RAFT controlled synthesis of six-armed biodegradable star polymeric architectures via a ‘core-first’ methodology

Six-armed biodegradable star polymers made from polystyrene (polySt), poly(polyethylene glycol) acrylate (polyPEG-A) and the block copolymer, polySt-b-polyPEG-A were synthesized using a ‘core-first’ methodology via RAFT polymerization. Disulfide linkages between the core and the arms conferred biodegradability on the stars. The star architectures were found to degrade rapidly on treatment with DL-dithiothreitol (DTT) and degrade more slowly in the presence of glutathione (GSH), the most abundant intracellular thiol tethered peptide. These star polymers were well characterized using gel permeation chromatography (GPC), nuclear magnetic resonance (NMR), electrospray ionization mass spectroscopy (ESI-MS) and dynamic light scattering (DLS).     

Synthesis of well-defined star polymers and star block copolymers from dendrimer initiators by atom transfer radical polymerization

Novel polyarylether dendrimers with 1,3,5-tri(4-hydroxyphenoxy)benzene core, polybenzylether interior, and benzyl 2-bromoisobutyrate surface group (CMGn-Br, n=1-3, with functionality of 6, 12, and 24, respectively) were prepared by convergent procedure. ATRP of tert-butyl acrylate (tBA) and styrene (St) with CMGn-Br dendrimer initiators in the presence of CuBr/pentamethyldiethylenetriamine catalytic system was investigated in detail, and a series of well-defined dendrimer-like star PtBA and PSt with precise arm numbers were synthesized under suitable conditions. The quantitative initiation of the dendrimer initiators was demonstrated by high initiation efficiency, ¹H NMR spectra, hydrolysis, and MALLS/SEC approach. Star block copolymers comprising PSt and PtBA segments with low polydispersity (1.08<M_w/M_n<1.18) were also successfully synthesized using functional macroinitiators by block copolymerization. In addition, the thermal properties of the resultant polymers were characterized by DSC and TGA.     

Synthesis of graphene-polystyrene nanocomposites via RAFT polymerization

Synthesis of graphene-polymer nanocomposites is of current interest due to their exceptionally physical and chemical properties. However, the ability to produce conductive inks/coatings retaining the properties of the graphene is still a major challenge. In this study, functionalized polydopamine-coated reduced graphene oxide (PDA/RGO) was reacted with a RAFT agent, 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT), to form macro-RAFT agents via an esterification reaction. The chemistry and kinetics of RAFT living/controlled polymerization of styrene was examined from the surface of these macro-RAFT agents to form polystyrene-grafted PDA/RGO (PS-g-PDA/RGO). By examination of the kinetics and GPC traces of the free and grafted polymer, living/controlled polymerization was confirmed. To examine the utility of this approach, the synthesized PS-g-PDA/RGO samples containing different amounts of grafted PS were employed in the preparation of graphene-PS nanocomposites by mixing them with commercial PS. Graphene was found to be well distributed in the PS matrix by this approach, increasing the decomposition temperature range. This research provides an efficient route towards the design and development of value-added functional graphene-PS composites.     

RAFT polymerization and thiol-ene modification of 2-vinyloxyethyl methacrylate: Towards functional branched polymers

A vinyl functional polymer, viz, poly(vinyloxyethyl methacrylate) (poly(VEMA) was synthesized by the RAFT polymerization of an asymmetric divinyl monomer, VEMA. This polymer, with pendant vinyloxyl groups, was subsequently reacted with three thiol compounds; 2-mercaptoethanol, cysteamine and 3-mercaptoproponic acid via the thiol-ene reactions. The resulting branched polymers contained hydroxyl, amino and carboxylic acid functionalities suitable for further reactions and conjugations.     

Synthesis and photoluminescent property of star polymers with carbzole pendent and a zinc porphyrin core by ATRP

Styrene-type monomer 9-(4-vinylbenzyl)-9H-carbazole (VBCz) and methacrylate-type monomer 2-(9H-carbazole-9-yl)-ethyl methacrylate (CzEMA) were polymerized to star polymers respectively via atom transfer radical polymerization (ATRP) using zinc 5,10,15,20-tetrakis(4-(2-methyl-2-bromopropoxy) phenyl) porphyrin as an initiator. The emission spectra of two star polymers (star poly(VBCz) and star poly(CzEMA)) in the solid state displayed red light emission, while those of two monomers showed blue light emission. The result demonstrates that effective energy transfer occurs from the carbazole to the Zn porphyrin core. However, two star polymers in DMF solution emit week red light and strong UV light at 350-400 nm, it points that energy transfer cannot occur from the carbazole to the Zn porphyrin core effectively. They exhibit good thermal stability with T_{d poly(VBCz)} = 374 °C and T_{d poly(CzEMA)} = 297 °C at 5% weight loss. The DSC curves show that the glass transition temperature of styrene-type (T_{g poly(VBCz)} = 177 °C) was better than that of methacrylate-type (T_{g poly(CzEMA)} = 148 °C).     

