bluuebunny/crossref_metadata_2025_split huggingface dataset:
Kinetic studies on a Ziegler catalyst system
Authors: Gilchrist A. 
Published: April 2003

AbstractMeasurements have been made on the kinetics of the polymerization of ethylene using Ziegler‐type catalysts made from zine di‐n‐butyl and titanium tetrchloride in hydrocarbon solvents. The rate of polymerization was proportional to the square of the ethylene pressure. At catalyst ratios (the molar ratio of zine butyl to titanium tetrachoride in the reaction mixture) less than 0.5, an initial, fast polymerzation was followed by a slower but more persistent polymerization. Both processes increased in velocity with increasing ratio up to 0.5; above this ratio the fast process was not observed while the rate of the slower one decreased with increasing ratio and was very small at unit ratio. At higher ratios the rate increased again as though to reach a limiting value when a very large excess of zine butyl was used. It has not been possible to devise more than speculative interpretations of these results. They suggest that at ratios greater than unity the reaction

may be completed and that the chain initiation step takes place between zine butyl and ethylene, both adsorbed equilibrium‐wise onto the solid, reduced titanium choloride. It is not yet known whether propagation of polymerization occurs along the catalyst surface or at single sites on the surface. If the former is the case and the mobile propagation center is an ion‐pair, the hydrocarbon end being negative and the positive charge lying in the surface, the mobility of this latter charge may limit the rate of propagation. It is possible to explain some of the results obtained most easily from these ideas.
The mechanism of the polymerization of diazoalkanes catalyzed by boron compounds
Authors: Bawn C. E. H. , Ledwith A. , Matthies P. 
Published: April 2003

AbstractA mechanism is proposed for the polymerization of aliphatic diazo compounds catalyzed by boren trifluoride and similar boron derivatives which satisfactorily accounts for the observed kinetic and molecular weight results. The final polymer molecule is shown to be a baron alkyl of the type . Such molecules are catalysts for the polymerization of fresh diazo compounds and in this manner it has been possible to prepare block copolymers of the type
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The mechanism of polymerization is shown to be similar to the polymerization of ethylene catalyed by aluminum alkyls and possibly to the Ziegler‐type polymerization of α‐olefins.
Polymerization of ethylene by soluble Ziegler catalysts
Authors: Bawn C. E. H. , Symcox R. 
Published: April 2003

AbstractKinetic measurements have been made on the polymerization of ethylene to low molecular weight products using a soluble catalyst formed by the interaction of sec‐butyl titanate and aluminum triethyl. A maximum in catalyst activity occured at ratio Al:Ti = 2.5:1 and a scheme is proposed to explain this observation. An interesting feature of the polymerization is the existence of a well‐defined cut‐off pressure of ethylene below which no polymerization occurred.
The polymerization of epoxides by metal halide catalysts
Authors: Colclough R. O. , Gee Geoffrey , Higginson W. C. E. , Jackson J. B. , Litt Morton 
Published: April 2003

AbstractEthylene, propylene, and styrene oxides have been polymerized by a range of metal halide catalysts, high molecular weight products being obtained from the two firstnamed. The limited quantitative evidence presented is consistent with the view that these polymerizations proceed by a coordinate propagation mechanism. The effective catalyst for this stage is essentially a metal alkoxide, formed in an initial cationic polymerization in which a small number of monomer units are attached to the metal atom. The propylene oxide polymers generally contain a fraction of crystalline material, but the proportion of this can be reduced practically to zero (with FeCl3 catalysis) by careful exclusion of water.
Synthesis of graft copolymers
Authors: Smets G. , Poot A. , Mullier M. , Bex J. P. 
Published: April 2003

