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Synthesis and thermal analysis of hafnium polyesters.

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Die Angewandte Makromolekulare Chemie 28 (1973) 145-151 ( N r . 429)
From the University of South Dakota, Department of Chemistry,
Vermillion, South Dakota, 57 069
Synthesis and Thermal Analysis of Hafnium Polyesters
By CHARLESE. CARRARER,
JR.
(Eingegangen am 8. August 1972)
SUMMARY:
The synthesis of hafnium polyesters by the solution and interfacial techniques
is presented :
C1-
P
I
f-C1
0
+
-
0
II
I1
eO-C-R-C-08
CP
0
0
II
I
-(-Hf-0-C-R-C-O+
ll
I
CP
CP
Thermal properties of the polyesters were studied by differential scanning calorimetry and thermal gravimetric analysis.
ZUSAMMENFASSUNG :
Hafniumhaltige Polyester wurden a d folgendem Weg durch Grenzfliichenpolykondensation hergestellt :
0
CP
I
C1-Hf-C1
I
+
-
0
II
QO-C-R-C-Oo
I
cp
0
0
II
-(-~f-O-C-R-C-O+
CP
II
I
CP
Die thermischen Eigenschrtften der Polyester wurden durch Differential-Thermoanalyse und thermogravimetrischeAnalyse untersucht.
1. Introduction
We have been interested in the synthesis of polymers containing metals in
their backbonesl-5. We recently reported the synthesis of titanium and
zirconium polyesters of form I via the interfacial and aqueous solution technique& 5. We now report synthesis of the analogous hafnium polyester. This is
the initial report of such a synthesis:
0
CP
I
Cl-M-C1
I
CP
+
0
1
I
11
eO-C-R--C-Oe
-
cp
0
II
fM-0-C-R-C-0
I
I
CP
0
li
j-
(1)
1
145
C. E. CARRAHER,
JR.
2. Experimental and Results
The following reagents were used as received : dicyclopentadienylhafnium dichloride (Strem Chemicals Inc., Danvers, Mass.); itaconic acid (a gift from Evans
Chemetics Inc., Darien, Conn.); azelaic acid (Emerox 1144; gift from Emery
Industries Inc., Cincinnati, Ohio); terephthaloyl chloride (Aldrich Chemical Co.,
Milwaukee, Wis.). Salts were generated by addition of equal equivalents of NaOH
and the diacids to water.
Polymerization and isolation procedures are similar to those reported elsewhere4-6.
Briefly, for interfacial systems, aqueous solutions of the dicarboxylate salts
are added to rapidly stirred organic solutions containing CpiHfClz. Aqueous syntheses were effected in an analogous manner except CpzHfClz is contained in water.
Product precipitates rapidly from the reaction solution. The products are washed
and dried to give white to light brown powders. Polymer synthesis is rapid and in
medium to good yield (Table 1).
Table 1. Results as a Function of Salt.
Azelaica
Itaconica
Terephthalic a
Azelaicb
Terephthalicb
57
56
58
71
99
7
26
7
28C
23
270
270
36.4
35.6
270
260
36.4
38.0
37.1
37.2
43
49
45
Reaction conditions : Disodium salts of dicarboxylic acids (0.000375 moles) in
water (3 ml) added to rapidly stirred (17 500 rpm, no load) solutions containing
CpzHfClz (0.000375 moles) in water (20ml) at 25°C with a stirring time of
10 seconds.
Reaction conditions : As recorded in a. except with CpzHfClz in 20 ml of CHC13
with a stirring time of 6 mins.
Corresponds to a weight-average molecular weight via light scattering of 1.3x 105
in 2-chloroethanol.
Recorded to the nearest 10°C of initial color change, c.f. Experimental.
Infrared spectra were obtained utilizing KBr pellets using a Beckman IR-12
over the range of 4000-200 cm-1. Elemental analyses for hafnium were carried out
by the author. Elemental analyses and infrared spectroscopy are both in agreement
with a repeating unit as illustrated by form 1.
Several general band assignments can be given. Bands around 800-830, 1010 to
1025 and 1440-1450 cm-1 are characteristic of the Cp-n-ring;a band about 3070 t o
3120 cm-l is assigned to the C-H stretching in n-Cp groups and an intense band in
the 1620-1680 cm-1 which is characteristic of the carbonyl stretching in the polyester. The down-field (energy wise) shift of the carbonyl stretching frequency in
146
Hafnium Polyesters
M-0-C
= O groups is well known and can be explained via electronegativity and
dn-pn arguments.