Synthesis and photoresponsive behavior of the high-Tg azobenzene polymers via RAFT polymerization

Well-defined photo-responsive alternating copolymers, poly(4-(N-maleimido)azobenzene-alt-styrene)s (PMSts), were successfully synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization. A divinyl monomer was used in this polymerization to prepare high molecular weight azobenzene polymers. These polymers had good solubility in most organic solvents, formed films well, and had high glass transition temperatures (T_g = 174–250 °C) and were heat resistant (T_d > 320 °C). The photo-induced trans–cis isomerization of the copolymers was examined in chloroform solution. Surface-relief-gratings (SRGs) formed on the polymer films were also investigated using illumination from a linearly polarized Kr⁺ laser beam.     

Alternate and random (co)polymers composed of anthracene and chloromethylstyrene units through controlled radical ring-opening polymerization: Synthesis,post-functionalization,and optical properties

A cyclic monomer containing the chloromethyl unit 10-methylene-9,10-dihydroanthryl-9-spiro(4′-chloromethylphenyl)cyclopropane (MDCMS) was polymerized using a controlled radical ring-opening polymerization via a reversible addition-fragmentation chain transfer (RAFT) process to afford a nonconjugated alternate polymer composed of anthracene and chloromethylstyrene (CMS) units. Well-defined random copolymers were obtained through the ring-opening RAFT copolymerization. Various functional groups were incorporated into the alternate polymer. The alternate polymer containing imidazole rings exhibited fluorescence quenching as a result of charge transfer. Fluorescence resonance energy transfer (FRET) was observed in the alternate polymers containing naphthalene and thiophene rings. The random copolymers obtained by copolymerization followed by post-functionalizations exhibited characteristic optoelectronic properties that differed from those of the alternate polymers.     

Synthesis and characterization of azobenzene-functionalized poly(styrene)-b-poly(vinyl acetate) via the combination of RAFT and “click” chemistry

Here, we described a strategy for preparing well-defined block copolymers, poly(styrene)-b-poly(vinyl acetate) (PS-b-PVAc), containing middle azobenzene moiety via the combination of the reversible addition-fragmentation chain transfer (RAFT) polymerization and “click” chemistry. Firstly, a novel RAFT agent containing α-alkyne and azobenzene chromophore in R group, 2-(3-ethynylphenylazophenoxycarbonyl)prop-2-yl-9H-carbazole-9-carbodithioate (EACDT), was synthesized and used to mediate the RAFT polymerization of styrene (St). Well-defined α-alkyne end-functionalized poly(styrene) (PS) was obtained. Secondly, the RAFT polymerization of vinyl acetate (VAc) was conducted using functionalized RAFT reagent with ω-azide structure in Z group, O-(2-azidoethyl) S-benzyl dithiocarbonate (AEBDC). Well-defined ω-azide end-functionalized poly(vinyl acetate) (PVAc) was obtained. Afterwards, the resulting α-alkyne terminated PS was coupled by “click” chemistry with the azide terminated PVAc. The block copolymer, PS-b-PVAc, was obtained with tailored structures. The products from each step were characterized and confirmed by GPC, ¹H NMR, IR and differential scanning calorimetry (DSC) examination. Kinetics of the trans-cis-trans isomerization from azobenzene chromophore in PS-b-PVAc and PS were investigated in CHCl₃ solutions.     

