Cage effect

Last updated
Free radicals in solvent can potentially react with a monomer within the solvent cage or diffuse out. CageEffect.tif
Free radicals in solvent can potentially react with a monomer within the solvent cage or diffuse out.

In chemistry, the cage effect [1] (also known as geminate recombination [2] ) describes how the properties of a molecule are affected by its surroundings. First introduced by James Franck and Eugene Rabinowitch [3] [4] in 1934, the cage effect suggests that instead of acting as an individual particle, molecules in solvent are more accurately described as an encapsulated particle. The encapsulated molecules or radicals are called cage pairs or geminate pairs. [5] [6] In order to interact with other molecules, the caged particle must diffuse from its solvent cage. The typical lifetime of a solvent cage is 10-11 seconds. [7] Many manifestations of the cage effect exist. [8]

Contents

In free radical polymerization, radicals formed from the decomposition of an initiator molecule are surrounded by a cage consisting of solvent and/or monomer molecules. [6] Within the cage, the free radicals undergo many collisions leading to their recombination or mutual deactivation. [5] [6] [9] This can be described by the following reaction:

[9]

After recombination, free radicals can either react with monomer molecules within the cage walls or diffuse out of the cage. In polymers, the probability of a free radical pair to escape recombination in the cage is 0.1 – 0.01 and 0.3-0.8 in liquids. [5] In unimolecular chemistry, geminate recombination has first been studied in the solution phase using iodine molecules [10] and heme proteins. [11] [12] In the solid state, geminate recombination has been demonstrated with small molecules trapped in noble gas solid matrices [13] and in triiodide crystalline compounds. [14] [15] [16]

Cage recombination efficiency

The cage effect can be quantitatively described as the cage recombination efficiency Fc where:

[9]

Here Fc is defined as the ratio of the rate constant for cage recombination (kc) to the sum of the rate constants for all cage processes. [9] According to mathematical models, Fc is dependent on changes on several parameters including radical size, shape, and solvent viscosity. [9] [17] [18] It is reported that the cage effect will increase with an increase in radical size and a decrease in radical mass.

Initiator efficiency

In free radical polymerization, the rate of initiation is dependent on how effective the initiator is. [6] Low initiator efficiency, ƒ, is largely attributed to the cage effect. The rate of initiation is described as:

[6]

where Ri is the rate of initiation, kd is the rate constant for initiator dissociation, [I] is the initial concentration of initiator. Initiator efficiency represents the fraction of primary radicals R·, that actually contribute to chain initiation. Due to the cage effect, free radicals can undergo mutual deactivation which produces stable products instead of initiating propagation – reducing the value of ƒ. [6]

See also

Related Research Articles

<span class="mw-page-title-main">Chemical reaction</span> Process that results in the interconversion of chemical species

A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking of chemical bonds between atoms, with no change to the nuclei, and can often be described by a chemical equation. Nuclear chemistry is a sub-discipline of chemistry that involves the chemical reactions of unstable and radioactive elements where both electronic and nuclear changes can occur.

<span class="mw-page-title-main">Polymerization</span> Chemical reaction to form polymer chains

In polymer chemistry, polymerization, or polymerisation, is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many forms of polymerization and different systems exist to categorize them.

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.

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 are emulsified in a continuous phase of water. Water-soluble polymers, such as certain polyvinyl alcohols or hydroxyethyl celluloses, can also be used to act as emulsifiers/stabilizers. The name "emulsion polymerization" is a misnomer that arises from a historical misconception. Rather than occurring in emulsion droplets, polymerization takes place in the latex/colloid particles that form spontaneously in the first few minutes of the process. These latex particles are typically 100 nm in size, and are made of many individual polymer chains. The particles are prevented from coagulating with each other because each particle is surrounded by the surfactant ('soap'); the charge on the surfactant repels other particles electrostatically. When water-soluble polymers are used as stabilizers instead of soap, the repulsion between particles arises because these water-soluble polymers form a 'hairy layer' around a particle that repels other particles, because pushing particles together would involve compressing these chains.

<span class="mw-page-title-main">Micelle</span> Group of fatty molecules suspended in liquid by soaps and/or detergents

A micelle or micella is an aggregate of surfactant amphipathic lipid molecules dispersed in a liquid, forming a colloidal suspension. A typical micelle in water forms an aggregate with the hydrophilic "head" regions in contact with surrounding solvent, sequestering the hydrophobic single-tail regions in the micelle centre.