Solubility studies were conducted in small test tubes containing about 0.001 g
of material with 2 ml of liquid. The tubes were occasionally hand shaken and
observed for a period of two weeks. The polyesters exhibit poor solubility characteristics being insoluble in most solvents tried. This presents a major obstacle to
the solution characterization of the products. For instance some polyesters appeared
to be soluble in triethylphosphate but on careful recovery (using a series of acetone
and ether washes complied with the use of a vacuum oven a t temperatures less
than 40°C) i t was found that degradation of the polyester occurred. We have
investigated the use of 2-chloroethanol as a solvent since most of the products
appear to be soluble in it.
The polyesters appear to be stable at room temperature for about a day. After
one day many of the products, after careful recovery, showed evidences of reaction
with the solvent. Increased temperature was found to decrease the time before
degradation was detected (detection was via infrared spectroscopy). Thus solutions
of polyester in 2-chloroethanol should be dealt with rapidly to minimize the importance of reaction with 2-chloroethanol. The solution measurements reported in this
work are believed to be derived from polymer solutions containing non-degraded
products. The solutions properties are still being studied and evaluated.
Weight-average molecular weight was obtained employing a Brice-Phoenix Light
Scattering Photometer-Model 2000 in 2-chloroethanol at 25 “C.
Softening ranges were measured using a Fisher-Johns Melting Point Apparatus
a t an approximate heating rate of 5°C per min. The products remained solid to
300°C. Generally they did change color within this range. The temperature at which
color changed is recorded in Table 1 under the heading of “Color Change Temperature”.
Thermal gravimetric analysis was conducted employing a 950 duPont TGA. Air
and nitrogen flows were about 0.3 liters per minute. Samples were ground to a fine
powder to aid in obtaining reproducable results (for use in both DSC and TGA).
Differential scanning calorimetry was carried out employing a duPont 900 DSC
cell fitted on a duPont Thermal Analyzer console employing a flow rate of about
0.3 liters per minute of air or nitrogen. A linear baseline compensator was used
with the DSC cell to insure a constant energy baseline. A Mettler H20 semimicro
balance was employed for the weighings of the DSC samples. Measurements were
obtained on samples contained in open aluminum cups to allow the free flow away
from the solid of volatilized gases thus more closely simulating the conditions under
which TGA studies were conducted.
3. Discussion
There are several reasons for employing Cp2HfClz as the hafnium containing
reactant. First, it is the only commercially available hafnium compound of the
form R2HfC12. Second, the analogous Cp2ZrCl2 and CpzTiCllz compounds have
been previously employed in the synthesis of analogous polyesters. Third,
CpzHfClz is stable in air and relatively stable t o hydrolysis. CpzHfCl2 presum-
147
C. E.CARRAHER,JR.
ably undergoes ionization in a manner analogous to CpZZrClz and CpzTiClz as
depicted in Eq. 2 to form I1 which subsequently
CpzMClz
HzO
CP~M+~
+
I1
CpzMOH+
+
CpzM(0H)z
(2)
I11
forms I11 which precipitates from solution4~59 7, 8. The rate of formation of I11
from I1 is experimentally observed to be CpzHfClz > CpzZrClz > CpzTiClz.
While solutions of CpzTi+z are stable for several hours a t room temperature,
solutions of CpzHf+Z should be kept no longer than 15min. The aqueous
solutions of CpzHf+z utilized in this study were used within 5 min after solution
was completed to minimize such a problem. CpzHf(0H)z precipitates rapidly
from aqueous solutions of CpzHfClz on addition of aqueous NaOH solutions.
This is consistent with CpzHf+z being the active species in aqueous solution
syntheses. The active hafnium species and site of polymerization is unknown
for the interfacial systems.
Fourth, CpzHfClz is used as a catalyst or cocatalyst in certain commercial
processes. Such a property might be imparted to polymers containing the
CpzHf moiety.
Lastly, analogous titanium and zirconium containing polyesters exhibited
medium to good thermal stabilities. Corresponding hafnium polyesters may
exhibit similar thermal stabilities. The oxidation of Group IV B organometallic
compounds has not been studied wellg. Many Group IV B dicyclopentadiene
dichlorides exhibit relatively good stability, being stable in air a t room
temperature. This may be due to the shielding of the metal atom by the
cyclopentadiene groups (which sandwich the metal atom) preventing many
reactants from reaching the metal atom and/or “out-lying’’ orbitals (filled or
vacant). The oxidation of dicyclopentadienyldialkyl (aryl)titanium compounds
has been reportedg.
