Journal of Analytical and Applied Pyrolysis, 2 (1980) 207-215
Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
PYROLYSISIAS CHROMATOGRAPHIC BEHAVIOUR
OF HEAD-TO-HEAD POLYSTYRENE AND HEAD-TO-HEAD
POLY(α-METHYLSTYRENE)
MINORU TANAKA, TATSUHISA SHIMONO, YOSHIKAZU YABUKI andTOSHIYUKI SHONO *Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadakami, Suita, Osaha 565 (Japan)(Received April 16th, 1980)
SUMMARY
Pyrolysis-gas Chromatographic investigations of head-to-head polymers of styrene anda-methylstyrene were carried out. The monomer yields of the head-to-head polymers arelower than those of the head-to-tail polymers. The pyrolysis products that directly reflect
the head-to-head linkages are produced on pyrolysis of head-to-head polymers. By the useof these pyrolysis products, the content of the head-to-head linkages was evaluated forpolystyrene, and the difference in the degradation mechanisms between the head-to-headand the head-to-tail polymers is discussed.
INTRODUCTION
In the conventional free-radical polymerization of asymmetric vinyl
monomers, head-to-tail (H-T) linkages normally predominate. It has beenreported that a substantial amount of the alternative head-to-head (H-H)
linkages [and also tail-to-tail (T-T) linkages] exist in certain polymers, suchas poly(vinyl fluoride) and poly(vinylidene fluoride) [1-3] , although a moreusual level is about 597o or less in other polymers. The importance of theinfluence of such structural variations on polymer properties is well established. A number of methods have been suggested that lead to chains withpredominantly H-H linkages and the properties often differ significantly
from those of normal H-T polymers (i.e., glass transition temperature, stability characteristic, flexibility, etc.) [4-11].
It is therefore important to evaluate the structural variations. Pyrolysisgas chromatography (Py-GC) is a useful method for the analysis of polymer
microstructures. In our previous work, the amounts of two units (1,2- and1,3-) existing in cationical1y polymerized poly(3-methy1-1-alkenes) wereevaluated by Py-GC [12]. In this work, the differences in the Py-GC
behaviour of the H-H and the H-T polymers of both styrene and ct-methylstyrene were studied and the amounts of the H-H and the H-T linkages inpolystyrenes were evaluated.

EXPERIMENTAL

SamplesHead-to-head polystyrenes (H-H PSt I and H-H PSt II) were synthesizedaccording to the methods reported by Inoue et al. [10] and Richards andScilly [13], respectively. H-H PSt III-VIII were obtained by hydrogenationof styrene-2,3-diphenylbutadiene copolymers with various compositions.
The contents of the H-H linkages of H-H PSt III-VIII were evaluated fromthe UV spectra (absorbance at 250 nm) of the copolymers before hydrogenation. H-T polystyrenes were prepared by the usual polymerization techniques.
H-H poly(α-methylstyrene) (H-H PMSt) was synthesized by the method
used for H-H PSt II [10], and H-T PMSt was obtained by anionic polymerization with potassium metal.
The H-H contents of the polymers and the polymerization conditions aregiven in Table 1.
Apparatus
A Curie-point pyrolyzer (Japan Analytical Industry JHP-2) was coupleddirectly to a gas chromatograph (Yanaco GC-180) with dual flame-ioniza-

tion detectors. The packed column used was a 2 m X 3 mm I.D. stainlesssteel tube containing 60-80 mesh Celite 545 coated with 10% PEG 20M orwith 10% silicone DC 550. The column temperatures were 80℃ or 200℃for the former and 185℃ for the latter. The flow-rate of the carrier gas(nitrogen) was 40 ml min-1.
A Shimadzu GC-6AM gas chromatograph was used for the analysis,equipped with a 40 m X 0.3 mm I.D. support-coated open-tubular (SCOT)
column with OV-17 as the stationary phase purchased from Shimadzu(Kyoto, Japan). The column temperature was increased from 50℃ to 200℃
at a rate of 6℃ min-1. The nitrogen flow-rate was 0.2 ml min-1.
The peak areas. were integrated by a Shimadzu Chromatopac E-1A integrator. Identification of the peaks on the pyrograms was carried out bycomparison of their retention data with those of known substances and/or
by Py-GC-mass spectrometry (Shimadzu PYR-10A and GC-MS-7000). Thedimers (except for 1,2diphenylethane) were prepared in our laboratory.
RESULTS AND DISCUSSION

Polystyrenes
When polystyrenes were prepared in the usual manner, the styrene monomer, the dimer (2,4-dipheny1-1-butene) and the trimer (2,4,6-triphenyl-1he3tene) Were observed as the major pyrolysis products. In this work, thepyrolysis products of monomer and dimer regions from polystyrenes having

H-H and H-T linkages were considered but trimers were not.
Figs. 1 and 2 show the pyrograms of H-H and H-T polystyrenes. Thepyrograms in Fig. 1 show the pyrolysis products with one benzene ring andthose in Fig. 2 show the pyrolysis products having two benzene rings whichcorrespond to the dimers. The difference between the H-H and the H-Tpolymers appeared on their pyrograms in both the monomer and the dinerregions. The H-H polymers produce compounds other than the styrene
monomer (i.e., toluene, ethylbenzene, n-propylbenzene, etc.) in relatively
large amounts, compared with the H-T polymer. The pyrolysis products
corresponding to the dimers (i.e., 1,2-diphenylethane, 1,4-diphenylbutane
and 1,4dipheny1-1-butene) which reflect directly the H-H structures areidentified as shown in Fig. 2. The pyrogram of poly(2,3-diphenylbutadiene)
is also shown in Fig. 2. For this polymer, the 2,3-diphenylbutadienemonomer is the major pyrolysis product. H-H polystyrenes prepared byhydrogenation of the 2,3diphenylbutadiene units have hardly any double
bonds in their main chains, because very little 2,3-diphenylbutadiene is observed in the pyrolysis products.
From the relative peak area ratios of the dimers, the pyrogram of H-H pst

