Macrocyclic polystrene (PSt) was produced in high yields by an intramolecular atom transfer radical coupling reaction of the chain ends. The slow addition of the dibrominated polystyrene (BrPStBr) ...precursors, prepared by atom transfer radical polymerization, into a refluxing tetrahydrofuran solution of copper(I) bromide, tris2-(dimethylamino)ethylamine, and copper (0) activated the alkyl bromides at the PSt chain ends, favoring an intramolecular ring closing reaction with only minor competition from intermolecular coupling. Comparison of the gel permeation chromatography traces of the BrPStBr precursor to the cyclic product showed a shift to lower apparent molecular weight values, consistent with the reduced hydrodynamic radius of the macrocycle. Quantitative activation of the chain ends was confirmed by 1H NMR spectroscopy, with the resonance due to the C(H)Br methine termini on the BrPStBr precursor absent on the NMR spectrum of the cyclic product. The cyclic structure was directly confirmed by matrix-assisted laser desorption ionization−time-of-flight spectrometry.
Monobrominated versions of poly(methyl methacrylate) (PMMABr) and poly(methyl acrylate) (PMABr) are prepared by atom transfer radical polymerization and subjected to a variety of chain end‐coupling ...reactions, with the goal of achieving high extents of coupling with minimal metal content. By using atom transfer radical coupling (ATRC) with radical trap assistance in conjunction with activators generated by electron transfer, high extents of coupling (Xc) could be achieved for both polymer classes without requiring copper(I) as an initial reactant, and replacing elemental copper with environmentally benign reducing agents (Xc > 0.85). Specifically, the highest extents of coupling are observed with ascorbic acid as the reducing agent along with copper(II) bromide as the pre‐catalyst. Overall, similar extents of coupling are observed while using ≈1% of the total metal of a traditional ATRC reaction for PMABr and ≈3% for PMMABr.
Activators generated by electron transfer are used to produce macro‐radicals from brominated versions of poly(methyl methacrylate) and poly(methacrylate). When carried out in the presence of a nitroso radical trap (specific to each polymer type), substantial dimerization occurs using only a small fraction of the metal catalyst typical of atom transfer radical‐type coupling reactions.
Dibrominated poly(methyl acrylate) (BrPMABr) was prepared by atom transfer radical polymerization (ATRP), and this unpurified reaction mixture was used directly in a radical trap-assisted atom ...transfer radical coupling (RTA-ATRC) reaction to achieve cyclic PMA by intramolecular chain-end coupling. When the polymerization-to-cyclization crossover was performed as a one pot-two step sequence simply by the addition of the radical trap 2-methyl-2-nitrosopropane (MNP) and increased amounts of ligand-catalyst complex, cyclic PMA could be formed in yields up to 65% by adjusting the total volume and reactant ratios during the intramolecular RTA-ATRC, based on gel permeation chromatography (GPC) analysis. In analogous intramolecular ATRC trials lacking the radical trap, cyclization was not successful and instead chain extension by continuation of ATRP was observed. When the ATRP reaction mixture containing the BrPMABr precursor was added drop-wise into a redox active solution containing MNP, yields of cyclic polymer were increased up to 85%. The effect of the ligand was studied, with the highest amounts of cyclic product obtained in intramolecular RTA-ATRC reactions using tris2-(dimethylamino)ethylamine (Me6TREN) as the ligand. Cyclic diblock copolymers were also prepared by this method, using telechelic poly(methyl acrylate-block-styrene-block-methyl acrylate) (PMA-b-PS-b-PMA) linear precursors prepared by sequential ATRP reactions.
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•Cyclic PMA was produced by the intramolecular RTA-ATRC of dibrominated PMA.•Cyclic diblock copolymers were obtained from linear BAB-type triblocks.•Cyclic polymers were verified by gel permeation chromatography and mass spectrometry.•Polymerization-to-cyclization was performed without purification of linear precursors.
