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Heat-resistant polymers containing the low molecular weight closo-carboranes. III. The preparation of polycarboranesiloxane polymers by alcoholysis

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JOURNAL OF APPLIED POLYMER SCIENCE VOL. 15, PP. 2645-2650 (1971)
Heat-Resistant Polymers Containing the Low
Molecular Weight closo-Carboranes. 111. The
Preparation of Polycarboranesiloxane
Polymers by Alcoholysis
R. E. ICESTING, K. F. JACKSON, and J. M. NEWMAN,
Chemical Systems Incorporated, Santa Ana, California 9,9705
synopsis
A new synthesis for carboranesiloxane polymers has been discovered. It involves
the alcoholysis of the bischlorodimethylsilylcarboranemonomers and the generation of
the HCl catalyst in situ. Alcoholysis is applicable to the synthesis of most carboranesiloxane polymers, with the probable exception of the SiB-1homopolymers of the larger
carboranes. The attack of the B-H moieties in the carborane cage can be minimized
both by the utilization of a tertiary alcohol and by the addition of excess acid when a
primary alcohol is employed.
INTRODUCTION
Crystalline carboranesiloxane polymers of the 5-SiB-1 type,
prepared by the FeC13-catalyzed bulk condensation of the chloro- (i.e.,
C1SiRzCB5H5CSiRzC1) and the methoxy- (i.e., CH30SiR2CB5H5CSiOCH3)
monomers, have been reported.'J Amorphous copolymers, made by the
incorporation of small quantities of larger carborane (-CB8H8Cor
m-CBloHloC-) into the predominantly -CB5H5C- siloxane backbone have
also been ~ r e p a r e d . ~The
. ~ 5-SiB-1 polymers, therefore, encompass a wide
range of materials bounded at one extreme by homopolymer waxes and on
the other by copolymer elastomers. These polymers are of considerable
interest because of their high thermal and oxidative ~ t a b i l i t y . ~Simplified
routes to replace the high temperature and vacuum syntheses hitherto
employed are currently being investigated. The present paper describes a
study initiated in December 1970 which appears to be applicable to the
preparation of all carboranesiloxane polymers with the probable exception
of the SiB-1 homopolymers of the large carboranes. It is a simple alcoholysis procedure which employs ambient temperatures, atmospheric pressure,
and a catalyst generated in situ.
2645
@ 1971 by John Wiley & Sons, Inc.
2646
KESTING, JACKSON, AND NEWMAN
EXPERIMENTAL
Polymer Synthesis
The new synthesis involves the addition of an alcohol to the 5chloro
monomer (i..e., 2,Pbis(chlorodimethylsilyl)-2,4-dicarb~c~s~heptaborane7) or mixtures thereof with the m-10-chloro monomer (i.e., l,lO-bis(chlorodimethylsilyl)-l,l0-dicarba4oso-decaborane-10). I n a typical example,
100 millimoles of 5-chloro monomer were placed in a flask to which 50 ml of
an anhydrous alcohol was added. I n another example, 50 ml of an alcohol
was added to a flask containing 80 millimoles of 5-chloro monomer and 20
millimoles of the m-10-chloro monomer. After standing for periods varying
between 20 and 200 hr, the precipitated polymers were extracted with
solvent, precipitated, washed with water, and dried in a vacuum oven.
This synthesis should be generally applicable to the preparation of SiB-1
polymers from the other small carboranes. The higher SiB polymers (i.e.,
SiB-2, -3, etc.) of C2Bs and C2B10, and most intermediate closo-carboranes
(with the possible exception of CzB9H11)are also accessibIe by this technique.
AnaIysis
Number-average molecular weights ATn were determined in chloroform
utilizing a vapor pressure osmometer (Mechrolab Model 201A). They are
accurate to =tlO%. Melting point ranges were determined on a FischerJohns melting point apparatus. Infrared analyses were obtained utilizing a
Perkin Elmer Model 21 spectrophotometer.
