Vinyl acetate polymerization processes

Vinyl alcohol does not exist as a monomer, but Herrmann and Haehnel (1) were able to obtain the desired product poly(vinyl alcohol) [9002-89-5] (PVA), by polymerizing vinyl acetate and then hydrolyzing the resultant poly(vinyl acetate). This process is employed for the commercial production of PVA even now. The principal concern of the discoverers was development of a suture for surgical operations the fiber then obtained was not suited for clothing use (2). 

Loop A continuous process for polymerizing aqueous emulsions of olefinic compounds such as vinyl acetate. Polymerization takes place in a tubular reactor (the loop) with recycle. Invented by Gulf Oil Canada in 1971 and further developed by several United Kingdom paint companies. It is now used for making copolymers of vinyl acetate with ethylene, used in solvent-free paints and adhesives. 

In 1952 W. J. Priest, in an important paper, laid out all of the basic qualitative features of the theory of homogeneous nucleation in emulsion polymerization as it is known today (12). This was based upon his studies of particle size distributions in vinyl acetate polymerization initiated by potassium persulfate (K2S20g) in the presence of varying amounts of different stabilizers and inhibitors at several temperatures. Priest proposed that (1) "polymerization in solution is the initial process" ... 

The effect of oxygen on the polymerization characteristics of the monomer is quite dramatic. In bulk polymerization of reasonably purified vinyl acetate, the process is autoaccelerated from the start and goes nearly to completion with a residual monomer content of 2-4%. When the monomer distillation is carried out in contact with air, inhibiting impurities form quite rapidly. These lead to dead-end polymerizations with 30-40% unreacted monomer left in the product [17]. 

Vinyl acetate polymerizes very easily by radical mechanism the technical method of polymerization is also a radical process. First, a typical radical polymerization scheme of the vinyl monomer takes place in the presence of an initiator, I, to yield a pair of free radicals R ... 

The reaction was performed in ethyl acetate solution, under nitrogen atmosphere, in the presence of the radical initiator (AIBN) and under the same reaction conditions (60°C) as used for the vinyl acetate polymerization. In particular, nitrogen atmosphere was used. Therefore, the reactions involved in the cross-linking process of a drying oil in the oxygen atmosphere were not present in this case. 

By about 1947 many of the major themes of free radical chemistry had emerged from this war-initiated polymer research data on structure and reactivity and establishment of the utility of competitive kinetics for determining relative rates of chain propagation steps in chain reactions by product analyses as I ve just described th first good rate constants for elementary steps in chain processes - here the first really reliable results were on vinyl acetate polymerization from Paul Bartlett s laboratory ( ) degradative chain transfer, first demonstrated by Bartlett and Altschul and showing how the length of kinetic... 

Poly(vinyl acetate) polymerization is accomplished by conventional processes, e.g., solution, bulk, or emulsion polymerization. Solution polymerization is favored because the subsequent alcoholysis reaction requires solvent addition. The polymerization step determines the ultimate molecular weight of the PVOH. Catalyst concentration, temperature, and solvent control the degree of polymerization acetaldehyde is an effective chain-transfer agent. It is the agent commonly used. 

The continuous bulk polymerization of methyl methacrylate was used as an example in Section 5.2. A stirred bulk polymerization like that used for styrene (Section 5.4) could be adapted for methyl methacrylate. A suspension process for poly(methyl methacrylate) was described in Section 5.4. The polymerization of ethyl acrylate most often is carried out in emulsion. A process such as that used for vinyl acetate is suitable (Section 16.4). Like vinyl acetate, the monomer is slightly water soluble, so true emulsion polymerization kinetics are not followed. That is, there is initiation of monomer dissolved in water in addition to that dissolved in growing polymer particles. Ethyl acrylate is distinguished by its rapid rate of propagation. Initiation of a 20% monomer emulsion at room temperature by the redox couple persulfate-metabisulflte can result in over 95% conversion in less than a minute. As with vinyl acetate polymerization, a continuous addition of monomer at a rate commensurate with the heat transfer capacity of the reactor is necessary in order to control the temperature. 

Almost all synthetic binders are prepared by an emulsion polymerization process and are suppHed as latexes which consist of 48—52 wt % polymer dispersed in water (101). The largest-volume binder is styrene—butadiene copolymer [9003-55-8] (SBR) latex. Most SBRlatexes are carboxylated, ie, they contain copolymerized acidic monomers. Other latex binders are based on poly(vinyl acetate) [9003-20-7] and on polymers of acrylate esters. Poly(vinyl alcohol) is a water-soluble, synthetic biader which is prepared by the hydrolysis of poly(viayl acetate) (see Latex technology Vinyl polymers). 

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). 

Continuous polymerization systems offer the possibiUty of several advantages including better heat transfer and cooling capacity, reduction in downtime, more uniform products, and less raw material handling (59,60). In some continuous emulsion homopolymerization processes, materials are added continuously to a first ketde and partially polymerized, then passed into a second reactor where, with additional initiator, the reaction is concluded. Continuous emulsion copolymerizations of vinyl acetate with ethylene have been described (61—64). Recirculating loop reactors which have high heat-transfer rates have found use for the manufacture of latexes for paint appHcations (59). 

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. At very low levels of stabilizer, eg, 0.1 wt %, the polymer does not form a creamy dispersion that stays indefinitely suspended in the aqueous phase but forms small beads that setde and may be easily separated by filtration (qv) (69). This suspension or pearl polymerization process has been used to prepare polymers for adhesive and coating appHcations and for conversion to poly(vinyl alcohol). Products in bead form are available from several commercial suppHers of PVAc resins. Suspension polymerizations are carried out with monomer-soluble initiators predominantly, with low levels of stabilizers. Suspension copolymerization processes for the production of vinyl acetate—ethylene bead products have been described and the properties of the copolymers determined (70). Continuous tubular polymerization of vinyl acetate in suspension (71,72) yields stable dispersions of beads with narrow particle size distributions at high yields. 

Copolymers of vinyl chloride, containing 5 to 40 percent vinyl acetate made by the inclusion of vinyl acetate in the polymerization process, have lower softening points and flow more easily than polyvinyl chloride. They are soluble in ketones, such as acetone, and certain esters for making film from solutions. They are used for phonograph records, rigid clear sheeting, and molding pov... 

A process for the preparation of porous polyvinyl alcohol gels in three steps is (1) suspension polymerization of vinyl acetate with diethylene glycol dimethacrylate in the presence of a diluent as porogen, (2) saponifying of the resulting porous polyvinyl acetate gel with an alkali, and then (3) subjecting... 

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. 

The free radical initiators are more suitable for the monomers having electron-withdrawing substituents directed to the ethylene nucleus. The monomers having electron-supplying groups can be polymerized better with the ionic initiators. The water solubility of the monomer is another important consideration. Highly water-soluble (relatively polar) monomers are not suitable for the emulsion polymerization process since most of the monomer polymerizes within the continuous medium, The detailed emulsion polymerization procedures for various monomers, including styrene [59-64], butadiene [61,63,64], vinyl acetate [62,64], vinyl chloride [62,64,65], alkyl acrylates [61-63,65], alkyl methacrylates [62,64], chloroprene [63], and isoprene [61,63] are available in the literature. 

For composites with polymerization-modified filler it is typical that the physico-mechanical characteristics should increase symbatically with the quantity of polymer which becomes attached to the filler in the polymerization process. This effect has been observed for polyethylene [293, 321], poly(vinyl chloride) coats [316], and in [336, 337] for kaolin coated with poly(vinyl acetate) and introduced into the copolymer of ethylene and vinyl acetate. 

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