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Analysis of Low Molecular Weight Homologues of Fiber-Forming Polycondensates.

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Analysis of Low Molecular Weight Homologues
of Fiber-Forming Polycondensates[**]
By V. Rossbach[*]
Dedicated to Professor Helmut Zahn on the occasion of his 65th birthday
Oligomers belong to the gray area between low molecular weight chemistry and macromolecular chemistry. Although they represent an undesirable “natural impurity” in fiber-forming polycondensates, they serve as useful model compounds for the corresponding polymers in fundamental research. Whereas for many years new classes of oligomers were being
made preparatively accessible and the isolation of higher oligomers in pure form was being
pursued, at the present time the emphasis is on analysis. By a combination of classical
chemical and instrumental methods of analysis from polymer and organic chemistry, the
identification of oligomers of unknown structure, the analytical control of their synthesis
and the determination of their content in technical polymers has meanwhile become a routine task.
1. Introduction
Apart from in a few special processes, the technical synthesis of polymers does not only yield macromolecules. On
the contrary, plastics, fibers, varnishes and adhesives often
contain low molecular weight precursors of the polymer as
“natural impurities”. They have the same structure as the
corresponding polymer molecules, only the number of repeat units from which they are constructed is smaller. Van
der Want and Staverrnann”’ introduced the term “oligomers” for these short-chain homologues of the polymer.
Kern‘*]later narrowed the concept of oligomers to be those
low molecular weight polymer homologues which differ
sufficiently in their properties so that they can be separated into individual chemical species. This definition is
now accepted. Nowadays an oligomer is understood as being a “chemically pure” substance, whereas with a polymer it is tacitly assumed that it can consist of polymer
chains of various lengths and frequently also contains
oligomer molecules.
In accordance with this definition of oligomers, their
classification depends on the efficiency of the methods of
separation used. Thus a polystyrene with 38 repeat units,
which can be separated chromatographically from its
neighboring polymer h o m o l ~ g u e s ~ would
~ “ ~ , be classified
as an oligomer. In the case of the fiber-forming polycondensates (cf. Fig. 1) one would have to speak of polymers
at much lower n-values (degree of oligomerization or polymerization), for here the lower polymer homologues with
molecular masses of 1000 to 2000 g/mol can no longer
be separated. In view of this, the definition by Zahn and
GleZt~mann‘~~’
appears less ambiguous. They define oligomers as those molecularly uniform initial members of
polymer-homologue series which do not yet have the typical physical structure (secondary and tertiary structure) of
the particular polymer. The higher members, for which this
definition no longer suffices, although they may be prepared in a pure form, can be termed pleionomers. In spite
of these various definitions, in this publication oligomers
are to be understood quite generally as the lower polymer
homologues.
As a result of studies by Zahn and his group, considerable experimental material about the oligomers of the fiber-
-
H
Polyethylene terephthalate
H-+NH--(CH,),-CO-],OH
Polyamide 6 (Polyhexanamide)
H--(NH--(CH&-NH-CO--(CH,)~-CO-],OH
Polyamide 6,6 (Polyhexamethylene adipamide, poly-e-caprolactam)
L
Jn
Poly(m-phenylene isophthalamide) (Nomex type)
r
L
J”
Poly(p-phenylene terephthalamide) (Kevlar type)
Deutsches Wollforschungsinstitut an der Technischen Hochschule
Veltmanplatz 8, D-5100 Aachen (Germany)
Present address:
Textilchemisches Laboratoriurn der Fachhochschule Niederrhein
Adlerstr. 32, D-4150Krefeld (Germany)
Based on a lecture delivered at the GDCh Extension Course, “Schutzgruppenchemie - Methoden zur Umsetzung polyfunktioneller Molekiile”, October 11, 1979 in Aachen (Germany).
Angew. Chem. In:. Ed. Engl. 20, 831-840 (1981)
O-CH2-CH2-O-C0
Polybutylene terephthalate
(Polytetramethylene terephthalate)
(‘1 Prof. Dr. V. Rossbach
[**I
i
r
L
J“
Poly(4,4’-methylenedicyclohexyldodecanamide) (Qiana type)
Fig. 1. Structural formulas of the most important fiber polymers of the polycondensate type.
0 Verlag Chemie GmbH. 6940 Weinheim, 1981
0570-0833/81/1010-0831 $02.50/0
831
forming polycondensates is available. In the course of
three decades, it has been possible to achieve the preparative isolation of the various oligomer classes and to spur
on the preparation of pure samples of the higher oligomers[4-81.
In recent times however, the emphasis has shifted
towards analysis of these oligomers.
2. Importance of Analysis
Analysis of oligomers from fiber-forming polycondensates is, for various reasons, of great scientific and technical significance. Lower polymer homologues serve as
model compounds for the elucidation of important textile
and polymer properties, such as dyeability, viscosity and
melting c h a r a c t e r i ~ t i c s [as
~ ~well
, ~ ~ as secondary and tertiary structure["] and thermal stability["1. Furthermore, oligomers are used to calibrate methods of investigation
used in polymer chemistry, e.g., as internal standards for
gel permeation chromatography[""I or in the calibration of
end-group determinations with polyesters['' b1 and polyamides['2c1.
