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Novel synthetic methods for condensation polymers using silylated nucleophilic monomers
Prog. Polym. Sci., Vol. 14, 173-193, 1989 Printed in Great Britain. All rights reserved.
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Copyright © 1989 Pergamon Press plc
NOVEL SYNTHETIC METHODS FOR CONDENSATION POLYMERS USING SILYLATED NUCLEOPHILIC MONOMERS YOSHIO IMAI and YOSHIYUKI OISHI
Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Meguro-ku, Tokyo 152, Japan CONTENTS I. Introduction 2. Polymers starting from N-silylated diamines 2.1. Synthesis of polyamides 2.2. Synthesis of polyimides 2.3. Synthesis o f polyureas 2.4. Synthesis of polyamines 2.5. Synthesis of polyazomethines 3. Polymers starting from O-silylated bisphenols 3.1. Synthesis of polyesters 3.2. Synthesis of polyethers 4. Polymers starting from S-silylated dithiols 5. Conclusion
References
173 174 174 182 184 185 187 187 188 189 190 192 192
I. I N T R O D U C T I O N
Silylation of organic compounds, substitution with trialkylsilyl groups, has found for many years wide application as an aid for chromatographic separation procedures through conversion of organic compounds into more volatile derivatives. The value of silylation as a new tool in synthetic organic chemistry has also become apparent during the last three decades, and the silylation method has created new organic synthesis tools directed toward highly specific and selective synthesis under milder reaction conditions. In the field of synthetic polymer chemistry, after pioneering research by Klebe in the mid 1960s on the synthesis of some condensation polymers with the use of N-silylated diamines, dramatic developments were not seen during the 1970s. Since the demonstration of polymer synthesis using O-silylated bisphenols by Kricheldorf in the late 1970s or at the beginning of the 1980s, this field of research has become active again and has entered a new growth stage. We have also investigated the synthesis of aromatic polyamides (aramids) and related polymers using N-silylated aromatic diamines since 1984, with interesting results. The present review deals with the application of the st~ation method as a new tool for the synthesis of various types of condensation polymers. 173
174
Y. IMAI and Y. OISHI
2. POLYMERS STARTING FROM N-SILYLATED DIAMINES N-Trialkylsilylated amines represent, next to trialkylsilyl halides, the most reactive class of organosilane compounds. They react with a variety of electrophiles such as carboxylic acid halides, acid anhydrides, isocyanates, benzyl halides, and aldehydes) These reactions are applicable for the synthesis of nitrogen-containing condensation polymers.
2.1. Synthesis of polyamides It was reported that the reaction of N-trimethylsilyl-substituted amines with acid chlorides yielded amides with the elimination of trimethylsilyl chloride, and not a combination of N-trimethylsilylamides and hydrogen chloride (eq. 1).2 The amide-forming reaction, therefore, can be extended to polycondensation between bifunctional monomer pairs leading to polyamides.
R-NHSiMe3
+
O
[
CLC-R'
I
-
0 II
R-NHC-R'
+ Me3SiCt
O
,
/'=
R--N-C-R' + I
(1) HCL
SiMe 3
We have successfully synthesized high molecular weight aramids by the low temperature solution polycondensation of N-silylated aromatic diamines with aromatic diacid chlorides (eq. 2). 3"4 The polymerization was carried out at - 1 0 ° C in amide-type solvents like N-methyl-2-pyrrolidone (NMP). Usually aramids are prepared by the polycondensation of aromatic diamines with aromatic diacid chlorides (eq. 3). 5'6 Me3SiNH-Ar-NHSiMe 3 +
I- NH-Ar- NHC-At'- C-]
-Me3S'CL---
HzN-Ar-NHz
-HCt
C t C - A r ' - C C I. II II O O
+
(2)
CtC-Ar'-CCL II II 0 0
[ - N H - A r - NHC-Ar'-C-111 " / O O ~
(3)
Figure 1 shows a comparison of the silylation method and the conventional diamine route for the synthesis of poly-p-phenyleneterephthalamide, Du Pont's "Kevlar" molecule, which was conducted in a mixture of NMP and hexamethylphosphoramide (HMPA). N-Silylated p-phenylenediamine reacted more
NOVEL SYNTHETIC METHODS FOR CONDENSATION POLYMERS
175
readily with terephthaloyl chloride than the parent p-phenylenediamine itself. The polycondensation using the N-silylated diamine proceeded more rapidly and afforded aramid with higher inherent viscosity, compared to the polymerization using the diamine. 3 10
~8
A
o~ 6
~
.
i>
~_ 4
B
~2 Z
0
I
2
I
I
I
4 6 8 REACTION TIME ( h )
I
I
10
I
FIG. 1. Comparison of time dependence of inherent viscosity of poly-p-phenyleneterephthalamide formed by the polycondensation of terephthaloyl chloride with (A) N,N'-bis(trimethylsilyl)-substituted p-phenylenediamine or with (B) p-phenylenediamine at a reactant concentration of 0.2 mol/l in H M P A - N M P (2 : I by volume) in the presence of lithium chloride between - 10 and - 5°C.
Table 1 summarizes the results for the synthesis of various aramids. In all cases, the silylation method was superior to the diamine route with respect to the inherent viscosity of the resulting aramids. 4 The high reactivity of N-silylated amines toward acid chlorides can be explained by the postulated mechanism as given below. Organosilane compounds are characterized by the fact that the silicon has a strong affinity for oxygen, fluoride, or chloride ion, and that the carbocation on the #-position to the silicon can be stabilized through the silicon a - n effect. Taking into consideration these particular features of organosilane compounds, the following nucleophilic addition-elimination two-step mechanism is proposed for the nucleophilic acyl substitution of an acid chloride with an N-trimethylsilylsubstituted amine (eq. 4). ~
iSiMe3
Ar' - C ~ " ~ " - , ~ f ~'- : NH - A t I CL
--'-'"
O - SIMe 3 I Ar'--C- NH-Ar I Ct.
NH-Ar =--
Ar'- C-;- 0 fl LI CLE.~ SiMe3
(4) "
Ar'--CNHII O
Ar
+
Me3SiCL
In the first step, the attraction of the carbonyl oxygen of an acid chloride to the silicon of an N-silylated amine facilitates the nucleophilic attack of the
176
Y.IMAIandY.OISHI TAnLEI. Synthesis of various
aramids* (ref. eqs 2 and
Monomers Ar (from diamine)
3)
Polymer rti.ht
Ar' (from diacid chloride)
(dl/g)
N-Silylation method
Diamine method
-~
~
2.45
1.03
_~
_~
3.19
0.94
-~
2.41
1.19
_~
_~
7.41:~
3.23~:
_.~CH2_~
-~
2.54
1.15
._.~CH2_~
.-~
3.21
1.40
3.9 _@_
,9
,.,0
*Polymerization was carried out in N-methyl-2-pyrrolidone containing lithium chloride at - 10 to - 5°C for 5 hr. tInherent viscosity was measured at a concentration of 0.5 g/dl in concentrated sulfuric acid at 30°C. :[:Polymerization was conducted in a mixture of N-methyl-2-pyrrolidone and hexamethylphosphoramide containing lithium chloride at - 10 to - 5 ° C for 12 hr.
nitrogen of an N-silylated amine at the carbonyl carbon of an acid chloride, thereby quickly giving rise to the tetrahedral intermediate. In the second step, the elimination of chloride ion from the intermediate is enhanced by the presence of the/~-silicon through the ~r-~ effect, affording rapidly the amine product along with trimethylsilyl chloride: Aromatic diamines with low reactivity, mainly due to their low basicity, usually produce only low molecular weight polyamides. However, they can be activated markedly by conversion to N-silylated diamine derivatives. This is illustrated by the following examples. Aromatic dianilino compounds (very weak bases) when substituted with trimethylsilyl groups, readily afforded N-phenylated aramids by high-temperature solution polycondensation with aromatic diacid chlorides in tetramethylene sulfone (eq. 5). 7.8 The inherent
NOVEL SYNTHETICMETHODS FOR CONDENSATIONPOLYMERS
177
TABLE 2. Synthesis of N-phenylated aramids (eq. 5) Monomers
Polymerization*
Polymer
Ar
Ar'
Temp.
Time
(from diamine)
(from diacid chloride)
(°C)
(hr)
r/i.ht (dl/g)
T~:~ (°C)
-~
160
24
1.65
225
~
200
6
2.21
255
--~'- 0~--
- ~
160
24
1.24
195
--~0--~-
--~
200
24
1.35
215
"-~
*Polymerization was carried out in tetramethylene sulfone. 1"Inherent viscosity was measured at a concentration of 0.5 g/dl in concentrated sulfuric acid at
30° C. :[:Determined by differential thermal analysis (DTA) at a heating rate of 10°C/min in air.
viscosities of the resulting N-phenylated aramids (Table 2) were much higher than those from the unsubstituted diamines. 9 The N-phenylated aramids have glass transition temperatures (T~) in the range of 195-255°C, and are soluble in a variety of solvents including dimethylformamide (DMF), m-cresol, and chloroform. 0 O II H MeaSiN-Ar-NSiM% + CLC-Ar'-CCI. I
I
Ph
Ph
-M%S,CL ,_
/r - N - A r -
0 N - II
L i Ph
i Ph
0 II
"]
C-Ar'-C- 1
(5)
-n
Again from the combinations of the N-silylated dianilino compounds and aromatic dicarbamoyl chlorides, N-phenylated aromatic polyureas could be obtained by high-temperature solution polycondensation, t° Tetrafluoro-m-phenylenediamine is another aromatic diamine having low basicity. This diamine could also be activated through N-silylation. The silylated derivative gave fluorine-containing aramids with reasonable inherent viscosities (0.28-0.47 dl/g) by low-temperature solution polycondensation with aromatic diacid chlorides (eq. 6). tl
Y. IMAI and Y. OISHI
178
_~ Me3SiNH
F
F" y
F -Me3SiCL
o*c
0 0 II II NHSiMe3 + CLC-Ar'-CCL -F r
F
~I~
0
0
II -I"
II
(6)
I-"" TO"F ""c -Ar'- c-I~" ~ F"~-/~" F F
Aromatic diacid chlorides of considerably lower reactivity than the usual diacid chlorides are also capable of yielding aramids with moderately high molecular weights using N-silylated diamines. An example is given for the fluorine-containing aramids from the low-temperature solution polycondensation of the N-silylated aromatic diamines with tetrafluoroisophthaloyl chloride (eq. 7)) 2 The fluorine-containing aramids have Tgs in the range of 245-280°C and dissolve readily in various organic solvents (Table 3). 0 II
F J.
0 II
Me3 SiNH -Ar-NHSiMe3 + CLC-~(-"~'I--CCL F 0 F 0 I, .L , 1 -Me3SiCt"/-NH-Ar-NHC T O Y C- I 6o*c
,.
