Polymer chemistry

Definition: Polymer chemistry is the branch of organic chemistry concerned with polymerization and the materials formed by polymerization. In a polymerization reaction, a large number of small molecules become chemically bonded together to form a much larger molecule. Polymer chemists work to produce new polymers with specific properties, to blend and compound existing polymer formulations for improved applicability, and to design the processes by which bulk quantities of specific polymers can be produced on an industrial scale. Polymer chemistry also includes the study and development of adhesives, rubbers, detergents, paints, and materials used for repair and insulation. A significant aspect of polymer chemistry relates to the now-ubiquitous presence of highly stable synthetic polymers in the environment. Polymer chemistry is therefore perhaps the most diverse field of industrial chemistry, due to the large number of compounds involved and the extent to which polymers are distributed throughout modern life.

Basic Principles

The essential science of polymer chemistry is organic chemistry, specifically the chemistry of polymerization reactions. In polymerization, a large number of small molecules become chemically bonded to each other to form a single large molecule, which may have a structure that is either linear or three-dimensional. As this applies generally, there are a large number of mineral forms based on silicon. However, the vast majority of polymers are based on carbon, with its innate ability to form stable extended chains. While silicon, being of the same periodic group as carbon, is capable of forming extended chain structures, carbon is unique among the elements in the number and variety of molecular structures that it can form. Thus, polymers are found not just in synthetic materials such as plastic and rubber but also in nature, as biopolymers such as cellulose and protein.

The key feature of polymerization reactions is that small molecules that may or may not be identical bond to each other in specific ways. In polyethylene and cellulose, for example, the polymer molecules are formed when individual molecules of ethylene and glucose, respectively, join together in a linear fashion. Each monomeric molecule has a single location at which the next molecule can be added to the chain. When more than one such site exists in the monomer, however, chains can become interconnected in a three-dimensional network rather than as a linear structure. It is thus entirely possible for a massive quantity of a suitable monomer to eventually become a single molecule through a polymerization reaction, although the likelihood of this happening is exceedingly small. In a typical polymerization reaction, many thousands of individual polymeric chains are forming at the same time independently of each other and are terminated when an unfavorable addition occurs or when an impurity is encountered that interrupts the chain-lengthening process.

Core Concepts

Addition Polymers. Addition polymers are the most common form of polymeric materials, the most readily recognizable one being polyethylene. In the formation of an addition polymer, a quantity of simple small molecules join up in a head-to-tail manner according to the nature of the site of addition. By controlling conditions, it is, for example, possible to produce an entire series of “linear alkane” molecules from ethylene: butane, hexane, octane, decane, and so on through chain lengths consisting of thousands of ethylene units. The ultimate molecular weights of such addition polymers are always multiples of the molecular weight of the starting monomer, or monomers when combinations of different monomeric materials are used in the formulation.

Condensation Polymers. Condensation polymers are similar to addition polymers in that monomeric units add together in a linear fashion. They differ, however, in that two different reaction sites, or functional groups, are normally required in the monomers used. The term condensation here refers to the structure of the linkages between monomer units in the polymeric molecule. In each one, it is as though the components of a small molecule such as water or hydrogen chloride have condensed together out of the parent functional groups. The ultimate molecular weights of condensation polymers are the combined weights of the monomers minus the corresponding amounts of condensation products, according to the formulation being used.

Cross-Linked Polymers. When more than one reactive site or functional group that can undergo an addition or condensation reaction is available in the same molecule, the resulting polymer molecules do not form linearly. Instead, each site can take part in a different polymerization chain reaction, and in this way, multiple polymer chains can become linked together in a very large three-dimensional molecular structure. The bonds between parallel polymerization chains are called cross-links. A complex system of polymeric materials that is of great importance in modern technological applications is the so-called epoxy resins, based on monomers in which the functional site is the epoxide group. Polymerization of these compounds occurs when the epoxide ring structure breaks open and adds to another in a chain reaction.

Copolymers. Copolymers are variations of the basic types of polymers. Their molecular structures depend on the formulation being used rather than the type of reaction that takes place. A copolymer would be formed when a combination of ethylene and propylene is used instead of just one of the two. The polymeric combination is normally indicated by a fractional subscript for each component. The copolymers provide the most common way of varying the physical properties of the resulting products, and the possible variations are essentially infinite.

