Showing papers in "Macromolecules in 2006"

Polymer Nanocomposites Containing Carbon Nanotubes

Abstract: We review the present state of polymer nanocomposites research in which the fillers are single-wall or multiwall carbon nanotubes. By way of background we provide a brief synopsis about carbon nanotube materials and their suspensions. We summarize and critique various nanotube/polymer composite fabrication methods including solution mixing, melt mixing, and in situ polymerization with a particular emphasis on evaluating the dispersion state of the nanotubes. We discuss mechanical, electrical, rheological, thermal, and flammability properties separately and how these physical properties depend on the size, aspect ratio, loading, dispersion state, and alignment of nanotubes within polymer nanocomposites. Finally, we summarize the current challenges to and opportunities for efficiently translating the extraordinary properties of carbon nanotubes to polymer matrices in hopes of facilitating progress in this emerging area.

Determining Beta-Sheet Crystallinity in Fibrous Proteins by Thermal Analysis and Infrared Spectroscopy

Abstract: We report a study of self-assembled beta-pleated sheets in B. mori silk fibroin films using thermal analysis and infrared spectroscopy. B. mori silk fibroin may stand as an exemplar of fibrous proteins containing crystalline beta-sheets. Materials were prepared from concentrated solutions (2−5 wt % fibroin in water) and then dried to achieve a less ordered state without beta-sheets. Crystallization of beta-pleated sheets was effected either by heating the films above the glass transition temperature (Tg) and holding isothermally or by exposure to methanol. The fractions of secondary structural components including random coils, alpha-helices, beta-pleated sheets, turns, and side chains were evaluated using Fourier self-deconvolution (FSD) of the infrared absorbance spectra. The silk fibroin films were studied thermally using temperature-modulated differential scanning calorimetry (TMDSC) to obtain the reversing heat capacity. The increment of the reversing heat capacity ΔCp0(Tg) at the glass transition fo...

Activators Regenerated by Electron Transfer for Atom Transfer Radical Polymerization of Styrene

Abstract: The amount of Cu-based catalysts in atom transfer radical polymerization (ATRP) of styrene has been reduced to a few ppm in the presence of the appropriate reducing agents such as FDA approved tin(II) 2-ethylhexanoate (Sn(EH)2) or glucose. The reducing agents constantly regenerate ATRP activator, the Cu(I) species, from the Cu(II) species, formed during termination process, without directly or indirectly producing initiating species that generate new chains. Moreover, the reducing agents allow starting an ATRP with the oxidatively stable Cu(II) species. The reducing/reactivating cycle may also eliminate air or some other radical traps in the system. This new catalytic system is based on regeneration of the activators for an ATRP by electron transfer and therefore was named activators regenerated by electron transfer (ARGET) ATRP. The optimum amount of reducing agent and minimal amount of ATRP Cu catalyst depend on the particular system. For example, styrene was polymerized with 10 ppm of CuCl2/Me6TREN and...

Discovery of cellulose as a smart material

Abstract: The past 10 years has witnessed a renewed interest in cellulose research and application, sparked mostly by technological interests in renewable raw materials and more environmentally friendly and sustainable resources. In this paper, we further expand the current knowledge in cellulose applications and technologies by reporting our discovery of cellulose as a smart material that can be used for biomimetic sensor/actuator devices and micro-electromechanical systems. This smart cellulose is termed electroactive paper (EAPap). It can produce a large bending displacement with low actuation voltage and low power consumption. The actuation phenomenon and its characteristics are illustrated in this paper. Because cellulose EAPap is ultra-lightweight, inexpensive, and biodegradable, it is advantageous for many applications such as micro-insect robots, micro-flying objects, micro-electromechanical systems, biosensors, and flexible electrical displays.