Oligomers have also proved to be useful tools for macromolecular chemistry in the development of new analytical
procedures. Thus it was possible to show, using the dimeric diol of ethylene terephthalate
that the reagent (I), m.p. = 181 " C ,is suitable for the sequential degradation (and therefore for sequential analysis) of
copolyesters containing ethylene terephthalate units['31.
The method for the sequential degradation of ethylene
terephthalate oligomers and polymers-binding of the reagent to the hydroxy end groups, cleavage of the Boc protective groups (Boc = tert-butyloxycarbonyl), thermal ester
aminolysis with ring closure-corresponds to the Edman
d e g r a d a t i ~ n "used
~ ~ in protein chemistry, in that a ring
compound (m.p. = 146°C) is formed which contains the
terminal monomer unit. The latter can thus be identified
and determined quantitatively. Furthermore, a new hydroxy end group is formed, to which the reagent can again
bind. The degradation can thus be repeated. Before this
method (the first to allow chemical sequencing of copolyesters) can be used to give satisfactory and complete analyses of products with unknown composition a number of
tasks still have to be completed, such as the preparation of
molecularly uniform fractions and degradation studies on
the corresponding cooligoesters.
It is obvious that, in order to act as model or calibration
compounds, oligomers have to be absolutely pure, i. e. molecularly homogeneous, which can only be ascertained by
efficient analytical methods. Often the task in hand is
namely to study changes in certain parameters as a function of chain length.
However, the importance of analysis of oligomers from
fiber-forming polycondensates is not confined to producing well-characterized model and calibration compounds
for textile and polymer chemistry. On the contrary, the determination of the low molecular weight polymer homologues is important in itself. Thus, for example, by analytical separation of the cyclic oligomers in polyethylene terephthalate1'5a1,polybutylene terephthalate[Isb1, polypropylene terephthalate['5b1, polyamide 6[I6l, polyamide 1l[i'al,
and polyamide 12['71it was possible to come closer to an
elucidation of the relationship between the chain conformation of the polymer and its tendency to form cyclic oligomers. There is an increasing tendency to use oligomer
analysis in applied problems e. g. the clarification of faults
in materials. A disproportionately large content of linear
oligomers in polyethylene terephthalate is an indication
that the material has been subjected to hydrolytic damage['81and the content of cyclic trimer (2) is actually a crite-
1
NH
.
NH-c H~-cH~-o-c'
H2N<HZ+<, -
I
832
II
0
1
rion of quality for polyethylene terephthalate. On account
of its low solubility in water the cyclic trimer (2) causes difficulties during the processing and finishing of polyethylene terephthalate fibers. During cooling of dye-baths used
in high-temperature dyeing, the oligomer (2) crystallizes
out in the water and on the fiber['91.The crystalline, sharpedged deposits which are thereby formed on the fiber surface (cf. Fig. 2) can damage guide rollers during further
processing of the fibers. In addition, oligomer crystals
which collect on rollers can discharge onto the material
and lower its quality.
Although it has been accepted, with resignation, that
one "has to live with oligomers"[201,it has recently been
shown possible to suppress oligomer formation in polyethylene terephthalateL2'I or to remove oligomers once
formed[221.
Angew. Chem. In!. Ed. Engl. 20, 831-840 (1981)
Fig. 2. Surface of a polyethylene terephthalate fiber before dyeing (left) and
after dyeing, with considerable oligomer deposition (right) (SEM micrograph
from Dr. P. Kusch, magnification 450 x ).
In the case of the other currently important fibrous polymers of the polycondensate type (cf. Fig. l), oligomers
present no very great problems during processing and finishing, since they all exhibit good solubility in water. This
is also true for the cyclic oligomers of polybutylene terephthalate, which, surprisingly, are about 3 to 5 times more
soluble in water than the sparingly soluble polyethylene
terephthalate o l i g o m e r ~ [ ~ ~ ~ .
I
I
The (readily soluble) oligomers from fiber-forming aliphatic polyamides can, however, give rise to dust formation if they are present in large quantities in the fiber polymer[z41.In the case of aromatic polyamides such as poly@phenylene terephthalamide) (Kevlar), oligomer analysis
has no direct practical importance since, on account of the
manufacturing process used, the commercially available fiber polymers contain no oligomer~[~’~.
In spite of the great importance which oligomers have as
model compounds, calibration compounds, and (undesirable) low molecular weight by-products in fiber-forming
polyesters and polyamides, it cannot be overlooked that
since about 1970 the number of publications on oligomer
preparation has declined steadily. This tendency arises
from the fact that in the last decade hardly any new polymers have achieved commercial importance, and the activities of the producers have therefore been concentrated on
modifying the polymers already available and optimizing
the processing procedures or developing new ones. Parallel to this development in the polymer field, interest in new
classes of oligomers is also declining. This development is
regrettable in as much as analytical methods are now available which allow even very complicated molecules to be
identified and complex oligomer mixtures to be separated.
