Vinyl acetate polymerizations initiation

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 initiators used m vinyl acetate polymerizations are the familiar free-radical types, Buffers are frequently added to emulsion recipes. Vinyl acetate emulsion polymerization recipes are usually buffered to pH 4-5. The pH of most commercially available emulsions is 4-6. 

Ten years later Napper and Alexander, in studying the kinetics of vinyl acetate polymerization in the presence of anionic, cationic and nonionic emulsifiers arrived at the same conclusions as Priest s, although they did not cite his work (13). They also observed an acceleratory effect of added emulsifier like that found by Baxendale et al. (8), but they seemed unaware of that work as well. They showed that when the charge on primary particles (due to initiator fragments) was opposite to that of the emulsifier, the rate of polymerization was slower than that in the absence of emulsifier. This was presumably due to the greater instability of the colloid formed and the consequent production of fewer polymerizing centers. 

Figures 7. Simulated start-up of vinyl acetate polymerization at low emulsifier level (0.01 mol/L H20) under closed-loop control with arbitrarily selected controller tuning constants and manipulation of initiator flow rate at 50°C conversion in R1—STD feedback (--------------------------) vs. DTC (----)...

Figure 24. Simulated response of third reactor of a continuous vinyl acetate polymerization to a step change in setpoint at high emulsifier feed concentration (0.06 mol/L H>0) and manipulation of initiator flow rate to the third reactor at 50°C ((---------------------] optimum PID) (-----) Z transform)...

Continuous Polymerization. A typical continuous flow diagram for the vinyl acetate polymerization is shown in Figure 12. The vinyl acetate is fed to the first reactor vessel, in which the mixture is purged with an inert gas such as nitrogen. Alternatively, the feed maybe purged before being introduced to the reactor (209). A methanol solution containing the free-radical initiator is combined with the above stream and passed directly and continuously into the first reactor from which a stream of the polymerization mixture is continuously withdrawn and passed to subsequent reactors. More initiator can be added to these reactors to further increase the conversion. 

The transfer reactions to the solvent and the initiator have been described for butadiene, isoprene, or vinyl acetate polymerizations using thermally decomposed hydrogen peroxide in methanol or rm-pentanol (Table 3.5)l55). The Mayo-Lewis equation has been applied... 

In the section concerning the synthesis of hydroxytelechelic polymers initiated by thermally or photochemically decomposed hydrogen peroxide, the molecular weight distribution of polymers has been found to be dependent on solution homogeneity. A unimodal distribution of molecular weights is observed in vinyl acetate polymerization (true solutions), a bimodal one was found for polydienes, and sometimes a tri-modal one for poly(methyl methacrylate) (non-regular solutions). 

The development takes into account transfer to monomer, transfer to polymer, and terminal double bond polymerization. For the vinyl acetate system where transfer to monomer is high, the generation of radicals by transfer to monomer is much greater than the generation of radicals by initiation, so that essentially all radicals present have terminal double bonds hence, effectively all dead polymer molecules also have a terminal double bond. Thus, for vinyl acetate polymerization, the terminal double bond polymerization can be significant, and has been built into the development. The equations for the moments of the molecular weight distribution and the average number of branches per polymer molecule are as follows ... 

The composition of a copolymer produced by simultaneous polymerization of two monomers is usually different from the composition of the comonomer feed from which it is produced. This shows that different monomers have different tendencies to undergo copolymerization. These tendencies often have little or no resemblance to their behavior in homopolymerization. Some monomers are more reactive in copolymerization than indicated by their rates of homopolymerization, and some monomers are less reactive. Thus, vinyl acetate polymerizes about twenty times as fast as styrene in a free-radical reaction, but the product in free-radical polymerization of a mixture of vinyl acetate and styrene is found to be almost pure polystyrene with practically no content of vinyl acetate. By contrast, maleic anhydride, which has very little or no tendency to undergo homopolymerization with radical initiation, undergoes facile copolymerization with styrene forming one-to-one copolymers. 

Table 6. Initiator polymerization rate, initiation rate and derived kp/kt values for vinyl acetate polymerization in various solvents at 30 °C17)...

Bilimoria et al. investigated the inhibition of radical initiated polymerization of vinyl acetate by tobacco smoke and some PAHs in 1973 (329). Their results indicated that vapor phase of smoke is an efficient inhibitor of vinyl acetate polymerization and that conjugated dienes like isoprene are responsible for the inhibition. There were no free radicals from tobacco smoke specifically identified in this research. Nisbet and Schmeller presented the results of this research at the 27th Tobacco Chemists Research Conference (TCRC) in 1973 (2789a). 

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]. 

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. 

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