F ~..~.F
(7)
~,
F
TABLE 3. Synthesis of fluorine-containing aramids* (eq. 7) Ar (from diamine)
Polymer
~,..1"
T,t
(dl/g)
(°C)
-•
0.43
245
--~
0.56~
275
CHz-~
0.63
280
--•0-•
0.82
280
-~--
*Polymerization was carried out in chloroform containing triethylamine hydrochloride at 60°C for 6 hr. $lnherent viscosity was measured at a concentration of 0.5g/dl in dimethylacetamide at 30°C. ~Determined by DTA at a heating rate o f 10°C/min in air. §Polymerization was carried out at 0°C for 6 hr.
NOVEL SYNTHETIC METHODS FOR CONDENSATION POLYMERS
179
The reaction of N,O-bistrimethylsilylated o-aminophenol with benzoyl chloride was also interesting. The acid chloride attacked selectively at the N-silylated amine site of the o-aminophenol and the corresponding amide product was formed exclusively. This occurred because the amino group of the o-aminophenol is activated toward acid chloride attack by silylation and the hydroxyl group, in contrast, is deactivated as discussed later. The reaction was successfully extended to the polycondensation of the N,N',O,O'-tetrakis-(trimethylsilyl)substituted bis-o-aminophenols with aromatic diacid chlorides. The polymerization was carried out at 0°C in an amide solvent to yield poly-o-hydroxyamides. These were then subjected to thermal cyclodehydration to afford polybenzoxazoles (eq. 8).13
Me3SiNH\Ar/NHSiMe3 MeaSiO/ \OSiMe 3
-Me 3 SiCt i~ O°C
MeOH.
[
O O II II + C k C - A r ' - CCL O O II II -NH\Ar/NHC-Ar'-C- ]
Me3SiO /
o ii
2~)0"C
J. o ii
[-NH\Ar/NHC-Ar'-C-] L HO /
-H20
XOSiMe3
J°
".OH
[ r.//N\^. /N%~ _ L "0/ "0
"] in
(8)
One of the remarkable features of the silylation method was that the polycondensation proceeded in homogeneous solution even in low-boiling solvents such as dichloromethane and tetrahydrofuran, as well as in high-boiling amide solvents. This occurred because the intermediate silylated poly-o-hydroxyamides are soluble in a wide range of solvents. It had earlier been especially difficult to prepare fluorine-containing polybenzoxazoles of high molecular weight from 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane, a weak nucleophile. However, the silylation method enabled formation of these polybenzoxazoles for the first time. The aromatic polybenzoxazoles thus obtained are one of the potential high temperature polymers, having Tgs in the range of 260-325°C (Table 4). 13 Although the following example does not deal with the synthesis of aramids, Katsarava et al. prepared aliphatic-aromatic polyamides of high molecular weights by the polycondensation of N-silylated lysine with aromatic diacid chlorides (eq. 9)J 4 They also showed that activated diesters such as 2,4-dinitrophenyl esters could be used in place of the diacid chlorides. The polyamides were then formed by elimination of O-silylated 2,4-dinitrophenol.
180
Y. IMA1and Y. OISHI Me3 SiNHCH(CHz)4 NHSiMe3 + I
COOE+, -,~e~SiCt 25°C
.
CLC--(CD:)-CCI. II ~ II 0 0
r-NHCH (CHz,4 N H C ~ C - - ] / I II ' ~ " COOEr 0
(9) II 0
/ Jn
~sp/C : 1.02 (HMPA)
TABLE4. Synthesisof aromatic poly-o-hydroxyamidesand polybenzoxazoles(eq. 8) Monomers Ar Ar' (from bis-o- (fromdiacid aminophenol) chloride)
Polymerization PolyamidePolybenzoxazole solvent* r/inht Tg:~ (dl/g) (°C)
-~-
Dimethylacetamide 0.50
290
-~
Dimethylacetamide 0.55
240
-~
Dimethylacetamide 0.64
260
- ~
Dimethylacetamide 0.65
310
']~/-~
Dimethylacetamide 0.86 Tetrahydrofuran 0.83 Dichloromethane 0.63
280
Dimethylacetamide 0.60
325
Dimethylacetamide 0.47
300
Dimethylacetamide 0.40
295
CH3 I ~ - ~ H 3 -~ CF~
~ ] - I F 3 -~
-
(50 : 50)
"
~ --~-~-- ~
OF3 I iC'~ ¢F3
*Polymerizationwas carried out at 0°C for 8 hr. tlnherent viscositywas measured at a concentration of 0~5g/dl in dimethylacetamideat 30°C. ~:Thermalcyclodehydrationwas conducted at 250°C for 30 hr/n v a c u o . T s was determined by thermomechanical analysis at a heating rate of 10°C/min in air.
NOVEL SYNTHETIC METHODS FOR CONDENSATION POLYMERS
181
Thus, through the aforementioned examples we are able to demonstrate the development of a versatile and promising method for the synthesis of a variety of high molecular weight aramids by the use of N-trimethylsilylated aromatic diamines. It is clear that the N-silylated diamines are far more reactive than the parent diamines toward aromatic diacid chlorides. Consequently, high molecular weight aramids could be obtained readily by the low-temperature solution polycondensation of these monomer pairs under milder reaction conditions. The N-silylated diamine method has several advantages over the conventional method: (1) High-purity N-silylated aromatic diamines can be obtained simply by distillation. (2) N-silylated aromatic diamines exhibited excellent solubility in organic solvents. This facilitates the polycondensation process in a variety of reaction media, ranging from low- to high-boiling and/or polar solvents. (3) The polycondensation proceeds under neutral reaction conditions with the elimination of trimethylsilyl chloride. (4) The trimethylsilyl chloride recovered from the polymerization system may be recycled as a silylating agent for the diamines. Aramid synthesis via the silylation method (eq. 2) is applicable to vacuum deposition polycondensation for the preparation of aramid thin films on appropriate substrates.lS Aramid thin films can be deposited, of course, by the conventional diamine route (eq. 3). However, a temperature of above 100°C was required to eliminate the hydrogen chloride formed during the polycondensation process. 16"17By the use of N-silylated diamines, the deposition polycondensation was achieved at a temperature near 60°C with the elimination of neutral trimethylsilyl chloride. Thereby polymer films of lower thermal stability, and erodible metal plates, can be used as the substrates and conveniently coated with aramid thin films. It is remarkable that the "Kevlar" molecule is normally highly crystalline, whereas the aramid film deposited by the silylation method is amorphous and transparent. In the silylation method described above, preformed N-trimethylsilylated diamines were used as the polymer-forming monomers (made by prior synthesis from aromatic diamines and trimethylsilyl chloride). In contrast, one-pot preparation of aramids has also been reported using an appropriate silylating agent. The solution polycondensation of 3,8-diaminophenanthridinone with terephthaloyl chloride in an N M P - H M P A mixture at 0°C in the presence of trimethylsilyl chloride effectively afforded an aramid having an inherent viscosity greater than 6.0 dl/g. ~s Another example is given for the synthesis of poly-p-benzamide by the self-polycondensation of p-aminobenzoic acid in pyridine in the presence of tetrachlorosilane which, in fact, acts as a condensing agent (eq. 10).'9 HzN-~COH
+ I/2S,CL4 0
120"C---
-Nit
C-
-In
r/i.h : 1.97 ( HzSO 4 )
I/2SiO 2
~- 2HCt
(10)
182
Y. IMAI and Y. OISHI
2.2. Synthesis of polyimides Aromatic polyimides are generally prepared in two steps; the ring-opening polyaddition of aromatic diamines to aromatic tetracarboxylic dianhydrides yielding soluble polyamic acid precursors, which in turn undergo subsequent thermal cyclodehydration to insoluble polyimides (eq. 11).6'20'21 HzN-Ar-NH2
_ / C O ,/CON~ + U-~O/~r \ C O / v I--Ar-NI-KX3... fCOOH -I/ / Ar'.~ L HOCC( CONH-Jn
DMAc r.,.
r
-HzO-3oo.c
..CO
CO. n
I-Ar-N.. "At ' / "N-/ L co" " c o ~" Jo
(11)
N-Silylated amines react with aromatic anhydrides in the same fashion. When an N-silylated amine is reacted with an anhydride, the intermediate ring-opened adduct, silylated amic acid, can eliminate silanol at higher temperature with the formation of an imide. ~ The application of the reaction to the synthesis of polyimides was reported by Klebe in the patent literature in 1967.22 The ringopening polyaddition of N-silylated aromatic diamines to aromatic dianhydrides afforded polyamic acid trimethylsilyl esters, which in turn were converted thermally to polyimides with the elimination of trimethylsilanol (eq. 12). ^/C0,,, ^ ,/CO x M%SiNH-&r-NHSiM% + U\co/~r \co/O
~o.c
=
- Me3 SiOH 2oo-c
r -At - NHCO\
L
/COOSiMe 3 ] Ar' Me3SiOCO/ \CONH- Jn
r /co\ /co\ -] m. I-Ar-Nx /.~.r'\ / N - I L CO CO J
(12)
We have reinvestigated the polyimide-forming reaction starting from N,N'bis(trimethylsilyl)-substituted bis(4-aminophenyl) ether and pyromellitic dianhydride in detail. 23The presence of trimethylsilyl groups provides solubility for the silylated polyamic acids in nonpolar solvents. Thus a variety of solvents including ethers and hydrocarbons can be used in place of the customary polar solvents such as dimethylacetamide (DMAc) and NMP, giving polymer with high inherent viscosity (Table 5). The thermal conversion of the silylated precursor polymer to polyimide proceeded more slowly and required higher temperatures for the completion of the imidization, compared with that of the parent polyamic acid.
NOVELSYNTHETICMETHODSFOR CONDENSATIONPOLYMERS
183
TABLE5. Synthesis of polyamic acid in various solvents* Solvent
Polymer
Remarks/;
(dl/g) Dimethylacetamide N-Methyl-2-pyrrolidone Bis(methoxyethyl) ether 1,4-Dioxane Tetrahydrofuran Chloroform Acetonitrile Nitrobenzene Toluene
1.77 1.46 0.92 0.80 0.97 0.99 0.88 0.81 0.92
S S S S S S P P P
*Polymerization was carried out with N,N'-bis(trimethylsilyl)-substitutedbis(4-aminophenyl) ether and pyromelliticdianhydride at 20°C for I hr and then at 50°C for 12hr. tInherent viscosity was measured at a concentration of 0.5 g/dl in dimethylacetamide at 30°C. ;~Appearanceof the polymerization:S, homogeneoussolution throughout the reaction; P, precipiration during the reaction. The polyimide synthesis by the N-silylated diamine method (eq. 12), as well as that by the conventional diamine route (eq. I l), can be used conveniently for the deposition of polyimide thin films on appropriate substrates such as metal and ceramic plates in a manner very similar to the vacuum deposition polycondensation of aramid thin films discussed previously. 24-26In the case of polyimide thin film deposition, an aromatic diamine component and an aromatic dianhydride were deposited in the first step on a substrate under high vacuum, and then the substrate was heated to induce imidization to form a polyimide thin film. Polycondensation with N-silylated diamines is also applicable to aromatic carboxylic acid derivatives having two different electrophilic functions on the aromatic ring. The polymerization of 4-chloroformylphthalic anhydride with N,N'-bis(trimethylsilyl)-substituted bis(4-aminophenyl) ether progressed through polycondensation and polyaddition leading to the polyamic acid trimethylsilyl ester (eq. 13). 27 Although polymerizations of this mixture in the solvents listed in Table 6, except for D M A c and N M P , proceeded with precipitation of the product, the resulting polymers had relatively high inherent viscosities. The silylated precursor polymer could be converted to the polyamide-imide by the same thermal treatment used for polyimide formation.
