Journal of Polymer Science Part A: Polymer Chemistry

Cover image for Vol. 44 Issue 3

1 February 2006

Volume 44, Issue 3

Pages 1049–1346

  1. Articles

    1. Top of page
    2. Articles
    3. Note
    1. Mechanistic investigations of antimony-catalyzed polycondensation in the synthesis of poly(ethylene terephthalate) (pages 1049–1059)

      Faissal-Ali El-Toufaili, Gunter Feix and Karl-Heinz Reichert

      Article first published online: 12 DEC 2005 | DOI: 10.1002/pola.21200

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      Antimony-catalyzed synthesis of polyethylene terephthalate was studied to elucidate its mechanism. A polycondensation mechanism was proposed, in which activation occurs by formation of a chelate ligand on antimony composed of hydroxyl- and ester alcoholic oxygens of the hydroxy ethylene ester end groups. Polycondensation rate determining step was concluded to be coordination of a second chain end to antimony. Low activity of antimony at high concentration of hydroxyl end groups was proposed to result from competition between these groups and the chelate structure leading to the transition state. Antimony was found to stabilize hydroxy ethylene ester end groups against degradation.

    2. Ligand effect in Ti-mediated living radical styrene polymerizations initiated by epoxide radical ring opening. III. Substituted sandwich metallocenes (pages 1060–1070)

      Alexandru D. Asandei and Isaac W. Moran

      Article first published online: 13 DEC 2005 | DOI: 10.1002/pola.21220

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      The ligand effect and reaction conditions for the living radical polymerization of styrene initiated by epoxide radical ring opening were investigated in a series of RCp2TiX2-substituted sandwich metallocenes (Cp = cyclopentadienyl). A weak dependence (R = H ∼ Et ∼ iPr ∼ tBu > indenyl ≫ pentamethylcyclopentadienyl) of the polymerization on the nature of the Cp substituent but an apparent X = Cl > Br ≪ F, CO, Me and RCp2TiCl2 > RCpTiCl3 trend were observed and were explained by a combination of steric and electronic effects.

    3. Gel effect in the bulk reversible addition–fragmentation chain transfer polymerization of methyl methacrylate: Modeling and experiments (pages 1071–1085)

      A. David Peklak, Alessandro Butté, Giuseppe Storti and Massimo Morbidelli

      Article first published online: 13 DEC 2005 | DOI: 10.1002/pola.21218

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      A model for the evaluation of the kinetics and the chain length distribution in living/controlled radical polymerization mediated by reversible addition–fragmentation chain transfer (RAFT) in bulk is presented. Using the free volume theory, the model accounts for the diffusion limitations over both termination and RAFT exchange reactions. Model predictions are compared to experimental results of methyl methacrylate polymerization with cumyl dithiobenzoate as a RAFT agent. It is shown that the polymerization retardation observed in living systems at large conversions is well predicted.

    4. Synthesis of polyisoprene–poly(methyl methacrylate) block copolymers via a two-electron-transfer mechanism (pages 1086–1092)

      Henryk Janeczek

      Article first published online: 13 DEC 2005 | DOI: 10.1002/pola.21224

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      Supramolecular complexes of alkali metals were used as catalysts in the polymerization of isoprene. In the reaction of isoprene with an alkali-metal complex, two electrons were transferred from the initiator to the isoprene monomer to form a dianion, which underwent instant protonation by the solvent and yielded a respective isoprene carbanion, which was capable of inducing isoprene polymerization. The obtained polyisoprene, having a living end group, was subsequently used to initiate methyl methacrylate polymerization in tetrahydrofuran.

    5. Epoxy resin containing polyphenylsilsesquioxane: Preparation, morphology, and thermomechanical properties (pages 1093–1105)

      Yong Ni and Sixun Zheng

      Article first published online: 13 DEC 2005 | DOI: 10.1002/pola.21222

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      Polyphenylsilsesquioxane (PPSQ) was incorporated into an epoxy resin to afford organic–inorganic composites, and two approaches were used to prepare composites with different morphologies. Phase separation induced by polymerization occurred in the physical blending system. However, nanostructured composites were obtained when a catalytic amount of aluminum triacetylacetonate was added to mediate the reaction between PPSQ and diglycidyl ether of bisphenol A. Composites with macroscopic and nanoscopic morphologies displayed quite different thermomechanical properties.

