Mechanistic Transformation Research Articles

Functionalized Polyisobutylene and Polyisobutylene‐Based Block Copolymers by Mechanistic Transformation from Cationic to Radical Process

AbstractThe strategy for the preparation of polyisobutylene‐based block copolymers via mechanistic transformation from cationic to radical polymerization is reported. This strategy involves the synthesis of 2‐bromo‐2‐methylpropanoyl‐terminated difunctional polyisobutylene macroinitiator (BiBB‐PIB‐BiBB) via consecutive cationic polymerization, in situ preparation of hydroxyl‐terminated polyisobutylene and its acylation by 2‐bromo‐2‐methylpropanoyl bromide. The Mn2(CO)10−triggered photo‐induced radical polymerization of styrene in bulk using this macroinitiator leads to the formation of multiblock copolymer, while predominantly triblock copolymer is generated during the polymerization of methyl methacrylate. The possibility to functionalize the polyisobutylene by pyrene via photo‐induced radical addition of 1‐bromomethyl pyrene in the presence of Mn2(CO)10 is also demonstrated in this work.

Regulating Co-O covalency to manipulate mechanistic transformation for enhancing activity/durability in acidic water oxidation.

Developing earth-abundant electrocatalysts with high activity and durability for acidic oxygen evolution reaction is essential for H2 production, yet it remains greatly challenging. Here, guided by theoretical calculations, the challenge of overcoming the balance between catalytic activity and dynamic durability for acidic OER in Co3O4 was effectively addressed via the preferential substitution of Ru for the Co2+ (Td) site of Co3O4. In situ characterization and DFT calculations show that the enhanced Co-O covalency after the introduction of Ru SAs facilitates the generation of OH* species and mitigates the unstable structure transformation via direct O-O coupling. The designed Ru SAs-CoO x catalyst (5.16 wt% Ru) exhibits enhanced OER activity (188 mV overpotential at 10 mA cm-2) and durability, outperforming most reported Co3O4-based and Ru-based electrocatalysts in acidic media.

Cationic β‐Scission of C−H and C−C Bonds for Selective Dimerization and Subsequent Sulfur‐Free RAFT Polymerization of α‐Methylstyrene and Isobutylene

AbstractA series of exo‐olefin compounds ((CH3)2C(PhY)−CH2C(=CH2)PhY) were prepared by selective cationic dimerization of α‐methylstyrene (αMS) derivatives (CH2=C(CH3)PhY) with p‐toluenesulfonic acid (TsOH) via β‐C−H scission. They were subsequently used as reversible chain transfer agents for sulfur‐free cationic RAFT polymerization of αMS via β‐C−C scission in the presence of Lewis acid catalysts such as SnCl4. In particular, exo‐olefin compounds with electron‐donating substituents, such as a 4‐MeO group (Y) on the aromatic ring, worked as efficient cationic RAFT agents for αMS to produce poly(αMS) with controlled molecular weights and exo‐olefin terminals. Other exo‐olefin compounds (R−CH2C(=CH2)(4‐MeOPh)) with various R groups were prepared by different methods to examine the effects of R groups on the cationic RAFT polymerization. A sulfur‐free cationic RAFT polymerization also proceeded for isobutylene (IB) with the exo‐olefin αMS dimer ((CH3)2C(Ph)−CH2C(=CH2)Ph). Furthermore, telechelic poly(IB) with exo‐olefins at both terminals was obtained with a bifunctional RAFT agent containing two exo‐olefins. Finally, block copolymers of αMS and methyl methacrylate (MMA) were prepared via mechanistic transformation from cationic to radical RAFT polymerization using exo‐olefin terminals containing 4‐MeOPh groups as common sulfur‐free RAFT groups for both cationic and radical polymerizations.

Cationic β-Scission of C-H and C--C Bonds for Selective Dimerization and Subsequent Sulfur-Free RAFT Polymerization of α-Methylstyrene and Isobutylene.

