Fast Initiation Research Articles

PULSED LASER UTILIZING A FLUORINE AND HYDROGEN MIXTURE

An investigation was made of fast initiation of the reaction F+H2=HF*+H in a short high-power electric discharge. The spectral composition of the stimulated emission and the time dependence of this emission, relative to the discharge current, were determined. Efficient initiation of the reaction occurred when the average energy of electrons in the discharge was ~1.5 eV. Under optimal conditions the output power was ~300 kW, the energy was ~510–2 J per pulse, and the electrical efficiency was ~8%.

Cationic polymerization of cyclic dienes. X. Cationic polymerization of 1‐vinylcyclohexene and 3‐methyl‐1,3‐pentadiene

Abstract1‐Vinylcyclohexene (VCH), which has one of the double bonds in the ring and the other outside the ring, was synthesized and polymerized by cationic catalysts. The reactivity of VCH was very large in the polymerizations catalyzed by boron trifluoride etherate (BF3OEt2) and stannic chloride–trichloroacetic acid complex. Similar to other cyclic dienes, the polymerization of VCH was a nonstationary reaction having a very fast initiation step. The polymerization proceeded by either a 1,2‐ or a 1,4‐propagation mode in which vinyl group was always involved. Particularly when BF3OEt2 was used as a catalyst, an intramolecular proton or an intramolecular hydride ion transfer reaction took place, resulting in the formation of methyl groups in the polymer. The degree of polymerization of polymer formed was about 10. This indicates the preponderance of monomer transfer reaction. To investigate the reason for the high reactivity of cyclic dienes, cationic copolymerizations of VCH and 3‐methyl‐cis/trans‐1,3‐pentadiene (cis/trans‐MPD) was carried out. The relative reactivity of monomers decreased in the order VCH > trans‐MPD > cis‐MPD. On the other hand, the resonance stabilization of monomers decreased in the order VCH > trans‐MPD > cis‐MPD. Therefore, it could be considered that the monomer reactivity is mainly determined by the stability of carbonium ion intermediate. The relative stability of carbonium ion must be VCH > trans‐MPD > cis‐MPD. Thus the influence of the conformation of ion on its stability was clearly demonstrated.

Propagation rate constant in cationic polymerization of cyclic dienes. I. Polymerization of cyclopentadiene with titanium tetrachloride–trichloroacetic acid

AbstractCationic polymerization of cyclopentadiene induced by titanium tetrachloride–trichloroacetic acid was investigated in a toluene solution at −69 to −77°C. All manipulations were handled under vacuum conditions. Time–conversion curves were determined accurately by following the exothermicity of the fast reaction in an adiabatic system. The polymerization kinetics were developed on the basis of a fast initiation reaction and a nonstationary‐state concentration (diminishing concentration) of active species, and the propagation rate constant k2 was determined by substituting either an initial rate of polymerization or a final conversion for the kinetic equations. k2 for the present system was determined to be 350 l./mole‐sec, which is larger than those so far reported for some vinyl monomers in cationic polymerization. The present method can be commonly applied to reactive monomers for the determination of k2. The nature of termination reaction is discussed in connection with the determination of k2.

Mechanism of the autoxidation of triethylborane. Part II. Importance of an autocatalytic initiation reaction in solution

Triethylborane was oxidized in the presence of variable amounts of iodine. Induction periods which were dependent upon the iodine–oxygen concentration ratio could be observed for the consumption of iodine, which was thereafter used up readily. In order to explain the experimental results, a mechanism including a fast autocatalytic initiation reaction is suggested.

Cationic polymerization of cyclic sulphides. I. Kinetic study of the polymerization of 3.3‐dimethylthietane initiated by triethyloxonium tetrafluoroborate

AbstractThe kinetics of the cationic polymerization of 3,3‐dimethylthietane with triethyloxonium tetrafluoroborate in methylene chloride at 20°C have been studied. The polymerization is characterized by a very fast initiation reaction. The propagation occurs most probably via cyclic sulphonium ions. The existence of a termination by reaction of the growing chains with sulphur atoms of the polymer chain was demonstrated by kinetic and molecular weight measurements. The propagation constants kp and termination constants kt at different temperatures were calculated.The enthalpies of activation for the propagation and termination reactions are, respectively, ΔH≠p = 12.5 kcal mole−1, ΔH≠t = 7.4 kcal mole−1, and the corresponding entropies of activation are ΔS≠p = ‐26 cal deg−1 mole−1, ΔS≠t = ‐49 cal deg−1 mole−1.

