Reversible Addition-fragmentation Chain Transfer Process Research Articles

A RAFT Analogue Olefin Polymerization Technique Using Coordination Chemistry

The present paper highlights analogies between one of the most efficient control radical polymerization techniques namely the reversible addition–fragmentation chain transfer process, and the catalyzed polyethylene chain growth, the only technique that can controlled olefins polymerization through coordination chemistry under catalytic conditions.

Synthesis of Heterotelechelic α,ω Dye-Functionalized Polymer by the RAFT Process and Energy Transfer between the End Groups

The synthesis of a vinyl polymer with two different fluorescent dye end groups using reversible addition-fragmentation chain transfer (RAFT) polymerization is described. Use of a pentafluorophenyl (PFP) activated ester chain transfer agent (CTA) provided a polymer with an R end group that was reactive toward amines and a dithioester ω end group. The R PFP ester was amidated with Oregon Green Cadaverin. This did not harm the ω dithioester, which was subsequently aminolyzed with an excess of n-propylamine in the presence of Texas Red-2-sulfonamidoethyl methanethiosulfonate, resulting in a disulfide bond connecting the second dye to the polymer chain. Excess dyes and side products were removed by thin layer chromatography (TLC). Gel permeation chromatography (GPC) using a UV-vis detector could verify the presence of each dye on the polymer chain and the absence of free dyes. The synthesis of the polymer by a living radical technique and the mild complementary conjugation methods conducted after polymerization at each end group allowed to introduce complex dye residues possessing high brightness and photostability. In particular, fluorescent dyes capable of acting as donor and acceptor for electronic excitation energy transfer were chosen. Time-resolved fluorescence measurements were used to determine the time constant of energy transfer between the end groups of isolated polymer chains. Assuming a Forster-type process, an average end- € to-end distance of 4.5 nm was calculated, which was in reasonable agreement with data obtained from light scattering.

A Strategy for Synthesis of Magnetic Nanoparticles with "Well-Defined" Polymers via Reversible Addition Fragmentation Chain Transfer Polymerization Under Ultrasonic Irradiation

The controlled graft modification of Fe3O4 magnetic nanoparticles has been achieved by reversible addition fragmentation chain transfer (RAFT) polymerization under ultrasonic irradiation. The nanoparticles of Fe3O4 were first prepared by a chemical co-precipitation method, and reacted with 3-aminopropyltriethoxylsilane (KH550), and subsequently with S-1-dodecyl-S'-(alpha,alpha'-dimethyl-alpha'-acetic acid) trithiocarbonate (DDACT) to serve as RAFT agent. The graft polymerization of methyl acrylate was carried out under ultrasonic irradiation (28 KHz, 60 W). The first-order kinetics and the polymers with narrow molecular weight distributions suggested the polymerization proceeded via RAFT process. The resultant products were characterized by Fourier transform infrared spectrometer (FT-IR), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), high performance particle sizer (HPPS) and Gouy magnetic balance. The resultant products with good paramagnetism could be well modified with the "well-defined" polymers via RAFT polymerization under ultrasonic irradiation, thereby providing a new method to well modify magnetic nanoparticles.

A Facile Approach for Preparation of Phenylphosphinic Acid-Functionalized PSt Microspheres by Emulsion Polymerization Using Amphiphilic Macro-RAFT Agent as Emulsifier

The phenylphosphinic acid-functionalized microspheres were successfully prepared by “one-pot” emulsion polymerization of styrene/divinylbenzene with a functional macromolecular reversible addition−fragmentation chain transfer (RAFT) agent, poly(3-[2-(acryloyloxy)ethoxy]-3-oxopropyl(phenyl)phosphinic acid)-block-polystyrene (P(AOPA)-b-PSt), as an amphiphilic emulsifier. The monodisperse microspheres were obtained with about 300 nm of the diameters and the clear core−shell structures, while phenylphosphinic acid groups were introduced into the shell via the RAFT process. The functionalized microspheres show high ability to coordinate with Cu2+ or Fe3+.

