Thermal polymerization inhibition

The presence of stable free radicals in the final polycondensate is supported by the observation that traces of (11) have a strong inhibiting effect on the thermal polymerization of a number of vinyl monomers. Radical polymerization was inhibited to a larger extent by a furfural resin than by typical polymerization inhibitors (34). Thermal degradative methods have been used to study the stmcture of furfural resinifted to an insoluble and infusible state, leading to proposed stmctural features (35). 

The presence of stable free radicals in the resin was further suggested by the strong inhibiting effect of traces of this product on the thermal polymerization of styrene. 

The inhibitors more commonly used are molecules which in one way or another react with active chain radicals to yield product radicals of low reactivity. The classic example is benzoquinone. As little as 0.01 percent causes virtual total suppression of polymerization of styrene or other monomers. This is true of both thermal and initiated polymerizations. Results of Foord for the inhibition of thermal polymerization of styrene by benzoquinone are shown in Fig. 22. The... 

Fig. 22.—Inhibition of the thermal polymerization of styrene at 90°C by benzoquinone. The log of the viscosity relative to that of pure monomer is here used as a measure of polymerization. The small induction period in the absence of quinone presumably was caused by spurious inhibitors present in the monomer. (Results of Foord. )...

Mitz and Usdin later (40) found that polymers lacking tyrosine residues showed a much lower level of activity. This activity was not inhibited by DFP. Also, Michaelis-Menten kinetics were not demonstrable with t3rrosine-free polymers. The authors emphasize that the thermal polymerization of a limited number of amino acids is a novel method for exploring the fundamental components of a catalytic polymer. Their approach thus complements the study of homopolymers or two-component polymers at one extreme and proteinoids at the other extreme. 

The simplest examples of CB-A antioxidant are quinones, and benzoquinone has widely been used to inhibit alkyl-radical reactions (such as the thermal polymerization of styrene on storage). They react with polymer alkyl radicals (R-) formed on chain scission or through attack on the backbone to give the reduced form of the radical as shown in Scheme 1.65. 

The role of sulphur-containing compounds in photopolymerization appears to have attracted some interest. Bis(j -methylpyridazinyl)-3,3 -disulphide has been found to initiate the photopolymerization of styrene but inhibits the thermal polymerization. The role of thiyl radicals (PhS-) in photoinitiated polymerization of vinyl monomers by aromatic thio-compounds has been postulated by several workers. In one study, flash photolysis was used to identify the nature of the radical. Sulphur-containing monomers such as 4-methyl-2-(vinylthio)thiazole and thiocyclanes have been photopolymerized and copolymerized with other vinyl monomers. Luca et al. have devised a mathematical model for the photopolymerization of 2,3-dimethylbutadiene and thiourea. 

Ryddy and Lazar demonstrated that oligostyryd radicals generated from styrene by benzoyl peroxide are stabilised sufficiently by 13X zeolite to be detected by ESR at temperatures upto SO C. This would indicate that 13X zeolite could inhibit or retard the radical pdlymerization of styrene at 30 °C. The themal polymerization of styrene is believed to be a radical reaction Hence it may be assumed that the presence of zeolite exerts no effect upon the rate of thermal polymerization. 

Thus, additives act in various ways. Benzoquinone produces a profound inhibition period in the thermal polymerization of styrene (Figure 20-5). Finally, styrene polymerizes at the same rate as for purely thermal polymerization. Obviously, all free radicals formed react instantaneously with the benzoquinone to produce inactive free radicals. It cannot be distinguished here without further experiments whether the benzoquinone forms a free radical by transfer or is incorporated into the reacting free radical ... 

O2 was found to inhibit the thermal polymerization of CN groups, altering the characteristics of the exotherm, whilst in air, the exotherm occurred at higher temperatures [176]. Sharp intense exotherms were found irrespective of the method of polymerization [177]. From IR evidence, CN absorption is substantially reduced during... 

Myrcene is a highly reactive compound that undergoes spontaneous thermal polymerization even in air. Polymerization is effectively inhibited in the cold... 

