Vinyl monomers polymerization kinetics stable radicals

In this cormection, systematic stndies were made of the inhibiting effect of many stable mono and poly radicals on the kinetics and mechanism of vinyl monomer polymerization. The efCtiency of nitroxyls as free radical acceptors has promoted their usage to explore the mechanism of polymerization by means of the inhibition technique. Usually, nitroxyls have time to only react with a part of the radicals formed at... 

Our kinetic work (10) showed that the small molecule radical produced by chain transfer with monomer had to be a stable radical. This was confirmed in the present paper by analysis of the isotope effect on the bulk polymerization rates. The isotope effect on molecular weights and rates unequivocally showed that almost 100% of the chain transfer involved the vinyl hydrogen. There is some evidence in the literature to support the idea of a stable vinyl radical. Phenyl acetylene acts as a retarder when copolymerized with styrene or methyl methacrylate (25). Thus the phenyl vinyl radical is very stable compared to the growing styryl or methacrylyl radical. 

The results definitely prove our hypotheses in the kinetic model for vinyl acetate emulsion polymerization (10), that vinyl radical, CH2=C-0Ac, is the major monomer radical formed and is a stable radical which reinitiates relatively slowly compared to the propagation step. 

New kinetic regularities at polymerization of vinyl monomers in homophase and heterophase conditions in the presence of additives of transition metal salts, azonitriles, peroxides, stable nitroxyl radicals and radical anions (and their complexes), aromatic amines and their derivatives, emulsifiers and solvents of various nature were revealed. The mechanisms of the studied processes have been estabhshed in the whole and as elementary stages, their basic kinetic characteristics have been determined. Equations to describe the behavior of the studied chemical systems in polymerization reactions proceeding in various physicochemical conditions have been derived. Scientific principles of regulating polymer synthesis processes have been elaborated, which allows optimization of some industrial technologies and solving most important problems of environment protection. 

Stable Radicals in the Polymerization Kinetics of Vinyl Monomers [11-18]... 

STABLE RADICALS IN THE POLYMERIZATION KINETICS OF VINYL MONOMERS... 

Henrici-Olive and Olive were the first to put forward the hypothesis that complexes are sometimes formed between the active centre and the monomer and or/solvent [45], As only the complex with monomer is capable of propagation, part of the centres is inhibited and the polymerization rate is reduced. This theory was found to be valid with styrene [46], but not with MMA [47]. Burnett called attention to the important circumstance that radicals solvated in various ways may react differently, or at least at different rates [47]. His conclusions were based on kinetic studies of MMA polymerization in various halogenated aromatics. In the copolymerization of butyl vinyl ether with methacrylates, complex formation between the active centre and condensed aromatics prior to monomer addition was observed by Shaik-hudinov et al. [48], The growing polymer forms a stable donor-acceptor complex with naphthalene, described by the formula. 

Aqueous dispersions of poly(vinyl acetate) and vinyl acetate-ethylene copolymers, homo- and copolymers of acrylic monomers, and styrene-butadiene copolymers are the most important types of polymer latexes today. Applications include paints, coatings, adhesives, paper manufacturing, leather manufacturing, textiles and other industries. In addition to emulsion polymerization, other aqueous free-radical polymerizations are applied on a large scale. In suspension polymerization a water-irnrniscible olefinic monomer is also polymerized. However, by contrast to emulsion polymerization a monomer-soluble initiator is employed, and usually no surfactant is added. Polymerization occurs in the monomer droplets, with kinetics similar to bulk polymerization. The particles obtained are much larger (>15 pm) than in emulsion polymerization, and they do not form stable latexes but precipitate during polymerization (Scheme 7.2). 

The solution polymerization of allyl acetate was studied in an effort to determine the effect of monomer concentrations on the reaction kinetics [14]. These studies were limited to the use of benzene. The growing allyl acetate radicals formed stable adducts with the solvent much as vinyl acetate does. The stabilized adduct is terminated by combination with a growing radical. The twinning reaction is said to account for the relatively high molecular weight of the polymer despite the fact that the reaction with benzene is a chain-transfer reaction. Ethyl acetate, on the other hand, would have been a preferable solvent... 

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