Polymerization

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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. [1] [2] [3] There are many forms of polymerization [4] and different systems exist to categorize them.

Contents

IUPAC definition for polymerization IUPAC definition for polymerization.png
IUPAC definition for polymerization

In chemical compounds, polymerization can occur via a variety of reaction mechanisms that vary in complexity due to the functional groups present in the reactants [3] and their inherent steric effects. In more straightforward polymerizations, alkenes form polymers through relatively simple radical reactions; in contrast, reactions involving substitution at a carbonyl group require more complex synthesis due to the way in which reactants polymerize. [3]

An example of alkene polymerization, in which each styrene monomer's double bond reforms as a single bond plus a bond to another styrene monomer. The product is polystyrene. Polystyrene formation.PNG
An example of alkene polymerization, in which each styrene monomer's double bond reforms as a single bond plus a bond to another styrene monomer. The product is polystyrene.

As alkenes can polymerize in somewhat straightforward radical reactions, they form useful compounds such as polyethylene and polyvinyl chloride (PVC), [3] which are produced in high tonnages each year [3] 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 copolymers (or co-polymers). [5]

Homopolymers


Copolymers

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; [3] molecules consisting of 3 monomer units, which can cyclize to form ring cyclic structures, or undergo further reactions to form tetramers, [3] or 4 monomer-unit compounds. Such small polymers are referred to as oligomers. [3] Generally, because formaldehyde is an exceptionally reactive electrophile it allows nucleophilic addition of hemiacetal 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, 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.

A classification of the polymerization reactions Polymerization classification ENmod.png
A classification of the polymerization reactions

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. [6] [7]

Step-growth polymers are formed by independent reaction steps between functional groups of monomer units, usually containing heteroatoms such as nitrogen or oxygen. Most step-growth polymers are also classified as condensation polymers, since a small molecule such as water is lost when the polymer chain is lengthened. For example, polyester chains grow by reaction of alcohol and carboxylic acid groups to form ester links with loss of water. However, there are exceptions; for example polyurethanes are step-growth polymers formed from isocyanate and alcohol bifunctional monomers) without loss of water or other volatile molecules, and are classified as addition polymers 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. [8]

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, cation, or anion. 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. [6] [7]

Chain-growth polymerization (or addition polymerization) involves the linking together of unsaturated monomers, especially containing carbon-carbon double bonds. 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, polypropylene, polyvinyl chloride (PVC), and acrylate. In these cases, the alkenes RCH=CH2 are converted to high molecular weight alkanes (-RCHCH2-)n (R = H, CH3, Cl, CO2CH3).

Other forms of chain growth polymerization include cationic addition polymerization and anionic addition polymerization. A special case of chain-growth polymerization leads to living polymerization. Ziegler–Natta polymerization allows considerable control of polymer branching.

Polymerization of ethylene Ethylene polymerization.png
Polymerization of ethylene

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, 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. [8]

The manner in which polymerization is conducted is a highly evolved technology. Methods include emulsion polymerization, solution polymerization, suspension polymerization, and precipitation polymerization. Although the polymer dispersity 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 [9] or ultraviolet light. Photopolymerization can also be a step-growth polymerization. [10] 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. [6] 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. [11]

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. [6] Several forms of 3D printing—including layer-by-layer stereolithography and two-photon absorption 3D photopolymerization—use photopolymerization. [12]

Multiphoton polymerization using single pulses have also been demonstrated for fabrication of complex structures using a digital micromirror device. [13]

See also

Related Research Articles

A monomer 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.

<span class="mw-page-title-main">Polymer</span> Substance composed of macromolecules with repeating structural units

A polymer is a substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeating subunits derived from one or more species of monomers. Due to their broad spectrum of properties, both synthetic and natural polymers play essential and ubiquitous roles in everyday life. Polymers range from familiar synthetic plastics such as polystyrene to natural biopolymers such as DNA and proteins that are fundamental to biological structure and function. Polymers, both natural and synthetic, are created via polymerization of many small molecules, known as monomers. Their consequently large molecular mass, relative to small molecule compounds, produces unique physical properties including toughness, high elasticity, viscoelasticity, and a tendency to form amorphous and semicrystalline structures rather than crystals.

