Poly reacetylation

In the above-mentioned example of the polymer-analogous saponification of poly(vinyl acetate) the reactant and the product differ in their properties, for example, in their solubility however, both compounds have the same average degree of polymerization. The poly(vinyl alcohol) obtained by saponification can, in principle, be esterified back to poly(vinyl acetate) with the original molecular weight the reacetylated polymer then has the same properties as the original material. The viscosity number may be used to check whether in fact any chain scission has occurred during the reaction sequence of saponification and reacetylation (see Example 5-1). 

Poly(vinyl Alcohol) by Transesterification of Poly(vinyl Acetate) Reacetylation of Poly(vinyl Aicohol)... 

It has been known for some time [see Ref. (176) for earlier work] that if poly(vinyl alcohol), produced by hydrolysis of poly(vinyl acetate) is reacetylated, the PVAc so obtained has a lower MW than the original PVAc prior to hydrolysis, though the MW of the material is not lowered any further by subsequent cycles of hydrolysis and reacetylation. Various explanations had been advanced for this phenomenon Wheeler explained it as a consequence of the presence of branches joined to the main chain through ester linkages which would be broken on hydrolysis and not re-formed on reacetylation. These branches were ascribed to chain transfer reactions with acetate groups, either in the polymer, or in monomer molecule subsequently polymerized at their double bonds. Transfer reactions by attack on hydrogen atoms other than those in... 

Lambe et al. (1978) studied the enhanced steric stabilization of polystyrene latices by poly(vinyl alcohol). This is included in this sub-section on copolymers because the samples studied were not fully hydrolysed. This means that the parent poly(vinyl acetate) from which they were derived was only partially (88%) hydrolysed (this is often accomplished by alcoholysis). The resultant polymer is not, however, a completely random copolymer because adjacent group effects influence the hydrolysis kinetics in such a way that some degree of blockiness is introduced. On average, these blocks consist of 2 ester groups to every 18 alcohol groups but blocks of average size 5-6 acetate groups are common. The chemical structure of the polymers should therefore formally represented by a structure intermediate between poly(vinyl acetate-6-vinyl dcohol) and poly(vinyl acetate-co-vinyl alcohol) rather than poly(vinyl alcohol) as such. The random (or statistical) copolymer can be prepared by partial reacetylation of fully hydrolysed poly(vinyl alcohol). 

Interestingly, a grade of poly(vinyl alcohol) which still contains approximately 20% vinyl acetate is more easily dispersed in water than 100% poly(vinyl alcohol). Reasonably random copolymers of vinyl acetate and vinyl alcohol can only be produced by reacetylation of completely hydrolyzed poly(vinyl acetate). 

In the field of poly(vinyl acetate) research, it was noted early that hydrolysis of the polymer to poly(vinyl alcohol) followed by reacetylation visually led to the production of poly(vinyl acetates) of lower average-molecular-weight distribution than that of the original polymer [45], The influence of the conditions of polymerization and of repeated hydrolysis and reacetylation will be discussed further. To be noted here is that Marvel and Inskeep [45] at that time, postulated that the observations were somehow related to aldehyde end-groups in poly-(vinyl acetate) and/or in poly(vinyl alcohol) which, in turn, could form acetals with hydroxyl groups of poly(vinyl alcohol). This concept was extended to the possibility that ketals also might form [46]. 

Commercial poly(vinyl acetate), as is well known, may be subjected either to acid or basic hydrolysis to form a completely hydrolized poly(vinyl alcohol). Upon reacetylation of this poly(vinyl alcohol), the regenerated poly(vinyl acetate) usually exhibits a reduction in molecular weight [47]. 

TABLE V Change in Viscosity of Poly(vinyl acetate) upon Alcoholysis and Reacetylation [47]... 

Poly (vinyl alcohol-co-vinyl acetate) polymers are surface active species which can be used to stabilise latex and oil in water dispersions. In order to understand the properties of these materials, it is necessary that their sequence distributions are well characterised. A number of NMR studies on the microstructure of PVA/PVAc copolymers have been made [51-53] (see also chapter 3). Moritani and Fujiwara [51], for example, have used proton and carbon-13 NMR spectroscopy to extract dyad distributions for a range of copolymers with different degrees of deacetylation. Samples were prepared using one of three routes direct saponification of PVAc alcoholyis of PVAc using sodium methoxide and reacetylation of PVA. From the polymer composition and the dyad distribution, the parameter rj was calculated for each polymer as follows ... 

Figure 2.8 Proportions of (OH, OAc) dyad as a function of hydroxyl group content for sets of poly(vinyl alcohol-co-vinyl acetate) copolymers prepared by different routes. ( x) Saponification (a) alcoholysis (o) reacetylation. The solid lines show calculated dyad proportions for ideal cases of alternating, random, and block copolymers. Reprinted with permission from [51]. (1977) American Chemical Society.

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