Solution polymerization of vinyl acetate

Solution Polymerization. Solution polymerization of vinyl acetate is carried out mainly as an intermediate step to the manufacture of poly(vinyl alcohol). A small amount of solution-polymerized vinyl acetate is prepared for the merchant market. When solution polymerization is carried out, the solvent acts as a chain-transfer agent, and depending on its transfer constant, has an effect on the molecular weight of the product. The rate of polymerization is also affected by the solvent but not in the same way as the degree of polymerization. The reactivity of the solvent-derived radical plays an important part. Chain-transfer constants for solvents in vinyl acetate polymerizations have been tabulated (13). Continuous solution polymers of poly(vinyl acetate) in tubular reactors have been prepared at high yield and throughput (73,74). 

A novel method of producing suspension polymers of poly(vinyl acetate) with a closely controlled molecular weight distribution involves the solution polymerization of vinyl acetate in methanol. By varying the ratio of monomer to methanol, a variety of molecular weight distributions may be prepared. The solution polymer is then added with agitation to an aqueous system containing poly(vinyl alcohol). The methanol is then distilled off to give a bead polymer [121]. 

Solution polymerization is of limited commercial utihty in free-radical polymerization but finds ready applications when the end use of the polymer requires a solution, as in certain adhesives and coating processes [i.e., poly(viityl acetate) to be converted to poly(viityl alcohol) and some acryhc ester finishes]. Solution polymerization is used widely in ionic and coordination polymerization. High-density polyethylene, poly butadiene, and butyl rubber are produced this way. Table 10.2 shows the diversity of polymers produced by solution polymerization, while Figure 10.2 is the flow diagram for the solution polymerization of vinyl acetate. 

PVA from solution polymerization of vinyl acetate with comonomers... 

Solvent transfer constants of various solvents for vinyl acetate are shown in Table 4.5 [16]. Polyvinyl acetate, as the raw material of polyvinyl alcohol for fiber, is produced by solution polymerization of vinyl acetate. In most cases, methanol is employed as the solvent. Solvent transfer constant, Cs, is one of the most important features for the selection of the solvent (see below in Section 4.2.2.5). 

In the industry today, solution polymerization of vinyl acetate is carried out using approximately 20% methanol, and the reaction is stopped at about 65%i conversion. Usually, a DP of approximately 2500 of polyvinyl acetate drops by conversion to PVA to about 1700. Once deacetylated, the repetition of acetylation and deacetylation does not change the DP [27]. 

Polyvinyl acetate (PVAc) is the largest volume polymer produced from a vinyl ester (1). In 1990, over 2.5 billion pounds of vinyl acetate monomer were produced in the United States alone (2). The bulk of this monomer was used for making PVAc and PVAc copolymers, which are widely used in water-based paints, adhesives, coatings, and binders for nonwoven paper products. PVAc is also the precursor to polyvinyl alcohol (PVA) and polyvinyl butyral, which cannot be made by direct polymerization. Methods of PVAc polymerization vary depending on the end use. Solution polymerizations of vinyl acetate in methanol are generally employed in processes in which PVAc is used as an intermediate in the production of PVA. PVAc latexes are generally made by emulsion polymerization, and PVAc in bead form is often synthesized by suspension polymerization (3,4). 

A cobalt(II)-chitosan chelate has been prepared by soaking a chitosan film in C0CI2 aqueous solution. The chitosan chelated Co(II) through both oxygen and nitrogen atoms in the chitosan chain. The tetracoordinated, high-spin Co(II)-chitosan chelate could be used as a catalyst, and the polymerization of vinyl acetate was carried out in the presence of Na2S03 and water at pH 7 and normal temperature. The polyvinyl acetate possessed a random structure [114,115]. 

PVAc may be produced by the polymerization of vinyl acetate (Equation 6.47). The viscosity of the solution continues to increase until the reaction is complete. Dilute polymer solutions are used to prevent the onset of autoacceleration because of the gel effect. 

The final product of emulsion polymerization is an emulsion —a stable, heterogeneous mixture of fine polymer beads in an aqueous solution, sometime called a latex emulsion. Water-based paints, for example, can be formed from the emulsion polymerization of vinyl acetate. In this process, I m of water containing 3% poly(vinyl alcohol) and 1% surfactant are heated to 60°C in a reaction vessel (see Figure 3.27) The temperature rises to around 80°C over a 4 to 5 hour period as monomer and an aqueous persulfate solution are added. The rate at which heat can be removed limits the rate at which monomer can be added. 

Commercial Hydrolysis Process. The process of converting poly (vinyl acetate) to poly(vinyl alcohol) on a commercial scale is complicated on account of the significant physical changes that accompany the conversion. The viscosity of the poly (vinyl acetate) solution increases rapidly as the conversion proceeds, because the resulting poly(vinyl alcohol) is insoluble in the most common solvents used for the polymerization of vinyl acetate. The outcome is the formation of a gel swollen with the resulting acetic acid ester and the alcohol used to effect the transesterification. 

Visible light induces hydrogen peroxide formation in ascorbic acid and ethyl eosine solution, and H202 initiates the polymerization of vinyl acetate 74,75). 

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. 

Emulsion Bottle Polymerization of Vinyl Acetate from Aqueous Solution at... 

The role of aromatic moieties in the polymerization of vinyl acetate is interesting since benzene, for example, may be considered a natural solvent for the solution of polymerization of vinyl esters. Yet, in the presence of oxygen, cumene (isopropylbenze) inhibits the autoxidation process which may use up oxygen. Thus, the onset of polymerization may be delayed. On the other hand. 

