8
Poly(thiol
esters)
Polymerization Reactions and New Polymers Downloaded from pubs.acs.org by WESTERN SYDNEY UNIV on 01/12/19. For personal use only.
H E I N R I C H G . B Ü H R E R and H A N S - G E O R G
ELIAS
Midland Macromolecular Institute, 1910 West St. Andrews Dr., Midland, Mich. 48640, and Swiss Federal Institute of Technology, Universitätstrasse 6, CH-8006, Zürich, Switzerland.
The
reported synthesis and properties of poly(thiol esters)
of the A-B and A-A/B-B
types are reviewed
for these
classes of compounds: 2 poly(α-mercapto acids), 7 poly(β -mercapto acids), 4 poly (ε-mercapto acids) and 44 type poly(thiol
esters).
A-A/B-B
The melting points of poly(thiol
esters) are generally lower than those of the corresponding polyamides and higher than those of the polyesters. are readily hydrolyzed
They
by alkali, which seems to be the
main reason for the lack of commercial interest in these materials.
" O o l y ( t h i o l esters) h a v e b e e n k n o w n f o r m o r e t h a n 25 years as stable, fiber-forming
p o l y m e r s ; f o r a b r i e f r e v i e w see G o e t h a l s ( I ) .
Still,
they h a v e a t t r a c t e d little a t t e n t i o n a m o n g scientists a n d h a v e not b e e n p r o d u c e d o n a c o m m e r c i a l scale.
I n this a r t i c l e , w e s u m m a r i z e these often
i n c o m p l e t e results t o get a better u n d e r s t a n d i n g of this class of p o l y m e r s . I n terms of t h e i r properties, p o l y ( t h i o l esters) l i e b e t w e e n ters a n d p o l y a m i d e s .
polyes
I n polyesters, o x y g e n atoms c a n b e r e p l a c e d f o r
m a l l y b y s u l f u r i n three different w a y s to g i v e p o l y ( t h i o l esters) ( I ) , p o l y ( t h i o n esters) ( I I ) , a n d p o l y ( d i t h i o esters)
o - £
R - C - S
-J
s
s
- £ R - C - 0
I
II
J
(III).
- £ R - C - S
^j-
III
A l t h o u g h p o l y m e r s of t y p e I I o r I I I h a v e n o t y e t b e e n m a d e , i t s h o u l d b e possible to synthesize t h e m f r o m the k n o w n p o l y ( i m i d o esters) (2)
or p o l y ( i m i d o t h i o l esters)
( 3 ) b y treatment w i t h h y d r o g e n sulfide. 105
106
POLYMERIZATION
NH
N H
II
r
II
S
i
4— R — 0 — C — R ' — C — 0 - f
NH
H S 2
S
II - i > 4 - R—O—C—R'—C—0 -4r
M
N H
II
r
REACTIONS A N D N E W POLYMERS
II
P o l y ( t h i o l esters)
—| H S r II II -4- ^ 4 - R - S - C - R ' - C - S 2
-I— R - S - C - R ' - C - S
-|
-4-
h a v e b e e n p r e p a r e d b y various methods,
often
analogous to p r e p a r a t i o n of polyesters, as is s h o w n here. A s w i t h p o l y a m i d e s , it is u s e f u l to d i s t i n g u i s h b e t w e e n p o l y ( t h i o l esters) of the A - B t y p e a n d the A - A / B - B type. poly(e-thiocaprolactone)
T y p i c a l examples are
( I V ) and poly(hexamethylene dithiol tereph
thalate) ( V ) .
S — ( C H ) — C O -J 2
5
S—(CH ) —SOC 2
CO
6
IV
V
Preparation of Poly (thiol esters) of the A-B
Type
T h e most g e n e r a l p r e p a r a t i o n of these c o m p o u n d s is the p o l y m e r i z a t i o n of the c o r r e s p o n d i n g thiolactones ( V I ) .
S—(CH ) —CO 2
>
m
S—(CH ) —CO 2
m
VI T h i o l a c t o n e s a n d s u b s t i t u t e d thiolactones w i t h four to seven r i n g atoms h a v e b e e n p r e p a r e d .
E x c e p t f o r y-thiolactones, they h a v e b e e n
polymerized.
