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ICAR ATRP of acrylonitrile under ambient and high pressure
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It is well known that well-defined polyacrylonitrile (PAN) with high molecular weight (Mw > 106 g·mol-1) is an excellent precursor for high performance carbon fiber. In this work, a strategy for initiators for a continuous activator regeneration atom transfer radical polymerization (ICAR ATRP) system for acrylonitrile (AN) was firstly established by using CuCl2·2H2O as the catalyst and 2,21-azobis(2-methylpropionitrile) (AIBN) as the thermal initiator in the presence of ppm level catalyst under ambient and high pressure (5 kbar). The effect of catalyst concentration and polymerization temperature on the polymerization behaviors was investigated. It is important that PAN with ultrahigh viscosity and average molecular weight (Mη = 1,034,500 g·mol-1) could be synthesized within 2 h under high pressure.
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... The synthesis of UHMW polymers via RDRP techniques was initially accomplished under high pressure and heterogeneous conditions.  Sumerlin and coworkers later reported the synthesis of UHMW poly (dimethylacrylamide) (PDMA) via a UV-mediated RAFT polymerization using trithiocarbonate (TTC) and xanthate chain-transfer agents. 14 Recently, enzyme-mediated RAFT polymerization has been reported to be successful in synthesizing UHMW homo-and copolymers in aqueous media. ...
This study demonstrates that the gradual and slow production of initiating radicals (i.e., hydroxyl radicals here) is the key point for the synthesis of ultra‐high molecular weight (UHMW) polymers via controlled radical polymerization. Hydrogen peroxide (H2O2) and ferrous iron (Fe2+) react via Fenton redox chemistry to initiate RAFT polymerization. This work presents two enzymatic‐mediated (i.e., Bio‐Fenton‐RAFT and Semi Bio‐Fenton‐RAFT) and one syringe pump‐driven Fenton‐RAFT polymerization processes in which the initiating radicals are carefully and gradually dosed into the reaction solution. The “livingness” of the synthesized UHMW polymers is demonstrated by chain extension and aminolysis experiments. Zimm plots obtained from static light scattering (SLS) technique are used to characterize the UHMW polymers. This Fenton‐RAFT polymerization provides access to polymers of unprecedented UHMW (Mw ~ 20 × 106 g mol−1) with potential in diverse applications. The UHMW polymers made via the controlled Fenton‐RAFT polymerization by using a syringe pump shows that it is possible to produce such materials through an easy‐to‐set up and scalable process. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 This research exhibits the importance of the controlled production of initiating radicals in the synthesis of ultra‐high molecular weight (UHMW) polymers. Herein, an enzyme (i.e., GOx) and a syringe pump are employed to gradually generate initiating radicals (i.e., hydroxyl radicals here) via Semi/Bio‐Fenton‐RAFT and continuous Fenton‐RAFT polymerizations, respectively. Use of syringe pump as a straightforward and robust approach shows significant potential for the synthesis of UHMW polymers by controlling the concentration of initiating radicals.
Supplementary techniques to atom transfer radical polymerization (ATRP) have been studied to reduce the concentration of catalytic complexes used, aiming at a less costly and more environmentally friendly process for synthesizing polymeric materials. Initiators for continuous activator regeneration ATRP (ICAR ATRP) is one technique in which a source of primary free radicals allows the reactivation of catalytic species used in the conventional ATRP. Due to the technological relevance of the topic, this work aims to expand the contribution to literature with kinetic modeling and simulation study for an ICAR ATRP system not yet explored. For this purpose, it was investigated solution ICAR ATRP of methyl acrylate with copper-based catalysts and azobisisobutyronitrile as ICAR initiator. The method of moments was employed to derive material balance equations. We find that the kinetic mechanism that best represented the trends of experimental data from literature was the one that considered specific side reactions of acrylates, such as backbiting and radical catalytic termination. In the model fitting procedure, a nonlinear optimization process had to be developed to estimate the values of kinetic rate constants not yet reported in the literature. The values obtained for the missing kinetic rate constants are consistent with similar experimental systems found in the literature. The deterministic model obtained was then employed to study the concentration profile maps. Simulations allowed verifying the slow release of primary free radicals from ICAR initiator to reduce the deactivators in activators species continuously. In addition, it was possible to confirm that the ICAR ATRP can be thermoregulated. Parametric analyses showed that variations in kinetic rate constants and reaction stoichiometry can shape monomer conversion and the molecular properties of the polymer (e.g., molecular weight, dispersity, and end functionality).
