Reversible Addition-fragmentation Transfer Polymerization Research Articles

Homogeneous near-infrared emissive polymeric nanoparticles based on amphiphilic diblock copolymers with perylene diimide and PEG pendants: self-assembly behavior and cellular imaging application

An amphiphilic diblock copolymer, poly(perylene diimide acrylate)-block-poly(poly(ethyleneglycol)methacrylate) (PPDA-b-P(PEGMA)), has been synthesized via the reversible addition fragmentation transfer polymerization (RAFT) method. The polymer shows self-assembly behavior in water due to the synergistic effects of the strong hydrophobic interactions and π–π stacking of perylene diimide (PDI) groups. Homogeneous polymer nanoparticles (PNPs) in aqueous solution with good water solubility and stability were formed with an average size of 64.3 ± 3.3 nm, revealed by dynamic light scattering (DLS). The PNPs showed near-infrared (NIR) emission at 660 nm instead of the traditional emission of individual PDI groups at 530 nm. The aggregation-enhanced π–π stacking and the resulting NIR emission of the PDI groups were demonstrated by spectroscopy and 1H-NMR characterization. Cellular imaging of human pancreatic cancer cells was conducted with the obtained PNPs. Confocal microscopy results showed that the PNPs were located specifically within the cell cytoplasm. This study provides a new design concept to take full advantage of polymer amphipathy to fabricate nanoparticles with NIR emission for applications in bio-imaging.

Light-Harvesting Polymers: Ultrafast Energy Transfer in Polystyrene-Based Arrays of π-Conjugated Chromophores

Energy transfer along a nonconjugated polymer chain is studied with a polystyrene-based copolymer of oligo(phenylene-ethynylene) (OPE) donor and thiophene-benzothiadiazole (TBT) acceptor pendants. The graft copolymers are prepared from reversible addition-fragmentation transfer polymerization (RAFT) and copper(I)-catalyzed azide-alkyne "click" reaction. The singlet energy transfer from donor to accept is studied via fluorescence emission and ultrafast transient absorption spectroscopy. Near unity quenching of the OPE excited state by the TBT moiety occurs on multiple time scales (2-50 ps) dependent on where the initial exciton is formed on the polymer.

Self‐Assembled Microspheres Driven by Dipole‐Dipole Interactions: UCST‐Type Transition in Water

A double hydrophilic block copolymer, poly(ethylene glycol)-poly(3-dimethyl (methacryloyloxyethyl) ammonium propane sulfonate) (PEG-SB), is synthesized by reversible addition-fragmentation transfer (RAFT) polymerization using PEG methyl ether (4-cyano-4-pentanoate dodecyl trithiocarbonate) as a chain transfer agent. PEG-SB forms multi-layered microspheres with dipole-dipole interactions of the SB side chains as the driving force. The PEG-SB polymers show an upper critical solution temperature (UCST) and the UCST is controllable by the polymerization degree. The PEG-SB microspheres are dissociated above the UCST and then monodispersed microspheres (∼1 μm) are obtained when the solution temperature is decreased below the UCST again. The disassociation/association of the microspheres is also controllable using the concentration of NaCl. These multi-responsive microspheres could be a powerful tool in the field of nano-biotechnology.

