Reversible Addition-fragmentation Chain Transfer Mechanism Research Articles

Synthesis of Multiresponsive Gold@Polymer‐Nanohybrid Materials Using Polymer Precursors Obtained by Photoiniferter RAFT Polymerization

AbstractCombining gold nanoparticles (GNP) with stimuli‐responsive polymers is a fascinating field for applications as sensor materials or in biomedical applications. The synthesis of multiresponsive Au@Polymer hybrid systems in an in situ approach using tailor‐made poly(2‐(dimethylamino)ethyl methacrylate) (P(DMAEMA)) precursor synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization is introduced. In particular, because of its multiresponsive behavior, P(DMAEMA) is of great interest and is used as the precursor for the synthesis. For the polymerization of the precursor, an approach is introduced that applies the photoiniferter RAFT (photoRAFT) mechanism applicable for tertiary amine‐bearing methacrylic monomers like DMAEMA. By varying the chain transfer agents (CTA), the influence of different RAFT Z‐groups on the solubility of the Au‐hybrids is studied. Aging effects are observed by transmission electron microscopy (TEM) analyses. A comprehensive dynamic light scattering (DLS) study reveals the thermoresponsive behavior of the Au‐hybrids in an aqueous solution. The influence of free polymer in solution on the suspension stability is observed. The capability of crosslinking using 1,5‐dibromopentane (DBP) and the CO2 sensitivity of the synthesized hybrids expands the area of their potential application.

Multicompartment Vesicles: A Key Intermediate Structure in Polymerization-Induced Self-Assembly of Graft Copolymers

The morphological evolution of graft copolymer-based nano-objects was monitored by light scattering and electron microscopy during their formulation in water by polymerization induced self-assembly using a photo-mediated reversible addition–fragmentation chain transfer mechanism. The copolymer models used were composed of a dextran backbone bearing poly(2-hydroxypropyl methacrylate) grafts of two degrees of polymerization (X). At a full monomer conversion, unilamellar vesicles (ULVs) and large compound nano-objects (LCNs) were formed when targeting X = 100 and 500, respectively. For X = 100, some spherical, worm-like, then jellyfish-like structures were progressively observed before the ULVs formation. For X = 500, electron cryotomography revealed an unprecedented intermediate morphology formed from the onset of self-assembly called a multicompartment vesicle (MCV) that fused to form LCN. The formation of MCV was attributed to a local phase separation between dextran and the residual 2-hydroxypropyl methacrylate inducing the appearance of multiple hydrophilic cores.

Reversible addition–fragmentation chain transferbased copolymers of acrylonitrile and alkyl acrylates as possible precursors for carbon fibers: synthesis and thermal behavior during stabilization

AbstractCopolymers of acrylonitrile and alkyl acrylates with various alkyl substituents (methyl, ethyl, butyl, 2‐ethylhexyl and lauryl) were synthesized via reversible addition–fragmentation chain transfer polymerization in dimethylsulfoxide. It is demonstrated that reactivity of alkyl acrylate in copolymerization decreases with increase of the length of the alkyl substituent. The molar fractions of homodiads and homotriads of acrylonitrile and the number‐average sequence length of acrylonitrile units decreases with a decrease in the acrylate content in the copolymer or with an increase in the length of the alkyl substituent in the acrylate. The copolymer composition was held constant throughout copolymerization in all the studied systems resulting in the formation of copolymers with high compositional homogeneity. Additionally the reversible addition–fragmentation chain transfer mechanism provides low dispersity of the copolymers. Both factors enable the effect of the alkyl substituent in alkyl acrylate on the thermal behavior of acrylonitrile copolymers to be carefully revealed. Being inert in the cyclization reaction, alkyl acrylates have no effect on the activation energy of the reaction. However, increase of the length of the alkyl substituent is followed by a slower evolution of the ladder structure. In contrast, in conditions of oxidative stabilization, the stabilization index increases in the order of increase of the number‐average sequence length of the acrylonitrile units in the copolymers. Thus, the use of alkyl acrylates with longer alkyl substituents provides an increase of chain flexibility and the formation of an enhanced ladder structure during oxidative stabilization which is profitable for carbon fiber production. © 2021 Society of Industrial Chemistry.

Controlled (Co)Polymerization of Methacrylates Using a Novel Symmetrical Trithiocarbonate RAFT Agent Bearing Diphenylmethyl Groups.

