Vinyl acetate bulk polymerizations

For the copolymerization of ethene and vinyl acetate, solution polymerization, suspension polymerization, emulsion polymerization and bulk polymerization may be used, but solution polymerization is preferred (1). A method of either continuous type or batch type may be employed. Methanol is generally used as the solvent. 

We will consider the MWD in two simple cases. The first is when chain transfer is sufficiently rapid to ensure that all other chain-stopping events can be ignored. In such a situation, whereas the compartmentalized nature of the reaction may affect the rate of initiation of new chains, it will not affect the lifetime distributions of the chains once they are formed. The MWD may then be found from the bulk formulas, provided only that the average number of free radicals per particle, is known. Such an approach has been used by Friis et al. (1974) to calculate the MWD evolved in a vinyl acetate emukion polymerization. These authors included in addition the mechanisms of terminal bond polymerization and of transfer to polymer (both of which cause broadening). The formulas required for the in corporation of these mechanisms could be taken from bulk theory. 

Suspension polymerization is designed to combine the advantages of both the bulk and solution polymerization techniques. It is one of the extensively employed techniques in the mass production of vinyl and related polymers. Suspension polymerization (also referred to as bead or pearl polymerization) is carried out by suspending the monomer as droplets by efficient agitation in a large mass (continuous phase) of nonsolvent, commonly referred to as the dispersion or. suspension medium. Water is invariably used as the suspension medium for all water insoluble monomers because of the many advantages that go with it. Styrene, methyl methacrylate, vinyl chloride, and vinyl acetate are polymerized by the suspension... 

Vinyl acetate is polymerized free radically in bulk, emulsion, or suspension. Bulk polymerization occurs at the boiling temperature of the monomer (72.5 C at 1 bar), and yields highly branched polymer because of chain transfer via the ester groups (see also Section 20.4.3). Commercially, the polymerization is taken to a specific yield and the residual monomer is removed by thin-layer evaporation. Alternatively, continuous polymerization can be carried out in a tower. But this method only produces moderate degrees of polymerization since the tower process requires that the polymer should flow and the flow temperature should lie below the decomposition tempera-... 

Polymerization Processes. Vinyl acetate has been polymerized industrially by bulk, solution, suspension, and emulsion processes (34). Perhaps 90% of the material identified as poly(vinyl acetate) or copolymers that are predominantly vinyl acetate are made by emulsion techniques. Detailed information is in patent and scientific Hterature and in procedures available in the brochures from monomer producing companies (15,34). 

Often a chain-transfer agent is added to vinyl acetate polymerizations, whether emulsion, suspension, solution, or bulk, to control the polymer molecular weight. Aldehydes, thiols, carbon tetrachloride, etc, have been added. Some emulsion procedures call for the recipe to include a quantity of preformed PVAc emulsion and sometimes antifoamers must be added (see Foams). 

Bulk Polymerizations. In the bulk polymerization of vinyl acetate the viscosity increases significantly as the polymer forms making it difficult to remove heat from the process. Low molecular weight polymers have been made in this fashion. Continuous processes are known to be used for bulk polymerizations (68). 

Suspension polymerization produces beads of plastic for styrene, methyl methacrviaie. viny l chloride, and vinyl acetate production. The monomer, in which the catalyst must be soluble, is maintained in droplet fonn suspended in water by agitation in the presence of a stabilizer such as gelatin each droplet of monomer undergoes bulk polymerization. In emulsion polymerization, ihe monomer is dispersed in water by means of a surfactant to form tiny particles held in suspension I micellcsK The monomer enters the hydrocarbon part of the micelles for polymerization by a... 

Free radical polymerization is a key method used by the polymer industry to produce a wide range of polymers [37]. It is used for the addition polymerization of vinyl monomers including styrene, vinyl acetate, tetrafluoroethylene, methacrylates, acrylates, (meth)acrylonitrile, (meth)acrylamides, etc. in bulk, solution, and aqueous processes. The chemistry is easy to exploit and is tolerant to many functional groups and impurities. 

Under conditions of low radical initiation, Graessely (21) has shown that the following set of equations describes the molecular weight and branching development in the bulk polymerization of vinyl acetate ... 

SOLUTION OF KINETIC EQUATIONS FOR LONG CHAIN BRANCHING IN BULK VINYL ACETATE POLYMERIZATION ... 

