Electrochemical Characterization of Myoglobin‐Polylysine Films at a Temperature Range of 6–80 °C

This work demonstrated for the first time that myoglobin cross‐linked in polylysine films is electrochemically active at 6 °C. At 6 °C, these protein films exhibited reversible reduction/oxidation peaks which are characteristic of Fe^III/Fe^II redox couple. The estimated current function densities (J=1.6×10^?4 C/V cm²), surface concentrations (Γ_T=0.10 nmol/cm²) and standard electron transfer constant (k_s=13.86 s^?1) at 6 °C for the data taken at a scan rate of 0.1 V/s were similar to those which were obtained at 10, 15 and 23 °C. Basically, this study shows a possible electrocatalytic application of these myoglobin/polylysine films, for example in low temperature sensing applications.     

X‐ray diffraction study of the thermal expansion behavior of cellulose triacetate I

Highly crystalline samples of cellulose triacetate I (CTA I) were prepared from highly crystalline algal cellulose by heterogeneous acetylation. X‐ray diffraction of the prepared samples was carried out in a helium atmosphere at temperatures ranging from 20 to 250 °C. Changes in seven d‐spacings were observed with increasing temperature due to thermal expansion of the CTA I crystals. Unit cell parameters at specific temperatures were determined from these d‐spacings by the least squares method, and then thermal expansion coefficients (TECs) were calculated. The linear TECs of the a, b, and c axes were α_a = 19.3 × 10^?5 °C^?1, α_b = 0.3 × 10^?5 °C^?1 (T < 130 °C), α_b = ?2.5 × 10^?5 °C^?1 (T > 130 °C), and α_c = ?1.9 × 10^?5 °C^?1, respectively. The volume TEC was β = 15.6 × 10^?5 °C^?1, which is about 1.4 and 2.2 times greater than that of cellulose I_β and cellulose III_I, respectively. This large thermal expansion could occur because no hydrogen bonding exists in CTA I. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 517–523, 2009     

Spectrophotometric investigation of the mechanism of polymerization of isobutylene with vanadium tetrachloride and visible light

The spectra of n-heptane solution of VCl₄, isobutylene, and their mixture at temperatures ranging from +25 to ?80°C in the range of wavelengths from 200 to 2000 nm were investigated. In the region of wavelengths of visible light (400–700 nm) in which the absorption of isobutylene alone and of VCl₄ (concentration lower than 2.2 × 10^?4 mole/l.) is practically zero, their mixtures exhibit an absorption which depends on the concentration of both components and on temperature. A colored complex of VCl₄ and isobutylene is thus obtained, the concentration of which increases with decreasing temperature and is in equilibrium with that of the starting components. The polymerization of isobutylene under the experimental conditions investigated here probably is initiated with the isobutylene–VCl₄ complex after its excitation with light or heat. At low temperatures (t < ?20°C), when the polymerization of isobutylene with VCl₄ virtually does not take place at all in the dark, only excitation with light is operative in the initiation, while at higher temperatures (t > +10°C) thermal excitation plays the predominant role.     

Mechanism and kinetics of the imidazolidinone nitroxide‐mediated free‐radical polymerization of styrene

Styrene radical polymerizations mediated by the imidazolidinone nitroxides 2,5‐bis(spirocyclohexyl)‐3‐methylimidazolidin‐4‐one‐1‐oxyl (NO88Me) and 2,5‐bis(spirocyclohexyl)‐3‐benzylimidazolidin‐4‐one‐1‐oxyl (NO88Bn) were investigated. Polymeric alkoxyamine (PS‐NO88Bn)‐initiated systems exhibited controlled/living characteristics at 100–120 °C but not at 80 °C. All systems exhibited rates of polymerization similar to those of thermal polymerization, with the exception of the PS‐NO88Bn system at 80 °C, which polymerized twice as quickly. The dissociation rate constants (k_d) for the PS‐NO88Me and PS‐NO88Bn coupling products were determined by electron spin resonance at 50–100 °C. The equilibrium constants were estimated to be 9.01 × 10^?11 and 6.47 × 10^?11 mol L^?1 at 120 °C for NO88Me and NO88Bn, respectively, resulting in the combination rate constants (k_c) 2.77 × 10⁶ (NO88Me) and 2.07 × 10⁶ L mol^?1 s^?1 (NO88Bn). The similar polymerization results and kinetic parameters for NO88Me and NO88Bn indicated the absence of any 3‐N‐transannular effect by the benzyl substituent relative to the methyl substituent. The values of k_d and k_c were 4–8 and 25–33 times lower, respectively, than the reported values for PS‐TEMPO at 120 °C, indicating that the 2,5‐spirodicyclohexyl rings have a more profound effect on the combination reaction rather than the dissociation reaction. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 327–334, 2003     

