The UC nmr spectra of low-density polyethylenes are compared to those of model copolymers and to calculated spectra based on model hydrocarbons. It is unequivocally demonstrated that the short branches are «-butyl, thus confirming the Roedel "backbiting" hypothesis.It is well known that the morphology and solid-state properties of polyethylene are critically dependent on the frequency of short-chain branches. It seems highly probable that they are also influenced to some degree by the length of these branches, as well as the trior tetrafunctional character of the branch points. There appears to have grown up during the past decade a presumption that the short-chain branch length is known and is no longer subject to question. This is borne out by the virtual absence of any current or recent studies of this problem. Nevertheless, a critical examination of the literature shows that the accepted conclusions, largely drawn from infrared and irradiation measurements, are ambiguous. Even where clear answers seem to have been provided, it is highly probable, as we shall show in this paper, that they are at least qualitatively in error.Carbon-13 Nmr Spectra of Saturated Hydrocarbons Carbon-13 magnetic resonance spectroscopy (cmr) promises to be extremely useful in the elucidation of alkane and large alkyl structures. This is due not only to the large range of chemical shifts (ca. 40 ppm) in which saturated carbons are found to occur, but also to the simple empirically derived relationships which allow the prediction of the chemical shift of any carbon for which the local structure is known.1-3 In general, it is found that carbons at or near a branch point are the least shielded in such structures, while methyl resonances are most frequently found at highest field. These conclusions suggest that 13C chemical shifts of polyethylenes should provide new information regarding the structure and extent of

The constant rm is the position of the maximum ordinate of the distribution, í the measure of standard deviation, and erfc-1 the inverse error function complement.Ackers showed for several gels that a plot of rv vs.erfc-1(Kd) was linear and that rm and í could be taken to be calibration constants for a given gel. In present work, the available values of Kd = (Ve -V0)/Vi were graphed in terms of the inverse error function complement, erfc-1(Kd),34 vs. the corresponding estimated molecular radii, rv, to yield the approximately linear (34) "Tables of the Error Function and Its Derivatives," National Bureau of Standards, Applied Mathematics Series 41,

Studies have been made of the near surface photooxidation of atactic polystyrene films prepared in the absence of air. The samples were photooxidized on exposure to air at two frequencies, 254 and 365 nm, using a calibrated mercury irradiation source with filters. Most studies were made at 40°C and as a function of irradition time with the reactions characterized by changes in molecular weight and composition. The former was evaluated by gel permeation chromatography and the latter by transmission Fourier transform infrared spectroscopy and by multiple-internal-reflectance infrared spectra using different angles and different crystals to evaluate compositions as a function of film depth. Species identified in photooxidation include the generation of hydroperoxides and the appearance of carbonyl bands with the latter identified by the spectral shift asociated with the exposure of the photooxidized polystyrene surface to ammonia. These results suggest that principal products of near-surface oxidation of polystyrene are carboxylic acids.

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The viscosity of carboxylic-acid-terminated polybutadiene increases markedly upon neutralization with mono-and divalent bases. These viscosity increases are related to apparent molecular weight, considering the association of the terminal salts as "polymerizing" the difunctional prepolymer. For the monovalent alkali-metal salts, simple end-group dimerization seems to occur with an average tJI of ""-25 kcal/mole. Neutralization with ions of higher valency leads to materials with higher viscosities.

SynopsisThe mechanical effects of association between pendant ionic groups are investigated in series of butadiene-lithium methacrylate copolymers, butadiene-methyl (2-methyl-5vinyl)-pyridinium iodide copolymers, and mixtures of these polyelectrolytes. Thermal and mechanical tests reveal a new transition above T, in the pyridinium copolymers, designated Tt*. Below this temperature the materials behave like covalently crosslinked elastomers. The complex between the metal carboxylates appears thermally dissociable a t all temperatures. The mixing of the oppositely charged polyelectrolytes leads to the formation of more stable intermolecular pyridinium carboxylate links. EXPERIMENTAL Polyelectrolyte Preparation and AnalysisIn a previous publication, l2 the mechanical behavior of three butadienemethacrylic acid copolymers, three butadiene-2-methyl-5-vinyl pyridine

Long and Short Branches in Low-Density Polyethylenes 507 on whether each rubber chain is anchored to the spherical domain(s) at no chain end, one chain end, or both chain ends and whether each rubber chain has enough mobility to flow into the lower density region to average out the density fluctuations.

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