    6. Cp2TiCl-catalyzed living radical polymerization of styrene initiated from peroxides (pages 1106–1116)

      Alexandru D. Asandei and Gobinda Saha

      Article first published online: 15 DEC 2005 | DOI: 10.1002/pola.21206

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      The effects of reaction conditions and initiator nature were investigated in the Cp2Ti(III)Cl-catalyzed styrene living radical polymerization initiated by peroxides. The reversible termination of the growing chains with Cp2Ti(III)Cl affords a linear dependence of Mn on conversion over a wide range of temperatures (60–120 °C) and with an optimum (Mw/Mn < 1.2) for St/BPO/Cp2TiCl2/Zn = 100/1/3/6 at 60–90 °C. The similarity of the polymerization kinetics from different peroxides and the reagent ratios required for a living process indicate that initiation occurs primarily by the Cp2Ti(III)Cl/peroxide redox reaction. These initiators ranked as BPO > TBPOB ∼ TBPO > CPO > TBPOEHC.

    7. Synthesis and thermoresponsive property of end-functionalized poly(N-isopropylacrylamide) with pyrenyl group (pages 1117–1124)

      Qian Duan, Yutaka Miura, Atsushi Narumi, Xiande Shen, Shin-Ichiro Sato, Toshifumi Satoh and Toyoji Kakuchi

      Article first published online: 15 DEC 2005 | DOI: 10.1002/pola.21208

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      End-functionalized poly(N-isopropylacrylamide) with the pyrenyl group, Py–PNIPAM, was prepared through the ATRP of N-isopropylacrylamide, using 1-pyreneyl-2-chloropropionate, PyCP, as the initiator and CuCl/tris[2-(dimethylamino)ethyl]amine as the catalyst system. The LCST of the aqueous Py–PNIPAM solution showed the low value of 21.7 °C, which was raised to ∼26 °C after the addition of β-cyclodextrin.

    8. Low potential electrosyntheses of free-standing poly(dibenzofuran) films in mixed electrolytes of boron trifluoride diethyl etherate and trifluoroacetic acid (pages 1125–1135)

      Jingkun Xu, Weiqiang Zhou, Bing Chen, Shouzhi Pu, Jingwu Wang and Yiqun Wan

      Article first published online: 15 DEC 2005 | DOI: 10.1002/pola.21210

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      Free-standing poly (dibenzofuran) (PDBF) films with good electrochemical behavior, good electrochromic properties, good thermal stability, good fluorescence properties especially in solid state, and conductivity of 100 S cm−1 were synthesized electrochemically by low potential direct anodic oxidation of dibenzofuran in mixed electrolytes of boron trifluoride diethyl etherate (BFEE) containing certain amount of trifluoroacetic acid (TFA). As-formed PDBF films were partly soluble in tetrahydrofuran (THF) or chloroform. To the best of our knowledge, this is the first report that free-standing PDBF films have been electrodeposited.

    9. Polymerization of 4-methylpentene and vinylcyclohexane by amine bis(phenolate) titanium and zirconium complexes (pages 1136–1146)

      Shimrit Gendler, Stanislav Groysman, Zeev Goldschmidt, Michael Shuster and Moshe Kol

      Article first published online: 15 DEC 2005 | DOI: 10.1002/pola.21225

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      The polymerization of the substituted olefins 4-methylpentene and vinylcyclohexane by amine bis(phenolate) titanium and zirconium complexes was investigated. Both titanium and zirconium complexes were found to be efficient catalysts for the polymerization of these monomers. The zirconium catalysts were more active than the titanium catalysts and yielded high-molecular-weight poly(4-methylpentene). The new zirconium catalysts were also investigated for the polymerization of 1-hexene. Their activity was found to be monomer-dependent: the most active catalysts toward 1-hexene included electron-withdrawing (Cl and Br) substituents on the ligand's phenolate rings, whereas the most active catalyst toward 4-methylpentene and vinylcyclohexane included methyl phenolate substituents.

    10. Monoaryloxo ytterbium(III) chlorides supported by β-diketiminato ligands as novel initiators for the living ring-opening polymerization of ε-caprolactone (pages 1147–1152)

      Mingqiang Xue, Hongmei Sun, Hui Zhou, Yingming Yao, Qi Shen and Yong Zhang

      Article first published online: 15 DEC 2005 | DOI: 10.1002/pola.21211

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      A novel kind of single-component catalysts, monoaryloxo ytterbium(III) chlorides supported by β-diketiminato ligands, were found to be efficient initiators for the living ring-opening polymerization of ε-caprolactone at a broad range of temperatures from 0 to 50 °C.