A series of exo-olefin compounds ((CH3)2C(PhY)-CH2C(=CH2)PhY) were prepared by selective cationic dimerization of α-methylstyrene (αMS) derivatives (CH2=C(CH3)PhY) with p-toluenesulfonic acid (TsOH) via β-C-H scision. They were subsequently used as reversible chain transfer agents for sulfur-free cationic RAFT polymerization of αMS via β-C-C scission in the presence of Lewis acid catalysts such as SnCl4. In particular, exo-olefin compounds with electron-donating substituents, such as a 4-MeO group (Y) on the aromatic ring, worked as efficient cationic RAFT agents for αMS to produce poly(αMS) with controlled molecular weights and exo-olefin terminals. Other exo-olefin compounds (R-CH2C(=CH2)(4-MeOPh)) with various R groups were prepared by different methods to examine the effects of R groups on the cationic RAFT polymerization. A sulfur-free cationic RAFT polymerization also proceeded for isobutylene (IB) with the exo-olefin αMS dimer ((CH3)2C(Ph)-CH2C(=CH2)Ph). Furthermore, telechelic poly(IB) with exo-olefins at both terminals was obtained with a bifunctional RAFT agent containing two exo-olefins. Finally, block copolymers of αMS and MMA were prepared via mechanistic transformation from cationic to radical RAFT polymerization using exo-olefin terminals containing 4-MeOPh groups as common sulfur-free RAFT groups for both cationic and radical polymerizations.

Effect of Pyrolysis Temperature on Mechanistic Transformation for Adsorption of Methylene Blue on Leached Rice‐Straw Biochar

AbstractToxicity characteristic leaching procedure (TCLP) leached biochar is studied for adsorption of methylene blue (MB). Rice straw biochar obtained from slow pyrolysis at 400, 500, and 600 °C, respectively, is TCLP leached to furnish leached biochar, BL4, BL5, and BL6. The leached biochar BL4–6 have been characterized for pH, CHN analysis, ash, zeta potential, surface area morphology and functional groups. Batch adsorption studies are optimized for pH (3–9), adsorbent dose (0.5–4 g L−1), and initial MB concentration (20–135 mg L−1). Nonlinear fitting to Langmuir, Freundlich, and Redlich‐Peterson adsorption isotherm with due statistical treatment and error function analysis is carried out. Leached biochar, BL4, BL5, and BL6, is characterized by the dominance of carboxylic acid, lactone, and phenols moieties, respectively. The MB adsorption on leached biochar exhibits maximum adsorption of 26.87, 51.34, and 18.83 mg g−1 for BL4, BL5, and BL6, respectively. The underlying mechanism for adsorption of MB using BL5, is characterized by non‐ionic lactone ring opening in presence of MB under alkaline conditions that is supported by X‐ray photoelectron spectroscopy(XPS) and Fourier transformed infrared spectroscopy (FTIR) studies. Desorption of MB and regeneration from BL5 is studied with methanol and 0.1 m HCl as stripping solvent for four cycles. Recovery of MB is better with methanol in comparison to 0.1 m HCl.

Synthesis of Block Copolymers by Mechanistic Transformation from Reversible Complexation Mediated Living Radical Polymerization to the Photoinduced Radical Oxidation/Addition/Deactivation Process.

A versatile strategy for the fabrication of block copolymers by the combination of two discrete living polymerization techniques—reversible complexation mediated living radical polymerization (RCMP) and photoinduced radical oxidation addition deactivation (PROAD) processes—is reported. First, RCMP is conducted to yield poly(methyl methacrylate) with iodide end groups (PMMA-I). In the following step, PMMA-I is used as macroinitiator for living PROAD cationic polymerization of isobutyl vinyl ether. Successful formation of the block copolymers is confirmed by 1H NMR, FT-IR, GPC, and DSC investigations.

Effect of local pH change on non-PGM catalysts – a potential-dependent mechanistic analysis of the oxygen reduction reaction

The potential-dependent mechanistic transformation of the oxygen reduction reaction originates from the local pH change.

