Inhibition and retardation

Inhibitors and retarders are used to stabilize monomers during storage or during processing e.g. synthesis, distillation). They are often used to quench polymerization when a desired conversion has been achieved. Tliey may also be used lo regulate or control the kinetics of a polymerization process. 

Inhibitors or retarders that give inert products are called ideal . The term ideal inhibitor has also been used to describe a species that stops all polymerization until such time as it is completely consumed i.e. the induction period) and then allows polymerization to proceed at the normal rate. However, in many cases the products formed during inhibition or retardation are not inert. Four 

The kinetics and mechanism of retardation and inhibition has been reviewed by Bamford, Ttidos and Fdldes-Berezsnich, Eastmond, Goldfinger et a/. and Bovey and Kolthoff.  

Common inhibitors include stable radicals (Section 5.3.1), oxygen (5.3.2), certain monomers (5.3.3), phenols (5.3.4), quinones (5.3.5), phenothiazine (5.3.6), nitro and nitroso-compounds (5.3.7) and certain transition metal salts (5.3.8). Some inhibition constants ikjkp) are provided in Table 5.6. Absolute rate constants (k ) for the reactions of these species with simple carbon-centered radicals arc summarized in Tabic 5.7. 

Tabic 5,6 Inhibition constants kj/k, 60 °C, bulk) for Various Inhibitors with 

Various inhibitors have been used in new determinations of rates of initiation in bulk polymerizations of acrylonitrile and solution polymerizations of styrene an improved procedure has been proposed.Aromatic aldonitrones, polymers with attached t-butyl nitroxide groups and charge-transfer complexes of anthracene have been studied as inhibitors. 

Retardation has already been mentioned in connection with degradative transfer it can arise also as a result of copolymerization. Another example has been quoted of a disulphide that can perform as a photosensitizer and also as a retarder of radical polymerization. A method proposed for overcoming interference by oxygen in photopolymerizable films involves its conversion to singlet oxygen by means of a dye-sensitizer and then its scavenging.  

Foldes-Berezsnich, M. Szesztay, E. Boros-Gyevi, and F. Tiidds, J. Polym. Sci., Polym. Chem. Ed., 1980, 18, 1223. 

The flow of publications on the subject of emulsion polymerization and related topics has continued unabated during the period covered by this Report. It would be impracticable to refer in the limited space available to all the items that have appeared during the past two years or so. Quite apart from the many items which have doubtless been missed, it has been necessary to be selective. 

Probably the most important international event concerned with emulsion polymerization that has occurred during the period covered by this Report is the Emulsion Polymerization Symposium arranged by the American Chemical Society and held in Las Vegas, 25—29 August, 1980. (The venue was to have been San Francisco, but it had to be changed at the last moment.) The symposium comprised 8 half-day sessions spread over 5 days. Each session consisted of an invited lecture plus approximately 5 contributed papers. The latter are available in preprint form, and, taken as a whole, these preprints represent an important addition to the literature pertaining to emulsion polymerization. As will appear evident subsequently, the subject matter of some of these preprint papers has also been published more extensively elsewhere. The principal impressions gained from the symposium and the preprints are of 

2 x 104 for CCT agent XXVIIb (with R = CH, L = isopropyl, pyridine) in the polymerization of methyl methacrylate at 60°C [Kowollik and Davis, 2000]. 

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, 

Hydrogen transfer can also take place to the acetate moiety  

Yamasaki et al. [152], reported that they successfully performed the radical polymerization of allylbiguanide hydrochloride in a concentrated, acid solution using either hydrochloric acid or phosphoric acid in the presence of a radical initiator at 50°C. The polymer was precipitated from the reaction solution through the addition of an excess amount of acetone. The molecular weight average of the product was 10,340-113,200, with a low polydispersity 1.04—1.68. 

In spite of degradative chain transferring, polyallyl compounds can be readily polymerized by a free-radical mechanism into three-dimensional lattices. High DP is not necessary to achieve growth in three dimensions. An example of such polyallyl compounds is triallyl phosphate  

Many other polyallyl derivatives are offered commercially for use in cross-linked films and are described in the trade literature. 

Free-radical polymerizations are subject to inhibition and retardation from side reactimis with various molecules [54]. Such polymerization suppressors are classified according to the effect that they exert upon the reaction. Inhibitors are compounds that react very rapidly with every initiating free radical as it forms. This prevents any polymerizatiOTi reaction from taking place until the inhibitor is completely cmisumed in the process. The reactions of inhibitors with initiating radicals result in formations of new free radicals. The newly formed free radicals, however, are too stable to initiate chain growths. As a result, well-defined induction periods exist. After the inhibitors are used up, polymerizations proceed at normal rates. 

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The effectiveness of inhibitors is measured in terms of the rate constant ratio kz/kp and the stoichiometric coefficient. The stoichiometric coefficient is the moles of radicals consumed per mole of inhibitor. These parameters may be determined by various methods. A brief description of the classical kinetic treatment for evaluating k7/kp follows. Consider the reaction scheme shown which describes ideal inhibition and retardation (Scheme 5.11). 

Fig. 3-9 Inhibition and retardation in the thermal, self-initiated polymerization of styrene at 100°C. Plot 1, no inhibitor plot 2, 0.1% benzoquinone plot 3, 0.5% nitrobenzene plot 4, 0.2% nitrosobenzene. After Schulz [1947] (by permission of Verlag Chemie GmbH and Wiley-VCH, Weinheim).

Eastmond, G. C., Chain Transfer, Inhibition and Retardation, Chap. 2 in Comprehensive Chemical Kinetics, Vol. 14A, C. H. Bamford and C. F. H. Tipper, eds., American Elsevier, New York, 1976c. 

The extent of inhibition and retardation depends on the amount of added lignin, the initiator used and the grafting temperature. Lignin in-situ of the lig-nocellulosic substrate effects inhibition or retardation only on using ceric ammonium sulfate as initiator. 

Fig. 1 Conversion vs. time for normal polymerization as well as inhibited and retarded free-radical polymerizations. (From Ref.. )...

Figure 6.12 Comparison of conversion-time plots for normal, inhibited, and retarded free-radical polymerization. Curve 1 normal polymerization curve 2 inhibition curve 3 retardation curve 4 inhibition followed by retardation.

Figure 6.11 Comparison of conversion-time plots for normal, inhibited, and retarded free-radical polymerization. Curve 1 normal polymerization in the absence of inhibitor/retarder. Curve 2 inhibition polymerization is completely stopped by inhibitor during the initial induction period, but at the end of this period with the inhibitor having been completely consumed, polymerization proceeds at the same rate as in normal polymerization (curve 1). Curve 3 retardation a retarder reduces the polymerization rate without showing an induction period. Curve 4 inhibition followed by retardation (After Ghosh, 1990).

Inhibition and Retardation. Certsun substances, when introduced into a polymerization system, will low donm or stop the reaction jl ey are called r rders or inhibitors, respectively. Both operate by the same reaction mechanism> the distinction being that, inhibitors are more effective in their action. Both react with active chain ends in such a way that either non-swtive resMstion products, are formed or, a radical of such low reactivity is proiiuced that no further reliction with monomer can take place. 

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