In contrast to this declining activity in the preparative
field, there have been numerous publications either touching on, or dealing with, the analytical aspects of oligomer
chemistry. However, these are almost entirely restricted to
the already well-known oligomer classes‘”. 15-22,24.26-371.
3. Special Aspects of Oligomer Analysis
As polymer homologues with a low degree of polymerization, oligomers occupy a place between monomers and
Angew. Chem. Int. Ed. Engl. 20,831-840 (1981)
polymers. They are thus, according to a definition of Lehn
et ~ f . [ ~to* be
~ , classified as “mesomolecules” i. e., they belong neither to the “micromolecules” of organic chemistry
(molecular mass < 500 g/mol), nor to the macromolecules
of polymer chemistry and biochemistry (molecular mass
> 5000 g/mol). This special position occupied by the oligomers also holds for their analysis: in many cases they
cannot be characterized either by the methods of organic
or of macromolecular chemistry.
This may be illustrated by two examples. In organic
chemistry, a standard method for the separation of complicated mixtures is gas chromatography. Since, like polymers, oligomers (especially the higher ones) cannot be vaporized without decomposition, this important analytical
principle generally cannot be applied to oligomer analysis.
Gas chromatography can only be used-as with the corresponding polymer-to analyze monomer units. For example, after trifluoroacetylation of the N,N’-diaryldiamines
from oligo- and p o l y a m i d e ~ [and
~ ~ ] trimethylsilylation of
w-amino acids[4o1they can be separated and identified, using gas chromatography. The methods of organic chemistry can only be used after the poly- and oligoamides have
been converted by hydrolysis into micromolecules.
Similarly, the methods of polymer analysis are also generally not applicable to oligomers. This can be illustrated
by an example from polyester analysis. A simple and reliable method for determining the hydroxy end groups in polyethylene terephthalate (3) is to react them with an excess
of 3,5-dinitrobenzoyl chloride (4), followed by hydrolysis
of the excess reagent with pyridine/water to 3,5-dinitrobenzoic acid (5), which is then determined potentiometri~aily[~’J.
CO-O-CHz-CHz-0
L
J”
I
NO?
Before the titration the polymer has to be precipitated
from the analysis solution with acetonelwater. Otherwise
the carboxy end-groups of the polymer would also be titrated, thus leading to erroneous values for the hydroxy
end group content. This procedure can only be used with
considerable reservations for the oligoesters, since they
have different solubility characteristics. In addition, the
solubility of the individual members differs considerably,
as shown in Table 1 for the dicarboxylic acid dimethyl esters of the polyethylene terephthalate series (cf. also Fig.
3)Whereas the lower members of this dimethyl ester series
are adequately soluble in the solvent dioxane, which is
833
Table 1. Solubility, melting point and elemental analysis of the dicarboxylic
acid dimethyl esters of the polyethyIene terephthalate series [42-44j.
n =Number of repeat units. DMF=N.N-dimethylformamide, TCE= 1,1,2,2tetrachloroethane.
-l
L
J”
~~
n
1
2
3
4
3
6
7
8
9
10
Solvent
M.p.
[“CI
Analysis
calc. [%]
found [%I
C
H
C
H
Dioxane
Dioxane
Dioxane
Dioxane
DMF
DMF
DMF
Dioxane/TCE
TCE
TCE
168-170
196-198
215-217
231-232
242- 243
248-249
250-252
251-253
62.17
62.28
62.33
62.37
62.39
62.41
62.42
62.43
62.44
62.44
254-256
4.70
4.53
4.45
4.41
4.35
4.33
4.32
4.30
4.29
4.28
62.12
62.24
62.11
62.22
62.27
62.54
62.57
62.22
62.26
62.36
4.62
4.50
4.32
4.38
4.21
4.16
4.40
4.21
4.21
4.39
well suited to analytical purposes, the higher members are
only soluble in such solvents as N.N-dimethylformamide
and 1,1,2,2-tetrachloroethane, which for various reasons
are less suitable for analytical studies. The higher salubility of the lower oligomers has often been made use of in
spectroscopic investigations.
Thus, for example, the UV spectrum of the polyethylene
terephthalate oligomers (in solution) was recorded long before the spectrum of polyethylene terephthalate itself[4s1.It
should be noted, however, that with the discovery of the
various fluorinated solvents such as 1,1,1,3,3,3-hexafluoroisopropyl alcohol for polyethylene terephthalate, polyamide 6 and polyamide 6,6 as well as 2,2,2-trifluoroethanoI
for aliphatic polyamides, suitable solvents are now also
available for the most important fiber polymers of the
polycondensate type.
The chemistry of low molecular weight polymer homologues, especially their analysis, had its beginnings in peptide chemistry and analysis. Zuhn et uf.f461described their
first synthetic oligoamides as “nylon-peptides”. This
should not, however, obscure the fact that there are great
differences between peptide analysis and oligomer analysis (including oligoamide analysis). These result from the
different chemical structure of these classes of compounds.