AbstractThe two‐step synthesis of graft copolymers was carried out by several methods. In the first method, the initial peroxidized polymer was prepared from a copolymer of methyl acrylate and acroyl chloride by recation with tert‐butyl hydroperoxide; the content of perester could be varied. The rate of decomposition of these perester groups and the degradation velocity of this first‐step polymer were determined in benzene solution at various temperatures. The activation energies were 30.1 kcal./mole, respectively, which values correspond to the scission of the OO linkage. Grafting experiments were carried out in dilute solutions (1%) in order to avoid any grafting by transfer to the polymer. The second‐step polymerization followed the square‐root law dependence with respect to the peroxide polymer concentration. This rate was also of the first order with respect to the monomer, i.e., styrene, concentration. The rate of grafting was also evaluated by quantitative fractionation of the reaction products (precipitation method), taking into account the weight per cent of styrene in these isolated graft copolymers. The rate of grafting was also of the first order with respect to the monomer concentration. A comparison of the total rate, Rp, and the rate of grafting, Rg, indicates that, at low perester‐content, Rg is about one‐half of Rp, while at high perester content, Rg is much lower than Rp as a result of the recombination of redicals. These kinetic result are discussed on the basis of a general reaction scheme. In the second method, first‐step polyners carrying tert‐butyl peroxide endgroups, were prepared on the basis of the decomposition of tert‐butyl hydroperoxide in the presence of cupric octoate. The influence of the concentration of this copper salt on the rate of polymerization has been considered. The first‐step polymers so produced can initiate the polymerization of a second monomer, so that block copolymers are obtained by chain extension, probably on both sides. In a third method, the initial peroxidic polymer was obtained by electrolysis of an aqueous solution of polymethacrylic acid. The experimental conditions are discussed. The hydroperoxide‐containing polymers, prepared and isolated at below 10°C., can be used for a second‐step polymerization with another monomer, e.g., acrylamide. Graft copolymers were thus obtained; these hydroperoxidic polymers are, however, relatively unstable, particularly because of the presence of neighboring acid groups.
Grafting of polycaprolactam on a polymethacrylate‐type backbone
Authors: Wichterle O. , Gregor V. 
Published: April 2003

AbstractRecently it has been found in our laboratories that ϵ‐caprolactam can be very readily polymerized by alkaline catalysts even at room temperature provided diacylamines, such as N‐acetylcaprolactam, are present. As part of the investigation of the mechanism of this reaction, we have prepared polymers of N‐methacryloylcaprolactam containing the active secondary imide groupings attached to a polymethacrylate‐type chain. Methacryloylcaprolactam itself will not, for obvious steric reasons, undergo polymerization, but its copolymers may readily, be obtained; thus it will form a copolymer with styrene with the parameters r1 = 1, r2 = 0. When this copolymer is allowed to react with caprolactam in the presence of alkaline catalysts, the monomer will enter at the sites of imide groupings. Simultaneously with the addition of the lactam to the copolymer, crosslinking of the polymer takes place which causes the formation of insoluble gels at the very beginning of the grafting. It is assumed that crosslinking is due to a reaction of the Claisen condensation type between two diacyl imide groups. This is supported by the fact that the relationship of the reciprocal specific viscosity and time is approximately linear in solution polymerization. The grafting of lactam which is not controlled by diffusion continues at an undiminished rate even after the gel formation. The rates of grafting and crosslinking are proportional to the concentration of sodium caprolactam added.
Influence of bond angle restrictions on polymer elasticity
Authors: Flory P. J. , Hoeve C. A. J. , Ciferri A. 
Published: April 2003

AbstractThe statistical distribution of end‐to‐end distances in polymer chains is re‐examined with particular attention to deviations from Gaussian behaviour. The nature of the error involved in the conventional inversion of the Langevin function is specified. The theory of the rubber elastic equation of state is generalized for the case where the chains display changes in internal energy with change in conformation. The temperature coefficient of the ratio f/T of the retractive force to the absolute temperature is shown to be directly related to the change with temperature of the mean‐square distance r between the ends of the free polymer chain. thus,

where fe = (∂E/∂L)V,T. represents the contribution of the internal energy to the retractive force, and [η]Θ is the intrinsic viscosity at the theta point. Preliminary experiments are reported on stress‐temperature coefficients for (I) crosslinked poly‐(dimethylsiloxane) and (II) polyethylene. For I, it is found that fe = 0, within experimental error, but for II, fe ≅ −f/2. Results for II are explicable on the basis of a potential energy difference of ca. 500 cal./mole between the gauche and trans forms, in agreement with spectropic results for lower hydrocarbons.
An internal friction study of polytetrafluoroethylene
Authors: McCrum N. G. 
Published: April 2003