Herein, we report a novel type of symmetrical trithiocarbonate chain transfer agent (CTA) based diphenylmethyl as R groups. The utilization of this CTA in the Reversible Addition-Fragmentation chain Transfer (RAFT) process reveals an efficient control in the polymerization of methacrylic monomers and the preparation of block copolymers. The latter are obtained by the (co)polymerization of styrene or butyl acrylate using a functionalized macro-CTA polymethyl methacrylate (PMMA) previously synthesized. Data show low molecular weight dispersity values (Đ < 1.5) particularly in the polymerization of methacrylic monomers. Considering a typical RAFT mechanism, the leaving groups (R) from the fragmentation of CTA should be able to re-initiate the polymerization (formation of growth chains) allowing an efficient control of the process. Nevertheless, in the case of the polymerization of MMA in the presence of this symmetrical CTA, the polymerization process displays an atypical behavior that requires high [initiator]/[CTA] molar ratios for accessing predictable molecular weights without affecting the Đ. Some evidence suggests that this does not completely behave as a common RAFT agent as it is not completely consumed during the polymerization reaction, and it needs atypical high molar ratios [initiator]/[CTA] to be closer to the predicted molecular weight without affecting the Đ. This work demonstrates that MMA and other methacrylic monomers can be polymerized in a controlled way, and with “living” characteristics, using certain symmetrical trithiocarbonates.

Fully photodegradable block copolymer nanoparticles for dual release of cargo and radicals

When exposed to radiation, photo-responsive block copolymer nanoparticles disassemble to liberate a possible carried guest. However, the particle disruption is rarely accompanied by a polymer chain disintegration into molecular fragments, that would facilitate clearance from the body. We describe herein a novel class of fully photodegradable all-acrylic poly(hydroxyethyl acrylate)-b-poly(o-nitrobenzyl acrylate) (PHEA-PNBA) nanoparticles. Their synthesis proceeds by dispersion polymerization-induced self-assembly (PISA) based on a reversible addition–fragmentation chain transfer mechanism. The self-immolation mechanism is initiated via a dual H-abstraction by the electronically excited o-nitrobenzyl pendant group, first, from the adjacent CH2, second, from CH of the polyacrylate chains. The former results in the release of the protecting group, while the latter initiates a cascade of reactions that fragment the whole polymer backbone. Simultaneous chain scission and cleavage of o-nitrobenzyl group combine in turn to induce a fast and complete degradation of the copolymer particles. Synthesis, structural and colloidal characterization of the diblock copolymer nanoparticles, photochemical mechanism governing self-immolation are described. Emphasis is placed on understanding the reaction parameters which are important in controlling degradation rates, in particular nature of continuous phase and degree of polymerization of the PNBA block. UV exposure is exploited to release a model guest (Nile red) from the particles. Additionally, the burst release of free radicals accompanying the chain degradation can serve to decompose a target molecule (methylene blue).

Reversible addition-fragmentation chain-transfer polymerization: Fundamentals and use in practice

Modern approaches to the synthesis of tailor-made macromolecules by radical polymerization proceeding through the reversible addition-fragmentation chain-transfer mechanism are considered. The mechanism of this process and the experimental and calculation methods for determining its main kinetic parameters are discussed. Particular emphasis is placed on the problems of designing copolymers of various microstructures, including random, gradient, and block copolymers.

Ternary copolymers of acrylic acid, N-isopropylacrylamide, and butyl acrylate: Synthesis and aggregative behavior in dilute solutions

Narrowly dispersed hydrophilic block and random copolymers of acrylic acid and N-isopropylacrylamide of various compositions and molecular masses are synthesized for the first time by controlled radical polymerization via the reversible addition-fragmentation chain-transfer mechanism. The copolymers are used for the synthesis of ternary copolymers with n-butyl acrylate. As shown by dynamic light scattering, the resulting amphiphilic copolymers can form stable dispersions in diluted aqueous solutions. It is found that the effective hydrodynamic radii of dispersion particles may be controlled via variation both in the primary structure of the ternary copolymer, i.e., the amount and sequence of hydrophilic and hydrophobic units, and in the external stimuli, i.e., the temperature and pH of a solution.

