Isothermal crystallization of poly(ethylene-terephthalate) of low molecular weight by differential scanning calorimetry: 1. Crystallization kinetics

The isothermal crystallization of poly(ethylene-terephthalate) (PETP) fractions, from the melt, was investigated using differential scanning calorimetry (d.s.c.). The molecular weight range of the fractions was from 5300–11750. Crystallization temperatures were from 498–513 K. The dependence of molecular weight and undercooling on several crystallization parameters has been observed. Either maxima or minima appear at a molecular weight of about 9000, depending on the crystallization temperature. The activation energy values point to the possibility of different mechanisms of crystallization according to the chain length. A folded chain process for the higher $\text{M}\text{?}{}_{\mathrm{<mtext>n</mtext>}}$ chains and an extended chain mechanism for the lower $\text{M}\text{?}{}_{\mathrm{<mtext>n</mtext>}}$ chains. The values of the Avrami equation exponent n vary from 2–4 depending on the crystallization temperature; non-integer values are indicative of heterogeneous nucleation. The rate constant K depends on T_c and $\text{M}\text{?}{}_{\mathrm{<mtext>n</mtext>}}$, showing maxima related to the T_c used. The plot of log K either vs. (ΔT)^?1 and (ΔT)^?2 or $\text{T}{}_{\mathrm{<mtext>m</mtext>}}\text{T(ΔT)}$ and $\text{T}{}^2{}_{\mathrm{<mtext>m</mtext>}}\text{T(ΔT)}{}^2$ is linear in every case.     

Synthesis of electroreactive polymers from poly (m,p-chloromethylstyrene) and copoly (m,p-chloromethylstyrene-styrene)

Polymerization, and copolymerization with styrene, of m,p-chloromethylstyrene have been carried out at 75°C, in chlorobenzene and in the presence of AIBN ($\text{[}\text{AIBN}\text{] ? 6 × 10}{}^{\mathrm{?2}}$, and $\text{12 × 10}{}^{\mathrm{?2}}\text{m}$, respectively). The polymer molecular weights, determined by g.p.c., are: $\text{M}\text{?}{}_{\mathrm{w}}\text{= 8670}$, $\text{M}\text{?}{}_{\mathrm{n}}\text{= 5860}$, and $\text{M}\text{?}{}_{\mathrm{w}}\text{/-M}{}_{\mathrm{n}}\text{= 1.48}$ for the homopolymer, poly(m,p-chloromethylstyrene), (1a); and $\text{M}\text{?}{}_{\mathrm{w}}\text{= 8805}$, $\text{M}\text{?}{}_{\mathrm{n}}\text{= 5144}$, and $\text{M}\text{?}{}_{\mathrm{w}}\text{/-M}{}_{\mathrm{n}}\text{= 1.71}$ for the copolymer, copoly(m,p-chloromethylstyrene-styrene), (2a). A series of phosphine derivatives of both 1a and 2a are prepared by the reaction of the polymers with either chlorodiphenylphosphine/lithium, or diphenylphosphine/potassium tert. butoxide. A number of other potentially electroreactive derivatives of 2a are obtained by reacting the polymer with 2-aminoanthraquinone, 3-N-methylamino-propionitrile, or 2-(2-aminoethyl) pyridine. The phosphinated polymers are reacted with bis-benzonitrilepalladium-(II) chloride to obtain a series of polymer-palladium(II) complexes containing 8.5–12.9% palladium. Similarly, reaction of the last-named bidentate polymeric ligand with cupric acetylacetonate, or cupric sulphate pentahydrate, produces polymer-copper(II) complexes having 5.8, or 3.3% copper, respectively. The inter/intra-chain nature of some of the side reactions during the derivatization of the chloromethylated polymers, and that of the complex formation between transition metal centres and macromolecular ligands, are briefly discussed in view of the experimental results.     

Degradation of nylon-6 in ethylene glycol: 2. Mathematical illustration of degradation

Glycolysis of nylon-6 under pressure at 200–300°C was studied. Reduction of the reaction time and a decrease of the yield of oligomers was observed with the temperature increase. The most advantageous conditions for obtaining oligoamides with $\text{M}\text{?}{}_{\mathrm{n}}\text{? 1000}$ were calculated. It was found that in the experimental area investigated linear regression equations describe the course of degradation adequately.     