RESULTS AND DISCUSSION
During the course of the methanolysis'J of a large amount of 5-chloro
monomer to produce 5-methoxy monomer, it was noticed by one of us
(J.M.N.) that a white solid precipitate formed in the methanol phase after
standing for several days. This precipitate was subsequently shown to be a
partially soluble linear polymer whose infrared spectrum exhibited absorption (not present in the case of the 5-SiB-1 homopolymer prepared by
anhydrous FeClrcatalyzed condensation) in the 7.2-7.9 pm range (maximum at 7.63 pm). This absorption is attributable to B-0 stretching from
B-0-R
groups. The latter are the result of t,he alcoholysis of the B-H
groups in the carborane cage.
A study was then made of the effects of the nature and concentration of
the R radical in ROH upon polymer properties (Table I). Where R is H or
a primary or secondary alkyl group, the B-H moieties in the carborane
cage are attacked after standing for several days at ambient temperature to
an extent sufficient to cause a substantial amount of B--OR group formation (polymers l to 6). Attack of the carborane cage is most severe when
water and methanol are employed (polymers 1 and 2). The polymers
produced in these cases were also found to be covalently crosslinked and
a
b
Methanol
1-Propanol
ZPropanol
Isoamyl
Cyclohexanol
Ethanol
Methanol
H~SOI(10 ml)
t-Amy1
t-Butyl
&Amy1
&Amy1
t-Amy1
thy1
2
3
4
5
6
7
8
9
10
11
12
13
14
3"
3"
3"
3"
3"
3"
1"
1"
1"
2"
1"
2"
1"
Class
white solid
sl. yellow solid
colorless liquid
sl. yellow liquid
6.67
6.54
4.6
5.28
white solid
white solid
white solid
white solid
white solid
white solid
5.67
5.33
1.0
2.0
3.4
5.67
sl. yellow solid
sl. yellow solid
8.53
12.4
12.4
White rubbery
solid
clear rubbery
solid
11.1
Concentration,
n ROH/n
monomer
Physical appearance
69 to 72
65 to 71
58 to 66
61 to 72
70 to 73
35 to 48
58 to 69
68 to 72
W
W
W
W
W
m
m
m
S
S
S
S
S
74 to 180
24 to 35
-1oto -5
-15to -10
30 to 40
S
54 to 180
Melting point
range, "C
Magnitude of
B-0
stretchingb
2180
2325
2732
2712
1760
2625
2200
2340
1020
1080
515
620
995
1630
a
t
~
10.0
11.0
12.6
12.5
8.0
12.0
10.0
11.0
4.7
5.0
1-2
2.0
4.6
7.5
DP
Polymers 1to 13 inclusive are 5SiB-1 homopolymers, polymer 14 is a SiB-1 copolymer containing 80 mole-% C2B5and 20 mole-% C2Blo.
s = Strong; m = medium; w = weak.
+
Water
Species
1
Polymer
no.
Alcohol
Polymer property8
TABLE I
Properties of 5-SiB-1 Polymers Prepared by Alcoholysisa
%
4
Ip
KESTING, JACKSON, AND NEWMAN
2648
hence incompletely soluble polymers. Broad melting ranges were therefore characteristic of these species. Interestingly, the addition of concentrated HzS04to methanol dramatically decreased the R-0 absorption in
the product (polymer 8). (Protonated methanol should, of course, be less
nucleophilic than unprotonated methanol, and hence less likely to attack
the electrophilic carborane cage.) However, when R was a 3” radical such
as tert-butyl or tert-amyl (polymers 9 to 14),a lesser amount of €3-0 absorption was observed, and the products were linear, completelysoluble polymers
with a molecular weight of about 2700 (i.e., a DP of about 12). For the
latter, an oversimplified reaction scheme can be envisioned as follows:
CH3
C&~!&--c&aHs
4””
i--c1 -k ROH
CH3
I
hH3
I
lHa
+
CHI
CHa
I
I
I
I
CH3
CHI
LH3
I1 or 111
Ha0 +
I
I
+
AH3
I11
I1
ROH
CHs
CHI
R O - - % - - C B s H 5 - - % 4 R HC1 (1)
CI-%-c2BsH6 i 4 R
+ HCl
CH3
+
+
RCl HaO+ C1CH3
CHa
I
(2)
+
H O - S i - C B s H ~ ~ i - O ~ i - C B ~ z ~ i - O HHC1+ ROH
I
CH3
CH3
I
(!!Ha
IV
+ RCl
LHa
(3)
Equation (1)suggests that if compound I1 were the more important intermediate, a maximum in MW should occur when 1mole of alcohol is reacted
with 1 mole of chloro monomer, since such is required for stoichiometry.