Firstly, peptides consist of heterogeneous amino acids,
whereas the most important oligoamides and oligoesters
are the lower members of the corresponding homopolymers (polyamide 6 and 6,6 or polyethylene terephthalate). It
is true that cooligoamides are known, e. g., from &-aminohexanoic acid“] and o-aminoundecanoic
but they
are of minor importance. Secondly, among the technically
important oligomers of the polycondensate type are some
in which - in contrast to peptides -the functional groups
leading to chain extension are distributed between two different monomer units. Combinations of practical importance are diamine/dicarboxylic acid and diol/dicarboxylic
acid. Such oligomers of the AABB type show a greater variety of structure than the oligorners (and peptides) in
which the chain-extending functional groups are contained
in the same monomer unit (AB type). Figure 3 illustrates
I*] IUPAC name for 6-aminocaproic acid.
834
Hydroxy a c i d s
Linear
mers
Diols
-
Dicarboxyiic a c i d s
Fig. 3. Structure of the possible oligomers of ethylene terephthalate
this in the case of the oligomers of ethylene terephthalate.
These characteristic differences in the structure of peptides as opposed to the technically important oligoesters
and oligoamides have various consequences for their analysis.
On account of the different chemical structure of the individual amino acids, the short-chain peptides differ quite
considerably from each other after every coupling step. As
a consequence, their purification and the proof of purity
do not present any great difficulties. In contrast to this, because of their homogeneous construction, the technically
important oligoesters and oligoamides resemble each other
chemically to such an extent, after a few coupling steps,
that it is hardly possible to purify and separate them using
simple methods (e.g. recrystallization). The classical criteria of purity ( e . g . elemental analysis, melting point) also
cannot be used (cf. Table 1). For cyclic oligomers this
statement is true only with limitations. It is true that elemental analysis is of little value, since all homologues must
give the same results, but the melting point determination
gives more information, because, in contrast to the linear
oligomers, the melting point does not gradually approach
that of the polymer (cf. Table 1). As Table 2 shows, in the
Table 2. Melting points, elemental analyses and molecular masses (uia mass
spectroscopy) of cyclic oligomers of butylene terephthalate [231. n = Number
of reueat units.
I
1
n
M.p.
Analysis
found [%I [a]
C
H
Molecular mass
Ig mol-‘1
calc.
found
65.50
65.40
65.13
65.40
5.50
5.90
5.49
5.48
440.40
660.60
880.80
1101.00
[“CI
2
3
4
5
199
171.5
250-251
207
440
660
880
[b]
[a] Calculated values C 65.45, H 5.49‘/0. [b] No molecular ion.
Angew. Chern. Int. Ed. Engl. 20, 831-840 (1981)
Table 3. Eluents for thin-layer chromatographic separation of oligomer mixtures.
case of the cyclic oligomers of polybutylene terephthalate,
the melting points develop discontinuously.
Analysis of the monomer units is much more critical for
peptides than for oligomers, since for complete characterization of peptides the sequence and configuration of the
amino acids have to be determined in addition to the
quantitative composition. In cases where the analysis of
the monomer units in oligomers is based on chemical
cleavage into these units, it has to be borne in mind that, in
the case of the lower cyclic polymer homologues, Flory’s
principle of equal reactivity of functional groups[481does
not always hold. After ring formation, not only can the
lower polymer homologues have a slower rate of hydrolysis compared to the polymer, as has long been known for
oligoamide~[~~
but
, ~ *also
~ , the reverse can occur: the ester
groups in the cyclic trimer of ethylene terephthalate hydrolyze in alkali 50 to 60 times faster than the ester groups in
the corresponding
End-group analysis of oligomers with chemical reagents
is simplified because, in contrast to peptides, the endgroups in any one series have chemically identical surroundings, independent of the chain length. This means
that the reactivity only varies within narrow limits‘4.
Peptides are usually synthesized with the aim of obtaining biologically active substances. In these cases, biological activity is the most unequivocal criterion of purity. In
the case of the oligomers of technically important polycondensates this aspect does not apply, since they exhibit no
biological activity.
Oligomer Type
Eluent
Linear oligomers with free or
blocked end groups and cyclic
oligomers of polyamide 6- and
6,6-type
sec-Butyl alcohol/formic acid/
water 75 :15 : 10 (“SBA”) o r
sec-butyl alcohol/lO% NH, 85 :15
(“SBN”)
Higher molecular weight linear
oligomers of polyamide 6,6-type
Amy1 alcohol/formic acid/water
60 :30 : 10
N-(2,4-Dinitrophenyl)oligomers
of polyamide 6- and 6,6-type
95 : 5 :1 (“CMA’)
Chloroform/rnethanol/acetic acid
Oligomers of Qiana- and Kevlartype
sec-Butyl alcohol/formic acid/
water/acetic acid 75:21.5: 15:8.5
(“WESBA”)
Cyclic oligomers of ethylene terephthalate (linear oligomers remain at origin)
Chloroform/ether 9 : I
Cyclic oligomers and linear oligomers of diol type of ethylene terephthalate
Benzene/dioxane 10: I
Linear oligomers of ethylene terephthalate (cyclic oligomers remain at origin)
Chloroform/ethanol 9 :1
Linear oligomers of diacid type of
ethylene terephthalate
65:25:10
Ethanol/triethylamine/glycol
Linear oligomers of hydroxy acid
type of ethylene terephthalate
I-Propanol/28% NH,/ammonia/
water 70 :25 :3
Cyclic oligomers of butylene terephthalate
Dioxane/toluene 1 :9
[a] Qiana and Kevlar, see Fig. 1.
oligomer synthesis, such as introduction of protective
groups, coupling, cleavage of protective groups and, if necessary, cyclization (cf. Fig. 4).
