formaldehyde - Sciencemadness Dot Org

FORMALDEHYDE POLYMERS 93
monomeric gas at higher temperatures. Under these conditions, complete
liquefaction of the polymers was not observed* 4 . Polymer obtained in
powdered form by warming a solution of liquid formaldehyde in dry
ether melts to a viscous liquid at 170-172°C in a sealed tube 91 . The
monomeric formaldehyde obtained by vaporizing eu-polyoxymethylene
shows little tendency to polymerize in the gaseous state because of its high
purity, and on chilling condenses to liquid formaldehyde 91 .
Polyoxymethylene glass prepared at —8Q°C is somewhat brittle at
ordinary temperatures, but in the plastic state is extremely tough. Polymer
obtained at higher temperatures is soft and may be easily drawn to
give long fibers or pressed into film. Staudinger reports that fine fibers
obtained in this way have a reversible elasticity of 10 per cent a4 ,
Eu-polyoxymethylene does not smell of formaldehyde under ordinary
conditions.' However, on long storage in a sealed container the odor of
formaldehyde is definitely detectable, indicating a gradual depoiymerization
to monomer. The polymer is insoluble in water but gradually dissolves
in dilute boiling alkali or sodium sulfite. On exposure to the air,
eu-polyoxymethylene powder absorbs up to 2 per cent water which cannot
be subsequently removed, even by vacuum desiccation over phosphorus
pentoxide. This is probably a case of physical adsorption, but might
possibly involve chemical reaction with formation of polyoxymethylene
glycols 91 .
According to Delepine 15 , the heat of combustion of this polymer is 122.9
kcal per formaldehyde (CH2O) equivalent. Its heat of formation as calculated
from this figure would be 40.3 kcal per mol of formaldehyde.
Preparation. The polymerization of pure liquid formaldehyde to
a solid polymer was first reported in 1892 by August Kekul^ 28 , who observed
the slow polymerization at — 20°C and the rapid, almost explosive,
reaction at higher temperatures.
For the preparation of eu-polyoxymethylene, best results are obtained
by use of pure, redistilled liquid formaldehyde (pages 24^26). This material
polymerizes to an opaque product when kept for several hours in a
nitrogen atmosphere at — S0°C. Oxygen inhibits the rate of polymer
formation, complete polymerization requiring up to several days when this
gas is present. Under these conditions, a clear glass-like polymer may be
obtained 64 . The inhibiting action of oxygen on polymerization reactions
at low temperature has also been reported in the case of vinyl acetate 68 .
At high temperatures polymerization is accelerated by oxygen.
Eu-polyoxymethylene can be obtained in powdered form by polymerisation
in a dry inert solvent. Good results are obtained by sealing 10-cc
quantities of a solution of one volume liquid formaldehyde and two volumes
anhydrous ether in a "Pyrex" tube, whieh is then allowed to come to room