    11. Synthesis and characterization of poly(phenylacetylene)s carrying oligo(ethylene oxide) pendants (pages 1153–1167)

      Junwu Chen, Kevin Ka-Leung Cheuk and Ben Zhong Tang

      Article first published online: 15 DEC 2005 | DOI: 10.1002/pola.21230

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      Amphiphilic poly(phenylacetylene)s bearing polar oligo(ethylene oxide) pendants were synthesized through organorhodium-catalyzed polymerization. Although the catalysts did not work well for the polymerizations of the monomers containing hydroxyl groups, they effected the polymerization of the monomers without hydroxyl groups, giving soluble polymers with a weight-average molecular weight up to ∼160 × 103 and a yield up to 99%.

    12. Inorganic–organic nanocomposites of polybenzoxazine with octa(propylglycidyl ether) polyhedral oligomeric silsesquioxane (pages 1168–1181)

      Yonghong Liu and Sixun Zheng

      Article first published online: 13 DEC 2005 | DOI: 10.1002/pola.21231

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      Octa(propylglycidyl ether) polyhedral oligomeric silsesquioxane (OpePOSS) was used to prepare polybenzoxazine (PBA-a) nanocomposites containing polyhedral oligomeric silsesquioxane (POSS). The crosslinking reactions involved with the formation of the organic–inorganic networks could be divided into two types: (1) the ring-opening polymerization of benzoxazine and (2) the subsequent reaction between the in situ formed phenolic hydroxyls of PBA-a and the epoxide groups of OpePOSS. The formation of the POSS-containing nanocomposites was evidenced by scanning electron, transmission electron, and atomic force microscopy. The thermal and dynamic mechanical analyses indicated that the organic–inorganic networks displayed improved properties.

    13. Preparation and properties of isobutylene–isoprene rubber–clay nanocomposites (pages 1182–1188)

      Makoto Kato, Azusa Tsukigase, Hiromitsu Tanaka, Arimitsu Usuki and Isamu Inai

      Article first published online: 13 DEC 2005 | DOI: 10.1002/pola.21233

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      Isobutylene isoprene rubber (IIR)-clay nanocomposites have been prepared successfully by melt intercalation with maleic anhydride-grafted IIR (Ma-g-IIR) and organophilic clay. The gas permeability coefficient decreased with an increase in clay content. Upon addition of 15 phr of clay, the gas permeability coefficient of the IIR-clay nanocomposite decreased to 40% of that of the matrix. The excellent gas barrier properties of the IIR-clay nanocomposite originated from nanometer level dispersion of the silicate layers. Because the dispersed silicate layers cause meandering of the gas diffusion paths, the permeability constant decreases.

    14. Synthesis and properties of new fluorene-based polyquinoxalines with an ether linkage in the main chain for light-emitting diodes (pages 1189–1198)

      Sung-Hyun Jung, Dong Young Kim, Hyun-Nam Cho and Dong Hack Suh

      Article first published online: 20 DEC 2005 | DOI: 10.1002/pola.21170

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      A new series of fluorene-based polyquinoxalines with an ether linkage in the main chain were prepared by the polycondensation reaction between tetraketone monomers and 3,3′,4,4′-tetraaminodiphenyl ether. The resulting polymers (P1P3) [P1 = poly(quinoxaline-co-9,9-dihexyl-2,7-dimethyl-9H-fluorene) P2 = poly(quioxaline-co-9,9-dihexyl-9-pentyl-2,7-di-p-tolyl-9H-fluorene) P3 = poly(quioxaline-co-9,9-bis- (4-methoxy-phenyl)-2,7-dimethyl-9H-fluorene)] showed good solubility in common organic solvents and also were thermally stable, showing a 5% weight loss up to 440 °C. Electroluminescence devices of the configuration indium tin oxide/ poly(3,4-ethylenedioxythiophene)/polymer:poly(N-vinylcarbazole) blend (2:8)/LiF/Al were fabricated with P1 or P2 and emitted blue light with electroluminescence peak wavelengths of 434 and 448 nm, respectively.