Cyclic poly(methyl methacrylate) (PMMA) is prepared by intramolecular radical trap‐assisted atom transfer radical coupling (RTA‐ATRC) of dihalogenated PMMA precursors. The inclusion of the radical ...trap nitrosobenzene (NBz) in the coupling sequence affords high yields of cyclic polymers, as observed by gel permeation chromatography and confirmed by 1H NMR and electrospray ionization mass spectra, which show the presence of the aromatic group from the NBz incorporated into the cycle. Analogous coupling reactions in the absence of the radical trap do not lead to appreciable cyclization or even intermolecular elongation, consistent with chain‐end sterics preventing radical–radical coupling as the predominant termination pathway. Thermolysis of the cyclic PMMA, possible because of the labile CO bond in the alkoxyamine linkage contained in the macrocycle, causes a reversion back to the linear form and is consistent with the role of the radical trap in the coupling sequence. Differential scanning calorimetry is also used to compare cyclic PMMA with its linear analog, with a marked increase in glass transition temperatures found after cyclization.
Cyclic poly(methyl methacrylate) is produced in an intramolecular radical trap‐assisted atom transfer radical coupling reaction. No cyclization is observed in analogous reactions lacking the radical trap. The dehalogenated linear precursor is prepared by atom transfer radical polymerization.
A nickel‐based catalytic system leading to high yields (≈80%) of polymer chain coupling has been developed. The extent of coupling (Xc) is found to be relatively high (>75%) and consistent across a ...wide range of catalyst loadings (0.1 – 1.0 equivalent with respect to the chain end), comparing favorably with other chain‐end joining methods. A mechanistic cycle is proposed to occur with benzylic chain‐end radical intermediates, consistent with the addition of a nitroso radical trap substantially lowering coupling yields. The effect of the polymer chain end (Cl vs Br) and the halide associated with the nickel catalyst is also probed.
Brominated polystyrene chains are effectively dimerized by a nickel‐catalyzed reductive coupling cycle, with high yields (80%) of chain‐end coupling observed across a wide range of conditions. The mechanism appears to involve the generation of polystyrene radicals which are captured by the nickel catalyst as part of catalytic cycle.
End-brominated poly(methyl methacrylate) (PMMABr) was prepared by atom transfer radical polymerization (ATRP) and employed in a series of atom transfer radical coupling (ATRC) and radical ...trap-assisted ATRC (RTA–ATRC) reactions. When coupling reactions were performed in the absence of a nitroso radical traptraditional ATRC conditionvery little coupling of the PMMA chains was observed, consistent with disproportionation as the major termination pathway for two PMMA chain-end radicals in our reactions. When 2-methyl-2-nitrosopropane (MNP) was used as the radical trap, coupling of the PMMA chains in this attempted RTA–ATRC reaction was again unsuccessful, owing to capping of the PMMA chains with a bulky nitroxide and preventing further coupling. Analogous reactions performed using nitrosobenzene (NBz) as the radical trap showed significant dimerization, as observed by gel permeation chromatography (GPC) by a shift in the apparent molecular weight compared to the PMMABr precursors. The extent of coupling was found to depend on the concentrion of NBz compared to the PMMABr chain ends, as well as the temperature and time of the coupling reaction. To a lesser extent, the concentrations of copper(I) bromide (CuBr), nitrogen ligand (N,N,N′,N′,N″-pentamethyldiethylenetriamine = PMDETA), and elemental copper (Cu) were also found to play a role in the success of the RTA–ATRC reaction. The highest levels of dimerization were observed when the coupling reaction was carried out at 80 °C for 0.5h, with ratio of 1:4:2.5:8:1 equiv of NBz: CuBr:Cu:PMDETA:PMMABr.
Carbon black-insulator composite chemiresistive vapor detectors have been prepared using dendrimers as the polymeric constituent of the composite. Amino-terminated dendrimer-carbon black composites ...exhibited an enhancement in detection sensitivity of ∼103 for volatile carboxylic acids as compared to nondendrimeric insulating polymer-carbon black composites. Similarly, protonated carboxylato-terminated and protonated amino-terminated dendrimer-carbon black composites showed an ∼103−104 increase in sensitivity for detection of volatile amines relative to the response of nondendrimeric insulating polymer-carbon black composites. The protonated amino-terminated dendrimer carbon black composite detectors exhibited a signal-to-noise ratio (S/N) of 22.4 ± 0.9 upon exposure to 2.7 ppb of butylamine in air, whereas poly(ethylene oxide)-carbon black composites exhibited a S/N of 3.5 ± 1.2 at 54 ppm of butylamine. The protonated amino-terminated dendrimer-carbon black detectors additionally exhibited relatively small responses to water vapor. Compositional diversity in an array of protonated amino-terminated dendrimeric vapor detectors was obtained by varying the type and generation of the dendrimer, and the type and concentration of the acid dopant. Fifteen analytes chosen from primary amines, branched amines, anilines, and non-amine organic analyte vapors were all robustly discriminated from each other by their different response patterns on the dendrimer-containing detector array. The signals produced by these 15 analytes additionally clustered into groups based on the chemical class of the analyte.
