Poly-alcohols Tautomerism

The small molecules used as the basic building blocks for these large molecules are known as monomers. For example the commercially important material poly(vinyl chloride) is made from the monomer vinyl chloride. The repeat unit in the polymer usually corresponds to the monomer from which the polymer was made. There are exceptions to this, though. Poly(vinyl alcohol) is formally considered to be made up of vinyl alcohol (CH2CHOH) repeat units but there is, in fact, no such monomer as vinyl alcohol. The appropriate molecular unit exists in the alternative tautomeric form, ethanal CH3CHO. To make this polymer, it is necessary first to prepare poly(vinyl ethanoate) from the monomer vinyl ethanoate, and then to hydrolyse the product to yield the polymeric alcohol. 

PVA Formation Reaction. Poly(vinyl alcohol) is itself a modified polymer being made by the alcoholysis of poly(vinyl acetate) under acid or base catalysis as shown in Equation 1 (6.7). This polymer cannot be made by a direct polymerization because the vinyl alcohol monomer only exists in the tautomeric form of acetaldehyde. This saponification reaction can also be run on vinyl acetate copolymers and this affords a means of making vinyl alcohol copolymers. The homopolymer is water soluble and softens with decomposition at about 200°C while the properties of the copolymers would vary widely. Poly(vinyl alcohol) has been widely utilized in polymer modification because ... 

For all practical purposes, monomeric vinyl alcohol exists only in its tautomeric form as acetaldehyde. Therefore, poly(vinyl alcohol) (PVAlc) is prepared by the hydrolysis of polymers of vinyl esters. For practical reasons, the starting material of choice is poly(vinyl acetate) (PVAc). Although hydrolysis may be carried out under acidic conditions, alkaline conditions in the presence of an alcohol are preferred. The reaction may be represented by... 

Vinyl alcohol is the enol of acetaldehyde. It occurs in small amounts in the tautomeric equilibrium so, acetaldehyde can be converted to poly(vinyl alcohol) in polar solvents with alkali alcoholates as initiators. But the process is not used commercially. 

Recently atactic poly (5 )-methyl-2-butylvinylketone has been reduced gradually by Merle etal [182] into copolymers of allyl-secondary alcohols and vinylketones they can exist, partially in the tautomeric cyclic hemiacetal form [182, 184]. 

Poly(vinyl alcohol) can be made from vinyl acetate in just two steps (as seen in Problem 27.44). Acetaldehyde can also be made from vinyl acetate (upon treatment with aqueous acid). Under these conditions, the acetate group is hydrolyzed, giving an enol, which tautomerizes to give acetaldehyde ... 

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