Chemistry Reference
In-Depth Information
Typical catalysts are cadmium acetate or zinc acetate. There are other routes to vinyl acetate as well,
based on ethylene.
Commercially, poly(vinyl acetate) is formed in bulk, solution, emulsion, and suspension
polymerizations by free-radical mechanism. In such polymerizations, chain transferring to the
polymer may be as high as 30%. The transfer can be to a polymer backbone through abstraction of
a tertiary hydrogen:
+
R
+
RH
n
n
O
O
O
O
It can also take place to the methyl proton of the acetate group:
+
R
RH
+
n
n
O
O
O
O
The polymer has a head to tail structure and is highly branched. It is quite brittle and exhibits cold
flow. This makes it useless as a structural plastic. It is, however, quite useful as a coating material and
as an adhesive for wood. The polymer is soluble in a wide range of solvents and swells and softens
upon prolonged immersion in water. At higher temperatures or at extended exposures to temperatures
above 70
C, the material loses acetic acid.
A number of copolymers are known where vinyl acetate is the major component. In coatings, vinyl
acetate is often used in copolymers with alkyl acrylates (line 2-ethylhexyl acrylate) or with esters of
maleic or fumaric acids. Such copolymers typically contain 50-20% by weight of the comonomer and
are usually formed by emulsion polymerization in batch processes. They are used extensively as
vehicles for emulsion paints.
Shaver and coworkers [
319
] investigated the mechanism of bis(imino)pyridine ligand framework
for transition metal systems-mediated polymerization of vinyl acetate. Initiation using azobisisobu-
tyronitrile at 120
C results in excellent control over poly(vinyl acetate) molecular weights
and polymer dispersities. The reaction yields vanadium-terminated polymer chains which can be
readily converted to both proton-terminated poly(vinyl acetate) or poly(vinyl alcohol). Irreversible
halogen transfer from the parent complex to a radical derived from azobisisobutyronitrile generates
the active species.
6.19 Poly(vinyl alcohol) and Poly(vinyl acetal)s
Vinyl alcohol monomer does not exist because its keto tautomer is much more stable. Poly(vinyl
alcohol) can be prepared from either poly(vinyl esters) or from poly(vinyl ethers). Commercially,
however, it is prepared exclusively from poly(vinyl acetate). The preferred procedure is through
