Multi-arm star polyisobutylenes: 3. New insight into the chemistry of star formation: Star-star coupling and core contraction

Details of the chemistry of linking living polyisobutylene (PIB⊕) pre-arms with divinylbenzene (DVB) to multi-arm PIB stars have been studied by triple-detector size-exclusion chromatography (SEC). Computer-aided deconvolution of multimodal differential refractive index (RI), laser light scattering (LLS), and ultraviolet (UV) traces obtained by SEC indicates the formation of complex product mixtures. The observations were interpreted in terms of inter-and intramolecular reactions of initially-formed active stars. Thus intermolecular or star-star couplings are proposed to lead to higher order (secondary, tertiary, etc.) stars, whereas intramolecular reactions result in core-contraction. The mechanism of these reactions has been outlined. The effect of PIB⊕ pre-arm length on star-star coupling was analyzed.     

Effect of reaction conditions on synthesis and properties of multiarm star-branched polyisobutylenes

A series of multiarm star-branched polyisobutylenes was synthesized from narrow polydispersity arms with molecular weights ranging from 12,000 to 60,000 g/mol, via living carbocationic polymerization using the cumyl chloride/TiCl₄/pyridine initiating system and divinylbenzene (DVB) as core-forming comonomer. The effect on star development of arm molecular weight, temperature, solvent composition, and DVB concentration was studied. The rate of star formation and the weight-average number of arms per star polymer, N̄_w, were found to scale inversely with arm molecular weight; N̄_w = 60 was attained for 13,100 g/mol arms, but N̄_w = 2.5 for 60,000 g/mol arms. It was established that star formation was much faster at −80°C compared to 23°C, regardless of solvent composition. For hexane : methyl chloride (MeCl) solvent compositions containing from 40 to 60 vol % MeCl, star–star coupling was observed at −80°C, but not at 23°C, even after 312 h; for the most polar 40 : 60 hexane : MeCl composition, star–star coupling was so extensive at −80°C that gelation was observed after only 44 h. The rate of star formation was found to be substantially higher in 60 : 40 hexane : MeCl compared to 60 : 40 hexane : methylene chloride (MeCl₂). Some reactions containing MeCl were immediately warmed to 23°C after DVB addition, and the MeCl thus volatilized was replaced with either MeCl₂ or hexane for the duration of the star-forming reaction. Slightly higher rates were consistently observed when MeCl₂ was the replacement solvent. The strong influence of initial MeCl content on rate of star formation was found to persist throughout the star-forming reaction, even when the solvent was immediately converted to 100% hexane. The fraction of arms that remained unlinked into stars was found to be higher at the higher temperature and at lower solvent polarity. Regardless of solvent or temperature, the residual arm fraction was approximately the same at a given stage of star development as measured by the average number of arms per star. One star sample was produced with the UV-transparent 2-chloro-2,4,4-trimethylpentane initiator; analysis showed that the residual arm fraction had approximately the same UV absorbance as the star fraction, indicating efficient crossover to DVB and the potential for approximately quantitative arm incorporation given sufficient time. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36 : 471–483, 1998     

Living carbocationic polymerization. XXX. One-pot synthesis of allyl-terminated linear and tri-arm star polyisobutylenes,and epoxy- and hydroxy-telechelics therefrom

Mono- and di-ended linear and three-arm star allyl-telechelic polyisobutylenes, PIB? A, A? PIB? A and (where A = ? CH₂? CH?CH₂) have been prepared by a rapid economical one-pot polymerization-functionalization process. The process involves the living polymerization of isobutylene (IB) by mono-, di-, or tri-functional initiating systems, specifically by allphatic and aromatic tert-ester and -ether/TiCl₄ combinations, followed by electrophilic functionalization of the living sites with allyl-trimethylsilane (ATMS). Structural characterization by ¹H-NMR spectroscopy and end group titration with m-chloroperbenzoic acid (m-CPBA) indicate quantitative end allylation even with relatively slowly initiating systems like DiOAcTMH₇/TiCl₄. Detailed kinetic analysis of the latter system indicates, unexpetedly, cationation to be rate determining. Quantitative derivatizations of the allyl termini have yielded mono-, di-, and tri-epoxy- and -hydroxy-telechelic PIBs. Strong rubbery networks have been made by curing the epoxy-telechelic PIBs with triethylene tetramine and by reacting the hydroxy-telechelic PIBs with MDI.     

