New intramolecular exciplexes between polynuclear aromatic hydrocarbons and head-to-head vs head-to-tail photodimerization of mesosubstituted anthracenes.

New dissymetrical non conjugated bichromophores linked by an oxygenated chain: (9-anthrylmethyloxymethyl)arenes[arenes:benzene(I_a),2-naphthalene(I_b),9-phenanthrene (I_c) and 1-pyrene (I_d) and (9-phenanthrylmethyloxymethyl)-1-pyrene (II)] have been prepared. I_c–d and II exhibit intramolecular exciplex fluorescence in methylcyclohexane at room temperature. Intramolecular photoadducts were not found but I_a–d yield a mixture of head-to-head and head-to-tail anthracenic photodimers.     

THE NOVEL MACROCYCLIC AND LINEAR-CHAIN THIOETHERS FROM PERCHLOROBUTADIENE AND DITHIOLES

Compounds 3a–c, 4a, b, 5a–c, and 6a, c were obtained from the reactions of perchlorobutadiene (1) with 1,4-butanedithiol (2a), 1,5-pentanethiol (2b), and 2.2′-(ethlene-dioxyl)diethanethiol (2c) in ethanol in the presence of sodium hydroxide. Compounds 7a, b were obtained from the reactions of thioethers 3a, b with m-chlorperbenzoic acid in CHCl ₃ .     

Ruthenium(II) carbonyl complexes of P,P and P,O donor diphosphine ligands,Ph2P(CH2)nPPh2 and Ph2P(CH2)nP(O)Ph2, n = 2, 3 and their activities in catalytic transfer hydrogenation reactions

The polymeric precursor [RuCl₂(CO)₂]_n reacts with the ligands, P∩P (a, b) and P∩O (c, d), in 1:1 M ratio to generate six-coordinate complexes [RuCl₂(CO)₂(?²-P∩P)] (1a, 1b) and [RuCl₂(CO)₂(?²-P∩O)] (1c, 1d), where P∩P: Ph₂P(CH₂)_nPPh₂, n = 2(a), 3(b); P∩O: Ph₂P(CH₂)_nP(O)Ph₂, n = 2(c), 3(d). The complexes are characterized by elemental analyses, mass spectrometry, thermal studies, IR, and NMR spectroscopy. 1a–1d are active in catalyzed transfer hydrogenation of acetophenone and its derivatives to corresponding alcohols with turnover frequency (TOF) of 75–290 h^?1. The complexes exhibit higher yield of hydrogenation products than catalyzed by RuCl₃ itself. Among 1a–1d, the Ru(II) complexes of bidentate phosphine (1a, 1b) show higher efficiency than their monoxide analogs (1c, 1d). However, the recycling experiments with the catalysts for hydrogenation of 4-nitroacetophenone exhibit a different trend in which the catalytic activities of 1a, 1b, and 1d decrease considerably, while 1c shows similar activity during the second run.     

Synthesis of polypropylene-co-p-methylstyrene copolymers by metallocene and Ziegler–Natta catalysts

This paper discusses the copolymerization reaction of propylene and p-methylstyrene (p-MS) via four of the best-known isospecific catalysts, including two homogeneous metallocene catalysts, namely, {SiMe₂[2-Me-4-Ph(Ind)]₂}ZrCl₂ and Et(Ind)₂ZrCl₂, and two heterogeneous Ziegler–Natta catalysts, namely, MgCl₂/TiCl₄/electron donor (ED)/AlEt₃ and TiCl₃. AA/Et₂AlCl. By comparing the experimental results, metallocene catalysts show no advantage over Ziegler–Natta catalysts. The combination of steric jamming during the consective insertion of 2,1-inserted p-MS and 1,2-inserted propylene (k₂₁ reaction) and the lack of p-MS homopolymerization (k₂₂ reaction) in the metallocene coordination mechanism drastically reduces catalyst activity and polymer molecular weight. On the other hand, the Ziegler–Natta heterogeneous catalyst proceeding with 1,2-specific insertion manner for both monomers shows no retardation because of the p-MS comonomer. Specifically, the supported MgCl₂/TiCl₄/ED/AlEt₃ catalyst, which contains an internal ED, produces copolymers with high molecular weight, high melting point, and no p-MS homopolymer. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2795–2802, 1999     