Enzyme-Initiated Reversible Addition–Fragmentation Chain Transfer Polymerization

Biocatalysis is promising for sustainable production of polymers. Enzyme-initiated reversible addition–fragmentation chain transfer (RAFT) polymerization is reported. Horseradish peroxidase (HRP) catalyzes oxidation of acetylacetone (ACAC) by hydrogen peroxide to generate ACAC radicals, which in the presence of a suitable chain transfer agent initiate efficient and well-controlled RAFT polymerization in aqueous buffer solution at room temperature. The versatility of HRP-initiated RAFT polymerization was demonstrated by controlled polymerization of a wide range of monomers, including both more and less activated monomers, under a variety of conditions, including both homogeneous solution polymerization and heterogeneous dispersion polymerization conditions. In all cases, the polymerization afforded excellent pseudo-first-order kinetics, predictable molecular weights, and narrow molecular weight distributions. Operation via RAFT mechanism of this HRP-initiated polymerization was confirmed by a combination o...

Polymerization Rate Considerations for High Molecular Weight Polyisoprene‐b‐Polystyrene‐b‐Poly(N,N‐dimethylacrylamide) Triblock Polymers Synthesized Via Sequential Reversible Addition‐Fragmentation Chain Transfer (RAFT) Reactions

The reversible addition‐fragmentation chain transfer (RAFT) polymerization mechanism is a powerful technique for synthesizing functional block polymers for myriad applications. Most kinetic studies regarding the RAFT mechanism have focused on low molecular weight homopolymer and block polymer syntheses using a dithiobenzoate chain transfer agent (CTA). Here, the polymerization kinetics are evaluated for a high molecular weight A‐B‐C triblock polymer system, polyisoprene‐b‐polystyrene‐b‐poly(N,N‐dimethylacrylamide) (PI‐PS‐PDMA), using a trithiocarbonate agent for application of these types of polymers. Importantly, it is demonstrated that the polymerization of polyisoprene is the step that generates the block with the largest dispersity for high molecular weight PI‐PS‐PDMA polymers. As such, the kinetics of isoprene polymerization must be altered systematically for desired nanostructures to be formed. In addition, it is established that the PS and PDMA block additions exhibit polymerization rate retardation, which is due to slow chain fragmentation of the CTA, as demonstrated by the magnitudes of the equilibrium constants for both the styrene and N,N‐dimethylacrylamide reactions, and as calculated using ab initio modeling. This elucidation of the nature of the controlled RAFT mechanism provides a critical handle for the more precise design and control of other next‐generation high molecular weight block polymer systems that are polymerized using the RAFT mechanism. image

Accelerated brush growth on nanoparticle surfaces by reversible addition‐fragmentation chain transfer polymerization

ABSTRACTIt is now well established that controlling the grafted chain lengths and densities on nanoparticle surfaces determines the effective interactions between particles, and their assembly. Here, we present unusual kinetic results for achieving grafted chain lengths longer than the free chains using reversible addition‐fragmentation chain transfer (RAFT) polymerization and discuss the limitations to obtaining polymer grafting density higher than ∼0.06 chains/nm2. We observe that surface initiated polymerization grows faster than the free chains in solution with high RAFT agent coverage (1.95 agents/nm2) on nanoparticles. The time‐dependence of graft density suggests that activation of the anchored chain transfer agent (CTA) is limited by the diffusion of macro‐radicals within growing grafts. Thus, radical transfer and exchange reactions become inefficient between grafts and free polymer, and convert the surface‐initiated RAFT mechanism to a free radical polymerization. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 1700–1705

Controlled copolymerization of acrylonitrile in bulk via the reversible addition-fragmentation chain-transfer mechanism

The pseudoliving radical binary copolymerization of acrylonitrile with methyl acrylate, styrene, n-butyl acrylate, and tert-butyl acrylate in bulk in the presence of the reversible addition-fragmentation chain-transfer agent dibenzyl trithiocarbonate is performed for the first time. The addition of trithiocarbonate makes it possible to prepare a narrowly dispersed visually optically transparent copolymer in a wide range of monomer-feed compositions even at limiting conversions. Conditions for the synthesis of acrylonitrile copolymers with controlled molecular masses and narrow molecular-mass distributions are ascertained. In the above copolymers, the trithiocarbonate group is shown to be located within the chain.

The influence of intermediate radical termination and fragmentation on controlled polymer synthesis via RAFT polymerization

Reversible addition-fragmentation chain transfer (RAFT) polymerization allows the design of tailored polymers by inducing addition and fragmentation reactions via a powerful kinetic mechanism; the process, however, is not well understood. A parametric analysis was conducted in conjunction with experimental data to provide insights into the RAFT mechanism, accounting for the key kinetics events in RAFT polymerization of styrene. The effects of overall, forward and backward fragmentations of intermediate radicals were analysed with experimental data and sensitivity investigations. Our results show that the intermediate radicals undergo relatively fast fragmentation and that the radical cleavage is symmetrical. These factors, critical in design and control, were found to profoundly influence the process kinetics and the final polymer product attributes.

