Nitroxide-mediated Polymerization Research Articles

Nitroxide mediated controlled radical polymerization of α‐Methylene‐γ‐Butyrolactone

AbstractThe controlled polymerization of α‐methylene‐γ‐butyrolactone (MBL, a naturally occurring sesquiterpene lactone found in tulips) was performed by nitroxide mediated polymerization (NMP) using the alkoxyamine Dispolreg 007 as the initiating system at 80 °C in 50 wt% DMSO. Features of a reversible‐deactivation radical polymerization (RDRP) were observed with a linear growth of the macro‐radical with conversion and a relatively low dispersity (Ð ≤ 1.45). The P(MBL) homopolymer chain ends were mostly re‐activatable as the chain extensions with styrene (S) and glycidyl methacrylate (GMA) in DMSO were successful. Interestingly, the polymerization of MBL initiated by 2‐methyl‐2‐[N‐tert‐butyl‐N‐(1‐diethoxyphosphoryl‐2,2‐dimethylpropyl)‐aminoxy]‐N‐propionyloxysuccinimide (known as NHS‐BlocBuilder) under various experimental conditions did not produce well‐defined P(MBL), possibly due to an excessively high equilibrium constant. However, the bulk copolymerization of MBL with 10–21 mol% of β‐myrcene (Myr, another renewable monomer from the terpene family) initiated by NHS‐BlocBuilder at 100–110 °C led to all the features of a controlled system. The SG1 chain end fidelity of the resulting P(MBL‐co‐Myr) copolymer was confirmed via 31P NMR and by re‐initiating successfully the copolymer chains with a mixture of GMA and Myr monomers at 120 °C in DMSO.

Thermadapt shape memory vitrimeric polymyrcene elastomer

Earlier studies of β-Myrcene (Myr)/ β-ketoester functional (acetoacetoxy)ethyl methacrylate (AAEMA) copolymers exhibited compositional drift, leading to ambiguity in microstructure/thermomechanical property correlations while shape memory recovery times were excessively long ~1 h. The terpene-based 1,3-diene Myr and AAEMA were copolymerized via nitroxide-mediated polymerization (NMP) with an initial Myr molar feed ratio ranging from 0.1 to 0.9. Reactivity ratios estimated by the Meyer-Lowry method were rMyr = 0.20 and rAAEMA = 0.22 with respective 95% confidence intervals of [0.13, 0.37] and [0.15, 0.23], suggesting alternating copolymerization. Bulk terpolymerization with styrene (Sty) afforded linear poly(Myr-stat-Sty-stat-AAEMA) prepolymers designed to modulate glass transition temperature Tg and stiffness. Subsequent cross-linking with isophorone diamine (IPDA) formed catalyst-free vinylogous urethane dynamic linkages. ATR-FTIR, dynamic mechanical analysis (DMA), and tensile measurements confirmed the recyclability of vitrimers through hot pressing at 110 °C, demonstrating minimal changes in rheological and mechanical properties even after four cycles. By heating the vitrimers to above their Tg (to ~45 °C), they exhibited a shape memory effect with high shape fixity and fast shape recovery (< 1 min). The vitrimers could undergo permanent shape reprogramming through reconfiguration of dynamic bonds at ~110 °C. This work shows the adaptability of Myr-based rubbers by vitrification and appropriate copolymerization for the synthesis of more sustainable elastomers with stimuli responsive properties.

Nitroxide-Mediated Controlled Radical Copolymerization of α-Trifluoromethylstyrenes with Styrenes.