Polymerization induced surface self-assembly (PISSA) technology is a state-of-the-art method in the fabrication of the hierarchical surface nanostructures. Previously, we demonstrated that reversible addition−fragmentation chain transfer (RAFT) polymerization-based PISSA can be used to construct hierarchical surface nanostructures on the silica particles. Atom transfer radical polymerization (ATRP) has many advantages, especially in the synthesis of the macroinitiator. In this research initiators for continuous activator regeneration (ICAR) ATRP-induced PISSA approach is used to fabricate surface nanostructures. Poly(ethylene glycol) (PEG) macroinitiator and poly(oligo(ethylene glycol) monomethyl ether methacrylate-co-2-(2-bromoisobutyrnyloxy) ethyl methacrylate) (P(OEGMA-co-BIEM)) polymer brush macroinitiator on silica particles were used to initiate ATRP of styrene in methanol/water mixture. With the chain extensions, POEGMA-g-PS graft copolymer brushes and “free” PEG-b-PS block copolymer chains make surface coassembly into surface nanostructures. With an increase in monomer feeding ratio, the surface morphology gradually changes from small-sized spherical surface micelles (s-micelles), to big-sized s-micelles, and to asymmetric layered structures. Kinetics studies indicate that with an increase in monomer conversion there is a transition from homogeneous to heterogeneous polymerization.
The novel approach for conducting ARGET ATRP based on simultaneous use of two copper complexes based on tris[2-(dimethylamino)ethyl]amine (Me6TREN), 2,2’-bipyridine (bipy) and tris(2-pyridylmethyl)amine (TPMA) in one pot is proposed. This approach allows to increase the rate of polymerization of acrylonitrile and to achieve polymers with high molecular weights. The performed electrochemical studies allowed establishing the possible mechanism of tandem catalysis where the more reducing complex mostly acts as activator determining the high polymerization rate while the second one reversible deactivates polymer chain preserving the control over the process. The influence of initiator nature and the ratio between copper catalysts on the polymerization rate and the molecular weight parameters of the samples was studied. It was shown that the proposed system may be applied for obtaining well-defined copolymers of acrylonitrile with methyl acrylate and dimethyl itaconate via ARGET ATRP mechanism using low copper concentrations.
This study realized a graphitic structure from the low-temperature carbonization of poly(acrylonitrile) (PAN) grafted onto a nanoparticle’s surface at a density of 0.3 chains nm⁻². The temperature was increased up to 800°C, which is 2,000°C lower than that in conventional graphitization processes. Two types of PAN-grafted nanoparticles with different core diameters of 30 and 150 nm were prepared and cast from a solution into films. The films comprised cores arranged in a face-centered cubic (FCC) lattice, which was maintained after the carbonization, albeit with shortened lattice sides. Elution of the silica cores by hydrofluoric acid etching produced carbonized PAN films with FCC-arranged pores. The carbonized PAN exhibited X-ray diffraction (XRD) peaks associated with the graphitic structure. Conversely, the same carbonization of similar PAN-grafted nanoparticles with a lower grafting density of 0.05 chains nm⁻² produced similar porous carbon films, but the graphitic structure was not evidenced by the associated XRD patterns. These results confirm that lower-temperature graphitization requires dense grafting of PAN chains onto a solid surface.
Biological macromolecules such as nucleic acids and proteins are homochiral, display monodisperse chain length with mega, multimillion, molecular weight and encode functions by their precise sequence via self-organization. The functions of biological macromolecules are either independent on molecular weight or, like in the case of ribosome, are programmed by the self-organization of their mega molecular weight. In spite of the fact that Staudinger coined the name macromolecule 100 years ago, self-organizable synthetic covalent and supramolecular mega macromolecules exhibiting the symptoms of biological macromolecules started to emerge only recently. This personal perspective will discuss, with examples mostly from our laboratory, methodologies for the synthesis of self-organizable covalent and supramolecular mega macromolecules for which functions are encoded, programmed and perfected via homochiral, sequence-defined and monodisperse components. Methodologies to generate them together with historical developments will be briefly discussed.
The flocculation efficiency of polyelectrolytes in high ionic strength environment is often affected and reduced due to shielding of the active ionizable functional groups, as well as changes in the surface chemistry of the solid slurry. To address this problem, a series of well-defined novel ABA triblock copolymers were employed for the flocculation of high ionic strength kaolin slurries at three different Ca2+ concentrations (0.05 M, 0.10 M, and 0.50 M). The primary focus was placed on the advancement in architecture, where the anionic functionalities were localized to the terminal ends. Typical commercial flocculants tend to have anionic functionalities randomly distributed throughout the polymer chain and hence a higher propensity towards condensed conformation and formation of insoluble species. In comparison to a control random copolymer, the ABA triblock copolymers were able to flocculate the kaolin suspension at a faster rate, particularly at the high Ca2+ concentrations of 0.10 M and 0.50 M. In addition, these polymers had significantly better clarification ability compared to control random copolymer, despite all increments in the Ca2+ concentration. ABA triblock copolymer architecture may therefore have potential as flocculants in high ionic strength applications.