Formation of 1,1,2,3,3-pentakis(arylthio)-1-propenes from tris(arylthio)cyclopropenyl cations and their conversion into 1,1,2,5,6,6-hexakis(arylthio)-(3E)-1,3,5-hexatriene

The reaction of tetrachlorocyclopropene (1) with arenethiols (2a–e), followed by treatmentwith perchloric acid, gave tris(arylthio)cyclopropenylium perchlorates (3a–c and e), 1,1,2,3,3-pentakis(arylthio)-1-propenes (4a–d), and 2,3,3-tris(arylthio)propenals (5a–d). The structures of tris(phenylthio)cyclopropenylium perchlorate (3a), 1,1,2,3,3-pentakis(phenylthio)-1-propene (4a), and 2,3,3-tris(o-tolylthio)propenal (5b) were analyzed by single-crystal X-ray diffraction studies. The yields depended significantly on the electron-withdrawing property of the substituents of the arenethiols and the molar ratio of 2 to 1. The reaction with 2,6-dimethylbenzenethiol (2e) gave only tris(2,6-dimethylphenylthio)cyclopropenylium perchlorate (3e) without the formation of 4e and 5e. Compounds 5a–d were produced by acid hydrolysis of 4a–d. Pyrolysis of 4a–d gave (3R,4S)-1,1,2,3,4,5,6,6-octakis(arylthio)-1,5-hexadienes (9a–d) and 1,1,2,5,6,6-hexakis(arylthio)-(3E)-1,3,5-hexatrienes (10a–d) together with diaryl disulfides (11a–d). Compound 10a was also produced by photolysis. © 1998 John Wiley & Sons, Inc. Heteroatom Chem 9:387–397, 1998     

Structures of 1‐hydrophenanthroimidazoles: building blocks in the synthesis of expanded‐ring bis(imidazoles)

The structures of 1H‐phenanthro[9,10‐d]imidazole, C₁₅H₁₀N₂, (I), and 3,6‐dibromo‐1H‐phenanthro[9,10‐d]imidazole hemihydrate, C₁₅H₈Br₂N₂·0.5H₂O, (II), contain hydrogen‐bonded polymeric chains linked by columns of π–π stacked essentially planar phenanthroimidazole monomers. In the structure of (I), the asymmetric unit consists of two independent molecules, denoted (Ia) and (Ib), of 1H‐phenanthro[9,10‐d]imidazole. Alternating molecules of (Ia) and (Ib), canted by 79.07 (3)°, form hydrogen‐bonded zigzag polymer chains along the a‐cell direction. The chains are linked by π–π stacking of molecules of (Ia) and (Ib) along the b‐cell direction. In the structure of (II), the asymmetric unit consists of two independent molecules of 3,6‐dibromo‐1H‐phenanthro[9,10‐d]imidazole, denoted (IIa) and (IIb), along with a molecule of water. Alternating molecules of (IIa), (IIb) and water form hydrogen‐bonded polymer chains along the [110] direction. The donor–acceptor distances in these N(imine)...H—O(water)...H—N(amine) hydrogen bonds are the shortest thus far reported for imidazole amine and imine hydrogen‐bond interactions with water. Centrosymmetrically related molecules of (IIa) and (IIb) alternate in columns along the a‐cell direction and are canted by 48.27 (3)°. The present study provides the first examples of structurally characterized 1H‐phenanthroimidazoles.     