Fluorinated polymers are important materials in everyday life; however, most monomers of widely used fluoropolymers are gaseous, and their polymerization is difficult in an ordinary laboratory. Therefore, partially fluorinated polymers have recently been reported. As an easy-to-handle fluorine-containing monomer, α-trifluoromethylstyrene (TFMST) can be used to produce partially fluorinated polymers with trifluoromethyl groups in the main chain; however, TFMST does not homopolymerize, and there are limited reports on its copolymerization with styrene (ST). In this study, we applied the controlled radical polymerization method, which is effective for the polymerization of ST, to the copolymerization of TFMST and ST. We also showed that nitroxide-mediated polymerization is effective. The content ratio of TFMST in the TFMST-ST copolymer can be controlled between 10% and 40% by changing its monomer ratio. Additionally, the polymerization of TFMST and ST with substituents was performed to increase structural variations. The thermal stability as well as water and oil repellency of the synthesized polymers with different composition ratios and substituents were also evaluated.

4D Printing of Adaptable “Living” Materials Based on Alkoxyamine Chemistry

Abstract4D printing has emerged as a powerful strategy capable of revolutionizing additive manufacturing by enabling objects to dynamically transform overtime on demand. Despite significant progress, the full potential remains unrealized, particularly in the utilization of dynamic covalent chemistry. This study introduces a new approach using a multifunctional cross‐linker with alkoxyamine functionalities for 4D printing. Digital light processing (DLP) is employed for high‐resolution printing of complex objects. Leveraging alkoxyamine bonds' dynamic and living characteristics, the printed structures can be further modified through nitroxide‐mediated polymerization (NMP) using styrene and nitroxide exchange reactions (NER). The resulting “living” printed structures exhibit the unique ability to undergo both “growth” and “degrowth”, dynamically adapting their size as well as the reduced Young's Modulus across a wide range (770 kPa–1.2 GPa). The chain extension by NMP and softening by NER are carefully characterized by IR and EPR spectroscopy. The presented approach opens avenues for the development of 4D printed structures with complex adaptive systems, showcasing enormous potential in a wide range of fields.

Comparing SG1 and TEMPO for NMP of n-butyl acrylate in miniemulsion to optimize the average particle size for rate and molecular control

The interplay between compartmentalization and phase transfer effects in NMP of n-butyl acrylate in miniemulsion. Optimal average particle sizes are identified for the two most important nitroxides and different reaction conditions.

A novel method of alkoxyamine homolysis activation via photochemical rearrangement.

We proposed the nitrone-oxaziridine rearrangement as a novel method for photochemical activation for the homolysis of alkoxyamine in nitroxide-mediated polymerization. The photoisomerization of the aldo-/ketonitrone-group into the oxaziridine one in 2,5-dihydroimidazole 3-oxide-based alkoxyamines was studied; the products of photolysis have been identified, and quantum yields were measured. Conversion of the nitrone group into the oxaziridine one was found to decrease the activation energy of alkoxyamine homolysis by ca. 10 kJ mol-1.

Beauty of Explicit Dispersity (Đ) Equations in Controlled Polymerizations.

Dispersity (Đ) as a critical parameter indicates the level of uniformity of the polymer molar mass or chain length. In the past several decades, the development of explicit equations for calculating Đ experiences a continual revolution. This viewpoint tracks the historical evolution of the explicit equations from living to reversible-deactivation polymerization systems. Emphasis is laid on displaying the charm of explicit Đ equations in batch reversible-deactivation radical polymerization (RDRP), with highlights of the relevant elegant mathematical manipulations. Some representative emerging applications enabled by the existing explicit equations are shown, involving nitroxide-mediated polymerization (NMP), atom transfer radical polymerization (ATRP), and reversible addition-fragmentation chain transfer (RAFT) polymerization systems. Stemming from the several outlined challenges and outlooks, sustained concerns about the explicit Đ equations are still highly deserved. It is expected that these equations will continue to play an important role not only in traditional polymerization kinetic simulation and design of experiments but also in modern intelligent manufacturing of precision polymers and classroom education.

