Vinyl groups polyethylene

The simplest linear-chain polymer is polyethylene (Fig. 22.3a). By replacing one H atom of the monomer by a side-group or radical R (sausages on Fig. 22.3b, c, d) we obtain the vinyl group of polymers R = Cl gives polyvinyl chloride R = CIT3 gives... 

The data of Table II indicate that the etch rates for CB and its "homologues"—TP, CO (or TO), and EPM—tend to increase monotonically with a decrease in vinylene (-CH=CH-) unsaturation. The elastomeric EPM was chosen instead of crystalline polyethylene as a model for the fully saturated CB to avoid a morphology factor in etch rates, as was observed with crystalline TB. The difference in etch rates for the partially crystalline TO and the elastomeric CO (ratio of about 1.2 1.0) is attributable more to a morphology difference between these polyoctenamers than to the difference in their cis/trans content. Cis/trans content had likewise no perceptible effect on etch rates in the vinyl-containing polybutadienes (see Table I) if there was a small effect, it was certainly masked by the dominant effect of the vinyl groups. 

We can incorporate short chain branches into polymers by copolymerizing two or more comonomers. When we apply this method to addition copolymers, the branch is derived from a monomer that contains a terminal vinyl group that can be incorporated into the growing chain. The most common family of this type is the linear low density polyethylenes, which incorporate 1-butene, 1-hexene, or 1-octene to yield ethyl, butyl, or hexyl branches, respectively. Other common examples include ethylene-vinyl acetate and ethylene-acrylic acid copolymers. Figure 5.10 shows examples of these branches. 

The vast majority of commercial polymers based on the vinyl group (H2C=CHX) or the vinylidene group (H2C=CX2) as the repeat unit are known by their source-based names. Thus, polyethylene is the name of the polymer synthesized from the monomer ethylene poly(vinyl chloride) from the monomer vinyl chloride, and poly(methyl methacrylate) from methyl methacrylate. 

The first-order rate constants for terminal vinyl groups and for trans-vinylene groups in linear polyethylene were observed by Dole and Williams (8) to be 20 to 30 times higher than our corresponding rates in the amorphous trans- and vinyl polybutadienes (Table III). On the other hand, the initial yields were lower than our G0 values because of the... 

Various authors—for example, Dole, Milner, and Williams (15) and Lyons (25)—have suggested that the decay of vinyl groups initially present in some types of high density polyethylene involves an end-linking process, these authors disagreeing only about the mechanism involved. If such were the case, some difference in solubility or elastic behavior above 140 °C. would be expected between low and high density poly-... 

The radical chain mechanism shown in Section 24.1 implies that polyethylene and polystyrene are composed of linear macromolecules—that is, that the carbons of the vinyl groups of the monomers are connected in a straight chain. In fact, two processes can cause individual macromolecules to have branched structures. The first of these occurs when a growing radical chain abstracts a hydrogen from a random position in the interior of another macromolecule. This process, called chain transfer, occurs when the radical does not find another monomer unit to which to add nor another radical... 

IR analyses of end-groups on some typical polymers are shown in Table 10 [17]. The first entry refers to a homopolymer (polyethylene) only. It contained about one terminal vinyl group for each terminal methyl group. The next two entries in the table refer to experiments in which propylene was added to produce a copolymer, and the reaction temperature was lowered to maintain the same approximate MW of the product. In this case, the concentration of methyl groups was greater than in the preceding case, because the branches were also detected as methyl groups... 

Although the production of molecular hydrogen was not monitored in this study, 13c NMR was an excellent technique to follow the production of internal double bonds, intermolecular links and the disappearance of terminal vinyl groups. Additionally, it was observed that saturated end groups are apparently produced during irradiation of polyethylene. 

The disappearance of terminal vinyl groups during irradiation of polyethylene has also been studied by many investigators who utilized infrared methods. The 13c NMR results reported in this study are similar to the infrared results reported by Okada and Mandelkern (6). Lyons (7,8) and Mandelkern (6) have proposed that vinyl disappearance is related to the formation of radiation induced links in polyethylene. Mandelkern has also proposed that vinyl disappearance is related to a decrease in the production of molecular hydrogen during irradiation (6). 

The formation of long chain Y branches appears to be related to the disappearance of terminal vinyl groups in irradiated polyethylene. 

Several modifications to the Charlesby—Pinner relationship that deal with deviation from the initial random distribution have been published [30-32]. In some cases a plot of s + Js against where k can vary from 0.42 to 0.55 leads to a better linear relationship for polyethylenes [28]. Vinyl group endUnking can also be taken into account with the following relationship [33] ... 

Vulcanized (cross-linked) polyethylene, also XLPE Elastomeric silicone with vinyl substituents Polydimethylsiloxane with vinyl groups Wide-angle X-ray scattering Woven rovings... 

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