Chain Stopper Research Articles

Synthesis and characterization of halato-telechelic bisphenol-A polycarbonates

Interfacial polycondensation of bisphenol-A and phosgene is a well-known way to prepare bisphenol-A polycarbonate, the molecular weight of which can be controlled by the addition of suitable amounts of phenol. When the chain stopper is a bifunctional compound, the way is open to the synthesis of telechelic polycarbonates. 4-Hydroxybenzoic acid, 4-hydroxybenzenesulfonic acid, 4 N, N′-dimethylaminophenol and the iodoammonium derivative have been used as bifunctional chain stoppers in the synthesis of acid and amino telechelic polycarbonates. The n.m.r. analysis of the final products has provided evidence for the successful synthesis of α,ω-tertiary amino and α,ω-quaternary ammonium polycarbonates. In contrast, fast transesterification reactions prevent α,ω-carboxylic and sulfonic acid polycarbonate from being prepared in a controlled way.

Calculating technique for formulating alkyd resins

A calculating technique has been developed for formulating alkyd resins composed, in general, of two monoacids (fatty acid and aromatic monoacid as the chain stopper), two diacids (diacid and diacid anhydride) and a blend of diol, triol and tetraol. The technique provides an explicit connection between the alkyd resin composition and the relevant parameters of the alkyd resin such as number average molecular weight, fatty acid content (or oil length) and hydroxyl number. For a fixed fatty acid content, the range of possible molecular weights for a given hydroxyl number can be determined and vice versa. A simple computer program can easily be devised using the derived equations, thus allowing rapid calculations. The equations are also suitable for combination with those based on the work of Flory, Stockmayer, Jonason, Gordon and Miller, and Macosko, from which a prediction of the chance for gelation can be made.

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Cationic polymerization of siloxanes kinetically controlled oligomerization in hexamethylcyclotrisiloxane – linear dimethylsiloxane systems

AbstractThe kinetically controlled distribution of linear oligomers in the cationic solution polymerization of hexamethylcyclotrisiloxane (D3) in the presence of hexamethyldisiloxane (MM), octamethyltrisiloxane (MDM), and decamethyltetrasiloxane (MD2M) as molecular chain stoppers was investigated by gas chromatography. At an early stage of the process the distribution was practically unaffected by consecutive reactions involving original stable products of oligomerization, which renders the distribution an important tool in the studies of the mechanism of cationic polymerization of siloxanes. Triple pattern distribution (the excess of every third homologue of the MDnM series) is observed even at nearly full monomer conversion. The distribution of this excess oligomers at low monomer conversion is highly specific as the amounts of each subsequent homologue is decreased by a constant factor. The factor is related to the relative reactivity of the siloxane bonds in the chain stopper and in the monomer. In principle, the results agree with the chain mechanism of polymerization involving kinetic chain transfer to the molecular chain stopper.Studies of the kinetically controlled distribution of linear oligomers in the oligomerization involving a chain stopper of unsymmetrical structure with respect to the reacting siloxane bond allow to determine the specificity of the combination of stopper fragments with a linear molecule. Low specificity showed by MDM constitutes evidence against the mechanism involving a silicenium ion intermediate.

Molecular weight control of polycarbonates obtained by interfacial polycondensation

AbstractResults for pilot plant scale experiments on the preparation of polycarbonates are presented. The effect of end‐groups content of the oligomer on the efficiency of polycondensation was quantitatively studied and procedures of molecular weight control with different chain stoppers have been investigated.It was found that for an efficient polycondensation of oligomer the contents of the reactive groups have to be as follows: minimum 30 mole‐% of OCOCl, and relation OCOCl:OH=0.5. The same applies for an effective and reproducible molecular weight control. The best effect of chain stopping is observed when a molecular weight regulator is added both in phosgenation and polycondensation stages.

Studies on polycarbonate preparation by the interfacial polycondensation method

Results are presented for pilot plant scale experiments on the preparation of polycarbonates. Effects of efficiency of mixing, catalyst, sodium hydroxide excess and chain stopper on the course of polycondensation were studied. It was found that a set of parameters can be selected such that the molecular weight of the polycarbonate depends only on the amount of chain stopper.

Polyamide‐imides

AbstractA novel preparation of polyamide‐imides from diphenylmethane diisocyanate and trimellitic anhydride in N‐methylpyrrolidone is described. Partial substitution of trimellitic anhydride with either maleopimaric acid, terephthalic acid, or by various di‐anhydrides permitted a correlation of structure with solution stability and thermal properties. Heat resistance improved with increasing imide content of the polyamide‐imide. Almost all solutions advanced in viscosity on storage at room temperature. The least stable solutions were those that deviated the most from 50–50 amide‐imide. However, the viscosity changes depended somewhat on the type of modifying ingredient. For example, polyamide‐imides modified by substitution of 10% PMDA (pyromellitic anhydride) or BPDA (benzophenone tetracarboxylic acid dianhydride) were less stable than those modified with 10% (cyclopentanetetracarboxylic acid dianhydride) CPDA or THFDA (tetrahydrofuran‐2,3,4,5‐tetracarboxylic acid dianhydride). The use of excess reagent and of monofunctional chain stopper was investigated as a means of controlling solution stability.