Guanidine and Amidine Organocatalysts for Ring-Opening Polymerization of Cyclic Esters

Abstract: 1,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD), N-methyl-TBD (MTBD), and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) are effective organocatalysts for the ring-opening polymerization (ROP) of cyclic esters such as lactide (LA), δ-valerolactone (VL), and e-caprolactone (CL). TBD is shown to polymerize LA, VL, and CL in a fast and controlled manner, whereas MTBD and DBU polymerized LA and addition of a thiourea cocatalyst led to the ROP of VL and CL being achieved. Each of the catalysts produced polymers displaying high end group fidelity, good correlation between theoretical and observed molecular weight, and linear relationships between conversion and molecular weight. The enhanced activity of TBD relative to MTBD and DBU is attributed to its bifunctionality, enabling the simultaneous activation of both the cyclic ester monomer and the alcohol group of the initiator/propagating species. Temperature-dependent NMR studies generated individual association constants for MTBD with benzyl alcohol and thiourea with VL. ...

End group effect on the thermal response of narrow-disperse poly (n -isopropylacrylamide) prepared by atom transfer radical polymerization

Abstract: Four series of narrow-disperse poly(N-isopropylacrylamide) (PNIPAM) with well-controlled molecular weights and with end groups of varying hydrophobicity were synthesized by room temperature atom tr...

Synthesis of Star Polymers by a Combination of ATRP and the “Click” Coupling Method

Abstract: Three-arm and four-arm star polystyrene (PS) polymers were synthesized by a combination of atom transfer radical polymerization (ATRP) and click coupling chemistry. The click reaction between an azido-terminated PS (PS−N3) and an alkyne-containing multifunctional compound proved to be fast and efficient. All coupling reactions were finished within 3 h, proven by the disappearance of signals from the azido groups in NMR spectra and the high yields of the coupled products by GPC analysis. For the model coupling reaction between a PS−N3 polymer and a dialkyne-containing compound, the final yield of the coupled PS−PS polymer was ca. 95%. When a PS−N3 polymer was reacted with a trialkyne-containing or tetraalkyne-containing compound, the yields of 3-arm star and 4-arm star polymers were around 90% and 83%, respectively. The influence of several parameters on the efficiency of the click coupling reaction was studied, including the molecular weight of the PS−N3 polymer, the presence of an added reducing agent, C...

Mechanism of Ion Transport in Amorphous Poly(ethylene oxide)/LiTFSI from Molecular Dynamics Simulations

Abstract: The mechanisms of lithium cation (Li+) and bis(trifluoromethane)sulfonamide anion (TFSI-) transport in poly(ethylene oxide) (PEO, Mw = 2380) melts were examined using molecular dynamics (MD) simulations over a wide range of salt concentrations and temperatures. MD simulations using a quantum-chemistry-based many-body polarizable force field yielded ion self-diffusion coefficients, electrolyte conductivity, ion aggregation, and the coordination environment of Li+ in good agreement with experiment. Lithium transport was found to arise from a combination of the subdiffusive Li+ motion along PEO chains, motion together with PEO segments and intersegmental Li+ hops from one PEO segment to another. The rate of intersegmental hops was found to correlate well with times at which Li+ motion crosses over from subdiffusive to diffusive behavior. The contribution of Li+ motion along PEO chains to the total Li+ transport was found to be approximately equal to the contribution from Li+ moving together with PEO segments...

Elastic Moduli of Ultrathin Amorphous Polymer Films

Abstract: The elastic moduli of ultrathin poly(styrene) (PS) and poly(methylmethacrylate) (PMMA) films of thickness ranging from 200 nm to 5 nm were investigated using a buckling-based metrology. Below 40 nm, the apparent modulus of the PS and PMMA films decreases dramatically, with an order of magnitude decrease compared to bulk values for the thinnest films measured. We can account for the observed decrease in apparent modulus by applying a composite model based on the film having a surface layer with a reduced modulus and of finite thickness. The observed decrease in the apparent modulus highlights issues in mechanical stability and robustness of sub-40 nm polymer films and features.

