Analytica Chimica Acta, 74 (1975) 119--124
c Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
PYROLYSIS--GAS CHROMATOGRAPHY OF VINYL
CHL.ORIDE-UMETHYL METHACRYLATE AND VINYLCHLORIDE-ACRYLONITRILE COPOLYMERS
M. TANAKA. F. NISHIMURA and T. SHONO
Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadakami, Suita, Osaka
565 (Japan)(Received 11th June l974)
Progress in polymerization techniques has gradually made it possible tocontrol the sequential structures of polymers. A typical example is the complexedcopolymerization1 of vinyl compounds with alkylaluminum halides or related metalhalides. This complexed copolymerization gives equimolar copolymers with analternating sequential structure. Some or these alternating copolymers have excellent
properties2.3, because of the regularity in their structural units. In the future, thesequential structure of polymers may be more easily controlled, and it must
become increasingly important to analyze the microstructure. Pyrolysis--gaschromatography is one of the most powerful methods for the analysis of themicrostructure of polymers.
The pyrolysis--gas chromatographic behaviours of cis-1,4-, trams-1,4- and1,2-polybutadienes4 and of random and alternating copolymers of acrylonitrile andbutadiene have already been reported5. In this paper, the relationships between
pyrolytic products and microstructures have been investigated by means ofpyrotysis-gas chromatography for copolymers of vinyl chloride with methyl methacrylate
or acrylonitrile.
EXPERIMENTAL

Materials

Random copolymers of vinyl chloride (VC)-methyl methacrylate (MMA)
and of VC-acrylonitrile (AN) were synthesized by bulk polymerization with azobis(isobutyronitrile) as initiator. Alternating VC-MMA copolymer6 was preparedin the presence of ethylaluminum dichloride-vanadium oxytrichloride, andalternating VC-AN copolymer7 in the presence of ethylaluminum dichloride byestablished methods.
Table I shows the copolymer compositions calculated from the elementalanalyses.

Apparatus and technique

A curie-point pyrolyzer (Nippon Bunseki Kogyo JHP-2) was used todecompose the polymers. This gives reproducible pyrograms because the fast temperature rise (curie-point of the ferromagnetic wire) results in more specific products

and any contamination from previous samples can be avoided by using a newwire each time. The pyrolyzer was coupled to a gas chromatograph (ShimazuGC-4BM) equipped with a dual flame-ionization detector. The samples were 10-20μg
In size.
The separation column was stainless steel tubing (75 cm x 3 mm i.d.), packed
with Porapak N (100--120 mesh). A column (5 cmx3 mm i.d.) packed with amixture of K2CO3 and Diasolid H(60--80 mesh) in a 1:2 weight ratio was placed
between the pyrolyzer and the separation column to precut acidic products onpyrolysis. The separation column was operated at 170℃ and the detector block
was maintained at 220℃. The flow rate of carrier gas (nitrogen) was 30 ml min-1
and the hydrogen and air pressures were 0.5 kg cm-2 and 1.0 kg cm-2
respectively. The peak areas were measured by a digital integrator (ShimazuITG-4A).
Identification of the peaks was carried out by comparison with the retention
data of known. substances and/or by direct combination of the pyrolysis-gas
chromatograph with a mass spectrometer (Nippon Bunko-Finnigan GC-95003100D)
RESULTS AND DISCUSSION

VC-MMA copolymer

Zutty and Welch8 reported that when VC-MMA copolymers are heated at150-200℃, an intramolecular lactonization from adjacent co-monomers occurs,
forming r,-butyrolactone groups in the polymer backbone concomitant with thequantitative elimination of methyl chloride (eqn, 1).