RAFT Polymerization of Vinylthiophene Derivatives and Synthesis of Block Copolymers Having Cross-Linkable Segments

The polymerization of three vinylthiophene derivatives, 2-vinylthiophene (2VT), 3-vinylthiophene (3VT), and 2,5-dibromo-3-vinylthiophene (DB3VT), was carried out by reversible addition−fragmentation chain transfer (RAFT) process using six different chain transfer agents (CTAs). The novel doubly polymerizable monomer, DB3VT, undergoes controlled radical polymerization via the RAFT process, followed by Suzuki coupling reaction. Two dithiobenzoate-type RAFT agents, phenylethyl dithiobenzoate (CTA 2) and cumyl dithiobenzoate (CTA 3), were the most efficient to obtain poly(DB3VT) with controlled molecular weights and low polydispersities (Mw/Mn = 1.05−1.15). Good control of the polymerization of DB3VT was confirmed by the linear increase in the molecular weight with the conversion and the ability to extend the chain by a second addition of the monomer. Chain extension from poly(methyl methacrylate) to DB3VT could be well controlled under suitable conditions and provided block copolymers having cross-linkable p...

Click chemistry meets polymerization: Controlled incorporation of an easily accessible ruthenium(II) complex into a PMMA backbone via RAFT copolymerization

The copper(I)-catalyzed azide-alkyne cycloaddition provided an easy and efficient access to a functionalized heteroleptic ruthenium(II) complex monomer. A grafted copolymer with the heteroleptic ruthenium(II) complex and methyl methacrylate (MMA) as comonomer was synthesized by reversible addition fragmentation chain transfer (RAFT) polymerization. The product was characterized by means of 1H NMR spectroscopy, UV/vis spectroscopy and size exclusion chromatography coupled with a photodiode array detector. The RAFT process itself led to a grafted copolymer with a low polydispersity index.

Preparation of nanocomposites by reversible addition‐fragmentation chain transfer polymerization from the surface of quantum dots in miniemulsion

AbstractHerein, we report the synthesis of quantum dots (QDs)/polymer nanocomposites by reversible addition‐fragmentation chain transfer (RAFT) polymerization in miniemulsions using a grafting from approach. First, the surfaces of CdS and CdSe QDs were functionalized using a chain transfer agent, a trisalkylphosphine oxide incorporating 4‐cyano‐4‐(thiobenzoylsulfanyl)pentanoic acid moieties. Using a free radical initiator (AIBN) to activate the RAFT process, a polystyrene (PS) block was grafted from the surface of the QDs. Quantum confinement effects were identified for the nanocomposite obtained, so attesting to the integrity of the QDs after the polymerization. Free PS chains were also present in the final nanocomposite, indicating that the RAFT polymerization from the surface of the QDs was accompanied by conventional free radical polymerization. After isolating the nanocomposite particles, a second poly(n‐butyl acrylate) block was tentatively grown from the initial PS block. The first results indicated a successful polymerization of the second polymer and show the potential of the current strategy to prepare block copolymers from the surface of the RAFT‐modified QDs. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5367–5377, 2009

Microwave assisted synthesis of high molecular weight polyvinylsilazane via RAFT process

Abstract The microwave-assisted synthesis of high molecular weight polyvinylsilazane (H-PVSZ) leads to a narrow molecular weight distribution at accelerated polymerization rates under controlled/living polymerization, while retaining the excellent controllability offered by reversible addition fragmentation chain transfer (RAFT) polymerization in a thermal reaction process. Under microwave irradiation, higher molecular weight H-PVSZ (6250 vs. 4370) was obtained with higher conversion (86.7 vs. 30.1%) than under conventional heating through a simple 3 h reaction of vinylcyclicsilazane with a molecular weight of 314 in the presence of dithiocarbamate derivatives (RAFT agent) and 2,2′-azobis-isobutyronitrile (AIBN, thermal initiator). H-PVSZ with a well-controlled high molecular weight is useful for synthesizing inorganic–organic diblock copolymer polyvinylsilazane-block-polystyrene (PVSZ-b-PS) with a higher molecular weight and lower polydispersity than by conventional heating of the PVSZ-derived product.

