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Alkyl Halides and Substitution Reactions
Published in Michael B. Smith, A Q&A Approach to Organic Chemistry, 2020
The chlorination of 2-methylpropane is a radical chain process. The process begins by exposure of Cl—Cl to UV light, which initiates homolytic cleavage and formation of two chlorine radicals in what is called a chain initiation step. The chlorine radical reacts with 2-methylpropane to give HCl (one product) and the carbon radical. A reaction that produces a neutral molecule and a new radical is a chain-carrier step or a chain propagation step. The chain carrier radical reacts with another molecule of Cl—Cl to produce the 2-chloro-2-methylpropane product and a chain-carrying chlorine radical in another chain propagation step. If two of the radicals collide, either a chlorine radical or a carbon radical as shown (a coupling reaction), they form a neutral molecule but do not produce a new radical chain carrier. Such a reaction stops the radical process and is called a chain termination step. To begin the process again, a new chain initiation step is necessary.
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
UV Radiation Processes (with Ruben Rivera)
Published in Jiri George Drobny, Radiation Technology for Polymers, 2020
In the chain propagation, mainly the monomer is consumed and the propagation rate depends on the monomer concentration, [M], and the concentration of polymeric radicals, [•Pn]. The quantity kp is the propagation rate constant.
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).