However, because of the side reaction indicated in eq. (2) (a reaction particularly likely when tertiary alcohols are employed), a somewhat higher
concentration of alcohol would be required to achieve this maximum value.
However, the fact that greater than stoichiometric quantities of alcohol can
be added without lowering the MW of the product indicates that compound
I11 may be more prevalent. I n either case, the possible deleterious effect
which an excess concentration of alcohol can have by virtue of a possible
attack of the B-H moieties in the carborane cages (polymer 9) remains an
important consideration.
It is significant that, although alcoholysis is applicable to the entire range
of SSiB polymers, including the 5-SiB-1 homopolymers (Fig. la) and 5SiB-1 copolymers containing 20 mole-% m-CzBlo (Fig. lb), the 10-SiB-1
homopolymers (Fig. lc) do not appear to be accessible by this method.
This appears to be primarily attributable to steric hindrance.
An examination of idealized molecular models (regular polyhedra) of the
above species shows that the carbon atoms in the CzBsH7 chloro monomer
PREPARATION OF POLYCARBORANE SILOXANES
2649
are encountered at the apex of two relatively small angles within the cage: a
wide angle of 10s' and a narrow angle of 63". In both cases a large exterior
angle results which can be expected to contribute to flexibility in the C-Si
bond. Both carbon atoms in the m-C2Blochloro monomer, on the other
hand, are situated at the apex of wide internal angles of 124.5', with the
result that the exterior angle is much smaller and hence the C-Si bond
more restricted than is the case for the 5-SiB polymers. This suggests that
v o m-C2Blo moieties cannot be made to adjoin one another in a SiB-1
Fig. 1. Polycarboranesiloxane polymers of the SiB-1 type.
polymer unless forcing conditions are employed. Copolymers incorporating both C2B5and CzBlomonomers apparently represent a situation intermediate between 5-SiB-1 and 10-SiB-1 homopolymers, since the MW of the
polymer containing 20 mole-% m-C2Blo (polymer 14) is lower than that of
the 5-SiB-1 homopolymer and higher than that of the 10-SiB-1 homopolymer, which does not appear to form under the present set of conditions.
I n other words, a m-CzBlomonomer can be joined to a CzBsmonomer, but
less readily than one CzBSmonomer to another of its own kind. This situation contributes to isolation of CzBlo monomers which in turn acts to
maximize randomness and hence elastomeric behavior on the copolymer.
Although low molecular weight polymers are obtained by alcoholysis, it is
obvious that, where high molecular weight is desired, further polymerization
can be effected by other techniques.
2650
KESTING, JACKSON, AND NEWMAN
The authors gratefully acknowledge the support of this study by the Office of Naval
Research. They would also l i e to express their gratitude to their colleagues Dr. R. E.
Williams and Dr. J. F. Ditter for helpful critical discussions.
References
1. R. E. Kesting, K. F. Jackson, E. B. Klusmann, and F. J. Gerhart, TR-2of Chemical
Systems Incorporated to Office of Naval Research, March 24,1970.
2. I d a , J. A p p l . Polym. Sci.,14,2525 (1970).
3. R. E. Kesting, K. F. Jackson, and J. M. Newman, TR-3 of Chemical Systems
Incorporated to Officeof Naval Research, January 1, 1971.
4. Idem, J. A p p l . Polym. Sci., 15,1527 (1971).
5. M. B. Roller and J. K. Gillham, paper presented at the meeting of the Amer.
Chem. SOC.in Washington, D. C., September, 1971.
Received June 2,1971
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molecular, alcoholysis, carborane, low, closs, iii, polymer, preparation, containing, resistance, heat, weight, polycarboranesiloxane
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