Synthesis of structured nanoparticles of styrene/butadiene block copolymers via RAFT seeded emulsion polymerization

The structured nanoparticles of styrene (St) and butadiene (Bd) block copolymers were prepared by RAFT seeded emulsion polymerization of butadiene. It was confirmed that the block copolymers of PSt-b-P(St-co-Bd) was formed with controlled molecular weight and rather low PDI at low composition of the P(St-co-Bd) segment. With more incorporation of butadiene, the branching reaction of polybutadiene became obvious, leading to higher PDI and positive deviation of M_n from the theoretical predication. At the gel point, the composition of the P(St-co-Bd) segment was estimated to be 0.72. After this, the gel fraction increased quickly. The morphology of structured nanoparticles could be largely tuned simply by the copolymer composition. With the composition of the P(St-co-Bd) segment increased from 0.37 to 0.92, the morphology within the structured particles changed from the polybutadiene domains-in-polystyrene matrix, perforated concentric-spherical layer, concentric-spherical multi-layers, bi-continuous, to broken layers of polystyrene in polybutadiene matrix. It was found that the morphology of the block copolymer within nanoparticles was dependent on d/L values, which was in excellent agreement with the theoretical prediction.     

RAFT polymerization of styrene mediated by naphthalene-containing RAFT agents and optical properties of the polymers

In this paper, we designed and synthesized five novel reversible addition–fragmentation chain transfer (RAFT) agents bearing naphthyl moieties in the Z or R groups, including 3,4,5-trimethoxy-benzyl dithio-2-naphthalenoate (TOBDN), 4-nitrobenzyl dithio-2-naphthalenoate (NBDN), 1-menaphthyl 4-cyanodithiobenzoate (NCDB), 1-menaphthyl dithiobenzoate (NDB) and 1-menaphthyl dithio-2-naphthalenoate (NDN). The RAFT polymerizations of styrene mediated by these RAFT agents with AIBN as the initiator at 80 °C were conducted and evaluated. Except for NCDB, the RAFT agents showed good control over the polymerization at different RAFT agent concentrations: the M_n,GPC increased linearly with the monomer conversion, and the PDIs of the polymers were relatively low (PDI = 1.20–1.50). The structure of RAFT agents bearing three different R groups with naphthyl as the Z group showed less effects on the polymerization rate, while those bearing different Z groups with 1-menaphthyl as the R group presented significant effects on the polymerization rates. The polymerization rate with phenyl as the Z group was higher than that with 2-naphthyl as the Z group, and it decreased significantly when using 4-cycno phenyl as the Z group. Retardation effects were observed with all the RAFT agents. ¹H NMR spectra and chain extension results confirmed that most of the polymer chains were “living”. Ultraviolet (UV) absorption of naphthyl moieties at the R group showed blue shifts compared with those of naphthyl at the Z group. The UV absorption intensity of PS was uniformly lower than that of the corresponding RAFT agent, while the fluorescence intensity of PS was higher than that of the corresponding RAFT agent.     

Synthesis and rheological properties of an associative star polymer in aqueous solutions

Rheological properties of aqueous solutions and hydrogels formed by an amphiphilic star block copolymer, poly(acrylic acid)-block-polystyrene (PAA₅₄-b-PS₆)₄, were investigated as a function of the polymer concentration (C_p), temperature, and added salt concentration. The water-soluble polymer synthesised by atom transfer radical polymerization (ATRP) was found to form hydrogels at room temperature at polymer concentrations, C_p, over 22 g/L due to the interpolymer hydrophobic association of the PS blocks. Increasing C_p leads to stronger elastic networks at room temperature that show a gel-to-solution transition with increasing temperature. Increase of ionic strength decreases the moduli compared with the pure hydrogel but did not affect the gel-sol transition temperature significantly. Small-angle X-ray experiments showed two distinct scattering correlation peaks for samples above the gelling C_p, which indicates the aggregates formed due to hydrophobic association. Upon heating the intensity of the scattering correlation peaks was found to decrease indicating the loss of the network structure due to thermal motion.     

A new approach to 3-miktoarm star polymers using a combination of reversible addition-fragmentation chain transfer (RAFT) and ring opening polymerization (ROP) via “Click” chemistry

The synthesis of AB₂-type miktoarm star polymers using a combination of reversible addition-fragmentation chain transfer (RAFT), ring opening polymerization (ROP) and “Click” chemistry was demonstrated in this work. An azide functional RAFT agent was used to polymerize butyl acrylate, polyethylene glycol acrylate and N-isopropylacrylamide monomers. Propargylamine was reacted with glycerine carbonate to obtain a dihydroxy functional alkyne compound which was used for the ring opening polymerization of ?-caprolactone (?-CL) and lactide. The resulting alkyne functional polycaprolactone (PCL) and polylactide (PLA) polymers were reacted with azide functional polymers in the presence of copper bromide (CuBr) catalyst to obtain miktoarm star polymers. The polymers were characterized by gel permeation chromatography (GPC), differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy and nuclear magnetic resonance (NMR) spectroscopy. The star polymers had low polydispersity (∼1.3) with well-defined structures. These polymers have a number of potential applications including crosslinking agents for polyurethane (PU) coatings for biodegradable and fouling release applications.     