Me 3SiNH - A r - NHSiMe 3 + CLCO - A r ' / C O \ o \CO / -Me3 SiCL,, 50% - M% SiOH 2OO*C
F /COOSiMe3] | - A r - NHCO- Ar'.., / L CONH- J. r ~.CO\ -I I-Ar-NHCO-Ar'~, /N- / L CO Jo
(13)
184
Y. IMAI and Y. OISHI TABLE 6. Synthesis of polyamide-amic acid in various solvents* Solvent
Polymer
Remarks~/
~/i.ht
(dl/g) Dimethylacetamide N-Methyl-2-pyrrolidone Acetonitrile Nitrobenzene Bis(methoxyethyl) ether 1,4-Dioxane Tetrahydrofuran Chloroform Benzene
!.82 1.48 I. 14 1.21 1.00 0.75 0.94 0.59 0.57
S S P P P P P P P
*Polymerization was carried out with N,N'-bis(trimethylsilyl)-substituted bis(4-aminophenyl) ether and 4-chloroformylphthalic anhydride at - 5°C for I hr and then at 50°C for 4 hr. flnherent viscosity was measured at a concentration of 0.5 g/dl in N-methyl2-pyrrolidone at 30°C. :~Appearance of the polymerization: S, homogeneous solution throughout the reaction; P, precipitation during the reaction.
Korshak and his group have reported that polyimides containing aliphatic groups, which were difficult to prepare in high molecular weight by the diamine route, could be obtained satisfactorily starting from N-silylated aliphatic diamines and aromatic dianhydrides.2s 2.3. Synthesis of polyureas The addition of N-silylated amines to isocyanates gives high yields of N-silylsubstituted urea derivatives, which are readily hydrolysed to the parent ureas) This reaction was utilized for the synthesis of aromatic polyureas by Klebe in 1964, who demonstrated the polyaddition of N-silylated aromatic diamines to aromatic diisocyanates to yield N-silylated polyureas (eq. 14).29The polymerization could be achieved in homogeneous solution even in solvents of low polarity due to the less polar nature of the N-silylated polyureas as compared with normal polyureas (Table 7).30 It is noteworthy that the polyureas derived from the methanolysis of the N-silylated polymers have low crystallinity and good solubility in organic solvents, in contrast to the polyureas prepared by the diamine-diisocyanate routeP ° Me3 S i N H - A r - NHSiMe 3 + O = C = N - A r ' - N =C =O O O H I 1 | -I~-Ar-~I-CNH-Ar'-NHC- I LMe3Si SiMe3 -n
I"
~.
r
~
~]
(14)
NOVEL SYNTHETIC METHODS FOR CONDENSATION POLYMERS
185
TABLE 7. Synthesis of an aromatic polyurea in various solvents*
Solvent
Polymer
~i.ht (dl/g)
Dimethyi sulfoxide Tetramethylene sulfone Tetrachloroethane Anisole Nitrobenzene Toluene
0.54 0.60 0.91 0.77 0.65 0.74
*Polymerization was carded out with N,N'-bis (trimethylsilyl)-substituted bis(4-aminophenyl) ether and 2,4-tolylene diisocyanate at 100°C for 10 hr. tlnherent viscosity was measured at a concentration of 0.5 g/dl in concentrated sulfuric acid at 300C.
2.4.
Synthesis of polyamines
Although alkyl halides are less reactive than carboxylic acid chlorides toward aminolysis, the alkyl halides are also capable of attacking N-silylated amines.' Klebe reported that benzyl chloride and an N-silylated amine underwent this substitution reaction at above 100°C and yielded benzylamine and trialklysilyl chloride quantitatively. 3t In this reaction, ammonium salts and a cyclic sulfone enhanced catalytically the rate of reaction. The reaction has also been extended to the synthesis of polyamines, which were difficult to obtain in high molecular weight by the dihalide--diamine route probably due to the occurrence of side reactions. The melt polycondensation of reactive dihalides of the benzyl chloride type with N-silylated piperazines at 150°C led to the formation of high molecular weight polyamines (eq. 15). 3~ Me
/-4 P . M e2SiNL_jNS ,M ezPh
+ Me
=
-Ell 2
(15)
C~ n
[7/] : 1.97 (ChLoroform)
We found that a similar substitution reaction occurred when activated aromatic halides were treated with N-silylated aromatic amines, giving aromatic secondary amines along with trimethylsilyl halides. 32 Accordingly, novel aromatic polyamines were successfully prepared from activated aromatic difluorides and N-silylated aromatic diamines (eq. 16). The solution polycondensation was carried out at 100-150°C in dimethyl sulfoxide (DMSO) in the presence of a
186
Y. IMAIand Y. OISHI
fluoride catalyst such as potassium or cesium fluoride. It produced aromatic polyamines with reasonable inherent viscosities (Table 8). The presence of the catalyst was essential for the preparation of high molecular weight polyamines. The use of the corresponding dichlorides in place of the activated aromatic difluorides yielded only low molecular weight polymers. Introduction of nitro groups onto the aromatic dihalides improved the reactivity, affording polyamines with higher inherent viscosities. These aromatic polyamines dissolve readily in organic solvents like N M P and have Tgs around 200 ° C, and therefore they may also be accepted as one of the potential engineering plastics. 32 Me3SiNH-Ar-NHSiMe 3 ÷ - Me3SiF
:[
--NH-Ar-
F-Ar'-F NH-Ar
:]
(16)
~
TABLE8. Synthesisof aromatic polyamines(eq. 16) Monomers
Polymerization*
Polymer
Ar' (from difluoride)
Temp./Time (°C/hr)
(dl/g)
-'~SOe"~
100/7-150/14
0.62
150/12
0.57
150/12
0.42
100/7-150/14
0.60
t/i.ht
Ar (from diamine) -'~
"-~
-'•CHt'-•
"
~ ' ~ --~CO~ --~
-•-CHf-• --~--O-~
100/24-200/48,
0.55
220/48t
0.37
100/24-200/48~
0.45
150/48
0.66
*Polymerization was carded out in dimethyl suifoxide in the presence of potassium fluoride. tInhercnt viscosity was measured at a concentration of 0.5 g/dl in N-methyl-2-pyrrolidoneat 30°C. :l:Polymerizationwas conducted in tetramethylenesulfone.
NOVEL SYNTHETIC METHODS FOR CONDENSATION POLYMERS
187
2.5. Synthesis of polyazomethines Most aromatic polyazomethines could not be obtained in high molecular weights due to their insoluble and infusible characteristics. Kurosaki and coworkers investigated extensively the preparation of aromatic polyazomethines and found new routes for these polymers by the use of N-silylated aromatic diamines. 33'34One method is the polycondensation of a combination of N,N'bis(trimethytsilyl)-substituted aromatic diamines and aromatic bis(diethyl acetal) compounds. The solution polymerization in NMP gave organic-soluble precursor polymers, which in the form of films were subsequently converted at 200°C to insoluble polyazomethines (eq. 17).33 Another method is the polyaddition of N,N,N',N'-tetrakis(trimethylsilyl)-substituted aromatic diamines to aromatic dialdehydes in N M P leading to soluble precursor polymers, followed by thermal conversion to insoluble polyazomethine films (eq. 18).34 In both cases, similar precursor polymers were formed in the first step, and the subsequent fl-elimination occurred with the formation of azomethine linkages along the polymer main chains. Me3SiNH-~NHSiMes + (EiO)2CH-<~-CH(OE+~)2 OEt -N I -'~
30°C"
N-CH I
MeaSiO 30°C
""
-N I Me3SiV
N-CH I SiMe 3
OEt CH--] /
( G H or SiMe 31
OSiMes CH-
,oo.c" 3. P O L Y M E R S
STARTING
FROM
O-SILYLATED
BISPHENOLS
Silylation of a hydroxyl group is normally a protection against electrophilic attack due to the strong affinity of silicon for oxygen. O-trialkylsilyated phenols (trialkylsiloxybenzenes), however, can be activated by means of chloride or fluoride ion via the equilibrium (eq. 19), and can thereby react with electrophiles such as carboxylic acid halides and activated aromatic halides. By the appli'cation of this information, Kricheldorf and his group have developed a new
188
Y. IMAI and Y. OISHI
synthetic method for aromatic polyesters and aromatic polyethers starting from O,O'-bistrimethylsilylated bisphenols. Ar-O-SiMe3+
X (~ - ~
Ar - 0 (~ + Me3SiX
(19)
3.1. Synthesis of polyesters Aromatic polyesters (polyarylates) are usually prepared either by the twophase polycondensation of bisphenols with diacid chlorides in organic solventaqueous alkaline solution systems using phase transfer catalysts (eq. 20), or by the melt polycondensation of bisphenol diacetates with dicarboxylic acids (eq. 21) or the similar melt polycondensation of A-B type monomers (eq. 22). 5.6 NaO-Ar-ONo
+ CLC-Ar'-CCt )) il 0 0
-.oc,
[-O-Ar-O~-Ar'-C-]
r.t.
L
o
CH3CO-Ar-O8 ~cH3
+
(20)
~ J°
HOCo-Ar)-COH8
- C%COOH,, [-O-Ar-OC-Ar'-C-] 2~o.c
CH3CO-,&r- COH
~)
~)
L
~
-¢ H3COOHi
3oo.c
~ J.
[ - O - A r - C -1
L
~) j.
(21) (22)
Kricheldorf et al. first demonstrated in 1979 that polyarylates could b¢ synthesized either from O-silylated bisphenols and diacid chlorides (eq. 23), or from trimethylsiloxybcnzoyl chlorides (eq. 24) by melt polycondensation at 200-300°C. 35 Me3SiO-Ar-OSiMe 3 + CLC-Ar'-CCL II I1 O O
-a,~sicL 25o'c
Me3SiO -Ar-CCL II 0
[-O-Ar-OC-Ar'-C-]
(23)
-Me3SiCL 300°C
(24)
/ L
II O
II / O ~"
r- O-Ar-C -1 ,. g J.