Kinetics of the graphite-oxygen reaction near 1000°K

The kinetics of oxidation of spectroscopic grade polycrystalline graphite have been studied by a flow method which allowed the partial pressure of oxygen (P_o2) to be changed abruptly during the course of the reaction. The changes were made in two alternative ways: by changing the total pressure of an oxygen/nitrogen mixture of constant composition or by changing the composition at constant total pressure. After the effect of diffusion resistance on the rate had been shown to be negligible and the effects of self-heating, burn-off and traces of gaseous impurities allowed for, the order of reaction (ń in the empirical expression: rate α ) was found to vary from 0.34 (at 993°K and mean ) to 0.19 (at 963°K and mean P_o2 = 245 torr). In certain circumstances, the response of the rate to changes in P_o2 is subject to a “memory” effect. This is attributed to fast, diffusion-controlled chemisorption of a minute amount of hydrogenous impurity on part of the active surface of the graphite.     

Molecular weight distribution and stereoregularity of polypropylenes obtained with Ti(OC4H9)4Al2(C2H5)3Cl3 catalyst system

The effects of temperature and catalyst homogeneity on the molecular weight distribution (MWD) and stereochemical regulation of polypropylenes produced by $\text{Ti(OC}{}_4\text{H}{}_9\text{)}{}_4\text{Al}{}_2\text{(}\text{C}{}_2\text{H}{}_5\text{)}{}_3\text{Cl}{}_3$ system have been investigated. The MWD of polymers obtained at temperatures below 21°C were unimodal and narrow ($\text{M}\text{?}{}_{\mathrm{w}}\text{M}\text{?}{}_{\mathrm{n}}\text{?2.0}$), whereas those obtained at temperatures higher than 31°C were bimodal with one narrow distribution and the other broad one ($\text{M}\text{?}{}_{\mathrm{w}}\text{M}\text{?}{}_{\mathrm{n}}\text{=18}$) at higher molecular weights. The existence of two different types of catalyst, one soluble with homogeneous catalytic centres and the other insoluble with heterogeneous catalytic centres was found in the polymerization at 41°C. At temperatures below 21°C only soluble catalyst was present and produced isotactic polypropylenes with [m]=0.65. The isospecific nature of soluble titanium-based catalyst is greatly contrasted to the syndiospecific nature of soluble vanadium-based catalyst.     

Ensembling and monolayer parameters for the dehydrogenation of cyclohexane over PtAl2O3 catalyst

The ensembling and the monolayer parameters, such as the number of atoms in an active center (n), the absolute efficiency of an active center (r_n), the number of migration regions (Z₀), average area of migration regions (Δ), fractional surface coverage (θ_c), and the average number of atoms in a region (v), were evaluated for the dehydrogenation of cyclohexane over $\text{Pt}\text{Al}{}_2\text{O}{}_3$ catalyst in vapor phase. The reaction was studied using alumina of two types in the temperature range 225–350 °C at two contact times. Monoatomic ensemble is the active center in the system studied. Dependence of probability of formation of atomic ensemble (P_n) on the average number of atoms in a migration region is also given.     

