Vinyl acetate emulsion polymerization mechanisms

The presence of these very small particles in samples polymerized to high conversions using different emulsifiers suggests that the predominant mechanism of particle nucleation is initiation in the aqueous phase, followed by flocculation with the particles of the main distribution. Such a mechanism would be reasonable for the emulsion polymerization of the more water-soluble monomers such as vinyl acetate or vinyl chloride indeed, such particles were found in vinyl acetate emulsion polymerizations (48). Nevertheless, this finding was not expected for the 50 50 styrene-butyl acrylate mixture, as both monomers are only sparingly soluble in water, and it suggests that initiation in the aqueous phase is far more prevalent, than has been thought. 

Friis and Hamielec (1975) have used GPC to study the MWD development in vinyl acetate and vinyl chloride emulsion polymerizations. For these monomers, the main chain-stopping mechanism is thought to ha transfer, and so the compartmentalize nature of the system is relatively unimportant. These workers found that the MWDs produced at early times, where branching reactions are unimportant, have a P value dose to 2, as expected for transfer-dominated reactions. 

Many different combinations of surfactant and protective coUoid are used in emulsion polymerizations of vinyl acetate as stabilizers. The properties of the emulsion and the polymeric film depend to a large extent on the identity and quantity of the stabilizers. The choice of stabilizer affects the mean and distribution of particle size which affects the rheology and film formation. The stabilizer system also impacts the stabiUty of the emulsion to mechanical shear, temperature change, and compounding. Characteristics of the coalesced resin affected by the stabilizer include tack, smoothness, opacity, water resistance, and film strength (41,42). 

Mechanisms. Because of its considerable industrial importance as well as its intrinsic interest, emulsion polymerization of vinyl acetate in the presence of surfactants has been extensively studied (75—77). The Smith-Ewart theory, which describes emulsion polymerization of monomers such as styrene, does not apply to vinyl acetate. Reasons for this are the substantial water solubiUty of vinyl acetate monomer, and the different reactivities of the vinyl acetate and styrene radicals the chain transfer to monomer is much higher for vinyl acetate. The kinetics of the polymerization of vinyl acetate has been studied and mechanisms have been proposed (78—82). 

Emulsion Polymerization. Poly(vinyl acetate) and poly(vinyl acetate) copolymer latexes prepared in the presence of PVA find wide appHcations in adhesives, paints, textile finishes, and coatings. The emulsions show exceUent stabiHty to mechanical shear as weU as to the addition of electrolytes, and possess exceUent machining characteristics. 

The presence of the emulsifier, and thus of the micelles, is not a necessary condition for emulsion polymerization. Styrene, the acrylates, vinyl acetate, and vinyl chloride can be polymerized in aqueous medium without an emulsifier [126]. The mechanism of the formation of monomer—polymer particles is in agreement with the ideas of Fitch and Tsai [139] which were refered to above. 

Unzueta et al. [18] derived a kinetic model for the emulsion copolymerization of methyl methacrylate (MMA) and butyl acrylate (BA) employing both the micellar and homogeneous nucleation mechanisms and introducing the radical absorption efficiency factor for micelles, F, and that for particles, Fp. They compared experimental results with model predictions, where they employed the values of Fp=10 and Fn,=10", respectively, as adjustable parameters. However, they did not explain the reason why the value of Fp, is an order of magnitude smaller than the value of Fp. Sayer et al. [19] proposed a kinetic model for continuous vinyl acetate (VAc) emulsion polymerization in a pulsed... 

Kinetics and Mechanism of the Emulsion Polymerization of Vinyl Acetate... 

Although theoretical models seem to be quite adequate for styrene emulsion polymerization in either batch reactors or CSTR s, such is not the case with other monomers like vinyl acetate, methyl acrylate, methyl methacrylate, vinyl chloride, etc. One of the early papers to discuss scane of the important mechanisms involved with these other moncaners was written by Priest ( ). He studied the emulsion polymerization of vinyl acetate and identified most of the key mechanisms involved. Priest s paper has been largely overlooked, however, perhaps because of the success of the Smith-Ewart approach to styrene. 

The mechanism of particle formation at submicellar surfactant concentrations was established several years ago. New insight was gained into how the structure of surfactants influences the outcome of the reaction. The gap between suspension and emulsion polymerization was bridged. The mode of popularly used redox catalysts was clarified, and completely novel catalyst systems were developed. For non-styrene-like monomers, such as vinyl chloride and vinyl acetate, the kinetic picture was elucidated. Advances were made in determining the mechanism of copolymerization, in particular the effects of water-soluble monomers and of difunctional monomers. The reaction mechanism in flow-through reactors became as well understood as in batch reactors. Computer techniques clarified complex mechanisms. The study of emulsion polymerization in nonaqueous media opened new vistas. 