The effect of synthesis conditions on the microheterogeneity of the butadiene-isoprene copolymer

The dependence of the microheterogeneity of butadiene-isoprene copolymers synthesized in the presence of a titanium-based catalytic system on the hydrodynamic regime used at the initial moment of formation of the reaction mixture of copolymerization is studied. When copolymers are synthesized with the direct addition of the catalyst to the solution of the comonomers, copolymers featuring a statistical distribution of units are formed, regardless of the composition of the initial comonomer mixture. In this case, the growth of copolymer chains obeys the Bernoulli statistics. Depending of the composition of the initial comonomer mixture, hydrodynamic factors promote different extents of deviation from the statistical growth of copolymer chains.     

New epoxide initiators for the controlled synthesis of functionalized polyisobutylenes

The mechanism of initiation was investigated in isobutylene (IB) polymerizations initiated by epoxidized α‐methylstyrene (MSE) and 1,2‐epoxy‐2,4,4‐trimethylpentane (TMPO) in conjunction with TiCl₄. The proposed mechanism predicts primary OH head groups and tertiary Cl end groups in the PIB. Model studies conducted with MSE/TiCl₄ and diisobutylene lead to ring closure yielding a substituted furanyl structure. Real‐time fiber‐optic refractive index monitoring was used to follow the initiation with the TMPO/TiCl₄ system. It was found that the cleavage of TMPO proceeds simultaneously by S_N1 and S_N2 mechanisms as proposed. The carbocation forming by the S_N1 route is proposed to initiate the polymerization of IB, but it was shown that excess TiCl₄ relative to TMPO was necessary for propagation. Isomerization and polyether formation by the S_N2 pathway lead to side reactions, reducing the initiating efficiency.     

The effect of impregnation conditions on the surface structure of silica-supported CuO catalysts

CuO/SiO₂ catalysts with varying amounts of copper were prepard using meso- and microporous silica supports at pH > 10 and pH = 4.5. Structural and textural changes were followed using X-ray diffraction, TG and DTA techniques. Impregnation for periods > 10 days at high pH produces crystalline catalysts with two distinct peaks at d-spacings of 2.33 and 2.03 Å resulting from a surface silicate which is structurally stable up to 800°C. At copper concentrations > 5% CuO also forms. Catalysts prepared at pH = 4.5 are amorphous to X-rays in spite of the presence of CuO which may either be < 50 Å or from a surface solid solution. The copper ammine complex, if adsorbed on mesoporous silica, attains its maximum coordination number as [Cu(NH₃)₄(H₂O)₂]²⁺, whereas on microporous silica it loses the two water molecules as a result of pore restrictions. The surface complex releases its coordinated ammonia exothermally in the temperature range 200–400°C, whereas chemisorbed ammonia is evolved endothermally at ～280°C. Ligand water is evolved at <200°C. An exotherm at ～545°C is observed for all catalysts, resulting form the shrinkage of the solid/void matrix which disappears upon aging. Increase of copper content to 22.7% at high pH lowered the temperature of constant weight attainment from 1000°C for the pure silica to 750°C.     

Peptide synthesis catalysed by papain immobilised on polymer supports: Effect of the macromolecular structure and reaction conditions on synthesis

Papain immobilised on different types of polymeric supports was used for the synthesis of peptides in aqueous-organic solvent mixtures. The effects of the nature of the polymer support, degree of crosslinking, nature and length of the spacer grouping between the polymer backbone and the point of attachment of the enzyme, and reaction conditions like pH, concentration of nucleophile and the immobilised enzyme content on the course of the synthesis were investigated. Divinylbenzene-crosslinked polystyrene, divinylbenzenecrosslinked polyacrylamide and N,N′-methylene-bis-acrylamide-crosslinked polyacrylamide systems immobilised with papain were used for these studies. An increase in the length of the spacer arm and an increase in hydrophilicity invariably resulted in an increase in the yield of the peptide synthesis. Papain immobilised on polystyrene-PEG supports and tetraethyleneglycol-crosslinked polystyrene supports was determined to be more efficient in effecting peptide synthesis when compared to other polystyrene-based supports.     

Constituents of crinoidea. 2. Isolation and structure of the novel type gangliosides from the feather star Comanthus japonica.