Dibrominated polystyrene (BrPStBr) was produced by atom transfer radical polymerization (ATRP) at 80°C, using the bifunctional initiator benzal bromide to afford the telechelic precursor. The ATRP ...reaction was stopped around 40% monomer conversion and directly converted into an radical trap-assisted atom transfer radical coupling (RTA-ATRC) reaction by lowering the temperature to 50°C, and adding the radical trap 2-methyl-2-nitrosopropane (MNP) along with additional catalyst, reducing agent, and ligand to match ATRC-type reaction conditions. In an attempt to induce intramolecular coupling, rather than solely intermolecular coupling and elongation, the total reaction volume was increased by the addition of varying amounts of THF. Cyclization, along with intermolecular coupling and elongation, occurred in all cases, with the extent of ring closure a function of the total reaction volume. The cyclic portion of the coupled product was found to have a 〈G〉 value around 0.8 by GPC analysis, consistent with the reduction in hydrodynamic volume of a cyclic polymer compared to its linear analog. Analysis of the sequence by 1H NMR confirmed that propagation was suppressed nearly completely during the RTA-ATRC phase, with percent monomer conversion remaining constant after the ATRP phase.
Cyclic polymers were produced by end-to-end coupling of telechelic linear polymers under dilute conditions in THF, using intramolecular atom transfer radical coupling or click chemistry. In addition ...to the expected shift to longer elution times on gel permeation chromatography (GPC) indicative of the formation of cyclic product, lower molecular weight species were consistently observed upon analysis of the unpurified cyclization reaction mixture. By systematically removing or altering single reaction components in the highly dilute cyclization reaction, it was found that THF itself was responsible for the low-molecular-weight material, forming oligomeric chains of poly(THF) regardless of the other reaction components. When the reactions were performed at higher concentrations and for shorter time intervals, conducive to intermolecular chain-end-joining reactions, the low-molecular-weight peaks were absent. Isolation of the material and analysis by ¹H NMR confirmed that poly(THF) was being formed in the highly dilute conditions required for cyclization by end-to-end coupling. When a series of mock cyclization reactions were performed with molar ratios of the reactants held constant, but concentrations changed, it was found that lower concentrations of reactants led to higher amounts of poly(THF) side product.
The synthesis of cyclic polystyrene (PSt) with an alkoxyamine functionality has been accomplished by intramolecular radical coupling in the presence of a nitroso radical trap. Linear α,ω-dibrominated ...polystyrene, produced by the atom transfer radical polymerization (ATRP) of styrene using a dibrominated initiator, was subjected to chain-end activation via the atom transfer radical coupling (ATRC) process under pseudodilute conditions in the presence of 2-methyl-2-nitrosopropane (MNP). This radical trap-assisted, intramolecular ATRC (RTA-ATRC) produced cyclic polymers in greater than 90% yields, possessing ⟨G⟩ values in the 0.8–0.9 range as determined by gel permeation chromatography (GPC). Thermal-induced opening of the cycles, made possible by the incorporated alkoxyamine, resulted in a return to the original apparent molecular weight, further supporting the formation of cyclic polymers in the RTA-ATRC reaction. Liquid chromatography–mass spectrometry (LC-MS) provided direct confirmation of the cyclic architecture and the incorporation of the nitroso group into the macrocycle. RTA-ATRC cyclizations carried out with faster rates of polymer addition into the redox active solution and/or in the presence of a much larger excess of MNP (up to a 250:1 ratio of MNP:C–Br chain end) still yielded cyclic polymers that contained alkoxyamine functionality.
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