In a complex apparatus, Gimesch and Schneider [30, 119] studied the suspension polymerization of vinyl acetate. Their procedure involved equipment which automatically added tempered water to the reacting system as heat was evolved as a result of the polymerization process. Thus they maintained isothermal reaction conditions. The rate of reaction could be followed by recording the water uptake of the equipment with time. The heat of polymerization was also determined (found to be 23 kcal/mole which was considered a satisfactory check of the literature value which is scattered around 21.4 kcal/mole). From this work, a somewhat different mechanism of the suspension polymerization process emerges than the widely accepted concept of the water-cooled bulk polymerization of small particles. It was noted that with an increase in the initiator concentration, there was the expected increase in polymerization rate. With increasing stirring rate, the rate of polymerization decreased. Along with the suspension polymerization, there was always a certain amoimt of imdesirable emulsion polymerization. It was postulated that in the process, free radicals, formed in a monomer drop may be extracted into the aqueous phase where they may act on dissolved vinyl acetate by kinetic processes unique to this system and different from the conventional mechanism of suspension polymerization. 

Most commonly, in the emulsion polymerization of vinyl acetate, anionic surfactants are used either alone or in combination with a protective colloid. Typical examples of surfactants which have found application are Aerosol OT (sodium dioctylsulfosuccinate), alkyl aryl sulfonate salts (e.g., Santomerse-3), sodium lauiyl sulfate, etc. A study of the kinetics of the vinyl acetate polymerization in the presence of sodium lauryl sulfate indicated that the rate of polymerization was proportional to the square root of the initiator concentration and the 0.25th power of the number of particles. The number of particles were proportional to the 0.5th 0.05 power of the surfactant concentration but independent of the level of potassium persulfate. The intrinsic viscosity of the final polymer was said to be independent of the initiator concentration and of the munber of polymer particles. These observations were said to suggest that the mechanism of the vinyl acetate polymerization in emulsion resembles that of vinyl chloride [153]. 

The study of Nomura et al. [143] is perhaps one of the most detailed investigations on the kinetics and mechanism of the emulsion polymerization of vinyl acetate published. The work presents a detailed evaluation of the interactions of monomer, surfactant, and initiator concentration in the course of the... 

Poly(vinyl acetate) (PVA) and ethylene-vinyl acetate (EVA) copolymer adhesives have much in common, yet represent extremes in the degree of sophistication of their production processes. Both products are stable suspensions in water of a film-forming polymer, the particles of which are generally spherical. They are made by emulsion polymerization, which uses a free-radical addition mechanism to polymerize the monomer in the presence of water and stabilizers. Vinyl acetate is the sole or major monomeric raw material. 

Wu and coworkers studied the silica-nanoparticle-stabilized emulsion polymerization of vinyl acetate, with the aid of a small amount of anionic reactive surfactant, 3-allyloxy-2-hydroxy-l-propanesulfonicacid sodium salt (HAPS) [118]. They argued that hydrogen bond interactions allowed for strong adhesion, and also commented on the mechanism of solids-stabilized emulsion polymerization. 

Various copolymers of ethylene with vinyl acetate are prepared by a free-radical mechanism in emulsion polymerizations. Both reactivity ratios are close to The degree of branching in these... 

Commercially, poly(vinyl acetate) is formed in bulk, solution, emulsion, and suspension polymerizations by a free-radical mechanism. In such polymerizations, chain transferring to the polymer may be as high as 30%. The transfer can be to a polymer backbone through abstraction of a tertiary hydrogen ... 

Copolymers as water dispersion have also been synthesized by emulsion polymerization in presence of poly(vinyl alcohol) as protective colloid and evaluated as wood adhesives. The corresponding polymer films have been subjected to mechanical characterization. An improvement in the water and solvent resistance has been observed due to the presence of drying oils. The cobalt acetate, if present as ceitalyst in the adhesive formulation, promotes cross-linking and produces a positive effect on the adhesive performance. 

A polyanrline-poly(butyl acrylate-vinyl acetate) composite exhibiting electroactivity and having a conductivity of 2.2 S/cm was prepared by emulsion polymerization. The composite was soluble in common organic solvents and a stable water-based dispersion could also be prepared. Films cast from aqueous media had exceptional mechanical properties and had excellent adhesion to steel [144]. From the same group, a polyaniline and polyvinyl alcohol electroactive composite has been synthesized by... 

Various copolymers of ethylene with vinyl acetate are prepared by free-radical mechanism in emulsion polymerizations. Both reactivity ratios are close to 1.0 [106]. The degree of branching in these copolymers is strongly temperature-dependent [107]. These materials find wide use in such areas as paper coatings and adhesives. In addition, some are hydrolyzed to form copolymers of ethylene with vinyl alcohol. Such resins are available commercially in various ratios of polyethylene to poly(vinyl alcohol), can range from 30% poly(vinyl alcohol) to as high as 70%. 

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