In addition to ascending thin layer chromatography, the
technique usually used, the descending method is also
used for some higher molecular weight oligomers.
There are various possibilities for the detection of the
compounds on thin layer chromatograms. If the oligomers
contain aryl groups ( e . g . in the oligomers of ethylene
terephthalate, butylene terephthalate, m- and p-phenyleneterephthalamide), the spots can be detected by quenching
of fluorescence under UV light, so long as thin layer plates
4. Monitoring the Course of Reaction
during Oligomer Synthesis
4.1. Synthesis of Oligomers in Solution
Synthesis of oligomers in s o l ~ t i o n [ ~is~the
. ~ ’most important method for their production besides isolation from the
technical polycondensate. The course of the synthesis can
be followed easily and rapidly with thin-layer chromatography. A large number of eluents are available for the
various oligomer series, some of them having remarkable
specificityl18.25.52.531
0
-0
ii
2-NH-( CHz),<-OH
b
,
ll
Z-NH-(C H2)-C-X
H~N-(CHZ)~-&Y
Fig. 4. General scheme for oligomer synthesis in solution, using an oligoamide of the AB type as example. a,a’:introduction of the protective
group; 6 : activation; c: coupling; d,d‘: removal of protective group; er cyclization.
As Table 3 shows, sufficient eluent mixtures are available for quantitatively following all important steps of an
Angew. Chem. Inr. Ed. Engl. 20. 831-840 (1981)
with a fluorescent indicator are used. In general, the “chlorine” method of Zahn and R e x r ~ t h ‘can
~~~
be used for
835
oligoamides. If they also contain free amino groups, oligoamides can be detected by spraying with ninhydrin. It has
to be remembered however, that with increasing chain
length (corresponding to a decreasing amino group content) the detection becomes less sensitive. With both methods the hue and intensity of the colors obtained on the thin
layer plates varies quite markedly between the individual
oligoamide series.
Determination of the unreacted amino groups using the
methods described above gives a direct estimate of the
amount of coupling.
The course of the reaction in the Merrifield synthesis is
especially easy to follow if easily identifiable protective
groups such as the 3,5-dimethoxy-a,a-dimethylbenzyloxycarbonyl group are used.
H
4.2. Oligomer Synthesis on Polymer Carriers
H3CO
Oligomer synthesis on polymer
only yields
pure oligomers if the reaction steps are kept under strict
analytical control. If an insolubfe carrier resin with chloromethyl groups is used, as in the classical Merrifield peptide synthesis, it is not easy to follow the course of the
reaction analytically. For example, in the synthesis of Eaminohexanoic acid oligomers the following steps (1 -4)
have to be checked1531.
1. Binding of the first amino acid on the resin:
--CH2-CI
+ HOOC+CH~),-NH-BOC
CH3 0
To determine the degree of binding to the polymer, the
protective group is cleaved and the carrier material washed
until the filtrate shows no UV adsorption. Cleavage and
washing are repeated to test completeness of reaction. The
combined filtrates are then condensed and the concentration of cleaved protective groups determined by UV spect r o s ~ o p y ~ ~ To
~ , ~the
' ~ . author's knowledge, this elegant
technique has not yet been applied to oligomer synthesis.
Attempts to synthesize ethylene terephthalate oligomers
on polymeric carriers have so far been without S U C C ~ S S I ~ ~ ] .
-+
--CH~-O-CO-(CH~)~-NH-BOC
(Boc = rert-butyloxycarbony1)
5. Determination of Oligomer Content in Polymers
N elemental analysis and Moore-Stein analysis of the total
amino-acid carrying resin can be used to follow the reaction analytically.
2. Total blocking with benzyl t h i o a l ~ o h o l ~
of~ excess
~]
chloromethyl groups which have not reacted with BOC-Faminohexanoic acid:
Technical fiber polymers of the polycondensate type almost all contain low molecular weight fractions, present in
the individual polymers to quite different extents (Table
This step can be checked by an S elemental analysis.
3. Cleavage of the Boc protective group from the resinbound &-aminohexanoic acid with trifluoroacetic acid in
CH2C12
-CHZ-O-CO---(CH2)5-N
H-BOC -t
--CH2-O-CO+CH2)5-NH2
Various methods are available for determining the resulting free amino groups. Lee and L o u d ~ n [ ~recently
'~
reviewed their efficacy. In the case of controlled pore glass,
the picric acid method is especially suitable. In this method, picric acid initially binds to the amino groups in stoichiometric amounts. The picrate is then cleaved with N.Ndiisopropylethylamine and the amount of amine-picrate
complex formed is determined spectrophotometrically at
358 nm.
The pyridinium chloride method[581is also useful. The
amino groups bind to the chloride ions of pyridinium chloride. Washing with triethylamine removes the chloride
ions, which are then determined potentiometrically.
