Branched Chain Molecules Research Articles

Rates of conformational transitions in branched chain molecules

A recently developed theory of the kinetics of conformational transitions based on a multi- dimensional extension of Kramers' rate theory is applied to branched polyethylene-like molecules. In particular, the influence of branching on the mechanism and rates of the trans - gauche transition is investigated. As in linear molecules, the reaction coordinate is a localized mode. Thus, the presence of an attached side chain introduces conformational rigidity into one of the tails connected to the transforming rotational angle, and the localized mode behaves akin to related linear molecules. In general, it is found that branches somewhat reduce the transition rate relative to the linear molecule; nevertheless the effect is fairly minor. Therefore, it is concluded that the time scale of conformational transitions in branched and linear chains is the same.

Origin of Gasoline-Range Hydrocarbons and Their Migration by Solution in Carbon Dioxide in Norton Basin, Alaska

Carbon dioxide from a submarine seep in Norton Sound, Alaska, carries a minor component of gas- and gasoline-range hydrocarbons. The molecular and isotopic compositions of the hydrocarbon gases and the presence of gasoline-range hydrocarbons indicate that these molecules are derived from thermal alteration of marine and/or nonmarine organic matter buried within Norton basin. In the gasoline-range hydrocarbons, individual cyclic and branched-chain molecules are much more abundant than straight-chain hydrocarbons. This distribution suggests that the hydrocarbon mixture is an immature, petroleumlike condensate of lower temperature origin than normal crude oil. The submarine seep provides a natural example in support of a carbon dioxide solution transport mechanism thought to be operative in the migration of hydrocarbons in certain reservoirs.

Quasi-elastic light scattering from branched polymers: 1. Polyvinylacetate and polyvinylacetate—microgels prepared by emulsion polymerization

Emulsion polymerization of vinylacetate leads to branched polymers which at high monomer conversions form microgels of the shape and size of the latex particles. Quasielastic light scattering measurements from samples in the pre-gel state give at small q 2 a linear angular dependence of D app = Г q 2 which resembles that of randomly branched chain molecules, where Г is the decay constant of the time correlation function. Extrapolation of D app towards zero scattering angle yields the translational diffusion constant D z . The diffusion constant follows the molecular weight dependence D z = 9.78 10 −5 M w −0.478. The diffusion constant of the microgels, i.e. at molecular weights M w > 14 10 6, remains constant because of the finite and constant size of the latex particles. The coefficients k f and k D in the concentration dependence of the frictional and diffusion coefficients are related according to the equation k D = k f − 2A 2M w − v ̄ where A 2 is the second virial coefficient and v̄ the partial specific volume of the particle. The coefficient k f is calculated from the experimentally determined quantities k D , A 2 and M w , and the result is compared with the theory by Pyun and Fixman. Accordingly the branched coils in the pre-gel state resemble soft spheres, but the microgels behave more like spheres of some rigidity.

Structures and properties of mixtures of branched chain compounds and lecithin: phytol, α-tocopherol (vitamin E), and phytanic acid

Phospholipid bilayers containing branched chain molecules, phytol (1), vitamin E (2), and phytanic acid (3), have been investigated by 31P nmr, esr, and differential scanning calorimetry (dsc).A 31P lanthanide induced shift study indicated varying permeabilities to Pr3+ in the order phytanic acid > vitamin E > phytol > egg yolk lecithin alone; the half-lives (in days) were 0.002, 0.14, 0.83, and 6.5, respectively. The activation energy for Pr3+ permeation through the egg yolk lecithin–phytol membrane was found to be 84.9 ± 0.8 kJ.The following esr order parameters, S3, were obtained using the extrinsic spin label, 5-doxylpalmitic acid, in oriented mixed multibilayers: S3 (1) = 0.29, S3 (2) = 0.50, and S3 (3) = 0.02.Differential scanning calorimetry revealed a lowering of the gel–liquid crystalline phase transition temperature, Tc, as the concentration of incorporated isoprenoid compound increases, with eventual disappearance of the endotherm. Specific entropy, s, calculated for dipalmitoyl lecithin +25 mol% 3 is 126 J kg−1 K−1 compared to s = 114.2 J kg−1 K−1 for 1and s = 85 J kg−1 K−1 for 2.