MALDI of synthetic polymers with labile end‐groups

Mass spectrometry is increasingly used in the field of synthetic polymers as a fast and accurate technique for end-group analysis. More particularly, matrix-assisted laser desorption/ionization (MALDI) has gained much popularity because it allows quite simple mass spectra to be obtained, displaying a single distribution for each polymeric species present in the sample, in contrast to electrospray ionization (ESI) which readily promotes multiple charging for most polymers. A soft ionization process, ensuring the integrity of the species upon transfer into gas phase ions, is however mandatory for polymer end-group analysis since information about the chain terminations mainly rely on the m/z values measured for polymer adducts. As compared to ESI, MALDI is sometimes suspected to be a quite "hard" ionization technique, leading to spontaneous dissociation of ionized species either in the source or during their flight time. This issue is of particular concern for polymers carrying so-called fragile end-groups arising from their mode of synthesis. In particular, controlled radical polymerization (CRP) processes, one of the most important advances in the field of polymer science during the last 20 years, allow the production of polymers with well-defined molecular distribution and low polydispersities, but they are all based on the low dissociation energy of the chemical bond between the last monomer and the terminating group. As a result, if macromolecules are activated while being ionized, this end-group is prone to fragmentation and ions measured in the mass spectra do no longer reflect the original chain composition. However, different results are reported in the literature about the ability of MALDI to generate intact ions from CRP synthetic polymers. This article reviews MALDI MS data reported for synthetic polymers produced by atom transfer radical polymerization (ATRP), reversible addition-fragmentation transfer polymerization (RAFT), and nitroxide-mediated polymerization (NMP), the three most studied CRP techniques. The general principle of each polymerization process, which defines the structure of the end-groups in both targeted macromolecules and species arising from eventual side-reactions, is first briefly presented. An overview of MALDI data reported for samples obtained upon polymerization of different monomers are then commented for each polymerization techniques with regards to the success of the ionization method to generate intact cationic adducts and its propensity to distinguish in-source fragments from polymerization side-products.

Catalyst assisted synthesis of initiator attached silica monolith particles via isocyanate-hydroxyl reaction for production of polystyrene bound chromatographic stationary phase of excellent separation efficiency

Dibutyltin dichloride (DBTDC) was used as a catalyst to chemically bind 4-chloromehtylphenylisocynate (4-CPI) to porous monolithic silica particles via isocyanate-hydroxyl reaction, and the reaction product was reacted with sodium diethyldithiocarbamate (SDDC) to yield initiator attached silica monolith particles. Reversible addition-fragmentation transfer (RAFT) polymerization was taken place on them to result in polystyrene attached silica particles that showed excellent separation efficiency when packed in a chromatographic column (1.0mm×300mm). The numbers of theoretical plates (N) of 56,500 is better than those of any commercially available HPLC or UHPLC column yet.

RAFT controlled synthesis of graphene/polymer hydrogel with enhanced mechanical property for pH-controlled drug release

A pH-sensitive, mechanically strong and thermally stable graphene/poly (acrylic acid) (graphene/PAA) hydrogel was prepared via reversible addition fragmentation transfer (RAFT) polymerizations in the presence of a cross-linking agent. The RAFT agent was covalently coupled onto graphene basal planes via an esterification reaction, with benzoic acid functionalities pre-attached on graphene with its aryl diazonium salt precursor. AFM and SEM analysis revealed the successful preparation of single layered graphene sheets and graphene/polymer hydrogels with pH controlled porous structures. Attenuated total reflection infrared (ATR-IR) and thermogravimetric analyzer (TGA) verified the successful stepwise preparation of graphene/PAA hydrogel. This graphene/PAA hydrogel was pH-sensitive and more mechanically elastic than the PAA hydrogel prepared without graphene. The pH sensitivity of the hydrogel was further utilized for controlled drug release. Doxorubicin was chosen as a model drug and loaded into the hydrogels. The drug loading and release experiment indicated that this hydrogel can be used to efficiently control drug release in the intestine environment (pH=7.4), better than release in a more acidic environment.

Synthesis of click‐reactive HPMA copolymers using RAFT polymerization for drug delivery applications

