摘要:

The interior particle morphology of dispersion-polymerized polystyrene particles was investigated by freeze fracture TEM (FFTEM) in order to compare the morphology of particles formed under two extreme mechanistic conditions: (a) particle growth by scavenging of dead polymer chains formed in solution (which gives rise to large particles of 5–10 μm) and (b) particle growth by oligomeric radical capture and polymerization inside the particle (giving rise to small particles of 1 μm or less). Although not generally recognized in the literature, ascribing mechanistic significance to an observed morphology requires a demonstration that the morphology is of the non-equilibrium type, i.e., the morphology depends upon how the particle was formed. An equilibrium morphology, by definition, must be independent of the particle formation route, and therefore carries no mechanistic information. In this case, large PS particles of 6.5 μm were found to have a nodular morphology that was unchanged on heating well aboveTg, indicating an equilibrium structure (the nodularity of amorphous polymers is briefly rationalized by comparison with literature results over the past 20 years). On the other hand, small PS particles of 1.3 μm had a pronounced radial structure that could be converted into the nodular morphology by the same heat treatment, or by dissolving and precipitating from a suitable solvent combination. This means that the small particles have a non-equilibrium morphology, from which we may attempt to draw mechanistic inferences consistent with the suspected growth mechanism.Keywords: dispersion-polymerized polystyrene, non-equilibrium polystyrene particle morphology, freeze fracture transmission electron microscopy.

ISSN:0008-4042    

DOI:10.1139/v95-215

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Polystyrene (PS) and poly(2-hydroxyethyl methacrylate) (PHEMA) macromonomers with eitherp-vinylbenzyl or methacrylate end groups were prepared by living anionic polymerizations of styrene and 2-(trimethylsilyloxy)ethyl methacrylate, respectively, followed by appropriate end-capping. They were radically copolymerized with 2-hydroxyethyl methacrylate and styrene, respectively, to afford PS-g-PHEMA and PHEMA-g-PS, having PS trunks with PHEMA branches or the reverse architecture.1H NMR spectra of the graft copolymers in deuterated methanol showed their micelle formation with PS and PHEMA chains as the core and the shell, respectively. In chloroform the reverse micelles were observed. Contact angle measurements of water on the graft copolymer films revealed that PS chains were generally dominant on the free surface. The films either cast from methanol or after immersion into water, however, were apparently wettable, indicating dominant PHEMA chains on the surface.Keywords: styrene, 2-hydroxyethyl methacrylate, graft copolymer, micelle formation, wettability.

ISSN:0008-4042    

DOI:10.1139/v95-216

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The interactions between poly(vinyl alcohol) and the surfactant sodium dodecyl sulphate in aqueous solutions have been investigated by means of static and dynamic fluorescence measurements. By static fluorescence measurements the onset of interaction, the critical aggregation concentration, is determined to be 6.5 mM at 20 °C and 7.0 mM at 40 °C. By dynamic fluorescence measurements without added quencher the influence on the lifetime of the fluorescent probe, pyrene, by the presence of the polymer is investigated. From measurements with an added quencher, dimethylbenzophenone, the surfactant aggregation numbers of the micellar aggregates formed upon interaction between the surfactant and the polymer and the quenching rate constant are obtained. At the two polymer concentrations, i.e., 0.2% and 2.0%, the surfactant aggregation numbers at low surfactant concentration are very small, but increase rapidly with increasing surfactant concentration. Comparisons are made with results from a system where poly(ethylene oxide) instead of poly(vinyl alcohol) is the polymer.Keywords: dynamic fluorescence quenching, polymer–surfactant interactions, aggregation numbers.

ISSN:0008-4042    

DOI:10.1139/v95-217

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Poly[4′-[[(2-methacryloyloxy)ethyl]ethylamino]azobenzene] (pMAEA) was prepared. Its reversible optical storage properties were studied and compared with the properties of previously reported poly[4-(2-methacryloyloxy)ethylazobenzene] (pMEA) and poly[4-nitrophenyl-4′-[[2-(methacryloyloxy)ethyl]ethyl-amino]phenyldiazene] (pDR1M). The effect of the dipole moment of the azobenzene moiety on the optical storage properties was studied by comparing the photochemically induced birefringence "writing" and "relaxation" behaviors. The writing and relaxation rates are both in the order pDR1M > pMAEA > pMEA. In addition, pDR1M, which contains donor/acceptor substituents in the azobenzene side chain, exhibits the highest saturation level of optically induced birefringence. The fraction of birefringence conserved after relaxation is lowest for pMEA, which has no donor/acceptor substituents in the azobenzene side chain, and highest for pDR1M.Keywords: polymers, azobenzene type, birefringence, photochemically induced orientation, reversible optical storage.

