Polyethene chains

Although both linear polyethene and isotactic polypropene are crystalline polymers, ethene-propene copolymers prepared with the aid of Ziegler catalysts are excellent elastomers. Apparently, a more or less random introduction of methyl groups along a polyethene chain reduces the crystallinity sufficiently drastically to lead to an amorphous polymer. The ethene-propene copolymer is an inexpensive elastomer, but having no double bonds, is not capable of vulcanization. Polymerization of ethene and propene in the presence of a small amount of dicyclopentadiene or 1,4-hexadiene gives an unsaturated heteropolymer, which can be vulcanized with sulfur in the usual way. 

The LCB in metallocene-catalysed ethene polymerisation is considered to occur via a copolymerisation reaction where a vinyl-terminated polyethene chain is reinserted into a growing chain. Thus, the choice of the catalyst used will be extremely crucial. When the prerequisites of LCB are fulfilled, the process conditions will then be even more important [44, 60]. 

FIGURE 2.4 A polymer molecule, in solution or in its relaxed state as a solid, is coiled and contorted in three dimensions. This diagram is a theoretical model of a polyethene chain with 500 monomer units. Source Treloar (1975). Reproduced with the permission of Clarendon Press. 

We have studied the alkane and alkene yields from the radiolysis of copolymers of ethylene with small amounts of propylene, butene and hexene. These are examples of linear low density polyethenes (LLDPE) and models for LDPE. Alkanes from Ct to C6 are readily observed after irradiation of all the polymers in vacuum. The distribution of alkanes shows a maximum corresponding to elimination of the short-chain branch. This is illustrated in Figure 8 for the irradiation of poly (ethylene-co-1-butene) containing 0.5 branches per 1,000 carbon atoms at 20 C. 

The polyethene produced in this way has from 100 to 1000 ethene units in the hydrocarbon chain. The polymer possesses a number of desirable properties as a plastic and is used widely for electrical insulation, packaging films, piping, and a variety of molded articles. Propene and 2-methylpropene do not polymerize satisfactorily by radical mechanisms. 

Figure 29-3 Representation of attractive interactions between the hydrogens in a crystallite of polyethene. This drawing is incomplete in that it does not show the interactions of the depicted chains with the other chains in front and behind.

Why should polypropene melt so much higher than polyethene (175° vs. 110°) The answer lies in the differences between the way the polymers crystallize. Polyethene crystallites have extended zig-zag chains that have very low barriers to rotation about the C-C bonds. Because of interferences between the methyl groups, polypropene does not crystallize in extended... 

Exercise 29-5 High-pressure polyethene (Section 10-8C) differs from polyethene made with the aid of Ziegler catalysts (Section 10-8D) in having a lower density and lower Tm. It has been suggested that this is due to branches in the chains of the high-pressure material. Explain how such branches may arise in the polymerization process and how they would affect the density and Tm. 

Exercise 29-6 Radical-induced chlorination of polyethene in the presence of sulfur dioxide produces a polymer with many chlorine and a few sulfonyl chlo-- ide (—S02CI) groups, substituted more or less randomly along the chains. Write suitable mechanisms for these substitution reactions. What kind of physical properties would you expect the chlorosulfonated polymer to have if substitution is carried to the point of having one substituent group to every 25 to 100 CH2 groups How may this polymer be cross-linked (A useful product of this general type is marketed under the name of Hypalon.)... 

Exercise 29-7 When polyethene (and other polymers) are irradiated with x rays, cross-links are formed between the chains. What changes in physical properties would you expect to accompany such cross-linking Would the polyethene become more flexible Explain. 

A nickel-based catalyst system, which produces, in the absence of comonomers, highly short-chain branched polyethene was developed by Brookhart et al. [23]. Independently, the groups of Brookhart [24, 25, 26] and Gibson [27, 28, 29, 30] developed efficient iron- and cobalt-based catalyst systems. Nickel or palladium is typically sandwiched between two a-di-imine ligands, while iron and cobalt are tridentate complexed with imino and pyridyl ligands. 

The constrained geometry catalyst (CGC), developed at Dow, was the first single-site catalyst [77, 78] discovered to be capable of producing long-chain branched polyethene. Since the announcement of the ability of CGC catalyst to produce small amounts of LCB, several conventional bis (cyclopentadi-enyl)-based metallocene complexes have been reported to produce low levels of LCB [59, 79, 80, 81, 82, 83]. 

Chain transfer may also occur to a carbon atom of the same polymer molecule five, six, or seven positions distant from the original reactive center [45]. This is called backbiting and is regarded as the mechanism of formation of short branches in polyethene polymerization [46,47],... 

These demands are fulfilled by membranes with a perfluorinated polyethene main chain with side-chains with sulfonic acid and/or carboxylic acid groups as produced by DuPont and Asahi Glass. 

Polymer analytical, spectroscopic and rheological data, compared with those of linear HDPE produced with a surface Cr(II)/silica catalyst, strongly suggest that the bifunctionally produced polyethene (BiPE) has a long chain-branched structure. 

The effects of introducing halogens in the 2 and 6 position of phenyl imine catalysts was also studied in diimine pyridine iron dichloride/MAO systems [13]. These catalysts afford linear products with a low olefin content, generally less than one (olefin) functionality per chain. The latter is due to a fast transfer of iron bound alkyl groups to the aluminum compounds that are present in excess. After hydrolysis, alkanes are obtained. When a high ratio of aluminum alkyl to iron catalyst is used, polyethene waxes are obtained due to the statistically favored alkyl group exchange between the metal species. 

Polyethylene (PE), also called polyethene, is the most common plastic and mainly used for plastic bags and bottles. It is a thermoplastic, with long hydrocarbon chains, made from the monomer ethylene (ethene). Many forms of it exist such as low density polyethylene (LDPE) and high density polyethylene (HOPE). Most of them have excellent chemical resistance to acids and bases, etc. and generally have the chemical formula (C2H4)n [14]. Figure 4 is a model of polyethylene [15]. 

A chain reaction has been set in motion and the —CH2—CH2— unit will repeat itself a thousand times or more in forming a huge molecule of the polymer, polyethylene or polyethene ( polythene ). 

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