Aggregation‐induced emission polymers via reversible‐deactivation radical polymerization

AbstractAggregation‐induced emission (AIE) is a unique phenomenon whereby aggregation of molecules induces fluorescence emission as opposed to the more commonly known aggregation‐caused quenching (ACQ). AIE has the potential to be utilized in the large‐scale production of AIE‐active polymeric materials because of their wide range of practical applications such as stimuli‐responsive sensors, biological imaging agents, and drug delivery systems. This is evident from the increasing number of publications over the years since AIE was first discovered. In addition, the ever‐growing interest in this field has led many researchers around the world to develop new and creative methods in the design of monomers, initiators and crosslinkers, with the goal of broadening the scope and utility of AIE polymers. One of the most promising approaches to the design and synthesis of AIE polymers is the use of the reversible‐deactivation radical polymerization (RDRP) techniques, which enabled the production of well‐controlled AIE materials that are often difficult to achieve by other methods. In this review, a summary of some recent works that utilize RDRP for AIE polymer design and synthesis is presented, including (i) the design of AIE‐related monomers, initiators/crosslinkers; the achievements in preparation of AIE polymers using (ii) reversible addition–fragmentation chain transfer (RAFT) technique; (iii) atom transfer radical polymerization (ATRP) technique; (iv) other techniques such as Cu(0)‐RDRP technique and nitroxide‐mediated polymerization (NMP) technique; (v) the possible applications of these AIE polymers, and finally (vi) a summary/perspective and the future direction of AIE polymers.

α‐Hydrogen spirocyclic alkoxyamines synthesis: ESR verified Nitroxides generation and their potential application in NMP polymerization

AbstractHerein, we report a synthetic methodology towards the construction of new cylic alkoxyamines without the prior obtention of their respective nitroxides. These systems are constituted by an indene core connected to a pyrrolidine nucleus through a spiro carbon atom, containing an adjacent α‐hydrogen to the N atom, in order to exhibit the 1‐azaspiro[4.4]nonane frame. Alkoxyamines were synthesised by a free radical‐based spirocyclization, starting from functionalized oxime ethers, involving aryl and neutral alkoxyaminyl radicals in 16–40 % yields. Then, the novel spirocyclic aminoxyl radicals (nitroxides) were detected in ESR spectra, when alkoxyamines were subjected by the thermal homolytic cleavage of their −C−O−N− bond. In general, the temperatures found were in accordance with the increase in the stability of the carbon‐centred radical for the same type of nitroxides. In addition, a spirocyclic alkoxyamine 8 b with α‐hydrogen was employed as initiators/mediators in the styrene bulk homopolymerization, obtaining high conversions and relatively narrow dispersity (Ð~1.5). This suggests that the synthetic methodology used in this research work to obtain α‐hydrogen spirocyclic alkoxyamines would have the potential to be applied in nitroxide‐mediated polymerization (NMP).

Enzyme assisted oxygen tolerance nitroxide-mediated miniemulsion polymerization

Nitroxide-mediated polymerization (NMP) is known to be a simple reversible deactivation radical polymerization (RDRP) method for preparing well defined polymers. In compared with other RDRP methods, NMP was highlighted by the clean and low toxic product which is favorable for some sensitive biological and electronic applications. Here we reported the first case of oxygen tolerance nitroxide-mediated miniemulsion polymerization, by introducing glucose oxidase (GOx) and ascorbic acid (AsAc) to the system. Without degassing treatment, the miniemulsion polymerization can be carried on using alkoxyamine 3-(((2-cyanopropan-2-yl)oxy) (cyclohexyl)amino)-2,2-dimethyl-3-phenylpropanenitrile (Dispolreg 007) as the initiator. The oxygen tolerance NMP miniemulsion exhibited similar living and control behavior to the degassing system, and the chain extension could also be performed without degassing. The characteristics of this enzyme assisted oxygen tolerant heterogeneous NMP may provide exciting new opportunities for increased versatility of NMP polymerization in preparation of polymeric nano-objects.