Effect of Molecular Weight on the Structure and Crystallinity of Poly(3-hexylthiophene)

Abstract: Recently, two different groups have reported independently that the mobility of field-effect transistors made from regioregular poly(3-hexylthiophene) (P3HT) increases strongly with molecular weight. Two different models were presented: one proposing carrier trapping at grain boundaries and the second putting emphasis on the conformation and packing of the polymer chains in the thin layers for different molecular weights. Here, we present the results of detailed investigations of powders and thin films of deuterated P3HT fractions with different molecular weight. For powder samples, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) were used to investigate the structure and crystallization behavior of the polymers. The GPC investigations show that all weight fractions possess a rather broad molecular weight distribution. DSC measurements reveal a strong decrease of the crystallization temperature and, most important, a significant decrease of the de...

Mechanical Properties and Structure of Polymer−Clay Nanocomposite Gels with High Clay Content

Abstract: The mechanical properties and structures of nanocomposite gels (NC gels), consisting of poly(N-isopropylacrylamide) (PNIPA) and inorganic clay (hectorite), prepared using a wide range of clay concentration (∼25 mol % against water) were investigated. All NC gels were uniform and transparent, almost independent of the clay content, Cclay. The tensile modulus (E) and the strength (σ) were controlled without sacrificing extensibility by changing Cclay. The E, σ, and fracture energy observed for as-prepared NC gels attained 1.1 MPa, 453 kPa, and 3300 times that of a conventional chemically cross-linked gel, respectively, and σ increased to 3.0 MPa for a once-elongated NC25 gel. From the tensile and compression properties, in addition to optical transparency, it was concluded that a unique organic/inorganic network structure was retained regardless of Cclay. The effects of Cclay on the tensile mechanical properties on the first and second cycles, the time-dependent recovery from the first large elongation and ...

Poly(vinyl alcohol)-Based Hydrogels Formed by “Click Chemistry”

Abstract: Novel poly(vinyl alcohols) (PVA) functionalized with pendant acetylene and azide groups were prepared by carbonyldiimidazole (CDI)-mediated couplings of the amines terminated with functional groups, 1-azido-2-aminoethane, propargylamine, or N-methylpropargylamine, to PVA. Low degrees (1−5%) of PVA modification were required in order to retain solubility in water. Azide-modified PVA and alkyne-modified PVA components were cross-linked by mixing of their solutions together with Cu(I) catalyst, a type of Huisgen's 1,3-dipolar azide−alkyne cycloaddition, recently defined as a powerful “click” chemistry. Reaction of the two different polymers results in a chemoselective coupling between alkynyl and azido functional groups with the multiple formation of triazole cross-links to give hydrogel formation. In another version the PVA-based hydrogels were obtained by cross-linking of alkyne-modified PVA with the telechelic bifunctional poly(ethylene glycol)−diazide cross-linker. The hydrogels prepared by these two met...

LLS and FTIR Studies on the Hysteresis in Association and Dissociation of Poly(N-isopropylacrylamide) Chains in Water

Abstract: Using a combination of static and dynamic laser scattering, we examined the association and dissociation of linear poly(N-isopropylacrylamide) (PNIPAM) chains in dilute aqueous solutions There exists a hysteresis in the temperature dependence of the average hydrodynamic radius (〈Rh〉), average radius of gyration (〈Rg〉), and apparent weight-average molar mass (Mw,app) in one heating-and-cooling cycle In the heating process, the chains first undergo intrachain contraction before interchain association to form stable aggregates at temperatures much higher than the lower critical solution temperature (LCST ∼ 32 °C) of PNIPAM in water In the cooling process before the solution temperature approaches the LCST, Mw,app remains a constant and both 〈Rg〉 and 〈Rh〉 increase, but the ratio of 〈Rg〉/〈Rh〉 decreases In other words, the aggregates undergo an unevenly swelling; namely, the periphery swells more than the center, and there is no chain dissociation FTIR spectra reveal that as the temperature increases, the