O t h e r g e n e r a l procedures
( t h i o l esters)
are the i n t e r n a l a d d i t i o n r e a c t i o n of ^ - u n s a t u r a t e d t h i o
for m a k i n g A - B t y p e
poly
acids ( V I I ) , H C = C H ( C H ) — C O S H -> 2
2
m
£ - S—(CH ) + —CO -J 2
m
2
VII a n d t h e 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 of o>-mercapto a c i d c h l o r i d e s b e a r -
8.
BUHBER A N D
ELIAS
Poly(thiol
107
esters)
i n g p r o t e c t i v e groups ( s u c h as b e n z y l , a n d t r i m e t h y l s i l y l ) o n the s u l f u r atom.
R S — (CH ) —COC1 2
-RCl
-£-S-(CH ) -CO-J 2
m
m
P o l y ( « - m e r c a p t o a c i d s ) . T h e simplest c o m p o u n d of this t y p e is poly(thioglycolide) (VIII)—poly(a-mercaptoacetic a c i d ) — w h i c h has b e e n p r e p a r e d i n different w a y s . -H 0 2
HS—CH —COOH 2
U), IX
(5),
(7)
(6),
-CH3OH
HS—CH —COOCH3 2
(4)
X
H C 2
I
base
I
0A
/
C H
-£s—ch coJ
°
2
S
VIII
XI C H
2
0 — C
\
base,
0
— C0 (4)
XII
2
(8)
-(CH ) SiCl 3
3
(CH ) Si—S—CH COCl 3
3
XIII
2
(9)
T h e condensation p o l y m e r i z a t i o n of I X a n d X yields oligomers o n l y (see T a b l e I ) . C o n d e n s a t i o n of I X gave d i t h i o g l y c o l i d e X I as a b y p r o d u c t (4). X I was also o b t a i n e d b y p y r o l y s i s of p o l y ( t h i o g l y c o l i d e ) (7). T h e p o l y m e r i z a t i o n of S-carboxy-a-mercaptoacetic a c i d a n h y d r i d e X I I u s i n g bases as initiators has b e e n s t u d i e d i n d e t a i l (4). It is s i m i l a r i n m a n y respects to the w e l l - k n o w n p o l y m e r i z a t i o n of N - c a r b o x y - a - a m i n o acid anhydrides.
108
POLYMERIZATION
REACTIONS
A N D N E W
POLYMERS
Table I. Monomer
Catalyst
Solvent
HS—CH —COOH
(Bulk)
H S — C H
(Bulk)
Ti(OBu)4
Chloroform
Cyclohexylamine
Dioxane
H 0
Dioxane
Amines
Tetrahydrofuran
N (C H )
2
2
— C O O C H 3
0
V o
Y
2
o
(CH ) Si—S—CH —COC1 3
0
3
2
5
3
—
2
—
In dioxane, 94.5°C Poly(thioglycolide)
is
readily
degraded
by
aqueous
alkali
and
amines.
I t is s o l u b l e w i t h o u t d e g r a d a t i o n o n l y i n d i c h l o r o a c e t i c a c i d
(DCA)
a n d hexafluoroacetone
sesquihydrate
The
(4).
viscosity-mo
l e c u l a r w e i g h t r e l a t i o n s h i p is M
= 0.018 MS,-
12
m l / g (25°C, D C A )
for the m o l e c u l a r - w e i g h t r a n g e M
of 5500 to 36000 D a l t o n s
w
Poly (a-mercaptopropionic
acids)
(XV),
or
(4).
poly (thiolactides),
w e r e s y n t h e s i z e d f r o m the c o r r e s p o n d i n g S - L e u c h s a n h y d r i d e s h a v i n g various optical purities ( I I ) . O *CH(CH ) 3
C
\
-co o
2
->
-£s*CH(CH )CO-J 3
-c XIV
o
XV
8.
109
Poly (thiol esters)
BUHRER A N D ELIAS
Poly (thioglycolides) Temp., °C
Yield, %
130
[rj], ml/g
100
140
—
25
94
100
97.4
25 -50
Reference
9
Liquid
(7)
—
3
"
U)
—
—
165-169
(8)
45
140-145
(10)
—
147-157
(8)
a
—
93.2
32.6
—
50*
—
mp, °C
n
—
15
100
x
440
6
157
c
—
(4)
—
(9)
In dichloroacetic acid, 25°C x (from light scattering)
b c
w
T h e p o l y m e r i z a t i o n i n d i o x a n e s o l u t i o n w i t h amines as initiators was v e r y s l o w a n d i n c o m p l e t e (see
Table II).
