Vinyl acetate polymerization retardation

Retarders and inhibitors differ only in the frequency with which propagating radicals react with them rather than with monomer and possibly also in the ability of the radicals resulting from such reactions to reinitiate. It is lo be expected, then, that a compound may not exert the same elTect in the polymerization of different monomers. For example, aromatic nitro compounds that are inhibitors in vinyl acetate polymerizations are classified as retarders in polystyrene syntheses. 

It was found by Burnett and Melville36 in 1947 that the radical polymerization of vinyl acetate was retarded in aromatic solvents. This retardation effect was confirmed by several researchers37-42. It is characterized by three features all of which cannot be simultaneously explained by the conventional kinetic scheme involving degradative chain transfer to solvent. (1) The rate of polymerization is markedly reduced in comparison with that in many aliphatic solvents. (2) The order with respect to initiator remains close to one-half over a wide range of initiator concentration. 

Buckminsterfullerene (Ceo) has attracted much interest, not only because of its structure and physical properties, but also because of its reaction behavior in radical polymerization systems. Ceo reacts rapidly with radicals to yield ESR-detectable radical species Ceo (465). Ceo was foimd to act as an effective inhibitor in vinyl acetate polymerization and all consumed Ceo molecules were incorporated into the polymeric chains during the polymerization. The relation between the induction period and the initiation rate revealed that one Ceo molecule can trap 15 radicals (466). The rate of polymerization of styrene and methyl methacrylate is also significantly retarded in the presence of Ceo (467-471). The average number of Ceo molecules incorporated in the polymer chains increases when increasing the Ceo concentration, maybe because of coupling of polsrmer-Ceo to give polymer-Ceo-Ceo-polymer (472). 

Polyalkyl ring-substituted phenols, such as 2,4,6-trimethyl-phenol act as more powerful retarders than phenol toward vinyl acetate polymerization. The mechanism for retardation may involve hydrogen abstraction followed by coupling of the phenoxy radical with other polymer radicals ... 

Aromatic nitro compounds act as inhibitors and show greater tendency toward more reactive and electron-rich radicals. Nitro compounds have very little effect on methyl acrylate and methyl methacrylate [5,10,11] but inhibit vinyl acetate and retard styrene polymerization. The effectiveness increases with the number of nitro groups in the ring [1 13]. The mechanism of radical termination involves attack on both the aromatic ring and the nitro group. The reactions are represented as follows ... 

In studies of the polymerization kinetics of triaUyl citrate [6299-73-6] the cyclization constant was found to be intermediate between that of diaUyl succinate and DAP (86). Copolymerization reactivity ratios with vinyl monomers have been reported (87). At 60°C with benzoyl peroxide as initiator, triaUyl citrate retards polymerization of styrene, acrylonitrile, vinyl choloride, and vinyl acetate. Properties of polyfunctional aUyl esters are given in Table 7 some of these esters have sharp odors and cause skin irritation. 

There are several reports scattered in the literature of the retarding effect of simple furan derivatives in the polymerization of a specific monomer. Hardy69, U6 found that furan, 2-furoic acid and its esters, and 5-substituted-2-furoie acids were strong retarders in the radical polymerization of vinyl acetate, but did not act likewise with styrene. He proposed that as a result of the reactions of the free radicals with the furan derivatives, dihydro- and tetrahydrofurans would form, but he did not produce any evidence to support these speculations. Clarke, Howard and Stock-... 

Anomolous results have been observed in some emulsion polymerizations—inverse dependencies of N, Rp, and Xn on surfactant concentration. Some surfactants act as inhibitors or retarders of polymerization, especially of the more highly reactive radicals from vinyl acetate and vinyl chloride [Okamura and Motoyama, 1962 Stryker et al., 1967]. This is most apparent with surfactants possessing unsaturation (e.g., certain fatty acid soaps). Degradative chain transfer through allyl hydrogens is probably quite extensive. 

These reactions resemple those by which anthracene can polymerize with certain monomers. The retardation caused by benzene during the polymerization of vinyl acetate can be attributed to the radical produced in (7) being unreactive because of resonance stabilization, so that some of the radicals are wasted and do not enter reaction (8). The benzene contents of the polymers are low and this type of co-polymerization can be studied quantitatively only by tracer techniques. 

Bartlett, P. D., and H. Kwart Dilatometric studies of the behavior of some inhibitors and retarders in the polymerization of liquid vinyl acetate. J. Am. Chem. Soc. 72, 1051 (1950). 

Chain transfer also occurs to the emulsifying agents, leading to their permanent incorporation into the product. Chain transfer to aldehydes, which may be formed as a result of the hydrolysis of the vinyl acetate monomer, tends to lower the molecular weight and slow the polymerization rate because of the lower activity of the radical that is formed. Thus, the presence of acetaldehyde condensates as a poly (vinyl alcohol) impurity strongly retards polymerization (91). Some of the initiators such as lauryl peroxide are also chain-transfer agents andlower the molecular weight of the product. 

The photo-induced polymerization of acrylonitrile, again in tetrahydrofuran solution, is greatly enhanced by the presence of benzophenone (46), which in this case exhibits a higher efficiency than TCMB. Both these compounds retarded the photoinduced polymerization of vinyl acetate in tetrahydrofuran, but did act as photoinitiators when toluene was used as solvent (46), possibly reflecting the differing abilities of the solvent-derived radicals to add to vinyl acetate. 

A lot of papers have been published on the effect of solvent on free radical polymerization rate. Studies on this effect have greatly been stimulated by (1) Norrish-Trommsdorff effect, (2) Q, e-scheme in copolymerization, (3) Retardation of the polymerization rate of vinyl acetate, (4) radical complex. 

The reduction in molecular weight of the polymer is slight as compared to that in many other solvents. Stockmayer et al.41,42 once interpreted this retardation effect in terms of copolymerization involving the aromatic ring, but the failure of the copolymerization of benzene with vinyl monomers was confirmed by the application of the isotope technique43-47. Therefore, the influence of aromatic compounds on the polymerization rate of vinyl acetate has remained unsolved. 

All aromatic solvents, including those without alkyl side chains such as chlorobenzene, ethyl benzoate, and benzene, retard the polymerization of vinyl acetate [37],... 

In a study using C-labeled benzene, it was found fliat at a degree of polymerization of 700, the average polymer molecule contained about 20 benzene units [37], Thus there is evidence of retardation attributable to a degradative chain-transfer process [37, 38]. In addition, there is evidence that actual copolymerization of vinyl acetate and benzene takes place. As a matter of fact, there may be controversy whether the observations discussed here are a result of chain transfer or of copolymerization. The point in question may be rather subtle [39], The facts, regardless of theoretical interpretation, are that aromatic solvents retard the polymerization of vinyl acetate, and the polymer contains substantial quantities of covalently bound solvent. 

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