Toward Photocontrolled Release Using Light-Dissociable Block Copolymer Micelles

Abstract: Micellar aggregates (core -shell micelles, vesicles, etc.) formed by amphiphilic block copolymers (BCPs) are being actively investigated as a nanocarrier system for controlled delivery of drugs and other biological substances. 1 Generally speaking, a good nanocarrier of drugs would be (1) stable during the circulation in the blood, allowing no release of encapsulated hydrophobic drugs, (2) able to accumulate specifically in the target site (e.g., tumors), and (3) once on target, able to release the carried drug quickly or in a controlled fashion. Each of these requirements has prompted a great deal of research efforts in the design, synthesis and exploitation of new BCPs. As far as the control of release is concerned, BCP micelles that can be disrupted by external stimuli, thus triggering the release of entrapped agents, are of particular interest. The stimuli studied so far not only include changes that BCP micelles in the body can experience, such as increase or decrease in pH, 2 in temperature 3 and exposure to oxidation reaction, 4 but also include sound 5 and light6-8 that can be applied from outside of the body for remote activation of the disruption of micelles. If the interaction of BCP micelles with light lead to their dissociation, photocontrolled release of encapsulated agents is possible. One obvious, and attractive, feature of stable, lightresponsive polymer micelles is that the release of the carried guest (not limited to drugs) could be started at required time (when light is applied) and at required location (where light exposure is directed to). This offers new perspectives in controlled release that would be difficult to realize using other stimuli. To obtain usable BCP-based nanocarriers for photocontrolled delivery applications, the development of effective designs of BCP structures and fundamental studies of their photoinduced dissociation and release processes are necessary. There are many studies of light-responsive micellar aggregates of small-molecule surfactants, 9-16 but very few reports on lightdissociable polymer micelles. 6-8 We first studied amphiphilic BCPs whose hydrophobic block is an azobenzene-containing side-chain liquid crystalline polymer (Azo-SCLCP). 6,7 We found that their core-shell micelles and vesicles in solution can be dissociated by UV light irradiation and re-formed upon visible light exposure, as a result of the reversible trans -ci photoisomerization of azobenzene side groups of the hydrophobic block. From a study of the mechanism of the reversible micellar dissociation, a general design principle for azobenzene-based light-dissociable BCP micelles has been suggested. 7 The azobenzene moiety should have a small (near zero) dipole moment in the trans form and a high dipole moment in the cis form (determined by the substituents on the azobenzene unit); when this is conjugated with a weakly hydrophilic block (determined by the chemical nature and the length of the block), the increase in polarity of the azobenzene polymer block under UV light (trans-cis isomerization taking place) can alter the hydrophilic/hydrophobic balance which results in dissociation of micelles, while the decrease in polarity upon visible light irradiation (reverse cis -trans isomerization occurring) shifts the hydrophilic/hydrophobic balance in the opposite direction which brings back the micelles. More importantly, these studies suggest that any chromophores whose photoinduced structural rearrangements gives rise to a significant change in dipole moment could be used in designing light-dissociable BCP micelles based on the same principle. If the photoinduced process is reversible, the micellar dissociation is reversible. More recently, we have demonstrated the efficiency of a different, and more general, strategy for designing BCPs whose micelles can undergo irreversible light-induced dissociation. 8

Exploration, Optimization, and Application of Supramolecular Thiourea−Amine Catalysts for the Synthesis of Lactide (Co)polymers

Abstract: The structural flexibility and efficacy of thiourea−amine catalysts for the supramolecular activation and ring-opening polymerization (ROP) of lactide are described. The nature of the hydrogen bonding group and its strength as well as the steric congestion have been altered, leading to shorter polymerization times, better control, and pathways to influence the stereochemistry of the resulting polymer. The tolerance to functionality and the mild conditions of the ROP mechanism allow for block copolymer synthesis by combination of nitroxide-mediated polymerization as well as reversible addition fragmentation and chain transfer polymerization using dual-headed initiators. Tandem hydrogen bond activation to organocatalyze ROP of lactide is an effective, versatile means to generate polymers with predictable molecular weights, narrow polydispersities, control of microstructure and a variety of complex architectures and block copolymers.