Stereospecific cationic RAFT polymerization of bulky vinyl ethers and stereoblock poly(vinyl alcohol) via mechanistic transformation to radical RAFT polymerization of vinyl acetate

The simultaneous control of molecular weight and tacticity in cationic polymerization remains challenging. Here, we propose a new approach for stereospecific living cationic polymerization by combining cationic reversible addition-fragmentation chain transfer (RAFT) polymerization with thiocarbonylthio compounds and stereospecific cationic polymerization of bulky vinyl ethers with Lewis acid catalysts. In this combined system, the molecular weight was controlled by the RAFT process between the growing cationic and dormant dithiocarbamate chain ends, whereas the tacticity was controlled by stereospecific propagation based on the steric hindrance of bulky side groups and counteranions derived from the Lewis acid catalysts. Herein, we demonstrate the availability of this approach in the cationic polymerization of various bulky vinyl ethers, such as tert‑butyl, benzyl, trimethylsilyl, and tert‑butyldimethylsilyl vinyl ether, using dithiocarbamates and BF3•OEt2 or EtAlCl2 in toluene at –78 °C. Isotactic-rich poly(vinyl ether)s with controlled molecular weights and dithiocarbamate chain-end groups were obtained from a series of bulky vinyl ethers and were converted into molecular-weight-controlled isotactic poly(vinyl alcohol) (mm ≥ 70%) with a high melting temperature (Tm ~ 230 °C). Furthermore, isotactic-b-atactic stereoblock PVA (mm/mr/rr = 75/25/5 and 23/48/29, Tm ~ 210 °C) was synthesized by mechanistic transformation from the stereospecific cationic RAFT polymerization of trimethylsilyl vinyl ether into the radical RAFT polymerization of vinyl acetate using diphenyl dithiocarbamate as the common RAFT agent followed by simultaneous deprotection of the silyl and acetyl groups.

Mechanistic Transformations Involving Radical and Cationic Polymerizations

Mechanistic transformation approach has been widely applied in polymer synthesis due to its unique feature combining structurally different polymers prepared by different polymerization mechanisms. Reported methods for the formation of block and graft copolymers through mechanistic transformation involve almost all polymerizations modes. However, certain polymerization processes require extensive purification processes, which can be time-consuming and problematic. Recent developments on controlled/living polymerizations involving radical and cationic mechanisms with the ability to control molecular weight and functionality led to new pathways for mechanistic transformations. In this mini-review, we systematically discussed relevant advances in the field through three main titles namely (i) from radical to cationic mechanism, (ii) from cationic to radical mechanism, and (iii) application of specific catalyst systems for both radical and cationic polymerizations.

Transition State Structure Modeling and Mechanistic Study of Purine Derivatives

Xanthine oxidase is one of the members of molybdo-enzymes known to catalyze the oxidative hydroxylation of various purine derivatives. The mechanistic transformation of the tetrahedral Michaelis-Menten type complex ((EOX)-[Mo(VI)-Oeq-CCRH]) to the product bound intermediate ((EOX)-[Mo(IV)-Oeq-CCR]) is expected to pass through the tetrahedral transition state complex ((EOX)-[Mo(VI)(-Oeq-CCRH---HRH---)(=SMo)]#). Although several accounts are made regarding the events taking place at the transition state, they have never been fully explored nor quite understood. Similarly, the description of the formation of Oeq-CCRH bond, cleavage of CCRH-HRH bond, migration of HRH to SMo terminal, and reduction of (EOX)-Mo(VI) to (ERED)-Mo(IV) was not well understood. In order to understand the events taking place during catalysis, new computational models have been used to perform an electronic structure calculation. The calculations were performed on the active site model compound bound to purine derivatives, mainly 2-hydroxy-6-methyl purine (HMP) that has never been investigated computationally. The transition state modeling also reveals a reactant like transition state for the reaction of HMP with the active site. Understanding the transition state geometry is a prime importance in developing a plausible reaction mechanism. From the four alternative paths, path one has small energy barrier compare to the three alternative paths. Understanding the mechanism may be used in the drug design process. From our mechanistic study HMP favor concerted mechanism rather than step wise mechanism.

