Reversible Addition Fragmentation Chain Transfer End-group Research Articles

Efficient Self-Immolative RAFT End Group Modification for Macromolecular Immunodrug Delivery.

The reversible addition-fragmentation chain-transfer (RAFT) polymerization provides access to a broad variety of biocompatible and functional macromolecules for diverse polymer-drug conjugates. Due to thiocarbonylthio groups at the ends of each growing polymer chain, they can straightforwardly be converted into disufilde-containing self-immolative motives for reversible drug conjugation by traceless linkers. This may be relevant for RAFT-polymerized poly(N,N-dimethylacrylamide) (pDMA), which has been demonstrated to provide similar properties as poly(ethylene glycol) (PEG) in terms of improving the drug's poor pharmacokinetic profile or enhancing its bioavailability. For that purpose, we established a highly efficient one-pot reaction procedure for introducing various functionalities including both primary and secondary amines and primary alcohols and demonstrated their reversible conjugation and traceless release from pDMA's polymer chain end. Next, a first polymer-drug conjugate with a Toll-like receptor agonist exhibited significantly increased activity in vitro compared to conventional irreversibly covalently fixed variants. Finally, α-ω-bifunctional dye or drug conjugates could be generated by a cholesterol-modified RAFT chain-transfer agent. It facilitated the polymer-drug conjugate's internalization at the cellular level monitored by flow cytometry and confocal imaging. This approach provides the basis for a variety of potentially impactful polymer-drug conjugates by combining versatile small molecular drugs with a plethora of available RAFT polymers through reductive-responsive self-immolative linkers.

Utilization of Poor RAFT Control in Heterogeneous RAFT Polymerization

Heterogeneous reversible addition–fragmentation chain transfer (RAFT) polymerization exhibits considerable promise in polymer and colloid synthesis. During the research of RAFT polymerization, good RAFT control is always an important goal to achieve while poor RAFT control is considered to be a lack of success. Herein, we develop heterogeneous RAFT polymerization from an unusual angle by deliberately using RAFT agents with poor RAFT controllability. The utilization of poor RAFT control in heterogeneous RAFT polymerization led to the formation of samples contain high-molecular-weight block copolymers, uncontrolled homopolymers, and the unreacted macro-RAFT agent. Remarkably, uniform polymer spheres with precise size could be obtained via heterogeneous RAFT polymerization, although molecular weight distributions of the obtained polymers were broad. In-depth studies of heterogeneous RAFT polymerization mediated by a binary mixture of RAFT agents with different controllability, the RAFT agent with good controllability dominated the polymerization while the RAFT agent with poor controllability was mainly inactivated or unreacted. On the basis of the unique mechanism provided, we have been able to prepare block copolymer assemblies with rich pendant RAFT groups on the surface by one-step heterogeneous RAFT polymerization using a macro-RAFT agent with divergent RAFT end groups. The reactivity of pendant RAFT groups on the surface provides a landscape for further functionalization of block copolymer assemblies. These proof-of-concept experiments demonstrate that the utilization of poor RAFT control in heterogeneous RAFT polymerization is a promising strategy to a variety of polymer particles that are challenging to be prepared by traditional heterogeneous RAFT polymerizations.

Self-Immolative RAFT-Polymer End Group Modification.

