This concluding chapter explains how polymers provide a convenient opportunity to study industrial aspects of chemical science. Indeed, considerations of economics, availability of feedstocks, use of specialized catalytic reactions, and similar matters are found throughout the chemical industry. However, polymers provide particularly instructive examples. It is an appropriate time to review polymers since there are many new aspects and applications in prospect. This is a very wide subject to cover and readers are encouraged to investigate more specialized bibliography for areas of interest. The chapter then offers some recommendations of reference books on all aspects of polymer science and technology, including series of volumes extensively filled with data.
Conclusions and further reading
This chapter reviews the recent developments in functional polymers. It begins by looking at high performance polymers, which refers to polymers with extreme mechanical strength and toughness. The strategy to achieve this is instructive, since enhancement of one specific property occurs at the expense of other properties and eventually the best practical system involves a trade-off between desired features and those attainable. The chapter then considers electrically conducting polymers. Polymers are best known for their effectiveness as electrical insulators, and electrical wiring throughout the world is now sheathed in plastic. The chapter also examines polymers with functionalized side chains, as well as polymeric photoresists. It assesses the role of polymers in the ecosystem, addressing key issues such as the bioproduction of useful technological polymers, remediation and biodegradability, and waste treatment and recycling.
General principles and historical aspects
This chapter provides an overview of polymers, which are formed by linking large numbers of small molecules together. Polymers are now ubiquitous in daily life. Not only have polymers found a wide use in structural and textile materials, polymer substitutes have also found a wide application in medicine. Moreover, synthetic polymers have had a sizeable impact in the field of fibres, plastics, and rubbers (elastomers). While the majority of polymers are synthetic polymers, there are natural polymers such as proteins, natural rubber, and cellulose, each of which could be fitted into a similar category. It was as a result of early attempts to modify these natural materials that the important understanding of polymer behaviour arose. The chapter then presents the general principles of polymerization; the statistical nature of polymer chains; and the general principles of industrial polymer synthesis.
General principles and historical aspects
This chapter examines chain polymerization. This class of polymer accounts for a large proportion of the synthetic polymer industry and includes the large-tonnage materials such as polyethene, polystyrene, polyvinylchloride (PVC), and acrylics. The reaction mechanisms, which affect the build-up of polymer molar mass (RMM), and other factors are different for alkene polymers and polyurethanes. The alkene systems involve chain reaction mechanisms and the class of materials is called chain polymers. Chain polymers can be prepared in one of three ways. In free-radical polymerization, the alkene double bond opens homolytically. In cationic polymerization, an electron-deficient species removes both electrons from the electron-rich double bond. In anionic polymerization, a species more electron-rich than the double bond increases electron density of the π bonds such that heterolytic fission gives a negatively charged propagating chain end-group. The chapter then considers commercial chain polymer syntheses; the synthesis of monomers; and the properties of chain-growth polymers.
Polymer properties and characterization
This chapter discusses the properties and characterization of polymers. The techniques most commonly used to determine polymer molar mass include end-group analysis, osmometry, light scattering, ultracentrifugation, sedimentation, viscometry, and chromatography. However, most of these involve rather lengthy procedures and in practice molar masses are obtained from high performance gel permeation chromatography (HPGPC) or viscosity measurements. It is important to recognize that the fundamental measurements of molar mass must be performed on dilute solutions so that intermolecular interactions can be ignored. The chapter then looks at polymer stereochemistry; structure-property relationships; and polymer processing. It also considers the thermal methods of polymer analysis, in which some physical property of a substance is measured as a function of temperature or time while the substance is subject to a controlled temperature programme. The most common techniques are differential scanning calorimetry, thermal gravimetry, dynamic mechanical analysis, dilatometry, heat-deflection temperature, and melt index.
David J. Walton and J. Phillip Lorimer
Polymers gives a thorough introduction to polymer chemistry, ranging from a historical perspective, through the development of high-tonnage materials earlier in the twentieth century, to modern high-performance materials that have a range of useful additional properties. Polymers are the archetypal modern materials, used in every aspect of everyday life. Chapters cover polymers, polymer properties and characterisation, chain polymerisation, step-growth polymers, three-dimensional networks, and functional polymers. The text also includes discussion of practical industrial aspects in the technology of these materials.
This chapter evaluates step-growth polymers. In the case of step-growth (condensation) polymers, the mechanism is simply an extension of the normal organic condensation reactions in which a small molecule is expelled as the link is built. This is a different situation to the chain polymerizations described in the previous chapter. It is assumed that most step polymerizations involve bimolecular reactions as key mechanistic processes. The chapter then looks at the kinetics of step polymerization. It also considers the commercial preparations of step-growth polymers. The industrial preparation of polyethylene terephthalate (PET) exploits the reversibility of esterification. A key feature in polymer technology is the manipulation of polymer properties by post-polymerization processing. PET behaviour is affected by its crystallinity.
This chapter focuses on three-dimensional networks, which are the toughest and most rigid materials, since the polymer chains are linked together in all directions to give effectively a single giant molecule. Three-dimensional networks do not melt, although segments may go through phase changes with temperature. They are insoluble, although lightly cross-linked ones can be solvent-swollen. They are therefore prepared in two stages, the first giving a processable intermediate that becomes the intractable final product in the second one. These principles are demonstrated for several systems, including Bakelite (phenol-formaldehyde polymers) and fibre glass (linear unsaturated polyesters), and also the vulcanization of rubber. Polymers which set hard after heating, usually because of a thermal cross-linking reaction, are called thermosets. The chapter then looks at electron beam cross-linking and physical cross-linking.