Polyfvinyl acetate

The degree of branching in PVAc is strongly dependent on the polymerization conditions. Differences in the degree of branching are thought to be one of the main factors responsible for substantial differences in properties between various commercial samples of PVAc or PVA.203 205 

The presence of hydrolyzable long chain branches in PVAc was established by McDowell and Kenyon206 in 1940. They observed a reduction in molecular weight obtained on successively hydrolyzing and reacetylating samples of PVAc. Only branches to the acetate methyl will be lost on hydrolysis of the polymer i.e. on conversion of PVAc to PVA. 

The proposal that PVAc also has non-hydrolyzable long chain branches stems from the finding that PVA also possesses long chain branches. No/akura et a/.171 07 suggested, on the basis of kinetic measurements coupled with chemical analysis, that chain transfer to PVAc involves preferential abstraction of backbone (methine) hydrogens (ca 5 1 v,v the acetate methyl hydrogens at 60 °C). 

H and nC NMR studies on PVAc or PVA also provide information on the nature of branches.30 1,204 508,209 Dunn and Naravane 0 and Bugada and Rudin204 proposed that the difference in intensity of the methylene and methine regions of the l3C NMR spectrum could be used as a quantitative measure of the non-hydrolyzable branches (short chain f long chain) in PVA. However, this approach has been questioned by Vercauteren and Dormers 4 because of the relatively large errors inherent hi the method. 

The extent of branching, of whatever type, is dependent on the polymerization conditions and, in particular, on the solvent and temperature employed and the degree of conversion. Nozakura et at.1 1 found that, during bulk polymerization of VAc, the extent of transfer to polymer increased and the selectivity (for abstraction of a backbone vs an acetoxy hydrogen) decreases with increasing temperature. 

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It may also be mentioned that a number of commercial polymers are produced by chemical modification of other polymers, either natural or synthetic. Examples are cellulose acetate from the naturally occurring polymer cellulose, poly(vinyl alcohol) from polyfvinyl acetate) and chlorosulphonated polyethylene (Hypalon) from polyethylene. 

Over the past years considerable attention has been paid to the dispersing system since this controls the porosity of the particle. This is important both to ensure quick removal of vinyl chloride monomer after polymerisation and also to achieve easy processing and dry blendable polymers. Amongst materials quoted as protective colloids are vinyl acetate-maleic anhydride copolymers, fatty acid esters of glycerol, ethylene glycol and pentaerythritol, and, more recently, mixed cellulose ethers and partially hydrolysed polyfvinyl acetate). Much recent emphasis has been on mixed systems. 

Polyfvinyl acetate) Resin derived from the polymerization of vinyl... 

Vinyl acetate h2c chococh3 Polyfvinyl acetate) Paint, adhesives, foams... 

Polyfvinyl acetate), uses of, 242 Poly(vinyl butyral), uses of, 1222 Poly(vinyl chloride), plasticizers in, 1216... 

Poly(vinyl alcohol) has the structure 10.67. Poly(vinyl acetate) is the fully esterified derivative of polyfvinyl alcohol), in which the -OH groups are replaced by -OCOCH3 groups. As indicated in Table 10.5, commercial polyvinyl sizes are effectively copolymers of polyfvinyl acetate) and polyfvinyl alcohol) that vary in the degree of saponification of the ester groups. These products may comprise 100% of either polymer, or combinations of the two monomers in any proportions. Crotonic acid (2-butenoic acid), widely used in the preparation of resins, may also be a component. This compound exhibits cis-trans isomerism (Scheme 10.17). The solid trans form is produced readily by catalysed rearrangement of the liquid cis isomer. 

Arsenic vesicants have been thickened with various substances to enhance deployment, increase their persistency, and increase the risk of percutaneous exposure. Thickeners include polyalkyl methacrylates (methyl, ethyl, butyl, isobutyl), polyfvinyl acetate), polystyrene, plexiglas, alloprene, polychlorinated isoprene, nitrocellulose, as well as bleached montan and lignite waxes. Military thickener K125 is a mixture of methyl, ethyl, and butyl polymethacrylates. When thickened, agents become sticky with a consistency similar to honey. Typically, not enough thickener is added to affect either the color or odor of the agent. 