Biofunctionalization of Metal–Organic Framework Nanoparticles via Combined Nitroxide‐Mediated Polymerization and Nitroxide Exchange Reaction

AbstractSurface engineering of metal–organic framework nanoparticles (MOF NPs), and enabling their post‐synthetic modulation that facilitates the formation of bio‐interfaces has tremendous potential for diverse applications including therapeutics, imaging, biosensing, and drug‐delivery systems. Despite the progress in MOF NPs synthesis, colloidal stability and homogeneous dispersity—a desirable property for biotechnological applications, stands as a critical obstacle and remains a challenging task. In this report, dynamic surfaces modification of MOF NPs with polyethylene glycol (PEG) polymer is described using grafting‐from PEGylation by employing nitroxide‐mediated polymerization (NMP) and inserting arginylglycylaspartic acid (RGD) peptides on the surface via a nitroxide exchange reaction (NER). The dynamic modification strategy enables tailoring PEG‐grafted MOF NPs of the type UiO‐66‐NH2 with improved colloidal stability, and high dispersity, while the morphology and lattice crystallinity are strictly preserved. The interaction of PEG‐grafted MOF NPs with human serum albumin (HSA) protein under physiological conditions is studied. The PEG‐grafted colloidal MOF NPs adsorb less HSA protein than the uncoated ones. Therefore, the described approach increases the scope of bio‐relevant applications of colloidal MOF NPs by reducing nonspecific interactions using NMP based PEGylation, while preserving the possibility to introduce targeting moieties via NER for specific interactions.

Modelling of polymerization kinetics and molar mass development in the nitroxide‐mediated polymerization (NMP) of styrene in supercritical carbon dioxide using the PC‐SAFT equation of state

AbstractA mathematical model for polymerization kinetics and molar mass development in the nitroxide‐mediated polymerization (NMP) of vinyl monomers in supercritical carbon dioxide (scCO2) has been developed. The method of moments is used for molar mass development. The perturbed‐chain statistical associating fluid theory (PC‐SAFT) equation of state is used to estimate the number of stable phases present at equilibrium in the reaction mixture, critical number average chain length at which polymer particles are formed, and monomer concentration in each phase. Pure and binary PC‐SAFT interaction parameters are estimated from liquid–liquid equilibrium (LLE) and liquid–vapour equilibrium (LVE) experimental data at 60 to 129°C. The effect of pressure on monomer conversion and molar mass development in the polymerization of styrene (Sty) using benzoyl peroxide (BPO) and 2,2,6,6‐Tetramethylpiperidinyl‐1‐oxyl (TEMPO) at 120°C and 300–500 bar is studied. It was observed that increasing pressure increases polymerization rate without significantly affecting molar mass development.

Toward an Automated Convergence Tool for Kinetic Monte Carlo Simulation of Conversion, Distributions, and Their Averages in Non-dispersed Phase Linear Chain-Growth Polymerization

The time evolution of (bio)chemical processes, specifically those involving distributed species as in polymer synthesis and recycling, can be obtained using Gillespie-based kinetic Monte Carlo simulations, provided that a sufficiently high (Monte Carlo) control volume is utilized with respect to the simulation targets (e.g., focus on only conversion/yield or a combination of the former and average molar masses, or even the combination with complete distributions with accurate tail prediction). For more detailed kinetics and more demanding simulations targets, currently, simulation results are mostly visually checked to decide which control volume to practically use, taking into account user time constraints. The present work puts forward a convergence strategy that avoids relying on subjective visual analysis to set the minimum control value that guarantees the minimization of the stochastic noise for several simulation target combinations to acceptable values. The strategy is illustrated for two (for simplicity, intrinsic) non-dispersed phase bulk chain-growth polymerization processes, one with high average chain lengths and a broad chain length distribution (CLD), that is, free-radical polymerization (FRP), and one with low average chain lengths and a narrow CLD, that is, nitroxide-mediated polymerization (NMP). It is showcased that the monomer conversion profile converges the fastest, with even the occurrence of noise-free simulation results in the absence of numerical convergence. A sufficiently accurate representation of the tail of the (number) CLD in FRP demands a sufficiently high control volume so that relative errors below 0.5% result for the z-based average chain length or molar mass (Mz). This need to inspect Mz convergence further holds under NMP, in general, reversible deactivation radical polymerization (RDRP) conditions, in which it is uncommon to report such higher order averages. An automated convergence check beyond threshold values is recommended to minimize the impact of possible fluctuations in certain simulation targets, specifically peak representations, for example, the initial spike in the dispersity plot in RDRP. The convergence results are supported by the number of radicals in the control volume with values much higher than 2 to accurately represent termination kinetics for specifically CLD prediction, already under intrinsic conditions.