Three anilides of 2,2′‐thiodibenzoic acid

The structures of N,N′‐bis(2‐methylphenyl)‐2,2′‐thiodibenzamide, C₂₈H₂₄N₂O₂S, (Ia), N,N′‐bis(2‐ethylphenyl)‐2,2′‐thiodibenzamide, C₃₀H₂₈N₂O₂S, (Ib), and N,N′‐bis(2‐bromophenyl)‐2,2′‐thiodibenzamide, C₂₆H₁₈Br₂N₂O₂S, (Ic), are compared with each other. For the 19 atoms of the consistent thiodibenzamide core, the r.m.s. deviations of the molecules in pairs are 0.29, 0.90 and 0.80 Å for (Ia)/(Ib), (Ia)/(Ic) and (Ib)/(Ic), respectively. The conformations of the central parts of molecules (Ia) and (Ib) are similar due to an intramolecular N—H...O hydrogen‐bonding interaction. The molecules of (Ia) are further linked into infinite chains along the c axis by intermolecular N—H...O interactions, whereas the molecules of (Ib) are linked into chains along b by an intermolecular N—H...π contact. The conformation of (Ic) is quite different from those of (Ia) and (Ib), since there is no intramolecular N—H...O hydrogen bond, but instead there is a possible intramolecular N—H...Br hydrogen bond. The molecules are linked into chains along c by intermolecular N—H...O hydrogen bonds.     

Synthesis of New Fused and Spiro 1,5‐Benzodiazepines

[1,3‐Dihydro‐4‐phenyl(1,5)benzodiazepin‐2‐ylidene]malononitrile 1_a was treated with formaline and some different primary amines to give the corresponding pyrimido(1,5)benzodiazepines 2_a–d . Treatment of compound 1_a with halo reagents yielded the corresponding pyrrolobenzodiazepines 3_a,b . The reaction of compound 1_a with active methylenes, bidentates, S,S‐ and N,S‐acetals afforded the corresponding spiro(1,5)‐benzodiazepines 4_a‐c–8_a,b , respectively.     

Synthesis of Fused Isolated Azoles and N-Heteroaryl Derivatives Based on 2-Methyl-3,4-dihydrothieno[3,4-d]pyrimidin-5-amine

In this work, we report the synthesis and characterization of new compounds derived from thieno[d]pyrimidines. The formation of isolated and fused thieno[d]pyrimidine derivatives was achieved via reacting 5-amino-(2-methyl)thieno[3,4-d]pyrimidin-4(3H)-one (3) with some selected reagents. The starting compound (2) was prepared in a quantitative yield using a modified procedure by conversion of the cyano group in 1 to the amide via hydrolysis using concentrated H₂SO₄. Methylthieno[3,4-d]pyrimidin-5-yl (8, 9, and 18), 3-phenylthieno[3,4-e][1,2,4]triazolo[4,3-c]pyrimidines (11–15) and tetrazolo[1,5-c]thieno[3,4-e]pyrimidine (16) have been synthesized in excellent isolated yield. The interaction of N-acetyl derivative 19 with benzaldehyde and/or some nitroso compounds afforded the chalcones and Schiff's bases derivatives 20 and 26a and c respectively. The latter compounds were used as key intermediates in the synthesis of N-acetylpyrazol-3-yl (21a and 21b), 6-phenylpyrimidine-2-(one)thione (22a and b), pyrazolo-1-carbothioamide (25), and thiazolidinone andβ β-lactam derivatives 28a and 28c and 30a and 30c respectively. The structures of these compounds were established by elemental analysis, infrared (IR), mass spectrometry (MS), and NMR spectral analysis.     