ABSTRACTThis study describes a versatile strategy combining reversible addition fragmentation transfer (RAFT) polymerization and click chemistry to synthesize well‐defined, reactive copolymers of N‐(2‐hydroxypropyl)methacrylamide (HPMA) for drug delivery applications. A novel azide containing monomer N‐(3‐azidopropyl)methacrylamide (AzMA) was synthesized and copolymerized with HPMA using RAFT polymerization to provide p(HPMA‐co‐AzMA) copolymers with high control of molecular weight (∼10–54 kDa) and polydispersity (≤1.06). The utility of the side‐chain azide functionality by Cu(I)‐catalyzed azide‐alkyne cycloaddition (CuAAC) was demonstrated by efficient conjugation (up to 92%) of phosphocholine, a near infrared dye, and poly(ethylene glycol) (PEG) with different substitution degrees, either alone or in combination. This study introduces a novel and versatile method to synthesize well‐defined click‐reactive HPMA copolymers for preparing a panel of bioconjugates with different functionalities needed to systemically evaluate and tune the biological performance of polymer‐based drug delivery. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 5091–5099

P51 Polymeric micelles for controlled delivery of hydrogen sulfide

With the discovery of the important biological roles of hydrogen sulfide (H2S), therapeutic applications of this gas have attracted growing attention. For medical applications of H2S, an increasing number of H2S donors, compounds which release H2S under physiological conditions, have been developed to evaluate the therapeutic potential of H2S. However, these low molecular weight molecules have intrinsic pharmacokinetic problems including poorly-controlled tissue distribution and rapid elimination from the blood circulation which may result in adverse effects and low therapeutic efficiency for practical applications. To overcome these limitations, polymeric micelles, spherical supramolecular assemblies formed from amphiphilic block copolymers with a diameter of 10–100 nm, have been used to improve pharmacokinetics, bioavailability and stability of drugs. In view of these feature, we sought to develop a micellar H2S delivery system. Here we report on novel polymeric micelles carrying H2S-releasing anethole dithiolethione (ADT) moieties. We prepared diblock copolymers having a hydrophilic poly (ethylene glycol) block and a hydrophobic ADT-bearing H2S-releasing block using reversible addition-fragmentation transfer (RAFT) polymerization. The resulting polymers self-assembled into monodisperse micelles with a diameter of 40 nm according to dynamic light scattering (DLS). Further characterization by transmission electron microscopy (TEM) confirmed that the polymers formed spherical micelles with narrow size distributions. The in vitro experiments using murine macrophages showed that the micelles enhanced lipopolysaccharide-induced nuclear factor-κB (NF-κB) activation and proinflammatory cytokine production. MTT assay revealed that the micelles have much lower toxicity compared to the small H2S donor ADT. This micellar system may offer a safe and efficient delivery of H2S in immunotherapy.

Preparation of Well-Defined Ibuprofen Prodrug Micelles by RAFT Polymerization

Nonsteroidal anti-inflammatory drugs (NSAIDs) are widely used to treat acute pain, fever, and inflammation and are being explored in a new indication in cancer. Side effects associated with long-term use of NSAIDs such as gastrointestinal damage and elevated risk of stroke, however, can limit their use and exploration in new indications. Here we report a facile method to prepare well-defined amphiphilic diblock copolymer NSAID prodrugs by direct reversible addition-fragmentation transfer (RAFT) polymerization of the acrylamide derivative of ibuprofen (IBU), a widely used NSAID. The synthesis and self-assembling behavior of amphiphilic diblock copolymers (PEG-PIBU) having a hydrophilic poly(ethylene glycol) block and a hydrophobic IBU-bearing prodrug block were investigated. Release profiles of IBU from the micelles by hydrolysis were evaluated. Furthermore, the antiproliferative action of the IBU-containing micelles in human cervical carcinoma (HeLa) and murine melanoma (B16-F10) cells was assessed.

Polymers Containing Highly Polarizable Conjugated Side Chains as High‐Performance All‐Organic Nanodielectric Materials

AbstractThe discovery of nanodipolar π‐conjugated oligomer‐containing polymers as high performance nanodielectric materials with high permittivity and low dielectric loss over a wide range of frequency (100 Hz–4 MHz) is reported. Terthiophene‐containing methacrylate polymers are synthesized by reversible addition fragmentation transfer (RAFT) polymerization. Both X‐ray and thermal studies indicate the formation of small crystalline domains of terthiophene side chains dispersed in amorphous matrix. The highly polarizable and fast‐responsive nanodipoles from the nanoscale crystalline domains (<2 nm) are believed to dictate the performance. These polymers uniquely satisfy nanodipole architectures conjectured two decades ago to guide the design of high performance nanodielectric materials. This unprecedented approach can be generalized to a variety of π‐conjugated oligomer‐containing polymers for the development of high energy density capacitor materials.