Two novel type gangliosides CJP2 and CJP3 have been obtained from the feather star Comanthus japonica. On the basis of methylation linkage analysis combined with ammonolysis and other chemical and spectroscopic evidence, the chemical structures of CJP2 and CJP3 were determined to be alpha-9-O-Me-NeuGc-(2-->3)-inositolphosphoceramide and alpha-9-O-Me-NeuGc-(2-->11)-alpha-9-O-Me-NeuGc-(2-->3)-inositolphosphoceramide, respectively. These gangliosides are unique in that they are inositolphosphoceramide derivatives possessing sialic acid; such gangliosides have not previously been identified. The presence of 9-O-methyl-N-glycolyl-neuraminosyl residues is also unique in naturally occurring gangliosides.     

不同制备条件对纳米Bi~2O~3发光的影响

本文用微乳液法合成了不同表面活性剂包覆的和反应物阴阳离子配比不同的纳米Bi~2O~3有机溶胶。发现阴阳离子配比对纳米体系的发光有很大影响, 不同的表面包覆剂对发光的影响则较小, 并对此进行了解释。     

The effect of processing conditions on the u.v. stability of polyolefins

It is shown that the lifetime of polyolefins in the presence of u.v. light is markedly dependent on the degree of oxidation occurring during the prior processing operation. This is influenced strongly by the presence of pro-oxidants and anti-oxidants, and their effect on subsequent u.v. life can be rationalized on the basis of their functions during processing.     

Impact of synthesis conditions on surface chemistry and structure of carbide-derived carbons

Carbide-derived carbons produced by chlorination of titanium carbide at 600, 800, or 1100 °C were subjected to a post-treatment at 600 °C in Ar, H₂, or NH₃ atmosphere. Experimental results suggest that the chlorination temperature influences the ordering of carbon in a manner that impacts specific surface area and porosity. Higher chlorination temperatures lead to higher total pore volume and increased ordering, but lower microporosity. The effect of post-treatments on surface chemistry is pronounced only for samples chlorinated at 600 °C; post-treatments in Ar are shown to be less effective for chlorine removal than those performed in H₂ or NH₃. Post-treatments in Ar result in a lower total pore volume compared to the ones in H₂ or NH₃ for the same chlorination temperature. Samples chlorinated at higher temperatures contained less oxygen functionalities than samples chlorinated at 600 °C, and showed correspondingly less desorption of H₂O, possibly due to diminished uptake of ambient water.     

The effect of gel structure on matrix orientation.

Four polymeric gels of different structure, low electroendosmosis (LE) agarose, highest electroendosmosis (HEEO) agarose, beta-carrageenan, and polyacrylamide, were studied by transient electric birefringence to determine the importance of various structural features on the orientation of the gels in an electric field. The two types of agarose, but not the polyacrylamide or beta-carrageenan, exhibited anomalous orientation effects. Both agarose and beta-carrageenan exhibited large birefringence signals, suggesting that the noncovalent hydrogen bonds joining the agarose fibers within the matrix allow the high degree of orientation of the gel. The spatial arrangement of the sugars of the agarose backbone is necessary for the anomalous orientation effects in reversing electric fields.     

The effect of N2O4 on the crystalline structure of cellulose

Depending on reaction conditions, the system cellulose–N₂O₄ may give two different unstable crystalline compounds, one being an ester (cellulose trinitrite), the second, an adduct of cellulose and HNO₃ (the Knecht compound). For these compounds, mechanisms of the formation of the crystalline phase as a result of topochemical reaction and self-organization are discussed. The different characteristics of structural transformations of the fiber under nitrosation and nitration are noted. The existence of polymorphic forms of the Knecht compound is suggested. These labile nitrogen-containing compounds make possible the regeneration of cellulose in its various modifications (cellulose I, II, IV, or amorphous cellulose) from the cellulose–N₂O₄ system. The formation of unstable compounds and their ability to crystallize in the reaction medium allows the passage from amorphous cellulose to its crystalline modifications II or IV under mild conditions. The causes of decrystallization of cellulose by N₂O₄ are established. © 1993 John Wiley & Sons, Inc.     

