Radical chain polymerization retardation

Nitroxyl radicals (AmO ) are known to react rapidly with alkyl radicals and efficiently retard the radical polymerization of hydrocarbons [7]. At the same time, only aromatic nitroxyls are capable of reacting with alkylperoxyl radicals [10,39] and in this case the chain termination in the oxidation of saturated hydrocarbons occurs stoichiometrically. However, in the processes of oxidation of alcohols, alkenes, and primary and secondary aliphatic amines in which the chain reaction involves the HOT, >C(0H)02 , and >C(NHR)02 radicals, possessing the... 

Finally, 2-vinyl furan 2a displays an intermediate behaviour in that it polymerizes slowly (because "normal" radicals formed from addition to the vinyl group are relatively stabilized), but gives modest DPs and limiting yields due to the fact that the furan rings pendant to the polymer chains act as radical traps which retard the polymenzation and inhibit it above a certain concentration (equivalent to a given polymer yield). 

We often want to prevent or retard free-radical reactions. For example, oxygen in the air oxidizes and spoils foods, solvents, and other compounds mostly by free-radical chain reactions. Chemical intermediates may decompose or polymerize by free-radical chain reactions. Even the cells in living systems are damaged by radical reactions, which can lead to aging, cancerous mutations, or cell death. 

Oxygen in the air oxidizes and spoils foods, solvents, and other compounds by free-radical chain reactions. Chemical intermediates may decompose or polymerize by free-radical chain reactions. Even the cells in living systems are damaged by radical reactions, which can lead to aging, cancerous mutations, or cell-death. We often want to prevent or retard free-radical reactions. Radical inhibitors are often added to food and chemicals to retard spoilage by radical chain reactions. Butylated hydroxyanisole (BHA) is often added to food as an antioxidant. It stops oxidation by reacting with radical intermediates to form a relatively stable free radical intermediate (BHA radical). The BHA radical can react with a second free radical to form an even more stable quinone with all its electrons paired (Scheme 4.64). 

Generally, the degradation of polymers is thought to be a free radical chain reaction through (3-scission of polymer chains. Ceo, described as a radical sponge [30], has high reactivity toward free radicals, in that it has 30 carbon-carbon double bonds and can trap more than 34 free radicals [30, 36-38], Thus, Ceo can trap the macromolecular or any other radicals created by the pyrolysis of polymers and then form a gelled ball network in situ. The network can increase the melt viscosity and consequently slow the combustion of polymeric materials. Thus, in another sense, Ceo naay be considered to be a gas phase flame retardant. 

Also, from UT to 58, sponsored by DOD, an extended study on polymerization kinetics was undertaken, particularly of allylic compounds, in cooperation with N. Gaylord, N. Field, H. Starkweather, A. Adicoff and M. Litt, rounding out our knowledge of degradative free radical chain transfer and retarded polymerization. This work led to studies of the mechanism of peroxide decomposition as a function of environments, especially of the cage effect, with W. 

Like the DT system, radicals must be generated by a conventional initiation to start and maintain polymerization in tbe RT system, too. The RT is a chain transfer reaction and does not basically change the radical concentration, and hence the steady-state kinetics, eqn [21], should hold. The catalyst radical (activator A") can tmdergo reactions with other radicals, possibly causing retardation from eqn [21] (see Section 3.05.8.2). 

Certain substances can be added to a polymerizing system which affect the rate of polymerization. Retarders are added to slow down the rate and inhibitors will completely stop polymerization. Both types of additive tend to act in the same way and differ only in their efficiency in rendering the growing chains inactive. The effect of the addition of a retarder or an inhibitor to a free-radical polymerization system is shown schematically in Fig. 2.6. 

We conclude this section by noting an extreme case of chain transfer, a reaction which produces radicals of such low reactivity that polymerization is effectively suppressed. Reagents that accomplish this are added to commercial monomers to prevent their premature polymerization during storage. These substances are called either retarders or inhibitors, depending on the degree of protection they afford. Such chemicals must be removed from monomers prior to use, and failure to achieve complete purification can considerably affect the polymerization reaction. 

An example of a commercial semibatch polymerization process is the early Union Carbide process for Dynel, one of the first flame-retardant modacryhc fibers (23,24). Dynel, a staple fiber that was wet spun from acetone, was introduced in 1951. The polymer is made up of 40% acrylonitrile and 60% vinyl chloride. The reactivity ratios for this monomer pair are 3.7 and 0.074 for acrylonitrile and vinyl chloride in solution at 60°C. Thus acrylonitrile is much more reactive than vinyl chloride in this copolymerization. In addition, vinyl chloride is a strong chain-transfer agent. To make the Dynel composition of 60% vinyl chloride, the monomer composition must be maintained at 82% vinyl chloride. Since acrylonitrile is consumed much more rapidly than vinyl chloride, if no control is exercised over the monomer composition, the acrylonitrile content of the monomer decreases to approximately 1% after only 25% conversion. The low acrylonitrile content of the monomer required for this process introduces yet another problem. That is, with an acrylonitrile weight fraction of only 0.18 in the unreacted monomer mixture, the low concentration of acrylonitrile becomes a rate-limiting reaction step. Therefore, the overall rate of chain growth is low and under normal conditions, with chain transfer and radical recombination, the molecular weight of the polymer is very low. 

Because of the low reactivity and tendency to undergo chain transfer, small additions of most aHyl compounds retard polymerization of typical vinyl monomers ia free-radical systems (1,3) and may be useful ia controlling molecular weight and stmcture ia polymers. 

Chain transfer, the reaction of a propagating radical with a non-radical substrate to produce a dead polymer chain and a new radical capable of initiating a new polymer chain, is dealt with in Chapter 6. There are also situations intermediate between chain transfer and inhibition where the radical produced is less reactive than the propagating radical but still capable of reinitiating polymerization. In this case, polymerization is slowed and the process is termed retardation or degradative chain transfer. The process is mentioned in Section 5.3 and, when relevant, in Chapter 6. 

Transfer to monomer is of particular importance during the polymerization of allyl esters (113, X=()2CR), ethers (113, X=OR), amines (113, X=NR2) and related monomcrs.iw, 8, lb2 The allylic hydrogens of these monomers arc activated towards abstraction by both the double bond and the heteroatom substituent (Scheme 6.31). These groups lend stability to the radical formed (114) and are responsible for this radical adding monomer only slowly. This, in turn, increases the likelihood of side reactions (i.e. degradative chain transfer) and causes the allyl monomers to retard polymerization. 

The hazards of a rigid classification of substances which may modify the course of a free radical polymerization are well illustrated by the examples of inhibitors and retarders which have been cited. The distinction between an inhibitor or retarder, on the one hand, and a co-monomer or a transfer agent, on the other, is not sharply defined. Moreover, if the substance is a free radical, it is potentially either an initiator or an inhibitor, and it may perform both functions as in the case of triphenylmethyl. If the substance with which the chain radicals react is a molecule rather than a radical, three possibilities may arise (i) The adduct radicals may be completely unreactive toward monomer. They must then disappear ultimately through mutual interaction, and we have a clear-cut case of either inhibition or retarda-... 

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