Nanostructured carbon arrays from block copolymers of polyacrylonitrile

Arrays of graphitic carbon nanoclusters were obtained by pyrolysis of nanoscale phase-separated block copolymers of polyacrylonitrile and poly(n-butyl acrylate). Upon heating in an inert atmosphere to temperatures ranging from approximately 400 to 1200 degrees C, polyacrylonitrile domains were converted into carbon nanoclusters, maintaining the overall shape and spacing, whereas the poly(n-butyl acrylate) phase was sacrificed. Preservation of the original nanoscale morphology of a block copolymer was possible only if pyrolysis was preceded by oxidation at temperatures of approximately 230 degrees C, in analogy with thermal stabilization of polyacrylonitrile precursor in the process used in the manufacturing of carbon fibers. Preorganization of the carbon precursor through self-assembly in block copolymers of polyacrylonitrile appears to be an attractive and robust strategy for templated synthesis of well-defined nanostructured carbon materials.     

Synthesis of hyperbranched degradable polymers by atom transfer radical (Co)polymerization of inimers with ester or disulfide groups

Degradable hyperbranched polymers with multiple alkyl halide chain ends were synthesized by the atom transfer radical polymerization of inimers containing ester (2‐(2′‐bromopropionyloxy)ethyl acrylate) or disulfide (2‐(2′‐bromoisobutyryloxy)ethyl 2′′‐methacryloyloxyethyl disulfide) groups. Both the homo‐ and copolymerizations (with styrene in the former case and methyl methacrylate in the latter) were studied. The hyperbranched polymers derived from the ester‐type inimer were hydrolytically degradable under basic conditions, whereas those derived from the disulfide‐containing inimer could be efficiently degraded in the presence of reducing agents such as tributylphosphine. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2009     

The shear modulus of the human vocal fold in a transverse direction

The aim of this study was to measure the shear modulus of the vocal fold in a human hemilarynx, such that the data can be related to direction of applied stress and anatomical context. Dynamic spring rate data were collected using a modified linear skin rheometer using human hemilarynges, and converted to estimated shear modulus via application of a simple shear model. The measurement probe was attached to the epithelial layer of the vocal fold cover using suction. A sinusoidal force of 3g was applied to the epithelium, and the resultant displacement logged at a rate of 1kHz. Force measurement accuracy was 20microg and position measurement accuracy was 4microm. The force was applied in a transverse direction at the midmembranous point between the vocal process and the anterior commissure. The shear modulus of the three female vocal folds ranged from 814 to 1232Pa. The shear modulus of the three male vocal folds ranged from 1021 to 1796Pa. These data demonstrate that it is possible to obtain estimates for the shear modulus of the vocal fold while preserving anatomical context. The modulus values reported here are higher than those reported using parallel plate rheometry. This is to be expected as the tissue is attached to surrounding structures, and is under natural tension.     

Preparation and functionalization of linear and reductively degradable highly branched cyanoacrylate‐based polymers

Ethyl cyanoacrylate (ECA) was polymerized radically in the presence of small amounts of trifluoroacetic acid as effective inhibitor of incidental anionic polymerization. Methyl methacrylate and other functional vinyl monomers (e.g., 2‐chloroethyl and 2‐bromoethyl methacrylate) were copolymerized with ECA, yielding linear ECA‐rich copolymers, which could readily undergo further modifications, for instance nucleophilic substitution with azide. In the presence of a disulfide‐containing dimethacrylate crosslinker and a chain transfer agent (CBr₄) during the free radical copolymerizations of ECA with methacrylates, highly branched ECA‐based polymers containing disulfide groups at the branching points were obtained prior to gelation. The polymers degraded upon addition of reducing agents. The prepared polymers, which contained peripheral (chain end) alkyl bromide groups as well as pendant alkyl chloride or bromide groups were then reacted with sodium azide, affording azide‐containing polymers that were reacted with functional alkynes under copper‐catalyzed “click” chemistry conditions. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 3683–3693     

Towards understanding monomer coordination in atom transfer radical polymerization: synthesis of [Cu(PMDETA)(π-M)][BPh4] (M = methyl acrylate, styrene, 1-octene, and methyl methacrylate) and structural studies by FT-IR and H NMR spectroscopy and X-ray crystallography

Cu^I complexes of the form [Cu^I(PMDETA)(π-M)][BPh₄] (where PMDETA = N,N,N′,N″,N″-pentamethyldiethylenetriamine, and M = vinyl monomer) were synthesized and isolated from solution as crystals with methyl acrylate (MA), styrene (Sty), and 1-octene (Oct). The interaction of the CC double bond of the vinyl monomer with Cu^I was characterized via FT-IR and ¹H NMR spectroscopy and single crystal X-ray crystallography. A fourth complex with methyl methacrylate (MMA) was synthesized and characterized spectroscopically, but no crystals suitable for X-ray structure analysis could be obtained. In all complexes, PMDETA acts as a tridentate ligand, while the pseudotetrahedral coordination geometry around Cu^I is completed by a π-interaction with the CC double bond of M in the presence of a non-coordinating counter-ion. A decrease in CC IR stretching frequencies of Δν(CC) = −110, −80, −109, and −127 cm⁻¹ for complexes with MA, Sty, Oct, and MMA, respectively, was observed upon coordination. No significant change in CC bond length was seen in the crystal structure for complexes with MA and Oct while a slight lengthening was observed for the Sty complex. The upfield shift of the vinyl proton resonances indicated the presence of significant π-back-bonding.     