Evaluation of the clenbuterol imprinted monolithic column prepared by reversible addition-fragmentation chain transfer polymerization

To make more homogenous organic monolithic structure, reversible addition-fragmentation chain transfer (RAFT) process was employed in the synthesis of the clenbuterol imprinted polymer. In the synthesis, the influence of synthetic conditions on the polymer structure and separation efficiency was studied. The result demonstrated that the imprinted columns prepared with RAFT process have higher column efficiency and selectivity than the columns prepared with conventional polymerization in the present study, which may result from the higher surface area, smaller pore size and the narrower globule size distribution in their structures. The result indicated that RAFT polymerization provided better conditions for the clenbuterol imprinted monolithic polymer preparation.

RAFT polymerization kinetics: How long are the cross‐terminating oligomers?

AbstractWe extend a new model for the kinetics of reversible addition‐fragmentation chain transfer (RAFT) polymerization. The essence of this model is that the termination of the radical intermediate formed by the RAFT process occurs only with very short oligomeric radicals. In this work, we consider cross‐termination of oligomers up to two monomers and an initiator fragment. This model accounts for the absence of three‐armed stars in the molecular weight distribution, which are predicted by other cross‐termination models, since the short third arm makes a negligible difference to the polymer's molecular weight. The model is tested against experiments on styrene mediated by cyano‐isopropyl dithiobenzoate, and ESR experiments of the intermediate radical concentration. By comparing our model to experiments, we may determine the significance of cross‐termination in RAFT kinetics. Our model suggests that to agree with the known data on RAFT kinetics, the majority of cross‐terminating chains are dimeric or shorter. If longer chains are considered in cross‐termination reactions, then significant discrepancies with the experiments (distinguishable star polymers in the molecular weight distribution) and quantum calculations will result. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3455–3466, 2009

Synthesis of hybrid TiO2 nanoparticles with well-defined poly(methyl methacrylate) and poly(tert-butyldimethylsilyl methacrylate) via the RAFT process

Surface of titania nanoparticles (TiO2) was modified by a coupling agent as 3-(trimethoxysilyl)propyl methacrylate (MPS) to form TiO2-MPS polymerizable particles. Methyl methacrylate (MMA) and tert-butyldimethylsilyl methacrylate (MASi) were radically polymerized through the immobilized vinyl bond on the surface in the presence of the reversible addition-fragmentation chain transfer (RAFT) agent 2-cyanoprop-2-yl dithiobenzoate using 2,2′-azobisisobutylnitrile (AIBN) as an initiator. FTIR spectroscopy confirmed the presence of the coupling molecule and the methacrylate groups on the surface. Thermogravimetric analysis and elemental analysis revealed a surface coverage of the coupling molecule of 2.0wt%. TGA measurements showed that grafted PMMA and PMASi were accounted for 10% and 4.8% of the particle mass, respectively. 1H NMR and SEC were used to verify the livingness of the polymerization. Transmission electron microscopy (TEM) was used to study the morphology of the particles before and after the surface grafting.

Synthesis of novel random and block copolymers of tert-butyldimethylsilyl methacrylate and methyl methacrylate by RAFT polymerization

Hydrophobic-hydrolysable copolymers consisting of methyl methacrylate (MMA) and tert-butyldimethylsilyl methacrylate (TBDMSMA) have been synthesized for the first time by Reversible Addition–Fragmentation chain Transfer (RAFT) polymerization technique using cumyl dithiobenzoate (CDB) and cyanoisopropyl dithiobenzoate (CPDB) as chain transfer agents (CTAs). The monomer reactivity ratios for TBDMSMA (r1=1.40±0.03) and MMA (r2=1.08±0.03) have been determined using a non-linear least-squares fitting method. Well-defined random copolymers PMMA-co-PTBDMSMA have been prepared. Then, the versatility of the RAFT process to synthesize silylated block copolymers with controlled molecular weights and low polydispersities has been demonstrated using two strategies: the synthesis of PMMA–SC(S)Ph or PTBDMSMA–SC(S)Ph as macro-chain transfer agent (macro-CTA) for use in a two step method or an one-pot method which consists in the successive addition of the two monomers. Diblock copolymers with narrow molecular weight distributions (PDI<1.2) were obtained from the one-pot method with number-average molecular weight values within the range 10,000–22,000gmol−1.

