EVALUA
TION OF COMPOSITIONAL DISTRIBUTIONS OFEVALUAALUATIONYMERS BY THIN-LAYERCOPOLYMERS
THIN-LAYERSTYRENE-MALEIC ANHYDRIDE COPOL
CHROMATOGRAPHYPYROLYSISCHROMATOGRAPHY
CHROMATOGRAPHY / PYROLYSIS - GAS CHROMATOGRAPHY
SIGEHIKO YAMAGUCHI *, JIRO HIRANO and YOSHIHIRO ISODA

Nippon Oil & Fats Co., Ltd, Tsukuba Research Laboratory, Tokodai 5-10, Thukuba,
Ibaraki, 300-26 (Japan)(Received October 14th, 1988; accepted January 9th, 1989)
ABSTRACTA method for the evaluation of the compositional distributions in styrene (St)-maleic
anhydride (MAN) copolymers by thin-layer chromatography/pyrolysis-gas chromatography
is described. A St-MAN copolymer sample (average St content = 75 mole %) was used after
methyl esterification by diazomethane. The fractionated samples of the original copolymer
were pyrolyzed at 590ºC using a Curie-point pyrolyzer and the resulting products wereseparated by gas chromatography. The resulting pyrograms were interpreted in terms of thecompositional distributions of the copolymer. The St content in the samples could bedetermined using the linear relationship between the St content in the samples and [St]/({St]+[MA]) ratio (where MA - methyl acrylate) in the pyrolytic products. The fractionatedsamples having higher RF values proved to have a higher St content.
Gas chromatography; maleic anhydride; polymers; pyrolysis; styrene; thin-layer chromatography.

INTRODU CTION
Pyrolysis-gas chromatography (Py-GC) has been widely used for thestructural characterization [1,2] and the compositional analysis [3,4] ofvarious polymeric materials.
For the characterization of polymers it is important to evaluate not only
the molecular weight but also the compositional distribution. Compositional
distribution has been evaluated by thin-layer chromatography (TLC) [5] or
gel-permeation chromatography (GPC) [6,7]. However, there is a need toestablish a direct and simple method to analyze the composition offractionated samples by other chromatographic methods. In these cases,
however, the sample size of each fraction is quite limited - often being less

t h a n s u b - m i l l i g r a m q u a n t i t i e s . I n t h i s p a p e r, P y - G C i s a p p l i e d t o t h ecompositional analysis of samples fractionated by TLC.
Styrene (St)-maleic anhydride (MAN) copolymer is important in the fieldof polymer conjugation drugs. The antitumor activity of neocarziostatin
conjugated with St-MAN copolymer has been noted recently [8], and theestablishment of a method of characterization was required. In the polymerization process, St and MAN monomer units are known to form chargetransfer complexes [9], and an alternating copolymer is synthesized from anequimolar mixture of the monomers. When the number of St units is larger
than that of MAN monomer units, excess St units combine with each otherto form blocks of St units in the polymer chain [10]. In this case, thecompositional distribution needs to be determined for a given batch of thecopolymer.
It is not easy to apply Py-GC to the analysis of polymers which havecarboxylic groups owing to the low pyrolytic recovery of the associatedmonomer units [11]. Therefore the St-MAN samples were esterified (toform methyl esters) prior to Py-GC analysis.
The MAN unit in the sample is easily converted to maleic acid if it is notesterified. In this study the St-MAN copolymers were esterified prior toTLC. Furthermore, the samples fractionated by preparative TLC wereanalyzed by Py-GC in order to elucidate the compositional distribution of
the original polymers.
EX PERI MENTAL

SampleSt-MAN copolymers (St content = 50, 67, and 75 mole %) obtained from
Arco Co. were used without further purification. The methyl esterification of
the sample was carried out according to a conventional procedure [12]. Theoriginal sample was refluxed in methanol and esterified with diazomethane.
The esterified samples were prepared for subsequent Py-GC analysis.