Synthesis and characterization of polyoctenamer with WCl6?e−?Al?CH2Cl2 catalyst system via ring‐opening metathesis polymerization

A typical low‐strain monomer, cyclooctene, was polymerized via ring‐opening metathesis polymerization with electrochemically produced active species. The structural properties of the polyoctenamer were determined by NMR, gel‐permeation chromatography and differential scanning calorimetry. Analysis of the polyoctenamer microstructure by ¹H and ¹³C NMR spectroscopy indicates that the polymer contains a highly cis stereoconfiguration of the double bonds (σ_c = 0.75). The resulting polymer is of low molecular weight and has a reasonably broad molecular weight distribution (M_w = 18 000, PDI = 1.9). The glass transition temperature and melting point of the polyoctenamer are ?11.3 °C and 36.5 °C respectively. Copyright © 2005 John Wiley & Sons, Ltd.     

Crystal structure of a liquid crystal non‐symmetric dimer: cholesteryl 4‐[4‐(4‐n‐butylphenylethynyl)phenoxy]butanoate

The crystal structure of cholesteryl 4‐[4‐(4‐n‐butylphenylethynyl)phenoxy]butanoate [phase sequence: Cr 155°C (46.1?J?g^?1) SmA 186.8°C (1.5?J?g^?1) TGB‐N* 204.7 (6?J?g^?1) I] has been solved from single crystal X‐ray diffraction data. The compound crystallizes in the monoclinic space group P2₁ with unit cell parameters: a?=?13.129(2), b?=?9.3904(10), c?=?17.4121(8)?Å, β?=?92.790(7)°, Z?=?2. The structure has been solved by direct methods and refined to R?=?0.0606 for 3?250 observed reflections. The bond distances and angles are in good agreement with the corresponding values for compounds containing phenyl and cholesterol moieties. The phenyl rings A and B are planar. The dihedral angle between the least‐squares planes of the two phenyl rings is 28°. The cholesterol moiety has the usual structure: the C and E rings have chair conformations, and the D and F rings adopt half‐chair conformations. The molecules in the unit cell are arranged in an antiparallel manner. The crystal structure is stabilized by an intermolecular C–H…O contact of 2.989(10)?Å.     

Temperature-induced changes in crystal lattice of bioaragonite of Tapes decussatus Linnaeus (Mollusca: Bivalvia)

The characteristics of bioaragonite of shells of recent T. decussatus during heating were studied by the means of TG-DTA-EGA (FTIR), XRD, XRF and FTIR. The mass loss recorded up to 2.5% appeared with the higher rates at 110–150, 200–250, 295–300, and 390–415°C at heating of 10°C min⁻¹ up to 500°C. IR analysis of the evolved gases revealed the emission of water and CO₂. The lattice constants tend to change with anisotropy character (parameters a and c diminish whilst b tends to grows) and with an overall contraction of cell volume (from 227.36 to 226.84 ?³) during heating was established. The peculiarity of bioaragonite was explained by substitution of H₂O and sulphate ion into the lattice. In spite of those substitutions, bioaragonite reveals an orthorhombic structure, which is preserved during the changes up to calcite formation above 380°C.     

Influence of chemical exchange on the temperature dependence of Eu(fod)3-induced shifts

Temperature dependences of the paramagnetic shifts induced by Eu(fod)₃ in ¹H NMR spectra of ethylene oxide in carbon disulphide solution are obtained in the temperature range from +40 to ? 100°C at 100 MHz and from +30 to ?60°C at 60 MHz. The influence of chemical exchange leads to a decrease of the observed paramagnetic shifts with decreasing temperature. It is shown that a modified Swift and Connick equation can be used to describe the observed dependences. Upper limits of the mean lifetimes of the Eu(fod)₃-ethylene oxide adduct are τ_p < 1·7 × 10^?8 s at 14 °C and τ_p < 1 × 10^?8 s at 20 °C, respectively. The corresponding activation energy is equal to V_a = 13·7 kcal/mol.     