4. Coupling of the second E-aminohexanoic acid molecule onto the resin with N,N-dicyclohexylcarbodiimide:
-C
Hz-O-CO--(C
H2)s-N H2
+ HOOC--(C H2)s-N
--CH,-O-CO--(CH~)5-NH-CO--(CH2)5-N
836
H-BOC -+
H-BOC
4).
Table 4. Content o f low molecular weight fractions in some fiber polymers
[3b, 23, 25, 61 a] (see Fig. 1).
Polymer
Low
molecular
weight
fraction
(wt.-Qh)
Extraction
solvent
Polyamide 6
Polyamide 6,6
Polyamide I 1
Qiana polyamide
Kevlar polyamide
Polyethylene terephthalate
Polybutylene terephthalate
11-12
2.0-2.2
ca. 7
2.4
0
1.3- 1.7
0.8- 1.9
Methanol
Glacial acetic acid
Glacial acetic acid
Butanol
Various solvents
Dioxane
Dichlorornethane, dioxane
These differences are not only due to the fact that during
polymer synthesis the lower polymer homologues are
formed in varying amounts, but rather that different technologies are used in their manufacture and processing.
Thus the reason no extraction residue is obtained from
technical Kevlar polyamide is to be found in the production process for the polymer, using condensation polymerization in solution and wet ~ p i n n i n g ' ~ ~ . ~ ~ ] .
The oligomers are often separated from the polymer in
order to estimate them. This can take place in two ways: by
extraction with a solvent for the oligomer or by reprecipitation of the polymer.
Extraction techniques are very common in oligoester
and oligoamide analysis and are applied in many different
ways. The solvents most often used are listed in Table 4. A
special problem, common to all extraction techniques is
Angew. Chem. Inr. Ed. Engl. 20, 831-840 (1981)
the complete removal of the low molecular weight fraction.
As was recently shown in the case of polyamide 6, when
determining the content of extractable matter on coarse
material ( e . g . cord and granules) the extraction times are
often not sufficient to completely remove the low molecular weight fraction[301.On the other hand, in some situations it is not required to extract exhaustively. With polyethylene terephthalate it often suffices to extract and determine the surface oligomers, since only these are responsible for problems arising during textile processing and finishing (cf. Section 2). Differentiation between surface
oligomer and total oligomer content can be carried out by
grading the extraction conditions with respect to solvent,
and time and temperature of extractiod3I1.
The second possibility for separating the low molecular
weight fraction consists in dissolving the whole polymer
sample (including the oligomers) in a suitable solvent and
then precipitating the high molecular weight fraction with
a precipitating agent which leaves the oligomer fraction in
solution (i. e. fractionation). Suitable solvent/precipitant
systems are e. g. 1,1,1,3,3,3-hexafluoroisopropylalcohol/
dioxane for polyethylene terephthalate[321,and 2,2,2-trifluoroethanol/ethanol for polyamide 6 and 6,6[631.
A problem with all such reprecipitation techniques is that, on addition of the precipitant, the coagulating polymer can occlude oligomers, making them unavailable for analysis.
There are various possibilities for quantitative determination of oligomers in extracts and reprecipitated solutions. The simplest is weighing of the solid residue after
evaporation of the solvents (gravimetry). It has to be borne
in mind here that foreign matter, such as stabilizers, spinning oils etc. is also included in the weighing and can
thereby falsify the results. Optical methods are more specific, e.g. UV spectroscopy for the oligomers of ethylene
tere~hthalate~
and
~ ~ refractometry
]
and interferometry for
the oligomers of polyamide 6 and 6,6[331.However, these
detection techniques are also liable to interference by foreign substances. In recent times, therefore, preference has
been given to analytical methods in which the oligorners
are separated from foreign matter before detection, e. g. by
thin layer chromatography, gel chromatography and adsorption chromatography, in the last case especially as
high pressure liquid chromatography (HPLC). These methods have the further advantage that they allow separation
of the various oligomers in the mixtures. Information is
thus obtained about the content of the individual oligomers in the polymer, which can further be used for the analysis of faults (cf. Section 2). A selection of various chromatographic methods which have been used for analytical
problems is given in Table 5.
High pressure liquid chromatography in particular has
made it possible to analyze technical extracts of chemically
very similar oligomers with great speed and precision. Figure 5 shows this for the monomeric cycloamides of the
Qiana-type, which differ only in their configuration.
4
1
1
L
*
q
CH30H
10
15
f [rninl
-
20
Fig. 5. HPLC chromatogram o f an extracted isomeric mixture o f the monomeric cycloamides of the Qiana-type [61a]. 1-4, see text.
By comparison with synthetically prepared cycloamides,
it can be shown that the peaks 1-4 in Figure 5 can be attributed to the isomers (6a)--(6d)161a1.
The search for suitable eluting agents is a problem with
HPLC. In the case of the ethylene terephthalate oligomers,
nano-thin layer chromatography can be used to advantage
for this purpose[651.
Table 5. Selection of chromatographic methods for quantitative oligomer analysis. GPC
matography, Ads = adsorption chromatography, TLC = thin layer chromatography.