Thermograms of the products are reproduced in Figures 1 to 3. Several
general results are apparant. First the products appear to exhibit relatively
good stability in both air and nitrogen. Several show 80% or greater weight
retension to 500 “C. For many industrial applications the amount of nonmetal
component remaining after heating in air is important. The end product of
thermal degradation in air is Hf027. To calculate the minimum remaining
nonmetal, one may simply calculate the percentage HfOz compared to the
overall weight of a chain. The final column in Table 1 contains this percentage
for each of the diacids utilized. The product from disodium azelate rapidly
loses weight in the 350 to 450°C region resulting in a weight retension of 56%
a t 600°C. If the hafnium is still all remaining as HfOz, this would correspond
to only 13% (minimum) remaining nonmetal. The product from disodium
148
E ndo
P
E xo
I
.........
~
I-
1
I
1I
0
-
2%-
. ..........
. . . .
- _ _ _ - - - - ....
................
...... ......... ‘..
T.-,-:
....--_....
.... ...
.. ..
..
..
I
200
I
400
I
600
Temperature ( O C )
Fig.
1.
DSC thermograms of the condensation products of CpaHfClz with disodium
, disodium azelate - . - . - . -, and disodium
terephthalate itsconate . . . . . . . . . . . on samples weighing 0.00 100 grams at a heating
rate of 20°C/MIN. The “Y-axis” sensitivity in air, the bottom entry of
each pair, was l.O”C/IN (0.04 MV/IN) and in nitrogen was 0.1 “C/IN
(0.004MV/IN) with a gas flow of about 0.31pm. Each “Y-axis” unit
represents one inch. The - - - - - - - line is A T = 0.
itaconate shows 21 yo (minimum) remaining nonmetal a t 1200 “C, while the
product from disodium terephthalate shows 25% (minimum) remaining
nonmetal a t 800°C. Thus caution should be exercised in interrupting thermal
stabilities from the sole basis of weight retension. Consideration should also
be given to the remaining product.
Second, thermal degradation is clearly occurring via different routes a t
temperatures greater than 300°C in air and nitrogen. Degradation in air is
accompanied by a large exotherm which has no correspondence in nitrogen.
Such exotherms have been attributed to degradation via an oxidative pathwaylo.
Third, the endothermic regions between the starting temperature and about
150°C are identical in air and nitrogen. There are no weight changes associated
with them. DSC thermograms of analogous titanium products often show
similar low temperature transitions. These are accompanied with charges in the
149
C. E. CARRAIIER,JR.
infrared spectra in the “H” bonding region. Because of the low amount of
product, such spectral studies were not made.
It is presently believed that degradative oxidation occurs a t the metal atom
in the polyester forming a labile product which proceeds, via a lower energy
route, to form volatile products. The mechanism(s) of such degradations, both
in air and nitrogen are currently being more closely studied.
I
1
%Wt.
Loss
60
80
0
300
600
Temperature (“C)
900
Fig. 2. TGA thermograms of samples listed in Fig. 1 in air.
0--.-
-
%Wt.
20-
-
40-
Loss
-
60-
-
80-
I
Fig. 3.
150
I
I
I
I
I
I
I
I
1
Hafnium Polyesters
1
2
3
4
6
7
9
10
C. CARRAHERand G. SCHERUBEL,
Makromol. Chem. 152 (1972) 61.
C. CARRAHERand J. REIMER,Polymer 13 (1972) 153.
C. CARRAHERand R . NORDIN,J. Polym. Sci. A-1 10 (1972) 521.
C. CARRAHER,
Eur. Polym. J. 8 (1972) 215.
C. CARRAHER,
J. Polym. Sci. A-1 9 (1971) 3661.
C. CARRAHER,
J. Chem. Ed. 46 (1969) 314.
C. CARRAHER,
unpublished results.
Y . ISRALLI,Bull. SOC.Chim. France 3 (1966) 837.
T. BRILKINA
and V. SHUSHUNOV,
Reactions of Organometallic Compounds with
Oxygen and Peroxides, CRC Press, 1969, pp. 164-168.
G. EHLERS
and K. FISCH,Applied Polymer Symposia, 8; International Symposium on Polymer Characterization, John Wiley, N.Y. 1969, p. 171.
151
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