Competing intermolecular interactions in some `bridge‐flipped' isomeric phenylhydrazones

To examine the roles of competing intermolecular interactions in differentiating the molecular packing arrangements of some isomeric phenylhydrazones from each other, the crystal structures of five nitrile–halogen substituted phenylhydrazones and two nitro–halogen substituted phenylhydrazones have been determined and are described here: (E)‐4‐cyanobenzaldehyde 4‐chlorophenylhydrazone, C₁₄H₁₀ClN₃, (Ia); (E)‐4‐cyanobenzaldehyde 4‐bromophenylhydrazone, C₁₄H₁₀BrN₃, (Ib); (E)‐4‐cyanobenzaldehyde 4‐iodophenylhydrazone, C₁₄H₁₀IN₃, (Ic); (E)‐4‐bromobenzaldehyde 4‐cyanophenylhydrazone, C₁₄H₁₀BrN₃, (IIb); (E)‐4‐iodobenzaldehyde 4‐cyanophenylhydrazone, C₁₄H₁₀IN₃, (IIc); (E)‐4‐chlorobenzaldehyde 4‐nitrophenylhydrazone, C₁₃H₁₀ClN₃O₂, (III); and (E)‐4‐nitrobenzaldehyde 4‐chlorophenylhydrazone, C₁₃H₁₀ClN₃O₂, (IV). Both (Ia) and (Ib) are disordered (less than 7% of the molecules have the minor orientation in each structure). Pairs (Ia)/(Ib) and (IIb)/(IIc), related by a halogen exchange, are isomorphous, but none of the `bridge‐flipped' isomeric pairs, viz. (Ib)/(IIb), (Ic)/(IIc) or (III)/(IV), is isomorphous. In the nitrile–halogen structures (Ia)–(Ic) and (IIb)–(IIc), only the bridge N—H group and not the bridge C—H group acts as a hydrogen‐bond donor to the nitrile group, but in the nitro–halogen structures (III) (with Z′ = 2) and (IV), both the bridge N—H group and the bridge C—H group interact with the nitro group as hydrogen‐bond donors, albeit via different motifs. The occurrence here of the bridge C—H contact with a hydrogen‐bond acceptor suggests the possibility that other pairs of `bridge‐flipped' isomeric phenylhydrazones may prove to be isomorphous, regardless of the change from isomer to isomer in the position of the N—H group within the bridge.     

Polymorphism of 2,5‐[(diphenylphosphanyl)methyl]‐1,1,2,4,4,5‐hexaphenyl‐1,4‐diphospha‐2,5‐diboracyclohexane

2,5‐[(Diphenylphosphanyl)methyl]‐1,1,2,4,4,5‐hexaphenyl‐1,4‐diphospha‐2,5‐diboracyclohexane shows polymorphism as two tetrahydrofuran (THF) disolvates [C₆₄H₅₈B₂P₄·2C₄H₈O, (Ia) and (Ib)] and pseudo‐polymorphism as its toluene monosolvate [C₆₄H₅₈B₂P₄·C₇H₈, (Ic)]. In each of polymorphs (Ia) and (Ib), the diphosphadiboracyclohexane molecule is located on a centre of inversion. The THF molecule of (Ib) is disordered over two sites, with a site‐occupation factor of 0.612 (8) for the major‐occupied site. Both structures crystallize in the same space group (P2₁/n), but they display a different crystal packing. For pseudo‐polymorph (Ic), although the space group is P2₁/c, which is just a different setting of the P2₁/n space group of (Ia) and (Ib), the crystal packing is completely different. Although the crystal packing in these three structures is significantly different, their molecular conformations are surprisingly the same.     

Syntheses and coordination behaviour of 2-(ortho-phosphinophenyl)-functionalised 1,3-dioxolanes and 1,3-dioxanes towards a [(COD)Rh]-complex fragment - models for immobilised complexes