Molecular weight distribution and thermal characterization of polydimethylsilmethylene

1,1,3,3-Tetramethyl-1,3-disilacyclobutane (I) was polymerized under the following conditions with H₂PtCl₆·6H₂O as catalyst: (a) addition of I dropwise to a large excess of catalyst at room temperature, producing [(CH₃)₃SiCH₂(CH₃)₂Si]₂O in 90% yield; (b) polymerization at room temperature in the presence of 10% water with 23 ppm Pt, yielding 9% conversion to low molecular weight polymer after 4 weeks; (c) polymerization in an open vessel (25°C., 7 ppm Pt, M?_n = 1.2 × 10⁵), a closed vessel (100°C., 28 ppm Pt, M?_n = 1.7 × 10⁵), in a closed tube after twice freezing and evacuating (25°C., 23 ppm Pt, M?_n = 2.9 × 10⁵); (d) polymerization in an oxygen atmosphere (25°C., 17 ppm Pt, M?_n = 2.7 × 10⁵). The molecular weight distributions of the polymers with M?_n = 1.2 × 10⁵ and 1.7 × 10⁵ was studied by gel-permeation chromatography. Ratios of M?_w/M?_n are 3.1 and 2.7, respectively. In both cases a long tail of high molecular weight polymer is evident. Interpretation of the molecular weight distributions is qualitatively discussed on the basis of a postulated seven-step mechanism. Water is shown to be a source of chain termination. Evidence is presented for the existance of ?SiOSi? and ?SiOH in the silmethylene polymers. Negligible cyclization occurs. Orders of thermal stability measured by DTA and TGA for polydimethylsilmethylene (A), polydimethylsiloxane (B), and polysiobutylene (C) are: in He, A > B > C; in air, in air, B > C ? A. A fractionally precipitated polydimethylsilmethylene had a weight loss of less than 5% by 600°C. by TGA analysis at 10°C./min. in He.     

Molecular weight distribution studies. I. Radiation-induced polymerization of liquid α-methylstyrene

The concentration of water in purified and BaO-dried α-methylstyrene was found to be 1.1 × 10^?4M. The radiation-induced bulk polymerization of the α-methylstyrene thus prepared was studied in the temperature range of ?20°C to 35°C. The polymerization rate varied as the 0.55 power of the dose rate. The theoretical molecular weights and molecular weight distribution were calculated from a proposed kinetic scheme and these values were then compared with those found experimentally. The agreement between these two was reasonably close, and therefore it was concluded that, from the molecular weight distribution point of view, the proposed kinetic scheme for the cationic polymerization of α-methylstyrene is an acceptable one. The rate constant for chain transfer to monomer k_f changed with temperature and was found to be responsible for the decrease in the molecular weight of the polymer with increase in temperature. k_f and k_p at 20°C were found to be 0.95 × 10⁴ l./mole-sec and 0.99 × 10⁶ l./mole-sec, respectively.     

Densities of liquid metals: calcium,strontium, barium

Abstract

We report a method of measuring the densities of liquids at intermediate temperatures which employs Archimedes' Principle in a two-sinker arrangement. This method is then used to measure the densities of pure liquid calcium, strontium, and barium. We find ρ(Ca) = 1.4931 ? 1.37 × 10^?4 T(°C) from 850 ? 950°C, ρ(Sr) = 2.5547 ? 2.83 × 10^?4 T(°C) from 780 ? 880°C, and ρ(Ba) = 3.5561 ? 2.99 × 10^?4 T(°C) from 730 ? 830°C, where the units are gm/cm³. We use relations critical constants for these liquids to estimated dρ/dT, and compare these values of dρ/dT with those for other liquid metals; we also compare our results with recent x-ray diffraction data for these liquid metals.     

Cationic polymerization of styrenes by activated covalent species. Direct 1H-NMR observation of complexes of 1-phenylethyl acetates with lewis acids

Complexes of boron trichloride, boron tribromide, and ethylaluminumdichloride with various acetates were directly observed by ¹H-NMR. Complexes of secondary and tertiary acetates which model macromolecular active species in polymerization of styrene and isobutene are stable at ?75°C, but decompose at temperatures above ?30°C to yield corresponding chlorides or bromides. The stability of complexes depends on the Lewis acid, the alkyl group in the ester, and the structure of acetate. Rates of the bimolecular exchange of complexes with excess acetate were calculated from dynamic NMR to be k_ex = 2 × 10¹ L mol^?1 s^?1 (?65°C) and k_ex = 5 × 10⁴ L mol^?1 s^?1 (?75°C) for 1-phenylethyl acetate with BCl₃ and EtAlCl₂, respectively.     