Conjugated donor-acceptor copolymer semiconductors with large intramolecular charge transfer : Synthesis, optical properties, electrochemistry, and field effect carrier mobility of thienopyrazine-based copolymers

Abstract: Several conjugated thieno[3,4-b]pyrazine-based donor−acceptor copolymers were synthesized by Stille and Suzuki copolymerizations, and their optical, electrochemical, and field-effect charge transport properties were characterized. The new copolymers, poly(5,7-bis(3-dodecylthiophen-2-yl)thieno[3,4-b]pyrazine) (BTTP), poly(5,7-bis(3-dodecylthiophen-2-yl)thieno[3,4-b]pyrazine-alt-2,5-thiophene) (BTTP-T), poly(5,7-bis(3-dodecylthiophen-2-yl)thieno[3,4-b]pyrazine-alt-9,9-dioctyl-2,7-fluorene) (BTTP-F), and poly(5,7-bis(3-dodecylthiophen-2-yl)thieno[3,4-b]pyrazine-alt-1,4-bis(decyloxy)phenylene) (BTTP-P), had moderate to high molecular weights, broad optical absorption bands that extend into the near-infrared region with absorption maxima at 667−810 nm, and small optical band gaps (1.1−1.6 eV). They showed ambipolar redox properties with low ionization potentials (HOMO levels) of 4.6−5.04 eV. The field-effect mobility of holes varied from 4.2 × 10-4 cm2/(V s) in BTTP-T to 1.6 × 10-3 cm2/(V s) in BTTP-F. These r...

Characterization of Polymer Nanocomposite Interphase and Its Impact on Mechanical Properties

Abstract: The structure of the interphase, a region between nanoparticle fillers and the bulk polymer matrix in a particle reinforced composite, was investigated using two different approaches. The polymer n...

Hierarchical modeling of polystyrene: From atomistic to coarse-grained simulations

Abstract: We present a hierarchical approach that combines atomistic and mesoscopic simulations that can generally be applied to vinyl polymers. As a test case, the approach is applied to atactic polystyrene (PS). First, a specific model for atactic PS is chosen. The bonded parameters in the coarse-grained force field, based on data obtained from atomistic simulations of isolated PS dimers, are chosen in a way which allows to differentiate between meso and racemic dyads. This approach in principle allows to study isotactic and syndiotactic melts as well. Nonbonded interactions between coarse-grained beads were chosen as purely repulsive. The proposed mesoscopic model reproduces both the local structure and the chain dimensions properly. An explicit time mapping is performed, based on the atomistic and CG mean-square displacements of short chains, demonstrating an effective speed up of about 3 orders of magnitude compared to brute force atomistic simulations. Finally the equilibrated coarse-grained chains are back mapped onto the atomistic systems. This opens new routes for obtaining well equilibrated high molecular weight polymeric systems and also providing very long dynamic trajectories at the atomistic level for these polymers.

Direct Synthesis of Thermally Responsive DMA/NIPAM Diblock and DMA/NIPAM/DMA Triblock Copolymers via Aqueous, Room Temperature RAFT Polymerization†

Abstract: We report aqueous, room temperature RAFT polymerization of N-isopropylacrylamide (NIPAM) and, subsequently, its block copolymerization utilizing a poly(N,N-dimethylacrylamide) macro-CTA. A series of thermally responsive AB diblock and ABA triblock copolymers have been prepared. These polymers contain hydrophilic N,N-dimethylacrylamide (DMA) A blocks of fixed molecular weight and temperature-responsive NIPAM B blocks of varied chain length. Using a combination of 1H NMR spectroscopy, T2 relaxation measurements, dynamic light scattering (DLS), and static light scattering (SLS), we demonstrate that these block copolymers are indeed capable of reversibly forming micelles in response to changes in solution temperature and that the micellar size and transition temperature are dependent on both the NIPAM block length and the polymer architecture (diblock vs triblock).