Controlled radical polymerization of cholesteryl acrylate and its block copolymer with styrene via the RAFT process

Reversible addition fragmentation chain transfer (RAFT) polymerization of cholesteryl acrylate (ChA) was conducted using S-1-dodecyl-S′-(α,α′-dimethyl-α′′-acetic acid)trithiocarbonate as CTA and AIBN as initiator in toluene at 80 °C. The polymerization was investigated at two different CTA concentrations (0.025 and 0.040 M). Polymerization of ChA with CTA concentration of 0.040 M proceeds in a controlled/living manner as evidenced by linear increase of the molecular weight with conversion and narrow polymer polydispersity (PDI < 1.2). With lower initial CTA concentration, namely 0.025 M, although poly(cholesteryl acrylate) (PChA) exhibiting narrow molecular weight distributions could be synthesized, the polymerization showed relatively low control with many termination products. Chain extension polymerizations were performed starting from either the PChA or the polystyrene (PS) block, and well-defined copolymers based on ChA and styrene were prepared. Thermal properties of PChA and PS- b-PChA copolymer were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), and the results showed that both PChA and PS- b-PChA are amorphous polymers. PChA begins to decompose at ca. 218 °C with maximum weight loss rate at 351 °C, while PS- b-PChA shows double weight loss rate peaks located at 345 and 415 °C, respectively.

Polygalactose Containing Nanocages: The RAFT Process for the Synthesis of Hollow Sugar Balls

Hollow poly(6-O-acryloyl-alpha-D-galactopyranose) (PAGP) nanospheres were prepared in a facile manner using the RAFT (reversible addition fragmentation chain transfer) process. Initially, an amphiphilic block copolymer, poly(lactide)-block-poly(6-O-acryloyl-alpha-D-galactopyranose) (PLA-b-PAGP), was synthesized using a poly(lactide) (PLA) macroRAFT agent. It was attained in high yields and displayed low PDI values. The block copolymers self-assembled in aqueous solution to form micelles with pendent galactose moieties covering the surface. By using hexandiol diacrylate the micelles were cross-linked at the nexus of the copolymer, creating stable aggregates. Aminolysis with hexylamine allowed the removal of the PLA core without any detrimental effect on the glycopolymer units to produce hollow nanocages. Characterization of these hollow "sugar balls" with transmission electron microscopy (TEM) showed the cross-linked micelles with a central void due to the removal of the hydrophobic block. These micelles are advantageous in drug delivery applications, especially those involving the liver, thanks to the pendent galactose functionalities covering the surfaces of the nanocages.

Orthogonal “Relay” Reactions for Designing Functionalized Soft Nanoparticles

The combination of reversible addition-fragmentation chain transfer (RAFT) chemistry followed by thiol-based "click" chemistry, known as an orthogonal "relay" reaction as one step complements the other, was used to produce surface-functionalized soft nanoparticles. Thiocarbonyl thio compounds were first used in the presence of vinyl monomers and a source of radicals to control the growth of the polymeric chains (via the RAFT process) and then reduced to thiols and utilized as a handle for functionalization of the resulting polymer chain ends (via a thiol-based click reaction). Both reactions occur under mild conditions and offer excellent control over the properties of the final product, and the thiol addition shows all the benefits of a click reaction, without requiring the use of a catalyst. This simple chemistry opens up the route to the production of a wide range of functional materials, and the concept is illustrated by the formation of nanoparticle-based gels, fluorescent-tagged particles, and protein-nanoparticle conjugates.

Living Radical Polymerization by the RAFT Process - A Second Update

This paper provides a second update to the review of reversible deactivation radical polymerization achieved with thiocarbonylthio compounds (ZC(=S)SR) by a mechanism of reversible addition–fragmentation chain transfer (RAFT) that was published in June 2005 (Aust. J. Chem. 2005, 58, 379–410). The first update was published in November 2006 (Aust. J. Chem. 2006, 59, 669–692). This review cites over 500 papers that appeared during the period mid-2006 to mid-2009 covering various aspects of RAFT polymerization ranging from reagent synthesis and properties, kinetics and mechanism of polymerization, novel polymer syntheses and a diverse range of applications. Significant developments have occurred, particularly in the areas of novel RAFT agents, techniques for end-group removal and transformation, the production of micro/nanoparticles and modified surfaces, and biopolymer conjugates both for therapeutic and diagnostic applications.