AbstractA torsion pendulum has been used to measure the internal friction of polytetrafluoroethylene. The crystallinity of the specimens varied form 48 to 92%, and the measurements were made over a wide temperature range. The site of each relaxation was assigned by observing the variation of relaxation strength and peak height with crystallinity. In this way amorphous relaxations were found to occur at 176.5°K. (0.60 cycle/sec.) and 399°K. (0.62 cycle/sec.). The crystallite relaxations occur at 291°K. (0.28 cycle/sec.) and the crystallite melting point, 600°K. It is concluded that the amorphous regions of polytetrafluoroethylene exist in three phases between 4.2°K. and the crystallite melting point. The supercooled liquid which exists below 600°K. is transformed into an amorphous solid at a temperature in the region of 400°K. This solid undergoes another transformation at approximately 176°K. to form a third phase which is stable down to 4.2°K. The crystallite realaxation centered at 291°K. consists of a series of disordering transitions including the well‐known first‐order transition. There is evidence that hindered rotation occurs within the crystallites at temperatures as low as 220°K.
Graft copolymer formation during the polymerization of vinyl acetate in the presence of polyvinyl alcohol
Authors: Hartley F. D. 
Published: April 2003

AbstractWhen aqueous dispersions of polyvinyl acetate are prepared by polymerizing vinyl acetate monomer, emulsified in water in the presence of polyvinyl alcohol and an alkali, with a water‐soluble persulfate catalyst, polyvinyl acetate‐polyvinyl alcohol graft copolymers are formed. The presence of graft copolymers in the dispersions was first established by the techinques of turbidimetric titration and adsorption and partition chromatography on paper. Subsequently the graft copolymers were separated according to species and at the same time the polyvinyl acetate was fractionated on a molecular weight basis, by using a continuous gradient elution chromatographic fractionation technique. The results obtained show that in the fractionation of polymers by chromatographic methods, solubility in the developing solvent is a more important differentiating property than adsorption on the supporting medium. Three dispersions prepared using identical quantities of vinyl acetate monomer and catalyst, but with varying quantities of polyvinyl alcohol, have been fractionated. Two different species of graft copolymer, one water‐soluble and of high polyvinyl alcohol content, and the other water‐insoluble and of low polyvinyl alcohol content, have been isolated from each of the dispersions. The relative amounts of the two species of graft copolymer and the moleculare weight distributions of the polyvinyl acetate were found to differ in the three dispersions. Mechanisms for the reactions giving rise to graft coplymer formation have been postulated which explain these differences. It is suggested that the watersoluble species of graft copolymer are formed in the initial stages of the reaction by the radicals from the decomposing catalyst which first abstract hydrogen atoms from the polyvinyl alcohol molecules to form polymer radicals which then initiate polymerization of vinyl acetate monomer. The water‐insoluble species of graft copolymer are formed at later stages in the reaction by transfer from growing polyvinyl acetate chains on to polyvinyl alcohol molecules, forming polymer radicals which can again initiate polymerizaion of vinyl acetate. In the infrared spectra of both species of graft copolymer an absorption maximum occurs at a wavelength of 8.8μ. An absorption maximum occurring at this wavelength is characteristic of a tertiary alcohol group, and suggests that the grafted polyvinyl acetate chains are attached to the chain carbon atoms of the polyvinyl alcohol molecules to which the hydroxyl groups are also attached.
Graft copolymerization by pre‐irradiation technique
Authors: Ballantine D. , Glines A. , Adler G. , Metz D. J. 
Published: April 2003

AbstractTrapped free radicals are formed when polyethylene is irradiated with cobalt‐60 gamma rays in the absence of any reactive species‐oxygen, monomer, etc. The net free radical population formed increases with increasing total dose, decreasing temperature of irradiation, and increasing crystallinity of the polymer. Upon addition of a monomer to these vacuum‐irradiated films, the monomer will graft copolymerize. Graft coplymers produced by this technique are the most homogeneous, compared to those formed using other radiation‐induced techniques. If oxygen is present during the irradiation many of the free radicals formed are consumed in reaction with the oxygen to produce peroxides, hydroperoxides, and other oxygen‐containing groups. The ratio of trapped free radicals to those reacted with oxygen increases with increasing crystallinity of the polyethylene. When films irradiated in air are subsequently contacted with monomer, grafting occurs, even at room temperature, and increases in rate at higher tamperatures. It is believed that the initiation of grafting in these films is accomplished by trapped radicals and peroxide decomposition. The graft copolymers formed by this technique are not as homogeneous as those formed in the vacuum irradiation technique, but are far more homogeneous than those formed in either of the simultaneous irradiation‐grafting techniques.
Synthèse des copolymères greffés à partir des polymères ayant subi l'action des radiations ionisantes. II. Influence des conditions de préirradiation sur la cinétique du greffage sur le polyéthylène
Authors: Chapiro Adolphe 
Published: April 2003

