difficulty relates to the diff...

Organometallic-Mediated Radical (Co)polymerization of γ-Methylene-γ-Butyrolactone: Access to pH-Responsive Poly(vinyl alcohol) Derivatives

Conjugated vinyl lactones commonly serve as precursors of polymers with pendant cyclic esters, which can undergo several chemical modifications. They have notably been copolymerized with “more acti...

Cycloalkyl-substituted salicylaldimine-nickel(II) complexes as mediators in controlled radical polymerization of vinyl acetate

Nickel(II) complexes of Schiff base derived from cycloalkylamines (cycloalkyl = cyclopentyl (cpen), cyclohexyl (chex), and cycloheptyl (chep)) were synthesized: [NiII(Lcpen)2] (1), [NiII(Lchex)2] (2), and [NiII(Lchep)2] (3). The Schiff base-NiII complexes 1-3 were characterized by FTIR, UV–Vis, elemental analysis, and computational methods. Electrodeposited films of complexes 1–3 were obtained by potential cycling CH2Cl2 on platinum electrode, and their electrochemical behavior were characterized by cyclic voltammetry. The polymerization of vinyl acetate (VAc) initiated by azobisisobutyronitrile (AIBN) at 55 °C were conducted according to an organometallic-mediated radical polymerization (OMRP) mechanism. The best conversions were obtained using a ratio of [VAc]/[AIBN]/[Ni] = 542/3.25/1, reaching 50, 69 and 85% in 12 h for 1, 2 and 3, respectively. The kinetic of polymerization mediated by complex 1 exhibited a linear dependence of ln([VAc]0/[VAc]) versus time, supporting a constant radical concentration; while for the complexes 2 and 3, the radical concentration was constant for a short period of time. The increase of molecular weights with the conversion coupled with low polydispersities indicate a certain level of control of the polymerization when using the complexes [NiII(LR)2] as controlling agents.

Straightforward Synthesis of Well-Defined Poly(vinylidene fluoride) and Its Block Copolymers by Cobalt-Mediated Radical Polymerization

The organometallic-mediated radical polymerization (OMRP) of vinylidene fluoride (VDF) using an alkyl cobalt(III) compound as initiator was recently proven successful for the controlled synthesis o...

The Role of a Metallocene Additive in Radically Initiated Polymerization when Solving Direct and Inverse Kinetic Problems

Polymerization of methyl methacrylate, initiated by benzoyl peroxide in the presence of titanocene dichloride, is considered from the point of view of formal kinetics. Based on the kinetic scheme of the process (which includes the reactions of classical radical polymerization, the reaction of benzoyl peroxide with titanocene dichloride, the reactions of the controlled radical polymerization of organometallic mediated radical polymerization (OMRP) and atom transfer radical polymerization (ATRP), the reaction of the formation of a coordinating active site and the coordinating chain propagation on a mathematical model of the kinetics of the process is created. This model also makes it possible to calculate the molecular-mass characteristics of poly(methyl methacrylate). As a result of the solution of the inverse kinetic problem at a temperature of 343 K, the values of the reaction rate constants of the kinetic scheme are found under which the discrepancy between the calculated models and experimental data is minimal. Using the developed model of the kinetics of the process, a numerical experiment is performed (i.e., a direct kinetic problem is solved). This problem revealed the following regularities of the process. (1) An increase in the initial concentration of titanocene dichloride at a constant initial concentration of benzoyl peroxide leads to an increase in the rate of consumption of benzoyl peroxide but not to an increase in the initial rate of the process compared to classical radical polymerization. (2) With an increase in the initial concentration of titanocene dichloride, the lifetime of the macroradicals at the initial stage of the process is reduced, and hence the molecular weight of the resulting polymethyl methacrylate is less than that of the polymethyl methacrylate obtained in the absence of titanocene dichloride, and it will increase during the process of approaching the final values. (3) During the polymerization of methyl methacrylate, initiated by benzoyl peroxide in the presence of titanocene dichloride, a smoothing gel effect (as in the case of the polymerization of methyl methacrylate initiated by benzoyl peroxide in the presence of ferrocene) does not occur since titanocene dichloride forms stable complexes with methyl methacrylate and, consequently, it participates in reactions consuming macroradicals to a lesser degree than ferrocene.

Organometallic‐Mediated Radical Polymerization of Vinylidene Fluoride

AbstractAn unprecedented level of control for the radical polymerization of vinylidene fluoride (VDF), yielding well‐defined PVDF (at least up to 14 500 g mol−1) with low dispersity (≤1.32), was achieved using organometallic‐mediated radical polymerization (OMRP) with an organocobalt compound as initiator. The high chain‐end fidelity was demonstrated by the synthesis of PVDF‐ and PVAc‐containing di‐and triblock copolymers. DFT calculations rationalize the efficient reactivation of both head and tail chain end dormant species.

Organometallic-Mediated Radical Polymerization of Vinylidene Fluoride.

An unprecedented level of control for the radical polymerization of vinylidene fluoride (VDF), yielding well-defined PVDF (at least up to 14 500 g mol-1 ) with low dispersity (≤1.32), was achieved using organometallic-mediated radical polymerization (OMRP) with an organocobalt compound as initiator. The high chain-end fidelity was demonstrated by the synthesis of PVDF- and PVAc-containing di-and triblock copolymers. DFT calculations rationalize the efficient reactivation of both head and tail chain end dormant species.

Ethylene/vinyl acetate-based macrocycles via organometallic-mediated radical polymerization and CuAAC ‘click’ reaction

Sequential organometallic-mediated radical polymerization and copper-catalyzed Huisgen dipolar cycloaddition ring-closure led to unique vinyl acetate and ethylene-containing macrocycles.