Poly /ethene

Cracking and isomerization reactions occur readily in acidic chloroaluminate(III) ionic liquids. A remarkable example of this is the reaction of poly(ethene), which is converted into a mixture of gaseous alkanes of formula (C Ff2n+2, where n = 3-5) and cyclic alkanes with a hydrogen to carbon ratio of less than two (Figure 5.1-4, Scheme 5.1-68) [99]. 

Scheme 5.1-68 The cracking of poly(ethene) in an ionic liquid.

The distribution of the products obtained from this reaction depends upon the reaction temperature (Figure 5.1-4) and differs from those of other poly(ethene) recycling reactions in that aromatics and alkenes are not formed in significant concentrations. Another significant difference is that this ionic liquid reaction occurs at temperatures as low as 90 °C, whereas conventional catalytic reactions require much higher temperatures, typically 300-1000 °C [100]. A patent filed for the Secretary of State for Defence (UK) has reported a similar cracking reaction for lower molecular weight hydrocarbons in chloroaluminate(III) ionic liquids [101]. An... 

This polymer has one of the simplest molecular structures ([CH2CH2— ] ) and is at present the largest toimage plastic material, having first been produced commercially in 1939 for use in electrical insulation. There is a difficulty over the nomenclature of this polymer. The lUPAC recommended name for the monomer is ethene, rather than the older ethylene. Hence the lUPAC name for the polymer is poly (ethene). However, this name is almost never used by chemists working with the material throughout this book, therefore, this polymer will be referred to by its more widespread name, poly(ethylene). 

The notion of a pnre chemical snbstance can be related to empirically identifiable properties (e.g. sharp melting and boiling temperatures) but is nowadays understood in theoretical terms that are abstract (Johnson, 2002 Taber, 2002a). So hydrogen, methane, diamond, sodium, sodium chloride and polythene - poly(ethene) - are all considered examples of single chemical substances, although they are very different... 

The separation of poly-ethene into fractions of different relative molecular mass, Using exclusion chromatography. — ... 

The larger hydrocarbon molecule has been broken down into a mixture of a shorter chain alkane plus an alkene. Both of these products are useful the alkane for the manufacture of petrol and the alkene for the manufacture of ethanol, by hydration, and plastics such as (poly)ethene. 

Polymers can be formed from compounds containing a c=c double bond. Alkenes, such as ethene, can undergo addition polymerisation to form a polymer. A polymer is a compound consisting of very long chain molecules built up from smaller molecular units, called monomers. The polymerisation of ethene, to form poly(ethene), is a free radical addition reaction. 

The polymer molecules, therefore, have the same empirical formula as those of the monomer. The conditions for the reaction depend upon whether low-density or high-density poly(ethene) is required. 

For low-density poly(ethene), a high pressure (over 1500 atmospheres), a temperature of 200 °C and a trace of oxygen as a catalyst are needed. 

For high-density poly(ethene), a pressure of 2 to 6 atmospheres, a temperature of 60 °C and a catalyst of titanium (IV) chloride and triethyl aluminium are needed. 

Poly(ethene) Plastic bags, food boxes, squeezy bottles, buckets, washing-up bowls... 

The second common method of synthesising polymers (Fig. 5.3) is chain (addition) polymerisation. The most common type of addition polymer is based on ethene CH2 = CHj in which the monomer contains at least one double (tt) bond which on being activated, by free radical attack say, opens up to produce two single sigma bonds and the homopolymer poly(ethene). (Note in Fig. 5.3 the resultant polymer backbone is joined together by carbon-carbon bonds, unlike the condensation polymer systems (Fig. 5.1).)... 

Very litde is reported regarding the effect of ultrasound on Ziegler-Natta polymerisation. The first report was by Mertes [97] who obtained a more uniform poly(ethene) in the presence of ultrasound. It was suggested this was as the result of a better dispersion of the catalyst and the prevention of catalyst deactivation (sweeping clean) in the presence of ultrasound. 

Rule 7 specifies CH=CH as the correct CRU in preference to =CH—CH=, since the former is bivalent, while the latter is tetravalent. The polymer -<-CH=CH—L is poly(ethene-1,2-diyl). 

Acetylene is polymerized to polyacetylene [IUPAC poly(ethene-l,2-diyl)] by Ziegler-Natta initiators such as titanium tetraisobutoxide with triethylaluminum [Ito et al., 1974 Shelburne and Baker, 1987 Shirakawa, 2001 Theophilou and Naarman, 1989]. Polymerization at... 

Figure 14.14 This model shows part of the poly(ethene) polymer chain...

Poly(ethene) is produced in three main forms ... 

The world production of all types of poly(ethene) is in excess of 52 million tonnes per year. 

Figure 14.15 These crates are made from poly(ethene).

Other alkene molecules can also produce substances like poly(ethene) for example, propene produces poly(propene), which is used to make ropes and packaging. 

If we use chloroethene (Figure 14.16a), the polymer we make is slightly stronger and harder than poly(ethene) and is particularly good for making pipes for plumbing (Figure 14.17). 

PVC is the most versatile plastic and is the second most widely used, after poly(ethene). Worldwide more than 27 million tonnes are produced annually. 

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