The thermal polycondensation of two different A-A and B-B type monomers, such as O,O'-bis(trimethylsilyl)-substituted 2,2-bis(4-hydroxyphenyl)propane and isophthaloyl chloride, suffers from distillation or sublimation of the more volatile monomer. This disrupts the stoichiometry of the monomer
NOVEL SYNTHETIC M E T H O D S FOR C O N D E N S A T I O N POLYMERS
189
pair and thereby reduces the degree of polycondensation. This disadvantage is obviously avoided if self-condensing A-B type monomers like trimethylsiloxybenzoyl chlorides are used. The molecular weight of the polymer obtained from the A-B type m-hydroxybenzoic acid derivative is indeed higher than that obtained from the O-silylated bisphenol and isophthaloyl chloride. Thus, the silylation method is most useful for the high-temperature synthesis of polyarylates from A-B type monomers. The self-polycondensations of rn- and p-siloxybenzoyl chlorides were investigated extensively in bulk or in solution. 36'37 The bulk polymerization of m-trimethylsiloxybenzoyl chloride at 250°C yielded the polyarylate having a molecular weight of 10,000-14,000, 36 whereas the polymerization of the p-oriented monomers in a high-boiling solvent at 320°C afforded the polymer with molecular weight in the range 20,000-50,000. 37 In the latter case, the polycondensation led at first to precipitation of the oligomer crystals, which continued chain growth to the polymer crystals with prolonged heating. Some catalysts such as triethylamine hydrochloride and benzyltriethylammonium chloride effectively accelerated the polycondensations, giving polyarylates of higher molecular weights. O-Silylated bisphenols also reacted with aryl phosphorodichloridates to yield aromatic polyphosphates, which are polyesters in a broad sense (eq. 2 5 ) . 39 The bulk polycondensation was conducted at temperatures between 180 and 300 ° C in the presence of a catalytic amount of benzyltriethylammonium chloride, affording polyphosphates with molecular weights up to 12,000. Me3SiO Ar OSiMe3 +
0 II
CL--P--C/ I OPh
(25)
0
Me3SCL. 280*C
F O Ar 0 ~ ] L ~)phan
3.2. Synthes# of polyethers Following the polyester synthesis, Kricheldorf et al. have investigated the bulk polycondensation of O,O'-bistrimethylsilyl-substituted bisphenols with activated aromatic difluorides (eq. 26). 39,4oThese polymerizations were successful only when potassium or cesium fluoride was used as catalyst at temperatures in the range of 270-350 ° C. They gave aromatic polyethers with molecular weights up to 17,500. Unfortunately, the reactivity enhancement was not sufficient for the polycondensation with the corresponding aromatic dichlorides; the aromatic difluorides, containing the fluorine atom (with a stronger affinity than chlorine for silicon), were indispensable reaction partners. The conventional synthetic procedure for aromatic polyethers is based on the high-temperature solution polycondensation of alkali metal salts of bisphenols with activated aromatic dihalides (eq. 27). 4~-43
190
Y. IMAI and Y. OISHI Me3SiO-Ar-OSiMe 3 ÷
F- Ar'- F
-Me3SiF f- O - A r - O 320. c ~ MO-Ar-OM
Ar'-
]
(26)
n
[-O-Ar-O-Ar'-]o
+ X-Ar'-X-MX=
(27)
M : No, K X:
F, CL
A general shortcoming of the conventional metal bisphenolate route is that the resulting polymers need to be purified from the metal salts byproducts and also from the expensive high-boiling point solvents. In contrast, the silylation method has the advantage that the molten polymers do not need purification from a large amount of metal salts and solvents prior to processing, although a catalytic amount of potassium or cesium fluoride exists as such in the molten polymers. A new class of aromatic polyether-esters was also synthesized from combinations of trimethylsiloxybenzoic acid trimethylsilyl esters and activated aromatic difluorides (eq. 28). 39 The melt polycondensation was effected at a temperature of 250-320°C in the presence of cesium fluoride as the catalyst, yielding polyether-esters with molecular weights in the range of 9000-11,000. Me3SiO-Ar-COSiMe3+ II O
320=C ="
4. P O L Y M E R S
STARTING
F-Ar'-F
O-Ar- O-Ar'O
FROM
°
S-SILYLATED
DITHIOLS
The silicon-sulfur bond has received relatively little attention from a synthetic point of view, although it appears that the reactivity of S-trialkylsilyated thiols may approach that of the N-silylated amines, and advantages from silylation may be found in the reaction of thiols with reactive halides and the like. We have compared the reactivity of S-trimethylsilylbenzenethiol (I), Ntrimethylsilylaniline (If), and O-trimethylsilylphenol (III) towards electrophiles. We found that the relative reactivity toward benzoyl chloride decreased in the order of II > I > III, and that toward 1-chloro-2,4-dinitrobenzene decreased in the order of I > II > I I I . ~ This finding was quite encouraging and we concluded that S,S'-bistrimethylsilyl-substituted dithiols were another class of promising polymer-forming reactive monomers, having similar attractive features to the N-silylated diamines discussed previously.
NOVELSYNTHETICMETHODSFOR CONDENSATIONPOLYMERS
191
This concept was applied for the first time in the polycondensation of Ssilylated aromatic dithiols with activated aromatic dihalides (eq. 29). ~ As shown in Table 9, aromatic polysulfides having moderate inherent viscosities of 0.6-0.7 dl/g were obtained when the S-silylated dithiols and bis(4-chloro-3-nitrophenyl) sulfone were heated at 50°C in H M P A . Bis(4-fluorophenyl) sulfone was found to be less reactive. Its solution polycondensation with the S-silylated dithiols was effected in tetramethylene sulfone at 220°C in the presence of cesium fluoride as the catalyst, giving polysulfides with inherent viscosities around 0.3 dl/g. 44 Aromatic polysulfides of this type are generally prepared either by the solution polycondensation of parent aromatic dithiols with activated aromatic dihalides or by two-phase polycondensation using phase transfer catalysts. 45 MeaSiS-Ar-SSiMe
3 + X-Ar'-X
- Me 3 SiX
X : F, CI.
TABLE9. Synthesis of aromatic polysulfides (eq. 29) Monomers Ar (from silylated dithiol)
Polymerizationt
Polymer
Dihalide*
Solvent
Temp. (°C)
Time (hr)
(dl/g)
A
HMPA
50
24
0.64
A
HMPA
50
24
0.67
-•0-•
A
HMPA
50
24
0.76
- ~
B
TMS
220
24
0.29~
B
TMS
220
24
0.30~
B
TMS
220
24
0.30~
-•
"-~O-~
*Dihalide A = bis(4-chloro-3-nitrophenyl)sulfone. Dihalide B = bis(4-fluorophenyl)sulfone. tPolymerization solvent: HMPA, hexamethylphosphoramide;TMS, tetramethylene sulfone. :[:Inherentviscositywas measured at a concentration of 0.05 g/dl in hexamethylphosphoramideat 30°C. §Measured at a concentration of 0.5 g/dl in N-methyl-2-pyrrolidoneat 30°C.
192
Y. IMAI and Y. OISHI
5. C O N C L U S I O N W e have been able to d e m o n s t r a t e t h a t the silylation m e t h o d , especially t h a t using N-silylated d i a m i n e m o n o m e r s , is a versatile a n d p r o m i s i n g m e t h o d for the synthesis o f a variety o f c o n d e n s a t i o n p o l y m e r s with high m o l e c u l a r weights. A n u m b e r o f synthetic studies o n c o n d e n s a t i o n p o l y m e r i z a t i o n using silylated nucleophilic m o n o m e r s are n o w u n d e r way. H o w e v e r , this field still r e m a i n s largely u n e x p l o r e d . T h e m o s t significant characteristic o f the silylation m e t h o d is t h a t the silylated nucleophilic m o n o m e r s have specific reactivity differences f r o m the p a r e n t nucleophiles. Therefore, this m e t h o d c a n find further new a p p l i c a t i o n s in the p r e p a r a t i o n o f novel c o n d e n s a t i o n p o l y m e r s a n d in the d e v e l o p m e n t o f new synthetic r o u t e s for v a r i o u s k n o w n p o l y m e r s . ACKNOWLEDGEMENTS T h e a u t h o r s a c k n o w l e d g e the s u p p o r t a n d skillful assistance o f Drs. M a s a - a k i K a k i m o t o a n d M u n i r a t h i n a P a d m a n a b a n a n d Messrs. Y u t a k a M a r u y a m a , A t s u s h i H a r a , a n d Shigeyuki H a r a d a . REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
10. 11. 12. 13. 14. 15. ! 6. 17. 18. 19. 20. 21. 22. 23. 24.
J.F. KLEBE,Adv. org. Chem. 8, 97 (1972). J.R. BOWSER,P. J. WILLIAMSand K. KURZ, J. org. Chem. 48, 4111 (1983). Y. Olsrn, M. KAKIMOTOand Y. IMAI,Macromolecules 20, 703 (1987). Y. OISHI,M. KAKIMOTOand Y. IMA1,Macromolecules 21, 547 (1988). P.W. MORGAN,Condensation Polymers by Inferfacial and Solution Methods, Interscience, New York (1965). P.E. CASSIDY,Thermally Stable Polymers, Dekker, New York (1980). M. KAKIMOTO,Y. Olsm and Y. IMAX,Makromolek. Chem. Rapid Commun. 6, 557 (1985). Y. OISHI,M. KAKIMOTOand Y. IMAI,J. Polym. Sci. Part A, Polym. Chem. 25, 2493 (1987). Y. IMA1,N. HAMAOKAANDM. KAKIMOTO,J. Polym. Sci. Polym. Chem. Edn. 22,1981(1984). Y. OISHI,M. PADMANABAN,M. KAKIMOTOand Y. IMAi,J. Polym. Sci. Part A, Polym. Chem.
25, 3387 (1987). Y. OISHI,S. HARADA,M. KAKXMOTOand Y. IMAI,Polym. Prepr. Jpn 36, 313 (1987). Y. OlSnl, S. HARADA,M. KAKIMOTOand Y. IMAI,53rd Chem. Soc. Jpn. Annual Meeting Prepr. 25 (1986). Y. MARUYAMA,Y. OIsni, M. KAKIMOTOand Y. IMAI,Macromolecules 21, 2305 (1988). R.D. KATSARAVA, D.P. KHARADZE, N. SH. JAPARIDZE, T.N. OMIADZE, L.M. AVALISHVlLiand M. M. ZAAUSHVlU,Makromolek. Chem. 186, 939 (1985). Y. TAKAHASHI,M. IIJIMA,K. INAGAWA,A. ITOH,Y. Olsm, M. KAKIMOTOand Y. IMAI,54th Chem. Soe. Jpn. Annual Meeting Prepr. 1549 (1987). R.M. IKEDA, R. J. ANGELO, F. P. BOETTCHER, R. N. BLOMBERGand M. R. SAMUEl.S,J. appl. Polym. Sci. 25, 1391 (1980). M. IIJ1MA,Y. TAKAHASHI,K. INAGAWAand A. ITOH, Shinku 28, 437 (1985). T. KANEDA, S. ISHIKAWA,H. DAIMON, T. KATSURA,M. UEDA, K. ODA and M. HORIO, Makromolek. Chem. 183, 417 (1981). P. STROHRIEGLand W. HEITZ, Makromolek. Chem. Rapid Commun. 6, 111 (1985). C.E. SROOG,J. Polym. Sci. macromolec. Rev. 11, 161 (1976). M.I. BESSONOV,M. M. KOTON,V. V. KUDRYAVTSEVand L. A. LAIUS,Polyimides, Plenum, New York (1987). E.M. BOLDEeUCKand J. F. KLEBE, U.S. Pat. 3303157 (1967). Y. Oisrn, M. KAKIMOTOand Y. IMA1,53rdChem. Soc.Jpn. AnnualMeetingPrepr. 26(1986). Y. TAKAHASH1,M. IIJIMA,K. INAGAWAand A. ITOH, Shinku 28, 440 (1985).