Well‐defined epoxide‐containing styrenic polymers and their functionalization with alcohols

Polymers containing electrophilic moieties, such as activated esters, epoxides, and alkyl halides, can be readily modified with a variety of nucleophiles to produce useful functional materials. The modification of epoxide‐containing polymers with amines and other strong nucleophiles is well‐documented, but there are no reports on the modification of such polymers with alcohols. Using phenyloxirane and glycidyl butyrate as low molecular weight model compounds, it was determined that the acid‐catalyzed ring‐opening of aryl‐substituted epoxides by alcohols to form β‐hydroxy ether products was significantly more efficient than that of alkyl‐substituted epoxides. An aryl epoxide‐type styrenic monomer, 4‐vinylphenyloxirane (4VPO), was synthesized in high yield using an improved procedure and then polymerized in a controlled manner under reversible addition‐fragmentation chain‐transfer (RAFT) polymerization conditions. A successful chain extension with styrene proved the high degree of chain‐end functionalization of the poly4VPO‐based macro chain transfer agent. Poly4VPO was modified with a library of alcohols and phenols, some of which contained reactive functionalities, e.g., azide, alkyne, allyl, etc., using either CBr₄ (in PhCN at 90 °C for 2–3 days) or BF₃ (in CH₂Cl₂ at ambient temperature over 30 min) as the catalyst. The resulting β‐hydroxy ether‐functionalized homopolymers were characterized using size exclusion chromatography, ¹H NMR and IR spectroscopy, and thermal gravimetric analysis. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1132–1144     

Stretched poly(methyl methacrylate) gel aligns small organic molecules in chloroform. stereochemical analysis and diastereotopic proton NMR assignment in ludartin using residual dipolar couplings and 3J coupling constant analysis

Poly(methyl methacrylate) (PMMA) gels prepared by copolymerizing methyl methacrylate (MMA) and various amounts of ethylene glycol dimethacrylate (EGDMA) in the presence of the radical initiator V-70 (2,2'-azobis(2,4-dimethyl-4-methoxyvaleronitrile)) can orient small organic molecules when swollen in NMR tubes with CDCl(3). The aligning properties of the stretched PMMA gels were evaluated by monitoring the quadrupolar splitting of the (2)H NMR signal of CDCl(3), and the aligning degree is proportional to the cross-linking density. Natural abundance one-bond (1)H-(13)C residual dipolar couplings (RDCs) for menthol measured in the gels depended on the cross-link density. The stereochemistry and assignment of the diastereotopic protons of the gastroprotective and nonsteroidal aromatase inhibitor sesquiterpene lactone ludartin, isolated from Stevia yaconensis var. subeglandulosa, were unambiguously determined using a combination of natural abundance one-bond (1)H-(13)C RDCs measured in a PMMA gel and a (3)J coupling constant analysis.     

Functionalization of cis-1,4-polyisoprene using hypervalent iodine compounds with tetrazole ligands

Hypervalent iodine(III) compounds with tetrazole ligands C₆H₅I(N₄CR)₂ (R  CH₃, C₆H₅, 4-CH₃C₆H₄) reacted, in the presence of elemental iodine, with the double bonds in cis-1,4-polyisoprene (polyIP) to afford iodo-tetrazolylated polymers. The alkyl-iodide groups in the products of the polyIP functionalization were utilized as macro chain-transfer agents for the iodine-transfer polymerization of methyl methacrylate, which yielded brush polymers with well-defined poly(methyl methacrylate) side chains. In addition, the iodo-tetrazolylated polymers were reacted with NaN₃ in DMF at room temperature, and it was noticed that, in addition to nucleophilic substitution, elimination reactions took place. However, the presence of azide groups was taken advantage of and successful click chemistry-type of grafting-onto reactions were carried out with alkyne-capped poly(ethylene oxide) in the presence of CuBr and N,N,N′,N″,N″-pentamethyldiethylenetriamine. The thermal decomposition of both the iodo-tetrazolylated and the azido-tetrazolylated polymers was exothermic, especially for the latter materials. © 2019 Wiley Periodicals, Inc. J. Polym. Sci. 2020 , 58, 172–180     

Highly active copper-based catalyst for atom transfer radical polymerization

Atom transfer radical polymerization (ATRP) generally requires a catalyst/initiator molar ratio of 0.1 to 1 and catalyst/monomer molar ratio of 0.001 to 0.01 (i.e., catalyst concentration: 1000-10,000 ppm versus monomer). Herein, we report a new copper-based complex CuBr/N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) as a versatile and highly active catalyst for acrylic, methacrylic, and styrenic monomers. The catalyst mediated ATRP at a catalyst/initiator molar ratio of 0.005 and produced polymers with well-controlled molecular weights and low polydispersities. ATRP occurred even at a catalyst/initiator molar ratio as low as 0.001 with copper concentration in the produced polymers as low as 6-8 ppm (catalyst/monomer molar ratio = 10(-5)). The catalyst structures were studied by X-ray diffraction and NMR spectroscopy. The activator CuIBr/TPEN existed in solution as binuclear and mononuclear complexes in equilibrium but as a binuclear complex in its single crystals. The deactivator CuIIBr2/TPEN complex was mononuclear. High stability and appropriate KATRP (ATRP equilibrium constant) were found crucial for the catalyst working under high dilution or in coordinating solvents/monomers. This provides guidance for further design of highly active ATRP catalysts.