Accelerated nitroxide-mediated polymerization of styrene and butyl acrylate initiated by BlocBuilder MA using flow reactors

Flow reactors enable the nitroxide-mediated radical polymerization (NMP) of styrene (St) and butyl acrylate (BA) initiated by BlocBuilder MA (BB MA) with additives in a highly controlled manner, and BA–St di-block copolymers were also synthesized.

DFT Calculations of Some Important Radicals Used in the Nitroxide-Mediated Polymerization and Their HOMO‒LUMO, Natural Bond Orbital, and Molecular Electrostatic Potential Comparative Analysis

DFT Calculations of Some Important Radicals Used in the Nitroxide-Mediated Polymerization and Their HOMO‒LUMO, Natural Bond Orbital, and Molecular Electrostatic Potential Comparative Analysis

Degradable Polystyrene via the Cleavable Comonomer Approach

Polystyrene (PS) is a major commodity polymer widely used in various applications ranging from packaging to insulation thanks to its low cost, high stiffness, and transparency as well as its relatively high softening temperature. Similarly to all polymers prepared by radical polymerization, PS is constituted of a C–C backbone and thus is not degradable. To confer degradability to such materials, the copolymerization of vinyl monomers with a cyclic monomer that could undergo radical ring-opening is an efficient method to introduce purposely cleavable bonds into the polymer backbone. Dibenzo[c,e]-oxepane-5-thione (DOT) is a cyclic thionolactone monomer known for its efficient copolymerization with acrylate derivatives but so far could not be incorporated into PS backbones. From a theoretical study combining density functional theory (DFT) and kinetic models using the PREDICI software, we showed that the modification of experimental conditions could overcome these limitations and that high molar mass degradable polystyrene (Mw close to 150 000 g·mol–1) could be prepared via statistical insertion of thioester groups into the polymer backbone. This copolymerization process is compatible with conventional free radical polymerization and reversible deactivation radical polymerization (RDRP) techniques such as nitroxide mediated polymerization (NMP). Thanks to favorable reactivity ratios allowing only a few mol % of thioester units to be randomly incorporated, there was no major modification of the thermal and mechanical properties of the PS. The degradation of such PS could be performed in tetrahydrofuran (THF) at room temperature (RT) in 1 h using 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as a base, leading to oligomers with Mn close to 2000 g·mol–1. We successfully demonstrate further applicability of these copolymerization systems for the phototriggered decomposition of PS in solution as well as the synthesis of cross-linked PS networks degradable into soluble side products.

Para fluoro-thiol clicked diblock-copolymer self-assembly: Towards a new paradigm for highly proton-conductive membranes

Sulfonated sPPFS-b-PBuA diblock and statistical copolymers based on 2,3,4,5,6-pentafluorostyrene PFS and butyl acrylate BuA were elaborated for Proton Exchange Membrane Water Electrolyser (PEMWE) purposes. The block copolymers (BCP) were synthetized by Nitroxide Mediated Polymerization NMP, a controlled radical polymerization technique that yields a well-defined molar mass and a low dispersity material. These diblock-copolymers have the ability to self-assemble due to the immiscibility of the two macromolecular segments PPFS and PBuA. Statistical copolymers of the similar chemical composition were synthetized by both controlled radical polymerization NMP in solution and by free radical polymerization FRP in emulsion as waterborne dispersed polymer with highest molar mass. The copolymers were sulfonated by a mild click-reaction, namely an organo-catalyzed nucleophilic substitution reaction with sodium 3-mercapto-1-propanesulfonate (SMPS) at low temperature. The morphology of the sulfonated diblock-copolymer BCP was studied by SAXS and AFM, revealing a nano-phase-segregated sulfonated membrane. The mechanical properties of the membranes were improved by ionic crosslinking with polybenzimidazole (PBI-OO). Finally, the conductive properties of the sulfonated BCPs and statistical copolymers were investigated as a function of parameters such as the morphology of the BCP, the molar mass, and the sulfonation degree of the materials.