Synthesis of Well-Defined Dawson-Type Poly(N,N-diethylacrylamide) Organopolyoxometalates.

An organotin-substituted Dawson-type phosphotungstate was covalently linked to a trithiocarbonate group by postfunctionalization methods. This organopolyoxometalate led to a series of polyoxometalate (POM)-poly(N,N-diethylacrylamide) hybrids in a controlled way through reversible addition-fragmentation chain transfer (RAFT) polymerization. Detailed comparison with and without the presence of POMs revealed that they do not profoundly disturb the RAFT mechanism, despite their oxidative power. The molar masses of the polymers were in the range of 10 to 100 kg mol-1 and molar mass dispersities of the composites were low (Mw /Mn <1.5). The POM building block in the hybrids retained the photocatalytic reactivity of the parent Dawson polyanion [P2 W18 O62 ]6- . Smaller, more homogeneous, and colloidally more stable silver nanoparticles were formed with the covalently linked POM-polymer compound than with its single unbound components.

Kinetic approach to modeling the radical polymerization of styrene in the presence dibenzyl trithiocarbonate

A mathematical model of the kinetics of the radical polymerization of styrene in the presence of dibenzyl trithiocarbonate by the reversible addition-fragmentation chain transfer mechanism with cross-termination of radicals and intermediates and quadratic termination of intermediates is developed. The adequacy of the polymerization mechanism underlying the model and the related mathematical formalism are tested by comparing the calculated and measured values of the average molecular-weight characteristics of resulting polystyrene.

Controlled radical (co)polymerization of (meth)acrylic esters via the reversible addition-fragmentation chain-transfer mechanism

The homopolymerization of acrylic and fluoroacrylic esters mediated by benzyl dithiobenzoate and dibenzyl trithiocarbonate proceeds in the controlled mode via the reversible addition-fragmentation chain-transfer mechanism, while the controlled radical polymerization of methacrylic esters is not effected under these conditions. The molecular-mass characteristics of the copolymers of acrylic and methacrylic esters may be satisfactorily controlled by benzyl dithiobenzoate-mediated copolymerization when the content of acrylic esters is no less than 50 mol %. If a reversible addition-fragmentation chain-transfer agent active with respect to only one of the monomers is used, compositionally homogeneous narrowly dispersed copolymers are formed via the azeotropic copolymerization of the monomers up to high conversions. The controlled copolymerization of N-vinylpyrrolidone and fluoroacrylates allows the synthesis of alternating narrowly dispersed amphiphilic copolymers with properties different from those of alternating copolymers with a broad molecular-mass distribution.

Controlled synthesis of multiblock copolymers by pseudoliving radical polymerization via the reversible addition-fragmentation chain-transfer mechanism

With the use of two classes of reversible addition-fragmentation chain-transfer agents—dithiobenzoates and trithiocarbonates—multiblock copolymers based on styrene and n-butyl acrylate, which are the best-studied monomers in these processes, are synthesized. It is shown that the polymers containing dithiobenzoate and trithiocarbonate groups are highly efficient for the synthesis of block copolymers, which is independent of the number of stages at which the polymeric RAFT agents are used in polymerization: In all cases, the polymeric RAFT agent is fully consumed in the polymerization of the “alien” monomer. The mechanism governing chain formation during the synthesis of multiblock copolymers, that is, the character of monomer insertion into the polymer chain, via one or both ends, is studied. It is found that the order of monomer loading determines the ratio of chains growing through one or two ends. The thermal stability of amphiphilic multiblock copolymers, their solubility in various solvents, and self-organizing ability are investigated.