<span class="mw-page-title-main">Polyethylene</span> Most common thermoplastic polymer

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 bottles, cups, jars, etc.). As of 2017, over 100 million tonnes of polyethylene resins are being produced annually, accounting for 34% of the total plastics market.

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 reactions are absent and the rate of chain initiation is also much larger than the rate of chain propagation. The result is that the polymer chains grow at a more constant rate than seen in traditional chain polymerization and their lengths remain very similar. Living polymerization is a popular method for synthesizing block copolymers since the polymer can be synthesized in stages, each stage containing a different monomer. Additional advantages are predetermined molar mass and control over end-groups.

<span class="mw-page-title-main">Dispersity</span> Measure of heterogeneity of particle or molecular sizes

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 inconsistent size, shape and mass distribution is called non-uniform. The objects can be in any form of chemical dispersion, such as particles in a colloid, droplets in a cloud, crystals in a rock, or polymer macromolecules in a solution or a solid polymer mass. Polymers can be described by molecular mass distribution; a population of particles can be described by size, surface area, and/or mass distribution; and thin films can be described by film thickness distribution.

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, usually water. Addition polymers can be formed by chain polymerization, when the polymer is formed by the sequential addition of monomer units to an active site in a chain reaction, or by polyaddition, when the polymer is formed by addition reactions between species of all degrees of polymerization. Addition polymers are formed by the addition of some simple monomer units repeatedly. Generally polymers are unsaturated compounds like alkenes, alkalines etc. The addition polymerization mainly takes place in free radical mechanism. The free radical mechanism of addition polymerization completed by three steps i.e. Initiation of free radical, Chain propagation, Termination of chain.

<span class="mw-page-title-main">Chain-growth polymerization</span> Polymerization technique

Chain-growth polymerization (AE) or chain-growth polymerisation (BE) is a polymerization technique where monomer molecules add onto the active site on a growing polymer chain one at a time. There are a limited number of these active sites at any moment during the polymerization which gives this method its key characteristics.

<span class="mw-page-title-main">Step-growth polymerization</span> Type of polymerization reaction mechanism

In polymer chemistry, step-growth polymerization refers to a type of polymerization mechanism in which bi-functional or multifunctional monomers react to form first dimers, then trimers, longer oligomers and eventually long chain polymers. Many naturally-occurring and some synthetic polymers are produced by step-growth polymerization, e.g. polyesters, polyamides, polyurethanes, etc. Due to the nature of the polymerization mechanism, a high extent of reaction is required to achieve high molecular weight. The easiest way to visualize the mechanism of a step-growth polymerization is a group of people reaching out to hold their hands to form a human chain—each person has two hands. There also is the possibility to have more than two reactive sites on a monomer: In this case branched polymers production take place.

<span class="mw-page-title-main">End group</span> Functional group at the extremity of an oligomer or other macromolecule

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 polymerization and free-radical types of polymerization, end-groups are commonly used and can be analyzed by nuclear magnetic resonance (NMR) to determine the average length of the polymer. Other methods for characterization of polymers where end-groups are used are mass spectrometry and vibrational spectrometry, like infrared and raman spectroscopy. These groups are important for the analysis of polymers and for grafting to and from a polymer chain to create a new copolymer. One example of an end group is in the polymer poly(ethylene glycol) diacrylate where the end-groups are circled.

<span class="mw-page-title-main">Radical polymerization</span> Polymerization process involving free radicals as repeating units

In polymer chemistry, free-radical polymerization (FRP) is a method of polymerization by which a polymer forms by the successive addition of free-radical building blocks. Free radicals can be formed by a number of different mechanisms, usually involving separate initiator molecules. Following its generation, the initiating free radical adds (nonradical) monomer units, thereby growing the polymer chain.