The syntheses are reported of the ether-phosphine ligands: 2-(ortho-diphenylphosphinophenyl)-1,3-dioxolane (1a), 2-(ortho-diisopropylphosphinophenyl)-1,3-dioxolane (1b), 2-(ortho-diphenylphosphinophenyl)-1,3-dioxane (1c), 2-(ortho-diisopropylphosphinophenyl)-1,3-dioxane (1d). Their reaction with [(COD)RhCl]₂ (COD: 1,5-cyclooctadiene) results in the formation of the mononuclear complexes: {chloro(COD)[2-(ortho-diphenylphosphinophenyl)-1,3-dioxolane]rhodium(I)} (2a), {chloro(COD)[2-(ortho-diisopropylphosphinophenyl)-1,3-dioxolane]rhodium(I)} (2b), {chloro(COD)[2-(ortho-diphenylphosphinophenyl)-1,3-dioxane]rhodium(I)} (2c), and {chloro(COD)[2-(ortho-diisopropylphosphinophenyl)-1,3-dioxane]rhodium(I)} (2d). The chloride ligands of compounds 2a and 2b were abstracted with TlPF₆, with accompanied insertion of an acetal oxygen atom of the ligands 1a and 1b into the coordination sphere of the metal centre, producing {(COD)[η²-P,O-2-(ortho-diphenylphosphinophenyl)-1,3-dioxolane]rhodium(I)}PF₆ (3a∗PF₆) and {(COD)[η²-P,O-2-(ortho-diisopropylphosphinophenyl)-1,3-dioxolane]rhodium(I)}PF₆ (3b∗PF₆). In contrast the dioxane analogues of 3, 3c∗BF₄ and 3d∗BF₄, were formed by reacting the ligands 1c, 1d with [Rh(COD)₂]BF₄. The ligands 1 and the complexes 2 serve as model compounds for their via acetalation to a polyvinylalcohol resin bound analogues. The complexes synthesised were employed as pre-catalysts in the hydroformylation reaction of 1-octene.     

Syntheses,characterization and catalytic activities of half-sandwich ruthenium complexes with naphthalene-based Schiff base ligands

Four ruthenium(II) p-cymene complexes with naphthalene-based Schiff base ligands [Ru(p-cymene)LCl] (2a–2d) have been synthesized and characterized. The half-sandwich ruthenium complexes were characterized by ¹H and ¹³C NMR spectra, elemental analyses, and infrared spectrometry. The molecular structures of 2a, 2b, and 2c were confirmed by single-crystal X-ray diffraction. Furthermore, these half-sandwich ruthenium complexes are highly active catalysts for the hydrogenation of nitroarenes to anilines using NaBH₄ as the reducing agent in ethanol at room temperature.     

Reactions ofo-tosylaminobenzaldehyde withp-aminobenzenesulfonamides. Synthesis and structures of 13-(4-aminosulfonylphenyl)-6,12-epimino-5,11-ditosyl-5,6,11,12-tetrahydrodibenzo[b,f]-1,5-diazocine and itsN-substituted derivatives

The reactions ofo-tosylaminobenzaldehyde (1) withp-aminobenzenesulfonamides (2a-c) yielded 13-(p-RNHSO₂C₆H₄)-6,12-epimino-5,11-ditosyl-5,6,11,12-tetrahydrodibenzo[b f]-1,5-diazocines (3a-c) (R=H (a); 2-thiazolyl (b); or 2,6-dimethoxy-4-pyrimidinyl 2,6-dimethoxy-4-pyrimidinyl (c). The structure of compound3c was confirmed by X-ray diffurction study. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2035–2039, November, 1997.     