Block copolymer nano-assemblies were synthesized via initiators for continuous activator regeneration atom transfer radical polymerization (ICAR ATRP) dispersion polymerization employing the CuBr2/tris(2-pyridylmethyl)amine catalyst in an alcoholic solvent at a relatively low temperature of 45 °C. The typical poly(ethylene glycol)-block-polystyrene (PEG-b-PS) nano-assemblies synthesized via ICAR ATRP dispersion polymerization are compared with those synthesized via RAFT dispersion polymerization under other similar conditions. It is found that the PEG45-b-PS nano-assemblies, e.g., including lamellas, vesicles and vesicular clusters, synthesized via ICAR ATRP dispersion polymerization have much complicated morphologies, which are different from those synthesized via RAFT dispersion polymerization. The reasons leading to the difference are searched, and it is conclusively supposed that the high Ð of the synthesized BCPs and the salt of the copper catalyst used in ICAR ATRP dispersion polymerization should be involved. This low temperature initiated ICAR ATRP dispersion polymerization is believed to be a valid method to prepare block copolymer nano-assemblies through polymerization-induced self-assembly.
Based on kinetic Monte Carlo simulations of the monomer sequences of a representative number of copolymer chains (approximate to 150,000), optimal synthesis procedures for linear gradient copolymers are proposed, using bulk Initiators for Continuous Activator Regeneration Atom Transfer Radical Polymerization (ICAR ATRP). Methyl methacrylate and n-butyl acrylate are considered as comonomers with CuBr2/PMDETA (N,N,N',N '',N ''-pentamethyldiethylenetriamine) as deactivator at 80 degrees C. The linear gradient quality is determined in silico using the recently introduced gradient deviation (< GD >) polymer property. Careful selection or fed-batch addition of the conventional radical initiator I-2 allows a reduction of the polymerization time with ca. a factor 2 compared to the corresponding batch case, while preserving control over polymer properties (< GD > approximate to 0.30; dispersity approximate to 1.1). Fed-batch addition of not only I-2, but also comonomer and deactivator (50 ppm) under starved conditions yields a < GD > below 0.25 and, hence, an excellent linear gradient quality for the dormant polymer molecules, albeit at the expense of an increase of the overall polymerization time. The excellent control is confirmed by the visualization of the monomer sequences of ca. 1000 copolymer chains.
The development of an atom transfer radical polymerization (ATRP) system without any transition metal catalyst for electronic and biomedical applications was considered to be in pressing need. Fluorescein (FL) was used as the organic photocatalyst for the polymerization of methyl methacrylate (MMA) via the proposed photoinduced electron transfer–atom transfer radical polymerization (PET–ATRP) mechanism. In the presence of electron donors provided by triethylamine (TEA), fluorescein can activate alkyl bromide and control radical polymerizations by a reductive quenching pathway. The polymerizations could be controlled by an efficient activation and deactivation equilibrium while maintaining the attractive features of “living” radical polymerization. The number-average molecular weight Mn,GPC increased with monomer conversion, and the controllability of molecular weight distributions for the obtained PMMA could be achieved in the polymerization processes. MALDI-TOF MS, 1H NMR spectroscopy and chain extension polymerizations show reserved chain-end functionality in the synthesized polymers and further confirm the “living” feature of the metal-free ATRP methodology. All these research results support the feasibility of the visible light mediated metal-free PET–ATRP platform for the synthesis of elegant macromolecular structures.
The thermal degradable poly(alkoxyamine) was synthesized through a novel nitroxide radical coupling step growth polymerization (NRC-SGP) mechanism. The monomers of 1,4-phenylene bis(2-bromo-2-methylpropanoate) (monomer 1) and 1,4-phenylene bis(2-bromopropanoate) (monomer 1′) with two bromide groups and 1,6-di(4-(2,2,6,6-tetramethylpiperidine-1-oxyl))-hexa-2,4-diyne (monomer 2) with two nitroxide radicals were first designed and synthesized. Then the NRC-SGP mechanism was investigated in detail by optimizing the factors such as polymerization time, temperature, solvents, catalysts, ligand, monomer concentration, and structures connected to halogen groups. The results showed that the termination by disproportionation was the major side reaction in the NRC-SGP mechanism, and the lower temperature (25 °C) would favor an important contribution. The proper combination of all factors could lead to an ideal NRC-SGP procedure. Finally, the thermal stability of formed poly(alkoxyamine) was monitored by TG, DSC and SEC instruments, and the results showed that the poly(alkoxyamine) would suffer a severe thermal degradation at the elevated temperature above 140 °C.