Facile preparation of a novel nickel-containing metallopolymer via RAFT polymerization

While the metallocene polymers were comprehensively studied, other metallopolymers are rarely explored. The major challenge is the lack of a synthetic platform for the preparation of metal coordinated derivatives, monomers, and polymers. Therefore, the development of a facile synthesis of new metal coordinated monomers and polymers is critically needed. A novel successfully synthesized methacrylate-containing nickel complex is reported in this communication. Controlled RAFT polymerizations are further carried out to prepare a series of side-chain nickel containing polymers with different molecular weight and narrow Polydispersity Index (PDI). This new metallopolymer performs specific electrochemical and excellent thermal properties. This study provides a novel and convenient strategy to prepare metallopolymer with controllable molecular weight, which has potential applications in assembled, catalytic and magnetic materials.     

Grafting polystyrene onto silica nanoparticles via RAFT polymerization

Reversible addition-fragmentation chain transfer (RAFT) polymerization was used to graft polystyrene (PS) onto silica nanoparticles. A novel route was used to prepare the RAFT agent, 2-butyric acid dithiobenzoate (BDB) by substitution of dithiobenzoate magnesium bromide with sodium 2-bromobutyrate under alkali condition in aqueous solution. Epoxy groups were covalently attached to silica nanoparticles by condensation reaction of 3-glycidyloxypropyltrimethoxysilane (GPS) with the hydroxyl on the silica particle surface. RAFT agent functionalized nanoparticles were produced by ring-open reaction of the epoxy group with the carboxyl group of BDB. Then, PS chains with controlled molecular weights and narrow polydispersities (less than 1.1) were grown from the RAFT agent anchored nanoparticle surface. FT-IR, transmission electron microscopy (TEM) and thermogravimetric analysis (TGA) results showed that PS chains grew from silica particles by surface RAFT polymerization.     

Core cross-linked star polymers via controlled radical polymerisation

Star polymers are comprised of multiple arms or branches radiating from a central point or core and have been of huge scientific interest since they were first prepared sixty years ago, as a result of their unique physical properties. Star polymers are not just an academic curiosity, but are currently employed or under investigation in a wide range of industries and commercial materials ranging from engine oils and coating technologies to contact lenses and biomedical devices. Although there are many different types of star polymers and methods for their synthesis, recent advances in the field of controlled radical polymerisation have enabled the facile production of complex star polymer architectures from a large range of monomer families, without the requirement of highly stringent reaction conditions. In particular, well-defined, nanometre scale core cross-linked star (CCS) polymers, which are readily accessible by controlled radical polymerisation techniques, have been increasingly prominent in the scientific literature. As a result, this feature article provides a comprehensive review covering the development, functionalisation, physical properties and application of core cross-linked star polymers prepared by controlled radical polymerisation and the arm-first approach.     

Advances in RAFT polymerization: the synthesis of polymers with defined end-groups

This paper provides an overview and discusses some recent developments in radical polymerization with reversible addition-fragmentation chain transfer (RAFT polymerization). Guidelines for the selection of RAFT agents are presented. The utility of the RAFT process is then illustrated with several examples of the synthesis of polymers with reactive end-groups. Thus, RAFT polymerization with appropriately designed trithiocarbonate RAFT agents is successfully applied to the synthesis of narrow polydispersity carboxy-functional poly(methyl methacrylate) and primary amino-functional polystyrene. Methods for removing the thiocarbonylthio end-group by aminolysis, reduction and thermal elimination are discussed. It is shown that the thiocarbonylthio end-group can be cleanly cleaved by radical induced reduction with tri-n-butylstannane, to leave a saturated chain end, or by thermolysis, to leave an unsaturated chain end.     

