![Polystyrene formation](https://upload.wikimedia.org/wikipedia/commons/1/10/Polystyrene_formation.PNG)
In
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 are ...
, polymerization (
American English
American English, sometimes called United States English or U.S. English, is the set of variety (linguistics), varieties of the English language native to the United States. English is the Languages of the United States, most widely spoken lan ...
), or polymerisation (
British English
British English (BrE, en-GB, or BE) is, according to Lexico, Oxford Dictionaries, "English language, English as used in Great Britain, as distinct from that used elsewhere". More narrowly, it can refer specifically to the English language in ...
), is a process of reacting
monomer molecules together in a
chemical reaction
A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...
to form
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 ...
chains or three-dimensional networks.
There are many forms of polymerization and different systems exist to categorize them.
In
chemical compound
A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
s, polymerization can occur via a variety of reaction mechanisms that vary in complexity due to the
functional group
In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the rest ...
s present in the reactants
and their inherent
steric effects
Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivity of ions ...
. In more straightforward polymerizations,
alkenes
In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond.
Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
form polymers through relatively simple
radical reactions; in contrast, reactions involving substitution at a
carbonyl group
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containing a ...
require more complex synthesis due to the way in which reactants polymerize.
Alkane
In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in which ...
s can also be polymerized, but only with the help of strong acids.
As alkenes can polymerize in somewhat straightforward radical reactions, they form useful compounds such as
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 bo ...
and
polyvinyl chloride (PVC),
which are produced in high tonnages each year
due to their usefulness in manufacturing processes of commercial products, such as piping, insulation and packaging. In general, polymers such as PVC are referred to as "homopolymers", as they consist of repeated long chains or structures of the same monomer unit, whereas polymers that consist of more than one monomer unit are referred to as
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 some ...
s (or co-polymers).
Other monomer units, such as formaldehyde hydrates or simple aldehydes, are able to polymerize themselves at quite low temperatures (ca. −80 °C) to form
trimers;
molecules consisting of 3 monomer units, which can
cyclize to form ring cyclic structures, or undergo further reactions to form
tetramer
A tetramer () (''tetra-'', "four" + '' -mer'', "parts") is an oligomer formed from four monomers or subunits. The associated property is called ''tetramery''. An example from inorganic chemistry is titanium methoxide with the empirical formula Ti ...
s,
or 4 monomer-unit compounds. Such small polymers are referred to as
oligomer
In chemistry and biochemistry, an oligomer () is a molecule that consists of a few repeating units which could be derived, actually or conceptually, from smaller molecules, monomers.Quote: ''Oligomer molecule: A molecule of intermediate relativ ...
s.
Generally, because formaldehyde is an exceptionally reactive electrophile it allows
nucleophilic
In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...
addition of
hemiacetal
A hemiacetal or a hemiketal has the general formula R1R2C(OH)OR, where R1 or R2 is hydrogen or an organic substituent. They generally result from the addition of an alcohol to an aldehyde or a ketone, although the latter are sometimes called hemike ...
intermediates, which are in general short-lived and relatively unstable "mid-stage" compounds that react with other non-polar molecules present to form more stable polymeric compounds.
Polymerization that is not sufficiently moderated and proceeds at a fast rate can be very hazardous. This phenomenon is known as
autoacceleration
Autoacceleration (gel effect, Trommsdorff–Norrish effect) is a dangerous reaction behavior that can occur in free-radical polymerization systems. It is due to the localized increases in viscosity of the polymerizing system that slow terminati ...
, and can cause fires and explosions.
Step-growth vs. chain-growth polymerization
Step-growth and chain-growth are the main classes of polymerization reaction mechanisms. The former is often easier to implement but requires precise control of stoichiometry. The latter more reliably affords high molecular-weight polymers, but only applies to certain monomers.
Step-growth
In step-growth (or step) polymerization, pairs of reactants, of any lengths, combine at each step to form a longer polymer molecule. The
average molar mass increases slowly. Long chains form only late in the reaction.
Step-growth polymers are formed by independent reaction steps between functional groups of monomer units, usually containing
heteroatoms
In chemistry, a heteroatom () is, strictly, any atom that is not carbon or hydrogen.