II is considered to be intermediate between those of H-H Pst I and H-T Pst.
This is illustrated in Fig. 1. It is assumed that H-H Pst II is not entirely composed of the H-H linkages, as Inoue et al. [10] reported. This result was supported by the lH and 13c NMR spectra of these three polymers (not shown).
All.H-T polystyrenes prepared by various polymerization techniques show
similar pyrograms and the relative amounts of the pyrolysis products arenearly identical. Even with H-T Pst obtained by radical polymerization only
small amounts of the H-H dimers are produced. This fact suggests theexistence of small amounts of H-H linkages in H-T Pst.
The lower monomer yield of H-H Pst I than that of H-T Pst results fromthe difference in their degradation mechanisms. By random main-chain scissions, the two radicals of (1) and (2) are produced from pure H-T Pst andthe three radicals of (3), (4) and (5) from completely H-H structured Pst.
The depolymerization of (1) to the monomer may be smoother and more
continuous than that of (3), which is predominantly produced from the H-H
linkages, because the secondary radical of (3) may "unzip" one monomer
unit, with the resulting production of a relatively unstable primary radical.
In addition, because of reproducing a relatively stable secondary radical, it isenergetically favourable for (3) to give the dimers such as 1,4-diphenylbutane, 1,4diphenyl-1-butene, etc.; the H-H polymer produces a large
amount of dimers. From the results obtained from differential thermogravimetry and thermal gravimetric analysis, Helbig et al. [14] have stated that
there is no significant difference in thermal stability between the H-H andthe H-T polymers owing to the lack of the fourth substituent on the carbon
atom of the H-H linkages in Pst. However, differences in the distributionof the pyrolysis products on the pyrograms between H-H Pst and H-T Pst
can apparently be observed and be qualitatively interpreted as mentioned
above. The relative peak area ratios for H-H Pst II are closer to those for
H-T Pst than to those for H-H Pst I. This Py-GC result agreed well with theNMR results.

Polymers with various compositions of H-H and H-T linkages were prepared by hydrogenation of the copolymers of styrene and 2,3-diphenyibutadiene. Their pyrograms are similar to that of H-H Pst II, although therelative amounts of the pyrolysis products differ according to the polymer.
There is a serious problem to be solved. In Fig. 2, the H-T dimer of 2,4diphenyl-1-butene (3) and the H-H dimer with M+ = 208 which cannot beidentified (4) have identical retention times measured on packed columns ofsilicone DC 550 and other commonly used stationary phases. It is impossible
for completely H-H structured Pst I to produce 2,4-diphenyl-1-butene onpyrolysis.
In order to separate the peaks of 2,4-diphenyl-1-butene and the unidentified H-H dimer, the glass capillary column was used. Fig. 3 shows typicalpyrograms of H-H Pst I, H-H Pst VI which has ca. 50% of H-H and H-Tlinkages, and radically polymerized H-T Pst obtained by the use of thecapmary column at a pyrolysis temperature of 358℃. The peaks of 2,4dipheny11-butene (3) and the unidentified H-H dimer (4) are separated well.
The fraction of the peak area of unidentified H-H dimer in the total peakareas of these two pyrolysis products is plotted against the H-H content ofeach Pst in Fig. 4. The content of H-H linkage means the value of the fraction of H-H and T-T linkages in all linkages, that is, (H-H + T-T)/(H-H +T-T + H-T). This peak area ratio of H-H Pst I1 (closed circle in Fig. 4) showsthat the content of H-H linkages in this polymer is ca. 18%. This is a value
comparable to that estimated from the 'H NMR spectrum. The peak area
ratios for all H-T polystyrenes were nearly zero. On the pyrograms, however,the H-H dimers are clearly observed although their amounts are very small. Itis concluded that these H-T polystyrenes do not consist of perfect H-Tlinkages but that the H-H contents of these H-T polymers are below 1%.

Poly (α-methylstyrene)
Fig. 5 shows the pyrograms of H-H and H-T poly(α-methylstyrenes) obtained at a pyrolysis temperature of 434℃. This H-H polymer is composed
entirely of H-H linkages as judged by the comparison of its 13C NMR spectrum with that of the H-T polymer, shown in Fig. 6. With poly(α-methylstyrenes), the monomer yields are higher than those of polystyrenes andother compounds are produced in only small amounts. The monomer yieldof H-H PMSt is smaller than that of H-T PMSt and the characteristic pyrolysis
products which reflect the H-H linkages, 2,5-diphenylhexane and 2,5-diphenyl-1-hexene, are observed in the dimer region on the pyrogram. The
major dimer from the H-T polymer is unidentified (M+ = 236, not 4-methyl2,4dipheny1-1-pentene). The difference in monomer yields may result fromthe ease of scission of the H-H linkage to produce a tertiary radical and thedifficulty in the continuous depolymerization to the monomer after therandom chain scission in the case of the H-H polymer, as described in thedegradation mechanisms of polystyrenes.
The apparent difference in the pyrograms between the H-H and the H-Tpolymers is due to the following structural factor. The H-H linkages in PMSt
are weakened by steric hindrance between the substituents of twoquatemary carbon atoms and the H-T polymer, having quaternary carbon
atoms in its main chain like PMSt, has a very high monomer yield. WithPMSt, polymers with different compositions of H-H and H-T linkages cannot
be prepared. Therefore, it is not possible to draw quantitative conclusions.