<span class="mw-page-title-main">Reversible addition−fragmentation chain-transfer polymerization</span> Chemical reaction that produces polymers

Reversible addition−fragmentation chain-transfer or RAFT polymerization is one of several kinds of reversible-deactivation radical polymerization. It makes use of a chain-transfer agent (CTA) in the form of a thiocarbonylthio compound to afford control over the generated molecular weight and polydispersity during a free-radical polymerization. Discovered at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) of Australia in 1998, RAFT polymerization is one of several living or controlled radical polymerization techniques, others being atom transfer radical polymerization (ATRP) and nitroxide-mediated polymerization (NMP), etc. RAFT polymerization uses thiocarbonylthio compounds, such as dithioesters, thiocarbamates, and xanthates, to mediate the polymerization via a reversible chain-transfer process. As with other controlled radical polymerization techniques, RAFT polymerizations can be performed under conditions that favor low dispersity and a pre-chosen molecular weight. RAFT polymerization can be used to design polymers of complex architectures, such as linear block copolymers, comb-like, star, brush polymers, dendrimers and cross-linked networks.

In step-growth polymerization, the Carothers equation gives the degree of polymerization, Xn, for a given fractional monomer conversion, p.

In polymer chemistry, chain termination is any chemical reaction that ceases the formation of reactive intermediates in a chain propagation step in the course of a polymerization, effectively bringing it to a halt.

<span class="mw-page-title-main">Photopolymer</span> Resin that cures when exposed to light of appropriate wavelengths

A photopolymer or light-activated resin is a polymer that changes its properties when exposed to light, often in the ultraviolet or visible region of the electromagnetic spectrum. These changes are often manifested structurally, for example hardening of the material occurs as a result of cross-linking when exposed to light. An example is shown below depicting a mixture of monomers, oligomers, and photoinitiators that conform into a hardened polymeric material through a process called curing.

In polymer chemistry, chain transfer is a polymerization reaction by which the activity of a growing polymer chain is transferred to another molecule:

In polymer chemistry, the kinetic chain length of a polymer is the average number of units called monomers added to a growing chain during chain-growth polymerization. During this process, a polymer chain is formed when monomers are bonded together to form long chains known as polymers. Kinetic chain length is defined as the average number of monomers that react with an active center such as a radical from initiation to termination.

In polymer chemistry, cationic polymerization is a type of chain growth polymerization in which a cationic initiator transfers charge to a monomer, which then becomes reactive. This reactive monomer goes on to react similarly with other monomers to form a polymer. The types of monomers necessary for cationic polymerization are limited to alkenes with electron-donating substituents and heterocycles. Similar to anionic polymerization reactions, cationic polymerization reactions are very sensitive to the type of solvent used. Specifically, the ability of a solvent to form free ions will dictate the reactivity of the propagating cationic chain. Cationic polymerization is used in the production of polyisobutylene and poly(N-vinylcarbazole) (PVK).

Expanding monomers are monomers which increase in volume (expand) during polymerization. They can be added to monomer formulations to counteract the usual volume shrinking to manufacture products with higher quality and durability. Volume Shrinkage is in first line for the unmeltable thermosets a problem, since those are of fixed shape after polymerization completed.

In organosulfur chemistry, the thiol-ene reaction is an organic reaction between a thiol and an alkene to form a thioether. This reaction was first reported in 1905, but it gained prominence in the late 1990s and early 2000s for its feasibility and wide range of applications. This reaction is accepted as a click chemistry reaction given the reactions' high yield, stereoselectivity, high rate, and thermodynamic driving force.

<span class="mw-page-title-main">Allyl glycidyl ether</span> Chemical compound

Allyl glycidyl ether is an organic compound used in adhesives and sealants and as a monomer for polymerization reactions. It is formally the condensation product of allyl alcohol and glycidol via an ether linkage. Because it contains both an alkene and an epoxide group, either group can be reacted selectively to yield a product where the other functional group remains intact for future reactions.