Copolymerization of vinyl monomers with Gd(OCOCCl3)3-(i-Bu)3Al-Et2AlCl

The polymerization of polar monomers such as methyl methacrylate (MMA), methyl acrylate (MA), methacrylonitrile (MAN), and acrylonitrile (AN) was carried out with gadolinium-based Ziegler–Natta catalysts [Gd(OCOCCl₃)₃-(i-Bu)₃Al-Et₂AlCl] in hexane at 50°C under N₂ to elucidate the effect of the monomer's HOMO(highest occupied moleculor orbital) and LUMO (lowest unoccupied molecular orbital) levels on the polymerizability. In the case of homopolymerization, all monomers were found to polymerize and the order of relative polymerizability was as follows: MM > MA > MAN > AN. On the other hand, the result of copolymerization of St with MMA shows that the values of the monomer reactivity ratios are r₁ = 0.06 and r₂ = 1.98 for St(M₁)/MMA(M₂). The monomer reactivity ratios of styrene (St), p-methoxystyrene (PMOS), p-methylstyrene (PMS), and p-chlorostyrene (PCS) evaluated as r₁ = 0.55 and r₂ = 1.07 for St(M₁)/PMOS(M₂), r₁ = 0.38 and r₂ = 0.51 for St(M₁)/PMS(M₂), and r₁ = 0.72 and r₂ = 1.25 for St(M₁)/PCS(M₂) were compared with those for St(M₁)/MMA(M₂). The copolymerization behavior is apparently different from the titanium-based Ziegler—Natta catalyst, regarding a larger monomer reactivity ratio of PCS. The lower LUMO level of PCS and MMA may enhance a back-donation process from the metal catalyst, therefore resulting in high polymerizability. These results are discussed on the basis of the energy level of the gadolinium catalyst and the HOMO and LUMO levels of the monomers. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 2591–2597, 1997     

Synthesis,characterization and cytotoxic activity of diorganotin complexes with Schiff base derived from salicylaldehyde and l-tyrosine

Seven diorganotin complexes with the Schiff bases derived from salicylaldehyde and l-tyrosine, R₂Sn[2-O-5-XC₆H₃CH?=?NCH(CH₂C₆ H₄OH-4)COO] (X?=?H (1), Br (2); R?=?Me (a), Et (b), Bu (c), Cy (cyclohexyl) (d)), were synthesized and characterized by elemental analysis, IR, ¹H and ¹³C NMR spectra, and the single-crystal X-ray diffraction. In methanol, the racemization of chiral center of l-tyrosinate fragment occurred and the racemic products were obtained. X-ray analyses of 1c, 1d, and 2a–2c showed that the tin atoms of the complexes exhibit distorted trigonal-bipyramidal geometries. In 1c, 1d, and 2c, the intermolecular O–H???O hydrogen bonds connected the molecules into 1-D supramolecular chain or a R₂²(20) macrocyclic dimer, and 2a and 2b formed the 2-D supramolecular network by the intermolecular Sn???O and O–H???O interactions. Bioassay results indicated that 1a, 1c, and 1d had moderate antibacterial activity against Escherichia coli and 1c, 1d, and 2c belonged to the efficient cytostatic agents against two human tumor cell lines (A549 and HeLa) and the activity tends to follow the order Cy > Bu?>?Et?>?Me for the R group attached to tin.     

Pseudopolymorphs of chelidamic acid and its dimethyl ester

Different tautomeric and zwitterionic forms of chelidamic acid (4‐hydroxypyridine‐2,6‐dicarboxylic acid) are present in the crystal structures of chelidamic acid methanol monosolvate, C₇H₅NO₅·CH₄O, (Ia), dimethylammonium chelidamate (dimethylammonium 6‐carboxy‐4‐hydroxypyridine‐2‐carboxylate), C₂H₈N⁺·C₇H₄NO₅⁻, (Ib), and chelidamic acid dimethyl sulfoxide monosolvate, C₇H₅NO₅·C₂H₆OS, (Ic). While the zwitterionic pyridinium carboxylate in (Ia) can be explained from the pK_a values, a (partially) deprotonated hydroxy group in the presence of a neutral carboxy group, as observed in (Ib) and (Ic), is unexpected. In (Ib), there are two formula units in the asymmetric unit with the chelidamic acid entities connected by a symmetric O—H...O hydrogen bond. Also, crystals of chelidamic acid dimethyl ester (dimethyl 4‐hydroxypyridine‐2,6‐dicarboxylate) were obtained as a monohydrate, C₉H₉NO₅·H₂O, (IIa), and as a solvent‐free modification, in which both ester molecules adopt the hydroxypyridine form. In (IIa), the solvent water molecule stabilizes the synperiplanar conformation of both carbonyl O atoms with respect to the pyridine N atom by two O—H...O hydrogen bonds, whereas an antiperiplanar arrangement is observed in the water‐free structure. A database study and ab initio energy calculations help to compare the stabilities of the various ester conformations.     