Kinetics of homogeneous butadiene polymerization catalyzed by TiI2Cl2/Al(iso‐C4H9)3,

AbstractKinetics of homogeneous butadiene polymerization initiated by TiI2Cl2/Al(iso‐C4H9)3 catalysts were studied at constant monomer concentration. A reaction mechanism involving fast initiation and living chains propagation with reversible deactivation of the active sites was developed. The active sites concentration and number average molecular weight during polymerization were calculated. The molecular weight distribution resulting from the assumed kinetic scheme was also considered. A method was developed to calculate number and weight average molecular weight of polymer at any moment after establishing of the deactivation‐reactivation equilibrium. The experimental findings are consistent with the presumed reaction mechanism.

Cationic Polymerization of Cyclic Dienes. VII. The Kinetic Observations on the Polymerization of 1,3-Cyclohexadiene

Abstract 1, 3-Cyclohexadiene was polymerized at 0°C in methylene chloride or benzene using boron trifluoride etherate or stannic chloride-trichloroacetic acid as catalyst. The former catalyst led to a stationary-state polymerization but the latter brought about a non-stationary-state polymerization in which the initiation reaction was very fast. The very fast initiation reaction is a consequence of a strong interaction between the monomer and the metal halide. Part of the added catalysts was made inactive by the interaction with the excess monomers.

A method of determining the active centers in cationic polymerization, applied to polymerization of dioxolane, trioxane, and formaldehyde

AbstractIn order to determine the concentration of active centers, polymerizations are terminated by addition of sodium alkoxide. This yields alkoxy endgroups which after acid hydrolysis of the isolated and purified polymers are determined by gaschromatography of the produced alcohol.In cationic polymerization of anhydrous formaldehyde at −78°C under certain reaction conditions a fast and quantitative initiation reaction and no kinetic chain termination have been observed. Presumably living polymers are obtained. The unexpected decrease in the rate of polymerization after a certain conversion is ascribed to hindrance of monomer diffusion in the crystalline polymer in which the active centers are imbedded.Cationic chainends have been detected in the 60Co γ‐ray induced polymerization of crystalline trioxane. This proves a cationic mechanism of chain propagation in the radiation‐induced polymerization of trioxane.In polymerization of 1.3‐dioxolane by HClO4 at 20°C a gradual but quantitative initiation reaction and no kinetic termination reaction have been observed. Termination by addition of alkoxide and determination of alkoxy endgroups also helped to distinguish between several different mechanisms of chain propagation proposed in the literature. It is assumed that the active center in dioxolane polymerization is a tertiary oxonium ion which propagates by an SN 2 reaction with monomer.

Radiation-induced polymerization by free ions. Part 1.—Polymerization of styrene under anhydrous conditions

The radiation-induced ionic polymerization of bulk styrene has been studied under extremely dry conditions. The rate Rp of polymerization increases with more rigorous drying of the monomer, and this effect is accompanied by a change in the exponent n for the dose-rate (I) dependence of Rp, from n values near unity to 0.6 (Rp∝In). A mechanism involving bimolecular termination of free ions is thereby indicated. The strong retardation of the polymerization by scavengers of high proton affinity such as water, diethyl ether, ammonia and trimethylamine in low concentrations (10–3–10–6 M) provides evidence that free cations are mainly responsible for the propagation. The addition of nitrous oxide does not affect Rp, and does not appear to prevent electron capture by styrene. Individual reactions steps for cationic and anionic mechanisms are discussed, and the initiation is considered to occur through the respective monomeric ion radicals M+˙ and M+.. It is concluded that the predominance of cationic over anionic polymerization is due to faster initiation and/or propagation processes in the former case.

Fatigue Crack Propagation in Metals

PEARSON1 has recently discussed the problem of the rate of growth of a fatigue crack. He tabulated values of the stress intensity factor K necessary to cause a crack to grow under a repeated tensile loading cycle at a certain velocity in a number of alloys and showed that the ratio of K to the Young's modulus E of the material was roughly constant. The fact that the fatigue crack growth rate characteristics bear no relation to any mechanical property other than E has been demonstrated previously by Frost2,3, who pointed out, however, that any such relationship applies only to pure metals or to alloys having the same growth rate characteristics as the base metal and in which the growth rate is independent of the tensile mean stress in the loading cycle. For example, although the growth rate characteristics of annealed and cold-rolled copper are the same, those of the various aluminium alloys may differ widely3,4. Complex alloys, such as the high-strength aluminium alloys, can have their crack growth characteristics affected markedly by microstructure; brittle second phase particles are known to provide easy fast fracture initiation sources. It is this introduction of elements of fast fracture into the growth process which causes the growth rate characteristics of the various aluminium alloys, having the same Young's modulus, to be different from each other and from the base metal and thus invalidates Pearson's generalization. Although the three copper-aluminium alloys tested by Pearson gave similar K/E values, a different value would certainly have been obtained for a zinc–aluminium alloy. To obtain similar K/E values for brass and aluminium alloy, for which he quotes similar K values, Pearson takes a value of E for brass similar to that for the aluminium alloy. Most authorities quote an elastic modulus of brass nearer to that of the titanium alloy (for which he quotes a K value different from that for brass) than to that of the aluminium alloy. It would seem, therefore, that the constancy of K/E obtained for the materials tested by Pearson must be considered fortuitous.

