Carbon-13 Observations of the Nature of the Short-Chain Branches in Low-Density Polyethylene

Abstract

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

Identification of Branches in Low-Density Polyethylenes by Fourier Transform Infrared Spectroscopy

“…This indicates that the considerable differences in polymerization conditions giving rise to a wide total branch concentration range has very little effect on the distribution of branch type. This is assured by the 13 C NMR results shown in Tabe III. Thus, most commercially available HPLDPEs should have this feature in common.…”

“…A comparison of the total branch concentrations of four HPLDPEs presented in Table III and Table VI shows that the 13 C NMR values are smaller than those of FTIR. This may be due to unassignable broad signals located around 13 and 8.…”

“…The signal intensity of all the carbons in the PE is thus considered calculable from /main+ 5(/p/2) + 3(/p/2), and the signal from the methyl is expected to have the intensity of (/p/2). Therefore, the branch concentration N(branches/1000 C) may be calculated from where N is the branch concentration determined by 13 C NMR, A, the absorbance value estimated by FTIR, T, the film thickness in em, and D, the film density in g cm- 3 . The values of K for various branch types are shown in Table II.…”

“…The extinction coefficients of the methyl symmetrical deformation bands for various branch types were determined by applying the FTIR difference spectrum method to the 13 where N is the branch concentration, /main the integral intensity of the main CH2 peak and lp is the integral intensity of the f:/-CH2 peak.…”

“…1 -4 For high pressure low-density polyethylenes (HPLDPEs), many studies have been conducted on SCBs by infrared spectroscopy,4-12 1 H or 13 C NMR spectroscopy, 13 -21 pyrolysis hydrogenation gas chromatography22-26 and y-radiolysis 27 -28 and show that there are ethyl, n-butyl and n-pentyl branches in HPLDPEs.…”