Effect of the solvent composition on the morphology of nano-objects synthesized via RAFT polymerization of benzyl methacrylate in dispersed systems

A hydrophilic macromolecular RAFT (reversible addition–fragmentation chain transfer) agent (macroRAFT agent) composed of 50 mol% methacrylic acid and 50 mol% poly(ethylene oxide) monomethyl ether methacrylate end-capped by a reactive trithiocarbonate group (P(MAA-co-PEOMA)) was used in the polymerization of benzyl methacrylate (BzMA) in different media, ethanol–water and 1,4-dioxane–water mixtures. Depending on the solvent composition, the polymerization showed features of either a dispersion polymerization (monomer soluble in the initial medium) or an emulsion polymerization (monomer insoluble in the initial medium). In all cases, the RAFT mechanism led to the in situ formation of well-defined amphiphilic P(MAA-co-PEOMA)-b-PBzMA block copolymers that self-assembled during the growth step into self-stabilized nano-objects, according to a polymerization-induced micellization process. For a given composition of the block copolymer, the final morphology depended strongly on the solvent composition. The presence of the organic co-solvent was favorable to the formation of fibers while an increased amount of water favored the formation of spherical particles. Compared to the ethanol–water system, in which the non-spherical objects existed only above 77–80 vol% of ethanol, in 1,4-dioxane–water mixtures the morphological transition was observed at a lower proportion of organic co-solvent (close to 20 vol%). For a given molar mass of the macroRAFT agent and an increased molar mass of the PBzMA block in a given solvent composition (ethanol–water, 95/5, v/v), the morphology changed from spheres to fibers and then to large spheres or vesicles. The molar mass window in which fibers were obtained was wider than that observed in pure water at pH 5 using the same macroRAFT agent [X. Zhang et al., Macromolecules, 2011, 44, 4149].

Pseudoliving polymerization of vinyl acetate mediated by reversible addition-fragmentation chain-transfer agents

The homopolymerization of vinyl acetate mediated by dithiobenzoates and trithiocarbonates as reversible addition-fragmentation chain-transfer agents is studied. The polymerization of vinyl acetate is characterized by some distinct features: (i) a substantial role of chain-termination reactions involving radical intermediates in the kinetics of the process that increases as the concentrations of the reversible additionfragmentation chain-transfer agent and the initiator increase and as temperature decreases and (ii) the occurrence of side reactions of chain transfer to monomers and polymers. The role of these reactions significantly increases with conversion of the monomer. Thus, in order to prepare a narrowly dispersed PVA via the reversible addition-fragmentation chain-transfer mechanism, the process should be conducted to small conversions (15–20%) at moderately high temperatures (80°C) and at a small molar excess of the reversible addition-fragmentation chain-transfer agent with respect to the initiator. A technique for the synthesis of block copolymers based on PVA and poly(n-butyl acrylate) via the reversible addition-fragmentation chain-transfer mechanism is developed.

Controlled synthesis of polyacrylonitrile via reversible addition-fragmentation chain-transfer pseudoliving radical polymerization and its thermal behavior

The polymerization of acrylonitrile mediated by various trithiocarbonates as reversible addition-fragmentation chain-transfer agents is studied. It is shown that, when polymerization is performed in DMSO, a narrowly dispersed PAN with a controlled molecular mass can be prepared. The pseudoliving radical polymerization of acrylonitrile is conducted for the first time via the reversible addition-fragmentation chaintransfer mechanism in carbon dioxide at an increased pressure. The structure of the polymers is investigated via NMR and IR spectroscopy. As shown by DSC and IR pyrolysis, the thermal behavior of PAN is determined by its molecular mass, the width of the molecular-mass distribution, and the conditions of synthesis. The incorporation of functional groups of the reversible addition-fragmentation chain-transfer agent into a macromolecule changes the structure of the polyconjugated system and makes it possible to control the conditions of its formation.

Modeling analysis of chain transfer in reversible addition‐fragmentation chain transfer polymerization

AbstractThe validity of simplifying the reversible addition‐fragmentation chain transfer (RAFT) polymerization as a degenerative chain transfer process was verified in this work. The simplified chain transfer mechanism enabled the direct modeling investigation of chain transfer coefficient in the RAFT polymerization. It also gave the analytical expressions for concentration, chain length, and polydispersity of various chain species. The comparison between the simulations based on chain transfer mechanism and those from general RAFT mechanism showed that this simplified mechanism can accurately predict RAFT polymerization in the absence of side reactions to adduct radicals other than fragmentation. However, significant errors are introduced at high conversion when side reactions to adduct are present. The chain transfer coefficient of RAFT agent is the key factor in RAFT polymerization. The polydispersity is more sensitive to chain transfer coefficient at low conversion. At high conversion, however, the polydispersity is mainly determined by termination, which can be controlled by RAFT agent concentration and the selection of initiator. At last, an analytical equation is derived to directly estimate chain transfer coefficient of RAFT agent from the experimental data. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.