References

  1. ↑ "Polymerization". The IUPAC Compendium of Chemical Terminology. 2014. doi: 10.1351/goldbook.P04740 .
  2. ↑ Young, Robert J. (1981). Introduction to polymers. London: Chapman and Hall. ISBN   0-412-22170-5. OCLC   8086791.
  3. 1 2 3 4 5 6 7 8 Clayden, Jonathan; Greeves, Nick; Warren, Stuart (2001). Organic chemistry. Oxford: Oxford University Press. pp. 1450–1466. ISBN   0-19-850347-4. OCLC   43338068.
  4. ↑ Manas, Chanda (2023). Introduction to Polymer Science and Chemistry:A Problem-Solving Approach (2nd ed.). CRC Press (published 2013). ISBN   978-1-4665-5385-9.
  5. ↑ Cowie, J. M. G. (2008). Polymers : chemistry and physics of modern materials. V. Arrighi (3rd ed.). Boca Raton: CRC Press. p. 4. ISBN   978-0-8493-9813-1. OCLC   82473191.
  6. 1 2 3 4 Allcock, H. R.; Lampe, Frederick Walter; Mark, James E. (2003). Contemporary polymer chemistry. Frederick Walter Lampe, James E. Mark (3rd ed.). Upper Saddle River, N.J.: Pearson/Prentice Hall. pp. 29–30. ISBN   0-13-065056-0. OCLC   51096012.
  7. 1 2 Fried, Joel R. (2003). Polymer science and technology (2nd ed.). Upper Saddle River, NJ: Prentice Hall Professional Technical Reference. p. 23. ISBN   0-13-018168-4. OCLC   51769096.
  8. 1 2 Jeremic, Dusan (2014). "Polyethylene". Ullmann's Encyclopedia of Industrial Chemistry . Weinheim: Wiley-VCH. pp. 1–42. doi:10.1002/14356007.a21_487.pub3. ISBN   9783527306732.
  9. ↑ McKenzie, Thomas G.; Fu, Qiang; Wong, Edgar H. H.; Dunstan, Dave E.; Qiao, Greg G. (23 June 2015). "Visible Light Mediated Controlled Radical Polymerization in the Absence of Exogenous Radical Sources or Catalysts" (PDF). Macromolecules. 48 (12): 3864–3872. Bibcode:2015MaMol..48.3864M. doi:10.1021/acs.macromol.5b00965. ISSN   0024-9297.
  10. ↑ Kaya, Kerem (January 2023). "A green and fast method for PEDOT: Photoinduced step-growth polymerization of EDOT". Reactive and Functional Polymers. 182: 105464. doi:10.1016/j.reactfunctpolym.2022.105464.
  11. ↑ Soto, Marc; Sebastián, Rosa María; Marquet, Jordi (2014). "Photochemical Activation of Extremely Weak Nucleophiles: Highly Fluorinated Urethanes and Polyurethanes from Polyfluoro Alcohols". J. Org. Chem. 79 (11): 5019–5027. doi:10.1021/jo5005789. PMID   24820955.
  12. ↑ Wang, Xifan; Schmidt, Franziska; Hanaor, Dorian; Kamm, Paul H.; Li, Shuang; Gurlo, Aleksander (May 2019). "Additive manufacturing of ceramics from preceramic polymers". Additive Manufacturing. 27: 80–90. arXiv: 1905.02060 . doi:10.1016/j.addma.2019.02.012. S2CID   104470679.
  13. ↑ Mills, Benjamin; Grant-Jacob, James A; Feinaeugle, Matthias; Eason, Robert W (17 June 2013). "Single-pulse multiphoton polymerization of complex structures using a digital multimirror device" (PDF). Optics Express. 21 (12): 14853–8. Bibcode:2013OExpr..2114853M. doi: 10.1364/oe.21.014853 . ISSN   1094-4087. PMID   23787672.
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