Raising Achievement by Teaching Thinking Skills (2001)
This TTA legacy document reports on development work undertaken on Thinking Skills and asks the question 'Can teaching Thinking Skills have a critical impact on children’s learning, leading to gains in achievement and self-esteem?'
4.5.1 The copolymer equation It can be shown that the rate of change of monomer concentration in any copolymerization is given by the equation where [M1] and [M2] are the concentrations of monomers 1 and 2 at any instant and r1
and r2., are reactivity ratios. The reactivity ratios represent the rate at which one type of gr
4.5 Copolymerization The alloying of metals to improve their properties is widespread and although many polymers used today are relatively pure (e.g. polystyrene, nylon), an increasing number are mixtures of two or more polymers. As with metals, one reason for doing this is to increase the range of properties. The major practical problem, however, is that homopolymers blend together with difficulty and even where blends are possible, as in some thermoplastics, phase separation can occur readily. This proble
4.4 Step growth polymerization 4.3.5 Co-ordination polymerization While most free radical and ionic polymerizations are carried out homogeneously, there is another important class of reaction which is often performed with solid catalysts. These reactions, discovered in the mid-fifties, have revolutionized polymer manufacture by permitting much less severe polymerization conditions than with other systems and by allowing a greater degree of control of polymer structure. Ziegler-Natta catalysts, as they are called, will convert vinyl and diene monomers 4.3.4 Ionic polymerization Free radicals are indiscriminate in the compounds they attack, and their non-selective nature in polymerization reactions leads to problems such as chain branching and transfer which affect the structure of the polymer produced. Anionic polymerization overcomes many of these problems. A typical commercial (but also see Box 8) anionic reaction is the polymerization of styrene using butyllithium, C4H9 4.3.3 Termination and transfer There are basically three ways in which chains terminate. The first is known as coupling and occurs when two free radicals join together. This can be represented by the general equation Such a mechanism significantly increases molecular mass, if it results in two polymer chains joining. This is the main mechanism which terminates the po 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
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:
Author(s): 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. Except for third party materials and otherwise stated (see terms and conditions), this content is made available under a Crea 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 = 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 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 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 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:
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 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 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 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: Introduction to Psychology
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What do your dreams mean? Do men and women differ in the nature and intensity of their sexual desires? Can apes learn sign language? Why can't we tickle ourselves? This course tries to answer these questions and many others, providing a comprehensive overview of the scientific study of thought and behavior. It explores topics such as perception, communication, learning, memory, decision-making, religion, persuasion, love, lust, hunger, art, fiction, and dreams. We will look at how these aspects
