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4.3.2 Propagation

Once a small number of chains have been started, propagation involves successive addition of monomer units to achieve chain growth. At each step the free radical is regenerated as it reacts with the double bond. So in the case of styrene the propagation step is

The free radical can also add on in a different way to produce

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4.3.1 Initiation

Initiation is the mechanism which starts the polymerization process. Vinyl monomers are quite easily polymerized by a variety of activating methods. Styrene, for example, can be converted to solid polymer simply by heating, and ultraviolet light can have exactly the same effect. Usually, however, an activating agent is used. This is an unstable chemical which produces active species that attack the monomer. A good example is benzoyl peroxide which splits up when heated:

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4.3 Chain growth polymerization

Chain growth polymerization is basically a three-stage process, involving initiation of active molecules, their propagation and termination of the active chain ends.

4.2 Chain and step growth

There are two basic ways of making chains. The first is to activate a small number of monomer units M which then successively consume other monomers. This mechanism is known as chain growth and is shown schematically in Figure 36 (a), where a monomer unit is activated by initiator I and forms a chain very quickly. After 75 per cent of the monomer molecules have reacted in this case the degree of polymerization n =
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4.1 Understanding the polymerization process

Converting monomer to long chain polymer is the final step in the polymer manufacturing sequence. Polymerization is usually highly favourable in thermodynamic terms, mainly on energetic grounds because ordering molecules into linked chains is a process where the entropy is decreased. Heat is always given out during polymerization owing to the very favourable energetics of reaction, a point you may have noticed if you have ever made GRP parts for your car, for example!

Advances in cataly
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3.4 The petrochemical industry

The four-fold increase in the price of oil in 1973–4, together with associated political events, proved a powerful stimulus in the development and exploitation of North Sea crude oil. Increasing the price of oil does not mean that the price of the final plastic moulding increases by the same amount. For example if oil prices were doubled again then naphtha prices would typically increase by about 80 per cent, although there is no simple and fixed gearing mechanism between the two prices. Th
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3.3.3 Higher aromatics

Benzene rings can be fused in various ways to create component parts for some of the complex aromatic repeat units shown in Table 5. One of the most important is bisphenol A, made by fusing two phenol rings with acetone:

This intermediate is important for a number of speciality polymers, for example, polycarbonate and epoxy resi
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3.3.2 Benzene, toluene and xylene

In addition to benzene itself, the catalytic reformer also produces ethylbenzene, toluene and the isomeric xylenes directly. The demand for ethylbenzene is always great as a source of styrene monomer, but toluene does not find great use apart from a relatively small application in polyurethane. This is why most toluene is de-alkylated to increase overall benzene production. A similar problem exists with the xylenes:

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3.3.1 Ethylene, propylene and butadiene

Nowadays ethylene is the most important building block for the chemical industry, particularly as a monomer in its own right, as a co-monomer with other vinyls, and as a source of vinyl monomers. It is the prime source for ethylene oxide, which is another major source of polymers, glycols and ethers. They can also be used to build up more complex C4 molecules and aromatics.

Some of the ways in which the ethylene molecule is modified to create other chemicals and polymers are
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3.3 Petrochemical intermediates and monomers

About 80 per cent of all petrochemicals end up in polymers, the most important building blocks being ethylene, propylene, butadiene and benzene. The first three can be polymerized directly but an important slice of their production is used to create more complex monomers. Ethylene is the progenitor of most vinyl monomers (Figure 35), so the pressure on ethylene supply is particularly strong compared for example to propyle
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3.2.2 Ethane cracking

Although ethane can be cracked thermally, the reaction is slow and does not necessarily yield ethylene at high severity. Careful control of reaction conditions, however, allows the reaction to occur

The yield of ethylene is typically nearly 50 wt% with the rest composed of unreacted ethane (40 per cent) and some methane and hydrogen (10 per cent). T
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3.2.1 Thermal cracking

The bulk of the major monomer and intermediate, ethylene (C2H4), is still produced in the UK by steam cracking without the use of catalysts. Paraffinic feedstocks are best for optimising ethylene yields, and the severity of cracking is specified by the rate of disappearance of a marker compound, usually n-pentane. The severity of the reaction can then be defined as follows:

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2.1.1 Memorising
How do we learn? Understanding ‘how’ is the key to learning more effectively. This unit looks at the three main categories of theories: the acquisitive, constructivist and experiential models of learning. There is no right way to learn but developing an active approach will ensure that you are open to new ideas.
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2 1 Different conceptions of learning
How do we learn? Understanding ‘how’ is the key to learning more effectively. This unit looks at the three main categories of theories: the acquisitive, constructivist and experiential models of learning. There is no right way to learn but developing an active approach will ensure that you are open to new ideas.
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1.3 The realities of war – a protest
In this unit we will consider how language can be used in different ways for different purposes. To do this we will use the theme of memorial and commemoration. In the first section we briefly discuss the life of the poet Siegfried Sassoon before examining both his poetry and prose. Through this we will see how he conveys meaning in different ways for different audiences using different forms. Following this we discuss more generally how different meanings can be conveyed using prose and poetic
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9.7 Debate 4: representation of Scotland
Sunset Song was written in the early 1930s and is still one of the best-known and most-debated Scottish novels. In this unit, we discuss whether Sunset Song succeeds as critique of capitalism and whether it has value as a work of literature separate from its propagandistic ambitions.
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8.2 Reviews of the first edition
Sunset Song was written in the early 1930s and is still one of the best-known and most-debated Scottish novels. In this unit, we discuss whether Sunset Song succeeds as critique of capitalism and whether it has value as a work of literature separate from its propagandistic ambitions.
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Financial Markets
Financial institutions are a pillar of civilized society, supporting people in their productive ventures and managing the economic risks they take on. The workings of these institutions are important to comprehend if we are to predict their actions today and their evolution in the coming information age. The course strives to offer understanding of the theory of finance and its relation to the history, strengths and imperfections of such institutions as banking, insurance, securities, futures, a
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Introduction to Political Philosophy
This course is intended as an introduction to political philosophy as seen through an examination of some of the major texts and thinkers of the Western political tradition. Three broad themes that are central to understanding political life are focused upon: the polis experience (Plato, Aristotle), the sovereign state (Machiavelli, Hobbes), constitutional government (Locke), and democracy (Rousseau, Tocqueville). The way in which different political philosophies have given expression to various
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Computer Games and Simulations for Investigation and Education, Spring 2005
Project-based subject in which students from multiple disciplines are encouraged to develop and investigate systems and ideas from their fields of study as they explore the process of building and testing models and simulations. Explores various modeling software packages, criteria for developing the most appropriate simulation for a given situation, and methods for evaluating the success and utility of models. Students with an education focus consider what and how people learn from simulations,
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