Organic chemistry
In practice, the term is usually used more specifically to indicate that non-carbon atoms have replaced carbon in the backbone of the molecula ...
such as nitrogen or oxygen. Most step-growth polymers are also classified as
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 form ...
s, since a small molecule such as water is lost when the polymer chain is lengthened. For example,
polyester
Polyester is a category of polymers that contain the ester functional group in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include natural ...
chains grow by reaction of
alcohol
Alcohol most commonly refers to:
* Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom
* Alcohol (drug), an intoxicant found in alcoholic drinks
Alcohol may also refer to:
Chemicals
* Ethanol, one of sev ...
and
carboxylic acid
In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic ...
groups to form
ester
In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ar ...
links with loss of water. However, there are exceptions; for example
polyurethane
Polyurethane (; often abbreviated PUR and PU) refers to a class of polymers composed of organic chemistry, organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethan ...
s are step-growth polymers formed from
isocyanate
In organic chemistry, isocyanate is the functional group with the formula . Organic compounds that contain an isocyanate group are referred to as isocyanates. An organic compound with two isocyanate groups is known as a diisocyanate. Diisocyan ...
and alcohol bifunctional monomers) without loss of water or other volatile molecules, and are classified as
addition polymer
In polymer chemistry, an addition polymer is a polymer that forms by simple linking of monomers ''without'' the co-generation of other products. Addition polymerization differs from condensation polymerization, which ''does'' co-generate a product ...
s rather than condensation polymers.
Step-growth polymers increase in molecular weight at a very slow rate at lower conversions and reach moderately high molecular weights only at very high conversion (i.e., >95%). Solid state polymerization to afford polyamides (e.g., nylons) is an example of step-growth polymerization.
Chain-growth
In chain-growth (or chain) polymerization, the only chain-extension reaction step is the addition of a monomer to a growing chain with an active center such as a
free radical
A daughter category of ''Ageing'', this category deals only with the biological aspects of ageing.
Ageing
Ailments of unknown cause
Biogerontology
Biological processes
Causes of death
Cellular processes
Gerontology
Life extension
Metabo ...
,
cation
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
, or
anion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
. Once the growth of a chain is initiated by formation of an active center, chain propagation is usually rapid by addition of a sequence of monomers. Long chains are formed from the beginning of the reaction.
Chain-growth polymerization (or addition polymerization) involves the linking together of unsaturated monomers, especially containing carbon-carbon
double bonds
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 ...
. The pi-bond is lost by formation of a new sigma bond. Chain-growth polymerization is involved in the manufacture of polymers such as
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 bo ...
,
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 and ...
,
polyvinyl chloride (PVC), and
acrylate
Acrylates (IUPAC: prop-2-enoates) are the salts, esters, and conjugate bases of acrylic acid. The acrylate ion is the anion C H2=CHC OO−. Often, acrylate refers to esters of acrylic acid, the most common member being methyl acrylate. These acry ...
. In these cases, the alkenes RCH=CH
2 are converted to high molecular weight alkanes (-RCHCH
2-)
n (R = H, CH
3, Cl, CO
2CH
3).
Other forms of chain growth polymerization include
cationic addition polymerization and
anionic addition polymerization
In polymer chemistry, anionic addition polymerization is a form of chain-growth polymerization or addition polymerization that involves the polymerization of monomers initiated with anions. The type of reaction has many manifestations, but tradi ...
. A special case of chain-growth polymerization leads to
living polymerization
In polymer chemistry, living polymerization is a form of chain growth polymerization where the ability of a growing polymer chain to terminate has been removed. This can be accomplished in a variety of ways. Chain termination and chain transfer r ...
.
Ziegler–Natta polymerization allows considerable control of
polymer branching.
![Ethylene polymerization](https://upload.wikimedia.org/wikipedia/commons/8/8c/Ethylene_polymerization.png)
Diverse methods are employed to manipulate the initiation, propagation, and termination rates during chain polymerization. A related issue is temperature control, also called
heat management
In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is al ...
, during these reactions, which are often highly exothermic. For example, for the polymerization of ethylene, 93.6 kJ of energy are released per mole of monomer.