Polymerization of a monomer in a solvent overcomes many of the disadvantages of the bulk process. The solvent acts as diluent and aids in the transfer of the heat of polymerization. The solvent also allows easier stirring, since the viscosity of the reaction mixture is decreased. Thermal control is much easier in solution polymerization compared to bulk polymerization. On the other hand, the presence of solvent may present new difficulties. Unless the solvent is chosen with appropriate consideration, chain transfer to solvent can become a problem. Further, the purity of the polymer may be affected if there are difficulties in removal of the solvent. Vinyl acetate, acrylonitrile, and esters of acrylic acid are polymerized in solution. 

Uses. The azobisnitriles have been used for bulk, solution, emulsion, and suspension polymerization of all of the common vinyl monomers, including ethylene, styrene vinyl chloride, vinyl acetate, acylonitrile, and methyl methacrylate. The polymerizations of unsaturated polyesters and copolymerizations of vinyl compounds also have been initiated by these compounds. 

Vinyl acetate has been polymerized by bulk, suspension, solution, and emulsion methods. It copolymerizes readily with some monomers but not with others,... 

Poly(vinyl pyrrotidone). Another commercial polymer with significant usage is PVP. It was developed in World War II as a plasma substitute lor blood. This monomer polymerizes faster in 50 water than it does in bulk, an abnormality inconsistent with general polymerization kinetics. This may be due to a complex with water that activates the monomer it may also be related to the impurities in the monomer that are difficult to remove. See also Vinyl Acetal Polymers. 

Thus these characterization results demonstrate that polymethacrylic acid does not function in the same manner as polyvinyl alcohol in the emulsion polymerization of vinyl acetate and that the adsorbed polymethacrylic acid can be separated from the bulk polymer, thus distinguishing it from grafted polymer. 

Viscometry The specific viscosity of each polymer from the bulk polymerization was measured in acetone at 30°C using an Ubbelohde dilution viscometer. Five concentrations in the range of 1.120 to 0.242 g/d poly(vinyl acetate) and polyvinyl trideuteroacetate) and 0.385 to 0.084 g/dl (poly(trideu-terovinyl acetate)) were run. Intrinsic viscosity was calculated by extrapolation of the Tlsp/c versus c plot to zero concentration. Number average molecular weights were calculated using the equation(20) [q] =1,0 x 10 1 [Mn] 0 72 which is in the mid range of the equations listed. 

Bulk Polymerization The conversion versus time plot for the bulk polymerizations of vinyl acetate and its deuterated analogues is shown in Figure 1. Vinyl tridueteroacetate has a conversion rate of 9.9 x 10 3/min which is identical with that of vinyl acetate (9.5 x 10 3/min) within the experimental error. However, trideuterovinyl acetate has a much higher conversion rate (1.69 x 10 2/min). The ratio of the rate of polymerization of tridueterovinyl acetate to the average of the other two monomers is 1.74 -. 03. 

Model and Nature of Termination Step for Bulk Polymerization of Vinyl Acetate The following is a reasonable kinetic model for the bulk polymerization of vinyl acetate. 

We have seen in Tables 1 and 2 that both the polymerization rate and molecular weight of trideuterovinyl acetate increased compared to that of vinyl acetate. Since the rate increase is identical for bulk and emulsion polymerization and we know that almost all termination in emulsion polymerization is between monomer and growing radical (10), the implication is that the same is true in bulk polymerization. While this conclusion is initially surprising, it supports our argument that the vinyl radical is reasonably stable (10). Otherwise the radical would reinitiate rapidly and its concentration would be too low for termination with it to be important. 

The bulk polymerization rate of trideuterovinyl acetate was found to be 1.78 times that of vinyl acetate. In emulsion polymerization the same result was found. The molecular weight of poly(trideuterovinyl acetate) was 2.59 times that of poly(vinyl acetate). The overall isotope effect on chain transfer to monomer was calculated to be 3.04. Chain transfer was shown to be 94% on the vinyl hydrogens and 6% on the acetyl hydrogens. The measurement of polymerization rates in emulsion polymerization showed that the chain transfer on acetyl hydrogens is kineti-cally insignificant. 

To check whether vinyl esters of strongly branched acids behave differently, mixtures of vinyl acetate and VV 911 in molar ratios of 1/3, 1/1, and 3/1 were polymerized in bulk to a conversion of about 10%, using benzoyl peroxide as initiator at 50°C. The reaction mixtures were then diluted with benzene, and the polymers were precipitated with methanol. After five further dissolutions in benzene and precipitation with methanol the polymers were freeze dried from their solutions in benzene and analyzed for carbon content. The results given in Figure 1 show that, at least for practical purposes, the assumption that r1 = r2 = 1 is valid, and at any time during the polymerization random copolymers are formed at any vinyl acetate-VV 911 ratio. 

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