Analytica Chimica Acta, 96 (1978) 359-365
Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

PYROLYSIS-GAS CHROMATOGRAPHY OF BUTADIENE CO-POLYMERSTATSUHISA SHIMONO, MINORU TANAKA and TOSHIYUKI SHONO*

Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamada-kami,
Suita, Osaha 565 (Japan)(Received 17th August 1977)

SUMMARY

With progress in polymerization techniques, many polymers having
controlled sequential structures have been produced. Some of these polymers
have specific properties resulting from their unique microstructures. Typicalexamples are the alternating vinyl co-polymers. The alternating isobutylenealkyl acrylate co-polymer is superior to acrylic rubbers in its resistanceto hydrolysis and heat [1]. Further, the alternating styrene-acrylonitrile
co-polymer has a high softening point and excellent mechanical properties
compared with the random co-polymers [2]. It is therefore important toinvestigate the microstructures of polymers. Pyrolysis-gas chromatography(p.g.c.) is one of the most powerful methods for the investigation of microstructures; p.g.c. has been applied successfully to the analysis of chain
branching in polyethylene or poly(vinyl chloride) and shown to be aneffective method of determining the short-chain branch content [3, 4].
The pyrolysis-gas chromatographic behaviours of polybutadienes [5]and butadiene-acrylonitrile [6] co-polymers have been reported. In thiswork, the relationships between the pyrolysis products and the microstructures for butadiene co-polymers (co-monomer = acrylonitrile, methacrylonitrile, methyl methacrylate, methyl acrylate, and methyl vinyl ketone) wereinvestigated by p.g.c. and the run number of each co-polymer was evaluated.

XPERIMENTAL

Materials
Alternating co-polymers of butadiene (BD) with methyl methacrylate(MMA) [7], methyl vinyl ketone (MVK) [8], methyl acrylate (MA) [9],

methacrylonitrile (MAN) [10] , and acrylonitrile (AN) [10] were synthesizedby complexed co-polymerization with Lewis acids. Random co-polymers
with different composition were prepared in low yield by radical polymerization with azobisisobutyronitrile as initiator.
Table 1 shows the co-polymer compositions calculated from elementalanalysis.

Apparatus
A Curie-point pyrolyzer (Nippon Bunseki Kogyo JHP-2) was coupleddirectly to a gas chromatograph (Shimazu GC-4BM) equipped with a dual
flame-ionization detector. Samples of 30-60μg and a stainless steel column(2 m X 3 mm i.d.) packed with 20% DOP on 60-80 mesh Celite 545 support
were used. The column temperature was 80℃. The flow rate of nitrogen
was 40 ml min-1; the hydrogen and air pressures were 0.5 kg cm-2 and1.0 kg cm-2, respectively. A column (5 cm X 3 mm i.d.) packed with amixture of K2CO3 and Diasolid H (60-80 mesh, 1 + 2, w/w) was placed
between the pyrolyzer and the separation column to precut acidic products
from nitrile co-polymers.
The peak areas were measured by a digital integrator (Shimazu ITG-4A).
Identification of the peaks was carried out by comparison with the retention
data of known substances.

RESULTS AND DISCUSSION
The BD monomer and the cyclic BD diner, 4-vinylcyclohexene (VCH),
obtained from successive 1,4-BD units are well known as the main pyrolysis
products of polybutadienes; the BD units are mostly linked 1,4 in polybutadienes prepared by conventional radical polymerization. The relationshipbetween the pyrolysis products (mainly VCH, BD, and co-monomer of BD)
and the monomer sequence distribution in some co-polymers is reported inthis paper.

Pyrogram and composition
Figure 1 shows the pyrograms of MMA-BD co-polymers at the pyrolysis
temperature of 590℃. Under the p.g.c. conditions used, lower hydrocarbons,BD, MMA, and VCH were the main pyrolysis products. There is a difference
between the pyrograms of alternating and random co-polymers; the VCH
peak is barely detectable in the pyrogram of the alternating co-polymer. Mostof the MMA units in MMA-BD co-polymers pyrolyze into MMA monomer
as in poly(methyl methacrylate). The yield of MMA at 590℃ was plottedagainst the MMA content of the co-polymer, where the yield is taken as theMMA mole fraction in the pyrolysis products, i.e., MMA/(MMA + BD + 2VCH)
as one mole of VCH corresponds to two moles of BD. A very good linear
relationship was obtained; this MMA yield therefore indicates the composition of MMA-BD co-polymers, including the alternating one. Fromleast-squares regression analysis, the regression line of the MMA contentof co-polymer (y) on the MMA yield described above (x) was y = 1.06x 9.93, and the error variance (Vyx) was 7.02.
The pyrolysis-gas chromatographic behaviour of MAN-BD and MMA-BD
co-polymers are closely similar. The yield of MAN, MAN/(MAN + BD + 2VCH),
indicated the composition of the co-polymers. In MVK-BD co-polymers, asmall quantity of acetone was formed in addition to lower hydrocarbons,.
BD, MVK, and VCH.
Polyacrylonitrile exhibits complicated pyrolytic reactions, i.e., eliminationof hydrogen cyanide or cyclization involving the C≡N group. Large quantities