Chimisorption de l'hydrogene par les composes d'insertion graphite-potassium

At low temperature, graphite-potassium intercalation compounds of stage higher than one behave as molecular sieves as regards hydrogen [1,2]. At ambient and higher temperature, however, these compounds including the first stage one, chemically fix hydrogen by a completely different process. This reaction, which has been the object of previous papers [3,4], has been further studied so as to remove some of the previous uncertainties and imprecisions.The brown compound KC₈ after fixing hydrogen becomes blue, and saturates for an $\text{H}\text{K}$ ratio of $\text{2}\text{3}$. The reaction is reversible but does present noticeable hysteresis. Radiocrystallographic examination of the hydrogenated product shows that it involves a pure ternary phase of formula $\text{KC}{}_8\text{H}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}$. The indexing of the 00l reflections indicates a second stage compound: each intercalated layer is composed of two metallic planes in the presence of hydrogen (Fig. 1). This stage change can be interpreted by the pleating of the graphite layers [5]. Neutrocrystallographic studies confirm the preceding results and allow (Table 2) to conclude that the hydrogen forms a layer between the two potassium planes (Fig. 2). The belonging to the second stage of the phase $\text{KC}{}_8\text{H}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}$ is further confirmed by the fact that the free graphite spacings are susceptible of fixing new alkaline metal layers (K, Rb or Cs) to form new compounds referred to as “bi-insertion compounds” [6].Attempts at hydrogen physisorption on the product during the course of chemical hydrogenation (Fig. 3), and the radiocrystallographic and kinetic measurements, clarify the finer points of the reaction. The hydrogen starts to be chemisorbed into the lattice of the compound KC₈ without any stage change (0A), then there appears progressively a second stage phase, unsaturated with hydrogen (AB). Saturation is only attained when the first stage phase has disappeared (BC).Low temperature hydrogen physisorption tests have been carried out on the second stage compound KC₂₄ during chemical hydrogenation. Figure 4 shows a linear decrease in the physisorption capacity, when the hydrogenation ratio increases. By extrapolation it is seen that this capacity should become zero for a chemisorption ratio $\text{H}\text{K}$ in the neighbourhood of $\text{2}\text{3}$.The results suggest the following hydrogenation model. Under the action of the gas, the intercalated metal forms double, hydrogenated layers, identical to those in the ternary $\text{KC}{}_8\text{H}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}$. There also appear three types of graphitic spacings: free intervals, those occupied by lacunar single layers and those occupied by hydrogenated double layers. These spacings are distributed in such a manner that there appears a binary phase KC_12n coexisting with a ternary phase $\text{KC}{}_{\mathrm{4N}}\text{H}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}$ of equal or neighbouring stages n and N. Stoichiometry imposes a relationship between n and N. Figure 5 represents the variation of N as a function of n for various values of the hydrogenation ratio. At saturation the system would once again become single phase: a ternary, $\text{KC}{}_{24}\text{H}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}$ of sixth stage.In Table 3 are given the strongest 00l reflections and the corresponding interplanar distances, characteristic of the compound KC_12n and $\text{KC}{}_{\mathrm{4N}}\text{H}\text{2}\text{3}$. The experimental data taken from the X-ray diagrams, obtained for three KC₂₄H_y, compounds are compared to the values predicted by our model in Table 4. It is seen that the positions of the observed peaks are in good agreement with the 00l reflections of the model, confirming the interpretation we propose.In conclusion, hydrogenation of the potassium layers of KC₈ or of KC₂₄ always leads to the formation of dense and hydrogenated metallic double layers of formula $\text{KH}\text{2}\text{3}$. This process thus creates ternaries of higher stage than the starting binary.     

Organic halides as cationic initiators

The reaction of organohalides with silver hexafluorophosphate or silver perchlorate in THF to generate cationic initiators has been examined. Reactivities in the order fluoride < chloride < bromide < iodide and $\text{alkyl « allyl ≈ benzyl}$ were found. Selected halides were examined in more detail, and it was shown that under controlled conditions living poly THF could be obtained. This was difunctional if p-xylylene dibromide or 1,4-dibromobut-2-ene was used, or monofunctional with conventional monohalides. Molecular weight distributions of $\text{M}{}_{\mathrm{w}}\text{M}{}_{\mathrm{n}}\text{? 1.2}$ were obtained. Experiments demonstrated that allyl bromide and p-methylbenzyl bromide initiated THF polymerization entirely additively but, with isopropyl iodide at room temperature, up to 10% proton transfer occurred.     

Kinetics of catalytic hydrodesulfurization of a petroleum residue in a batch-recycle trickle bed reactor

The kinetics of catalytic hydrodesulfurization of an atmospheric residuum were investigated in a batch-recycle trickle bed reactor with a commercial catalyst. The global rate equation determined was R = Catalyst effectiveness factors η, remaining activity ε, effective pore diffusivities D_e and activation energies E have been determined quantitatively. The data have also been interpreted in terms of the two parallel, first-order reactions model and the respective parameters have been determined. Solid—liquid chromatography fractions of three hydrocarbon and sulfur compound groups have been determined in addition to the asphaltenes content of the feedstock and the hydrotreated product. These data are discussed and conclusions are drawn with respect to their behaviour during HDS and a reaction network is proposed which explains the complicated interrelations involved.     

