Molecular weight vinyl acetate polymerization

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,... 

One way to distinguish between the two possibilities is to study the isotope effect on the kinetics of vinyl acetate polymerization and on the polymer molecular weight. The deuterium isotope eftect has been ascribed to the difference in the zero point energies of the stretching vibrations of the C-H and C-D bond (11). The rate of a reaction in which deuterium is transferred is slower than that of the corresponding reaction for hydrogen, since the C-D bond has a lower zero point energy. 

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 ... 

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. 

Janowicz disclosed that molecular weight reductions result from addition of the cobaloxime derivative (3) to most acrylates, but little or no effect is seen in vinyl acetate polymerizations. 

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. 

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). 

The kinetics of vinyl acetate emulsion polymeriza tion in the presence of alkyl phenyl ethoxylate surfactants of various chain lengths indicate that part of the emulsion polymerization occurs in the aqueous phase and part in the particles (115). A study of the emulsion polymerization of vinyl acetate in the presence of sodium lauryl sulfate reveals that a water-soluble poly(vinyl acetate)—sodium dodecyl sulfate polyelectrolyte complex forms, and that latex stabihty, polymer hydrolysis, and molecular weight are controlled by this phenomenon (116). 

Partially hydrolyzed poly(vinyl alcohol) grades are preferred because they have a hydrophobic /hydrophilic balance that make them uniquely suited for emulsion polymerization. The compatibUity of the residual acetate units with the poly(vinyl acetate) latex particles partly explains the observed stabilization effect. The amount of PVA employed is normally 4—10% on the weight of vinyl acetate monomer. The viscosity of the resulting latex increases with increasing molecular weight and decreasing hydrolysis of the PVA (318). 

Among the different pressure sensitive adhesives, acrylates are unique because they are one of the few materials that can be synthesized to be inherently tacky. Indeed, polyvinylethers, some amorphous polyolefins, and some ethylene-vinyl acetate copolymers are the only other polymers that share this unique property. Because of the access to a wide range of commercial monomers, their relatively low cost, and their ease of polymerization, acrylates have become the dominant single component pressure sensitive adhesive materials used in the industry. Other PSAs, such as those based on natural rubber or synthetic block copolymers with rubbery midblock require compounding of the elastomer with low molecular weight additives such as tackifiers, oils, and/or plasticizers. The absence of these low molecular weight additives can have some desirable advantages, such as ... 

In a very recent development, Debuigne et at. of vinyl acetate at 30 °C mediated by Co"(acac)2 (121). They obtained predictable molecular weights up to Mn 100000 and dispersities < 1.3 and proposed a polymerization mechanism analogous to that shown in Scheme 9.27. The complex... 

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