Fig. 2. Typical example of the variation of the diffusion coefficient with concentration in concentrated polymeric solutions. The system shown is polyfvinyl acetate) and toluene. (Reproduced with permission from Ju el al., 1981.)...

Manufacture. PVBs are manufactured by a variety of two-stage heterogeneous processes. In one of these an alcohol solution of polyfvinyl acetate) and an acid catalyst are heated to 60-80°C with strong agitation. As the poly(vinyl alcohol) forms, it precipitates from solution. As the reaction approaches completion the reactants go into solution. When the reaction is complete, the catalyst is neutralized and the PVB is precipitated trom solution with water, washed, and centrifuged and dried. Resin from this process has very low residual vinyl acetate and very low levels of gel from intennolecular acetalization. 

Polyfvinyl acetate) (PVAc) latexes produced by batch and continuous emulsion polymerization were used in this study. Details for the apparatus and the polymerization procedure can be found in Penlidis et al. (6,12,K3). Samples taken during the reaction were subsequently analyzed to follow conversion- and particle growth-time histories. The batch experimental runs were designed to yield similar conversion-time histories but different particle sizes. Conversion was measured both off-line, by gravimetric analysis, and on-line using an on-line densitometer (a U-tube DPR-YWE model with a Y-mode oscillator with a PTE-98 excitation cell and a DPR-2000 electronic board by Anton Paar, Austria). A number of runs were repeated to check for reproducibility of the results. Four batch runs are described in Table I below and their conversion histories are plotted in Figure 1. 

When such comparisons are made it becomes clear that the reactivities of radicals, monomers, or transfer agents depend on the particular reaction being considered. It is not possible to conclude, for example, that polyfvinyl acetate) radical will always react x times more rapidly than polystyrene radical in addition reactions or y times as rapidly in the atom abstraction reactions involved in chain transfer. Similarly the relative order of efficiency of chain transfer agents will not be the same for all radical polymerizations. This is because resonance, sleric, and polar influences all come into play and their effects can depend on the particular species involved in a reaction. 

Monomers that yield radicals in which the unpaired electron isextensively delocalized have ground state structures that are themselves resonance stabilized. The important factor is the relative stability of the product radical, however, because a single electron is more easily delocalized than one in a C=C double bond. Thus resonance stabilization causes as increase in monomer reactivity and a decrease in reactivity of the resulting polymer radical. Styrene is more reactive toward polymerization than vinyl acetate, for example, and the propagation rate in the former polymerization is much slower than in the radical synthesis of polyfvinyl acetate). 

Fig. 1. Solubility coefficients for a number of gases in various polymers above their j ass transition temperature. poly methyl vinyl ether) polyfvinyl acetate) a poly(methyl vinyl ketone) poly-(methyl acrylate) OFEP o poly-(dimethyl siloxane) o polytphenyl siloxane). The solid line is that for natural rubber (Van Amerongen and Barret, see Ref. l, p. 65)...

Poly(vinyl alcohol) is typically obtained by alcoholysis of poly(vinyl esters), for example from polyfvinyl acetate) and methanol in the presence of NaOH. The process can be completed or only partially conducted. In this latter case a copolymer (alcohol/ester) is obtained. Other synthetic procedures are used, most of them also based on the hydrolysis of poly(vinyl esters). Poly(vinyl alcohol) is typically used in the atactic form, but isotactic or syndiotactic poly(vinyl alcohols) also are known. 

Treatment of poly(vinyl alcohol) with aldehydes and ketones leads to the formation of polyfvinyl acetals) and polyfvinyl ketals), of which only the former products are of any commercial significance Figure 14.7). 

Scheme 55 Polyfvinyl acetate) oligomers bearing an alcohol chain end...

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