A repeat unit or repeating unit , or mer, is a part of a

polymer A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...

whose repetition would produce the complete polymer chain (except for the end groups) by linking the repeat units together successively along the chain, like the beads of a necklace. A repeat unit is sometimes called a mer (or mer unit) in

polymer chemistry Polymer chemistry is a sub-discipline of chemistry that focuses on the structures, chemical synthesis, and chemical and physical properties of polymers and macromolecules. The principles and methods used within polymer chemistry are also applic ...

. "Mer" originates from the Greek word ''meros'', which means "a part". The word polymer derives its meaning from this, which means "many mers". The mer is not the same thing as a

monomer A monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or two- or three-dimensional network in a process called polymerization. Classification Chemis ...

—a mer is a repeating unit within a larger molecule, whereas a monomer is an actual molecule that exists independently, either prior to

polymerization In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many fo ...

or after decomposition.Callister, William D. (2007). ''Materials science and engineering : an introduction'' (7th ed.) New York : John Wiley & Sons.

Overview

One of the simplest repeat units is that of the addition polymer

polyvinyl chloride Polyvinyl chloride (alternatively: poly(vinyl chloride), colloquial: vinyl or polyvinyl; abbreviated: PVC) is the world's third-most widely produced synthetic polymer of plastic (after polyethylene and polypropylene). About 40 million tons of ...

, - H₂-CHClsub>n-, whose repeat unit is - H₂-CHCl. In this case the repeat unit has the same atoms as the monomer vinyl chloride CH₂=CHCl. When the polymer is formed, the C=C

double bond In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betw ...

in the monomer is replaced by a C-C single bond in the polymer repeat unit, which links by two new bonds to adjoining repeat units. In condensation polymers (see examples below), the repeat unit contains fewer atoms than the monomer or monomers from which it is formed. The subscript "n" denotes the degree of polymerisation, that is, the number of units linked together. The

molecular mass The molecular mass () is the mass of a given molecule, often expressed in units of daltons (Da). Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. The derived quan ...

of the repeat unit, M_R, is simply the sum of the atomic masses of the

atom Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...

s within the repeat unit. The molecular mass of the chain is just the product nM_R. Other than monodisperse polymers, there is normally a molar mass distribution caused by chains of different length. In copolymers there are two or more types of repeat unit, which may be arranged in alternation, or at random, or in other more complex patterns.

Other vinyl polymers

Polyethylene Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging (plastic bags, plastic films, geomembranes and containers including bott ...

may be considered either as - H₂-CH₂-sub>n- with a repeat unit of - H₂-CH₂, or as CH₂-sub>n-, with a repeat unit of - H₂. Chemists tend to consider the repeat unit as - H₂-CH₂ since this polymer is made from the monomer

ethylene Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon–carbon bond, carbon–carbon doub ...

(CH₂=CH₂). More complex repeat units can occur in vinyl polymers - H₂-CHRsub>n-, if one hydrogen in the ethylene repeat unit is substituted by a larger fragment R.

Polypropylene Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer Propene, propylene. Polypropylene belongs to the group of polyolefin ...

- H₂-CH(CH₃)sub>n- has the repeat unit - H₂-CH(CH₃)

Polystyrene Polystyrene (PS) is a synthetic polymer made from monomers of the aromatic hydrocarbon styrene. Polystyrene can be solid or foamed. General-purpose polystyrene is clear, hard, and brittle. It is an inexpensive resin per unit weight. It i ...

has a chain where the

substituent In organic chemistry, a substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. The suffix ''-yl'' is used when naming organic compounds that contain a single bond r ...

R is a phenyl group (C₆H₅), corresponding to a

benzene Benzene is an Organic compound, organic chemical compound with the Chemical formula#Molecular formula, molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar hexagonal Ring (chemistry), ring with one hyd ...

ring minus one hydrogen: - H₂-CH(C₆H₅)sub>n-, so the repeat unit is - H₂-CH(C₆H₅).

Condensation polymers: repeat unit and structural units

In many condensation polymers, the repeat unit contains two structural units related to the

comonomer In polymer chemistry, a comonomer refers to a polymerizable precursor to a copolymer aside from the principal monomer. In some cases, only small amounts of a comonomer are employed, in other cases substantial amounts of comonomers are used. Furt ...

s which have been polymerized. For example, in polyethylene terephthalate (PET or "polyester"), the repeat unit is -CO-C₆H₄-CO-O-CH₂-CH₂-O-. The polymer is formed by the

condensation reaction In organic chemistry, a condensation reaction is a type of chemical reaction in which two molecules are combined to form a single molecule, usually with the loss of a small molecule such as water. If water is lost, the reaction is also known as a ...

of the two monomers terephthalic acid (HOOC-C₆H₄-COOH) and ethylene glycol (HO-CH₂-CH₂-OH), or their chemical derivatives. The condensation involves loss of water, as an H is lost from each HO- group in the glycol, and an OH from each HOOC- group in the acid. The two structural units in the polymer are then considered to be -CO-C₆H₄-CO- and -O-CH₂-CH₂-O-.

References

{{reflist Polymer chemistry