More recently, the polymerization of several cyclic disulfides has been investigated by Endo et d. The thermal polymerization of 1,2-dithiane (DT) did not proceed at monomer concentrations below 4.0 mol 1" The polymerization was inhibited by addition of radical inhibitors, indicating that the propagation proceeds by a radical intermediate. The molecular weight of the polymers increased with reaction time. It was proposed that the cyclic polymer is formed mainly by back biting reaction mechanism during the polymerization (Scheme 36). ... 

The polymerization of diacrylate monomers functionalized with Schiff base complexes of Cu(IT), Pd(n), and Zn(ll) has been investigated. Thermal polymerizations of monomer 17 (M=Pd and Zn) were successful however, polymers were not obtained when flie copper analog was subjected to tiie same conditions. Photopolymerization of these monomers in the presence of a titanium initiator and their copolymerization with monoacrylate organic monomers were also examined. In aU cases, the copper complexes inhibited the polymerization reaction. The palladium-containing polymers exhibited liquid crystalline properties. ... 

For example, in the case of thermal polymerization of styrene [1], benzoquinone acts as an inhibitor. When the inhibitor has been consumed, polymerization regains its momentum and proceeds at the same rate as in the absence of the inhibitor. Nitrobenzene [1] acts as a retarder and lowers the polymerization rate, whereas nitrosobenzene [1] behaves differently. Initially, nitrosobenzene acts as an inhibitor but is apparently converted to a product which acts as a retarder after the inhibition period. Impurities present in the monomer may act as inhibitors or retarders. The inhibitors in the commercial monomers (to prevent premature thermal polymerization during storage and shipment) are usually removed prior to polymerization or, alternatively, an appropriate excess of initiator may be used to compensate for their presence. 

As discussed below, the reactivity of one site often interferes with desirable chemical reactions to be conducted at another she. It should also be noted that model reactions can sometimes be performed on the N-dly hosphoranimines precursors, (6) but, because these small molecules also contain reactive N-Si and P-O bonds, they are not always suhable prototypes. Furthermore, vAAle derivatization of these precursors can be a suhable means of prq aring new polymers with substituent diversity, some groups interfere with or inhibit the thermal polymerization. Thus, our work as described here focuses on the chemistry of the preformed polymer sfystems. 

U.S. 4,168,982. Photopolymerizable Compositions Containing Nitroso Dimers to Selectively Inhibit Thermal Polymerization. Pazos, Jose F. (E. I. Du Pont de Nemours and Company). September 25, 1979. Cl. 430/281.1 430/917 522/16 522/18 522/28 522/63 522/65 522/76 522/121 522/167 Appl. December 7, 1977. Thermally stable photo-polymerizable compositions comprise (i) at least one nongaseous ethylenicaUy unsaturated compound, (ii) a nitroso dimer which is a noninhibitor of free-radical polymerization but thermally dissociates to nitroso monomer which is an inhibitor of free-radical polymerization, and (iii) an organic, radiation-sensitive free-radical generating system. 

Lithium hexafluoroarsenate is thermally stable [54, 55] but shows environmental risks due to possible degradation products [56-58], even though it is itself not very toxic. Its LD 50 value is similar to that of lithium perchlorate [55]. Just like lithium hexafluorophosphate, it can initiate the polymerization of cyclic ethers. Polymerization may be inhibited by tertiary amines [59], or 2-methylfuran [60], yielding highly stable electrolytes. 

The proposed polymerization mechanism is shown in Scheme 9.12. Thermal decomposition of the hexasubstituted ethane derivative yields hindered tertiary radicals that can initiate polymerization or combine with propagating species (primary radical termination) to form an oligomeric macroinitiator. The addition of the diphenylalkyl radicals to monomer is slow (e.g. k[ for 34 is reported as KT M"1 s l at 80 °C84) and the polymerization is characterized by an inhibition period during which the initiator is consumed and an oligomeric macroinitiator is formed. The bond to the Cl I formed by addition to monomer is comparatively thermally stable. 

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