Initiating and propagating radicals

The addition of certain substances suppresses the polymerization of monomers. These substances act by reacting with the initiating and propagating radicals and converting them to either nonradical species or radicals of reactivity too low to undergo propagation. Such polymerization suppressors are classified according to their effectiveness. Inhibitors stop every radical, and polymerization is completely halted until they are consumed. Retarders are less efficient and stop only a portion of the radicals. In this case, polymerization occurs,... 

Inhibitors are substances that stop radical polymerization. Those have quite a rapid action on initiator and propagator radicals. They transformed them either into a non radical form or into a radical with low reactivity in propagation reaction. They will block the radical polymerization over a so-called induction period that will vary according to its concentration in the reaction environment and to the experimental conditions (temperature, catalyst, etc.). Beyond this induction period, polymerization will occur at the same rate as it would if the inhibitor was not present, (ref. 8). 

Direct detection of intermediate species in chemical reactions of polymer materials, (a) The identification and the structure of the initiating and propagating radicals and the kinetics of the formation are important for clarifying polymerization mechanisms, (b) The free radicals formed by irradiation of polymers with ionizing... 

Radical polymerization is initiated by a free radical, which subsequently adds to a vinyl or diene monomer to produce a propagating radical. To obtain information about the structure and concentrations of initiating and propagating radicals in radical polymerizations, use of ESR spectroscopy has called the interest of physical or polymer chemists. However, ESR measurements on these radicals in solution poly merizati on were found to be difficult, except for the case where polymers precipitated, because otherwise the concentrations of the radicals were too low. Thus, these measurements had to be limited to polymerization systems in highly viscous solutions or in the solid state, where the disappearance of free radicals by bimolecular reactions is suppressed. Bresler et al. -i7) succeeded for the first time in obtaining ESR spectra of free radicals which were produced in homogeneous bulk polymerization of methyl methacrylate (MMA), methyl acrylate (MA) and vinyl acetate (VAc) at conversions of 50-60% (in the gel state). 

Certain substances, when added to the polymerization system, may react with the initiating and propagating radicals converting them either to nonradical species or to less reactive radicals to undergo propagation. Such additives are classified according to their effectiveness ... 

FIGURE 4 21 The initiation and propagation steps in the free radical mechanism for the chlorination of methane Together the two propaga tion steps give the overall equation for the reaction... 

Write equations for the initiation and propagation steps for the formation of dichloromethane by free radical chlorination of chloromethane... 

Chain-Growth Associative Thickeners. Preparation of hydrophobically modified, water-soluble polymer in aqueous media by a chain-growth mechanism presents a unique challenge in that the hydrophobically modified monomers are surface active and form micelles (50). Although the initiation and propagation occurs primarily in the aqueous phase, when the propagating radical enters the micelle the hydrophobically modified monomers then polymerize in blocks. In addition, the hydrophobically modified monomer possesses a different reactivity ratio (42) than the unmodified monomer, and the composition of the polymer chain therefore varies considerably with conversion (57). The most extensively studied monomer of this class has been acrylamide, but there have been others such as the modification of PVAlc. Pyridine (58) was one of the first chain-growth polymers to be hydrophobically modified. This modification is a post-polymerization alkylation reaction and produces a random distribution of hydrophobic units. 

Fig. 3. Polymerization initiation and propagation by radiation-generated free radicals. A is the initiating radical produced by irradiating the Hquid coating. (1) represents the Hquid monomer—unsaturated polymer reactive coating system. R is functional. (2) is the growing polymer chain (free radical). The cured...

In the period 1910-1950 many contributed to the development of free-radical polymerization.1 The basic mechanism as we know it today (Scheme 1.1), was laid out in the 1940s and 50s.7 9 The essential features of this mechanism are initiation and propagation steps, which involve radicals adding to the less substituted end of the double bond ("tail addition"), and a termination step, which involves disproportionation or combination between two growing chains. 

Some characteristics of free-radical terpolymerization of tri-butylstannyl methacrylate, styrene and maleic anhydride governed by the pentacoordination state of the tin atom are reported in Refs. 95),96). It is shown that a coordination-bound monomer has a considerable effect on chain initiation and propagation. Copolymerization mainly involves the participation of complex-bound monomers. 

Ionic polymerisation forms high Molecular weight products and reactions can be easily carried out at room temperature or low temperature. Ionic polymerisations are chain reactions and are analogous to radical chain reactions. They also involve initiation and propagation steps. 

It is unfortunate that many workers have not appreciated how essential a clue to the kinetics can be provided by the kinetic order of the whole reaction curve. The use of initial rates was carried over from the practice of radical polymerisation, and it can be very misleading. This was in fact shown by Gwyn Williams in the first kinetic study of a cationic polymerization, in which he found the reaction orders deduced from initial rates and from analysis of the whole reaction curves to be signfficantly different [111]. Since then several other instances have been recorded. The reason for such discrepancies may be that the initiation is neither much faster, nor much slower than the propagation, but of such a rate that it is virtually complete by the time that a small, but appreciable fraction of the monomer, say 5 to 20%, has been consumed. Under such conditions the overall order of the reaction will fall from the initial value determined by the consumption of monomer by simultaneous initiation and propagation, and of catalyst by initiation, to a lower value characteristic of the reaction when the initiation reaction has ceased. 

The ability to conduct radical reactions without the use of tin reagents is important. Allylic triflones have been used to conduct allylation reactions on a range of substrates (39) as a replacement for allyltributylstannane (Scheme 28). The main limitation was that unactivated or trisubstituted triflones failed to undergo reactions. In other nontin radical methods, arenesulfonyl halides have been used as functional initiators in the CuCl/4,4 -dinonyl-2, 2 -bipyridine-catalysed living atom-transfer polymerization of styrenes, methacrylates, and acrylates.The kinetics of initiation and propagation were examined with a range of substituted arylsulfonyl halides with initiator efficiency measured at 100%. 

Scheme 5.1 Initiation and propagation steps for radical-based hydrosilylation...

Combustion is a chain reaction that can be initiated and propagated by free radicals such as the hydroxyl free radical. The hydroxyl radical may be produced by reaction of oxygen with macroalkyl radicals as shown below ... 

Polymerization of the monomer in solution undoubtedly takes place but does not contribute significantly, since the monomer concentration is low and propagating radicals would precipitate out of aqueous solution at very small (oligomeric) size. The micelles act as a meeting place for the organic (oil-soluble) monomer and the water-soluble initiator. The micelles are favored as the reaction site because of their high monomer concentration (similar to bulk monomer concentration) compared to the monomer in solution. As polymerization proceeds, the micelles grow by the addition of monomer from the aqueous solution whose concentration is replenished by dissolution of monomer from the monomer droplets. A simplified schematic representation of an emulsion polymerization system is shown in Fig. 4-1. The system consists of three types of particles monomer droplets, inactive micelles in which... 

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