Chemistry Reference
In-Depth Information
Determination of microstructure and reactivity ratios revealed a strong inherent tendency to form
alternating copolymers.
Jordan [
103
] described copolymerization of ethylene with vinyl ethers and with vinyl fluoride.
The catalyst used was
(
ortho
-phospheno-arenesulfonate)PdMe(pyridine). The reaction was
illustrated as follows:
Ar
Ar
Me
P
Pd
N
O
S
O
O
+
OR
n
where Ar = 2-OMe-Ph
OR
OR
6.5.3 Copolymers of Propylene with Dienes
Although presently lacking industrial importance, alternating copolymers can be made from propyl-
ene and butadiene [
104
] and also from propylene and isoprene [
105
]. Copolymers of propylene and
butadiene form with vanadium- or titanium-based catalysts combined with aluminum alkyls.
The catalysts have to be prepared at very low temperature (
70
C). Also, it was found that a presence
of halogen atoms in the catalyst is essential [
75
]. Carbonyl compounds, such as ketones, esters, and
others, are very effective additives. A reaction mechanism based on alternating coordination of
propylene and butadiene with the transition metal was proposed by Furukawa [
104
].
6.5.4 Copolymers of Ethylene with Vinyl Acetate
Various copolymers of ethylene with vinyl acetate are prepared by free-radical mechanism in
emulsion polymerizations. Both reactivity ratios are close to 1.0 [
106
]. The degree of branching in
these copolymers is strongly temperature-dependent [
107
]. These materials find wide use in such
areas as paper coatings and adhesives. In addition, some are hydrolyzed to form copolymers of
ethylene with vinyl alcohol. Such resins are available commercially in various ratios of polyethylene
to poly(vinyl alcohol), can range from 30% poly(vinyl alcohol) to as high as 70%.
Vinyl acetate residues in ethylene-vinyl acetate copolymers reduce regularity of polyethylene.
This reduces crystallinity in the polymer. Materials containing 45% vinyl acetate are elastomers and
can be cross-linked with peroxides.
6.5.5
Ionomers
Another group of commercial copolymers of ethylene is those formed with acrylic and methacrylic
acids, where ethylene is the major component. The copolymerizations are carried out under high
pressures. These materials range in comonomer content from 3 to 20%. Typical values are 10%.
A large proportion of the carboxylic acid groups (40-50%) are prereacted with metal ions like sodium