NOVEL SYNTHETIC METHODS FOR CONDENSATION POLYMERS 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45.
193
J.R. SALEM,F. O. SEQUEDA,J. DURAN, W. Y. LEE and R. M. YANG, J. Vac. Sci. Technol. A4, 369 (1986). Y. TAKAtL~Sm, M. ImM^, K. INAOAWA, A. ITOH, Y. OISHI, M. KAKIMOTO and Y. IMAI, Polym. Prepr. Jpn 36, 304 (1987). Y. OlSH1, M. KAKIMOTOand Y. IMAI, Polym. Prepr. Jpn 36, 315 (1987). V.V. KORSHAK, S. V. VINOt3gADOVA, YA. S. VYC,ODSKI, Z. M. NAGIEV, YA. G. URMAN, S. G. ALEKS~VA and I. YA. SLONIUM, Makromolek. Chem. 184, 235 (1983). J . F . KLEBE, J. Polym. Sci. Part B 2, 1079 (1964). Y. OIsm, M. PADMANASAN,M. KXgIMOTO and Y. IMAX,Polym. Prepr. Jpn 36, 317 (1987). J . F . KLESE, J. Polym. Sci. Part A 2, 2673 (1964). Y. OlSl-n, M. PADMXNAaAN,M. KAK1MOTOand Y. IMAI, Polym. Prepr. Jpn 36, 318 (1987). T. MUNETOH, K. MIVAZAWA,T. M^TSUMOTOand T. KUROSAKI, Polym. Prepr. Jpn 36, 325 (1987). K. MlVAZAWA,T. MU~ETOH, T. MXTSUMOTOand T. KUROSAKI, Polym. Prepr. Jpn 36, 324 (1987). H.R. KgICHELDORF and G. ScnwAgz, Polym. Bull. I, 383 (1979). H . R . KRICHELDORF, Q. Z. ZANG and G. SCHWAgZ, Polymer 23, 1821 (1982). H . R . KRICHELDORF and G. SCnWARZ, Makromolek. Chem. 184, 475 (1983). H . R . KRICHELDORF and H. KOZIEL, J. macromolec. Sci. Chem. A23, 1337 (1986). H . R . KRICHELDORF and G. BIER, J. Polym. Sci. Polym. Chem. Edn. 21, 2283 (1983). H . R . KRICHELDORF and G. BIER, Polymer 25, 1151 (1984). R.N. JOHNSON,A. G. FARNHAM,R. A. CLENDINNING,W. F. HALEand C. N. MERRIAM,J. Polym. Sci. Part A-I 5, 2375 (1967). T.E. ATTWOOD, D. A. BARR, T. K,NG, A. B. NEWTON and J. B. ROSE, Polymer 18, 359 (1977). T.E. ATTWOOD, P.C. DAWSON, J . L . FREEMAN, L.R. HOV, J.B. ROSE and P.A. STANILAND, Polymer 22, 1096 (1981). A. HARA, Y. OISHI, M. KAK,MOTO and Y. IMA,, Polym. Prepr. Jpn 36, 316 (1987). Y. IMAI and M. UEDA, Crown Ethers and Phase Transfer Catalysis in Polymer Science, (L. J. MATH,AS and C. E. CARRAHER, JR. Eds), pp. 121-137, Plenum, New York (1984).
0079-6700/89 $0.00 + .50
Copyright © 1989 Pergamon Press plc
NOVEL SYNTHETIC METHODS FOR CONDENSATION POLYMERS USING SILYLATED NUCLEOPHILIC MONOMERS YOSHIO IMAI and YOSHIYUKI OISHI
Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Meguro-ku, Tokyo 152, Japan CONTENTS I. Introduction 2. Polymers starting from N-silylated diamines 2.1. Synthesis of polyamides 2.2. Synthesis of polyimides 2.3. Synthesis o f polyureas 2.4. Synthesis of polyamines 2.5. Synthesis of polyazomethines 3. Polymers starting from O-silylated bisphenols 3.1. Synthesis of polyesters 3.2. Synthesis of polyethers 4. Polymers starting from S-silylated dithiols 5. Conclusion
References
173 174 174 182 184 185 187 187 188 189 190 192 192
I. I N T R O D U C T I O N
Silylation of organic compounds, substitution with trialkylsilyl groups, has found for many years wide application as an aid for chromatographic separation procedures through conversion of organic compounds into more volatile derivatives. The value of silylation as a new tool in synthetic organic chemistry has also become apparent during the last three decades, and the silylation method has created new organic synthesis tools directed toward highly specific and selective synthesis under milder reaction conditions. In the field of synthetic polymer chemistry, after pioneering research by Klebe in the mid 1960s on the synthesis of some condensation polymers with the use of N-silylated diamines, dramatic developments were not seen during the 1970s. Since the demonstration of polymer synthesis using O-silylated bisphenols by Kricheldorf in the late 1970s or at the beginning of the 1980s, this field of research has become active again and has entered a new growth stage. We have also investigated the synthesis of aromatic polyamides (aramids) and related polymers using N-silylated aromatic diamines since 1984, with interesting results. The present review deals with the application of the st~ation method as a new tool for the synthesis of various types of condensation polymers. 173
174
Y. IMAI and Y. OISHI
2. POLYMERS STARTING FROM N-SILYLATED DIAMINES N-Trialkylsilylated amines represent, next to trialkylsilyl halides, the most reactive class of organosilane compounds. They react with a variety of electrophiles such as carboxylic acid halides, acid anhydrides, isocyanates, benzyl halides, and aldehydes) These reactions are applicable for the synthesis of nitrogen-containing condensation polymers.
2.1. Synthesis of polyamides It was reported that the reaction of N-trimethylsilyl-substituted amines with acid chlorides yielded amides with the elimination of trimethylsilyl chloride, and not a combination of N-trimethylsilylamides and hydrogen chloride (eq. 1).2 The amide-forming reaction, therefore, can be extended to polycondensation between bifunctional monomer pairs leading to polyamides.
R-NHSiMe3
+
O
[
CLC-R'
I
-
0 II
R-NHC-R'
+ Me3SiCt
O
,
/'=
R--N-C-R' + I
(1) HCL
SiMe 3
We have successfully synthesized high molecular weight aramids by the low temperature solution polycondensation of N-silylated aromatic diamines with aromatic diacid chlorides (eq. 2). 3"4 The polymerization was carried out at - 1 0 ° C in amide-type solvents like N-methyl-2-pyrrolidone (NMP). Usually aramids are prepared by the polycondensation of aromatic diamines with aromatic diacid chlorides (eq. 3). 5'6 Me3SiNH-Ar-NHSiMe 3 +
I- NH-Ar- NHC-At'- C-]
-Me3S'CL---
HzN-Ar-NHz
-HCt
C t C - A r ' - C C I. II II O O
+
(2)
CtC-Ar'-CCL II II 0 0
[ - N H - A r - NHC-Ar'-C-111 " / O O ~
(3)
Figure 1 shows a comparison of the silylation method and the conventional diamine route for the synthesis of poly-p-phenyleneterephthalamide, Du Pont's "Kevlar" molecule, which was conducted in a mixture of NMP and hexamethylphosphoramide (HMPA). N-Silylated p-phenylenediamine reacted more
NOVEL SYNTHETIC METHODS FOR CONDENSATION POLYMERS
175
readily with terephthaloyl chloride than the parent p-phenylenediamine itself. The polycondensation using the N-silylated diamine proceeded more rapidly and afforded aramid with higher inherent viscosity, compared to the polymerization using the diamine. 3 10
~8
A
o~ 6
~
.
i>
~_ 4
B
~2 Z
0
I
2
I
I
I
4 6 8 REACTION TIME ( h )
I
I
10
I
FIG. 1. Comparison of time dependence of inherent viscosity of poly-p-phenyleneterephthalamide formed by the polycondensation of terephthaloyl chloride with (A) N,N'-bis(trimethylsilyl)-substituted p-phenylenediamine or with (B) p-phenylenediamine at a reactant concentration of 0.2 mol/l in H M P A - N M P (2 : I by volume) in the presence of lithium chloride between - 10 and - 5°C.
Table 1 summarizes the results for the synthesis of various aramids. In all cases, the silylation method was superior to the diamine route with respect to the inherent viscosity of the resulting aramids. 4 The high reactivity of N-silylated amines toward acid chlorides can be explained by the postulated mechanism as given below. Organosilane compounds are characterized by the fact that the silicon has a strong affinity for oxygen, fluoride, or chloride ion, and that the carbocation on the #-position to the silicon can be stabilized through the silicon a - n effect. Taking into consideration these particular features of organosilane compounds, the following nucleophilic addition-elimination two-step mechanism is proposed for the nucleophilic acyl substitution of an acid chloride with an N-trimethylsilylsubstituted amine (eq. 4). ~
iSiMe3
Ar' - C ~ " ~ " - , ~ f ~'- : NH - A t I CL
--'-'"
O - SIMe 3 I Ar'--C- NH-Ar I Ct.
NH-Ar =--
Ar'- C-;- 0 fl LI CLE.~ SiMe3
(4) "
Ar'--CNHII O
Ar
+
Me3SiCL
In the first step, the attraction of the carbonyl oxygen of an acid chloride to the silicon of an N-silylated amine facilitates the nucleophilic attack of the
176
Y.IMAIandY.OISHI TAnLEI. Synthesis of various
aramids* (ref. eqs 2 and
Monomers Ar (from diamine)
3)
Polymer rti.ht
Ar' (from diacid chloride)
(dl/g)
N-Silylation method
Diamine method
-~
~
2.45
1.03
_~
_~
3.19
0.94
-~
2.41
1.19
_~
_~
7.41:~
3.23~:
_.~CH2_~
-~
2.54
1.15
._.~CH2_~
.-~
3.21
1.40
3.9 _@_
,9
,.,0
*Polymerization was carried out in N-methyl-2-pyrrolidone containing lithium chloride at - 10 to - 5°C for 5 hr. tInherent viscosity was measured at a concentration of 0.5 g/dl in concentrated sulfuric acid at 30°C. :[:Polymerization was conducted in a mixture of N-methyl-2-pyrrolidone and hexamethylphosphoramide containing lithium chloride at - 10 to - 5 ° C for 12 hr.
nitrogen of an N-silylated amine at the carbonyl carbon of an acid chloride, thereby quickly giving rise to the tetrahedral intermediate. In the second step, the elimination of chloride ion from the intermediate is enhanced by the presence of the/~-silicon through the ~r-~ effect, affording rapidly the amine product along with trimethylsilyl chloride: Aromatic diamines with low reactivity, mainly due to their low basicity, usually produce only low molecular weight polyamides. However, they can be activated markedly by conversion to N-silylated diamine derivatives. This is illustrated by the following examples. Aromatic dianilino compounds (very weak bases) when substituted with trimethylsilyl groups, readily afforded N-phenylated aramids by high-temperature solution polycondensation with aromatic diacid chlorides in tetramethylene sulfone (eq. 5). 7.8 The inherent
NOVEL SYNTHETICMETHODS FOR CONDENSATIONPOLYMERS
177
TABLE 2. Synthesis of N-phenylated aramids (eq. 5) Monomers
Polymerization*
Polymer
Ar
Ar'
Temp.