Cationic Polymerization via Activation of Alkoxyamines Using Photoredox Catalysts

AbstractCationic polymerization via mesolytic cleavage of alkoxyamines was investigated using photoredox catalysts under irradiation with visible light of various wavelengths. The carbon–oxygen bond of the alkoxyamine derived from p‐methoxystyrene (pMOS) and (2,2,6,6‐tetramethylpiperidin‐1‐yl)oxyl (TEMPO) was cleaved by an excited cationic iridium catalyst under blue LED irradiation to generate the pMOS cation, which efficiently initiated the cationic polymerization of vinyl ethers and styrene derivatives to give the polymers with a pMOS unit at the initiating end. Temporal control of the polymerization was achieved by switching the light on and off. Poly(pMOS) with a terminal TEMPO unit obtained by nitroxide‐mediated radical polymerization was subsequently employed as a macroinitiator for the photoredox‐catalyzed cationic polymerization to afford block copolymers via a mechanistic transformation from radical to cationic polymerization using alkoxyamine as the mutual initiating site.

Controlled Synthesis of Block Copolymers by Mechanistic Transformation from Atom Transfer Radical Polymerization to Iniferter Process

A straightforward transformation protocol combining two distinct living polymerization methods for the controlled synthesis of block copolymers is described. In the first step, bromo-terminated poly(methyl methacrylate) is prepared by atom transfer radical polymerization (ATRP). Then, a bromide end group is substituted with a triphenylmethyl (trityl) functionality under visible light irradiation using dimanganese decacarbonyl (Mn2 (CO)10 ) photochemistry. The resulting polymers with trityl end groups are used as macroiniferter for the polymerization of styrene and tert-butyl acrylate (tBA) to yield desired block copolymers with narrow molecular weight distribution. Moreover, the amphiphilic copolymers with acrylic acid functionalities are obtained by the hydrolyzation of poly(tert-butyl acrylate) containing block copolymers with trifluoroacetic acid.

Direct through anionic, cationic, and radical active species: Terminal carbon–halogen bond for “controlled”/living polymerizations of styrene

ABSTRACTIn this work, we examined the synthesis of novel block (co)polymers by mechanistic transformation through anionic, cationic, and radical living polymerizations using terminal carbon–halogen bond as the dormant species. First, the direct halogenation of growing species in the living anionic polymerization of styrene was examined with CCl4 to form a carbon–halogen terminal, which can be employed as the dormant species for either living cationic or radical polymerization. The mechanistic transformation was then performed from living anionic polymerization into living cationic or radical polymerization using the obtained polymers as the macroinitiator with the SnCl4/n‐Bu4NCl or RuCp*Cl(PPh3)/Et3N initiating system, respectively. Finally, the combination of all the polymerizations allowed the synthesis block copolymers including unprecedented gradient block copolymers composed of styrene and p‐methylstyrene. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 465–473

Polymer Synthesis with More Than One Form of Living Polymerization Method.

Controlled radical polymerization (CRP) or controlled/living radical polymerization has revolutionized the polymer industry as a tool for the preparation of a wide variety of polymers. This process enables the preparation of polymers with good control of molecular weight, narrow polydispersity, and a range of architectures including block and graft copolymers, star polymers, and other functional polymers. The mechanistic transformation reaction provides a great opportunity to tune chemical and physical properties of copolymers. It can be applied to combine different homopolymers using post-modification techniques or by the use of a dual initiator, allowing the combination of mechanistically distinct polymerization reactions. This review will cover CRP transformations including the synthesis of block copolymers with both linear structures (AB, ABA, (AB) n , multiblocks, etc.) and branched macromolecular architectures (graft, miktoarm star, and dendritic-like), which are obtained by a combination of more than one form of living polymerization reaction.

Block Copolymers by Mechanistic Transformation from PROAD to Iniferter Process.

A facile strategy for synthesizing block copolymers by the combination of two different living polymerization techniques, namely, photoinduced radical oxidation/addition/deactivation (PROAD) and iniferter processes is described. In the first step, PROAD polymerization of isobutyl vinyl ether using bromotriphenylmethane, dimanganese decacarbonyl (Mn2 (CO)10 ), and diphenyliodonium bromide (Ph2 I+ Br- ) is carried out to yield polymers with triphenylmethyl (trityl) end groups. These prepolymers are used as macroiniferters in thermally induced free radical polymerization of vinyl monomers such as methyl methacrylate, tert-butyl acrylate, and styrene, resulting in the formation of corresponding block copolymers free from homopolymers. The precursor polymer and final block copolymers are characterized by 1 H NMR, FT-IR, GPC, and DSC analyses.