On the Degradation of Branched Chain Molecules

A procedure for the derivation of the molecular weight distribution in a degrading branched polymer is developed. In the explicit calculations only sizes up to about twice the distance between branch points are considered. This is adequate to describe the rates of volatilization in the pyrolysis of polyethylenes. Expressions are given for an arbitrary distribution of branch lengths. If the rate of bond scission is uniform, the over-all rate is reduced in comparison with the linear case; the branches, if short, have a protective effect. The differences between C–H linkages on tertiary and secondary C's reverse the situation. However, the reaction pattern remains unchanged. The rates exhibit maxima which are only insignificantly displaced by the presence of (short) branches. Unless the reaction mechanism treated is profoundly altered or the differences between the two types of bonds are much larger than in small molecules, the existence of branches, per se, does not account for the observed differences in the kinetics of pyrolysis of polyethylene and the linear polymethylene.

Branched-Chain High Polymers

IF two bifunctional molecules such as ethylene glycol and adipic acid are esterified in equal proportions, linear super-ester molecules are produced. In a similar way polyamides and polyurethanes may be produced, the chemistry of the process being such that only linear polymers are, in fact, formed. On the other hand, if one of the reactants contain three or more functional groups, for example, glycerol, then instead of linear molecules, branched molecules are first formed ; but these branched products soon join up into three-dimensional networks which are insoluble and, therefore, not suitable for detailed investigation. Between these two extremes there must inevitably be produced molecules containing branched chains which, if the chains are not indefinitely long, should be soluble. Owing to the rapidity with which the network stage is reached, it has not hitherto been possible to isolate and characterize branched-chain molecules possessing long branches. Yet it is of great interest to know something of the thermo-dynamic and hydrodynamic behaviour of solutions of such molecules. The only branched molecule of which the structure has been determined is amylopectin. Here, however, it is not possible to vary the length of the branches while keeping the number fixed. It is, moreover, just the kind of variation that must be produced if the necessary information about branched-chain molecules is to be obtained. The present communication describes the attempts that have been made to synthesize, characterize and measure the properties of branched-chain super-ester molecules.

Statistical Mechanics of High Polymer Solutions

The configurational partition function for random mixtures containing any numbers of components which can consist of simple, simple chain, branched chain, or closed ring molecules is examined. Using a general statistical method a set of partial differential equations is obtained for the appropriate combinatory factor. The integrability of this set of equations is examined. This provides a criterion by which it can be decided in what cases a mathematically precise value for the combinatory factor can be obtained rigorously by solving the set of partial differential equations. In these cases general formulae are given for the combinatory factor. It is shown that precise formulae can be deduced rigorously (a) for all binary mixtures whether the components consist of simple, simple chain, branched chain, or closed ring molecules ; (b) mixtures of any number of components containing not more than one high polymer species, which can consist of closed ring equally well as of simple chain or branched chain molecules ; and (c) mixtures of any number of components containing more than one high polymer species provided these consist only of simple chain or branched chain molecules. It is also shown that even in the case in which the condition of integrability is not satisfied, an approximation, which involves negligible error for high polymer molecules, indicates that the general formula must still provide a good approximation practically. The approximations inherent in the physical model are also considered.

Statistical Thermodynamics of Mixtures

FROM some work employing the Bethe technique, I deduced1 a formula for the number of configurations of mixtures of p-mer and single molecules on a lattice. Afterwards, a kinetic derivation of this formula, together with its generalization to the case of a mixture of simple or branched chain molecules of any number of species, was given by Guggenheim2 using the principle of detailed balancing. The formula in question can be written in the form: where qiz is the number of sites which are closest neighbours of a molecule of species i which contains ri submolecules (a slight modification of the definition covers the case of flexible open-chain molecules which can bend back on themselves), and the other symbols have their customary meanings. The value of qi is given by