Factors influencing the synthesis and the post-modification of PEGylated pentafluorophenyl acrylate containing copolymers

Copolymerization reactions involving oligoethylene glycol acrylate (OEGA) or diethylene glycol acrylate (DEGA) with pentafluorophenyl acrylate (PFPA) have been performed by reversible addition fragmentation transfer (RAFT) polymerization. The effect of the reaction conditions on the nucleophilic acyl substitution reactions of PFPA was studied using a model amine (furfuryl amine). The resulting PEG/PFP functional copolymers were then used as scaffolds to produce a library of polymers by reaction with a range of amines.

Effective Delivery of siRNA into Cancer Cells and Tumors Using Well-Defined Biodegradable Cationic Star Polymers

Cancer is one of the most common causes of death worldwide. Two types of cancer that have high mortality rates are pancreatic and lung cancer. Despite improvements in treatment strategies, resistance to chemotherapy and the presence of metastases are common. Therefore, novel therapies which target and silence genes involved in regulating these processes are required. Short-interfering RNA (siRNA) holds great promise as a therapeutic to silence disease-causing genes. However, siRNA requires a delivery vehicle to enter the cell to allow it to silence its target gene. Herein, we report on the design and synthesis of cationic star polymers as novel delivery vehicles for siRNA to silence genes in pancreatic and lung cancer cells. Dimethylaminoethyl methacrylate (DMAEMA) was polymerized via reversible addition-fragmentation transfer polymerization (RAFT) and then chain extended in the presence of both cross-linkers N,N-bis(acryloyl)cistamine and DMAEMA, yielding biodegradable well-defined star polymers. The star polymers were characterized by transmission electron microscopy, dynamic light scattering, ζ potential, and gel permeation chromatography. Importantly, the star polymers were able to self-assemble with siRNA and form small uniform nanoparticle complexes. Moreover, the ratios of star polymer required to complex siRNA were nontoxic in both pancreatic and lung cancer cells. Treatment with star polymer-siRNA complexes resulted in uptake of siRNA into both cell lines and a significant decrease in target gene mRNA and protein levels. In addition, delivery of clinically relevant amounts of siRNA complexed to the star polymer were able to silence target gene expression by 50% in an in vivo tumor setting. Collectively, these results provide the first evidence of well-defined small cationic star polymers to deliver active siRNA to both pancreatic and lung cancer cells and may be a valuable tool to inhibit key genes involved in promoting chemotherapy drug resistance and metastases.

Gold Nanoparticles Decorated by Amphiphilic Block Copolymer as Efficient System for Drug Delivery

A drug delivery system based on gold nanoparticles (Au NPs) stabilized by an thiol-functionalized amphiphilic diblock copolymer was developed. The copolymer, poly(ethylene glycol)-b-poly(n-butyl acrylate) (MPEG-b-PBA), was synthesized by Reversible Addition-Fragmentation Transfer (RAFT) polymerization and the thiocarbonylthio group at the end of the hydrophobic block PBA was further reduced to thiol group. Pyrene, used as a model molecule for the hydrophobic drugs, was loaded in the hydrophobic interior of the shell around Au NPs by physical entrapment. The analysis performed by UV-Vis spectrophotometer show that the obtained Au NPs-pyrene nanocomposites have good stability in the aqueous solution and high drug loading capacity in comparison with Au NPs decorated with small molecular ligands. In addition, both their loading capacity and release behavior were found closely related to the chain length of hydrophobic block. In conclusion, we report here a versatile and simple procedure to fabricate Au NP-polymer hybrids that is applicable to various amphiphilic copolymers synthesized by RAFT polymerization and useful to produce efficient Au NP based drug carriers.