=
gel permeation chromatography, HPLC
=
high pressure liquid chro~~
Area of Application
Principle o f Separation
Mobile Phase
Detection
Linear monomers and oligomers of polyamide 6-, 6,6- and 12-type
Cyclic monomers and oligomers of polyamide 6- and 6,6-type
Cyclic monomers and oligomers of polyamide I I - and 12-type
Isomeric mixtures o f the cycloamides of
the Qiana-type
Cyclic trimer of ethylene terephthalate
Cyclic oligomers o f ethylene terephthalate
Cyclic oligomers of ethylene terephthalate
GPC after reaction with 1fluoro-2,4-dinitrobenzene
GPC
0.05 N HCVMethanol
UV-VIS spectroscopy
0.1 N HCI
UV spectroscopy
G PC
o-Iert-Butylphenol
No information
HPLC
MethanoVwater
Refractometry
TLC
GPC
Benzene/dioxane
Chloroform
Quenching of fluorescence
Refractometry
Ads.
UV spectroscopy
HPLC
Chloroform/ether,
chloroform/ethanol, CH2Cl,/hexane
MethanoVwater
UV spectroscopy
Ads
Hexane/dioxane
UV spectroscopy
Cyclic oligomers of butylene terephthalate
Cyclic oligomers of ethylene terephthalate
Angew. Chem. Int. Ed. Engl. 20. 831-840 (1981)
Ref.
837
08
NH
NH
I
C0
-
( 6 a ) , cis, cis.
m. p. = 254.257 'C
(CHdio-
......-.
..--5
0.6
I
co
_......
1 6 h ) , p r o b a b l e conformer of f 6 ~ )
-T
( 6 ~ )trans,
.
trans,
m. p. = 310-314 "C
(r
L.2
OL
0
0.2
0
NH
I
co-
(CH2)io
I
a
( 6 d j , cis, trans,
m.p.= 2 6 9 - 2 7 2
A
O C
co
The errors associated with separation of oligomers can
be avoided by using techniques which enable the low molecular weight homologues to be determined directly along
with the polymer. Examples of this are gas chromatoand IR spectrophot~metric[~~~
determination of
&-caprolactamin polyamide 6, or the determination by adsorption chromatography of the cyclic oligomers of ethylene terephthalate'*']. For routine absolute determinations,
this method of determining the low molecular weight polymer homologues in the presence of polymer appears particularly promising.
The chromatographic techniques described above can be
used in conjunction with authentic reference compounds
for the identification of oligomers in polymers. Thus in the
N.N-dimethylformamide extracts of poly@-phenyleneterephthalamide), manufactured according to the relevant
company patent for Kevlarr621,N,N'-dibenzoylphenylenediamine and the dibenzoyl derivative (7) of the monomeric
diamine (m.p. >385"C) could be identified, along with
higher condensed products, by means of thin layer chrornat~graphy['~l
(cf. Fig. 6).
I71
The formation of these benzoylated low molecular
weight homologues can be attributed to the use of benzoic
B
C
Fig. 6. Thin layer chromatogram of an N.N-dimethylformamide extract of
poly-@-phenyleneterephthalamide) (A) and the reference compounds N,Wdibenzoylphenylenediamine (B) and (7) (C) (Eluent: WESBA, cf. Table 3).
acid, as prescribed in the patentf6'], as a molecular weight
regulator in the polycondensation.
6. Characterization of Oligomers
The methods described above for following the course
of reactions in solution (Section 4.1) and for determining
the oligomer content of the polymer (Section 5) can also be
applied to characterization of the oligomers. They have to
be supplemented, however, by further techniques. The
methods normally used in organic chemistry, i. e. elemental
analysis plus UV, IR and NMR spectroscopy, generally
are sufficient to assign an oligomer to a certain series. Further information is given by the processes used to determine the monomeric units of the relevant polymers. Table
6 lists a selection of methods for quantitative analysis of
monomeric units.
In Table 6 the N-propionylpropylamides of &-aminohexanoic acid and the methyl esters of terephthalic acid
have been included as especially useful model compounds
("oligomers free of end groups") for polyamide 6 and
polyethylene terephthalate, respectively[3b1.
If the chain length and exact oligomer type are to be determined (cf. Fig. 3 in the case of the ethylene terephthalate oligomers) the methods of organic chemistry and polymer chemistry as described above are not sufficient. Fur-
Table 6. Selection of methods for quantitative analysis of monomeric units.
Area of Application
Preparatory Steps
Principle of Separation
Determination
Ref.
Propylamine and propionic acid in N-propionyl-propylamides of aminocarboxylic
acids, especially &-aminohexanoicacid
Various diamines, amino carboxylic acids
and dicarboxylic acids from aliphatic POlyamides; aromatic diamines from aliphatic-aromatic polyamides
Methyl ester groups in polyethylene terephthalate
Terephthalic acid units in polyesters
Hydrolysis with H,SO,
Distillation
Titration
[661
Trifluoroacetylation, methylation or trimethylsilylation
after hydrolysis with HCI
Gas chromatography
Retention time (reference
compounds)
139, 40, 671
Hydrazinolysis with formation of methanol
Hydrazinolysis with formation of terephthalic acid dihydrazide and monohydrazide
Gas chromatography
Retention time
[681
N o separation
Polarograph y
~ ~ 7 0 1
Hydrolysis with HCI
Ion exchange
Moore-Stein analysis
1711
&-Aminohexanoicacid units
838
Angew. Chem.
lnf.Ed. Engl. 20, 831-840 (1981)
ther chemical and instrumental methods have to be applied.
