Polymerisation inhibitors (US: polymerization inhibitors) are chemical compounds added to
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
Mo ...
s to prevent their auto-polymerisation. Unsaturated monomers such as
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 ...
s,
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 ...
,
butadiene
1,3-Butadiene () is the organic compound with the formula (CH2=CH)2. It is a colorless gas that is easily condensed to a liquid. It is important industrially as a precursor to synthetic rubber. The molecule can be viewed as the union of two viny ...
and
styrene
Styrene () is an organic compound with the chemical formula C6H5CH=CH2. This derivative of benzene is a colorless oily liquid, although aged samples can appear yellowish. The compound evaporates easily and has a sweet smell, although high concen ...
require inhibitors for both processing and safe transport and storage. Many monomers are purified industrially by
distillation
Distillation, or classical distillation, is the process of separation process, separating the components or substances from a liquid mixture by using selective boiling and condensation, usually inside an apparatus known as a still. Dry distilla ...
, which can lead to thermally initiated polymerisation. Styrene for example is distilled at temperatures above 100 °C whereupon it undergoes thermal polymerisation at a rate of ~2% per hour. This polymerisation is undesirable, as it can
foul the
fractionating tower, it is also typically exothermic which can lead to a
runaway reaction
Thermal runaway describes a process that is accelerated by increased temperature, in turn releasing energy that further increases temperature. Thermal runaway occurs in situations where an increase in temperature changes the conditions in a way t ...
and potential explosion if left unchecked. Once initiated polymerisation is typically
radical
Radical may refer to:
Politics and ideology Politics
*Radical politics, the political intent of fundamental societal change
*Radicalism (historical), the Radical Movement that began in late 18th century Britain and spread to continental Europe and ...
in mechanism and as such many polymerisation inhibitors act as radical scavengers.
Inhibitors vs retarders
The term 'inhibitor' is often used in a general sense to describe any compound used to prevent unwanted polymerisation, however these compounds are often divided into 'retarders' and 'true inhibitors'. A true inhibitor has a well defined
induction period
An induction period in chemical kinetics is an initial slow stage of a chemical reaction; after the induction period, the reaction accelerates. Ignoring induction periods can lead to runaway reactions.
In some catalytic reactions, a pre-catalyst n ...
during which no noticeable polymerisation takes place. They are consumed during this period and once gone polymerisation occurs as normal. Retarders display no induction period but provide a permanent decrease in the rate of polymerisation, while themselves being degraded only slowly. Attempts have been made to define the difference quantitatively in terms of reaction rate. In an industrial setting compounds from both classes will usually be used together, with the true inhibitor providing optimal plant performance and the retarder acting as a failsafe.
Inhibitors for processing
True inhibitors
Radical polymerisation of unsaturated monomers is generally propagated by C-radicals. These can be effectively terminated by combining with other radicals to form neutral species and many true inhibitors operate through this mechanism. In the simplest example oxygen can be used as it exists naturally in its
triplet state
In quantum mechanics, a triplet is a quantum state of a system with a spin of quantum number =1, such that there are three allowed values of the spin component, = −1, 0, and +1.
Spin, in the context of quantum mechanics, is not a mechanical ...
(i.e. it is a
diradical
In chemistry, a diradical is a molecular species with two electrons occupying molecular orbitals (MOs) which are degenerate. The term "diradical" is mainly used to describe organic compounds, where most diradicals are extremely reactive and in ...
). This is referred to as air inhibition and is a
diffusion-controlled reaction
Diffusion-controlled (or diffusion-limited) reactions are reactions in which the reaction rate is equal to the rate of transport of the reactants through the reaction medium (usually a solution). The process of chemical reaction can be considered ...
with rates typically in the order of 10
7–10
9 mol
−1 s
−1, the resulting peroxy radicals (ROO•) are less reactive towards polymerisation. However air stabilisation is not suitable for monomers with which it can form explosive peroxides, such as vinyl chloride. Other stable radicals include
TEMPO
In musical terminology, tempo (Italian, 'time'; plural ''tempos'', or ''tempi'' from the Italian plural) is the speed or pace of a given piece. In classical music, tempo is typically indicated with an instruction at the start of a piece (often ...
and
TEMPOL
4-Hydroxy-TEMPO or TEMPOL, formally 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl, is a heterocyclic compound. Like the related TEMPO, it is used as a catalyst and chemical oxidant by virtue of being a stable aminoxyl radical. Its major appeal ove ...