Synthesis of phenylphosphinic acid-containing amphiphilic homopolymers by reversible addition-fragmentation transfer (RAFT) polymerization and its aggregation in water

A novel amphiphilic phosphorus-containing polymer was prepared by RAFT polymerization of 3-[2-(acryloyloxy)ethoxy]-3-oxopropyl(phenyl) phosphinic acid (AOPA). The monomer was first synthesized by esterification of 3-[hydroxy(phenyl)phosphoryl]propanoic acid and 2-hydroxyethyl acrylate, and then the polymerizations were performed at 60 °C. The polymerization was well controlled, and the polymers with “well-defined” structures were successfully synthesized. The polymers can self-assemble to form the micelles in distilled water due to the special amphiphilic structure, and the shell of the micelles could be cross-linked by the coordination of phosphinic acid with cations. The property may promote the polymers to be used in the ionic exchange for the environment protection.     

Synthesis and characterization of hyperbranched azobenzene-containing polymers via self-condensing atom transfer radical polymerization and copolymerization

We report on the synthesis of an azobenzene-containing inimer 6-{4-[4-(2-(2-bromoisobutyryloxy)hexyloxy)phenylazo]phenoxy}hexyl methacrylate (I) and used it to prepare hyperbranched homopolymer and copolymers by self-condensing vinyl polymerization (SCVP) and copolymerization (SCVCP) with its precursor 6-{4-[4-(6-hydroxyhexyloxy)phenylazo]phenoxy}hexyl methacrylate (M) using atom transfer radical polymerization (ATRP). Depending on the comonomer ratio, γ=[M]₀/[I]₀, branched polymethacrylates with number-average weights between 8000 and 20,000 and degree of branching (DB) between 0.08 and 0.49 were obtained by SCVCP, as evidenced by GPC and ¹H NMR analysis. In addition, the photochemical properties of the polymers were also studied by UV-vis spectra and found the structure of polymers affect obviously the trans-cis isomerization properties of the branched polymers.     

Allyl functionalized telechelic linear polymer and star polymer via RAFT polymerization

Reversible addition-fragmentation chain transfer (RAFT) polymerization of styrene using bisallyl trithiocarbonate as a chain transfer agent (CTA) was studied. The polymerization exhibited first-order kinetics and the molecular weight increased linearly with increase of monomer conversion. Well defined allyl-functionalized telechelic polystyrene (PS), poly(tert-butyl acrylate) (PtBA) and corresponding triblock copolymers, polystyrene-b-poly(n-butyl acrylate)-b-polystyrene (PS-b-PnBA-b-PS) and poly(tert-butyl acrylate)-b-polystyrene-b-poly(tert-butyl acrylate) (PtBA-b-PS-b-PtBA) were prepared as characterized with GPC and NMR analysis. The allyl-end groups of the telechelic PS have been switched to 1,2-dibromopropyl groups quantitatively by bromine addition reaction, further substitution of the bromide with azide was also made. Furthermore, star PS with allyl-end-functionalized arms was synthesized by RAFT polymerization of divinyl benzene using allyl-functionalized PS as a macro-CTA via arm-first approach. Star polymer with a thiol-functionalized core was generated by aminolysis reaction of the star polymer and ethylenediamine. As a result, difunctionalized star polymer with allyl and thiol groups was obtained and was used as a stabilizer for the formation of gold nanoparticles.     

Synthesis of poly(vinyl alcohol) combs via MADIX/RAFT polymerization

Poly(vinyl acetate) combs have been prepared via macromolecular design via interchange of xanthate (MADIX)/reversible addition-fragmentation chain-transfer (RAFT) polymerization using xanthate functionalized polymer cores. The comb backbones were prepared using well-defined poly(vinyl alcohol) PVA polymers with a degree of polymerization of 20, 100 and 170, respectively. Functionalization with xanthates via R-group or a Z-group approach resulted in the formation of macromolecular MADIX agents. While Z group designed macromolecular xanthate agents appeared to inhibit the polymerization of vinyl acetate (VAc), R group designed macromolecular xanthate agents achieved to mediate efficiently the bulk polymerization of VAc affording PVAc combs. However, the growth of the combs was accompanied at low conversions by the formation of linear polymer chains as a result of the constant initiation (AIBN) and shoulders, which can be attributed to intermolecular coupling reactions. The proportions of single chains and termination products were observed to increase with the degree of polymerization of the macromolecular MADIX agents broadening the molecular weight distribution. As a result of a stable ester link between the branches and the PVA backbone, the branched PVAc architectures were finally hydrolyzed to afford poly(vinyl alcohol) combs.