H o w e v e r , a r a p i d solid-state
p o l y m e r i z a t i o n was o b s e r v e d d u r i n g the a t t e m p t e d s u b l i m a t i o n or r e c r y s t a l l i z a t i o n of the m o n o m e r X I V . Poly(/?-mercapto
acids).
Poly(/3-mercaptopropionic acid)
has b e e n p r e p a r e d this w a y (12)
(XVI)
(see also T a b l e I I I ) : O II
H S C H C H C O O H + CICOEt + 2
2
1 2
-fsCH CH C0j 2
2
3
o
1HSCH CH C0—O—COEt] + 2
NEt
+
NEt H+Cl3
EtOH
+
C0
2
XVI An
alpha-substituted poly(/?-mercaptopropionic
acid)
has
been
110
POLYMERIZATION
Table II.
REACTIONS AND N E W
Poly(thiolactides) Temp., Yield, [rj] /c, °C % ml/g sp
Monomer
Solvent
POLYMERS
Catalyst
mp, Refer°C ence
CH-C /
\ O Dioxane
D29L71
11
n-Hexylamine
25
16
37.7°
133
(11)
"
—
17.4"
(460
(34)
25
60
40*
—
—
(34)
h
Gummy Vsvec/c, 25°C, tetrachloroethane-phenol, c = 0.0014 g/ml Same as c = 0.0044 g/ml Same as ' c = 0.0006 g/ml * Same as * c = 0.0005 g/ml
f
0
h
0
O
HSOC—(O)
COSH
> { - ^ ) — C — S — S - J XXVIII
w i t h a m m o n i u m p e r s u l f a t e - s o d i u m metabisulfite. T h e a d d i t i o n p r o d u c t of d i t h i o l s e b a c i c a c i d a n d 1-hexyne w a s also p r e p a r e d (22), b u t its structure is u n k n o w n . Properties
and
Applications
H i g h - m o l e c u l a r - w e i g h t p o l y ( t h i o l esters) s h o w fiber-forming p r o p e r ties. T h i s section covers t h e a v a i l a b l e d a t a c o n c e r n i n g m e l t i n g p o i n t s , c r y s t a l l i n i t y , s o l u b i l i t y , a n d s t a b i l i t y of this class of esters, a n d compares t h e m w i t h polyesters a n d p o l y a m i d e s . I n f o r m a t i o n a b o u t spectra is also reviewed. M e l t i n g P o i n t s . T h e m e l t i n g points of the various p o l y m e r s are given i n Figures 1 through 4 (more data are i n Tables I - I V a n d V I ) . I n these f o r m u l a s , X m a y b e O , S, o r N H .
-Px(CH ) CO J2
f- X ( C H ) X O C (CH ) CO - J
m
2
6
2
£ - X ( C H ) X O C (CH ) CO -J 2
m
-flX(CH ) XOC 2
m
2
4
—(o)—
c o
-J
m
124
POLYMERIZATION
REACTIONS AND N E W POLYMERS
M e l t i n g points of polyesters a n d p o l y a m i d e s w e r e t a k e n f r o m the Brandrup, J . , Immergut, E . H . , "Polymer Handbook," W i l e y , N e w York, 1967.
T h e d a t a d o not represent t r u e t h e r m o d y n a m i c m e l t i n g points
b u t h a v e b e e n d e t e r m i n e d for p o l y m e r s of u n k n o w n degree of c r y s t a l linity
by
methods
such
as d i f f e r e n t i a l t h e r m a l analysis, p o l a r i z a t i o n
m i c r o s c o p y , c a p i l l a r y t u b e , a n d others. F i v e conclusions c a n b e d r a w n f r o m the d a t a . ( 1 ) T h e m e l t i n g points of p o l y ( t h i o l esters) l i e b e t w e e n those p o l y esters a n d p o l y a m i d e s ( F i g u r e s 1-4). T h e y m e l t 30° to 7 0 ° C h i g h e r t h a n d o the c o r r e s p o n d i n g polyesters, the shape of the curves b e i n g v e r y s i m i l a r . T h i s b e h a v i o r is most l i k e l y because of a decreased flexibility O O of the - C - S - b o n d , c o m p a r e d w i t h the - C - O - b o n d . tions are p o l y ( t h i o g l y c o l i d e ) , w h i c h melts 6 0 ° C b e l o w (Figure 1), and poly(decamethylene l o w e r m e l t i n g p o i n t of colide)
T h e o n l y excep poly(glycolide)
dithioladipate) (Figure 3).
poly (thioglycolide)
compared
with
The
poly(gly-
is p r o b a b l y caused b y the h e l i c a l c o n f o r m a t i o n of the latter.
mp
[°c] 400
300
200
100
0
1
2
3
4
5
6
m
Figure 1. Melting points of polymers with the monomer unit^:X(CH ) COh A X = NH; • X = O; • X = S;—m.p. of polyethylene 2 m
8.