, which are exceedingly effective radical scavengers. Certain compounds marketed as true inhibitors, such as
p-phenylenediamine
''p''-Phenylenediamine (PPD) is an organic compound with the formula C6H4(NH2)2. This derivative of aniline is a white solid, but samples can darken due to air oxidation. It is mainly used as a component of engineering polymers and composites lik ...
s,
phenothiazine
Phenothiazine, abbreviated PTZ, is an organic compound that has the formula S(C6H4)2NH and is related to the thiazine-class of heterocyclic compounds. Derivatives of phenothiazine are highly bioactive and have widespread use and rich history. T ...
and
hydroxylamines like HPHA and
DEHA
The Deha, sometimes pronounced as Dahiya, Dhaya, Dhea, Daiya and Dheya are a caste found in India, and have other backward caste status in Haryana. They forms the largest jat community in Haryana.
Origin
According to some traditions, the c ...
, are also thought to react through the intermediary of
aminoxyl radicals. Not all inhibitors are radicals however, with
quinone
The quinones are a class of organic compounds that are formally "derived from aromatic compounds uch as benzene or naphthalene
Uch ( pa, ;
ur, ), frequently referred to as Uch Sharīf ( pa, ;
ur, ; ''"Noble Uch"''), is a historic city in the southern part of Pakistan's Punjab province. Uch may have been founded as Alexandria on the Indus, a town founded by Alexand ...
by conversion of an even number of –CH= groups into –C(=O)– groups with any necessary rearrangement of double ...
s and
quinone methide
A quinone methide is a type of conjugated organic compound that contain a cyclohexadiene with a carbonyl and an exocyclic methylidene or extended alkene unit. It is analogous to a quinone, but having one of the double bonded oxygens replaced wit ...
s being important examples.
Retarders
Certain
hydroxylamine
Hydroxylamine is an inorganic compound with the formula . The material is a white crystalline, hygroscopic compound.Greenwood and Earnshaw. ''Chemistry of the Elements.'' 2nd Edition. Reed Educational and Professional Publishing Ltd. pp. 431–43 ...
s and
p-phenylenediamine
''p''-Phenylenediamine (PPD) is an organic compound with the formula C6H4(NH2)2. This derivative of aniline is a white solid, but samples can darken due to air oxidation. It is mainly used as a component of engineering polymers and composites lik ...
s, may act as retarders. For styrene,
nitrophenol Nitrophenols are compounds of the formula HOC6H5−x(NO2)x. The conjugate bases are called nitrophenolates. Nitrophenols are more acidic than phenol itself.
Mono-nitrophenols
with the formula HOC6H4NO2. Three isomeric nitrophenols exist: ...
compounds such as
dinitro-ortho-cresol
Dinitro-''ortho''-cresol (DNOC) is an organic compound with the structural formula CH3C6H2(NO2)2OH. It is a yellow solid that is only slightly soluble in water. DNOC and some related derivatives have been used as herbicides.
Preparation
This co ...
and
di-nitro-sec-butylphenol (DNBP) have long been the important,
however they are coming under regulatory pressure due to their high toxicity.
Inhibitors for transport & storage
Purified monomers stored at ambient temperatures are of less risk of polymerising and as such the most highly reactive inhibitors are rarely used at this stage. In general compounds are chosen which can be easily removed immediately prior to industrial polymerisation to make plastics. Compounds bearing a
hydroxy group
In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...
, which can be removed by an alkali wash, tend to dominate. Examples include
4-tert-butylcatechol (TBC),
4-methoxyphenol (MEHQ),
butylated hydroxytoluene
Butylated hydroxytoluene (BHT), also known as dibutylhydroxytoluene, is a lipophilic organic compound, chemically a derivative of phenol, that is useful for its antioxidant properties. BHT is widely used to prevent free radical-mediated oxidatio ...
(BHT) and
hydroquinone
Hydroquinone, also known as benzene-1,4-diol or quinol, is an aromatic organic compound that is a type of phenol, a derivative of benzene, having the chemical formula C6H4(OH)2. It has two hydroxyl groups bonded to a benzene ring in a ''para'' ...
(HQ).
See also
*
Anti-skinning agent - These agents prevent polymerisation in paints and varnishes by binding to, and thus inhibiting, the action of
oil drying agent An oil drying agent, also known as siccative, is a coordination compound that accelerates ( catalyzes) the hardening of drying oils, often as they are used in oil-based paints. This so-called "drying" (actually a chemical reaction that produces an ...
s
*
Tubulin polymerisation inhibitors - chemotherapy drugs that interfere with the tubulin system
References
{{Reflist
Monomers