Synthesis of Block Copolymers Based on N-alkyl Substituted Acrylamide via Combination of Reversible Addition-Fragmentation Transfer Polymerization and Click Chemistry

Chain transfer agents of azido terminated trithiocarbonate and alkyne terminated trithiocarbonate were synthesized by esterification reaction with trithiocarbonate (2-dodecylsulfanylthiocarbonylsulfanyl-2-methylpropionic acid, DMP). The kinetics of reversible addition-fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide (NIPAAm) with the above three kinds of thiocarbonate as RAFT agents showed that chain transfer activity of azido-DMP and alkyne-DMP was similar, which was lower than that of DMP. Homopolymer (poly(N,N-dimethylacrylamide) (PDMA-C ≡ CH)) and block copolymer (P(DMA-b-NIPAAm)-N3) were successfully prepared by RAFT polymerization with desired structure. The subsequent preparation of triblock P(DMA-b-NIPAAm-b-DMA) was carried out by the click reaction. The molecular weight of P(DMA-b-NIPAAm-b-DMA) was the summation of molecular weight of reactant polymers and the molecular weight distribution was small (PDI<1.15), proving the successful click reaction.

Emulsion polymerization of vinyl acetate using iodine-transfer and RAFT radical polymerizations

This study deals with control of the molecular weight and molecular weight distribution of poly(vinyl acetate) by iodine-transfer radical polymerization and reversible addition-fragmentation transfer (RAFT) emulsion polymerizations as the first example. Emulsion polymerization using ethyl iodoacetate as the chain transfer agent more closely approximated the theoretical molecular weights than did the free radical polymerization. Although 1H NMR spectra indicated that the peaks of α- and ω-terminal groups were observed, the molecular weight distributions show a relatively broad range (Mw/Mn = 2.2–4.0). On the other hand, RAFT polymerizations revealed that the dithiocarbamate 7 is an excellent candidate to control the polymer molecular weight (Mn = 9.1 × 103, Mw/Mn = 1.48), more so than xanthate 1 (Mn = 10.0 × 103, Mw/Mn = 1.89) under same condition, with accompanied stable emulsions produced. In the Mn versus conversion plot, Mn increased linearly as a function of conversion. We also performed seed-emulsion polymerization using poly(nonamethylene L-tartrate) as the chiral polyester seed to fabricate emulsions with core-shell structures. The control of polymer molecular weight and emulsion stability, as well as stereoregularity, is also discussed. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Z-Group Ketone Chain Transfer Agents for RAFT Polymer Nanoparticle Modification via Hydrazone Conjugation

A ketal-containing trithiocarbonyl compound has been synthesized and characterized as a chain transfer agent (CTA) in Reversible Addition Fragmentation Transfer (RAFT) polymerization. The ketal functionality does not interfere with RAFT polymerization of acrylate monomers, which proceeds as previously reported to yield macro-CTA polymers and block co-polymers. Post-polymerization ketal cleavage revealed ketone functionality at the polar terminus of an amphiphilic block co-polymer. Hydrazone-formation was facile in both organic solution as well as in aqueous buffer where polymer nanoparticle assemblies were formed, indicating a conjugation/end-functionalization yield of 40-50%. Conjugation was verified with fluorescein, biotin and Gd-DOTA derivatives, and though the trithiocarbonate linkage is hydrolytically labile, we observed stable conjugation for several days at pH 7.4. and 37°C. As expected, streptavidin binding to biotinylated polymer micelles was observed, and size-change based relaxivity increases were observed when Gd-DOTA hydrazide was conjugated to polymer micelles. Cell-uptake of fluorescently labeled polymer micelles was also readily tracked by FACS and fluorescence microscopy. These polymer derivatives demonstrate a range of potential theranostic/biotechnological applications for this conveniently accessible keto-CTA, which include ligand-based nanoparticle targeting and fluorescent/MR nanoparticle contrast agents.