Copoly(m,p-chloromethylstyrene-50% styrene): Copolymer molecular weights,fractionation and derivatization

Copolymerization of an equimolar mixture of m,p-chloromethylstyrene ($\text{M}$₁) and styrene ($\text{M}$₂) was carried out in chlorobenzene in the presence of AIBN at 80°C. Molecular weight analysis (by g.p.c.) of the resulting polymer samples was performed at various conversions. $\text{M}\text{?}{}_{\mathrm{<mtext>w</mtext>}}$, $\text{M}\text{?}{}_{\mathrm{<mtext>n</mtext>}}$, and ($\text{M}\text{?}{}_{\mathrm{<mtext>w</mtext>}}\text{M}\text{?}{}_{\mathrm{<mtext>n</mtext>}}$) value of 21 300, 13 800 and 1.54 were obtained at 8.9% conversion. At higher conversions, the value of $\text{M}\text{?}{}_{\mathrm{<mtext>w</mtext>}}$ remained effectively constant while $\text{M}\text{?}{}_{\mathrm{<mtext>n</mtext>}}$ decreased to 9200 at ca. 80% conversion, and then increased to 12 000 at about 100% conversion (16 h), and to 13 700 if the polymer solutions were maintained at 80°C for an additional 44 h. These results suggest that, although the termination step initially involves the combination of polymer radicals, at high conversions a large number of very low molecular weight, and unsaturated, polymer molecules are formed possibly by disproportionation involving polymer radicals and primary radicals. The unsaturated polymer molecules are subsequently polymerized by growing polymer radicals towards the end of the polymerization. It was noticed that further reaction occurred after complete depletion of monomer, involving radical attack on the unsaturated polymer molecules. Other reactions including chain transfer to polymer will also be important at high polymer concentrations. A copolymer of $\text{M}$₁ and $\text{M}$₂ was separated into four fractions on a preparative scale, and molecular weight analysis of the resulting polymer samples provided more evidence of the above interpretation. G.p.c. analysis of several derivatives of a copolymer of $\text{M}$₁ and $\text{M}$₂ showed that most molecular weights were much lower than that of the starting polymer. These results in some cases may reflect the chemical or dimensional changes introduced into the polymer molecules during derivatization.     

Cloud-point curves of the polystyrene-cyclohexane system near the critical point

Cloud-point curves for solutions of five polystyrene samples, including three well-fractionated polystyrenes, in cyclohexane have been examined near their critical points. Even for a solution of polystyrene characterized by $\text{M}{}_{\mathrm{w}}\text{M}{}_{\mathrm{n}}\text{<1.03}$, the critical point determined by the phase-volume method is generally situated on the right hand branch of the cloud-point curve. The precipitation threshold concentration is appreciably lower than the critical concentration, while the threshold temperature slightly deviates from the critical temperature. The agreement of the precipitation threshold point with the critical point has been found for a solution of polystyrene characterized by M_w=20×10⁴ and $\text{M}{}_{\mathrm{w}}\text{M}{}_{\mathrm{n}}\text{<1.02}$ in cyclohexane. The η(φ) function derived from critical miscibility data is expressed by $\text{χ(φ) = 0.2798+}\text{67.50}\text{T}\text{+0.3070φ+0.2589φ}{}^2$, which yields θ of 33.2°C and ψ₁ of 0.22.     

Random-coil configurations of alicyclic polyformals

A sample of poly (trans-1,4-cyclohexylene-dimethylene-oxymethylene oxide) (PTCDM) was synthesized by condensation of trans-1,4-cyclohexane dimethanol and paraformaldehyde using p-toluene sulphonic acid as catalyst. A fraction having M_n=6500 and a melting point of 86°C was isolated and purified; its n.m.r. spectrum does not change with temperature in the range 20°–50°C which indicates a rigid distribution of methylene substituents in the cyclohexane ring; its dipole moment, measured in benzene solution at several temperatures between 20° and 60°C, yielded values of $\text{D}{}_{\mathrm{n}}\text{=}\text{[μ}{}^2\text{]}\text{nm}{}^2\text{=0.17–0.21}$ and a temperature coefficient dln $\text{{gm}{}^2\text{}}\text{d}\text{T}\text{= 5.5 × 10}{}^{\mathrm{?3}}\text{K}{}^{\mathrm{?1}}$, similar to those reported in the literature for acyclic polyformals. Agreement between experimental and calculated (using rotational isomeric states theory) values is satisfactory.     

Electrochemical determination of the standard Gibbs free energy change of the graphite-Oxygen reactions

A calcia doped zirconia solid oxide electrolyte galvanic cell was investigated in the temperature range of 700–950°C to directly and more accurately measure the standard Gibbs free energy change of the following reactions: $\text{CO(g) +}\text{1}\text{2}\text{O}{}_2\text{(g) = CO}{}_2\text{(g)}$C_(graphite) + CO₂(g) = 2 CO(g). The results obtained in the present investigation are well within the scatter band of the calorimetric and spectroscopic data.     

