POLYVINYL ESTERS

POLY (VINYL ACETATE), PVAc

Vinyl acetate monomer is most conveniently prepared by catalytic addition of acetic acid to acetylene, which may be
accomplished by a liquid-phase or a vapour-phase process. The vapour-phase synthesis is more in commercial practice
and is accomplished by passing a mixture of acetylene and acetic acid through a reaction tube packed with such
catalysts as cadmium acetate or zinc acetate at a temperature of 220-250°C. The monomer formed is purified by
distillation.

Manufacturing
The polymer is widely used in an emulsion form and hence, emulsion polymerization of vinyl acetate is widely
employed for commercial purposes. The commercial emulsion polymerization is usually accomplished at a temperature
of 50-70°C in a reactor with all provisions for temperature control. For desirable control of particle size and particle
size distribution in the emulsion or latex and to regulate chain branching, a measured stream of the monomer
and initiator is slowly and steadily added over a period of 3-4 h into the polymerization system and the
polymerization is usually stopped after about 4-6 h.

Properties
Because of soft and gummy nature(Tg = 300C) of poly(vinyl acetate), the polymer has poor dimensional stability. It
rarely finds an application as moulded or formed objects and is seldom processed like common thermoplastics.
Poly(vinyl acetate) as formed commercially is atactic in nature and hence amorphous and it is transparent if free from
contaminants such as emulsifiers. It is soluble in benzene, acetone and chloroform. Poly (vinyl acetate) is also widely
miscible with methanol.

Applications
1. Adhesives for wood, decorative laminates and related items
2. Sizing material for paper and textiles;
3. Gum application (chewing gum)

Poly(vinyl alcohol), PVOH

Poly(vinyl acetate) is also used as the intermediate for the commercial production of (i) poly (vinyl alcohol) and (ii)
poly (vinyl acetals). The monomer, vinyl alcohol, does not exist in the free state and all attempts to make it result in the
formation of the stable tautomer, acetaldehyde.

Poly (vinyl alcohol) is prepared by the alcoholysis of poly (vinyl acetate) as shown above. The process is often
inappropriately termed as "hydrolysis". Methanol is preferred over ethanol for the alcoholysis which is better carried out
with alkaline catalysts (NaOH, NaOCH 3, etc.) than with acidic catalysts. The process is accomplished much quicker (< 1
h) at a higher temperature (say, 60°C) than at or near room temperature (6-8 h at 20-25 0C). The average degree of
polymerization of poly (vinyl alcohol) is considerably lower than that of the parent poly (vinyl acetate) as because the
alcoholysis process splits the branch units formed via the acetate groups.
Properties
Polymers with a degree of alcoholysis close to or higher than 85% (85% or more of the acetate groups of poly (vinyl
acetate) transformed into alcohol (OH) groups) are readily soluble in water. For polymers with >90% of the substituents
transformed into OH groups by alcoholysis, water-solubility follows a falling trend with increasing degree of
alcoholysis.
Poly (vinyl alcohol) is normally unsuitable as a material for thermoplastic applications. It tends to decompose before
melting, liberating moisture and forming chain unsaturations. Extended heating leads to discolouration and
insolubilization.

Applications
1. Water soluble grades are used as thickening agents for aqueous suspensions and emulsions
2. Films are useful as water-soluble packages
3. As nonionic surfactant, it is extensively used as a stabilizer in suspension and emulsion polymerization
4. As a binder, adhesive and sizing agent in ceramic, paper and textile industry
5. Fibre is useful as a textile material with washability and quick drying properties.

POLY(VINYL ACETALS)

Poly(viny1 acetals) are products of interaction between poly(viny1 alcohol) and various aldehydes. Products based on
reactions involving formaldehyde, acetaldehyde, propionaldehyde and butyraldehyde are separately known as
poly(viny1 formal), poly(viny1 acetal), poly (vinyl propional) and poly(viny1 butyral) respectively. Reactions usually
involve condensation between 1,3-hydroxyl groups of poly(viny1 alcohol) and the aldehyde molecules. Besides
containing the acetal structures formed,

the derived polymer molecules also contain some residual hydroxyl groups as well as some residual acetate groups
retained in the polymer due to incomplete alcoholysis of the original poly(viny1 acetate) to poly(viny1 alcohol). Among
the poly(vinyl acetals), only poly(vinyl butyral) is important considering commercial aspects. It finds extensive use as
safety glass interlayer, particularly because of its outstanding clarity, toughness, photo stability, moisture insensitivity
and exceptional adhesion to glass. The polymer may be prepared by the direct process from poly(viny1 acetate); but for
safety glass purposes it is most conveniently prepared from poly(viny1 alcohol). The catalyst most commonly used is
H2SO4. If the reaction is done using aqueous solution of poly(viny1 alcohol) and butyraldehyde to give a homogeneous
system to begin with, poly(viny1 butyral) precipitates out as it is formed. On the other hand, if one begins with an initial
inhomogeneous system, i.e., a suspension of poly(viny1 alcohol) in water-ethanol mixture and requisite proportions of
butyraldehyde and H2SO4 catalyst, one obtains a homogeneous solution at the completion of the reaction. Poly(viny1
butyral) is precipitated from the solution by addition of water under stirring conditions, washed and dried. For safety
glass application, about 25% of the hydroxyl groups are usually left un-reacted, and about 30-40 wt.% of a plasticizer,
such as dibutyl sebacate or triethylene glycol di-2-ethyl butyrate, are commonly used.