5 What is a Polymer? Polymer is a term used to describe large molecules consisting of repeating structural units, or monomers, connected by covalent chemical bonds. The term is derived from the Greek words: polys meaning many, and meros meaning parts. A key feature that distinguishes polymers from other molecules is the repetition of many identical, similar, or complementary molecular subunits in these chains. These subunits, the monomers, are small molecules of low to moderate molecular weight, and are linked to each other during a chemical reaction called polymerization.
6 The quantity x represents the number of repeat units in the chain, and is called the degree of polymerization. There are end groups on the chain that are different from the repeat units, but these usually represent a negligible portion of the molecule, so they are seldom drawn.
7 Monomer ? A monomer (from Greek mono "one" and meros "part") is a small molecule that may become chemically bonded to other monomers to form a polymer. Examples of monomers are hydrocarbons such as the alkene and arene homologous series. Here hydrocarbon monomers such as phenylethene and ethene form polymers used as plastic like poly(phenylethene) (commonly known as polystyrene) and polyethylene. Other commercially important monomers include acrylic monomers such as acrylic acid, methyl methacrylate, acrylamide, propylene oxide, lactams, lactones and lactides.
8 Amino acids are natural monomers, and polymerize to form proteins. Glucose monomers can also polymerize to form starches, amylopectins and glycogen polymers. In this case the polymerization reaction is known as a dehydration or condensation reaction (due to the formation of water as one of the products) where a H atom and a (-OH) group are lost to form H 2 O and an oxygen molecule bonds between each monomer unit. Lower molecular weight compounds built from monomers are also referred to as dimers, trimers, tetramers, pentamers, octamers, 20-mers, etc. if they have 2, 3, 4, 5, 8, or 20 monomer units, respectively. Any number of these monomer units may be indicated by the appropriate prefix, eg, decamer, being a 10-unit monomer chain or polymer. Polymers with relatively low number of units are called oligomers.
11 Like the old story about the elephant and the blind men, the way people classify polymers depends their experience. For example, an organic chemist is interested in the detailed arrangement of atoms in the chain, while a structural engineer only considers a table of physical attributes such as tensile strength or density. There is no uniform system of classification of polymers. The terminology has evolved along with polymer science, and there are numerous exceptions to categories, as well as widely used historical terms or trade names lacking information content.
12 Polymer Classification based On Repeat Units The repeat units can all be identical, in which case the compound is a homopolymer. If the repeat units are different, the result is a copolymer. For copolymers, the situation gets complicated quickly. At the limit of extreme complexity, there are biopolymers such as proteins and DNA that have exactly defined sequences of many repeat units to serve the purposes of the organism. Synthetic copolymers are seldom so elegant; however, there are still many possibilities for even simple systems. Consider a copolymer made from just two ingredients. There are infinite ways in which the two can be sequenced along the backbone. Here are some limiting cases:
15 Molecular Classification The structures of polymers are conveniently represented by the repeating chain formula, which shows the arrangement of bonds and atoms. The repeat units often contain recognizable functional groups that can be used to describe the polymer. This terminology often emphasizes the functional groups that were involved in the synthesis of the polymer from its monomers, although the usage is seldom exact. Some examples:
17 A polymer is often named according to the monomer that was used to form it. This is why the polymer consisting of only a long chain of CH 2 groups is called polyethylene, not polymethylene. The polyamide containing 6 carbons is known as polycaprolactam.
20 Classification Based on Physical Properties Processability: Some polymers can be readily melted and then molded into any shape. These are known as thermoplastics, and they usually have linear or branched architectures. Most thermoplastics can also be dissolved in suitable solvents. Some other polymers decompose on heating before they can melt. These are known as thermosets, and they are usually crosslinked and therefore insoluble. To form a part out of a thermoset, one usually synthesizes the polymer in the mold itself. Once the polymer has cured, the only way to reshape the part is by machining (e.g., grinding, drilling, etc.) Physical performance: Polymers that stretch and rebound are called rubbers or elastomers. These materials are usually crosslinked, either by covalent bonds, or, in several modern cases, by noncovalent forces such as H-bonds. Other solid polymers are known simply as plastics, adhesives, or fibers, depending on their application. The word resin is a generic term for polymer, although occasionally it indicates a thermoset.
21 A common physical measurement that provides distinctive curve shapes for the different physical classes of polymers is called tensile testing. The measurement is carried out by stretching a sample of polymer at a controlled rate, and measuring the amount of force required. The curve stops when the sample breaks, and the area under the curve is a measure of the energy required to make this sample fail.