The polymerization of vinyl acetate in solution may be carried out to produce lacquers, chewing gum bases, and adhesives. Usually the polymers are used... 

An azeotropic mixture of methyl acetate and methanol is a common coproduct from the manufacture of poly(vinyl alcohol) from poly(vinyl acetate) in the presence of methanol and sodium methoxide. The polymerization of vinyl acetate in this azeotropic solution, with the total removal of oxygen from the system, and using conventional free radical initiators is said to afford high-molecular weight poly(vinyl acetate) [106]. 

TABLE XIV The Polymerization of Vinyl Acetate in Methanol Solution [103]... 

The determinination of the rate of polymerization of vinyl acetate in solution has been carried out in a mercury recording dilatometer. In this procedure, the solution of the monomer and 2,2 -azobis(2,4-dimethylvaleronitrile) is placed in the dilatometer, degassed, and sealed at a pressure of approximately 10 mm Hg. The dilatometer is maintained at 50.0 0.02°C. The total shrinkage is calculated using 0.892 gm/ml as the density of the monomer and 1.166 gm/ml as that of the polymer. The rate of polymerization is usually determined from the rate of shrinkage in the conversion range of 5-7% [62]. 

In organic solutions, polyethylene oxides of molecular weight less than 1000 or copolymers of PEO may act as molecular weight regulators of the polymerization of vinyl acetate. For 100 gm of the monomer, from 0.5 to 20 gm of the... 

According to O Donnell et al. [130], the emulsion polymerization of vinyl acetate follows the Smith-Ewart theory of emulsion polymerization [131] because the rate of polymerization is independent of the total amount of monomer present, the rate is a function of the 0.6th power of the emulsifer concentration, and the rate of emulsion polymerization is a function of the 0.7th power of the initiator concentration instead of the expected 0.4th power. In this work poly(vinyl alcohol), 88% hydrolyzed with a medium molecular weight (i.e., Du Font s Elvanol 52-22), was used as the only externally added emulsifier. Light-scattering studies indicated that this emulsifier formed no aggregates in the aqueous solution. These latter observations may, however, have been made at room temperature and not at the reaction temperature [1]. The conversion versus time curve was essentially linear up to 80% conversion. 

When reviewing the published literature on the emulsion polymerization of vinyl acetate, one is struck with seemingly contradictory data presented by many reputable research teams. Some of these results published may not be strictly comparable because of variations in the polymerization recipes used. For example, the effect of the emulsifiers on the rate of polymerization may have a profound effect on the course of the reaction. In a persulfate-initiated system using no other surfactant, it has been postulated that the free radicals formed fixrm the decomposition of the initiator combine with the monomer in solution. As polymer forms, aggregates develop which absorb more monomer and the number of particles increases up to a constant value (at about 5% conversion). Then, while the number of particles remains constant at 1.7 X 10 per ml, the reaction rate increases. Ultimately, as a last stage of the reaction, the rate begins to drop off. The latex formed in this process is said to consist of particles of great uniformity with a diameter of 0.26 fim [137]. 

This observation seems to be in line with the Smith-Ewart concepts. The adsorption of surfactants on the surfaces of latex particles influences the capture by the particles of low-molecular-weight polymers formed in the aqueous solution. This in turn affects the reaction kinetics and the formation of new particles. The number of free radicals per particle, which is usually considered to be constant during the major phases of an emulsion polymerization, seems to vary considerably during the polymerization of vinyl acetate [139]. 

In their review of the effects of poly(vinyl alcohol) on the polymerization of vinyl acetate. Dunn and co-workers [145] point out that because of the variations in the distribution of vinyl acetate blocks in poly(vinyl alcohol), similar grades from different manufacturers differ in effect on emulsion polymerizations. For example Elvanol (product of Du Pont Co.) has a retarding effect when compared to Gehsenol (product of Nippon Gosei Co.). The difference in structure may also affect the ease with which micelles may form in poly(vinyl alcohol) solutions. 

Very much as in the case of vinyl acetate, the solution polymerization of allyl acetate in benzene leads to the addition of the growing radical chains to benzene to produce stable aromatic adducts with a tendency to terminate by combination. This explains the retention of the degree of polymerization in the expected range (DP = 14.3-14.9) and the fact that a polymer chain may contain more than one aromatic group [14]. 

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... 

As a result of its highly exothermic nature, bulk polymerization of vinyl acetate poses problems at high conversions. The properties of the resulting polymer are susceptible to deterioration due to chain branching. Therefore, bulk polymerization of vinyl acetate is usually stopped at 20 to 50% conversion. Thereafter, the unreacted monomer is either distilled off or the polymer precipitated with a suitable solvent (methanol, ethanol). Poly (vinyl acetate) is manufactured primarily by free-radical-initiated emulsion and, sometimes, solution polymerization. 

There are four kinds of polymerization processes bulk, solution, emulsion, and suspension polymerization. As Table 4.7 shows [24], the heat of polymerization of vinyl acetate is high compared to other monomers hence, the control of temperature is difficult in bulk polymerization. In the case of emulsion and suspension polymerization, it is somewhat troublesome to separate dispersed polyvinyl acetate particles from the aqueous medium, and it is necessary to remove the emulsifier and stabilizer completely because these substances induce problems in the process of fiber-making. 

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