BUHRER AND
ELIAS
125
Poly (thiol esters)
T h e l o w m e l t i n g p o i n t of p o l y ( d e c a m e t h y l e n e d i t h i o l a d i p a t e ) m a y be c a u s e d b y the l o w m o l e c u l a r w e i g h t ([iy] cnci3 25
(2)
10 g / m l ) .
=
T h e m e l t i n g points of a l i p h a t i c p o l y ( t h i o l esters)
(Figures 2
a n d 3 ) l i e b e l o w that of p o l y e t h y l e n e ( 1 4 3 ° C ) a n d t e n d to increase w i t h
mp
(°c)
-m 6
8
10
Figure 2. Melting points of polymers with the monomer unit -frX(CH ) XOC(CHt) CO i7 A X = NH; MX — O; • X = S;—m.p. of polyethylene t
e
m
:
i n c r e a s i n g n u m b e r of m e t h y l e n e groups p e r m o n o m e r u n i t . t r u e for some p o l y ( t h i o l a c t o n e s ) ; see F i g u r e 1.
T h i s is not
(3) T h e i n t r o d u c t i o n of a l i c y c l i c parts i n t o the p o l y ( t h i o l ester) c h a i n increases c o n f o r m a t i o n a l r i g i d i t y a n d , c o n s e q u e n t l y , the m e l t i n g p o i n t . T h i s c a n b e seen f r o m the m e l t i n g points of p o l y ( h e x a m e t h y l e n e S(CH ) SOC(CH ) CO -J 2
6
2
4
XXIX 113°-115°C
£s(CH ) SOC—(F)—COJ 2
6
£ S C H
2
— C H
2
S 0 C - ^ H y - C 0 j
XXX
XXXI
188-215°C
302-310°C
126
POLYMERIZATION
REACTIONS AND N E W POLYMERS
d i t h i o l a d i p a t e ) ( X X I X ) , p o l y ( h e x a m e t h y l e n e dithiol-£rans-cyclohexane-l, 4-dicarboxylate)
(XXX),
and
p o l y (l,4-dimethylene-£rans-cyclohexane
dithiol-£rans-cyclohexane-l,4-dicarboxylate) ( X X X I ) .
mp (°C)
300
200
100
6
8
10
Figure 3. Melting points of polymers with the monomer unit-tXtCHdnXOCfCHthCOl? AX = NH; • X = O; • X = S;—m.p. of polyethylene I n X X X I , w h e n the r r a n s - d i m e r c a p t a n is r e p l a c e d b y a 70:30 m i x t u r e of trans a n d cis isomers, the m e l t i n g p o i n t is l o w e r e d to (4)
P o l y ( t h i o l esters)
with
240°-255°C.
a t e r e p h t h a l i c m o i e t y i n the c h a i n
( F i g u r e 4 ) h a v e m e l t i n g points h i g h e r t h a n p o l y e t h y l e n e .
These melt
i n g points decrease w i t h i n c r e a s i n g n u m b e r of m e t h y l e n e groups i n the monomer unit.
P o l y ( t h i o l esters) w i t h t e r e p h t h a l i c units a l w a y s m e l t at
h i g h e r temperatures t h a n do the i s o p h t h a l i c isomers ( T a b l e V I ) . (5)
A l l - a r o m a t i c p o l y ( t h i o l esters), s u c h as
poly(4,4'-biphenylene
d i t h i o l t e r e p h t h a l a t e ) ( X X X I I ) h a v e v e r y h i g h m e l t i n g points •SOC
^5>-
c o
}.
(34).
XXXII >460°C
F r o m these results, it is e v i d e n t that a n u m b e r of a r o m a t i c o r a l i c y c l i c p o l y ( t h i o l esters) h a v e m e l t i n g points s u i t a b l e for m a k i n g fibers. V e r y few Table I V ) .
glass-transition temperatures h a v e b e e n p u b l i s h e d
(see
8.
