Chemistry Reference
In-Depth Information
The products consist exclusively of poly(1-olefin) units. Polymerizations of 2, 3 and 4 octenes with
TiCl
3
/Al(C
2
H
5
)
3
at 80
C for instance, result in the same high molecular weight homopolymer, poly
(1-octene) [
319
].
Also, it was reported that addition of some transition metal compounds of Group VIII to
Ziegler-Natta catalysts enhances isomerization of 2-olefins [
321
,
329
]. When nickel compounds
are added to TiCl
3
-(C
2
H
5
)
3
Al they react and form TiCl
3
-NiX
2
-(C
2
H
5
)
3
Al. X can be a chloride, an
acetylacetonate, or a dimethyl glyoximate [
328
]. The product is an efficient isomerization catalyst.
4.6 Polymerization of Aldehydes
Aldehyde polymers were probably known well over 100 years ago [
322
-
324
]. In spite of that,
polyoxymethylene is the only product from aldehyde polymerization that is produced in large
commercial quantities. Formaldehyde polymerizes by both cationic and anionic mechanisms. An
oxonium ion acts as the propagating species in cationic polymerizations [
322
,
323
]. In the anionic
ones, the propagation is via an alkoxide ion.
Most polymerizations of aldehydes are conducted in aprotic anhydrous solvents because proton-
yielding impurities are very efficient chain transferring agents. Polymerization of formaldehyde can be
looked upon as an exception [
323
]. When protonic solvents are used, however, the resultant polymer has
a very wide molecular weight distribution. Usually, therefore, solvents with low dielectric constants are
preferred. In all cases of aldehyde polymerizations, the polymers precipitate from solution as they form.
Choice of low dielectric solvents contributes to this. In spite of that, the polymerizations often keep on
going as rapidly as the monomers are added to the reaction mixtures. In addition, only low molecular
weight polyoxymethylene forms in some high dielectric solvents, like dimethylformamide, and the
reaction is quite sluggish. Higher aldehydes fail to polymerize in dimethylformamide.
As stated above, the propagation reaction continues after polymer precipitation. That is due to the
fact that the precipitated macromolecules are highly swollen by the monomers. This was shown in
polymerizations of
-butyraldehyde in heptane [
326
]. The physical state of the polymers and the
surface areas of their crystalline domains, therefore, influence the paths of the polymerizations.
The solubility of the initiators in the solvents can affect strongly the molecular weights of the
resultant products and their bulk densities [
323
]. The whole reaction can even be affected by small
changes in the chemical structures of the initiators or by changes in the counterions [
323
]. Chain
transferring is the most important termination step in aldehyde polymerizations.
n
4.6.1 Cationic Polymerization of Aldehydes
The cationic polymerizations of formaldehyde can be carried out in anhydrous media with typical
cationic initiators. Initiation takes place by an electrophilic addition of the initiating species to the
carbonyl oxygens. This results in formations of oxonium ions [
323
]:
+
R
R
R
CH
2
O
O
O
The oxonium ions may react as oxygen-carbon cations. The propagation steps consist of attacks by
the electrophilic carbon atoms upon the carbonyl oxygens of the highly polar formaldehyde molecules:
R
+
R
O
O
O
O