Time
(from diamine)
(from diacid chloride)
(°C)
(hr)
r/i.ht (dl/g)
T~:~ (°C)
-~
160
24
1.65
225
~
200
6
2.21
255
--~'- 0~--
- ~
160
24
1.24
195
--~0--~-
--~
200
24
1.35
215
"-~
*Polymerization was carried out in tetramethylene sulfone. 1"Inherent viscosity was measured at a concentration of 0.5 g/dl in concentrated sulfuric acid at
30° C. :[:Determined by differential thermal analysis (DTA) at a heating rate of 10°C/min in air.
viscosities of the resulting N-phenylated aramids (Table 2) were much higher than those from the unsubstituted diamines. 9 The N-phenylated aramids have glass transition temperatures (T~) in the range of 195-255°C, and are soluble in a variety of solvents including dimethylformamide (DMF), m-cresol, and chloroform. 0 O II H MeaSiN-Ar-NSiM% + CLC-Ar'-CCI. I
I
Ph
Ph
-M%S,CL ,_
/r - N - A r -
0 N - II
L i Ph
i Ph
0 II
"]
C-Ar'-C- 1
(5)
-n
Again from the combinations of the N-silylated dianilino compounds and aromatic dicarbamoyl chlorides, N-phenylated aromatic polyureas could be obtained by high-temperature solution polycondensation, t° Tetrafluoro-m-phenylenediamine is another aromatic diamine having low basicity. This diamine could also be activated through N-silylation. The silylated derivative gave fluorine-containing aramids with reasonable inherent viscosities (0.28-0.47 dl/g) by low-temperature solution polycondensation with aromatic diacid chlorides (eq. 6). tl
Y. IMAI and Y. OISHI
178
_~ Me3SiNH
F
F" y
F -Me3SiCL
o*c
0 0 II II NHSiMe3 + CLC-Ar'-CCL -F r
F
~I~
0
0
II -I"
II
(6)
I-"" TO"F ""c -Ar'- c-I~" ~ F"~-/~" F F
Aromatic diacid chlorides of considerably lower reactivity than the usual diacid chlorides are also capable of yielding aramids with moderately high molecular weights using N-silylated diamines. An example is given for the fluorine-containing aramids from the low-temperature solution polycondensation of the N-silylated aromatic diamines with tetrafluoroisophthaloyl chloride (eq. 7)) 2 The fluorine-containing aramids have Tgs in the range of 245-280°C and dissolve readily in various organic solvents (Table 3). 0 II
F J.
0 II
Me3 SiNH -Ar-NHSiMe3 + CLC-~(-"~'I--CCL F 0 F 0 I, .L , 1 -Me3SiCt"/-NH-Ar-NHC T O Y C- I 6o*c
,.
F ~..~.F
(7)
~,
F
TABLE 3. Synthesis of fluorine-containing aramids* (eq. 7) Ar (from diamine)
Polymer
~,..1"
T,t
(dl/g)
(°C)
-•
0.43
245
--~
0.56~
275
CHz-~
0.63
280
--•0-•
0.82
280
-~--
*Polymerization was carried out in chloroform containing triethylamine hydrochloride at 60°C for 6 hr. $lnherent viscosity was measured at a concentration of 0.5g/dl in dimethylacetamide at 30°C. ~Determined by DTA at a heating rate o f 10°C/min in air. §Polymerization was carried out at 0°C for 6 hr.
NOVEL SYNTHETIC METHODS FOR CONDENSATION POLYMERS
179
The reaction of N,O-bistrimethylsilylated o-aminophenol with benzoyl chloride was also interesting. The acid chloride attacked selectively at the N-silylated amine site of the o-aminophenol and the corresponding amide product was formed exclusively. This occurred because the amino group of the o-aminophenol is activated toward acid chloride attack by silylation and the hydroxyl group, in contrast, is deactivated as discussed later. The reaction was successfully extended to the polycondensation of the N,N',O,O'-tetrakis-(trimethylsilyl)substituted bis-o-aminophenols with aromatic diacid chlorides. The polymerization was carried out at 0°C in an amide solvent to yield poly-o-hydroxyamides. These were then subjected to thermal cyclodehydration to afford polybenzoxazoles (eq. 8).13
Me3SiNH\Ar/NHSiMe3 MeaSiO/ \OSiMe 3
-Me 3 SiCt i~ O°C
MeOH.
[
O O II II + C k C - A r ' - CCL O O II II -NH\Ar/NHC-Ar'-C- ]
Me3SiO /
o ii
2~)0"C
J. o ii
[-NH\Ar/NHC-Ar'-C-] L HO /
-H20
XOSiMe3
J°
".OH
[ r.//N\^. /N%~ _ L "0/ "0
"] in
(8)
One of the remarkable features of the silylation method was that the polycondensation proceeded in homogeneous solution even in low-boiling solvents such as dichloromethane and tetrahydrofuran, as well as in high-boiling amide solvents. This occurred because the intermediate silylated poly-o-hydroxyamides are soluble in a wide range of solvents. It had earlier been especially difficult to prepare fluorine-containing polybenzoxazoles of high molecular weight from 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane, a weak nucleophile. However, the silylation method enabled formation of these polybenzoxazoles for the first time. The aromatic polybenzoxazoles thus obtained are one of the potential high temperature polymers, having Tgs in the range of 260-325°C (Table 4). 13 Although the following example does not deal with the synthesis of aramids, Katsarava et al. prepared aliphatic-aromatic polyamides of high molecular weights by the polycondensation of N-silylated lysine with aromatic diacid chlorides (eq. 9)J 4 They also showed that activated diesters such as 2,4-dinitrophenyl esters could be used in place of the diacid chlorides. The polyamides were then formed by elimination of O-silylated 2,4-dinitrophenol.
180
Y. IMA1and Y. OISHI Me3 SiNHCH(CHz)4 NHSiMe3 + I
COOE+, -,~e~SiCt 25°C
.
CLC--(CD:)-CCI. II ~ II 0 0
r-NHCH (CHz,4 N H C ~ C - - ] / I II ' ~ " COOEr 0
(9) II 0
/ Jn
~sp/C : 1.02 (HMPA)
TABLE4. Synthesisof aromatic poly-o-hydroxyamidesand polybenzoxazoles(eq. 8) Monomers Ar Ar' (from bis-o- (fromdiacid aminophenol) chloride)
Polymerization PolyamidePolybenzoxazole solvent* r/inht Tg:~ (dl/g) (°C)
-~-
Dimethylacetamide 0.50
290
-~
Dimethylacetamide 0.55
240
-~
Dimethylacetamide 0.64
260
- ~
Dimethylacetamide 0.65
310
']~/-~
Dimethylacetamide 0.86 Tetrahydrofuran 0.83 Dichloromethane 0.63
280
Dimethylacetamide 0.60
325
Dimethylacetamide 0.47
300
Dimethylacetamide 0.40
295
CH3 I ~ - ~ H 3 -~ CF~
~ ] - I F 3 -~
-
(50 : 50)
"
~ --~-~-- ~
OF3 I iC'~ ¢F3
*Polymerizationwas carried out at 0°C for 8 hr. tlnherent viscositywas measured at a concentration of 0~5g/dl in dimethylacetamideat 30°C. ~:Thermalcyclodehydrationwas conducted at 250°C for 30 hr/n v a c u o . T s was determined by thermomechanical analysis at a heating rate of 10°C/min in air.
NOVEL SYNTHETIC METHODS FOR CONDENSATION POLYMERS
181
Thus, through the aforementioned examples we are able to demonstrate the development of a versatile and promising method for the synthesis of a variety of high molecular weight aramids by the use of N-trimethylsilylated aromatic diamines. It is clear that the N-silylated diamines are far more reactive than the parent diamines toward aromatic diacid chlorides. Consequently, high molecular weight aramids could be obtained readily by the low-temperature solution polycondensation of these monomer pairs under milder reaction conditions. The N-silylated diamine method has several advantages over the conventional method: (1) High-purity N-silylated aromatic diamines can be obtained simply by distillation. (2) N-silylated aromatic diamines exhibited excellent solubility in organic solvents. This facilitates the polycondensation process in a variety of reaction media, ranging from low- to high-boiling and/or polar solvents. (3) The polycondensation proceeds under neutral reaction conditions with the elimination of trimethylsilyl chloride. (4) The trimethylsilyl chloride recovered from the polymerization system may be recycled as a silylating agent for the diamines. Aramid synthesis via the silylation method (eq. 2) is applicable to vacuum deposition polycondensation for the preparation of aramid thin films on appropriate substrates.lS Aramid thin films can be deposited, of course, by the conventional diamine route (eq. 3). However, a temperature of above 100°C was required to eliminate the hydrogen chloride formed during the polycondensation process. 16"17By the use of N-silylated diamines, the deposition polycondensation was achieved at a temperature near 60°C with the elimination of neutral trimethylsilyl chloride. Thereby polymer films of lower thermal stability, and erodible metal plates, can be used as the substrates and conveniently coated with aramid thin films. It is remarkable that the "Kevlar" molecule is normally highly crystalline, whereas the aramid film deposited by the silylation method is amorphous and transparent. In the silylation method described above, preformed N-trimethylsilylated diamines were used as the polymer-forming monomers (made by prior synthesis from aromatic diamines and trimethylsilyl chloride). In contrast, one-pot preparation of aramids has also been reported using an appropriate silylating agent. The solution polycondensation of 3,8-diaminophenanthridinone with terephthaloyl chloride in an N M P - H M P A mixture at 0°C in the presence of trimethylsilyl chloride effectively afforded an aramid having an inherent viscosity greater than 6.0 dl/g. ~s Another example is given for the synthesis of poly-p-benzamide by the self-polycondensation of p-aminobenzoic acid in pyridine in the presence of tetrachlorosilane which, in fact, acts as a condensing agent (eq. 10).'9 HzN-~COH
+ I/2S,CL4 0
120"C---
-Nit
C-
-In
r/i.h : 1.97 ( HzSO 4 )
I/2SiO 2
~- 2HCt
(10)
182
Y. IMAI and Y. OISHI
2.2. Synthesis of polyimides Aromatic polyimides are generally prepared in two steps; the ring-opening polyaddition of aromatic diamines to aromatic tetracarboxylic dianhydrides yielding soluble polyamic acid precursors, which in turn undergo subsequent thermal cyclodehydration to insoluble polyimides (eq. 11).6'20'21 HzN-Ar-NH2
_ / C O ,/CON~ + U-~O/~r \ C O / v I--Ar-NI-KX3... fCOOH -I/ / Ar'.~ L HOCC( CONH-Jn
DMAc r.,.
r
-HzO-3oo.c
..CO
CO. n
I-Ar-N.. "At ' / "N-/ L co" " c o ~" Jo
(11)
N-Silylated amines react with aromatic anhydrides in the same fashion. When an N-silylated amine is reacted with an anhydride, the intermediate ring-opened adduct, silylated amic acid, can eliminate silanol at higher temperature with the formation of an imide. ~ The application of the reaction to the synthesis of polyimides was reported by Klebe in the patent literature in 1967.22 The ringopening polyaddition of N-silylated aromatic diamines to aromatic dianhydrides afforded polyamic acid trimethylsilyl esters, which in turn were converted thermally to polyimides with the elimination of trimethylsilanol (eq. 12). ^/C0,,, ^ ,/CO x M%SiNH-&r-NHSiM% + U\co/~r \co/O
~o.c
=
- Me3 SiOH 2oo-c
r -At - NHCO\
L
/COOSiMe 3 ] Ar' Me3SiOCO/ \CONH- Jn
r /co\ /co\ -] m. I-Ar-Nx /.~.r'\ / N - I L CO CO J
(12)
We have reinvestigated the polyimide-forming reaction starting from N,N'bis(trimethylsilyl)-substituted bis(4-aminophenyl) ether and pyromellitic dianhydride in detail. 23The presence of trimethylsilyl groups provides solubility for the silylated polyamic acids in nonpolar solvents. Thus a variety of solvents including ethers and hydrocarbons can be used in place of the customary polar solvents such as dimethylacetamide (DMAc) and NMP, giving polymer with high inherent viscosity (Table 5). The thermal conversion of the silylated precursor polymer to polyimide proceeded more slowly and required higher temperatures for the completion of the imidization, compared with that of the parent polyamic acid.