The extent of this cyclization depends on the arrangement of the monomer
units in the copolymers. The copolymers listed in Table I were heated undervacuum at 200℃ until lactonization was complete, and no further weight loss wasobserved. From this weight loss, the fraction of the methyl methacrylate units
cyclized, f c(MMA), was calculated. These experimental f c (MMA) values were ingood agreement with the theoretical ones given from a statistical treatment9.10
based on the expected number of adjacent MMA and VC residues, as shown in
TableII.
Figure 1 shows typical pyrograms of random (R2) and alternating VC-MMA
copolymers (pyrolysis at 541 ℃). The yields of MMA monomer, methyl chloride

and methanol on the pyrogram are summarized in Table m, and expressed interms of mole percentage to MMA in the copolymer. In the case or the randomcopolymers, it is reasonable that the MMA yield decreases with the decrease ofMMA content in the sample. The yields of methyl chloride and methanol reachmaxima near 50 mole-%. of MMA content. But the alternating copolymer gives
only negligible amounts of MMA and methanol, though it yields methyl chloridein amounts between those of R2 and R3.
These results may be interpreted as follows. The alternating copolymer
contahs only a VC-MMA unit in its chain, and has a larger fc(MMA) value
than the random copolymers with compositions close to the alternating one, as
shown in Table II. It is therefore assumed that the production of MMA monomerfrom the alternating copolymer is reduced because of both the absence of theunzipping reaction from the successive MMA unit and the lesser extent of theuncyclized MMA units. There is no difference in the methyl chloride yields of therandom and alternating copolymers, hence methyl chloride must be produced byother mechanisms predominantly occurring in the random copolymer in addition
to the lactonization reaction. Reactions (2) anc1 (3) were proposed by McNeil andNeilll from the results of the pyrolysis for PVC- PMMA blends, but they could
not confirm the production of methanol. In the present tests of pyrolysis at 541℃,
the 1:1 PVC-MMA blend gave methyl chloride, methanol and MMA in molar
ratios of 0.393, 0.166 and 1.000, respectively; but methyl chloride and methanol
were obtained from neither PVC nor PMMA. As shown in eqns. (3), methanol isgiven by the attack or hydrogen chloride from dehvdrochlorination of VC units
on the MMA--MMA unit together with equal amounts of methyl chloride. If themethanol yield (TableⅢ) is considered, the contributjor1 of eqn. (3) to theformation of methyl chloride cannot be neglected in the case of the random
copolymers. It was found, as mentioned above, that the microstructural differences
are reflected on the pyrograms.

VC-AN copolymer

The thermal degradation of polyacrylonitrile and acry]onitrile containing
copolymers has been studied extensively12--14, and many kinds of degradation
products including dimers have been found. In the present work, the random andalternating VC-AN copolymers listed in Table I were pyrolyzed at 536℃, but did
not give diners as degradation products (Fig. 2). The main products of the random
copolymers are AN monomer and acetonitrile. Figure 3 shows the relationshipbetween the molar ratio of acetonitrile to AN monomer on the pyrogram andthe VC content or the copolymer. This ratio increases suddenly as the VC content
exceeds 50 mole-%. It is suggested that AN monomer is produced from AN--AN
units at least. In fact, the alternating copolymer having only a VC--AN unit gives
no peak of AN monomer on the pyrogram, but only acetonitrile.
It is interesting to note that methyl chloride is obtained from the alternating
samples as a main product but hardly at all from the random samples. The
alternating copolymer gives a resinous solid on heating at 320℃. for 3 min undera nitrogen atmosphere. When this resinous solid is pyrolyzed at 536℃, the main
product is acetonitrile and only a little methyl chloride is formed. It is assumed
from this result that methyl chloride is eliminated first, and that the residual
substance has a structure from which acetonitrile is produced easily on pyrolysis.
Further work is being done to elucidate the specific degradation mechanismof the alternating VC-AN copolymer.
SUMMARY
Pyrolysis-gas chromatographic investigations have been carried out on
VC-MMA and VCAN copolymers with a curie-point pyrolyzer. The pyrograms
distinctly show the difference in sequential structures of these copolymers. MMA
monomer and methanol are scarcely formed from the alternating VC-MMAcopolymer; the alternating VC-AN polymer yields mainly methyl chloride, which
is not obtained from the random samples. The microstructure of copolymers in
relation to their degradation products is discussed.