Ethyl-branched polyethylene

Fig. 5. Characteristic ratio of ethyl branched polyethylenes as a function of 1-butene content SoUd symbols are RIS values for statistic weights t = 0 ( ) and 0.43 ( ). Open symbols represent various experimental values from sources listed in [161]. Reprinted with permission from Macromolecules, 24 6205 (1991). Copyright 1991 American Chemical Society...

Linear Polyethylene with Randomly Distributed Ethyl Branches (Hydrogenated Polybutadiene)... 

The half-widths of 37-39 and 78-88 Hz, respectively, for the crystalline and amorphous phases are significantly larger than 18 and 38 Hz for those of the bulk-crystallized linear polyethylene (cf. Table 1). This is caused by incorporation of minor ethyl branches. The molecular alignment in the crystalline phase is slightly disordered, and the molecular mobility in the amorphous phase will therefore be promoted. With broadening of the crystalline and amorphous resonances, the resonance of the interphase also widens in comparison to that of bulk-crystallized linear polyethylene samples. This shows that the molecular conformation is more widely distributed from partially ordered trans-rich, conformation to complete random conformation, characteristic as the transition phase from the crystalline to amorphous regions. 

Branching may be produced deliberately by copolymerizing the principal monomer with a suitable comonomer. Ethylene and 1-butene can be copolymerized with a diethylaluminum chloride/iitanium chloride (Section 9.5) and other catalysts to produce a polyethylene with ethyl branches ... 

Tirpak, G. a. Position of ethyl branches in conventional polyethylene made by the free radical process. J. Polymer Sci. 3B, 371 (1965). 

High-density polyethylene has less spurious branching than the low-density material, about 0.3-5 ethyl branches per 1,000 carbon atoms. The much lower extent of branching in HOPE enables closer packing of the polymer chains in the solid state. Closer packing explains both the higher density and the higher crystallinities observed for this material as compared to those of low-density polyethylene. 

LCB-PE (long chain branched polyethylene) is not accessible through simple ethyl-ene/a-olefin copolymerization. Therefore, a bifunctional bimetallic catalyst was developed, in which one active center oligomerizes ethene to long-chain a-olefins, while the other copolymerizes them with ethene. 

A major example of the second branched polymer type is the polyethylene that is made by free radical polymerization at temperatures of 100-300°C and pressures of 1,000-3,000 atm. The extent of branching varies considerably depending on reaction conditions and may reach as high as 30 branches per 500 monomer units. Branches in polyethylene are mainly short branches (ethyl and butyl) and are believed to result from intramolecular chain transfer during polymerization (described later in Chapter 5). This branched polyethylene, also called low-density polyethylene (LDPE), differs from linear polyethylene (high-density polyethylene, HDPE) of a low-pressure process so much so that the two materials are generally not used for the same application. 

Figure 6.9 Mechanism of ethyl and butyl branching in polyethylene. This is a very important process in the free-radical, high-pressure polymerization of ethylene producing branched polyethylene of lower density and crystallinity. (After Ghosh, 1990.)...

Hydrogenation pyrolysis has been applied to the determination of the composition of copolymers of a-olefins, the sequence of monomer units and the manner in which they are added (head-to-head and head-to-tail) [253]. Mikhailov et al. [251] used Py—GC to investigate the structure of low- and high-density polyethylenes and copolymers of ethylene with propylene. The pyrolysis products were hydrogenated. The method made it possible to examine alkanes up to Cjo, which facilitates the investigation of the polymer chain structure. The isoalkanes identified corresponded to the branched polyethylene structure. It has been established that the ethyl and butyl side-chains occur most frequently in polyethylenes. 

LLDPE(2) Linear low density polyethylene with 17 ethyl branchings per 1000 carbon atoms... 

Despite these observations, analysing the SEC Rl-elution volume traces with a linear polyethylene calibration curve appears to be valid, in that the short-chain branches did not substantially alter the molecular dimensions. In particular, hydrogenated polybutadienes with 2-5% ethyl branches, an excellent model system for n-butene-1 copolymer systems, elute at the same retention volume as HDPE with the same molecular mass, and they have been used widely as molecular mass standards for HDPE. Unfortunately, the effect... 

Figure 4.4 High field C-NMR spectra of low-density polyethylene. (A) Ethyl branched copolymer (B) hexyl branched copolymer (C) LDPE.

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