Concomitant but disappearing: two polymorphs of 1,4‐bis(tribromomethyl)benzene

The title compound, C₈H₄Br₆, (I), initially crystallized from deuterochloroform as the comcomitant polymorphs (Ia) (prisms, space group P2₁/n, Z = 2) and (Ib) (hexagonal plates, space group C2/c, Z = 4). The molecules in both forms display crystallographic inversion symmetry. All further attempts to crystallize the compound led exclusively to (Ib), so that (Ia) may be regarded as a `disappearing polymorph'. Surprisingly, however, the density of (Ia) is greater than that of (Ib). The only significant difference between the molecular structures is the orientation of the CBr<sub>3</sub> groups. The molecular packing of both structures is largely determined by Br...Br interactions, although (Ia) also displays a C—H...Br hydrogen bond and both polymorphs display one Br...π contact. For (Ia), six of the eight contacts combine to form a tube‐like substructure parallel to the a axis. For (Ib), the two shortest Br...Br contacts link `half' molecules consisting of C—CBr₃ groups to form double layers parallel to (001) in the regions z≃, .     

The surfactant-sensitized analytical reaction of niobium with some thiazolylazo compounds

Cationic surfactants, such as cetylpyridinium bromide (CPB), sensitize the color reaction of Nb(V) with 1-(2-benzothiazolylazo)-2-hydroxy-3-naphthoic acid (I_a), 5-(benzothiazolylazo)2,5-naphthalenediol (I_b), 5-(2-benzothiazolylazo)8-hydroxyquinoline (I_c) and 4-(2- benzothiazolylazo)2, -biphenyldiol (I_d) reagents. The formation of a ternary complex of stoichiometric ratio 1:2:2 (Nb-R-CPB) is responsible for the observed enhancement in the molar absorptivity and the Sandell sensitivity of the formed complex, when a surfactant is present. The ternary complex exhibits absorption maxima at 649, 692, 661 and 612 nm, (=3.35×10⁴, 3.59×10⁴, 4.46×10⁴ and 2.79×10⁴ l mol⁻¹ cm⁻¹) on using reagent I_a, I_b, I_c, and I_d, respectively. Beer’s law is obeyed between 0.05 and 2.50 μg ml⁻¹, while applying the Ringbom method for more accurate results is in the range from 0.20 to 2.30 μg ml⁻¹. Conditional formation constants in the presence and absence of CPB for niobium complexes have been calculated. On the basis of a detailed spectrophotometric study, the nature of the chromophoric reagent–surfactant interaction and the peculiar features of the sensitization by CPB are discussed.     

Ethylene–hexene copolymerization by heterogeneous and homogeneous Ziegler–Natta catalysts and the “comonomer” effect

Homopolymerization of ethylene and 1-hexene and their copolymerizations were compared to investigate the influence of α-olefin on the enhancement of ethylene polymerization rate (R_p), which is often referred to as the “comonomer” effect. With the two homogeneous Ziegler–Natta catalysts, Et[Ind]₂ZrCl₂/MAO and (π-C₅H₅)₂ZrCl₂/MAO (MAO = methylaluminoxane), hexene causes reduction of R_p—in other words a negative “comonomer” effect. In the case of the high activity MgCl₂ supported TiCl₃ catalysts there is a slight positive “comonomer” effect; the R_p increases by 25 to 70% with the addition of 15 mol % of hexene. The “comonomer” effects in there catalyst systems are much smaller than that observed for the classical TiCl₃ catalyst. © 1993 John Wiley & Sons, Inc.