Atom transfer radical polymerization (ATRP) is a versatile and robust tool to synthesize a wide spectrum of monomers with various designable structures. However, it usually needs large amounts of transition metal as the catalyst to mediate the equilibrium between the dormant and propagating species. Unfortunately, the catalyst residue may contaminate or color the resultant polymers, which limits its application, especially in biomedical and electronic materials. How to efficiently and economically remove or reduce the catalyst residue from its products is a challenging and encouraging task. Herein, recent advances in catalyst separation and recycling are highlighted with a focus on (1) highly active ppm level transition metal or metal free catalyzed ATRP; (2) post-purification method; (3) various soluble, insoluble, immobilized/soluble, and reversible supported catalyst systems; and (4) liquid-liquid biphasic catalyzed systems, especially thermo-regulated catalysis systems. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
The generation of carbon-centered radicals from alkyl bromides through an oxidative quenching pathway using perylene as an organic visible-light photocatalyst is described. This methodology is used to initiate the radical polymerization of methyl methacrylate and other functionalized vinyl monomers. The polymers possess bromide chain-end groups that can be used to reinitiate polymerization to produce block copolymers. Control over the polymerization propagation can be achieved through pulsed light sequences while the ability to use natural sunlight to promote carbon–carbon bond formation produces polymers with dispersity as low as 1.29.
Carbon fibers have been processed from gel spun polyacrylonitrile copolymer on a continuous carbonization line at Georgia Tech (GT) with a tensile strength in the range of 5.5–5.8 GPa, and tensile modulus in the range of 354–375 GPa. This combination of strength and modulus is the highest for any continuous fiber reported to date, and the gel spinning route provides a pathway for further improvements in strength and modulus for mass production of carbon fibers. At short gauge length, fiber tensile strength was as high as 12.1 GPa, which is the highest value ever reported for a PAN based carbon fiber. Structure analysis shows random flaws of about 2 nm size, which results in limiting tensile strength of higher than 20 GPa. Inter-planar turbostratic graphite shear modulus in high strength carbon fibers is 30 GPa, while in graphite the corresponding value is only 4 GPa.
Photoinduced metal-free atom transfer radical polymerization has been successfully extended to the synthesis of polyacrylonitrile (PAN) with predictable molecular weights and low dispersities. This was achieved using phenothiazine derivatives as photoredox catalysts, which activate dormant alkyl bromides to reversibly form propagating radicals. Both H-1 NMR spectroscopy and chain-end extension polymerization show highly preserved Br chain-end functionality in the synthesized PAN.
In this work, high molecular weight polyvinyl acetate (PVAc) (Mn,GPC = 123,000 g/mol, Mw/Mn = 1.28) was synthesized by reversible addition-fragmentation chain transfer polymerization (RAFT) under high pressure (5 kbar), using benzoyl peroxide and N,N-dimethylaniline as initiator mediated by (S)-2-(ethyl propionate)-(O-ethyl xanthate) (X1) at 35 °C. Polymerization kinetic study with RAFT agent showed pseudo-first order kinetics. Additionally, the polymerization rate of VAc under high pressure increased greatly than that under atmospheric pressure. The “living” feature of the resultant PVAc was confirmed by 1H NMR spectroscopy and chain extension experiments. Well-defined PVAc with high molecular weight and narrow molecular weight distribution can be obtained relatively fast by using RAFT polymerization at 5 kbar. © 2015 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2015
We report the first ever use of electrochemically mediated atom transfer radical polymerization (eATRP) employing a bipolar electrochemical method for the fabrication of both gradient and patterned polymer brushes. A potential gradient generated on a bipolar electrode allowed the formation of a concentration gradient of a Cu(I) polymerization catalyst through the one-electron reduction of Cu(II) , resulting in the gradient growth of poly(NIPAM) brushes from an initiator-modified substrate surface set close to a bipolar electrode. These polymer brushes could be fabricated in three-dimensional gradient shapes with control over thickness, steepness, and modified area by varying the electrolytic conditions. Moreover, by site-selective application of potential during bipolar electrolysis, a polymer brush with a circular pattern was successfully formed. Polymerization was achieved using both a polar monomer (NIPAM) and a nonpolar monomer (MMA) with the eATRP system. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.