    15. Synthesis of poly(glycolic acid-alt-12-aminododecanoic acid): The thermal polymerization kinetics of sodium N-chloroacetyl-12-aminododecanoate (pages 1199–1213)

      E. Botines, L. Franco, X. Ramis and J. Puiggalí

      Article first published online: 20 DEC 2005 | DOI: 10.1002/pola.21226

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      A thermal polycondensation method involving the formation of metal halide salts as a driving force is effective for preparing a new poly(ester amide) with an alternate disposition of glycolic and aminododecanoic acid units. The polymer can be obtained with weight-average molecular weights higher than 30,000, depending on the polymerization conditions, and shows a semicrystalline nature. Isoconversional procedures have been performed to study the polymerization kinetics in solid and liquefied phases.

    16. New antibacterial cationic surfactants prepared by atom transfer radical polymerization (pages 1214–1224)

      Sandrine Lenoir, Christophe Pagnoulle, Christophe Detrembleur, Moreno Galleni and Robert Jérôme

      Article first published online: 20 DEC 2005 | DOI: 10.1002/pola.21229

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      Antibacterial surfactants were prepared by the quaternization of the amino groups of a poly(ethylene-co-butylene)-b-poly[2-(dimethylamino)ethylmethacrylate] (PEB-b-PDMAEMA) diblock copolymer by octyl bromide. The diblock copolymer was synthesized by the ATRP of DMAEMA initiated by an activated bromide-end-capped poly(ethylene-co-butylene). The optimal experimental conditions for the synthesis required a mixed CuCl/CuCl2 (10 wt %) catalyst ligated by 1,4,7,10,10-hexamethyl-triethylenetetramine in toluene at 50 °C. The quaternization of the poly[2-(dimethylamino)ethylmethacrylate] block of PEB-b-PDMAEMA by octyl bromide resulted in the micellization of the amphiphilic copolymer in water and provided an effective antibacterial surfactant at a low concentration (150 μg/mL).

    17. Alternating polyester/fluoroalkene copolymers: Combining high hydrophobicity with degradability (pages 1225–1232)

      Sachin Borkar, Ayusman Sen and Jeffrey R. Shallenberger

      Article first published online: 19 DEC 2005 | DOI: 10.1002/pola.21212

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      2-Methylene-1,3-dioxepane (MDO) was copolymerized with fluoroalkenes (RfCH[DOUBLE BOND]CH2) using AIBN as an initiator. The obtained polymers are effectively alternating copolymers of caprolactone and fluoroalkene. In thin films, the perfluoroalkyl tails segregate to the air-side thereby presenting a hydrophobic surface. Blends of this material with polycaprolactone and polycarbonate polymers also form hydrophobic films.

    18. Quinone transfer radical polymerization of styrene: Synthesis of the actual initiator (pages 1233–1244)

      Antoine Debuigne, Jean-Raphaël Caille and Robert Jérôme

      Article first published online: 15 DEC 2005 | DOI: 10.1002/pola.21234

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      The quinone transfer radical polymerization (QTRP) of styrene has been successfully initiated at 100 °C by α-hydroxy ketone derivatives of phenanthrenequinone added with a catalytic amount of cobalt(II) acetylacetonate. These initiators, which mimic the dormant species proposed in the QTRP mechanism, are effective control agents for the radical polymerization of styrene; this gives credit to the recently proposed mechanism. Moreover, the induction period, which has been systematically observed in the presence of phenanthrenequinone, is eliminated when an α-hydroxy ketone derivative is used instead.

    19. Synthesis, characterization, and thermal properties of a novel pentaerythritol-initiated star-shaped poly(p-dioxanone) (pages 1245–1251)

      Hai-Xia Huang, Ke-Ke Yang, Yu-Zhong Wang, Xiu-Li Wang and Jun Li

      Article first published online: 15 DEC 2005 | DOI: 10.1002/pola.21240

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      A novel star-shaped poly(p-dioxanone) (ssPPDO) was synthesized by the ring-opening polymerization of p-dioxanone initiated by pentaerythritol with stannous octoate as a catalyst in bulk. The number of initiating hydroxyl groups was close to the theoretical value (four [BOND]OH groups). The thermal stability of ssPPDO could be improved by an increase in its chain length. ssPPDO had the same crystalline structure as the linear poly(p-dioxanone) (PPDO) but a lower crystallization temperature and melting point than linear PPDO with similar molecular weights.

