Polymer Chemistry | Research & Encyclopedia Articles

Polymer Chemistry

Polymer chemistry is the field of study concerned with the production, classification, and modification of macromolecules or polymers. Polymers are produced by a polymerization reaction of simpler units called monomers. While polymers have been utilized for centuries, it was not until the twentieth century that their chemistry was understood. Many of the organic materials on Earth are polymers. These include natural materials like nucleic acids, protein, rubber, and cotton, as well as synthetic materials like nylon, polyester and polyvinyl chloride.

Polymers have been used by humans for centuries. However, the field of polymer chemistry has been around only since the 1800s. The first experiments in the development of this branch of chemistry were performed by technologists who attempted to convert natural polymers into more useful derivatives. In 1839, Charles Goodyear produced the first significant development when he created a process for making cross-linked, or vulcanized, rubber. This material was harder and more useful than the native material. Other scientists developed processes to make plaster derivatives, thermoplastics and artificial silks. The first synthetic polymer, called Bakelite (a polymer made from phenol and formaldehyde) was invented by Leo Baekeland in 1907.

In the early part of the 1900s advances in polymer technology were achieved largely without the advantage of understanding the chemistry behind them. The groundwork for modern understanding of polymer science was laid by Nobel laureate Hermann Staudinger in the 1920s. He theorized that the structure of these compounds were long, chainlike molecules and not aggregates or cyclic compounds as previously thought. In 1928, Staudinger=s models were confirmed by Meyer and Mark who used x-ray techniques to show the dimensions of natural rubber. By the 1930s most polymer scientists accepted Staudinger=s model of polymeric molecules. The development and of polymer chemistry since the 1940s has been extensive. Today, polymers are used in nearly every facet of life.

For an area of science to develop, a method of naming compounds is required to facilitate communication between researchers in the field. The materials studied in polymer chemistry however, are more complex than those of traditional areas of chemistry and standard naming systems were inadequate. In the early years of polymer chemistry, names given to compounds were not consistent. Some were named after the origin of their starting materials, others were named after their inventor. Eventually, a source-based naming system was adopted. In this system, polymeric compounds are named by adding the prefix Apoly@ to the name of their starting monomers. For example, polystyrene is the material that results from the polymerization of styrene monomers. This nomenclature system is slowly being replaced by a structure-based system that was adopted by the International Union of Pure and Applied Chemistry (IUPAC).

Polymer science is concerned with the classification, creation, and modification of macromolecules. An important aspect of polymer classification is the structure of polymers. In general, polymers can be grouped into three structural groups, linear, branched, and cross-linked. Linear polymers are long, chainlike molecules covalently bonded in a rigid manner. Side groups are also chemically bonded to atoms in the chain to give the polymer its unique characteristics. Examples of linear polymers include polyethylene, polyvinyl chloride and polypropylene. Depending on the location of the side groups and the nature of the starting monomers, linear polymers can have various structural isomers denoted as isotactic, syndiotactic, or atactic. These isomers exhibit different characteristics such as variable melting temperatures and viscosities.

Branched polymers are similar to linear polymers however, they have irregularly spaced extensions of the polymer chain that are attached to the primary polymer backbone. These polymers tend to occupy more volume per unit and have reduced densities. An example of this type of polymer is low density polyethylene. Linear and branched polymers are classified as thermoplastics which means they will soften when heated and harden when cooled. Crosslinked polymers, which are the third type of structural group of polymers, are thermoset polymers which do not soften with heat but decompose. Cross-linked polymers consist of two or more chains that are covalently bonded. An example of this type of polymer is vulcanized rubber.

Although chemical composition determines some characteristics of polymers, the bulk properties of polymers are dependent on the organization of their chains. While the chains of crosslinked polymers are covalently bonded, linear and branched polymers interact with weaker bonds such as van der Waals forces and hydrogen bonding. These bonds are responsible for characteristics such as melting and boiling points. The strength of these bonds is determined by the organization of the polymer chains. Amorphous polymers have chains that are random and disordered. As more of the chains are caused to line up in an ordered manner, the polymer becomes more crystalline.