Poly(thiol
BUHRER A N D ELIAS
Crystallinity.
127
esters)
M o s t p o l y ( t h i o l esters) are q u i t e c r y s t a l l i n e .
tallinity can be lowered by using nonlinear monomers—for H S C H ( C H ) C H - ( C H ) S H (31)— 2
2
5
2
x-ray p a t t e r n of
4
or b y c o p o l y m e r i z a t i o n ( 2 9 ) .
a uniaxially oriented poly(e-thiocaprolactone)
indicates that this p o l y m e r adopts
Crys-
example, The (17)
a p l a n a r structure w i t h
extended
T h e sparse d a t a a v a i l a b l e s h o w no m a r k e d
difference
chains. Solubility.
b e t w e e n polyesters a n d p o l y ( t h i o l esters) i n t h e i r b e h a v i o r t o w a r d or g a n i c solvents.
G o o d solvents for m a n y p o l y ( t h i o l esters)
are c h l o r i
n a t e d h y d r o c a r b o n s , p h e n o l , a n d o r g a n i c acids s u c h as d i c h l o r o a c e t i c acid.
W i t h the e x c e p t i o n of p o l y ( t h i o g l y c o l i d e )
tide)
(11),
and poly(thiolac-
the p r o b l e m of d e g r a d a t i o n b y o r g a n i c solvent a c t i o n was
not s t u d i e d s y s t e m a t i c a l l y . coesters)
(4)
S o l u b i l i t y is i n c r e a s e d i n p o l y ( t h i o l ester
(34).
Stability.
O n l y few
studies of the s t a b i l i t y of p o l y ( t h i o l esters)
mp
PC)
100-
0-J
•
1
2
4
1
1
6
1
1
8
r*m
1
10
Figure 4. Melting points of polymers with the monomer unit^X(CH ) XOC —Jgfy-COlr A X = NH; • X = O; • X = S;—m.p. of polyethylene 2 m
128
POLYMERIZATION
against c h e m i c a l agents h a v e b e e n m a d e .
REACTIONS AND N E W
POLYMERS
It appears t h a t the esters are
v e r y stable against h y d r o l y s i s i n w a t e r or acids b u t r e a d i l y react w i t h aqueous a l k a l i ( 2 8 ) .
A m i n o l y s i s w i t h p r i m a r y a n d secondary amines is
v e r y fast a n d y i e l d s the l o w - m o l e c u l a r - w e i g h t
amides.
Poly (thiogly
c o l i d e ) is also d e g r a d e d b y d i m e t h y l f o r m a m i d e a n d t e r t i a r y amines
(4).
A p p l y i n g results f r o m l o w - m o l e c u l a r - w e i g h t thiolesters a n d esters to the c o r r e s p o n d i n g p o l y m e r s s h o u l d g i v e s l i g h t l y s l o w e r a c i d h y d r o l y s i s , e q u a l l y fast a l k a l i n e h y d r o l y s i s , a n d a m u c h faster a m i n o l y s i s of p o l y ( t h i o l esters) c o m p a r e d w i t h polyesters (24, 35,
36).
N o d a t a are a v a i l a b l e c o n c e r n i n g the t h e r m a l s t a b i l i t y of p o l y ( t h i o l esters). Infrared Spectra. 17,
show
28)
the
T h e i n f r a r e d spectra of p o l y ( t h i o l esters) (4, characteristic
- C -
stretch at
1675-1700
11,
cm" . 1
O B a n d s at 900-1000 c m "
1
are a t t r i b u t e d to the - C - S - stretch v i b r a t i o n , O
a n d those at 1100-1200 c m U V and O R D Spectra.
1
to the - C - C - stretch v i b r a t i o n (37). T h e U V spectra of t h i o l esters h a v e a peak
at about 230 n m ( l o g e 7T ->
type.
7T*
esters)
(17
3.6)
a s c r i b e d to a t r a n s i t i o n of
(38)
the
Similar absorption maxima were found i n poly (thiol
a n d 21).
