A monomer (// MON-ə-mər; 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.
Monomers can be classified in many ways. They can be subdivided into two broad classes, depending on the kind of the polymer that they form. Monomers that participate in condensation polymerization have a different stoichiometry than monomers that participate in addition polymerization:
Other classifications include:
- natural vs synthetic monomers, e.g. glycine vs caprolactam, respectively
- polar vs nonpolar monomers, e.g. vinyl acetate vs ethylene, respectively
- cyclic vs linear, e.g. ethylene oxide vs ethylene glycol, respectively
The polymerization of one kind of monomer gives a homopolymer. Many polymers are copolymers, meaning that they are derived from two different monomers. In the case of condensation polymerizations, the ratio of comonomers is usually 1:1. For example, the formation of many nylons requires equal amounts of a dicarboxylic acid and diamine. In the case of addition polymerizations, the comonomer content is often only a few percent. For example, small amounts of 1-octene monomer are copolymerized with ethylene to give specialized polyethylene.
- Ethylene gas (H2C=CH2) is the monomer for polyethylene.
- Other modified ethylene derivatives include:
- Epoxide monomers may be cross linked with themselves, or with the addition of a co-reactant, to form epoxy
- BPA is the monomer precursor for polycarbonate
- Terephthalic acid is a comonomer that, with ethylene glycol, forms polyethylene terephthalate.
- Dimethylsilicon dichloride is a monomer that, upon hydrolysis, gives polydimethylsiloxane.
- Ethyl methacrylate is an acrylic monomer that, when combined with an acrylic polymer, catalyzes and forms an acrylate plastic used to create artificial nail extensions
Some of the main biopolymers are listed below:
For polynucleic acids (DNA/RNA), the monomers are nucleotides, each of which is made of a pentose sugar, a nitrogenous base and a phosphate group. Nucleotide monomers are found in the cell nucleus. Four types of nucleotide monomers are precursors to DNA and four different nucleotide monomers are precursors to RNA.
Isoprene is a natural monomer that polymerizes to form natural rubber, most often cis-1,4-polyisoprene, but also trans-1,4-polymer. Synthetic rubbers are often based on butadiene, which is structurally related to isoprene.