Vinyl acetate polymerization side reactions

Some monomers are also polymerized by a cationic mechanism in a series of steps not too unlike those of anionic chain-growth. Initiators are often Lewis acids such as AICI3. The polymerization is not quite as straightforward as anionic, because for one thing cationic intermediates are subject to more side reactions. Common monomers that undergo cationic polymerization include styrene, isobutylene, and vinyl acetate. Some commercial products... 

One version of the gas phase process was developed by National Distillers Products (now Quantum Chemical) in the USA and another independently in Germany by Bayer together with Hoechst. In both versions, ethylene is reacted with acetic acid and oxygen on a palladium-containing fixed-bed catalyst at 5-10 bar and 175-200°C to form vinyl acetate and water. The explosion limit restricts the O2 content in the feed mixture so that the ethylene conversion is relatively small ( 10%). The acetic acid conversion is 20-35% with selectivi-ties to vinyl acetate of up to 94% (based on C2H4) and about 98-99% (based on AcOH). The most important side reaction of this process is the total oxidation of ethylene to carbon dioxide and water. Other by-products are acetaldehyde, ethyl acetate and heavy ends. After a multistep distillation the vinyl acetate purity is 99.9% with traces of methyl acetate and ethyl acetate that do not affect the subsequent use in polymerization. 

Ethylene, vinyl acetate, and acrylonitrile are polymerized in this way. The redox initiated polymerization of acrylonitrile is an example of precipitation polymerization where the polyacrylonitrile formed is insoluble in water and separates as a powder. This can lead to undesirable side reactions known as popcorn polymerizations when tough cross-linked nodules of polymer grow rapidly and foul the feed lines in industrial plants. 

This system is suitable for the polymerization of polar functional monomers and the synthesis of polymers of various shapes. Figure 2.30 shows the living polymerization of polar monomers containing Si, r-butyldimethylsilyl vinyl ether. Polymerization was carried out using Eti sAlCli s in toluene at 0 °C in the presence of ethyl acetate.The C—0-Si bond is known to be readily hydrolyzed by free acid, and the use of added base was effective for inducing living polymerization without such side reactions. The molecular weight increased in direct proportion to the conversion, and the Mw /Mn was as low as 1.04. 

In vinyl acetate (VA) bulk or solution polymerization systems, side reactions (e.g., chain transfer or termination) will inevitably occur to produce highly branched poly(vinyl acetate) (PVA), based on the nonconjugated nature of the propagating radical. However, the polymerization of VA in nanochannels of [Cu2(terephthalate)2ted] effectively suppresses chain branching during the polymerization, and this results in a constrained chain growth in the narrow 1-D nanochannels [26]. 

Similar statistical limits also apply to the irreversible reaction of polymeric side groups with bifunctional low-molar-mass compounds. If all groups are in the 1,3-position, then theoretically, l/e of the groups cannot react (see Appendix A23). This corresponds to a maximum theoretical conversion of 86.5% in the reaction of poly(vinyl alcohol) with aldehydes to produce poly(vinyl acetals) ... 

When depropagation takes place at an elevated temperature, at a rate that is equal to the propagation in a free-radical polymerization, then the temperature of the reaction is a ceiling temperature (see Chap. 3). Termination can take place by disproportionation. Secmidary reactions, however, may occur in the degradation process depending upon the chemical structure of the polymer. Such side reactions can, for instance, be successive eliminations of hydrochloric acid, as in poly(vinyl chrolide), or acetic acid as in poly(vinyl acetate). 

ATRP is successfully employed in the polymerization of a large variety of vinyl monomers such as styrenes, methacrylates, acrylates, acrylonitrile, and some others [2,9-15]. However, at present, available catalytic systems seem to be unsuitable for the less reactive monomers such as ethylene, olefines, vinyl chloride, and vinyl acetate. In the polymerization of monomers with strong electron-donating groups such as /7-methoxy styrene, some side reactions arising from the involvement of cationic intermediate are observed. Acrylic and methacrylic acids are also not prone to ATRP because they form Cu(II) carboxylates, which are inefficient deactivators. However, hydroxy derivatives such as hydroxyethyl acrylate and hydroxyethyl methacrylate can be polymerized by ATRP. 

---