Immobilization of RAFT agents on silica nanoparticles utilizing an alternative functional group and subsequent surface‐initiated RAFT polymerization

AbstractSilica–polystyrene core‐shell particles were successfully prepared by surface‐mediated reversible addition fragmentation chain transfer (RAFT) polymerization of styrene monomer from the surfaces of the silica‐supported RAFT agents. Initially, macro‐RAFT agents were synthesized by RAFT polymerization of γ‐methacryloxypropyltrimethoxysilane (MPS) in the presence of chain transfer agents (CTAs). Immobilization of CTAs onto the silica surfaces was then performed by reacting silica with macro‐RAFT agents via a silane coupling. Grafting of polymer onto silica forms core‐shell nanostructures and shows a sharp contrast between silica core and polymer shell in the phase composition. The thickness of grafted‐polymer shell and the diameter of core‐shell particles increase with the increasing ratio of monomer to silica. A control experiment was carried out by conventional free radical emulsion copolymerization of MPS‐grafted silica and styrene under comparable conditions. The resulting data provide further insight into the chemical composition of grafted‐polymers that are grown from the silica surface through RAFT process. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 467–484, 2009

Synthesis of Well-Defined Alternating Copolymers by RAFT Copolymerization of N-Vinylnaphthalimide

A novel nonconjugated N-vinyl monomer, N-vinylnaphthalimide (NVNPI), was copolymerized with various comonomers via reversible addition-fragmentation chain transfer process. Two different chain transfer agents (CTAs), O-ethyl-S-(1-ethoxycarbonyl) ethyldithiocarbonate (CTA 1) and benzyl 1-pyrrolecarbodithioate (CTA 2), were compared for these copolymerizations with 2,2′-azobis(isobutyronitrile) as an initiator. The structures of the resulting copolymers were characterized by 1H and 13C NMR spectroscopy, suggesting the sufficient incorporation of NVNPI, when methyl acrylate and N-isopropylacrylamide (NIPAAm) were used as comonomers. The copolymerization of NVNPI and NIPAAm using CTA 2 afforded well-defined copolymers with a predominantly alternating structure, controlled molecular weights (Mn = 2000−10700), and low molecular mass distributions (Mw/Mn = 1.21−1.29). Characteristic optical properties of the naphthalimide-containing copolymer were investigated by UV−vis and fluorescence spectroscopic methods.

Z-RAFT star polymerization of styrene: Comprehensive characterization using size-exclusion chromatography

Reversible addition–fragmentation chain transfer (RAFT) polymerizations of styrene in bulk at 80°C using tri-, tetra-, and hexafunctional trithiocarbonates, in which the active RAFT groups are linked to the core via the stabilizing Z-group, were studied in detail. These Z-RAFT star polymerizations of styrene showed excellent molecular weight control up to very high monomer conversions and star sizes of more than 200kDa. The application of high pressure up to 2600bar was found to significantly increase the relative amount of living star polymer. Not even at very high monomer conversions and for large star molecules, a shielding effect of growing arms hampering the RAFT process could be identified. Absolute molecular weights of star polymers using a conventionally calibrated SEC setup were determined with high precision by using a mixture of linear and star-shaped RAFT agents. When using phenylethyl as the leaving R-group, well-defined star polymers that perfectly match the theoretical predictions were formed. However, when using benzyl as the leaving group, a pronounced impact of monomer conversion on the star polymer topology was observed and pure star polymers with the expected number of arms could not be obtained.

RAFT Polymerization Kinetics: Combination of Apparently Conflicting Models

We propose a model for the kinetics of reversible addition−fragmentation chain transfer (RAFT) polymerization. The essence of this model is that the termination of the radical intermediate formed by the RAFT process occurs only with the shortest active radicals. This model accounts for the absence of 3-armed stars predicted by other cross-termination models since the short radical makes a negligible difference to the overall molecular weight. The model is tested against experiments on styrene at 60 °C with cyanoisopropyl dithiobezoate (CPDB) as the RAFT agent. The predicted rate coefficients are consistent with slow fragmentation of the RAFT intermediate, and the overall concentration of radicals is consistent with ESR experiments. Overall, it demonstrates that the two conflicting models that have been proposed so far can actually coexist.