NOVELSYNTHETICMETHODSFOR CONDENSATIONPOLYMERS
183
TABLE5. Synthesis of polyamic acid in various solvents* Solvent
Polymer
Remarks/;
(dl/g) Dimethylacetamide N-Methyl-2-pyrrolidone Bis(methoxyethyl) ether 1,4-Dioxane Tetrahydrofuran Chloroform Acetonitrile Nitrobenzene Toluene
1.77 1.46 0.92 0.80 0.97 0.99 0.88 0.81 0.92
S S S S S S P P P
*Polymerization was carried out with N,N'-bis(trimethylsilyl)-substitutedbis(4-aminophenyl) ether and pyromelliticdianhydride at 20°C for I hr and then at 50°C for 12hr. tInherent viscosity was measured at a concentration of 0.5 g/dl in dimethylacetamide at 30°C. ;~Appearanceof the polymerization:S, homogeneoussolution throughout the reaction; P, precipiration during the reaction. The polyimide synthesis by the N-silylated diamine method (eq. 12), as well as that by the conventional diamine route (eq. I l), can be used conveniently for the deposition of polyimide thin films on appropriate substrates such as metal and ceramic plates in a manner very similar to the vacuum deposition polycondensation of aramid thin films discussed previously. 24-26In the case of polyimide thin film deposition, an aromatic diamine component and an aromatic dianhydride were deposited in the first step on a substrate under high vacuum, and then the substrate was heated to induce imidization to form a polyimide thin film. Polycondensation with N-silylated diamines is also applicable to aromatic carboxylic acid derivatives having two different electrophilic functions on the aromatic ring. The polymerization of 4-chloroformylphthalic anhydride with N,N'-bis(trimethylsilyl)-substituted bis(4-aminophenyl) ether progressed through polycondensation and polyaddition leading to the polyamic acid trimethylsilyl ester (eq. 13). 27 Although polymerizations of this mixture in the solvents listed in Table 6, except for D M A c and N M P , proceeded with precipitation of the product, the resulting polymers had relatively high inherent viscosities. The silylated precursor polymer could be converted to the polyamide-imide by the same thermal treatment used for polyimide formation.
Me 3SiNH - A r - NHSiMe 3 + CLCO - A r ' / C O \ o \CO / -Me3 SiCL,, 50% - M% SiOH 2OO*C
F /COOSiMe3] | - A r - NHCO- Ar'.., / L CONH- J. r ~.CO\ -I I-Ar-NHCO-Ar'~, /N- / L CO Jo
(13)
184
Y. IMAI and Y. OISHI TABLE 6. Synthesis of polyamide-amic acid in various solvents* Solvent
Polymer
Remarks~/
~/i.ht
(dl/g) Dimethylacetamide N-Methyl-2-pyrrolidone Acetonitrile Nitrobenzene Bis(methoxyethyl) ether 1,4-Dioxane Tetrahydrofuran Chloroform Benzene
!.82 1.48 I. 14 1.21 1.00 0.75 0.94 0.59 0.57
S S P P P P P P P
*Polymerization was carried out with N,N'-bis(trimethylsilyl)-substituted bis(4-aminophenyl) ether and 4-chloroformylphthalic anhydride at - 5°C for I hr and then at 50°C for 4 hr. flnherent viscosity was measured at a concentration of 0.5 g/dl in N-methyl2-pyrrolidone at 30°C. :~Appearance of the polymerization: S, homogeneous solution throughout the reaction; P, precipitation during the reaction.
Korshak and his group have reported that polyimides containing aliphatic groups, which were difficult to prepare in high molecular weight by the diamine route, could be obtained satisfactorily starting from N-silylated aliphatic diamines and aromatic dianhydrides.2s 2.3. Synthesis of polyureas The addition of N-silylated amines to isocyanates gives high yields of N-silylsubstituted urea derivatives, which are readily hydrolysed to the parent ureas) This reaction was utilized for the synthesis of aromatic polyureas by Klebe in 1964, who demonstrated the polyaddition of N-silylated aromatic diamines to aromatic diisocyanates to yield N-silylated polyureas (eq. 14).29The polymerization could be achieved in homogeneous solution even in solvents of low polarity due to the less polar nature of the N-silylated polyureas as compared with normal polyureas (Table 7).30 It is noteworthy that the polyureas derived from the methanolysis of the N-silylated polymers have low crystallinity and good solubility in organic solvents, in contrast to the polyureas prepared by the diamine-diisocyanate routeP ° Me3 S i N H - A r - NHSiMe 3 + O = C = N - A r ' - N =C =O O O H I 1 | -I~-Ar-~I-CNH-Ar'-NHC- I LMe3Si SiMe3 -n
I"
~.
r
~
~]
(14)
NOVEL SYNTHETIC METHODS FOR CONDENSATION POLYMERS
185
TABLE 7. Synthesis of an aromatic polyurea in various solvents*
Solvent
Polymer
~i.ht (dl/g)
Dimethyi sulfoxide Tetramethylene sulfone Tetrachloroethane Anisole Nitrobenzene Toluene
0.54 0.60 0.91 0.77 0.65 0.74
*Polymerization was carded out with N,N'-bis (trimethylsilyl)-substituted bis(4-aminophenyl) ether and 2,4-tolylene diisocyanate at 100°C for 10 hr. tlnherent viscosity was measured at a concentration of 0.5 g/dl in concentrated sulfuric acid at 300C.
2.4.
Synthesis of polyamines
Although alkyl halides are less reactive than carboxylic acid chlorides toward aminolysis, the alkyl halides are also capable of attacking N-silylated amines.' Klebe reported that benzyl chloride and an N-silylated amine underwent this substitution reaction at above 100°C and yielded benzylamine and trialklysilyl chloride quantitatively. 3t In this reaction, ammonium salts and a cyclic sulfone enhanced catalytically the rate of reaction. The reaction has also been extended to the synthesis of polyamines, which were difficult to obtain in high molecular weight by the dihalide--diamine route probably due to the occurrence of side reactions. The melt polycondensation of reactive dihalides of the benzyl chloride type with N-silylated piperazines at 150°C led to the formation of high molecular weight polyamines (eq. 15). 3~ Me
/-4 P . M e2SiNL_jNS ,M ezPh
+ Me
=
-Ell 2
(15)
C~ n
[7/] : 1.97 (ChLoroform)
We found that a similar substitution reaction occurred when activated aromatic halides were treated with N-silylated aromatic amines, giving aromatic secondary amines along with trimethylsilyl halides. 32 Accordingly, novel aromatic polyamines were successfully prepared from activated aromatic difluorides and N-silylated aromatic diamines (eq. 16). The solution polycondensation was carried out at 100-150°C in dimethyl sulfoxide (DMSO) in the presence of a
186
Y. IMAIand Y. OISHI
fluoride catalyst such as potassium or cesium fluoride. It produced aromatic polyamines with reasonable inherent viscosities (Table 8). The presence of the catalyst was essential for the preparation of high molecular weight polyamines. The use of the corresponding dichlorides in place of the activated aromatic difluorides yielded only low molecular weight polymers. Introduction of nitro groups onto the aromatic dihalides improved the reactivity, affording polyamines with higher inherent viscosities. These aromatic polyamines dissolve readily in organic solvents like N M P and have Tgs around 200 ° C, and therefore they may also be accepted as one of the potential engineering plastics. 32 Me3SiNH-Ar-NHSiMe 3 ÷ - Me3SiF
:[
--NH-Ar-
F-Ar'-F NH-Ar
:]
(16)
~
TABLE8. Synthesisof aromatic polyamines(eq. 16) Monomers
Polymerization*
Polymer
Ar' (from difluoride)
Temp./Time (°C/hr)
(dl/g)
-'~SOe"~
100/7-150/14
0.62
150/12
0.57
150/12
0.42
100/7-150/14
0.60
t/i.ht
Ar (from diamine) -'~
"-~
-'•CHt'-•
"
~ ' ~ --~CO~ --~
-•-CHf-• --~--O-~
100/24-200/48,
0.55
220/48t
0.37
100/24-200/48~
0.45
150/48
0.66
*Polymerization was carded out in dimethyl suifoxide in the presence of potassium fluoride. tInhercnt viscosity was measured at a concentration of 0.5 g/dl in N-methyl-2-pyrrolidoneat 30°C. :l:Polymerizationwas conducted in tetramethylenesulfone.
