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Chapter 1
Polymers
1.1 Introduction
Polymers can be characterized by many methods that find applications in organic chemistry, such as, for example, nuclear magnetic resonance, infrared spectroscopy, or liquid chromatography. On the other hand, there are several methods that find utilization almost exclusively in the field of polymer chemistry. Examples include light scattering, dilute solution viscometry, size exclusion chromatography, and flow field flow fractionation.
Polymer is a substance composed of macromolecules, that is, molecules built of a big number of small molecules linked together by covalent bonds. The entirely manmade polymers (synthetic polymers) are relatively new materials that did not exist a hundred years ago. The first synthetic polymer, phenol-formaldehyde resin, Bakelite, appeared shortly before World War I. Further synthetic polymers, developed before World War II, were neoprene, nylon, poly(vinyl chloride), polystyrene, polyacrylonitrile, and poly(vinyl butyral); poly(vinyl butyral) was first used in automotive safety glass to prevent flying glass during car accidents and continues to be used for this important application. World War II encouraged further development of polymers as a result of war shortages and demands for new materials with enhanced properties. Other important polymers included polytetrafluoroethylene (Teflon), polysiloxanes (silicones), polyester fibers and plastics such as poly(ethylene terephthalate) (PET), aromatic polyamides (Kevlar), and polyetheretherketone (PEEK). Nowadays, the synthetic polymers are used in a variety of applications covering, for example, electronics, medical uses, communications, food, printing inks, aerospace, packaging, and automobiles.
Synthetic polymers can be classified as thermoplasts, which soften under heat and can be reversibly melted and dissolved, and thermosets, which, by the action of heat or chemical substances, undergo chemical reaction and form insoluble materials that cannot be melted or dissolved. Mixtures of molecules of relatively low molar mass (hundreds to thousands g/mol) that are able to react mutually or with other compounds and form cross-linked materials are often called synthetic resins. The term oligomer refers to a polymer molecule with relatively low molar mass (roughly below 10,000 g/mol) whose properties vary significantly with the removal of one or a few of the units. Besides synthetic polymers, many polymers can be found in the nature. Various polysaccharides (e.g., cellulose, starch, dextran, hyaluronic acid) represent an important group of biopolymers (natural polymers); some of them are an essential part of food or have other important applications. Proteins are other examples of biopolymers, which represent a specific and tremendously rising field of research, where the use of efficient analytical tools is necessary for the characterization and process development of protein therapeutics.
1.2 Molecular Structure of Polymers
The terms configuration and conformation are used to describe the geometric structure of a polymer and are often confused. Configuration refers to the molecular structure that is determined by chemical bonds. The configuration of a polymer cannot be altered unless chemical bonds are broken and reformed. Conformation refers to the order that arises from the rotation of molecules about the single bonds. If two atoms are joined by a single bond, then rotation about that bond is possible since it does not require breaking the bond. However, a rotation about a double bond is impossible. The term conformation refers to spatial structure of a macromolecule in dilute solution. Depending on the thermodynamic quality of solvent and properties of a polymer chain, the polymer may adopt a random coil, compact sphere-like shape or highly extended rod-like conformation. The terms topology or architecture often refer to the polymer chain arrangement with respect to branching.
The part of a macromolecule from which the macromolecule is built is called a monomer unit while the smallest part of a macromolecule that repeats periodically is called a structural repeating unit. Polymers can consist of one or more kinds of monomer unit. The former are called homopolymers, the latter copolymers. Synthetic polymers are usually varied mixtures of molecules of different molar mass (M) and often also of different chemical composition and/or molecular architecture. That is, they are nonuniform (polydisperse) materials. Polydispersity means that a given property, such as molar mass, spans a continuous range. Various possible nonuniformities are outlined in the following:
- Molar mass.
- Chemical composition: A random copolymer contains a random arrangement of the monomers and can be denoted schematically as -A-B-A-B-A-A-B-B-B-B-A-B-B-A-B-. The particular macromolecules can differ in their overall chemical composition as well as in the sequential arrangement of monomers in the polymer chain. A block copolymer contains linear blocks of monomers of the same type -A-A-A-A-A-A-A-A-A-B-B-B-B-B-B- and the possible heterogeneity includes various block length or existence of homopolymer fractions. A graft copolymer contains a linear main chain consisting of one type of monomer with branches made up of other monomers, when the molecules may differ in the number, position, and length of the branches. An alternating copolymer consists of regularly alternating units -A-B-A-B-A-B-A-B-A-B- such as, for example, in the well-known Nylon 66 (– CO– (CH2)4– CO– NH– (CH2)6– NH– )n and the heterogeneity is limited to the molar mass and end groups. The ch...