Polymerization of poly(dimethylsiloxane) macromers: 1. Copolymerization with styrene

The synthesis and characterization of methacrylate-ended macromers ($\text{M}\text{?}{}_{\mathrm{<mtext>n</mtext>}}$ 500 to 10 000) and their copolymerization with styrene (M₂) is described. The experimental errors in the values of the reactivity ratios r₁ render them meaningless. Values of r₂ can be determined with more precision and increase from 1.06 to 1.55 as the molecular weight of the macromer increases. This behaviour is due to steric effects, not diffusion-controlled propagation. It is shown that the assumptions that $\text{1 > r}{}_1\text{[}\text{M}{}_1\text{]}\text{[}\text{M}{}_2\text{]}$ and $\text{r}{}_2\text{>}\text{[}\text{M}{}_1\text{]}\text{[}\text{M}{}_2\text{]}$ are only valid for macromers of $\text{M}\text{?}{}_{\mathrm{<mtext>n</mtext>}}\text{> ca. 10 000}$.     

Carbon as a catalyst in oxidation reactions and hydrogen halide elimination reactions

Carbon catalyses many reactions, mainly oxidation reactions with oxygen and with halogens, e.g. $\text{SO}{}_2\text{+}\text{1}\text{20}{}_2\text{→ SO}{}_3$, or CO + Cl₂→ COCl₂. It is known, however, that different carbons behave quite differently in the reduction of oxygen on fuel cell cathodes. Therefore the catalytic activity of carbons has been studied in other reactions. A convenient test reaction is the oxidation of dilute aqueous sulphurous acid. It became apparent that all catalytically active carbons contain small quantities of nitrogen, and inactive carbons such as wood charcoal or carbon blacks can be rendered highly active by treatment with N H₃ or HCN at elevated temperatures. Photoelectron spectra indicate that the catalytic activity increases parallel to the incorporation of a nitrogen species which is pyridine-like, i.e. incorporated in the aromatic layers. Treatment with NH₃ at 900 °C leads also to massive gasification of the carbons, increasing their surface area. Other reactions studied included the oxidation of aqueous oxalic acid and of methanol to formaldehyde. A quite different type of reaction is the elimination of hydrogen chloride from 1-chloroalkanes, e.g. 1 -chlorobutane. Again, activity changes in parallel to nitrogen content. Reaction products are olefins, dimers of the alkyl groups, and a polymer on the catalyst surface. The formation of alkyl dimers, e.g. n-octane in the case of n-butylchloride, suggests that radicals are involved in the reaction.     

Studies of cyclic and linear poly(dimethyl siloxanes): 2. Preparative gel permeation chromatography

A preparative gel permeation chromatographic (g.p.c.) instrument has been constructed and used to separate broad fractions of cyclic poly(dimethyl siloxanes) into sharp fractions with heterogeneity indices $\text{M}\text{?}{}_{\mathrm{w}}\text{M}\text{?}{}_{\mathrm{n}}\text{= 1.05 ± 0.02}$. The number-average molecular weights $\text{M}\text{?}{}_{\mathrm{n}}$ of the cyclic polymer fractions obtained were as high as 50 000, corresponding to number-average numbers of skeletal bonds $\text{n}\text{?}{}_{\mathrm{n}}$ up to 1300. The concentrations of linear poly(dimethyl siloxanes) in all but the highest molecular weight cyclic polymer fractions prepared are believed to be negligible. The preparative g.p.c. instrument was also used to obtain some sharp fractions of linear poly(dimethyl siloxanes).     

In situ laser raman spectroscopy of the sulfiding of Moγ-Al2O3 catalysts

Raman spectra of sulfided $\text{Mo}\text{γ-}\text{Al}{}_2\text{O}{}_3$ catalysts were obtained using in situ techniques for two sulfiding methods. For samples sulfided by 10% $\text{H}{}_2\text{S}\text{H}{}_2$ at 400 °C, MoS₂ structures were observed. A stepwise sulfiding using 10% $\text{H}{}_2\text{S}\text{H}{}_2$, with spectra recorded at 150, 250, and 350 °C, resulted in observation of molybdenum oxysulfide, reduced molybdate, and surface “MoS₂” phases. Reexposure of these samples to air led to radical modification of the oxysulfide structures as well as transformation of some sulfide phases. A model incorporating terminal and bridging MoS bonding and anion vacancies is proposed. This model is based on the conversion of isolated and aggregated molybdate and MoO₃ species to oxysulfide and reduced molybdenum phases. Conversion of reduced molybdenum phases to sulfides is observed to be slow.