Catalysis. Many polymers are produced using a catalyst to mediate the reaction process. A catalyst is a compound or material that speeds up a chemical reaction by reducing the energy barriers that reacting components must overcome, while the catalyst itself is not changed in the reaction. Catalysts normally function by forming a complex with the monomers, bringing them directly into the proper orientation for the reaction to occur between them. In the production of polyethylene, for example, the catalyst material joins with units of ethylene without forming a chemical bond to those units. They can then easily form a bond between themselves, during which process the active end of the growing chain is released from the catalyst, which can then add another ethylene unit to undergo further reaction. Eventually, the long polymeric chain is released completely from the catalyst, which can then be recovered unchanged from its initial form.

Reaction Conditions. Most polymers are produced on an industrial scale, requiring the utilization of great amounts of energy. Accordingly, many processes are carried out in the gas phase, as this is the state of matter that is easiest to manipulate in large quantities. This also facilitates the separation of product materials from the gaseous process stream. Reactions can also be carried out in liquid-phase conditions. In other cases, components of the polymer in liquid form are combined in place and allowed to react, producing the polymeric material in its final shape. This is the method typically used in molding operations such as those used for the manufacture of modern aircraft components from advanced composite materials.

Biopolymers. Biopolymers are polymeric materials that are produced by living organisms. In plants, the most abundant biopolymers are celluloses, starches, and lignins. Celluloses and starches are produced by the polymerization of glucose molecules that are the product of photosynthesis. Lignins are hard structural materials produced by other biochemical pathways in plant metabolism. In animals, a wide variety of biopolymers are produced, including numerous proteins and structural materials such as collagen and DNA. These are the products of enzyme-mediated condensation reactions with combinations of simple amino acids, bases, and sugars.

Fiber and Matrix Applications. A very significant field of polymer chemistry focuses on the development of new polymers and polymerizing formulations to be formed into fibers and matrix materials for advanced composite materials. These may be produced from biopolymers, as is the case with carbon fiber produced from cotton, or from synthetic polymers, as is the case with the polyamide material known as Kevlar. These and other fibers of mineral origin are encased in a matrix of cross-linked polymer resin, creating structures of low weight and extremely high strength. A great deal of chemical awareness is required for their development, since the fiber and resin materials must be chemically inert with respect to each other and to other materials with which they come into contact.

Applications Past and Present

Explosives and Propellants. While explosives and propellants are a minor aspect of polymer chemistry, they have nevertheless played a significant role in the history of the field. The first successful synthetic polymer, nitrocellulose, falls into this category. An effective method of its synthesis was discovered accidentally by the Swiss German chemist Christian Friedrich Schönbein in 1846. Because it burned rapidly and with almost no smoke, nitrocellulose was quickly identified as a desirable alternative to the highly smoky black powder used in the production of gunpowder. Nitrocellulose thus became known as “gun cotton,” and a furious competition ensued among European nations seeking synthetic methods for producing it that would circumvent Schönbein’s patents. Since that time, a number of other explosives and propellants, such as Cordite, have been developed, all of them based on nitrocellulose.

Fibers and Fabrics. Other researchers quickly developed other uses for nitrocellulose, as well as additional methods of modifying cellulose to produce other plastics. The first and most commercially successful of these variations on nitrocellulose became the material known as celluloid, which as the first fully formable plastic became the foundation of the film industry and a plethora of objects that could be produced en masse with the new material. A further refinement of processes based on cellulose is the viscose process, by which cellulose is modified into cellulose xanthate and then recovered. The process, developed in order to find a method of creating artificial silk, produced the fine-fiber materials known as rayon and its many variants.

A great many other synthetic polymers have been developed for use in fiber and fabric production. Each particular polymer has its own unique characteristics and physical properties that make it useful. One of the most common fabric polymers is nylon, of which there are innumerable variants. Nylon is a polyamide, meaning that the repeating units of its molecular structure are connected by amide bond systems. Nylon is another material originally developed as a substitute for the natural biopolymer silk; it has since found use in many other fabric applications in clothing, sheet goods, tents, boat sails, and others.

The variety of diamine and diacid molecular structures that are known lends itself to the development of a tremendous variety of polyamide materials. One of the most resilient of these is the material known as Kevlar. The material, like the more ordinary nylons, can be formed into fibers that have an exceptionally high tensile strength due to the intertwining of the polymer molecules in the material, rather like so many coil springs that have become muddled together. The fibers thus have the ability to dissipate a great deal of energy along their length when impacted perpendicularly by a blunt object such as a bullet.