Single Wall Carbon Nanotube/Polyethylene Nanocomposites: Nucleating and Templating Polyethylene Crystallites

Abstract: The crystallization kinetics and resulting morphology of polyethylene (PE) in the presence of single wall carbon nanotubes (SWNT) are investigated in isotropic and aligned composites. A hot coagulation method was developed to incorporate SWNT loadings as high as 30 wt % with uniform distribution. Thermal analysis interpreted using the Avrami equation showed that the nanotubes provide nucleation sites to PE and accelerate the PE crystal growth rate while reducing the crystal dimensionality from spherulitic to disk-shaped. Nucleating on SWNT bundles significantly increases the shish density during melt fiber spinning, so that the PE microstructure is shish kebab with straight (not twisted) lamellae in 1 wt % SWNT-HDPE composite fibers. By comparing the orientations of SWNT and PE produced by various processing conditions, we show that SWNT bundles template the growth of PE crystals by imposing a growth direction perpendicular to the SWNT. This provides a new route toward controlling the SWNT-polymer interface and thereby the physical properties of nanocomposites.

Compatibilizing Bulk Polymer Blends by Using Organoclays

Abstract: We have studied the morphology of blends of PS/PMMA, PC/SAN24, and PMMA/EVA and compared the morphologies with and without modified organoclay Cloisite 20A or Cloisite 6A clays. In each case we found a large reduction in domains size and the localization of the clay platelets along the interfaces of the components. The increased miscibility was accompanied in some cases, with the reduction of the system from multiple values of the glass transition temperatures to one. In addition, the modulus of all the systems increased significantly. A model was proposed where it was proposed that in-situ grafts were forming on the clay surfaces during blending and the grafts then had to be localized at the interfaces. This blending mechanism reflects the composition of the blend and is fairly nonspecific. As a result, this may be a promising technology for use in processing recycled blends where the composition is often uncertain and price is of general concern.

A Versatile Method To Prepare RAFT Agent Anchored Substrates and the Preparation of PMMA Grafted Nanoparticles

Abstract: Well-defined polymer brushes grafted onto silica nanoparticles were prepared by reversible addition−fragmentation chain transfer polymerization (RAFT). A versatile RAFT agent, 4-cyanopentanoic acid...

Effect of Areal Chain Density on the Location of Polymer-Modified Gold Nanoparticles in a Block Copolymer Template

Abstract: A strategy for controlling the location of gold nanoparticles within block copolymer domains through varying the surface coverage of gold nanoparticles by end-attached polymer ligands is described. Gold nanoparticles coated by short thiol end functional polystyrene homopolymers (PS-SH) (Mn = 3.4 kg/mol) are incorporated into a poly(styrene-b-2-vinylpyridine) diblock copolymer template (PS-b-P2VP) (Mn = 196 kg/mol), the P2VP block of which has a more favorable interaction with a bare gold particle surface than does the PS block. The areal chain density of the PS-SH ligands on gold particles is varied by changing the mole ratio of PS-SH chains to gold atoms. It is found that the areal density of PS chains on the gold particles is critical to controlling their location in block copolymer templates. PS-coated gold nanoparticles with PS chain areal density higher than 1.6 chains/nm2 are dispersed in PS domains of PS-b-P2VP while they are segregated along the interface between PS and P2VP domains of PS-b-P2VP f...

Theory of Phase Separation in Polymer Nanocomposites

Abstract: The microscopic polymer reference interaction site model theory of polymer nanocomposites composed of flexible chains and spherical nanoparticles has been employed to study second virial coefficients and spinodal demixing over a wide range of interfacial chemistry, chain length, and particle size conditions. For hard fillers, two distinct phase separation behaviors, separated by a miscibility window, are generically predicted. One demixing curve occurs at relatively low monomer−particle attraction strength and corresponds to a very abrupt transition from an entropic depletion attraction-induced phase separated state to an enthalpically stabilized miscible fluid. The homogeneous mixture arises via a steric stabilization mechanism associated with the formation of thin, thermodynamically stable bound polymer layers around fillers. The second demixing transition occurs at relatively high monomer−particle adsorption energy and is inferred to involve the formation of an equilibrium physical network phase with l...