    20. Surface-initiated atom transfer radical polymerization grafting of poly(2,2,2-trifluoroethyl methacrylate) from flat silicon wafer surfaces (pages 1252–1262)

      Renxu Chen, Wei Feng, Shiping Zhu, Gianluigi Botton, Beng Ong and Yiliang Wu

      Article first published online: 15 DEC 2005 | DOI: 10.1002/pola.21232

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      Poly(2,2,2-trifluoroethyl methacrylate) (PTFEMA), a partially fluorinated polymer, was directly grafted from silicon wafer surfaces by surface-initiated atom transfer radical polymerization (ATRP). The polymer layer thickness increased linearly with monomer conversion and reached more than 100 nm. The grafted PTFEMA chains were “living” and allowed the extension of a second block of PMMA. A novel surface-attachable difunctional initiator was synthesized with which the grafting density was doubled from 0.48 to 0.86 chains/nm2. The scheme shows the reactions that involved in the surface ATRP grafting of the fluorinated polymer.

    21. Pd(II)-catalyzed polymerization of optically active norbornene carboxylic acid esters (pages 1263–1270)

      Gwang-Su Byun, Sang Youl Kim and Iwhan Cho

      Article first published online: 15 DEC 2005 | DOI: 10.1002/pola.21227

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      (−)-(1S,2R)-Norbornene-2-carboxylic acid methyl esters (alkyl = Me, Bz, L-menthyl, or D-menthyl) were successfully prepared by the Diels–Alder reaction of cyclopentadiene with (R)-(−)-pantolactone-O-yl acrylate followed by epimerization and column chromatography. The enantiomeric excess was 99.9%. These monomers were polymerized by Pd(II)-based catalysts, and high yields of the polymers were obtained. The methyl ester gave an optically active polymer of high optical rotation (monomer [α]D = −24.7, polymer [α]D = −98.5). This high rotation was not observed with methyl esters prepared by the transesterification of menthyl esters. The stereoregular polymer exhibited notable resonance peaks at 39 ppm in 13C NMR spectra.

    22. Synthesis and micellization of amphiphilic poly(sebacic anhydride)–poly(ethylene glycol)–poly(sebacic anhydride) block copolymers (pages 1271–1278)

      Na Zhang and Sheng-Rong Guo

      Article first published online: 15 DEC 2005 | DOI: 10.1002/pola.21235

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      Amphiphilic biodegradable block copolymers [poly(sebacic anhydride)–poly(ethylene glycol)–poly(sebacic anhydride)] were synthesized by the melt polycondensation of poly(ethylene glycol) and sebacic anhydride prepolymers. The copolymers were characterized with 1H NMR, Fourier transform infrared, gel permeation chromatography, and differential scanning calorimetry. 1H NMR spectra carried out in CDCl3 and D2O were used to demonstrate the existence of hydrophobic PSA domains as the core of the micelle. In aqueous media, the copolymers formed micelles after precipitation from water-miscible solvents. The effects on the micelle sizes due to the micelle preparation conditions, such as the organic phase, droppings rate of the polymer organic solution into the aqueous phase, and copolymer concentrations in the organic phase, were studied.

    23. Poly(alkylthio-p-phenylenevinylene): Synthesis and electroluminescent and photovoltaic properties (pages 1279–1290)

      Jianhui Hou, Benhu Fan, Lijun Huo, Chang He, Chunhe Yang and Yongfang Li

      Article first published online: 19 DEC 2005 | DOI: 10.1002/pola.21242

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      Two alkylthio-substituted poly(p-phenylenevinylene) derivatives, poly(2-octylthio-p-phenylenevinylene) (OT–PPV) and poly[5-methoxy-2-(2′-ethyl-hexylthio)-p-phenylenevinylene] (MEHT–PPV), were synthesized by a Heck coupling reaction. A narrow photoluminescence emission band was observed for the MEHT–PPV film and solution, and the electroluminescence (EL) spectra of OT–PPV and MEHT–PPV showed very narrow bandwidths with full widths at half-maximum of 40 and 47 nm, respectively. The maximum EL efficiency of the single-layer polymer light-emitting diode based on MEHT–PPV reached 1.48 cd/A. The polymer solar cell based on OT–PPV/[6,6]-phenyl-C61 butyric acid methyl ester showed a power conversion efficiency of 1.4% under the illumination of AM1.5 (80 mW/cm2).