The polymerization of styrene by perchloric acid

The rates and degrees of polymerization of styrene by perhcloric acid have been measured dilatometrically at 25, 0 and –30°C in solution in 1, 2-dichloroethane ( DCE ), carbon tetra­chloride and mixtures of these and other solvents. The rates, throughout the conversion are found proportional to the first power of the monomer concentration, and to the initial concentration of perchloric acid. They are unaffected by traces of water, but show a strong solvent effect. In DCE + CCl 4 mixtures the rates correlate simply with the dielectric constant of the mixture; other solvents show specific solvent effects especially nitro compounds which inhibit. The molecular weights are very low (1000 to 6000) and give evidence of two rapid transfer processes, spontaneous and to monomer. The polymer contains both saturated and unsatu­rated chains, the fraction of the former correlating with the fraction stopped by the monomer transfer process. The catalyst is not consumed. Further doses of monomer are polymerized at virtually the original rate, but are not added to the first formed chains. The polymerization is interpreted as a ‘transfer-dominated’ living polymerization, with a fast initiation and no termination. The overall rate is therefore taken to be that of the propagation step. Rate constants and activation energies for this and for the transfer pro­cesses are derived.

Polymerization of butadiene in the presence of triethylaluminum and n‐butyl titanate

AbstractMeasurement of butadiene absorption at constant volume, and of polymer formation, indicated that the rate of polymerization was first order in monomer concentration, up to 1.5 mole/l., and also in initial total catalyst concentration, for given molar ratio of alkyl to titanium ester. The overall activation energy was 11.5 kcal./mole. A fast initiation, accompanied by the evolution of ethylene or a mixture of ethane and ethylene, a relatively slow propagation, and chain transfer to monomer, appeared to be the main steps of the reaction. Termination was essentially absent. Insoluble, gellike material was formed roughly at 20% conversion and was attributed to a random crosslinking, involveing very few crosslinks per molecule, or possibly trifunctional branching. The molecular weight distribution was very narrow. A polymer of over 80% in 1,2 units, having a predominantly syndiotactic structure and about 10% crystallinity, was obtained. In the absence of monomer, the catalyst components reacted rapidly to form complexes, accompanied by strong ethane evolution. A complex series of reactions leading to the possible formation of more than one active species was indicated. The active catalyst suffered bimolecular decomposition on prolonged standing. For optimal molar ratio of alkyl to ester of 5/1, the concentration of active species was estimated as having an upper limit of 4.8 × 10−6 mole/l.

Kinetics of Pyrolysis of Alkyl Hydroperoxides and Their O–O Bond Dissociation Energies

It is shown that group additivity rules lead to very reliable estimates (±1 kcal) for the ΔHf° of both alkyl peroxides and hydroperoxides. Previously published data for t-BuO2H are unique in showing a discrepancy of 6.3 kcal/mole from the estimated data. From the ΔHf° for the ROOH and the activation energies of pyrolysis of ROOR it is shown that the bond dissociation energies D(RO–OH) are about 42 kcal/mole for alkyl R. On this basis the direct split ROOH→RO+OH is shown to contribute negligibly to the observed kinetics of decomposition of t-BuOOH in solution. Instead the decomposition is shown to be compatible with a chain involving t-BuO2 and t-BuO radicals via the fairly well established reaction: 2 t-BuO2→2 t-BuO+O2. The rate law for such a chain is shown to lead to 43 power dependence on t-BuO2H in good accord with the kinetic data. Absolute values of the chain rate are also in good accord with the data. It is further shown that very reactive solvents (olefins, etc.) may convert RO2 to RO directly, changing the rate to 32 order with a lower activation energy (∼28 kcal). Published data from studies of tetralin-OOH and cyclohexane-OOH are shown to be in good accord with the proposed mechanism. Allylic hydroperoxides are discussed briefly and considered from the standpoint of good H-atom donors. The styrene induced decomposition of cyclohexane-OOH is considered in detail and a new, fast initiation step is discussed: 2φCH=CH2+ROOH→φĊH–CH2–CH2–CH2φ+RO2·.