Impact of end-group association and main-chain hydration on the thermosensitive properties of hydrophobically modified telechelic poly(N-isopropylacrylamides) in water

Abstract: We examine the influence of the macromolecule chain length on the cloud point temperature (Tcp) and the temperature of the coil-to-globule transition (TM) in aqueous solutions of hydrophobically modified (HM) telechelic poly(N-isopropylacrylamides) (PNIPAM) ranging in concentration from 0.01 to 35 g L-1 (0.1−310 mmol of NIPAM L-1). The telechelic HM-PNIPAM samples with n-octadecyl termini were obtained by RAFT polymerization of NIPAM in dioxane in the presence of S-1-n-octadecyl-S‘-(α,α‘-dimethyl-α‘ ‘-N-n-octadecylacetamide)trithiocarbonate as a chain transfer agent. Their molar mass (Mn) ranged from 12 000 to 49 000 g mol-1 with a polydispersity index lower than 1.20. The cloud point temperatures, measured by monitoring the temperature-induced changes in scattering intensity, decreased significantly with increasing polymer concentration, this effect being more pronounced with decreasing polymer molar mass. In contrast, the temperature of the PNIPAM chain coil-to-globule collapse (30 ± 1 °C) was only slig...

Rapid Generation and Control of Microporosity, Bimodal Pore Size Distribution, and Surface Area in Davankov-Type Hyper-Cross-Linked Resins

Abstract: Synthesis of Davankov-type hyper-cross-linked resin beads has been carried out using gel-type and permanently porous poly(divinylbenzene-co-vinylbenzyl chloride) (DVB−VBC) precursor resins without any external electrophile. Of the Lewis acids examined (FeCl3, AlCl3, SnCl4), FeCl3 was by far the most effective catalyst. Rather remarkably in the case of a gel-type 2 mol % DVB−VBC precursor, extensive microporosity was generated within only 15 min of initiating the cross-linking reaction, yielding a surface area (N2 sorption/BET) of ∼1200 m2 g-1, which rose steadily to a maximum approaching 2000 m2 g-1 after 18 h. In the case of porous DVB−VBC precursor resins, the presence of discrete pores in the precursor, accompanied by a micropore fraction on hyper-cross-linking, yields resins with a clearly bimodal distribution of pores. Manipulation of the Friedel−Crafts reaction variables and the structure of the precursor resin allows final resin products to be prepared with surface areas in the range ∼300−2000 m2 g-1.

Anthracene−Maleimide-Based Diels−Alder “Click Chemistry” as a Novel Route to Graft Copolymers

Abstract: Using the Diels−Alder (DA) “click chemistry” strategy between anthracene and maleimide functional groups, two series of well-defined polystyrene-g-poly(ethylene glycol) (PS-g-PEG) and polystyrene-g-poly(methyl methacrylate) (PS-g-PMMA) copolymers were successfully prepared The whole process was divided into two stages: (i) preparation of anthracene and maleimide functional polymers and (ii) the use of Diels−Alder reaction of these groups First, random copolymers of styrene (S) and chloromethylstyrene (CMS) with various CMS contents were prepared by the nitroxide-mediated radical polymerization (NMP) process Then, the choromethyl groups were converted to anthryl groups via the etherifaction with 9-anthracenemethanol The other component of the click reaction, namely protected maleimide functional polymers, were prepared independently by the modification of commercially available poly(ethylene glycol) (PEG) and poly(methyl methacrylate) (PMMA) obtained by atom transfer radical polymerization (ATRP) usin