Particle and breath figure formation of triblock copolymers having self‐complementary hydrogen‐bonding units

AbstractNovel triblock copolymers having self‐complementary hydrogen‐bonding units were synthesized by using reversible addition–fragmentation transfer polymerization. As characterized by dynamic light scattering and atomic force microscopy, these polymers formed noncovalently crosslinked polymer particles and showed an aggregation behavior by intermolecular and intramolecular interactions. At low concentration, polymers formed nanoparticles, and the particle diameter increased with increasing polymer concentration. Well‐ordered hexagonal microstructures were prepared by “Breath Figure” technique with the triblock copolymers. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Thermo-Reversible Hydrogel for Nucleus Pulposus Replacement: Feasibility under Static Loading in a mild Papain-induced Disk Degeneration Model

Introduction Nucleus pulposus (NP) replacement by the application of injectable hydrogels seems a straightforward approach for tissue engineering. We investigate a thermo-reversible hydrogel (TH-RHG), based on a modified polysaccharide with a thermo-reversible polyamide (poly(N-isopropylacrylamide = pNIPAM). Using “click chemistry” and reversible addition fragmentation transfer (RAFT) polymerization,2,3 the gel is made to behave as a liquid at room temperature and hardens at &gt;32°C. To inject the hydrogel, a mild papain disk degeneration model (PDDM) is employed that creates a cavity in the intervertebral disk. Here, we investigate the performance of the TH-RHG in situ in bovine IVD with preconditioned human mesenchymal stem cells (hMSC) with rhGDF-5. Materials and Methods IVDs were harvested from six calf tails, aged 6 to 9 months, which were received from a local abattoir within 3 to 5 hour of slaughter. The bovine IVDs of intermediate size (max. height of 2 cm and diameter of 1.5 cm) were used included the bony endplates and cultured in vitro for 16 days. Utilizing our previously established PDDM was necessary to create a cavity in the NP. For the papain injection, a 25 G needle was used to inject centrally the enzyme into the bovine IVD. Simultaneously, primary hMSCs expanded from bone marrow obtained from five patients undergoing spine surgery (ethically approved) were seeded at 4M cells/mL in 3-D in the TH-RHG and preconditioned with 100 ng/mL rhGDF-5 for 7 days. Afterwards, the TH-RHG was reversed to a fluid and injected into the IVD organ culture and kept under static loading of 0.1 MPa for 7 days using custom-designed specimen chambers.1 Experimental designs are as follows (i) fresh d0 control PBS injection (ii) PDDM + PBS injection (iii) PDDM + TH-RHG + bovine NPCs (Cell control) (iv) PDDM + TH-RHG + hMSC.4 Magnetic resonance imaging (MRI) was employed to image the hydrogel-filled cavity before and after loading at day 9 and day 16, and RT-PCR comparison of the hMSCs at day 1, 8, and 15 investigated the changes in gene expression. On the same days of cell sampling, cell viability (trypan blue) was determined. Sulfated glycosaminoglycan synthesis (DMMB assay) and DNA content (Picogreen assay) from anterior and posterior annulus fibrosus (AF) segments of each disk at day 16 were performed. Results MRI images confirmed a central positioning of the TH-RHG into the PDDM. A considerable drop in volume across all groups in the NP region and consequently of disk height was observed between day 9 and day 16. The RT-PCR results of injected hMSCs showed significant differences between day 8 and day 15 for versican and Sox9 relative to day 1 (Fig. 1). The results of bovine AF gene expression relative to control AF showed significant differences between groups for ACAN, Col1, Col2, IL-b1, versican, ADAMTS-4, Casp-8, MMP13, and MMP3. Cell viability of injected cells dropped from around ∼100% down to ∼87% for bovine cells and ∼86% for hMSC after 7 days in 3D culture. It further dropped to ∼72% after organ culture. GAG/DNA ratio showed significant difference across groups irrespective of posterior or anterior location. Conclusion MRI imaging of TH-RHG seems a promising protocol for noninvasively observing the gel performance in vitro. 10% TH-RHG is a feasible scaffold for in vitro 3D preconditioning of hMSCs and can be used at the same time for direct injection into the IVD. However, the TH-RHG has been shown to be unable to bare static load leading to a drop of cell viability. Relative gene expression of the hMSC phenotype after “co-culture”, however, points towards “discogenic” differentiation.5 Acknowledgements This project is funded by HansJörg Wyss Start up Award 2011/Project no. 102646/AOSpine International. I confirm having declared any potential conflict of interest for all authors listed on this abstract No Disclosure of Interest None declared Chan, et al. Spine 2011. Mortisen, et al. Biomacromolecules 2010;11(5):1261–1272 Mortisen, et al. PCT2009, 3-3-2009 2009;141 Gantenbein-Ritter, et al. European Spine Journal 2011;20:962–971 Peroglio, et al. European Spine Journal 2011;epub