Reversible modifications of reversible addition-fragmentation chain transfer (RAFT)-polymerization derived end groups are usually limited to reductive degradable disulfide conjugates. However, self-immolative linkers can promote ligation and traceless release of primary and secondary amines as well as alcohols via carbonates or carbamates in β-position to disulfides. In this study, these two strategies are combined and the concept of self-immolative RAFT-polymer end group modifications is introduced: As model compounds, benzylamine, dibenzylamine, and benzyl alcohol are first attached as carbamates or carbonates to a symmetrical disulfide, and in a straightforward one-pot reaction these groups are reversibly attached to aminolyzed trithiocarbonate end groups of RAFT-polymerized poly(N,N-dimethylacrylamide). Quantitative end group modification is confirmed by 1 H NMR spectroscopy, size exclusion chromatography, and mass spectrometry, while reversible release of attached compounds under physiological reductive conditions is successfully monitored by diffusion ordered NMR spectroscopy and thin layer chromatography. Additionally, this concept is further expanded to protein-reactive, self-immolative carbonate species that enable reversible bioconjugation of lysozyme and α-macrophage mannose receptor (MMR) nanobodies as model proteins. Altogether, self-immolative RAFT end group modifications can form the new basis for reversible introduction of various functionalities to polymer chain ends including protein bioconjugates and, thus, opening novel opportunities for stimuli-responsive polymer hybrids.

RAFT Emulsion Polymerization for (Multi)block Copolymer Synthesis: Overcoming the Constraints of Monomer Order

Synthesis of (multi)block copolymers using reversible addition–fragmentation chain transfer (RAFT) polymerization generally suffers from the limitation that the order of the blocks must be considered. Herein, syntheses of block copolymers by RAFT polymerization using trithiocarbonate RAFT species are conducted as solution, miniemulsion, and emulsion polymerizations to demonstrate that the issue of monomer order for styrene and butyl methacrylate can be largely overcome in emulsion polymerization under carefully chosen conditions. The presence of monomer droplets in emulsion polymerization—in addition to polymer particles that constitute the locus of polymerization—leads to a reduction in the ratio of RAFT end groups to monomer at the locus of polymerization. Consequently, fragmentation of the RAFT adduct radical in the “backward” (“wrong”) direction is associated with fewer monomer additions, thus minimizing the impact of this undesired kinetic event. It is demonstrated that RAFT emulsion polymerization can be exploited to prepare an alternating pentablock copolymer composed of methacrylates (with 10 mol % styrene) and styrene without consideration of monomer order, thereby significantly broadening the scope of RAFT polymerization for multiblock copolymer synthesis.

In Situ Spectroscopic Studies of Highly Transparent Nanoparticle Dispersions Enable Assessment of Trithiocarbonate Chain-End Fidelity during RAFT Dispersion Polymerization in Nonpolar Media.

We report the synthesis of highly transparent poly(stearyl methacrylate)-poly(2,2,2-trifluoroethyl methacrylate) (PSMA–PTFEMA) diblock copolymer nanoparticles via polymerization-induced self-assembly (PISA) in nonpolar media at 70 °C. This was achieved by chain-extending a PSMA precursor block via reversible addition–fragmentation chain transfer (RAFT) dispersion polymerization of TFEMA in n-tetradecane. This n-alkane has the same refractive index as the PTFEMA core-forming block at 70 °C, which ensures high light transmittance when targeting 33 nm spherical nanoparticles. Such isorefractivity enables visible absorption spectra to be recorded with minimal light scattering even at 30% w/w solids. However, in situ monitoring of the trithiocarbonate RAFT end-groups during PISA requires selection of a weak n → π* band at 446 nm. Conversion of TFEMA into PTFEMA causes a contraction in the reaction solution volume, leading to an initial increase in absorbance that enables the kinetics of polymerization to be monitored via dilatometry. At ∼98% TFEMA conversion, this 446 nm band remains constant for 2 h at 70 °C, indicating surprisingly high RAFT chain-end fidelity (and hence pseudoliving character) under monomer-starved conditions. In situ 19F NMR spectroscopy studies provide evidence for (i) the onset of micellar nucleation, (ii) solvation of the nanoparticle cores by TFEMA monomer, and (iii) surface plasticization of the nanoparticle cores by n-tetradecane at 70 °C. Finally, the kinetics of RAFT chain-end removal can be conveniently monitored by in situ visible absorption spectroscopy: addition of excess initiator at 70 °C causes complete discoloration of the dispersion, with small-angle X-ray scattering studies confirming no change in nanoparticle morphology under these conditions.