ISSN:0008-4042    

DOI:10.1139/v95-218

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The H/D exchange reactions occurring on collision of protonated alkylbenzenes with C2H5OD, CH3OD, and D2O at low collision energies in the quadrupole collision cell of a hybrid tandem mass spectrometer have been studied for 22 alkylbenzenes. The exchange reactions with D2O are much less efficient than the exchange reactions with C2H5OD or CH3OD; this difference is rationalized on energetics grounds. The H/D exchange of the protonated alkylbenzenes with C2H5OD or CH3OD leads, typically, to 40–70% deuterium incorporation in the protonated alkylbenzene and, for most cases, to significant ion signals corresponding to exchange of the added hydrogen and the aromatic hydrogens. Thus, the method is a feasible approach to counting the number of aromatic hydrogens in alkylbenzenes. The extent of exchange decreases with increasing size of the alkyl group in monoalkylbenzenes for reasons that are not readily understood.Keywords: protonated alkylbenzenes, H/D exchange, reactive collisions.

ISSN:0008-4042    

DOI:10.1139/v95-219

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The thermal reaction of benzene thiol withrac-1-(4aS*,5R*,8S*,8aR*-5,8-methano-1,2,3,4,4a,5,8,8a-octahydro-4a-naphthyl)ethanone (10b), anendo-5-acetylnorborn-2-ene with a particularly close distance between one olefinic carbon and the keto carbon, and with its 8aS* epimer10a, has been investigated. Besides the conventional unrearranged adducts16a,band17a,bof the thiol to the C=C bonds of10a,b, an adduct18was formed from10bthat involved a [1,3]-acetyl shift; no other products were formed. The dependence of adduct ratios on thiol concentration is consistent with two competing reactions of an intermediate radical14bformed by addition of thiyl radical to the C=C bond of10b, namely, abstraction of hydrogen from thiol (which is the conventional reaction path) vs. intramolecular attack of the radical on the keto group inducing the [1,3] shift of the latter. This shift constitutes an intramolecular variety of the known alkyl transposition reaction of ketones initiated by a reversible attack of an alkyl radical on a keto carbonyl group to generate an intermediatetert-alkoxyl radical; however, it is very much faster than the intermolecular reaction and corresponds to an effective molarity between 107and 1010. These findings have a bearing on the mechanism of photorearrangements homologous to the ODPM (oxa-di-π-methane) photorearrangement, such as ODPE (oxa-di-π-ethane) and higher homologues, in that they support a two-step mechanism involving this type of acyl shift.Keywords: oxadi-π-methane photorearrangement; oxadi-π-ethane photorearrangement; acetyl shift, non-allylic [1,3]; thiol, addition to C=C bonds; norbornenes.

ISSN:0008-4042    

DOI:10.1139/v95-220

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Under cationic conditions using triflic acid as the initiator, it is possible to oligomerize β-trichlorosilylstyrene2to low molecular weight oligomers14with a maximum degree of polymerization of about 9. Termination of the process was shown to occur by an intramolecular Friedel–Crafts reaction, leading to highly functionalized, indane-terminated oligomers9,12, etc. At lower temperatures, the reaction is diastereoselective. The oligomerization process was shown to require electron-withdrawing groups on silicon; the replacement of Cl spectator ligands with alkoxy or alkyl groups led to protiodesilylation. The mechanism for formation of the indane-terminated oligomers is discussed.Keywords: cationic polymerization, β-silylstyrene, stereoselective oligomerization, indene termination.