Aqueous polymerization of acrylamide initiated by oxygenated cobalt(III) triethylenetetramine complex

Polymerization of acrylamide was thermally initiated by the oxygenated cobalt( III ) triethylenetetramine complex. Rate, conversion, and molecular weights obtained are favorable comparable to those initiated by K₂S₂O₄ and K₂S₂O₈ initiators. An induction period is about 3 mins. The value of K_df at 60°C is 6.75 × 10^?5 s^?1, and the chain transfer to monomer constant is 1.2 × 10^?5. The rate dependence obtained are a half order on the initiator concentration and a 1.38 order on the monomer concentration. The mole fraction of combination termination occurred in the overall termination reactions evaluated is 0.746.     

Reaction of tetramethyl-1,2-dioxetane with triphenylphosphine: Activation parameters for the formation of 2,2-dihydro-4,4,5,5-tetramethyl-2,2,2-triphenyl-1,3,2-dioxaphospholane

The reaction of tetramethyl-1,2-dioxetane ( 1 ) and triphenylphosphine ( 2 ) in benzene-d₆ produced 2,2-dihydro-4,4,5,5-tetramethyl-2,2,2-triphenyl-1,3,2-dioxaphospholane ( 3 ) in ?90% yield over the temperature range of 6–60°. Pinacolone and triphenylphosphine oxide ( 4 ) were the major side products [additionally acetone (from thermolysis of 1 ) and tetramethyloxirane ( 5 ) were noted at the higher temperatures]. Thermal decomposition of 3 produced only 4 and 5 . Kinetic studies were carried out by the chemiluminescence method. The rate of phosphorane was found to be first order with respect to each reagent. The activation parameters for the reaction of 1 and 2 were: Ea ? 9.8 ± 0.6 kcal/mole; ΔS^≠ = ?28 eu; k_30° = 1.8 m^?1sec^?1 (range = 10–60°). Preliminary results for the reaction of 1 and tris (p-chlorophenyl)phosphine were: Ea ? 11 kcal/mole, ΔS^≠ = ?24 eu, k_30° = 1.3 M^?1sec^?1 while those for the reaction of 1 and tris(p-anisyl)phosphine were: Ea ? 8.6 kcal/mole, ΔS^≠ = ?29 eu, k_30° = 4.9 M^?1 sec^?1.     

1‐Methyl‐4‐nitraminopyridinium nitrate and 4‐nitraminopyridinium methanesulfonate

In the title compounds, C₆H₈N₃O₂⁺·NO₃^? and C₅­H₆­N₃­O₂⁺·­CH₃SO₃^?, respectively, the cations are almost planar; the twist of the nitr­amino group about the C—N and N—N bonds does not exceed 10°. The deviations from coplanarity are accounted for by intermolecular N—H?O interactions. The coplanarity of the NHNO₂ group and the phenyl ring leads to the deformation of the nitr­amino group. The C—N—N angle and one C—C—N angle at the junction of the phenyl ring and the nitr­amino group are increased from 120° by ca 6°, whereas the other junction C—C—N angle is decreased by ca 5°. Within the nitro group, the O—N—O angle is increased by ca 5° and one O—N—N angle is decreased by ca 5°, whereas the other O—N—N angle remains almost unchanged. The cations are connected to the anions by relatively strong N—H?O hydrogen bonds [shortest H?O separations 1.77 (2)–1.81 (3) Å] and much weaker C—H?O hydrogen bonds [H?O separations 2.30 (2)–2.63 (3) Å].     