NOVEL SYNTHETIC METHODS FOR CONDENSATION POLYMERS
187
2.5. Synthesis of polyazomethines Most aromatic polyazomethines could not be obtained in high molecular weights due to their insoluble and infusible characteristics. Kurosaki and coworkers investigated extensively the preparation of aromatic polyazomethines and found new routes for these polymers by the use of N-silylated aromatic diamines. 33'34One method is the polycondensation of a combination of N,N'bis(trimethytsilyl)-substituted aromatic diamines and aromatic bis(diethyl acetal) compounds. The solution polymerization in NMP gave organic-soluble precursor polymers, which in the form of films were subsequently converted at 200°C to insoluble polyazomethines (eq. 17).33 Another method is the polyaddition of N,N,N',N'-tetrakis(trimethylsilyl)-substituted aromatic diamines to aromatic dialdehydes in N M P leading to soluble precursor polymers, followed by thermal conversion to insoluble polyazomethine films (eq. 18).34 In both cases, similar precursor polymers were formed in the first step, and the subsequent fl-elimination occurred with the formation of azomethine linkages along the polymer main chains. Me3SiNH-~NHSiMes + (EiO)2CH-<~-CH(OE+~)2 OEt -N I -'~
30°C"
N-CH I
MeaSiO 30°C
""
-N I Me3SiV
N-CH I SiMe 3
OEt CH--] /
( G H or SiMe 31
OSiMes CH-
,oo.c" 3. P O L Y M E R S
STARTING
FROM
O-SILYLATED
BISPHENOLS
Silylation of a hydroxyl group is normally a protection against electrophilic attack due to the strong affinity of silicon for oxygen. O-trialkylsilyated phenols (trialkylsiloxybenzenes), however, can be activated by means of chloride or fluoride ion via the equilibrium (eq. 19), and can thereby react with electrophiles such as carboxylic acid halides and activated aromatic halides. By the appli'cation of this information, Kricheldorf and his group have developed a new
188
Y. IMAI and Y. OISHI
synthetic method for aromatic polyesters and aromatic polyethers starting from O,O'-bistrimethylsilylated bisphenols. Ar-O-SiMe3+
X (~ - ~
Ar - 0 (~ + Me3SiX
(19)
3.1. Synthesis of polyesters Aromatic polyesters (polyarylates) are usually prepared either by the twophase polycondensation of bisphenols with diacid chlorides in organic solventaqueous alkaline solution systems using phase transfer catalysts (eq. 20), or by the melt polycondensation of bisphenol diacetates with dicarboxylic acids (eq. 21) or the similar melt polycondensation of A-B type monomers (eq. 22). 5.6 NaO-Ar-ONo
+ CLC-Ar'-CCt )) il 0 0
-.oc,
[-O-Ar-O~-Ar'-C-]
r.t.
L
o
CH3CO-Ar-O8 ~cH3
+
(20)
~ J°
HOCo-Ar)-COH8
- C%COOH,, [-O-Ar-OC-Ar'-C-] 2~o.c
CH3CO-,&r- COH
~)
~)
L
~
-¢ H3COOHi
3oo.c
~ J.
[ - O - A r - C -1
L
~) j.
(21) (22)
Kricheldorf et al. first demonstrated in 1979 that polyarylates could b¢ synthesized either from O-silylated bisphenols and diacid chlorides (eq. 23), or from trimethylsiloxybcnzoyl chlorides (eq. 24) by melt polycondensation at 200-300°C. 35 Me3SiO-Ar-OSiMe 3 + CLC-Ar'-CCL II I1 O O
-a,~sicL 25o'c
Me3SiO -Ar-CCL II 0
[-O-Ar-OC-Ar'-C-]
(23)
-Me3SiCL 300°C
(24)
/ L
II O
II / O ~"
r- O-Ar-C -1 ,. g J.
The thermal polycondensation of two different A-A and B-B type monomers, such as O,O'-bis(trimethylsilyl)-substituted 2,2-bis(4-hydroxyphenyl)propane and isophthaloyl chloride, suffers from distillation or sublimation of the more volatile monomer. This disrupts the stoichiometry of the monomer
NOVEL SYNTHETIC M E T H O D S FOR C O N D E N S A T I O N POLYMERS
189
pair and thereby reduces the degree of polycondensation. This disadvantage is obviously avoided if self-condensing A-B type monomers like trimethylsiloxybenzoyl chlorides are used. The molecular weight of the polymer obtained from the A-B type m-hydroxybenzoic acid derivative is indeed higher than that obtained from the O-silylated bisphenol and isophthaloyl chloride. Thus, the silylation method is most useful for the high-temperature synthesis of polyarylates from A-B type monomers. The self-polycondensations of rn- and p-siloxybenzoyl chlorides were investigated extensively in bulk or in solution. 36'37 The bulk polymerization of m-trimethylsiloxybenzoyl chloride at 250°C yielded the polyarylate having a molecular weight of 10,000-14,000, 36 whereas the polymerization of the p-oriented monomers in a high-boiling solvent at 320°C afforded the polymer with molecular weight in the range 20,000-50,000. 37 In the latter case, the polycondensation led at first to precipitation of the oligomer crystals, which continued chain growth to the polymer crystals with prolonged heating. Some catalysts such as triethylamine hydrochloride and benzyltriethylammonium chloride effectively accelerated the polycondensations, giving polyarylates of higher molecular weights. O-Silylated bisphenols also reacted with aryl phosphorodichloridates to yield aromatic polyphosphates, which are polyesters in a broad sense (eq. 2 5 ) . 39 The bulk polycondensation was conducted at temperatures between 180 and 300 ° C in the presence of a catalytic amount of benzyltriethylammonium chloride, affording polyphosphates with molecular weights up to 12,000. Me3SiO Ar OSiMe3 +
0 II
CL--P--C/ I OPh
(25)
0
Me3SCL. 280*C
F O Ar 0 ~ ] L ~)phan
3.2. Synthes# of polyethers Following the polyester synthesis, Kricheldorf et al. have investigated the bulk polycondensation of O,O'-bistrimethylsilyl-substituted bisphenols with activated aromatic difluorides (eq. 26). 39,4oThese polymerizations were successful only when potassium or cesium fluoride was used as catalyst at temperatures in the range of 270-350 ° C. They gave aromatic polyethers with molecular weights up to 17,500. Unfortunately, the reactivity enhancement was not sufficient for the polycondensation with the corresponding aromatic dichlorides; the aromatic difluorides, containing the fluorine atom (with a stronger affinity than chlorine for silicon), were indispensable reaction partners. The conventional synthetic procedure for aromatic polyethers is based on the high-temperature solution polycondensation of alkali metal salts of bisphenols with activated aromatic dihalides (eq. 27). 4~-43
190
Y. IMAI and Y. OISHI Me3SiO-Ar-OSiMe 3 ÷
F- Ar'- F
-Me3SiF f- O - A r - O 320. c ~ MO-Ar-OM
Ar'-
]
(26)
n
[-O-Ar-O-Ar'-]o
+ X-Ar'-X-MX=
(27)
M : No, K X:
F, CL
A general shortcoming of the conventional metal bisphenolate route is that the resulting polymers need to be purified from the metal salts byproducts and also from the expensive high-boiling point solvents. In contrast, the silylation method has the advantage that the molten polymers do not need purification from a large amount of metal salts and solvents prior to processing, although a catalytic amount of potassium or cesium fluoride exists as such in the molten polymers. A new class of aromatic polyether-esters was also synthesized from combinations of trimethylsiloxybenzoic acid trimethylsilyl esters and activated aromatic difluorides (eq. 28). 39 The melt polycondensation was effected at a temperature of 250-320°C in the presence of cesium fluoride as the catalyst, yielding polyether-esters with molecular weights in the range of 9000-11,000. Me3SiO-Ar-COSiMe3+ II O
320=C ="
4. P O L Y M E R S
STARTING
F-Ar'-F
O-Ar- O-Ar'O
FROM
°
S-SILYLATED
DITHIOLS
The silicon-sulfur bond has received relatively little attention from a synthetic point of view, although it appears that the reactivity of S-trialkylsilyated thiols may approach that of the N-silylated amines, and advantages from silylation may be found in the reaction of thiols with reactive halides and the like. We have compared the reactivity of S-trimethylsilylbenzenethiol (I), Ntrimethylsilylaniline (If), and O-trimethylsilylphenol (III) towards electrophiles. We found that the relative reactivity toward benzoyl chloride decreased in the order of II > I > III, and that toward 1-chloro-2,4-dinitrobenzene decreased in the order of I > II > I I I . ~ This finding was quite encouraging and we concluded that S,S'-bistrimethylsilyl-substituted dithiols were another class of promising polymer-forming reactive monomers, having similar attractive features to the N-silylated diamines discussed previously.
NOVELSYNTHETICMETHODSFOR CONDENSATIONPOLYMERS
191
This concept was applied for the first time in the polycondensation of Ssilylated aromatic dithiols with activated aromatic dihalides (eq. 29). ~ As shown in Table 9, aromatic polysulfides having moderate inherent viscosities of 0.6-0.7 dl/g were obtained when the S-silylated dithiols and bis(4-chloro-3-nitrophenyl) sulfone were heated at 50°C in H M P A . Bis(4-fluorophenyl) sulfone was found to be less reactive. Its solution polycondensation with the S-silylated dithiols was effected in tetramethylene sulfone at 220°C in the presence of cesium fluoride as the catalyst, giving polysulfides with inherent viscosities around 0.3 dl/g. 44 Aromatic polysulfides of this type are generally prepared either by the solution polycondensation of parent aromatic dithiols with activated aromatic dihalides or by two-phase polycondensation using phase transfer catalysts. 45 MeaSiS-Ar-SSiMe
3 + X-Ar'-X
- Me 3 SiX
X : F, CI.
TABLE9. Synthesis of aromatic polysulfides (eq. 29) Monomers Ar (from silylated dithiol)
Polymerizationt
Polymer
Dihalide*
Solvent
Temp. (°C)
Time (hr)
(dl/g)
A
HMPA
50
24
0.64
A
HMPA
50
24
0.67
-•0-•
A
HMPA
50
24
0.76
- ~
B
TMS
220
24
0.29~
B
TMS
220
24
0.30~
B
TMS
220
24
0.30~
-•
"-~O-~
*Dihalide A = bis(4-chloro-3-nitrophenyl)sulfone. Dihalide B = bis(4-fluorophenyl)sulfone. tPolymerization solvent: HMPA, hexamethylphosphoramide;TMS, tetramethylene sulfone. :[:Inherentviscositywas measured at a concentration of 0.05 g/dl in hexamethylphosphoramideat 30°C. §Measured at a concentration of 0.5 g/dl in N-methyl-2-pyrrolidoneat 30°C.
192
Y. IMAI and Y. OISHI
5. C O N C L U S I O N W e have been able to d e m o n s t r a t e t h a t the silylation m e t h o d , especially t h a t using N-silylated d i a m i n e m o n o m e r s , is a versatile a n d p r o m i s i n g m e t h o d for the synthesis o f a variety o f c o n d e n s a t i o n p o l y m e r s with high m o l e c u l a r weights. A n u m b e r o f synthetic studies o n c o n d e n s a t i o n p o l y m e r i z a t i o n using silylated nucleophilic m o n o m e r s are n o w u n d e r way. H o w e v e r , this field still r e m a i n s largely u n e x p l o r e d . T h e m o s t significant characteristic o f the silylation m e t h o d is t h a t the silylated nucleophilic m o n o m e r s have specific reactivity differences f r o m the p a r e n t nucleophiles. Therefore, this m e t h o d c a n find further new a p p l i c a t i o n s in the p r e p a r a t i o n o f novel c o n d e n s a t i o n p o l y m e r s a n d in the d e v e l o p m e n t o f new synthetic r o u t e s for v a r i o u s k n o w n p o l y m e r s . ACKNOWLEDGEMENTS T h e a u t h o r s a c k n o w l e d g e the s u p p o r t a n d skillful assistance o f Drs. M a s a - a k i K a k i m o t o a n d M u n i r a t h i n a P a d m a n a b a n a n d Messrs. Y u t a k a M a r u y a m a , A t s u s h i H a r a , a n d Shigeyuki H a r a d a . REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
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