Copolymerization of 2,6-dimethyl-4-bromophenol with 2-methyl-4-bromo-6-allylphenol and other p-bromophenols

A series of poly(phenylene oxide) copolymers was prepared from 2,6-dimethyl-4-bromophenol and 2-methyl-4-bromo-6-allylphenol by the Price method of interfacial oxidative coupling. The physical and chemical properties of the copolymers were correlated with the molar proportions of the bromophenols. The thermosetting rates were proportional to the concentration of the allyl function and were of interest in the development of high-temperature thermosetting resins and adhesives. The preparation and copolymerization of 2-methyl-6-(α-methallyl)-4-bromophenol were also investigated. Homopolymerization of 2-methyl-4-bromophenol or 2,2′-methylenebis-(4-bromo-6-methylphenol) under the same conditions did not occur to any significant degree. When these bromophenols were used in attempted copolymerizations with 2,6-dimethyl-4-bromophenol, only low molecular weight homopolymers of the latter were obtained. The effect of these nonpolymerizing bromophenols as chain stoppers is discussed in connection with the mechanism for this type of polymerization.

Mechanism of antioxidant action in the stabilization of hydrocarbon systems

AbstractFour antioxidant classifications, based on differences in the method by which they exert their effect, are noted. The mechanism of the chain‐stopping action is discussed in terms of four different reactions which have been proposed. The participation of the antioxidant in the mechanism of inhibited thermal oxidation is reviewed. The hydrogen donation mechanism as the first step in the chain‐stopping mechanism has been investigated by observing the effect of replacing the active hydrogen of the antioxidant by deuterium. Contrary to the findings of some prior workers, a kinetic isotope effect has been demonstrated with a secondary aryl amine antioxidant as well as with phenols. These results, together with other supporting evidences reported in the literature, indicate that abstraction of a hydrogen from the antioxidant is the initial step in the usual mechanism by which antioxidants serve as chain stoppers in the oxidation of hydrocarbon systems.

Oxidation and Antioxidant Action in Rubber Vulcanizates

Abstract The variation of rate of oxygen absorption in the constant rate stage with oxygen concentration involves both a square-root and a first-power term for most rubber vulcanizates, although the latter term may be negligible in some cases. A mechanism of oxidation and antioxidant action has been postulated which is consistent with the observed behavior, and which indicates that the antioxidant may function in at least four ways, some beneficial and some harmful. Direct oxygen attack on the antioxidant may produce radicals capable of initiating oxidation chains. Initiation by this mechanism appears to be more important with amine antioxidants than with phenols. The antioxidant may participate in the propagation stage by a chain-transfer mechanism. While this effect alone for a typical antioxidant would probably have only a small effect on the over-all rate of oxidation, it might also affect in a favorable way the secondary reactions which determine changes in physical properties. Amines also appear to be more inclined to chain transfer than are phenols. Termination by a chain-stopping mechanism is one way in which antioxidants retard oxidation. Phenols appear to be more efficient than amines in this connection. Destruction of peroxide by antioxidant-induced decomposition to stable products reduces initiation by peroxide decomposition, and is thus another important mechanism of antioxidant action. The amines are more efficient than the phenols in this regard, and the presence of carbon black is essential for the amines to exhibit their maximum effectiveness. Carbon black is known to promote the decomposition of peroxide to free radicals, but in the presence of amine antioxidants it also promotes the decomposition to stable products. This dual effect of carbon black results in a reversal of effectiveness of amines and phenols in gum and black stocks. The phenols (which function best as chain stoppers) are more effective in the gum stocks; the amines (which function best by reducing peroxide initiation by directing the decomposition to stable products) are more effective in the black stocks.

On the Oscillation of the Anchor Chain

It is well known that the anchor chain, when paid out, suffers oscillations so severe that it is obliged to reduce the payout speed. Its cause is quite clearly the resonating phenomenon of the disturbing period of the links of the chain paid out with that of the chain, under tension and with travelling speed, of the span between the gipsy wheel and the chain stopper. This fact is treated theoretically in the present note, arid, the natural period of the chain being obtained, the relation between the pay-out velocity and the number of links within the span is given for the resonating condition. Some suggestions are also described for the purpose of reducing the oscillations.