RESISTANCE OF POLYFLUORINATED COMPLETE ESTERS
mal degradation is usually the temperature at which
1799
the weakest chemical bonds in the molecule are
broken [6]. It is commonly believed that, in thermal
degradation of esters, the primary event is cleavage of
ester bonds. Actually the temperature at which the
sample loses 5, 10, or 15% of its weight is arbitrarily
chosen as the degradation onset temperature.
1
250
200
150
2
The figure shows that, with the same acyl groups,
trimethylolpropane derivatives IV VII are more sta-
ble, as in the case of nonfluorinated esters. The ther-
mal stability of other complete esters of trimethylol-
propane and neopentylene glycol can be estimated by
extrapolation.
100
50
3
4
5
6
7
8
9
Length of acyl residue, number of C atoms
Note that, among complete perfluoroheptanoates I,
VI, and XIII, pentaerythritol tetraester I is the most
stable, which well correlates with the data of [2].
Temperature of the onset of the weight loss vs. length of
the acyl residue: (1) complete trimethylolpropane esters
IV VII (y = 18.12x + 80.51, R = 0.996) and (2) co-
xy
mplete neopentylene glycol esters XI XIV (y = 12.09x +
It is known that introduction of chlorine into vari-
ous organic molecules enhances their thermal stability
[7]. When studying thermograms of I XVII, we also
found that, at the same length of the acyl chain, esters
II, VIII, IX, XV, and XVI prepared by esterification
of pentaerythritol, trimethylolpropane, and neopentyl-
ene glycol with -chloroperfluoroalkanoic acids are
more stable than esters of series A and C (Table 3).
As for the influence of the structure of polyhydric
alcohol on the resistance of esters II, VIII, IX, XV,
and XVI to thermal oxidation, it is similar to that
observed in [2] and in this study for esters of series A:
Chlorinated tetraester II is the most stable, triesters
VIII and IX are less stable, and diesters XV and XVI
are the least stable.
68.22, R = 0.995).
xy
col with polyfluorinated carboxylic acids exhibit
higher resistance to thermal oxidative degradation
than their nonfluorinated analogs. The difference
between the nearest homologs of the polyfluorinated
ester series in the degradation temperature is appreci-
ably larger than for the nonfluorinated analogs.
EXPERIMENTAL
The IR spectra were recorded on a Perkin Elmer
Spectrum One Fourier spectrometer in a thin layer for
esters I and III XII and in the diffuse reflection
1
(DRA) mode for II. The H and 19F NMR spectra
Among the examined complete esters, compounds
III, X, and XVIII are the least stable. This conclusion
was based not only on the temperatures of 5% weight
loss in thermogravimetric analysis. We also took into
account the temperatures of complete burn-out of
samples of III, X, and XVII.
were taken on a Bruker DRX 400 spectrometer in
CDCl3, internal references TMS and hexafluoroben-
zene, respectively. The 19F NMR chemical shifts are
given relative to CFCl3, with upfield shifts considered
as positive. The differential thermal gravimetric anal-
ysis was performed on a Director-MOM device with
1
programmed heating at a rate of 5 deg min , at a
Along with ester fragments, compounds III, X, and
XVII contain ether bonds, and the polyfluorinated
groups in them have a branched structure with two
side trifluoromethyl groups. The structural features of
III, X, and XVII are responsible for their decreased
resistance to thermal oxidation. The revealed trends
are similar to those observed with nonfluorinated
esters. For example, Dufaure et al. [3] studied the
resistance of octyl and 2-ethylhexyl oleates to thermal
oxidation and found that the linear derivative was
more stable than that with the side ethyl groups: The
onset of weight loss was observed at 175 and 155 C,
respectively.
sample weight of 100 mg and measurement error of
0.5%. Elemental analysis was performed on a CHN
EA1108 Carlo Erba automatic analyzer.
Typical procedures for preparing esters I XVII.
a. From polyol and fluoroalkanoic acid. To a mix-
ture of 6.7 g of trimethylolpropane and 32.1 g of per-
fluorobutyric acid, heated to the melting point, 1 ml
of sulfuric acid was added dropwise, after which the
heating was continued for 2 h. The resulting mixture
was washed with water, sodium carbonate solution,
and again water, dried over CaCl2, and distilled in
an oil-pump vacuum; yield of ester IV 29.6 g (82%),
bp 152 154 C (5 mm Hg).
Our results show that complete esters of penta-
erythritol, trimethylolpropane, and neopentylene gly-
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 11 2006
Gorbunova