AbstractOn a étudié l'influence de la dose at de l'intensité de la préirradiation sur la réaction de greffage des monomères vinyliques sur des films de polyméthylène ayant été préallablement irradiés en présence d'air. Plusieurs types de rayonnements out été utiliés pour l'irradiation des films: rayons gamma du cobalt‐60, rayons‐X de 37 kV et électrons accélérés de 2 MeV. Les films irradiés étaient ensuite chauffés sous vide, en présence de monomère, à 80 et à 135°C. Au cours de la réaction de greffage, les films grossissaient régulièrement dans lels trois dimensions sans que la forme de leur contour extérieur soit modifiée měme pour des taux de greffage très élevés. On a trouvé que la vitesse de greffage à une température donnée était une fonction linéaire du temps de préirradiation jusqu' à des doses de l'ordre de 10 mégarads; pour des doses plus élevées, la vitesse de greffage tend vers une valeur limite. D'autre part les films préirradiés en présence d'air et irradiés ensuite sous vide, perdent leur activité pour le greffage. Pour une dose d'irradiation donnée, la vitesse de greffage est d'autant plus élevée que l'intensité à laquelle a été effectuée la préirradiation est plus petite. L'ensemble de ces résultats confirme l'hypothèse déjà exprimée antèrieurement, selon laquelle l'irradiation en présence d'air produirait dans le polymère des groupements peroxydiques. Ces peroxydes macromoléculaires sont eux‐měmes sensibles à la radiolyse et se décomposent pour des doses élevées. La cinétique de la réaction de greffage est conforme à ne réaction de polymérisation amorcée par les macroperoxydes. Cette polymérisation s'effectue dans un milieu polymère très visqueux, ce qui ralentit la réaction de terminaison et produit des perturbations analogues à celles que l'on observe lors de l'apparition de l'“effet de gel.” De plus la nécessité pour le monomère de pénétrer à l'intérieur du film de polymère, introduit une complication supplémentaire, la concentration instantanée du monomère sur les lieux de la réaction étant contrǒlée par diffusoin. L'influence des branches latérales existant dans le polyéthylène “haute pression” sur la peroxydation radiochimique a été mise en évidence par une étude comparative entre le greffage sur le polyéthlène “basse pression” et sur du polyproylène. Les résultats obtenus montrent que la présence d'hydrogènes tertiaires dans le polypropylène favorise la formation d'hydroperoxydes, tandis que des structures biperoxydiques dominent dans le polyéthylène à chaǐne linéaire. On discute l'ensemble de ces résultats.
Préparation des copolymères greffés du polytetrafluoroéthylène (Teflon) par voie radiochimique
Authors: Chapiro Adolphe 
Published: April 2003

AbstractIl a été trouvé antérieurement que l'on pouvait greffer différents polymères sur le polytétrafluoroéthylène, en soumettant à l'action des radiations ionisantes ce polymère en présence de monomères vinyliques. Cependant, comme le Téflon est une substance chimiquement très inerte, aucun monomère ne peut pénétrer à l'intérieur de ce polymère dans les conditions usuelles et le greffage est généralement limité à une zone superficielle de celui‐ci. Or on a trouvé que si l'on greffe sur le Téflon dans des conditions expérimentales bien définies un monomère capable de gonfler le produit greffé obtenu, le greffage se poursuit de proche en proche et on obtient finalement un copolymère greffé dont la structure est homogène dans toute la masse. Cette réaction a été étudiée en greffant sur des films de Téflon du styrolène et du méthacrylate de méthyle. Au cours du greffage les films de polymère grossissent régulièrement dans les trois dimensions sans que la forme de leur contour extérieur soit modifiée. Cependant, si la teneur en polystyrolène dans les films dépasse 90% en poids, ceux‐ci tendent à se déchirer à la suite des tensions provoquées par le gonflement du film greffé dans le styrolène. La cinétique de cette réaction est contrǒlée par la diffusion du monomère dans les films partiellement greffés. Les courbes de conversion font apparaǐtre une accélération de la réaction au cours du temps. Elles peuvent ětre représentées par une équation empirique de la forme C% = Atα + B où t représente le temps et où l'exposant α a une valeur généralement comprise entre 2 et 3. Si la vitesse de diffusion du monomère est inférieure à sa vitesse de polymérisation, le greffage est limité à une zone superficielle des films; si par contre on réduit la vitesse de polymérisation en maintenant constante la diffusion, le greffage dans toute la masse du film devient possible. On a étudié l'influence de différentes paramètres sur la cinétique de la réaction. C'est ainsi que l'intensité du rayonnement, la concentration du monomère et la température jouent un rǒle fondamental dans la réaction. Des films de Téflon de différentes épaisseurs ont été greffés dans ces conditions. Une irradiation intermittente favorise également le greffage dans la masse. Quelques propriétés des films de Téflon greffés de polystyrolène dans toute leur masse ont été étudiées. Ces films ne gonflent que faiblement dans le benzène, měme pour des teneurs en polystyrolène de l'ordre de 70%; ils se ramollissent vers 150° sans fondre et conservent une grande inertie chimique. On discute l'ensemble de ces résultats.
Molekulargewichte und molekulargewichtsverteilungen unverzweigter und verzweigter polystyrole
Authors: Meyerhoff G. , Cantow M. 
Published: April 2003

