Vinyl acetate transfer agent

This is a kinetic resolution that works by enantioselective acylation of the unwanted enantiomer of the alcohol. The reaction is therefore an ester exchange and vinyl acetate is an efficient acetate transfer agent since the other product is vinyl alcohol better known as the enol of acetaldehyde so the reaction is irreversible. 

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. 

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). 

Chain transfer also occurs to the emulsifying agents, leading to their permanent iacorporation iato the product. Chain transfer to aldehydes, which may be formed as a result of the hydrolysis of the vinyl acetate monomer, tends to lower the molecular weight and slow the polymerisation rate because of the lower activity of the radical that is formed. Thus, the presence of acetaldehyde condensates as a poly(vinyl alcohol) impurity strongly retards polymerisation (91). Some of the initiators such as lauryl peroxide are also chain-transfer agents and lower the molecular weight of the product. 

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

Vinyl iodides are considerably more reactive than bromides in the vinylations. It may be presumed that chlorides are not generally useful, with one exception noted below, since they have not been employed in the reaction. The bromides are usually reacted with a palladium acetate-triphenyl- or tri-o-tolyl-phos-phine catalyst at about 100 C. The reaction will occur without the phosphine if a secondary amine is present. Vinyl iodides will react in the absence of a phosphine even with only a tertiary amine present.48 37 The iodides are so reactive, in fact, that reactions occur even at room temperature if potassium carbonate is the base and tetra-zi-butylammonium chloride is used as phase transfer agent in DMF solution when palladium acetate is the catalyst.88... 

Troth (32) has studied the polymerization of vinyl acetate in the presence of triphenylmethane and observed the effects discussed above. In practice, there are complications resulting from reactions involving the initiator radicals and the transfer agent. These complications were found also when carbon tetrabromide was used as a transfer agent in the polymerization of styrene in this case, the bromine contents of the polymers were determined by neutron activation analysis (17). 

It is evident that the values of the transfer constants are dependent on the nature both of the attacking radicals and of the transfer agent itself, and that similar effects should be expected during the synthesis of graft copolymers by chain transfer methods. For example, with respect to toluene the chain transfer constant is a little greater for methyl methacrylate radicals than for styrene radicals on the contrary, with respect to halogenated solvents (CC14) the polystyrene radical is much more effective in the removal of a chlorine atom. Vinyl acetate chains are far more effective than either of the other two polymer radicals. 

Table 10.1 presents typical specifications for a polymerization-grade product, as well as some physical properties. Prohibited impurities refer to inhibitors (croton-aldehyde, vinyl acetylene), chain-transfer agents (acetic acid, acetaldehyde, acetone) and polymerizable species (vinyl crotonate), while methyl and ethyl acetate impurities are tolerated. 

Chemical Transformations. Operations that modify a mer, such as saponifying poly (vinyl acetate) to poly (vinyl alcohol), should be considered. The introduction of chain transfer agents, 1, which limit molecular weight, need consideration for many cases. The symbol i for initiator might be introduced. However, details of these operations are beyond the scope of the present work. 

A very efficient transfer agent in polymerizations of styrene, vinyl acetate and methyl methacrylate is CBr4. 

Table 10.1 lists approximate transfer constants of some common agents in polymerization of styrene, methyl methacrylate, and vinyl acetate. 

Vinyl acetate was polymerized in a free-radical reaction. The initial monomer concentration was 1 mol/liter and its concentration after I h was 0.85 mol/liter. Chloroform was present as a chain transfer agent, with concentrations 0.01 mol/liter at time zero and 0.007 mol/liter after I h. What is the chain transfer constant C in this case ... 

Prochiral Compounds. The enantiodifferentiation of prochi-ral compounds by lipase-catalyzed hydrolysis and transesterification reactions is fairly common, with prochiral 1,3-diols most frequently employed as substrates. Recent reports of asymmetric hydrolysis include diesters of 2-substituted 1,3-propanediols and 2-0-protected glycerol derivatives. The asymmetric transesterification of prochiral diols such as 2-0-benzylglycerol and various other 2-substituted 1,3-propanediol derivatives is also fairly common, most frequently with Vinyl Acetate as an irreversible acyl transfer agent. 

The asymmetric transesterification of cyclic me o-diols, usually with vinyl acetate as an irreversible acyl transfer agent, includes monocyclic cycloalkene diol derivatives, bicyclic diols, such as the ej o-acetonide in eq 12, bicyclic diols of the norbomyl type, andorganometallic l,2-bis(hydroxymethyl)ferrocenepossessing planar chirality. 

In the absence of transfer agent, when the rate of radical desorption from the partides and the reactivity of monomer are vtxy high, as with vinyl acetate and vinyl chloride, the ccmdition is fuliilled. For such... 

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