ISSN:0008-4042    

DOI:10.1139/v95-221

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

One- and two-electron transfer reactions of 3,5-di-tert-butylcatechol and the corresponding quinone were studied by fast kinetic spectroscopy coupled with laser photolysis and pulse radiolysis, and by cyclic voltammetry in aqueous solutions. Photoionization of catechol yields the same semiquinone radical as the formate-radical-induced quinone reduction in the pulse radiolysis experiment. The neutral semiquinone radical deprotonates at pKr = 6.0 ± 0.1, as deduced from the pH induced changes in the radical spectra. The two-electron reduction potentials of quinone and catechol were measured by cyclic voltammetry in aqueous solutions containing 25% methanol in the pH range 3–14. The pH dependence has two linear parts with slopes of 0.056 and 0.03 V/pH, intersecting at pKa = 10.0. This is in excellent agreement with pKa = 10.05 ± 0.05 obtained spectrophotometrically. The reduction potential of 3,5-di-tert-butyl quinone,E7 = 0.01 ± 0.04 V, was obtained from the one-electron transfer equilibrium of the quinone with the oxygen/superoxide redox couple by pulse radiolysis. The rate constant of the reaction of quinone with the superoxide radical,k = 1.2 × 109 M−1s−1, is higher than those of the superoxide reduction by catechols and phenols (typically ~ 105 M−1s−1); thus, lignino-quinones could be efficient scavengers of superoxide under the conditions relevant for the photo-induced yellowing of lignin-rich paper. The reaction witheffectively bleaches yellow quinone and generates metastable furanone, which hydrolyses to muconic acid, thus completely eliminating yellow quinone. 3,5-Di-tert-butylquinone also undergoes rapid bleaching with ascorbate,k = 600 ± 100 M−1s−1, in methanol. The reaction has a 1:1 stoichiometry and leads to complete reduction of quinone to catechol with concomitant oxidation of ascorbate to dehydroascorbate. This unusual selectivity and the fact that the reaction of the milder oxidant 3,5-di-tert-butylquinone is an order of magnitude faster than that of stronger oxidantp-benzoquinone (k = 60 ± 10 M−1s−1) suggest that a nucleophilic attack of quinone at the ascorbate double bond initiates the reaction.Keywords: superoxide, lignin, photochemistry, quinone.

ISSN:0008-4042    

DOI:10.1139/v95-222

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

New mono- and disubstituted cholesteryl derivatives of itaconic acid have been prepared and their thermotropic liquid crystalline behaviour examined. The monosubstituted derivative has been homopolymerized, and also copolymerized with a non-chiral mesogen 4-cyanophenyl-4′-(6-acryloyl oxyhexyloxy) benzoate. Examination of the series of copolymers prepared, using differential scanning calorimetry and hot-stage polarizing microscopy, showed that when the content of the cholesteryl itaconate was high, both a smectic-A phase (SA) and a cholesteric phase (N*) were present. It was found that the SAphase could be eliminated by lowering the content of the cholesteryl itaconate in the copolymers, giving samples that displayed only the N* phase over a much wider temperature range. The samples in the N* phase also exhibited selective reflection of visible light that changed from short to long wavelengths as the samples were cooled from the isotropic melt. These colours can be locked into the glassy state of the polymer by quenching below the glass transition temperature, but only if the SAphase is absent. It was also noted that at high contents of the cholesteryl itaconate the selective reflection appears to occur in the ultraviolet region.Keywords: itaconic acid, copolymers, liquid crystalline polymers, cholesteric phases, selective reflection.

ISSN:0008-4042    

DOI:10.1139/v95-223

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Reaction of 1,4-bis(trimethylsilylethynyl)benzene with CCl3COCl and Zn gave 1,4-bis(4′,4′-dichloro-2′-trimethylsilyl-3′-oxo-1′-cyclobutenyl)benzene (7), which reacted with concentrated H2SO4at 87 °C to give 1,4-bis(2′-trimethylsilyl-3′,4′-dioxo-1′-cyclobutenyl)benzene (8). Photolysis of8in CDCl3with 350 nm light gave the bisketene9, which on further photolysis formed the title tetraketene10, as identified by its spectral properties. Upon heating of10the successive closure of the two bisketene moieties, first to give the mono(cyclobutenedione)9and then to reform8in a slower step, was observed by NMR and UV spectroscopy. Reaction of10with MeOH gave the tetraester11, which was assigned theerythroconfiguration at each of the succinate moieties.Keywords: ketenes, photolysis, cyclobutenediones, esterification, silicon.

ISSN:0008-4042    

DOI:10.1139/v95-224

出版商:NRC Research Press    

年代:1995

数据来源: NRC