Chain-growth polymerization (AE) or chain-growth polymerisation (BE) is a polymerization technique where unsaturated 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">Radical anion</span> Free radical species

In organic chemistry, a radical anion is a free radical species that carries a negative charge. Radical anions are encountered in organic chemistry as reduced derivatives of polycyclic aromatic compounds, e.g. sodium naphthenide. An example of a non-carbon radical anion is the superoxide anion, formed by transfer of one electron to an oxygen molecule. Radical anions are typically indicated by .

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.

The degree of polymerization, or DP, is the number of monomeric units in a macromolecule or polymer or oligomer molecule.

Atom transfer radical polymerization (ATRP) is an example of a reversible-deactivation radical polymerization. Like its counterpart, ATRA, or atom transfer radical addition, ATRP is a means of forming a carbon-carbon bond with a transition metal catalyst. Polymerization from this method is called atom transfer radical addition polymerization (ATRAP). As the name implies, the atom transfer step is crucial in the reaction responsible for uniform polymer chain growth. ATRP was independently discovered by Mitsuo Sawamoto and by Krzysztof Matyjaszewski and Jin-Shan Wang in 1995.

<span class="mw-page-title-main">Reversible addition−fragmentation chain-transfer polymerization</span>

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.

<span class="mw-page-title-main">Branching (polymer chemistry)</span> Attachment of side chains to the backbone chain of a polymer

In polymer chemistry, branching is the regular or irregular attachment of side chains to a polymer's backbone chain. It occurs by the replacement of a substituent on a monomer subunit by another covalently-bonded chain of that polymer; or, in the case of a graft copolymer, by a chain of another type. Branched polymers have more compact and symmetrical molecular conformations, and exhibit intra-heterogeneous dynamical behavior with respect to the unbranched polymers. In crosslinking rubber by vulcanization, short sulfur branches link polyisoprene chains into a multiple-branched thermosetting elastomer. Rubber can also be so completely vulcanized that it becomes a rigid solid, so hard it can be used as the bit in a smoking pipe. Polycarbonate chains can be crosslinked to form the hardest, most impact-resistant thermosetting plastic, used in safety glasses.

A Norrish reaction in organic chemistry is a photochemical reaction taking place with ketones and aldehydes. Such reactions are subdivided into Norrish type I reactions and Norrish type II reactions. The reaction is named after Ronald George Wreyford Norrish. While of limited synthetic utility these reactions are important in the photo-oxidation of polymers such as polyolefins, polyesters, certain polycarbonates and polyketones.

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, ν, 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.

<span class="mw-page-title-main">Radical (chemistry)</span> Atom, molecule, or ion that has an unpaired valence electron; typically highly reactive

In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemically reactive. Many radicals spontaneously dimerize. Most organic radicals have short lifetimes.

Living free radical polymerization is a type of living polymerization where the active polymer chain end is a free radical. Several methods exist. IUPAC recommends to use the term "reversible-deactivation radical polymerization" instead of "living free radical polymerization", though the two terms are not synonymous.

In 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).

Radical disproportionation encompasses a group of reactions in organic chemistry in which two radicals react to form two different non-radical products. Radicals in chemistry are defined as reactive atoms or molecules that contain an unpaired electron or electrons in an open shell. The unpaired electrons can cause radicals to be unstable and reactive. Reactions in radical chemistry can generate both radical and non-radical products. Radical disproportionation reactions can occur with many radicals in solution and in the gas phase. Due to the reactive nature of radical molecules, disproportionation proceeds rapidly and requires little to no activation energy. The most thoroughly studied radical disproportionation reactions have been conducted with alkyl radicals, but there are many organic molecules that can exhibit more complex, multi-step disproportionation reactions.

Ionic polymerization is a chain-growth polymerization in which active centers are ions or ion pairs. It can be considered as an alternative to radical polymerization, and may refer to anionic polymerization or cationic polymerization.

