Monomers and Polymers
Monomers and polymers are fundamental concepts in chemistry and materials science, where monomers are simple molecules that link together to form larger, complex structures known as polymers. This process, termed polymerization, involves various chemical reactions that enable the bonding of monomer units, resulting in diverse materials with specific properties. For instance, polyethylene is created from ethylene monomers, while proteins are synthesized from amino acid monomers through intricate biological processes mediated by enzymes.
The characteristics of a polymer depend heavily on the types and arrangements of its monomer units, including the presence of substituent groups. Polymers can be linear or cross-linked; linear polymers are typically thermoplastic, whereas cross-linked polymers are more rigid and do not melt upon heating. Additionally, the synthesis of polymers plays a crucial role in both living organisms, where it contributes to the formation of essential biomolecules like DNA and RNA, and in the production of advanced composite materials used in high-performance applications such as aerospace.
Understanding the relationship between monomers and polymers is essential, as these materials are integral to many aspects of life and technology, showcasing the versatility and significance of polymeric substances in natural and engineered systems.
Monomers and Polymers
FIELDS OF STUDY: Organic Chemistry; Biochemistry
ABSTRACT
The basic relationship of monomers and polymers is defined, and the importance of polymeric materials in both living and nonliving systems is elaborated. Polymers are formed by simple molecules bonding together to create large and often complex molecules that have specific properties and serve specific functions.
The Nature of Monomers and Polymers
All polymerization reactions are synthesis reactions in which a product molecule is formed by the combination and chemical bonding of reactant molecules. The molecular structure of a polymer consists of repeating monomer units, monomers being the reactant molecules specific to a polymerization reaction. For example, the monomer of polyethylene is the simple ethylene molecule, C2H4. The monomer of cellulose is the sugar glucose, C6H11O6, formed through the process of photosynthesis. The monomer of a protein molecule is the set of twenty amino acids from which it is produced, according to the genetic code in the DNA molecule—which is itself a complex polymer made up of repeating units of four nucleotides.
Formation of Polymers
Polymers can be formed from very simple monomers in a straightforward manner, by bonding the monomer molecules together in a head-to-tail fashion. The properties of the polymer are determined by the number and type of substituent groups, or side chains, on the monomer molecules. Combinations of different monomers can also be used. The "linear" polymers produced in this way tend to be resilient materials that are thermoplastic, meaning they can be softened by heating. Cross-linking in the polymerization process occurs when there is more than one active site in the monomer molecules, allowing each one to take part in different polymerization chain reactions. Cross-linked polymers are generally hard materials that, when heated, will decompose before ever reaching the temperature at which they would melt (the "glass transition temperature"). Cross-linking can be so extensive and complete that, in principle, the contents of an entire cross-linkable resin could become a single, extremely large molecule.
The bonds between monomer molecules in a polymer can be formed by many different reactions. Dehydration reactions create bonds by eliminating the components of water from two different monomer molecules to form ester or amide linkages, as in the peptide bond between two amino acids. Chemical bonds between monomers with carbon-carbon double bonds (C=C bonds) can be formed by the addition of a catalyst molecule with an unpaired electron, which breaks apart the double bond in one of the monomers so the catalyst can bond to one of the carbon atoms, thus leaving an unpaired electron on the other carbon atom to repeat the process with the next monomer. In fact, all types of bond-forming reactions can be applied to the synthesis of polymers from monomers.
In biological systems, polymerization is mediated by enzymes. Photosynthesis converts carbon dioxide, water, and solar energy into the molecule α-d-glucose, which is then polymerized to produce a broad range of compounds, from simple disaccharides (two-sugar units) to large cellulose molecules. Amino acids are another major monomeric component of biological systems, used to synthesize the polymeric structure of all proteins and enzymes. Technically, a protein is not a polymer but a "copolymer," produced through polymerization of a number of different monomers (the amino acids). The synthesis of proteins is enzyme mediated and involves both DNA and RNA. In a similar manner, DNA and RNA are produced by the copolymerization of a combination of four different nucleotides with inorganic phosphate groups. The pattern for all of the many thousands of proteins required by living systems is contained in the genetic code, which is the sequence of nucleotides that make up the DNA molecule.
Advanced Composite Materials
A special application of monomers and polymers is the production of advanced composite materials. Typically these materials are prepared from a structured array of mineral (such as aluminum, glass, basalt, or carbon) or polymeric (such as nylon or Kevlar polyamide) fibers bound in a matrix of thermosetting resin, which is a resin that becomes permanently set when heated. These are the materials of choice for a wide variety of applications, especially for the manufacture of aircraft and other structures requiring the combination of extreme strength and low weight. Formation of advanced composite materials requires advanced training, though the principles are fairly simple. These materials do not exist in nature, and they would not exist without monomers and polymers.
PRINCIPAL TERMS
- bonding: the formation of a link between two atoms as a result of the interaction of their valence electrons.
- dehydration reaction: a chemical reaction in which hydrogen and oxygen atoms are removed from the reactants and combine to form water.
- molecule: the basic unit of a compound, composed of two or more atoms connected by chemical bonds.
- polymerization: a process in which small molecules bond together in a chain reaction to form a polymer, which is a much larger molecule composed of repeating structural units.
- synthesis reaction: a chemical reaction in which two or more reactants combine to form a single, more complex product; also, a reaction executed with the goal of creating a specific product or products.
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