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Photochemical Smog
Published in Stanley E. Manahan, Environmental Chemistry, 2022
The hydroxyl radical may react with other organic compounds, maintaining the chain reaction. Gas-phase reaction chains commonly have many steps. Furthermore, chain-branching reactions take place in which a free radical reacts with an excited molecule, causing it to produce two new radicals. Chain termination may occur in several ways, including reaction of two free radicals, adduct formation with nitric oxide or nitrogen dioxide (which, because of their odd numbers of electrons, are themselves stable free radicals), or reaction of the radical with a solid particle surface. 2HO•→H2O2HO•+NO2+M→HNO3+M
Characteristics of Polymers and Polymerization Processes
Published in Manas Chanda, Plastics Technology Handbook, 2017
Chain polymerization involves three processes: chain initiation, chain propagation, and chain termination. (A fourth process, chain transfer, may also be involved, but it may be regarded as a combination of chain termination and chain initiation.) Chain initiation occurs by an attack on the monomer molecule by a free radical, a cation, or an anion; accordingly, the chain polymerization processes are called free-radical polymerization, cationic polymerization, or anionic polymerization. (In coordination addition or chain polymerization, described below separately, the chain initiation step is, however, assumed to be the insertion of the first monomer molecule into a transition metal–carbon bond.) A free radical is a reactive substance having an unpaired electron and is usually formed by the decomposition of a relatively unstable material called an initiator. Benzoyl peroxide is a common free-radical initiator and can produce free radicals by thermal decomposition as
Photochemical Smog
Published in Stanley Manahan, Environmental Chemistry, 2017
leaving an aldehyde or ketone. The hydroxyl radical may react with other organic compounds, maintaining the chain reaction. Gas-phase reaction chains commonly have many steps. Furthermore, chain-branching reactions take place in which a free radical reacts with an excited molecule, causing it to produce two new radicals. Chain termination may occur in several ways, including reaction of two free radicals 2HO•→H2O2
Atom transfer radical polymerization initiated by activator generated by electron transfer in emulsion media: a review of recent advances and challenges from an engineering perspective
Published in Journal of Dispersion Science and Technology, 2023
Mohammed Awad, Ramdhane Dhib, Thomas Duever
Several researchers contributed to the development of CRP techniques to produce polymers with relatively good control of the molecular weight distribution (MWD).[11–14] The inherent features of the CRP polymerization technique include well-controlled molecular weight (MW), narrow MWD, chain-end functionality and chain architecture, boost the synthesis of polymers.[14,15] CRP combines the advantages of both living ionic polymerization and FRP.[15–17] The reaction mechanism of CRP is fundamentally similar to that of FRP. The initiation of growing polymer chains takes place as the monomer reacts with the primary radicals. Since chain propagation has a high reaction rate, it is considered the primary chain growth. Lastly, the chain termination reactions consist of either combination, disproportionation, or both.[18–20]
Thermal analysis of the styrene bulk polymerization and characterization of polystyrene initiated by two methods
Published in Chemical Engineering Communications, 2019
Xin-Miao Liang, Hui-Chun Jiang, Jiang-Lai Fang, Min- Hua, Xu-Hai Pan, Jun-Cheng Jiang
Chain termination occurs in two ways: monoradical and biradical-coupling termination. Monoradical termination occurs for the following three reasons: (1) free radicals react with impurities; (2) free radicals collide with the surface of the metal reactor, termed “Stick wall” termination; and (3) free radicals are surrounded by a highly viscous polymer. Similarly, biradical-coupling termination occurs for the following two reasons: (1) two free radicals are linked into a polymer molecular chain, termed coupling termination; and (2) free radical captures the hydrogen atom of another free radical to produce two polymer chains, called disproportionation termination.
Low density polyethylene tubular reactor control using state space model predictive control
Published in Chemical Engineering Communications, 2021
D. Muhammad, Z. Ahmad, N. Aziz
In this process, LDPE is produced via high pressure free radical polymerization of ethylene gas. There are many reaction steps involved in the mentioned polymerization process leading to the complex polymer structure with short and long chain branches. Brandolin et al. (1996) and Agrawal et al. (2006) study had been referred to obtain the reaction mechanisms and kinetic parameters for this work. Generally, the free radical polymerization reaction mechanism involves five fundamental reactions:Initiator decomposition - Free radicals are formed due to the breakdown of initiators such as organic peroxides and oxygen. These free radicals are reactive intermediate with an unpaired electron.Chain Initiation – The combination of free radicals with monomer molecules will form polymer radicals. These polymer radicals (also called live polymer) will be the preliminary chain for the polymerization process.Chain Propagation – Polymer radicals propagate by reacting with other monomer molecules in a successive series of reactions to form polymer chains.Chain termination - Polymer radicals are terminated when two polymer radicals of the same or different chain length are combined (termination by combination) or disproportionate to form dead polymer chains (termination by disproportionation).Chain Transfer - Polymer radicals can undergo reaction with the monomer (chain transfer to monomer), agents (chain transfer to agents), and polymer molecules (chain transfer to polymer). It can also break away (e.g., beta scission) or jump on the same or another polymer chain (e.g., back-biting).