Vinyl acetate polymerization chain transfer

In the Soviet study110, the following elementary stages were taken into account in the kinetic scheme of vinyl acetate polymerization chain transfer to the monomer, solvent, and polymer, and chain termination caused by the disproportionation of radicals. It was assumed that long-chain branches could be formed by chain transfer both to the acetate group hydrogen atoms and to the main chain hydrogen. 

Among the effects that poly(vinyl alcohol) may have in emulsion polymerization systems are adsorption on the poly(vinyl acetate) latex, chain-transfer reactions, enhancement of the initiation rate since it increases the rate of decomposition of potassium persulfate, the oxidation of poly(vinyl alcohol) by the initiator, solubilization of monomer and polymer [148]. 

In the polymerization of vinyl acetate, chain transfer to the acetate portion of the monomer is an important aspect of the process (see Chapter 7 of this volume). In the case of the allyl acetate polymerization, chain transfer to the acetate moiety is considered negligible as compared to the degradative chain-transfer process [14, 15]. 

It is interesting to note that the type of polymerization techniques (conventional emulsion polymerization versus microemulsion polymerization) can have a significant influence on the chain transfer reactions and, consequently, polymer properties [59], For example, in the microemulsion polymerization of vinyl acetate, the chain transfer reaction of a polymeric radical to monomer is the predominant mechanism that terminates the free radical reactivity. Thus, the resultant polyvinyl acetate exhibits a lower degree of branching than that produced by the conventional emulsion polymerization process, in which the chain transfer reaction of a polymeric radical to polymer is significant. 

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). 

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 chain-transfer agent is added to vinyl acetate polymerizations to control the polymer molecular weight,... 

Chain Transfer. At the molecular scale, vinyl acetate polymerizations generally are understood as free-radical polymerizations, but are characterized in particular by a relatively large amount of chain transfer. 

Investigation has shown that chain transfer to polymer occurs predominantly on the acetate methyl group in preference to the chain backbone one estimate of the magnitude of the predominance is 40-fold (92,93). The number of branches per molecule of poly(vinyl acetate) polymerized at 60°C is ca 3, at 80% conversion. It rises rapidly thereafter and is ca 15 at 95% conversion and 1—2 x 104 number-average degrees of polymerization. 

Chain transfer with a monomer occurs, for instance, in the early stages of a vinyl acetate polymerization ... 

When we combine this observation with the autoaccelerating tendencies of the system, the chain-transfer reactions to both the monomer and the polymer on one of the several positions which leads to branched-chain formation, and the possible reactivation of dead polymer molecules by hydrogen abstraction with monomeric free radicals [78], the complexity of the kinetics of vinyl acetate polymerization may be appreciated. Similar factors may be involved not only in the polymerization of other vinyl esters, but also in the fiee-radical polymerization of other types of monomers. 

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 ... 

Keddie, D.J., et al. 2012. Chain transfer kineties of acid/base switchable N-aryl-N-pyridyl dithiocarbamate RAFT agents in methyl acrylate. N-Vinylcarbazole and Vinyl Acetate Polymerization. Macromolecules 45(10) 4205—4215. 

The transfer coefficients of disulfides are extremely low for styrene and methyl methacrylate (see Table 4), but are close to one for the vinyl acetate polymerization. In general the xanthogens and thiurams have higher chain transfer ability, which has been attributed to the iniferter mechanism described above. Monosulfides have lower transfer coefficients in comparison to disulfides. This may refiect steric factors and the relative strength of the C—S bond, which is significantly stronger than the S—S bond. 

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. 

Backbiting is an intramolecular chain transfer reaction. If transfer reactions occur between different chains, long-chain branched polymers are formed. Well-known examples include ethylene and vinyl acetate. Vinyl acetate polymerization could lead to gel formation under certain conditions. It should be pointed out that chain transfer to polymer reaction alone generates only T-type branch structures that do not result in gel formation. Theoretically, some mechanism such as radical termination by combination that brings two chains together to form H-type branch structures is an essential condition for gelation. 

On the other hand, when the propagating radicals (pol5rvinyl acetate, ethylene, and vinyl chloride) have very high reactivity, the Cm is usually large. In the case of vinyl acetate polymerization [26], chain transfer to monomer has been generally attributed to transfer from the acetoxy methyl group ... 

Buffers are frequently added to emulsion recipes and serve two main purposes. The rate of hydrolysis of vinyl acetate and some comonomers is pH-sensitive. Hydrolysis of monomer produces acetic acid, which can affect the initiator, and acetaldehyde which as a chain-transfer agent may lower the molecular weight of the polymer undesirably. The rates of decomposition of some initiators are affected by pH and the buffer is added to stabilize those rates, since decomposition of the initiator frequently changes the pH in an unbuffered system. Vinyl acetate emulsion polymerization recipes are usually buffered to pH 4—5, eg, with phosphate or acetate, but buffering at neutral pH with bicarbonate also gives excellent results. The pH of most commercially available emulsions is 4—6. 

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). 

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