References

  1. ↑ Chemistry (IUPAC), The International Union of Pure and Applied. "IUPAC - cage effect (C00771)". goldbook.iupac.org. doi: 10.1351/goldbook.c00771 . Retrieved 2022-03-28.
  2. ↑ Chemistry (IUPAC), The International Union of Pure and Applied. "IUPAC - geminate recombination (G02603)". goldbook.iupac.org. Retrieved 2022-03-28.
  3. ↑ Rabinowitch, Franck (1934). "Some remarks about free radicals and the photochemisty of solutions". Transactions of the Faraday Society. 30: 120–130. doi:10.1039/tf9343000120.
  4. ↑ Rabinowitch, E (1936). "The collison[sic] mechanism and the primary photochemical process in solutions". Transactions of the Faraday Society. 32: 1381–1387. doi:10.1039/tf9363201381.
  5. 1 2 3 Denisov, E.T. (1984). "Cage effects in a polymer matrix". Macromolecular Chemistry and Physics. 8: 63–78. doi:10.1002/macp.1984.020081984106.
  6. 1 2 3 4 5 6 Chanda, Manas (2013). Introduction to Polymer Science and Chemistry: A problem solving approach. New York: CRC Press. pp. 291, 301–303.
  7. ↑ Herk, L.; Feld, M.; Szwarc, M. (1961). "Studies of "Cage" Reactions". J. Am. Chem. Soc. 83 (14): 2998–3005. doi:10.1021/ja01475a005.
  8. ↑ "Radical cage effects" (PDF).
  9. 1 2 3 4 5 Braden, Dale, A. (2001). "Solvent cage effects. I. Effect of radical mass and size on radical cage pair recombination efficiency. II. Is geminate recombination of polar radicals sensitive to solvent polarity?". Coordination Chemistry Reviews. 211: 279–294. doi:10.1016/s0010-8545(00)00287-3.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  10. ↑ Schwartz, Benjamin J.; King, Jason C.; Harris, Charles B. (1994), Simon, John D. (ed.), "The Molecular Basis of Solvent Caging", Ultrafast Dynamics of Chemical Systems, Dordrecht: Springer Netherlands, pp. 235–248, doi:10.1007/978-94-011-0916-1_8, ISBN   978-94-011-0916-1 , retrieved 2022-03-28
  11. ↑ Chernoff, D A; Hochstrasser, R M; Steele, A W (1980-10-01). "Geminate recombination of O2 and hemoglobin". Proceedings of the National Academy of Sciences. 77 (10): 5606–5610. doi: 10.1073/pnas.77.10.5606 . ISSN   0027-8424. PMC   350115 . PMID   6932659.
  12. ↑ Rohlfs, R J; Olson, J S; Gibson, Q H (1988-02-05). "A comparison of the geminate recombination kinetics of several monomeric heme proteins". Journal of Biological Chemistry. 263 (4): 1803–1813. doi: 10.1016/s0021-9258(19)77948-4 . ISSN   0021-9258. PMID   3338995.
  13. ↑ Apkarian, V. A.; Schwentner, N. (1999-06-09). "Molecular Photodynamics in Rare Gas Solids". Chemical Reviews. 99 (6): 1481–1514. doi:10.1021/cr9404609. ISSN   0009-2665. PMID   11849000.
  14. ↑ Cerullo, Giulio; Garavelli, Marco (2017-05-27). "Caught in the act". Nature Chemistry. 9 (6): 506–507. doi:10.1038/nchem.2780. ISSN   1755-4349. PMID   28537591.
  15. ↑ Poulin, Peter R.; Nelson, Keith A. (2006-09-22). "Irreversible Organic Crystalline Chemistry Monitored in Real Time". Science. 313 (5794): 1756–1760. doi: 10.1126/science.1127826 . PMID   16946037. S2CID   35002522.
  16. ↑ Xian, Rui; Corthey, Gastón; Rogers, David M.; Morrison, Carole A.; Prokhorenko, Valentyn I.; Hayes, Stuart A.; Miller, R. J. Dwayne (2017-03-27). "Coherent ultrafast lattice-directed reaction dynamics of triiodide anion photodissociation". Nature Chemistry. 9 (6): 516–522. doi:10.1038/nchem.2751. hdl: 20.500.11820/52dbea74-99b4-454b-aac2-56c7be20947b . ISSN   1755-4349. PMID   28537597.
  17. ↑ Noyes, R.M. (1954). "A Treatment of Chemical Kinetics with Special Applicability to Diffusion Controlled Reactions". J. Chem. Phys. 22 (8): 1349–1359. Bibcode:1954JChPh..22.1349N. doi:10.1063/1.1740394.
  18. ↑ Noyes, R.M. (1961). "Effects of diffusion rates on chemical kinetics". Progr. React. Kinet. 1: 129–60.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.