Synthesis of block copolymers by mechanistic transformation from photoinitiated cationic polymerization to a RAFT process

A novel synthetic strategy for the synthesis of block copolymers based on mechanistic transformation from photoinitiated cationic polymerization to radical addition fragmentation transfer polymerization is presented.

Diverse approaches to star polymers via cationic and radical RAFT cross-linking reactions using mechanistic transformation

Core cross-linked star polymers were synthesizedviacationic RAFT polymerization and three different approaches in combination with a radical RAFT mechanism.

Ultrasound assisted multicomponent reactions: A green method for the synthesis of N-substituted 1,8-dioxo-decahydroacridines using β-cyclodextrin as a supramolecular reusable catalyst in water

We demonstrate a superficial method for the synthesis of N-substituted 1,8-dioxo-decahydroacridines using β-cyclodextrin as a supramolecular, biodegradable, and reusable catalyst in aqueous medium. The reaction product is in excellent yield with moderate to excellent selectivity. The mechanistic transformation presumably proceeds via a one-pot, multicomponent cyclization of dimedone in the presence of aromatic aldehydes and aromatic amines/INH, undergoing a tandem Michael addition reaction. The proposed approach in this study provides a highly efficient and environmentally benign route to N-substituted 1,8-dioxo-decahydroacridines.

Distinct metal-exchange pathways of doped Ag25 nanoclusters.

Atomically precise metal nanoclusters (NCs) containing more than one type of metal atom (i.e., doped or alloyed), due to synergistic effects, open new avenues for engineering the catalytic and optical properties of NCs in a manner that homometal NCs cannot. Unfortunately, it is still a major challenge to controllably introduce multimetallic dopants in NCs, understanding the dopants' positions, mechanism, and synergistic effects. To overcome these challenges, we designed a metal-exchange approach involving NCs as molecular templates and metal ions as the source of the incoming dopant. In particular, two structurally similar monodoped silver-rich NCs, [MAg24(SR)18]2- (M = Pd/Pt and SR: thiolate), were synthesized as templates to study their mechanistic transformation in response to the introduction of gold atoms. The controllable incorporation of Au atoms into the MAg24 framework facilitated the elucidation of distinct doping pathways through high-resolution mass spectrometry, optical spectroscopy and elemental analysis. Interestingly, gold replaced the central Pd atom of [PdAg24(SR)18]2- clusters to produce predominantly bimetallic [AuAg24(SR)18]- clusters along with a minor product of an [Au2Ag23(SR)18]- cluster. In contrast, the central Pt atom remained intact in [PtAg24(SR)18]2- clusters, and gold replaced the non-central Ag atoms to form trimetallic [AuxPtAg24-x(SR)18]2- NCs, where x = 1-2, with a portion of the starting [PtAg24(SR)18]2- NCs remaining. This study reveals some of the unusual metal-exchange pathways of doped NCs and the important role played by the initial metal dopant in directing the position of a second dopant in the final product.

Polyethylene-g-poly(cyclohexene oxide) by Mechanistic Transformation from ROMP to Visible Light-Induced Free Radical Promoted Cationic Polymerization

A novel polymerization mechanism transformation strategy, combining ring-opening metathesis polymerization (ROMP) and visible light induced cationic polymerization, is successfully applied for the synthesis of polyethylene-graft-poly(cyclohexene oxide) (PE-g-PCHO). First, cis-cyclooctene (COE) was polymerized via ROMP in the presence of a chain transfer agent and quantitatively hydrobrominated to give bromo functional polyethylene (PE-Br). Subsequent irradiation of PE-Br in the visible range using dimanganese decacarbonyl (Mn2(CO)10) and diphenyl iodonium hexafluorophosphate (Ph2I+PF6–) as radical generator and oxidant, respectively, initiated cationic polymerization of cyclohexene oxide (CHO) resulting in the formation of PE-g-PCHO. The effect of irradiation time and Mn2(CO)10 concentration on the grafting density and efficiency was evaluated. Both the precursor polymers and the corresponding graft copolymers were characterized by 1H NMR and Fourier transform infrared (FT-IR) spectroscopy, gel-permeation chromatography (GPC), and differential scanning calorimetric (DSC) analyses.