[
The manner in which polymerization is conducted is a highly evolved technology. Methods include ]emulsion polymerization Emulsion polymerization is a type of radical polymerization that usually starts with an emulsion incorporating water, monomer, and surfactant. The most common type of emulsion polymerization is an oil-in-water emulsion, in which droplets of monomer ...
, solution polymerization
Solution polymerization is a method of industrial polymerization. In this procedure, a monomer is dissolved in a non-reactive solvent that contains a catalyst or initiator.
The reaction results in a polymer which is also soluble in the chosen solv ...
, suspension polymerization, and precipitation polymerization
In polymer science, precipitation polymerization is a heterogeneous polymerization process that begins initially as a homogeneous system in the continuous phase, where the monomer and initiator are completely soluble, but upon initiation the form ...
. Although the polymer dispersity
In chemistry, the dispersity is a measure of the heterogeneity of sizes of molecules or particles in a mixture. A collection of objects is called uniform if the objects have the same size, shape, or mass. A sample of objects that have an inconsi ...
and molecular weight may be improved, these methods may introduce additional processing requirements to isolate the product from a solvent.
Photopolymerization
Most photopolymerization reactions are chain-growth polymerizations which are initiated by the absorption of visible or ultraviolet light. The light may be absorbed either directly by the reactant monomer (''direct'' photopolymerization), or else by a ''photosensitizer'' which absorbs the light and then transfers energy to the monomer. In general, only the initiation step differs from that of the ordinary thermal polymerization of the same monomer; subsequent propagation, termination, and chain-transfer steps are unchanged.
In step-growth photopolymerization, absorption of light triggers an addition (or condensation) reaction between two comonomers that do not react without light. A propagation cycle is not initiated because each growth step requires the assistance of light.
Photopolymerization can be used as a photographic or printing process because polymerization only occurs in regions which have been exposed to light. Unreacted monomer can be removed from unexposed regions, leaving a relief polymeric image. Several forms of 3D printing
3D printing or additive manufacturing is the Manufacturing, construction of a three-dimensional object from a computer-aided design, CAD model or a digital 3D modeling, 3D model. It can be done in a variety of processes in which material is ...
—including layer-by-layer stereolithography
Stereolithography (SLA or SL; also known as vat photopolymerisation, optical fabrication, photo-solidification, or resin printing) is a form of 3D printing technology used for creating models, prototypes, patterns, and production parts in a lay ...
and two-photon absorption 3D photopolymerization—use photopolymerization.
Multiphoton polymerization using single pulses have also been demonstrated for fabrication of complex structures using a digital micromirror device
The digital micromirror device, or DMD, is the microoptoelectromechanical system (MOEMS) that is the core of the trademarked DLP projection technology from Texas Instruments (TI). Texas Instrument's DMD was created by solid-state physicist and ...
.
See also
* Cross-link
In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...
* ''In situ'' polymerization
* Metallocene
A metallocene is a compound typically consisting of two cyclopentadienyl anions (, abbreviated Cp) bound to a metallic element, metal center (M) in the oxidation state II, with the resulting general formula Closely related to the metallocenes are ...
* Plasma polymerization
Plasma polymerization (or glow discharge polymerization) uses plasma sources to generate a gas discharge that provides energy to activate or fragment gaseous or liquid monomer, often containing a vinyl group, in order to initiate polymerization. ...
* Polymer characterization
Polymer characterization is the analytical branch of polymer science.
The discipline is concerned with the characterization of polymeric materials on a variety of levels. The characterization typically has as a goal to improve the performance of t ...
* Polymer physics Polymer physics is the field of physics that studies polymers, their fluctuations, mechanical properties, as well as the kinetics of reactions involving degradation and polymerisation of polymers and monomers respectively.P. Flory, ''Principles of ...
*
* Ring-opening polymerization
In polymer chemistry, ring-opening polymerization (ROP) is a form of chain-growth polymerization, in which the terminus of a polymer chain attacks cyclic monomers to form a longer polymer (see figure). The reactive center can be radical, anion ...
* Sequence-controlled polymer
A sequence-controlled polymer is a macromolecule, in which the sequence of monomers is controlled to some degree. This control can be absolute but not necessarily. In other words, a sequence-controlled polymer can be uniform (its dispersity Ð is ...
s
* Sol-gel
References
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