Polymers can be further characterized by specific bulk properties such as rheology, solubility, viscosity and molecular weight. The rheology of a polymer is indicative of its physical characteristics. A polymer may be glassy, leathery, rubbery or liquid. Its rheological characteristics help determine for what applications a polymer can be used. Similarly, the solubility of a polymer in a solvent is another important characteristic. This is particularly critical for polymers that are the basis for paints and coatings. The viscosity of a polymer is a measure of its thickness. It is the most widely used measurement for characterizing polymers and determining their molecular weight. The molecular weight of a polymer is indicative of the average number of monomers that make-up a polymer chain. Other methods of molecular weight determination include gel permeation chromatography, end-group analysis, and osmometry.

To ensure the quality of polymeric materials, specific tests have been developed by the American Society for Testing and Materials (ASTM). Stress tests can measure the amount of force that can be applied to a material before it breaks. This test gives an indication of how well the material will perform for certain applications. Other physical tests measure such parameters as tensile strength, impact strength, compressive strength, hardness, elasticity and electrical properties.

Polymer chemistry is such an important area of study because nearly every material in the world is polymeric. This includes such diverse materials as the nucleic acids that make-up the DNA of living things, the paint used to coat walls, and plastics used everyday. Many of these materials contain one type of monomer and are considered homopolymers. Some materials are known as copolymers because they contain two or more different monomeric units. In general, polymers can be classified as either natural or synthetic depending on how they are produced.

The first polymers to be used by humans were naturally occurring polymers. There is a wide variety of naturally occurring polymers and their science and production is still largely not understood. The primary categories for natural polymers include polysaccharides, proteins, nucleic acids, and polyisoprenes.

Polysaccharides are the most abundant organic compounds on the earth. They are linear or branched polymers that are composed of repeating carbohydrate units. They are used by living organisms as food, and also provide structure. Important types of polysaccharides include cellulose, starch, and chitin. Proteins are more complex polymers made up of repeating amino acid units. These polymers are critical to all living systems providing structure, and catalyzing biochemical reactions. Nucleic acids are also polymers, composed of repeating units called nucleotides. A nucleotide is a combination of a ribose or deoxyribose sugar, a purine or pyrimidine base, and a phosphate group. They are the building blocks for genetic information in all living organisms. The final class of important natural polymers is polyisoprenes. These are hard plastics such as rubber produced by various plants.

Numerous synthetic polymers have been developed over the years. Two general methods for their production including condensation polymerization and addition polymerization. These methods generally apply for both homopolymers and copolymers. Condensation polymerization is a method in which two types of monomers are joined together with a characteristic loss of atoms. This can involve a reaction between a di-alcohol and a di-acid. Examples of polymers produced in this way include polyesters, polyamides, polyurethanes, polyethers, and polysulfones.

Addition polymerization involves the joining of monomers without the loss of atoms. In this reaction, there are three key steps including initiation, propagation, and termination. In the initiation phase, some monomers are converted to free radicals. These free radicals react with other monomers during the propagation step, growing the polymeric chains and creating more free radicals. The polymerization continues through the termination step in which free radical production stops. Various materials are produced in this method including polyacrylates, polyvinyl chloride, polytetrafluoroethylene (Teflon), polyethylene, and polypropylene.

While polymers themselves have useful characteristics they are typically modified to improve their characteristics. Some of the first useful polymers were chemically modified natural polymers such as vulcanized rubber and cellulose acetate. Synthetic polymers are also modified by having various materials added. Fillers are added to polymers to make them stronger, more workable and reduce costs. They include materials such as wood, chopped paper, cotton, glass flakes, chalk, or sand. Reinforcement materials such as fiber glass are also added to polymeric materials. Other property modifiers include plasticizers which improve polymeric films, stabilizers that make polymers more temperature and UV resistant, colorants, biocides, and flame retardants which help prevent burning.

Polymers are used in many forms and for numerous applications. Polymeric fibers have become important in the textile industry. Particularly notable have been materials like polyester, nylon, cotton and polyurethane. Elastomers are another form of polymeric materials. These are rubbery materials used for everything from tires to carpets to gloves. Polybutadiene, polyacrylates and polyisoprene are important examples. Films and sheets of polymers have been used for packaging materials, movie films, and construction. Most notable are regenerated cellulose films. Polymeric foams have been used for such things as building insulation, flotation devices and furniture cushions. Polystyrene, polyvinyl chloride, and polyethylene are all examples. Molded plastics are another area where polymers find application. Additionally, the paints and coatings industry is based on polymers.

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