Methyl branching in polyethylene

The measurement of the methyl absorption at 1378 cm (7.26 pm) in polyethylene can serve as a good estimation of branching. However, interference from the methylene absorption at 1368 cm (7.31 pm) makes it difficult to measure the 1368 cm (7.31 pm) band, especially in the case of relatively low methyl contents. 

Neilson and Holland [9] associate the amorphous phase absorption of polyethylene at 1368 cm (7.31 pm) and 1304 cm (7.69 pm) with the trans-trans conformation of the polymer chain about the methylene group. Therefore, the intensities of these two absorptions are proportional to one another. By placing an annealed film (approximately 0.3 - 0.4 mm) of high-density polyethylene (HOPE) in the reference beam of a double beam spectrometer and a thin, quenched film of the sample in the sample beam, most of the interference at 1368 cm (7.31 pm) can be removed. The method has the advantage that it is not necessary to have complete compensation for the 1368 cm (7.31 pm) band since a correction for uncompensation at 1378 cm (7.25 pm) can be applied based on the intensity of the 1368 cm (7.31 pm) absorption. 

A calibration for the methyl absorption based on mass spectrometric studies of such gaseous products produced during electron bombardment of polyethylene has demonstrated irradiation-induced detachment of complete alkyl units [10]. In addition to saturated alkanes characteristic of the branches, small quantities of 

Methyl group content of LDPE has been determined with a standard deviation of 0.8% provided methylene group absorptions were compensated by polyethylene of similar structure [13]. 

Nishoika and co-workers [14] determined the degree of chain branching in LDPE using proton NMR, Eourier transform (ET) NMR (ET-NMR) at 100 MHz and ET-NMR at 25 MHz with concentrated solutions at approximately 100 °C. The methyl concentrations agreed well with those of infrared based on the absorbance at 1378 cm (7.25 pm). 

A regression analysis of IR, differential thermal analysis and X-ray diffraction data by Laiber and co-workers [12] for high-density polyethylene showed that, as synthesis conditions varied, the number of methyl groups varied from 0 to 15 per 1000 carbon atoms and the degree of crystallinity varied from 84% to 61%. 

Calibration for the methyl absorption based on mass spectrometric studies of such gaseous products produced during electron bombardment of polyethylene has demonstrated irradiation-induced detachment of complete alkyl units [15]. In addition to saturated alkanes characteristic of the branches, small quantities of methane, other paraffins, and olefins were simultaneously evolved. It was suggested that extraneous paraffins result from cleavage of the main chain [15,16]. Nerheim [17] has described a circular calibrated polymethylene wedge for the compensation of CH2 interferences in the determination of methyl groups in polyethylene by IR spectroscopy. 

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Scheme 4 Precise methyl branching in polyethylene (from [73])...

Smith JA, Brzezinska KR, Valenti DJ, Wagener KB (2000) Precisely Controlled Methyl Branching in Polyethylene via Acyclic Diene Metathesis (ADMET) Polymerization. Macromolecules 33 3781-3784. 

Smith, J.A., Brzezinska, K.R., Valenti, D.J. and Wagener, K.B. (2000) Precisely controlled methyl branching in polyethylene via acyclic diene metathesis (ADMET) polymerization. Macromolecules 33, 3781-3794. 

This thermal behavior is completely opposite from the model polyethylenes that we have synthesized via ADMET. All the endotherms for the methyl-branched ADMET polyethylenes are sharp in comparison with their chain-made... 

This covers methods that depend upon the fact that branching introduces groupings with different chemical structure from that of the repeat units of linear chain, namely branch-points and end-groups. These can sometimes be detected and estimated by physical or chemical methods. However, short branches as well as long ones introduce these groups, and it may not be justifiable to attribute them, or all of them, to long branches. Methyl groups in polyethylene are a case in point. 

