Chemistry Reference
In-Depth Information
5.7.1 Polymerization of Trioxane
Trioxane is unique among the cyclic acetals because it is used commercially to form
polyoxymethylene, a polymer that is very much like the one obtained by cationic polymerization
of formaldehyde. Some questions still exist about the exact mechanism of initiation in trioxane
polymerizations. It is uncertain, for instance, whether a cocatalyst is required with strong Lewis acids
like BF
3
or TiCl
4
.
The cationic polymerization of trioxane can be initiated by protonic acids, complexes of organic
acids with inorganic salts, and compounds that form cations [
70
]. These initiators differ from each
other in activity and in the influence on terminations and on side reactions. Trioxane can also be
polymerized by high-energy radiation [
70
]. In addition, polymerizations of trioxane can be carried
out in the solid phase, in the melt; in the gas phase, in suspension, and in solution. Some of these
procedures lead to different products, however, because variations in polymerization conditions can
cause different side reactions.
Polymerizations in the melt above 62
C are very rapid. They come within a few minutes to
completion at 70
C when catalyzed by ten moles of boron trifluoride. This procedure, however is only
useful for preparation of small quantities of the polymer, because the exothermic heat of the reaction
is hard to control.
Typical cationic polymerizations of trioxane are characterized by an induction period. During
that period only oligomers and monomeric formaldehyde form. This formaldehyde, apparently,
results from splitting the carbon cations that form in the primary steps of polymerization. The
reaction starts after a temperature dependent equilibrium concentration of
formaldehyde is
reached [
70
].
CH
2
RO
O
O
RO
O
O
CH
2
RO
O
O
+
Several reaction mechanisms were proposed. One of them is based on the concept that Lewis
acids, like BF
3
coordinate directly with an oxygen of an acetal. This results in ring opening that is
induced to form a resonance stabilized zwiter ion [
71
]:
O
O
+
BF
3
O
BF
3
O
O
O
O
O
O
OOO
CH
2
BF
3
BF
3
Resonance stabilizations of the adjacent oxonium ions lead to formations of carbon cations that are
believed to be the propagating species. Propagations consist of repetitions of the sequences of addition
of the carbon cations to the monomer molecules and are followed by ring opening. The above
mechanism has to be questioned, however, because rigorously dried trioxane solutions in cyclohexane
fail to polymerize with BF
3
O(C
4
H
9
)
3
catalyst [
72
]. The same is true of molten trioxane [
73
]. It appears,