of both acetonitrile and lower hydrocarbons and quantities of the monomers
were formed on the pyrolysis of AN-BD co-polymers. Figure 2 shows thepyrogram of the alternating AN-BD co-polymer at 590℃. Porapak N wasused as a column packing, because the separation of AN from acetonitrile
was impossible with the DOP column. The mole fraction (AN + acetonitrile)/(AN + acetonitrile + BD + 2VCH) in the degradation products indicated thecomposition of these AN-BD co-polymers.
MA-BD co-polymers gave the monomers, and also a great deal of methanolfrom scission of the side-chain. This phenomenon is especially notable at thelow pyrolysis temperature. Figure 3 gives the pyrograms of the alternating
MA-BD co-polymer pyrolyzed at 590 and 710℃. The production ofmethanol decreased greatly at 710℃; the yield MA/(MA + BD + 2VCH)
at 710℃ indicated the composition of both the alternating and the random
MA-BD co-polymers.
Table 2 shows the results of regression analysis for five BD co-polymers,
where y is a co-monomer content of BD co-polymer and x is a co-monomer
yield on pyrolysis. It is reasonable that the MA-BD co-polymer, with thehigh tendency to undergo side-chain scission, has the large error variance, Vyx
VCH/BD molar ratio and run number
Figure 4 shows the VCH/BD molar ratio in the degradation products ofeach MMA-BD co-polymer pyrolyzed at 386, 590, and 710℃. The otherfour BD co-polymers also gave the same tendency as the MMA-BD copolymer. The VCH/BD molar ratio changes with the pyrolysis temperatureand the co-polymer composition in the case of random co-polymers. Thefact that in the case of the alternating co-polymers the values of the VCH/BDmolar ratio are nearly zero may be interpreted as follows. The pyrolysis
temperature dependence of the VCH/BD molar ratio results from thenumber of sites where a random chain scission occurs. The number ofrandom chain scissions increases with increase in pyrolysis temperature.
Consequently, the higher the pyrolysis temperature, the smaller the probabilityof VCH production. The dependence on the co-polymer composition isa result of the difference in BD sequence distribution. Because the numberof successive BD units decreases with decreased BD content in the random
co-polymer, the lower the BD content, the smaller the probability of VCH
production. The alternating co-polymer contains only BD-co-monomer
units in its chain, and has a VCH/BD ratio of nearly zero.
The relationship between this VCH/BD molar ratio and the BD sequence
distribution was further investigated. Let PBBB, PBBM, PMBB, and PMBM
represent the probabilities of a BD unit in the center of triads BD-BD-BD,
BDMBD-M, M-BD-BD, and M-BD-M, respectively, where M is a comonomer. If the probability of the BD unit in the center of each triad
degrading to products other than BD and VCH is constant, and if the BD
units in the center of BD-BD-BD, BD-BD-M, and M-BD-BD triads
degrade to VCH and BD in the ratio k1/(1 -kl), k2/(1 -k2), and k2/(1 -k2),
respectively, theVCH/BD molar ratio, Q, can be given as

In this equation, each P value is calculated from the monomer reactivity
ratios shown in Table 3 and the monomer feed ratio [11], and Q is estimatedfrom the pyrograms. The values of k1 and k2 Can therefore be calculated.
For the pyrolysis of MMA-BD co-polymers at 590℃, the calculated Q
values when k1 = 0.11 and k2 = 0.067 in eqn. (1) agreed very well with theQ values obtained from the pyrograms; the correlation coefficient was 0.997.
Table 3 gives the values of

k1 and k2 for five BD co-polymers. All the kl

values are nearly 0.1, as is k1, for polybutadiene obtained by conventional
radical polymerization. The value of k2 decreases in the order MMA-BD >MAN-BD > AN--BD > MVK-BD > MA-BD; this order may result fromthe stability of the radical produced after chain scission at position (1)shown below.

The tertiary radical is produced in the cases of MMA- and MAN-BD copolymers (R = CH3) and the secondary radical in the other three co-polymers(R = H). Therefore, the scission at this position is easier in the former andthe k2 Values (the probability of the dimer formation) become larger thanin the latter.
The run number is a parameter [12] for characterizing sequence distribution in co-polymers. The run number is 1 for a homopolymer and 100
for a completely alternating co-polymer; eqn. (1) can be rewritten as

where R = run number, and X = BD mole%. When the values of kl and k2
are known in eqn. (2), the run number of a BD co-polymer can be calculated
by using the values of Q and X obtained from the pyrogram. The run numbersof five BD co-polymers are shown in Table 4. Each alternating co-polymer
has an R value near 100 and the alternating sequential structure of each is
confirmed; the difference between the alternating co-polymer and therandom one with the composition of ca. 50 mole% is obvious.
As described above, the VCH/BD molar ratio in the pyrolysis productsof BD co-polymers indicates the sequence distribution of co-polymers, andthe run number can be determined from it; the run numbers obtained werereasonable for the co-polymers studied.