I n a d d i t i o n , i n t h e U V s p e c t r u m of
poly(thio-
l a c t i d e ) ( X V ) , a s h o u l d e r at 279 n m ( l o g e = 2.5 i n C H C 1 ) (11) 3
may
b e t h e result of a n — » T T * t y p e a b s o r p t i o n . O R D a n d C D measurements of o p t i c a l l y a c t i v e thiolacetates poly(thiolactides)
(XV)
( J J ) , and poly(c-thiolactones)
(21)
(39),
show two
C o t t o n effects, one c e n t e r e d a r o u n d 2 3 0 - 2 4 0 n m a n d t h e other at about 280 n m . those
T h e O R D spectra of the p o l y m e r s are n e a r l y i d e n t i c a l w i t h
of l o w - m o l e c u l a r - w e i g h t
model
compounds,
thus i n d i c a t i n g the
absence of a h e l i c a l c o n f o r m a t i o n of the p o l y m e r s i n s o l u t i o n . Applications.
P o l y ( t h i o l esters) c a n be g o o d s t a r t i n g materials for
p r o d u c i n g fibers w h e n the esters h a v e an i n h e r e n t viscosity {77} > 0 . 3 or > 0 . 7 5 g / d l (16).
(27)
i n v o l v i n g a three- to
T h e y c a n b e c o l d - d r a w n to o r i e n t e d
fivefold
g/dl fibers
or even h i g h e r increase i n l e n g t h .
Poly(a,«-dimethyl-^-thiopropiolactone) has b e e n m e l t - s p u n at 1 8 5 ° C to
give
a
fiber
w h i c h , after d r a w i n g , h a d a t e n a c i t y of
1.4
g/den.
( p o l y e t h y l e n e terephthalate, 4 - 7 g / d e n . ) a n d a n i n i t i a l m o d u l u s of 12 g/den. 80%.
( 3 0 - 1 3 0 g / d e n . ) (16).
T e n s i l e recovery at 10% e l o n g a t i o n w a s
N o i n f o r m a t i o n is a v a i l a b l e a b o u t p o l y ( t h i o l esters)
w i t h higher
m e l t i n g points, s u c h as p o l y ( h e x a m e t h y l e n e d i t h i o l t e r e p h t h a l a t e ) . F i l m s c a n b e cast f r o m p o l y ( t h i o l esters) w i t h i n h e r e n t viscosities {77} >
0.5 g / d l (16),
Moderately
b u t n o d a t a a b o u t their properties are a v a i l a b l e .
c r y s t a l l i n e p o l y ( t h i o l esters)
may
prove
valuable in
8.
BUHRER A N D ELIAS
129
Poly (thiol esters)
p r o d u c i n g m o l d i n g plastics. T h e fast r e a c t i o n b e t w e e n p o l y ( t h i o l esters) a n d amines has b e e n u s e d to increase t h e s u l f u r content o f c a s e i n - a n d l y s i n e - c o n t a i n i n g p o l y peptides
(40).
Conclusions T h e m a i n differences b e t w e e n p o l y ( t h i o l esters) o n t h e o n e h a n d a n d p o l y a m i d e s a n d polyesters o n t h e other m a y b e s u m m a r i z e d this way: (a)
P o l y ( t h i o l ester) synthesis is m o r e c o m p l i c a t e d a n d expensive
since reactions u s i n g d i a c i d s d i r e c t l y as m o n o m e r s a r e n o t p o s s i b l e ; i t is necessary t o use either t h e a c i d chlorides o r t h e p h e n o l esters. (b)
D i m e r c a p t a n s a r e m o r e expensive t h a n glycols o r d i a m i n e s .
( c ) T h e m a i n a d v a n t a g e o f p o l y ( t h i o l esters) over polyesters is t h e formers higher melting point. H o w e v e r , p o l y m e r s p r e p a r e d f r o m thiolactones m e l t a t a t e m p e r a ture t o o l o w f o r fiber u s e . C o n s i d e r i n g these p o i n t s , i t is u n d e r s t a n d a b l e w h y p o l y ( t h i o l esters) h a v e n e v e r a t t r a c t e d c o m m e r c i a l interest.
F u r t h e r m o r e , most p o l y ( t h i o l
esters) seem to b e u n s t a b l e against alkalis.
T h e odors o f t h e b y - p r o d u c t s
of h y d r o l y s i s a n d t h e r m a l d e g r a d a t i o n are u n d e s i r a b l e too.
I t is, n e v e r
theless, possible that, i n some cases as yet u n k n o w n , they m a y offer s o m e advantages over other p o l y m e r s .
Literature
Cited
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R E C E I V E D February 2, 1972.