Intrinsic Acid–Base Properties of a Hexa‐2′‐deoxynucleoside Pentaphosphate,d(ApGpGpCpCpT): Neighboring Effects and Isomeric Equilibria

The intrinsic acid‐base properties of the hexa‐2′‐deoxynucleoside pentaphosphate, d(ApGpGpCpCpT) [=(A1?G2?G3?C4?C5?T6)=(HNPP)^5?] have been determined by ¹H NMR shift experiments. The pK_a values of the individual sites of the adenosine (A), guanosine (G), cytidine (C), and thymidine (T) residues were measured in water under single‐strand conditions (i.e., 10 % D₂O, 47 °C, I=0.1 M , NaClO₄). These results quantify the release of H⁺ from the two (N7)H⁺ (G?G), the two (N3)H⁺ (C?C), and the (N1)H⁺ (A) units, as well as from the two (N1)H (G?G) and the (N3)H (T) sites. Based on measurements with 2′‐deoxynucleosides at 25 °C and 47 °C, they were transferred to pK_a values valid in water at 25 °C and I=0.1 M . Intramolecular stacks between the nucleobases A1 and G2 as well as most likely also between G2 and G3 are formed. For HNPP three pK_a clusters occur, that is those encompassing the pK_a values of 2.44, 2.97, and 3.71 of G2(N7)H⁺, G3(N7)H⁺, and A1(N1)H⁺, respectively, with overlapping buffer regions. The tautomer populations were estimated, giving for the release of a single proton from five‐fold protonated H₅(HNPP)^±, the tautomers (G2)N7, (G3)N7, and (A1)N1 with formation degrees of about 74, 22, and 4 %, respectively. Tautomer distributions reveal pathways for proton‐donating as well as for proton‐accepting reactions both being expected to be fast and to occur practically at no “cost”. The eight pK_a values for H₅(HNPP)^± are compared with data for nucleosides and nucleotides, revealing that the nucleoside residues are in part affected very differently by their neighbors. In addition, the intrinsic acidity constants for the RNA derivative r(A1?G2?G3? C4?C5?U6), where U=uridine, were calculated. Finally, the effect of metal ions on the pK_a values of nucleobase sites is briefly discussed because in this way deprotonation reactions can easily be shifted to the physiological pH range.     

Automatic Geometry Optimization and Vibrational Analysis in External Electric Field: Ethylene

Stationary points of the INDO energy hypersurface for various orientations of ethylene in external electric fields of the strength F=0, 2, 4, 6, 8 and 10 × 10¹⁰ V m^?1 were found and their characteristics studied by the force constant matrix analysis. Energies, structural parameters, charges, Wiberg indices and dipole moments are presented. The only stable orientation of the ethylene molecule is that for which the C? C bond is parallel to the field direction up to F=6 × 10¹⁰ V m^?1 (orientation (a) in Fig. 1). Above this value the molecule is structurally unstable and it decomposes to the hydride anion and the C₂H₃⁺ cation. Rotational instability was found for two perpendicular orientations of the C? C bond with respect to the field vector, in which the field vector was parallel and perpendicular to the molecular plane. Pseudorotations with negative eigenvalues of force constant matrices lead to the stable orientation (a). No stationary points were found when the angle between the C? C bond and the field vector was between 0 and 90°. The five longest wavelength vibrational bands are presented for selected orientations and field strengths.     

Alkylidinphosphane und -arsane. I [P ≡ C?S]−[Li(dme)3]+ – Synthese und Struktur

Alkylidynephosphanes and -arsanes. I [P ≡ C? S]^?[Li(dme)₃]⁺ – Synthesis and Structure O,O′-Diethyl thiocarbonate and bis(tetrahydrofuran)-lithium bis(trimethylsilyl)phosphanide dissolved in 1,2-dimethoxyethane, react below 0°C to give ethoxy trimethylsilane and tris(1,2-dimethoxyethane-O,O′)lithium 2λ³-phosphaethynylsulfanide – [P≡C? S]^? [Li(dme)₃]⁺ – ( 1a ). Apart from bis(trimethylsilyl)sulfane or carbon oxide sulfide, dark red concentrated solutions of λ³-phosphaalkyne 1 are also obtained from reactions of carbon disulfide with bis(tetrahydrofuran)-lithium bis(trimethylsilyl)phosphanide or with the homologous lithoxy-methylidynephosphane ( 2 ) [1]. The ir spectrum shows two absorptions at 1762 and 747 cm^?1 characteristic for the P≡C and C? S stretching vibrations. The nmr parameters {δ(³¹P) ? 121.3; δ(¹³C) 190.8 ppm; ¹J_CP 18.2 Hz} resemble much more values of diorganylamino-2λ³-phosphaalkynes than those of bis(1,2-dimethoxyethane-O,O′)lithoxy-methylidyne-phosphane ( 2a ). As found by an X-ray structure analysis (P2₁/c; a = 1192.6(16); b = 1239.1(19); c = 1414.8(26) pm; β = 105.91(13)° at ?100 ± 3°C; Z = 4 formula units; wR = 0.064) of pale yellow crystals (mp. + 16°C) isolated from the reaction with O,O′-diethyl thiocarbonate, the solid is built up of separate [P≡C? S]^? and [Li(dme)₃]⁺ ions. Typical bond lengths and angles are: P≡C 155.5(11); C? S 162.0(11); Li? O 206.4(17) to 220.3(20) pm; P≡C? S 178.9(7)°.     

