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

, a repeat unit or repeating unit (or mer) is a part of a

polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...

whose repetition would produce the complete polymer chain (except for the

end-group End groups are an important aspect of polymer synthesis and characterization. In polymer chemistry, they are functional groups that are at the very ends of a macromolecule or oligomer (IUPAC). In polymer synthesis, like condensation polymerizati ...

s) 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). "Mer" originates from the

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

word ''meros'', which means "a part". The word polymer derives its meaning from this, which means "many mers". A repeat unit (mer) is not to be confused with the term

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

, which refers to the small molecule from which a polymer is synthesized.Callister, William D. (2007). ''Materials science and engineering : an introduction'' (7th ed.) New York : John Wiley & Sons. One of the simplest repeat units is that of the addition polymer polyvinyl chloride, - H₂-CHClsub>n-, whose repeat unit is - H₂-CHCl. In this case the repeat unit has the same atoms as the monomer

vinyl chloride Vinyl chloride is an organochloride with the formula H2C=CHCl. It is also called vinyl chloride monomer (VCM) or chloroethene. This colorless compound is an important industrial chemical chiefly used to produce the polymer polyvinyl chloride (PVC ...

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 betwee ...

in the monomer is replaced by a C-C

single bond In chemistry, a single bond is a chemical bond between two atoms involving two valence electrons. That is, the atoms share one pair of electrons where the bond forms. Therefore, a single bond is a type of covalent bond. When shared, each of ...

in the polymer repeat unit, which links by two new bonds to adjoining repeat units. In

condensation polymer In polymer chemistry, condensation polymers are any kind of polymers whose process of polymerization involves a condensation reaction (i.e. a small molecule, such as water or methanol, is produced as a byproduct). Condensation polymers are for ...

s (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 (''m'') is the mass of a given molecule: it is measured in daltons (Da or u). Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. The related quant ...

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

atomic mass The atomic mass (''m''a or ''m'') is the mass of an atom. Although the SI unit of mass is the kilogram (symbol: kg), atomic mass is often expressed in the non-SI unit dalton (symbol: Da) – equivalently, unified atomic mass unit (u). 1&n ...

es of the

atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, a ...

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 The molar mass distribution (or molecular weight distribution) describes the relationship between the number of moles of each polymer species (Ni) and the molar mass (Mi) of that species. In linear polymers, the individual polymer chains rarely hav ...

caused by chains of different length. In

copolymer In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are ...

s 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 b ...

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 double bonds). Ethylene ...

(CH₂=CH₂). More complex repeat units can occur in

vinyl polymer In polymer chemistry, vinyl polymers are a group of polymers derived from substituted vinyl () monomers. Their backbone is an extended alkane chain . In popular usage, "vinyl" refers only to polyvinyl chloride (PVC). Examples Vinyl polymers ...

s - 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 propylene. Polypropylene belongs to the group of polyolefins a ...

- 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, aromatic hydrocarbon styrene. Polystyrene can be solid or foamed. General-purpose polystyrene is clear, hard, and brittle. It is an inexpensive resin pe ...

has a chain where the substituent R is a

phenyl group In organic chemistry, the phenyl group, or phenyl ring, is a cyclic group of atoms with the formula C6 H5, and is often represented by the symbol Ph. Phenyl group is closely related to benzene and can be viewed as a benzene ring, minus a hydrogen ...

(C₆H₅), corresponding to a benzene 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

s, the repeat unit contains two

structural unit In polymer chemistry, a structural unit is a building block of a polymer chain. It is the result of a monomer which has been polymerized into a long chain. There may be more than one structural unit in the repeat unit. When different monomers are ...

s 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. Furthe ...

s which have been polymerized. For example, in

polyethylene terephthalate Polyethylene terephthalate (or poly(ethylene terephthalate), PET, PETE, or the obsolete PETP or PET-P), is the most common thermoplastic polymer resin of the polyester family and is used in fibres for clothing, containers for liquids and food ...

(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 Terephthalic acid is an organic compound with formula C6H4(CO2H)2. This white solid is a commodity chemical, used principally as a precursor to the polyester PET, used to make clothing and plastic bottles. Several million tonnes are produced an ...

(HOOC-C₆H₄-COOH) and

ethylene glycol Ethylene glycol ( IUPAC name: ethane-1,2-diol) is an organic compound (a vicinal diol) with the formula . It is mainly used for two purposes, as a raw material in the manufacture of polyester fibers and for antifreeze formulations. It is an o ...

(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