CO2-responsive polyacrylamide copolymer vesicles with acid-sensitive morpholine moieties and large hydrophobic RAFT end-group

Sequential Reversible Addition Fragmentation Chain Transfer (RAFT) polymerizations using 2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride (VA-044) and 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT) were used to give amphiphilic polyacrylamide block copolymers containing N-(2-morpholin-4-ylethyl)acrylamide (MEA), where the morpholine moieties are CO2-responsive. The order in which monomers were polymerized determined the placement of the large hydrophobic RAFT end group with more complex ABA′ type self-assembly (e.g. patterned and large worm-like vesicles, large compound micelles) obtained when the dodecyl trithiocarbonate end-group was attached to the hydrophilic poly(MEA) block. Cleaving the hydrophobic end group reverts self-assembly to simpler spherical vesicles observed in the triblock of the same chemical composition, but with the RAFT end group attached to the hydrophobic poly(tert-butyl acrylamide) block. Ionization of the hydrophilic poly(MEA) block through flushing with CO2 irreversibly shifts self-assembly towards lower order morphologies with spherical micelles being more favoured than vesicles.

Polymerization-Induced Self-Assembly: The Effect of End Group and Initiator Concentration on Morphology of Nanoparticles Prepared via RAFT Aqueous Emulsion Polymerization.

Polymerization-induced self-assembly (PISA) is a widely used technique for the synthesis of nanoparticles with various morphologies including spheres, worms, and vesicles. The development of a PISA formulation based on reversible addition-fragmentation chain transfer (RAFT) aqueous emulsion polymerization offers considerable advantages such as enhanced rate of polymerization, high conversion and environmentally friendly conditions. However, this formulation has typically produced spheres as opposed to worms and vesicles. Herein, we report the formation of vesicle morphology by increasing the RAFT end-group hydrophobicity of the macromolecular chain transfer agent or manipulating the radical initiator concentration used in the aqueous emulsion polymerization PISA formulation. Additionally, decreasing the molecular weight of the hydrophobic polystyrene domain in these vesicles leads to the formation of worms. This work demonstrates that RAFT end-group hydrophobicity and radical initiator concentration are key parameters which can be exploited to enable access to sphere, worm, and vesicle morphologies via the RAFT aqueous emulsion polymerization.

Reversible addition-fragmentation chain transfer polymerization of alkyl-2-cyanoacrylates: An assessment of livingness

Alkyl 2-cyanoacrylates (CAs) are primarily used as instant adhesives, including those sold under the Loctite brand. The adhesive action can be inhibited with acid stabilizers allowing radical polymerization to be employed. The following article details the first attempted controlled/living radical polymerization of alkyl CAs: Reversible addition fragmentation chain transfer (RAFT) polymerization mediated by a poly(methyl methacrylate) dithiobenzoate macroRAFT agent for three different CA monomers (ethyl 2-cyanoacrylate, n-butyl 2-cyanoacrylate, and 2-phenylethyl cyanoacrylate) allowed the preparation of the first block copolymers of this challenging but commercially important monomer class. Nevertheless, GPC with UV detection indicated significant loss of the RAFT end-group for all three CAs limiting control/living character. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 1397–1408

Selective Uptake of a Fructose Glycopolymer Prepared by RAFT Polymerization into Human Breast Cancer Cells

A new methacrylic fructose glycomonomer is synthesized and copolymerized with N-isopropyl acrylamide by reversible addition fragmentation chain transfer (RAFT) poly-merization. By additional copolymerization of the analog mannose, glucose, and galactose glycomonomers, a set of glycopolymers is obtained which vary in the type of sugar attached to the polyacrylamide backbone. The glycopolymers are subsequently deprotected and characterized by size exclusion chromatography, FT-IR and NMR spectroscopy, elemental analysis, as well as turbidimetry, revealing the thermoresponsive character of all synthesized glycopolymers. The deprotected glycopolymers are subsequently labeled with a Rhodamine B derivative, utilizing the thiol-functionalities derived from the RAFT endgroups. As concluded from the ArlamaBlue assay, the glycopolymers are not cytotoxic. Finally, cellular uptake studies reveal a higher uptake of the fructose polymer into MDA-MB-231 breast cancer cells compared to the other glycopolymers, which demonstrates the high potential of fructosylated polymers for potential applications in the targeted treatment of breast cancer.