Some methods, originally developed for the determination of the degree of branching in polyethylene, were extended to C2-C3 copolymers with low C3 content. One such method, proposed by Slovinski et al. (71), involves measuring the absorbance ratio of the bands at 7.25 and 7.30 p. The calibration curve reported by these authors, however, is not correct4, since they used three hydrocarbons (octacosane, hexatriacontane and octapentacontane) which contain only terminal methyl groups and these have a much lower absorptivity (a little more than one half) than methyl branches (39). A similar, but more reliable method was proposed by Rohmer (64) she uses the same... 

The degree of crystallinity and spheralite density of PLA also increased with an increase in the number of branches. This view is not supported by other works in the field. The presence of short chain branches in polyethylene delayed the onset of nucleation and the growth of crystalline stractures. Star-chain branched PA-11 had low crystallization rate because star-branched core and its adjacent chains were unable to crystallize. Methyl groups may still be included in the PE orthorhombic crystal lattice, but with increased methyl group content, polymer gradually looses its ability to crystalUze when the methyl content reaches 20 wt%. If short-chain brarrches increase in size to 1-butane, 1-hexane, 1-octane, the crystallization is even more severely hampered. ... 

Wagener, K.B., Valenti, D., Hahn, S.F., ADMET modeling of branching in polyethylene. The effect of a perfectly-spaced methyl group. Macromolecules 1997, 30 6688-6690. 

Relating peak intensities and characteristics of iso-alkanes for low-density polyethylene to those of reference model copolymers, they determined methyl, ethyl and butyl branch contents in low-density polyethylenes. Liebman and co-workers [68] reported a comparable study on short chain branches in polyethylenes by fused-silica capillary PHGC and C-NMR spectroscopy. 

By NMR spectroscopy, and by comparison with copolymers containing 1-propene and 1-butene comonomer units (E/P and E/B copolymers), typical HDPE samples were shown to contain on the order of 0.5 2.5 methyl branches per thousand backbone carbon atoms longer branches were not detected and are assumed to be absent [47]. Combined NMR and IR evidence indicates that the number of methyl branches in HDPE samples does not correlate with attainable crystallinity values, which implies that the methyl branches can be largely accommodated in the crystalline domains of polyethylene [47, 48]. 

Py-GC Fourier-transform (FT) nuclear magnetic resonance (NMR) spectroscopy is another complementary technique that provides more detailed information on the structure of polymers than does Py-GC alone. As an example of the application of this technique Leibman and co-workers [2] applied it to a study of short-chain branching in polyethylene (PE) and polyvinylchloride (PVC). The nature and relative quantities of the short branches along the polymer chains were determined, providing detailed microstructural information. Down to 0.1 methyl, ethyl, or w-butyl branches per 1000 methylene groups can be determined by this procedure. Other structural defects can also be determined, thus providing significant information on polymer microstructure that is not otherwise readily obtained. 

Figures 59 and 60 show the ATR-FTIR spectra recorded from areas C and D, respectively, from the cross-section of catheter sample 1. Both spectra show features that indicate that each is essentially a polyethylene. The shoulder seen at 2,960 cm-1 (due to terminal methyl groups in the branching) in Figure 59 indicates that the polymer is probably a low-density polyethylene (LDPE). Other weak absorption bands are also evident in this spectrum between 1,800-1,600 cm-1 and 1,300-1,000 cm-1, which are not attributable to polyethylene. These may well indicate it is a copolymer or contains an additive. The sharp well-resolved doublet at 730/720cm 1 evident in the infrared spectum of Figure 60 and lack of shoulder at 2,960 cm"1 indicate that this layer is probably...

More importantly, understanding the chemistry of PP requires you to know the critical difference between PP and the polyethylenes—the asymmetry of the PP molecules. backbone. In polyethylene, every carbon looks like every other carbon in the chain. In PP, the polymer linkage is between succeeding double-bonded carbons, like polyethylene. But, the methyl group survives as a branch on every second carbon in the PP backbone chain. See Figure 23—7.) Furthermore, the orientation of that branch is crucial to the properties of the polymer. See Figure 23-8.)... 

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