    24. Synthesis and characterization of polyimides derived from (3-amino-2,4,6-trimethylphenyl)-(3′-aminophenyl)methanone and aromatic dianhydrides (pages 1291–1298)

      Z. X. Li, L. Q. Lin, W. M. Zhang, T. Wu and J. L. Pu

      Article first published online: 21 DEC 2005 | DOI: 10.1002/pola.21241

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      A series of polyimides were derived from a new diamine of (3-amino-2,4,6-trimethylphenyl)-(3′-aminophenyl)methanone and aromatic dianhydrides. Most of the polyimides had good solubility both in strong aprotic solvents and in common organic solvents. The polyimides exhibited high glass-transition values and excellent thermal properties, with an initial thermal decomposition temperature above 470 °C and glass-transition temperatures in the range of 280–320 °C. The polyimide films also exhibited good mechanical properties as well as transparency in the visible-light region, with transmittance higher than 80% at 450 nm and a cutoff wavelength lower than 370 nm.

    25. Copolymerization of vinylidene difluoride with hexafluoropropene in supercritical carbon dioxide (pages 1299–1316)

      Uwe Beginn, Reza Najjar, Jürgen Ellmann, Rostislav Vinokur, Roland Martin and Martin Möller

      Article first published online: 21 DEC 2005 | DOI: 10.1002/pola.21214

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      The free radical copolymerization of vinylidene difluoride and hexafluoropropene in supercritical carbon dioxide is presented. The obtained copolymers possessed bimodal molecular weight distributions, and their average comonomer composition are well described by the Lewis–Mayo equation with the copolymerization parameters rVDF = 4.8 and rHFP = 0.

    26. New combined polymer/chromium approach for investigating the phototransformations involved in hologram formation in dichromated poly(vinyl alcohol) (pages 1317–1325)

      Fatma Djouani, Yaël Israëli, Lawrence Frezet, Agnès Rivaton, Roger A. Lessard and Michèle Bolte

      Article first published online: 21 DEC 2005 | DOI: 10.1002/pola.21243

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      An increase in the insoluble fraction of dichromated poly-(vinyl alcohol) film upon exposure proves, for the first time, that hologram formation results from crosslinking proceeding through coordination bonds around chromium(V). The crosslinking is enhanced when the polymer is doped by ammonium dichromate [poly(vinyl alcohol)/ammonium dichromate] with respect to potassium dichromate [poly(vinyl alcohol)/potassium dichromate]. This enhancement, involving additional chelating sites, explains a well-known experimental feature: ammonium dichromate doped poly(vinyl alcohol) yields better holograms than those obtained from potassium dichromate.

    27. Synthesis and characterization of novel perfluorinated polyimides with 3D-controlled structures for photonic applications (pages 1326–1342)

      Jin Kyu Park, Dong Hun Lee, Bok Ju Song, Jae Buem Oh and Hwan Kyu Kim

      Article first published online: 21 DEC 2005 | DOI: 10.1002/pola.21209

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      We have designed and synthesized novel perfluorinated polyimides with 3D-controlled structure. We successfully developed the new synthetic routes to diamines with pendant bulky perfluorinated aromatic units in a multi-step synthetic procedure. Novel perfluorinated polyimides were prepared in a two-step reaction in NMP solution. The refractive index and birefringence of the resultant polyimides are 1.5858–1.6452 and 0.01–0.005, respectively. The refractive index of polyimide decreases with increasing the fluorine content. Surprisingly, the pendant ether linkage is not a crucial factor in reducing the polyimide birefringence. Their birefringence is comparatively very low, compared with that of previous polyimides.

  2. Note

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    2. Articles
    3. Note
    1. You have free access to this content
      Role of back-biting in the stereoselectivity of Ni(II)-catalyzed butadiene polymerization (pages 1343–1346)

      Chiara Costabile, Stefania Pragliola and Pasquale Longo

      Article first published online: 21 DEC 2005 | DOI: 10.1002/pola.21228

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      Remarkable information on the relevance of back-biting to determining the stereoselectivity in the polymerization of butadiene is reported. The first insertion of the butadiene unit in the metal–methyl σ bond is preferentially 1,4-trans, whereas during chain propagation, more than 90% of the monomer is in a 1,4-cis arrangement. The experimental results have been rationalized by the fact that during the first insertion, anti–syn isomerization of the allyl group is favored.