One-Pot Preparation of Inert Well-Defined Polymers by RAFT Polymerization and In Situ End Group Transformation.

A one-pot procedure that straightforwardly combines reversible addition-fragmentation chain transfer (RAFT) polymerization and end group transformation to remove the RAFT end groups is developed for the synthesis of well-defined poly(meth)acrylates and polyacrylamides with inert end groups. This procedure only requires the addition of an amine at the end of the standard RAFT polymerization procedure, which avoids the separation and purification of the intermediate polymers and, hence, extremely reduces the working time and utilized amount of solvents. Upon addition of the amine, a thiol group is formed by aminolysis of the thiocarbonylthio group, which subsequently undergoes Michael addition with unreacted monomer leading to an inert thioether functionalized polymer.

RAFT preparation and the aqueous self-assembly of amphiphilic poly(octadecyl acrylate)- block -poly(polyethylene glycol methyl ether acrylate) copolymers

Reversible addition-fragmentation chain transfer (RAFT) polymerisation was used to prepare novel amphiphilic poly(octadecyl acrylate)-block-poly(polyethylene glycol methyl ether acrylate) (P(ODA)-b-P(PEGA-OMe)) copolymers. These polymers were reduced to remove the dodecyl RAFT end-group, via radical-induced reduction using a hypophosphite salt, N-ethylpiperidine hypophosphite (EPHP). The resulting brush-like diblock copolymers were characterised by proton nuclear magnetic resonance (1H NMR) and gel permeation chromatography (GPC). Investigation of the self-assembly behaviour of both the reduced and RAFT-group functional polymers in excess water determined by dynamic light scattering (DLS), cryo-transmission electron microscopy (cryo-TEM) and cross-polarised light microscopy (CPLM), indicated that all the copolymers form micellar aggregates in excess water. The hydrophobic dodecyltrithiocarbonate RAFT end-group, which is located on the terminal end of the hydrophilic domain, was found to influence the self-assembly behaviour of the copolymers in water.

Sequential flow process for the controlled polymerisation and thermolysis of RAFT-synthesised polymers

The RAFT (Reversible Addition-Fragmentation Chain Transfer) process greatly enhances the control over radical polymerisations, while leaving behind a thiocarbonylthio end-group. Thermolysis presents a convenient and efficient way of removing the thiocarbonylthio end-group from RAFT polymers, without the use of additional reagents. This paper describes a simple two-step flow process for the synthesis of RAFT polymers followed by the subsequent removal of the RAFT end-group via thermolysis, without the need for isolating intermediates. A range of different polymers based on styrene, acrylates, methacrylates and acrylamides were synthesised with different RAFT agents and successfully tested for thermolysis at temperatures between 220 and 250 °C in a stainless steel tube flow reactor, resulting in complete removal of thiocarbonylthio end-groups.

Synthesis of alkyne functional cyclic polymers by one-pot thiol–ene cyclization

We demonstrate the versatility of polymers made by reversible addition–fragmentation chain transfer (RAFT) to be cyclized in one-pot using the thiol–ene reaction. Hexylamine was used to convert the RAFT end-group to a free thiol which rapidly reacted with the acrylate on the other chain-end of the polymer to form a cyclic polymer. This procedure allowed a wide range of polymers (polystyrene, poly(tert-butyl acrylate), poly(N-isopropylacrylamide) and poly(N,N-dimethylacrylamide)) to be cyclized with close to 80% cyclic formation. These cyclic polymers all contained a reactive alkyne functional group. Attempts to use the thio–bromo reaction to cyclize polystyrene were not as successful as the thiol–ene cyclization reaction, producing less than 25% cyclic.

