Chemistry Reference
In-Depth Information
The monomer,
vinylidine
chloride,
can
be
prepared
by
dehydrochlorination
of
1,1,2-trichloroethylene:
Cl
Cl
Cl
-HCl
400
o
C
Cl
Cl
It is a colorless liquid that boils at 32
C. Also, it is rather hard to handle as it polymerizes on
standing. This takes place upon exposure to air, water, or light. Storage under an inert atmosphere
does not completely prevent polymer formation.
Poly(vinylidine chloride) can be formed in bulk, solution, suspension, and emulsion polymerization
processes. The products are highly crystalline with regular structures and a melting point of 220
C.
The structure can be illustrated as follows:
Cl
Cl
Cl
Cl
Cl
Cl
This regularity in structure is probably due to little chain transferring to the polymer backbone
during polymerization. Such regularity of structure allows close packing of the chains and, as a result,
there are no effective solvents for the polymer at room temperature.
Copolymerization of vinylidine chloride with vinyl chloride reduces the regularity of the structure.
It increases flexibility and allows processing the polymer at reasonable temperatures. Due to
extensive crystallization, however, that is still present in 85:15 copolymers of vinylidine chloride
with vinyl chloride, they melt at 170
C. The copolymerization reactions proceed at slower rates than
do homopolymerizations of either one of the monomers alone. Higher initiator levels and
temperatures are, therefore, used. The molecular weights of the products range from 20,000 to
50,000. These materials are good barriers for gases and moisture. This makes them very useful in
films for food packaging. Such films are formed by extrusion and biaxial orientation. The main
application, however, is in filaments. These are prepared by extrusion and drawing. The tensile
strength of the unoriented material is 10,000 lb/in.
2
and the oriented one 30,000 lb/in.
2
.
Vinylidine chloride is also copolymerized with acrylonitrile. This copolymer is used mainly as a
barrier coating for paper, polyethylene, and cellophane. It has the advantage of being heat sealable.
6.18 Poly(vinyl acetate)
Vinyl acetate monomer can be prepared by reacting acetylene with acetic acid:
O
O
+
OH
O
The reaction can be carried out in a liquid or in a vapor phase. A liquid phase reaction requires
75-80
C temperatures and a mercuric sulfate catalyst. The acetylene gas is bubbled through
glacial acetic acid and acetic anhydride. Vapor phase reactions are carried out at 210-250
C.