Although mass spectroscopy was used by Repid7'] in
1968 to characterize the cyclic diesters of terephthalic acid
and ethylene glycol, it is only in recent years that it has become a standard method in oligomer research. This is due
firstly to further refinement of the a p p a r a t u ~ ~and
~ ~also
.~~~,
to the fact that since that time reliable data on the fragmentation of the various classes of compounds in the mass
spectrometer have become available. This enables the
structure of the higher oligomers to be clarified even
2
- x lo6
E
( E : end-group content in meq/kg; M : molecular mass in g/mol)
M
=
In addition, the modified oligomers can be tested for homogeneity using the methods described previously. An
example of such a procedure is the reaction of ethylene
terephthalate oligomers (9)-(12) containing hydroxy
groups with 4-nitronaphthyl 1-isocyanate (8),as described
by Nissen"' bl.
(81
( 9 ) , R 2 = H;
OXH2-CHz-O-CO
O-C'H2-CHz-OH
--+
O-C H2-C H2-O-C 0
R' =
@
( l o ) , R 2 = CH3
0 - C H2-C H2-O-C 0-NH-R'
( l l ) , x = 1; ( I ? ) , x = 2
NO?
though they are completely fragmented in the mass spectrometer and hence give no molecular ion.
Determination of the molecular mass by identifying the
relevant molecular ion is especially applicable to the cyclic
oligomers, which can be sublimed without decomposition.
This technique has been used successfully for the cyclic
dimer[36b1
and t ~ i r n e r of
~ ~ethylene
~ ~ l terephthalate, the cyclic oligomers of butylene terephthalate up to the tetramer[23,37b1
(cf. Table 2) and the cyclic monomeric and oligomeric amides of various polyamides (Qiana-typeC6la1,
polyamide 6 - t y ~ e @ ' ~
polyamide
I,
4-, 7-, 12- and 4,lO-, 6,66,lO-, 11,6-, 12,12-type[6"1. Mass spectrometry is therefore
a useful supplement to the methods of molecular weight
determination frequently used in oligomer analysis, uiz. vapor pressure osmometry and end group analysis[5z1,since
the latter methods are not universally applicable. Vapor
pressure osmometry yields values with good reproducibility if the oligomers have a cyclic structure or the endgroups are blocked with non-ionic residues, but it cannot
be used for linear oligomers with ionic end groups, e. g. for
the linear oligomer of E-aminohexanoic acid and hexamethylene adipamide with unprotected end-groups. However, for these oligomer series a number of chemical and instrumental methods of end-group determination are available, which respond specifically to amino, carboxy and
hydroxy e n d - g r ~ u p s [ ~ ~ ] .
All these methods have been developed, however, for
the corresponding polymers and therefore in some cases
they have to be modified before being used with oligomers.
Considerable information is obtained from methods of endgroup determination which are based on an "oligomeranalogous reaction", i. e. a reaction in which, according to
Staudinger's criterion, chemical modification takes place
without changing the number of repeat units. With this
technique it is not only possible to determine the molecular mass according to the following equation:
Angew. Chem. Int. Ed. Engl. 20, 831-840 (1981)
The molecular mass of the oligomers can be calculated
from the absorption of the introduced chromophore. In
addition, information about the homogeneity of the oligomers can be obtained from the melting point, elemental
analysis, and chromatographic behavior. Another oligomer-analogous reaction which has proved valuable for
analysis is the reduction of cyclic oligoamides to cyclic
oligoamines. This was first used by Spoor and Zahn[761to
determine the oligomers of polyamide 6. In the case of the
cis,truns-isomeric cycloamides (6) of the Qiana-type, this
technique proved to be especially useful for characterization since the resulting isomeric diazacycloalkanes (13)
can, in contrast to the cycloamides, be distinguished by
thin layer
For the identification of known oligomers, e . g . in extracts from technical polymers, mention should be made of
the clearly arranged table by R ~ t h e ' ~This
~ ] .lists physical
data and literature references for all oligomers which were
characterized up to ca. 1970.
Thanks are expressed to Prof: Dr. H . Zahn for introducing
the author to oligomer chemistry. The financial support of
the Minister fur Wissenschaji und Forschung des Landes
Nordrhein- Westfalen, Deutsche Forschungsgemeinschaft
(Grant No. Ro 432/2) and the Verband der chemischen Industrie is acknowledged with thanks.
839
Received: December 12, 1979 [A 385 IE]
Supplemented: August I I , 1981
German version: Angew. Chem. 93, 866 (1981)
Translated by Dr. E. Finnimore, Aachen (Germany)
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Angew. Chem. Int. Ed. Engl. 20. 831-840 (1981)
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