Macromol. React. Eng. 6–7/2012

Cover: The Reversible Addition-Fragmentation chain Transfer (RAFT) method is a convenient and versatile process to control free radical polymerization. A key feature of this process is that the so-called RAFT end groups are retained in the polymer product, which may be unfavorable in certain applications. One of the methods for removing the RAFT end group is based on the radical-induced reduction of the end group in the presence of an H-atom source. Herein we describe a novel continuous flow process for this radical-induced reduction. Further details can be found in the article by C. H. Hornung,* A. Postma, S. Saubern, J. Chiefari* on page 246.

From NMP to RAFT and Thiol‐Ene Chemistry by In Situ Functionalization of Nitroxide Chain Ends

A straightforward, novel strategy based on the in situ functionalization of polymers prepared by nitroxide-mediated polymerization (NMP), for the use as an extension toward block copolymers and post-polymerization modifications, has been investigated. The nitroxide end group is exchanged for a thiocarbonylthio end group by a rapid transfer reaction with bis(thiobenzoyl) disulfide to generate in situ reversible addition-fragmentation chain transfer (RAFT) macroinitiators. Moreover, not only have these macroinitiators been used in chain extension and block copolymerization experiments by the RAFT process but also a thiol-terminated polymer is synthesized by aminolysis of the RAFT end group and subsequently reacted with dodecyl vinyl ether by thiol-ene chemistry.

Preparation of macro reversible addition-fragmentation chain transfer copolymers and their application in pigment dispersion

By using reversible addition–fragmentation chain transfer (RAFT) controlled polymerization, we prepared styrene–maleic anhydride copolymers with RAFT end groups. The resulting copolymers were characterized by Fourier transform infrared and 1H-NMR spectroscopy. The molecular weights and molecular weight distributions were determined with gel permeation chromatography. The dispersing ability of the hydrolyzed macro reversible addition–fragmentation chain transfer (macro-RAFT) copolymers was evaluated. An organic phthalocyanine blue pigment (P.B.15 : 3) was dispersed with the macro-RAFT copolymers by vigorous stirring and ultrasonic processing. The properties (particle size and polydispersity index) of the pigment dispersions were measured. The results show that with prolonged ultrasonication time, the particle size and polydispersity index decreased first and increased slightly later. There were no remarkable differences in the particle size and particle size distribution between the macro-RAFT copolymer and poly(styrene-co-maleic anhydride) dispersed pigment dispersions. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Switchable Reversible Addition–Fragmentation Chain Transfer (RAFT) Polymerization in Aqueous Solution, N,N-Dimethylacrylamide

Reversible addition–fragmentation chain transfer (RAFT) polymerization of a “more activated monomer” (MAM), N,N-dimethylacrylamide (DMAm), has been successfully achieved in aqueous solution using an acid/base switchable pyridyl-substituted dithiocarbamate RAFT agent. The effect of strength (pKa) and stoichiometry of the acid used to switch the RAFT agent on the molecular weight and dispersity of poly(N,N-dimethylacrylamide) (PDMAm) were examined, making use of high-throughput protocol. Best control was achieved with a stoichiometric amount of the strongest acid investigated, p-toluenesulfonic acid monohydrate. Use of weaker acids (higher pKa) or less than stoichiometric amounts of acid with respect to RAFT agent resulted in broader molecular weight distributions. These results can be rationalized in terms of the extent of protonation of the pyridine nitrogen of the RAFT agent (pKa ∼ 3.13). The preparation of unimodal low dispersity block copolymers of PDMAm with the “less-activated” monomers (LAMs) N-vinylcarbazole (NVC), vinyl acetate (VAc), and N-vinylpyrrolidone (NVP) demonstrated the fidelity of the RAFT end group in these experiments and provides a further example of the utility of switchable RAFT polymerization.