AbstractZur Bestimmung von Molekulargewichtsverteilungen muss man die genaue Beziehung zwischen Molekulargewicht und Viskositätszahl kennen. Für Polystyrole erhielten wirdiese Beziehung aus Sedimentation‐und Diffusionsmessungen. Im niedrigeren Molekulargewichtsbereich fanden wir, dass anionische Polystyrole nach Szwarc eine etwas kieinere Viskositätszahl aufweisen wie gewöhnliche Polystyrole. Das normale Polystyrol zeight in diesem Bereich keinen merklichen Verzweigungseinfluss. Jedoch vermindert die Verzweigung [η] für Molekulargewichte > 500 000 beträchtlich. Hier unterscheidet sich das isotaktische Polystyrol vom gewöhnlichen ataktischen, das immer etwas verzweigt ist. Es zeigt sich, dass gelöstes isotaktisches Polystyrol sich wie ideales unverzweightes Polystyrol verhält. Die Beziehung [η] = f(M) als Funktion der Verzweigung kann man durch Kombination der Viskositätstheorie von Debye‐Bueche mit der Konfigurationsstatistik von Zimm‐Stockmayer erhalten. Wir führen dies für das nicht‐ideale Lösungsmittel Toluol durch. Danach ergibt sich, dass bei gewöhnlichen Polystyrolen zwischen M = 105 bis M = 106 etwa nach jeweils 4 × 103 monomeren Einheiten ein Verzweigungspunkt auftritt. Im Gebiet von Mw = 105 bis 1,5 × 106 führten wir verschiedene Fraktionierungen aus. Es wurde dabei die Dreiecksfraktionierung benutzt, die uns als die zur Zeit wirksamste Methode erscheint. Dabei zeigte sich, dass es fast unmöglich ist, für niedrige Molekulargewichte eine genaue Verteilungskurve zu erhalten. Nur durch Zusammenfassung osmotischer, ebullioskopischer und viskosimetrischer Messungen konnten wir verlässliche Resultate rezielen. Das höchste von uns fraktionierte Präparat zeigte einigen Verzweigungseinfluss, der für die höheren Fraktionen stärker wird. Dann ist die Grösse [η] kein Mass mehr für das Molekulargewicht der einzelnen Fraktionen. Daher bestimmten wir Mw für jede Fraktion mit Hilfe der Lichtstreuung. Die sich derart ergebenden Molekulargewichtsverteilungen werden diskutiert.
Properties of isotactic, atactic, and stereoblock homopolymers, random and block copolymers of α‐olefins
Authors: Natta G. 
Published: April 2003

AbstractThe study of several methods of controlling the molecular weight in the stereospecific polymerization of α‐olefins leads us to a better understanding of the neture of coordinated ionic catalysis. Some physical and chemical factors that influence the steric composition of polymers have been examined and a kinetic interpretation of stereoblock Polymers is proposed. A continuous variation of physical properties, particularly of crystallinity, melting point, and densities, and mechanical properties is observed as functions of the length of the steically homogeneous section in stereoblock polymer. This length is related to the number of steric irregularities of the chain. A new method to prepare block copolymers of different monomers is described; these copolymer show evidence of crystallinity due to long sequences of the same monomeric units, but properties different from those of physical mixtures of homopolymer and from those of random copolymer. The production of random copolymer has been examined, particularly of amorphous ethylene‐propylene copolymer having interesting elastomeric properties.