Cocrystallization in blends of random tetrafluoroethylene fluorinated copolymers: The effect of the chain structure and crystallization conditions

The possibility of the cocrystallization of random fluorinated tetrafluoroethylene copolymers was investigated with differential scanning calorimetry and wide‐angle X‐ray scattering. In particular, mixtures composed of poly(tetrafluoroethylene)‐co‐(hexafluoropropylene) containing 8 or 1 mol % comonomer or poly(tetrafluoroethylene)‐co‐perfluoromethylvinylether (2–10 mol % comonomer) were examined. The extent of cocrystallization was determined by the difference in the comonomer content, being higher when the difference was lower, and it was favored when quenching from the melt state was adopted. Nevertheless, a key to determining the extent of cocrystallization was the behavior of counits with respect to inclusion or exclusion from the crystal lattice: when the components were different with respect to this behavior, they were not likely to be miscible in the crystal state even if the difference in the comonomer content was low. Moreover, the similarity in the crystallization rates between the components played an important role: the cocrystallization decreased as the difference in the crystallization rate increased until, when the difference became high enough, the blend became immiscible. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1477–1489, 2002     

Investigation of the effect of reaction conditions on the synthesis of multiarm-star polyisobutylene-polystyrene block copolymers

New multi-arm star block copolymers comprising of rubbery polyisobutylene (PIB) midsegment and glassy polystyrene (PS) end blocks have been synthesized by carbocationic polymerization using a new multifunctional initiator, hexaepoxy squalene (HES), with TiCl₄ coinitiator, di-t-butylpyridine (DtBP) as a proton trap and N,N-dimethylacetamide (DMA) as an electron pair donor in methylcyclohexane (MeCHx)/methyl chloride (MeCl) solvent mixtures at −80 °C. It was found that reaction conditions, such as solvent composition, HES/isobutylene (IB) ratio and TiCl₄ concentration, have profound influence on initiator efficiency and functionality. Living conditions were achieved in the presence of DMA in MeCHx/MeCl 60/40 v/v, while in the absence of DMA, the M_n-conversion plot showed a considerable intercept. Depending on the reaction conditions, the PIB midblocks had 3-10 arms. Reaction rates increased with increasing solvent polarity and TiCl₄ concentration. Living narrow molecular weight distribution PIBs (M_w/M_n=1.1-1.2) were reacted with styrene (St) solution containing DtBP and DMA to yield multiarm-star PIB-PS block copolymers. Blocking was evidenced by SEC analysis and copolymers with 8.9-28.6 wt.% PS, M_n∼164,000-609,000 g/mol and M_w/M_n=1.32-1.88 were successfully synthesized.     

Synthesis of SnO_2 nanorods from aqueous solution:The effect of preparation conditions on the formed patterns

<正>SnO_2 nanorods were deposited on the Si substrates in an aqueous solution containing both SnCl_4 and CO(NH_2)_2.It is found that different self-assembled patterns of SnO_2 nanorods can be obtained by changing the deposition conditions such as the molar ratio of CO(NH_2)_2 to SnCl_4 and the pretreatment of the substrate.Scattered SnO_2 nanorods,for example,can be changed into flower-like patterns when the molar ratio of CO(NH_2)_2 to SnCl_4 is raised,and well-aligned nanorod arrays can be formed when the pretreatment of the substrate is changed.In addition,some interesting patterns,e.g.tree-like patterns,can also be observed.     

Effects of template synthesis conditions on TiO2 gel structuring

Studies have been made on the effects of template synthesis conditions (initial substances, SA/TiO₂ ratio, and pH) on TiO₂ gel formation; under certain conditions, a nanoperiodic structure is formed in the presence of a cationic surfactant SA (ethonium, cetyltrimethylammonium chloride) and in the presence of anionic SA (sodium dodecylsulfat, and dodecylbenzenesulfonic acid). A feature of the TiO₂ gels made in the presence of a crown ether (dibenzo-18-crown-6) is that it contains a nanoperiodic structure and a crystalline one (anatase). L. V. Pisarzhevskii Institute of Physical Chemistry, National Academy of Sciences of Ukraine, 31 Prospekt Nauki, Kiev 03039, Ukraine. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 36, No. 4, pp. 251–256, July–August, 2000.     

Fourier transform IR spectroscopical investigations of the anodic oxide films on aluminum: Part. 2. Anodizing conditions effect on the coating

The aim of the present, second part of FT-IR spectroscopical investigation was the effect of the anodizing parameters on the anodic oxide coating structure on aluminium.From the spectroscopical data the following conclusions has been drawn: the molecular structure of the coating strongly depend on the electrolyte used for anodizing, the electrolyte concentration and current density does not change the mechanism of the coating formation; however its effect is the same as that of time of anodizing.