Polybromostryl macromers and their anionic polymerization

The decomposition of polybromostyryl carbanions (PBS^?), obtained by anionic polymerization of 4-bromostyrene in tetrahydrofuran (THF), was investigated in the dark in a temperature range of ?6–?21°C. It was accompanied by the evolution of bromine anions and by the formation of polymeric allylic carbanions (λ_max = 575 nm; ε_max = 6800 eq^?1·L·cm^?1). The reaction mechanism was elucidated. The rate constant of the unimolecular rate-determining step of the process was 1.3 × 10^?5 s^?1 and 9.7 × 10^?5 s^?1 at ?21 and ?6°C, respectively. Its apparent energy of activation E_app = 18.38 Kcal/mol. The polybromostyrenes with allylic carbanions at their ends may decompose further. Their “dark” decomposition yielded 1,3-butadiene-1,3-diphenyl-macromers. The mechanisms of decomposition of the PBS^? carbanions and the dark decomposition of the polybromostyryl allylic carbanions are analogous. The rate constant of the latter process was 2.5 × 10^?6 s^?1 at ?6°C. The anionic polymerization of prepared macromers can be initiated in THF at ?78°C by α-methylstyryl carbanions, which do not react, however, with PBS^? carbanions. “Comblike” polymacromers were prepared in which each branch had a molecular weight of about 50,000. The overall molecular weight of the polymacromer was estimated to be about 1 × 10⁶. It has been assumed that the 2–1 mode of addition to the diene group of the macromer is predominant during its polymerization. The 3–4 mode of addition followed by proton shift represents the termination step. The 4–3 mode of addition was ruled out on the basis of spectroscopic evidence.     

Silolene-Bridged zirconocenium polymerization catalysts

A new silolene-bridged compound, racemic (1,4-butanediyl) silylene-bis (1-η⁵-in-denyl) dichlorozirconium ( 1 ) was synthesized by reacting ZrCl₄ with C₄H₈Si (IndLi)₂ in THF. 1 was reacted with trialkylaluminum and then with triphenylcarbenium tetrakis (penta-fluorophenyl) borate ( 2 ) to produce in situ the zirconocenium ion ( 1 ⁺). This “constraint geometry” catalyst is exceedingly stereoselective for propylene polymerization at low temperature (T_p = ?55°C), producing refluxing n-heptane insoluble isotactic poly(propylene) (i-PP) with a yield of 99.4%, T_m = 164.3°C, δH_f = 20.22 cal/g and M?_w = 350 000. It has catalytic activities of 10⁷?10⁸ g PP/(mol Zr · [C₃H₆] · h) in propylene polymerization at the T_p ranging from ?55°C to 70°C, and 10⁸ polymer/(mol Zr · [monomer] · h) in ethylene polymerization. The stereospecificity of 1 ⁺ decreases gradually as T_p approaches 20°C. At higher temperatures the catalytic species rapidly loses stereochemical control. Under all experimental conditions 1 ⁺ is more stereospecific than the analogous cation derived from rac-dimethylsilylenebis (1-η⁵-indenyl)dichlorozirconium ( 4 ). The variations of polymerization activities in ethylene and in propylene for T_p from ?55°C to +70°C indicates a Michaelis Mention kinetics. The zirconocenium-propylene π-complex has a larger insertion rate constant but lower thermal stability than the corresponding ethylene π-complex. This catalyst copolymerizes ethylene and propylene with reactivity ratios of comparable magnitude r_E ? 4r_p. Furthermore, r_E.r_p ? 0.5 indicating random copolymer formation. Both 1 and 4 activated with methylaluminoxane (MAO) exhibit much slower polymerization rates, and, under certain conditions, a lower stereo-selectivity than the corresponding 1 ⁺ or 4 ⁺ system. © 1994 John Wiley & Sons, Inc.