Photo-Induced Macromolecular Functionalization of Cellulose via Nitroxide Spin Trapping

Cellulose is the most abundant organic raw material on the planet. Due to its renewability and biodegradability it is currently attracting much interest for the production of biofuels or platform chemicals. In addition, recent applications in the field of materials science have appeared, arising from the low density and the excellent thermal and mechanical properties of cellulose, particularly in the production of composites. However, there are still some major drawbacks to using cellulose including its water-absorbing nature and its poor compatibility with other materials, for example, synthetic polymers. To combat these problems, grafting synthetic polymers onto cellulose is the most straightforward method to alter its surface properties and thus to control the wettability, adhesion, or hydrophobicity of the biopolymer. Although grafting from methods such as surface-initiated nitroxide-mediated polymerization (NMP), atom transfer radical polymerization (ATRP),reversible addition–fragmentation transfer polymerization (RAFT), or ring-opening polymerization (ROP)are considered as the most efficient approaches, particularly in terms of grafting density, efficient grafting to methods have recently produced very good results. Cellulose has been successfully modified with preformed polymers by both hetero-Diels–Alderand 1,3-dipolar nitrile imine-ene cycloadditions. In the latter case, light was used as the grafting trigger. Importantly, employing light offers temporal and spatial control of the reaction. In the present contribution, we introduce a very facile grafting to protocol based on the generation of radicals at the surface of cellulose by mild UV irradiation (λmax ∼ 311 nm) of an immobilized photoinitiator, followed by radical trapping with a nitroxide-functionalized polymer (see Scheme 1). Previously, nitroxide radical coupling was employed to efficiently link polymer strands, however via a copper-catalyzed mechanism using ATRP-made polymers to generate reactive radicals. A rather similar philosophy based on spin capturing was reported, employing nitrones that after a first radical reaction generate a nitroxide able to undergo a second radical coupling.

Fabrication and characterization of thermoresponsive Fe3O4@PNIPAM hybrid nanomaterials by surface-initiated RAFT polymerization

A versatile route applied to the synthesis of thermoresponsive magnetite nanoparticles involved the formation of nanoparticles by coprecipitation of Fe2+/Fe3+ in the presence of an alkaline solution, followed by attachment of the reversible addition-fragmentation transfer (RAFT) agents onto the surface of the Fe3O4 nanoparticles via the electrostatic interactions, subsequent grafting from polymerization of N-isopropylacrylamide (NIPAM) through surface-initiated RAFT polymerization. The surface-initiated RAFT polymerization can be conducted in a well-controlled manner, as revealed by the linear kinetic plot, linear evolution of number-average molecular weights (M n ) versus monomer conversions, and the relatively narrow molecular weight distributions (M w /M n < 1.5) of the grafted PNIPAM chains. The as-synthesized hybrid nanoparticles showed thermoresponsive behavior and were characterized by FTIR, XPS, TGA, VSM, DLS, and TEM, etc.