Pyrolysis-gas chromatographic conditions
An inductive heating Curie-point pyrolyzer (Japan Analytical Industry,Model JHP-2) was attached to a gas chromatograph (Hitachi 163). Less than
0.1 mg of a sample was wrapped with a piece of foil, the Curie temperatureof which was 590 º C, and subjected to pyrolysis. A glass capillary column(50 m ×0.25 mm I.D., Sperco Co., SOFA HL, FFAP) was used for the gas
chromatographic separation. Nitrogen was used as the carrier gas at aflow-rate of 1 ml/min in the capillary column with a 92 : 1 split. Scavenger
gas at 60 ml/min was added at the column outlet just before the flameionization detector. The column temperature was maintained at 100 º C.

Identification of the peaks was carried out by a Py-GC/mass spectrometric (MS) system (JEOL, JMS-DX3O3), in the electron impact mode (70eV).

TLC conditions
An Iatroscan (Iatron, TH-10) system with a flame ionization detector wasused to obtain a thin-layer chromatogram of the fractionation of thesamples. The TLC separation of the samples was carried ou with rods coatedwith silica gel (Chromarod S) using chloroform-ethyl acetate (8 : 2, v/v)
solvent.
A preparative TLC plate (Merck, silica gel 60 F-254) was used for the
preparation of the sample fractions under the same separation conditions asmentioned above. The main components of the original sample were developed between the RF values of 0.5-0.75 on the TLC plate. Then threefractions were prepared at RF=0-0.05, 0.5-0.7 and 0.7-0.75. The
fractionated samples were extracted with acetone, and the extract was thensubjected to Py-GC analysis.
RESULTS AND DISCUSSION

Compositional analysis by Py-GC
The pyrogram of the St-MAN copolymer after methyl esterification isshown in Fig. 1. In the conventional splitting mode operation, the sample
composition actually entering the capillary column sometimes differs fromthe original one depending on the volatility of each component [13]. Therefore, in this work only the low boiling point pyrolytic products weresubjected to analysis. The main pyrolytic products of the sample aremethanol, methyl acrylate (MA), toluene and styrene.
These products are associated with the corresponding chemical structures
shown in Fig. 2. MA and methanol, and St and toluene are formed fromMAN and St units in the sample, respectively. In Table 1, the proportions ofthese four main pyrolytic products are shown for three copolymer samples(St content = 50, 67 and 75 mole %). As the number of St units increases in
the St-MAN copolymer, the content of St in the pyrolytic products increases, while that of methanol, MA and toluene decreases. The reason why

toluene decreases as the number of St units increases is believed to be thatthe pyrolytic yield of toluene from St-MAN dyad in the polymers is higher
than that from the St-St dyad.
Tsuge et al. have observed that, in the case of St-MA copolymers, there islittle boundary effect in the dimer products, therefore the dyad concentration in the polymer can be evaluated directly using the associated dimer
concentration in the pyrolytic products [14]. In this study, as shown in Fig.
3, we found that there is a linear relationship between the [St]/([St] + [MA])ratio in the pyrolytic products and the St monomer unit concentration in thesamples. The composition of St-MAN copolymers can be determined byusing the linear relationship presented in Fig. 3. The standard deviation andthe coefficients of variation of [St]/[MA] ratio for five runs are 1.8 and
5.7%, respectively.

Py-GC evaluation on TLC separation
The sample (St content = 75 mole %) was developed by preparative TLC
and the three fractionated samples (Nos. 1-3), which developed at different
R F values on TLC plate (No. 1 = 0-0.05, No. 2 = 0.5-0.7, No. 3 = 0.7-0.75)
were prepared for Py-GC analysis. The chromatograms of the preparatively
fractionated samples measured by the Iatroscan are shown in Fig. 4. The RFvalues of sample No. 1 range very widely between 0 and 0.8, and those ofsamples No. 2 and 3 are about 0.55-0.90 and 0.75-1, respectively. In Fig. 5,
the relationships between the St content in the samples and the associatedR F values are shown. The St content is determined by Py-GC using therelationships shown in Fig. 3. The result indicates that the samples having
higher St contents have higher PF values. The compositional distributions ofSt-MAN copolymer can therefore be evaluated using the chromatogram
shown in Fig. 4. Thus, the compositional distributions of polymers caneasily be evaluated by TLC/ Py-GC.