The Kinetic Solvent Effect on 1,3-Dipolar Cycloaddition of 2,2,5,5-Tetramethyl-3-imidazoline-3-oxide-1-oxyl.

The use of 2,2,5,5-tetramethyl-3-imidazoline-3-oxide-1-oxyl (1) as a controlling agent in Nitroxide Mediated Polymerization allows for activation of alkoxyamine homolysis by 1,3-dipolar cycloaddition of a vinyl monomer [Edeleva et al. Chem. Commun. 2019, 55, 190-193]. Polymerization can be carried out in a medium with different polarity and hydrogen bonding capacity which affects the rate of 1,3-dipolar cycloaddition reaction. In the present study, the solvent effect on the rate of this reaction was investigated by the electron paramagnetic resonance and density functional theory for the six different dipolarophiles, e. g. styrene, n-butyl acrylate, acrylonitrile, methyl vinyl ketone, maleic anhydride, and N-phenyl maleimide. The rate of 1,3 dipolar cycloaddition of non-polar styrene was found to be slightly dependent on solvent, while one order of magnitude decrease in rate coefficient is observed for other dipolarophiles when going from hexane to methanol.

Covalent Adaptable Networks Based on Dynamic Alkoxyamine Bonds

AbstractCovalent adaptable networks (CANs) introduce a new paradigm to polymer science, by making static network polymers dynamic and thereby recyclable, reprocessable, and self‐healing. The critical feature in CANs is the presence of dynamic covalent linkages within the network structure. A variety of such linkages are introduced into CANs, making the respective networks responsive to various stimuli, such as light, temperature, or pH. Here, CANs based on alkoxyamines as dynamic covalent bonds are reviewed. Alkoxyamines uniquely combine the ability to dynamically form, break, and reform covalent bonds with the possibility to initiate reversible‐deactivation radical polymerization. Polymer networks based on alkoxyamines are therefore both adaptive and quasi‐living, able to remodel the network structure by nitroxide‐exchange reactions (NER) and extend the network structure by nitroxide‐mediated polymerization (NMP). In this review, the concepts behind CANs are first introduced and the properties of nitroxides and derived alkoxyamines are discussed. A special focus is set on the ability to tune the response of alkoxyamines to different stimuli, through alteration of their structure. In addition, possible side reactions during dynamic bond exchange and limitations for polymerization are critically reviewed. Subsequently, examples of alkoxyamine‐based CANs responsive to different stimuli, such as temperature, light, or chemical triggers, are discussed. Properties and applications of CANs based on alkoxyamines are then discussed. Finally, an outlook is provided on challenges that need to be addressed as well as opportunities that lie within these “living” CANs.

One‐For‐All Polyolefin Functionalization: Active Ester as Gateway to Combine Insertion Polymerization with ROP, NMP, and RAFT

This work develops the Polyolefin Active-Ester Exchange (PACE) process to afford well-defined polyolefin-polyvinyl block copolymers. α -Diimine Pd(II)-catalyzed olefin polymerizations were investigated through in-depth kinetic studies in comparison to an analog to establish the critical design that facilitates catalyst activation. Simple transformations lead to a diversity of functional groups forming polyolefin macroinitiators or macro-mediators for various subsequent controlled polymerization techniques. Preparation of block copolymers with different architectures, molecular weights, and compositions was demonstrated with ring-opening polymerization (ROP), nitroxide-mediated polymerization (NMP), and photoiniferter reversible addition-fragmentation chain transfer (PI-RAFT). The significant difference in the properties of polyolefin-polyacrylamide block copolymers was harnessed to carry out polymerization-induced self-assembly (PISA) and study the nanostructure behaviors.