Chemistry Reference
In-Depth Information
a transesterification reaction using methyl or ethyl alcohols. Alkaline catalysts yield rapid
alcoholyses. A typical reaction employs about 1% of sodium methoxide and can be carried to
completion in 1 h at 60
C. The product is contaminated with sodium acetate that must be removed.
The reaction of transesterification can be illustrated as follows:
CH
3
O
O
+
n
n
n
O
O
O
O
O
O
O
The branches of poly(vinyl acetate) that form during polymerization as a result of chain transfer-
ring to the acetate groups cleave during transesterification. As a result, poly(vinyl alcohol) is lower in
molecular weight than its parent material.
Poly(vinyl alcohol) is very high in head to tail structures, based on NMR data. It shows the
presence of only a small amount of adjacent hydroxyl groups. The polymer prepared from amorphous
poly(vinyl acetate) is crystalline, because the relatively small size of the hydroxyl groups permits the
chains to line-up into crystalline domains. Synthesis of isotactic poly(vinyl alcohol) was reported
from isotactic poly(vinyl ethers), like poly(benzyl vinyl ether), poly(
t
-butyl vinyl ether), poly
(trimethylsilyl vinyl ether), and some divinyl compounds.
Poly(vinyl alcohol) is water soluble. The hydroxyl groups attached to the polymer backbone,
however, exert a significant effect on the solubility. When the ester groups of poly(vinyl acetate) are
cleaved to a hydroxyl content of 87-89%, the polymer is soluble in cold water. Further cleavage of
the ester groups results in a reduction of the solubility and the products require heating of the water to
85
C to dissolve. This is due to strong hydrogen bonding that also causes unplasticized poly(vinyl
alcohol) to decompose below its flow temperature. On the other hand, due to hydrogen bonding the
polymer is very tough.
Poly(vinyl acetals) are prepared by reacting poly(vinyl alcohol) with aldehydes. Reactions of poly
(vinyl alcohol) with ketones yield ketals. These are not used commercially.
Not all hydroxyl groups participate in formations of acetals and some become isolated. A typical
poly(vinyl acetal) contains acetal groups, residual hydroxyl groups, and residual acetate groups from
incomplete transesterification of the parent polymer.
Poly(vinyl acetal)s can be formed directly from poly(vinyl acetate) and this is actually done
commercially in preparations of poly(vinyl formal). A typical reaction is carried out in the presence
of acetic acid, formalin, and sulfuric acid catalyst at 70
C:
70
o
C
+
O
n
O
O
O
O
O
+ H
2
O
O
O
OH
Poly(vinyl butyral), on the other hand, is prepared from poly(vinyl alcohol) and butyraldehyde.
Sulfuric acid is used as the catalyst. Commercially only poly(vinyl formal) and poly(vinyl butyral)
are utilized on a large scale in coating materials.