Self‐Assembly of Hydrophilic Homopolymers: A Matter of RAFT End Groups

Unusual self-assembly behavior is observed for a range of hydrophilic homopolymers. This self-assembly behavior is contrary to the expected behavior of such hydrophilic polymers and instead mimics more commonly reported amphiphilic block copolymers. It is proposed that the unique combination of hydrophobic end groups at both the α and ω chain end accounts for this unusual self-assembly behavior. Complex internal polymer micelles are spontaneously formed when hydrophilic homopolymer polyelectrolytes and neutral polymers (with a weight fraction of the hydrophobic end groups <10 wt%) are directly dissolved in water. The homopolymers, poly[2-(diethylamino)ethyl methacrylate], poly(N-isopropylacrylamide), and poly(ethoxyethylacrylate) are synthesized by reversible addition-fragmentation chain-transfer (RAFT) polymerization using S'-1-dodecyl-(S')-(α,α'-dimethyl-α″-acetic acid) trithiocarbonate (DDMAT) and its derivatives as chain transfer agents (CTAs). A range of polyelectrolyte homopolymers with different terminal groups are designed and synthesized, which under acidic aqueous solution direct the self-assembly to form well-defined nanostructures. This assembly behavior was also observed for neutral polymers, and it was determined that the structure of the hydrophobic end groups (and thus choice of RAFT CTA) are very important in facilitating this unusual self-assembly behavior of hydrophilic homopolymers. It is proposed that the functionality of commonly used CTAs such as DDMAT, can affect the solution association of the resultant homopolymers and can in fact afford ABA' type polymers, which can undergo self-assembly to form higher-order nanostructures.

Characterization of New Amphiphilic Block Copolymers of N‐Vinyl Pyrrolidone and Vinyl Acetate, 1 – Analysis of Copolymer Composition, End Groups, Molar Masses and Molar Mass Distributions

AbstractNew amphiphilic block copolymers consisting of N‐vinyl pyrrolidone and vinyl acetate were synthesized via controlled radical polymerization using a reversible addition/fragmentation chain transfer (RAFT)/macromolecular design via the interchange of xanthates (MADIX) system. The synthesis was carried out in 1,4‐dioxane as process solvent. In order to get conclusions on the mechanism of the polymerization the molecular structure of formed copolymers was analysed by means of different analytical techniques. 13C NMR spectroscopy was used for the determination of the monomer ratios. End groups were analysed by means of matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry. This technique was also used to determine possible fragmentations of the RAFT end groups. By means of a combination of size exclusion chromatography, 13C NMR and static light scattering molar mass distributions and absolute molar masses could be analysed. The results clearly show a non‐ideal RAFT mechanism.magnified image

Synthesis of Core - Shell Nanoparticles with Polystyrene Core and PEO Corona from Core-Crosslinked Micelles by the RAFT Process

Core–shell nanoparticles have been synthesized by core crosslinking of micelles. The underlying block copolymer, poly(oligo(ethylene glycol methyl ether methacrylate))-block-polystyrene (POEGMA-b-PS), was synthesized successfully by the reversible addition–fragmentation chain transfer (RAFT) process, using POEGMA as a macro-RAFT agent. The block copolymers were self-assembled into micelles in aqueous media and the resulting micelles and the RAFT endgroup, located in the core of the micelle, were used for the subsequent crosslinking step using a crosslinker, divinyl benzene (DVB). The rate of the crosslinking reaction was found to be slow with less than 20% conversion being achieved after 72 h. Nevertheless, crosslinked micelles were obtained and only a small fraction of free block copolymers remained. Cytotoxicity tests confirmed the biocompatibility of the prepared core-crosslinked micelles. In addition the crosslinked micelles were taken up by L929 cells without causing any signs of cell damage.