Fibers produced from polymers are essential in the assembly of advanced composite materials. Carbon fiber, in particular, is extremely valuable in this application. Carbon fiber is typically produced by one of two methods. One method involves the carbonization of pitch, a viscous residue obtained from coal tar. The other, which involves the carbonization of cellulose fibers usually obtained from cotton, is the more common method of the two. Cellulose is a carbohydrate material in which each carbon atom in the material is bonded to the components of a water molecule. In the carbonization process, these are driven off as water molecules, leaving the bare carbon skeleton intact. The result is carbon fibers that are extremely versatile and become immensely strong when encased and compressed in a resin matrix. Polymeric fibers used in such applications are limited in number due to the basic restriction that the fiber material must be chemically inert with respect to the matrix material. With the exceptions of carbon fibers and Kevlar fibers, the vast majority of advanced composite materials incorporate fibers of mineral origin. This does not preclude the importance of carbon and Kevlar fiber, however. Carbon-fiber advanced composites are the material of choice for the construction of high-performance devices such as modern jet aircraft and are constantly being researched for adaptation to other roles. Kevlar fiber holds something of a companion position and is used often in the production of advanced composite structures designed to withstand impact, such as blast doors and safety seats in aircraft and other military equipment.

Matrix Materials. The other component of an advanced composite material is the matrix within which the fiber structure is bound. These are typically epoxy-based resins that polymerize in place with heating, called thermosetting resins. As the polymerization occurs, the liquid resin becomes hard and extremely stable. Further heating does not soften the material, but above a certain temperature the material begins to break down and decompose chemically. In contrast, a thermoplastic resin is solid below its glass-transition temperature but becomes progressively softer and eventually liquid as the temperature increases. Both types of resins are used in advanced composite structures, according to the conditions that the final product will be required to withstand. In the production phase, the fiber and matrix combination is typically consolidated under reduced pressure, using the even distribution of atmospheric pressure as the external applied force.

Thermoplastics. The most useful feature of polymers, without question, has been their ability to be molded into any desired shape. This is a unique ability of thermoplastic materials, which become liquid when heated and then solidify on cooling. Typically, a solid polymer feedstock is heated to its liquid form and then formed into a desired shape under pressure, the shape being retained when the material cools below its glass-transition temperature. The addition of other components to the formulation can change a hard, brittle plastic into a resilient, even soft, malleable material. A variety of techniques and methods has been developed for manufacturing objects from fresh and recovered plastics. These include primarily injection molding for three-dimensional objects and blow molding for sheet-like objects such as plastic bags and bottles. In the former, liquefied polymeric material completely fills the interior cavity of a mold, while in the latter, a bubble of the material is literally blown outward to coat the inside of the mold. Injection-molded objects may then be further modified by standard machining practices, while blow-molded pieces are produced as the final product without further modification.

Foams. Cross-linked polymers that form three-dimensional molecular networks typically expand to fill space as the gaseous by-products of polymerization exert pressure internally. This feature also allows those materials to be used to produce objects of a particular shape or materials that fulfill a specific purpose. Foam rubber typically is not actual rubber but a resilient type of three-dimensional polymer foam. Similarly, Styrofoam is composed of small granules of polystyrene that have been expanded as an entrained solvent is driven off with heating. When the correct amount of such materials is allowed to expand within a mold, an object with the corresponding shape is produced. Foams are noted for their insulating properties, and the application of liquid components by spraying onto the interior surfaces of walls is commonly used to produce urethane foam insulation.

Social Context and Future Prospects

It is impossible to think of the modern world without polymers and plastics. Some have described the twentieth century as the “plastic century,” beginning with the commercialization of naturally occurring biopolymers. These fairly humble beginnings rapidly evolved into the modern polymer and plastics industry that touches every aspect of twenty-first-century life, from the most advanced plastics used in modern technology in developed nations to the recycled footwear made in Africa from discarded plastic soda bottles to the “plastic dump” where errant plastic items collect in the middle of the northern Pacific Ocean.

As an industry, polymer chemistry has an indefinite future. Relatively little work is focused on the development of new polymers, except for very specific applications. Rather, the majority of the work of polymer chemists is in developing new and better formulations for existing polymers and determining the best ways to recover and utilize plastics that have been discarded after their original use. The focus on polymer chemistry has centered on the development of green technologies that will have a considerably smaller environmental impact than their predecessors. Polymer chemists will therefore be developing polymerization processes that use less toxic and less polluting materials, many of which will demand the complete restructuring or replacement of existing processing facilities. They will also be intent on the development of safely biodegradable polymers, many of which will incorporate cellulose and other biopolymers rather than stable synthetic polymers derived from petroleum. Another significant aspect of polymer chemistry that will become increasingly important is its use in advanced composite materials, used in modern aircraft almost to the exclusion of all other materials, as well as in an increasing number of other applications demanding high strength and low weight.

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