POLYMER LETTERS VOL. 4, PP. 1007-1010 (1966)

REMARKS ON THE ARTICLE O F J. L. BINDER, “CYCLIC STRUCTURES IN DIOLEFIN POLYMERS”

From his IR and NMR results, Binder contended that polymers from butadiene a s well as isoprene contain, in addition to their normal micro- structure units, cyclic structures (1). His main evidence for th i s conten- tion was based on the NMR absorption near 1 pprn which he interpreted as due to’hydrogens in fused cyclohexane rings in the cyclic structures. In the NMR spectrum of a Ziegler-type high cis-polyisoprene at high spec- trometer gain, h e observed peaks near 1 ppm which he attributed to cyclic structures in the polymer. He a l so suggested that the presence of the cyclic structure in isoprene and butadiene polymers might be responsible for the observed discrepancy from 100% in total found value of normal microstructure units of the polymers by an IR method.

We would l ike to suggest an alternative interpretation for the NMR re- su l t s as follows:

(1) In the spectra of 172-polybutadiene and 3,4-polyisoprene7 the strong peaks near 1 ppm which Binder assigned to hydrogens in fused cyclohex- ane rings are actually from the absorption of the underlined methylene hydrogens in the following structure units:

I I -CH - CH - CH = CH and -CH - CH - C(CH 3) = CH 2, respectively.

The peaks near 2 pprn of both polymers should be assigned to teritiary hydrogens. There is no observable peak due to cyclic structures in either polymer.

(2) In the ladder polymers, the original peak assignments have also to be readjusted. Strong peaks at approx. 0.8 pprn in both ladder poly- mers are actually due to methyl groups attached to the fused cyclohexane rings. The peaks originally assigned to -CH3 and -CH2- in 3,4-ladder polymer a t approx. 1.5 and 2.0 ppm are, respectively, due to the methyl- ene and tertiary hydrogens on the substituted cyclohexane rings. Because no absorption is shown in the usual olefinic region, there i s not much un- cyclic polymer left in th i s ladder polymer. With these reassignments, the NMR results do not furnish any evidence on the presence of cyclic structures in either l72-polybucadiene or 3,4-polyisoprene. I t should a l so be pointed out that the legends of Figure 3 and Figure 5 of the original article are interchanged.

(3) The peaks near 1 ppm in Ziegler-type cis-1,4-polyisoprene which Binder attributed to cyclic structures, a re really the satell i te peaks from the intense peaks of methyl and methylene groups in the polymer a s well

1007

1008 POLYMER LETTERS

TMS

I

I , . , . , . . . . l . . . . , . . . . l . . . I . , . I . . . l . . . . , . . . . l . . . . , . . . . l . . . . , . . . . I . . ,

3.0 2.0 WMIO 1.0 0

Fig. 1. NMR Spectrum of pure tetramethyl ethylene. The outer pair of satellite peaks approx. 1 ppm from the main peak is from C The other two pairs of sharp peaks are spin sideband.

coupling.

as methyl groups of the TMS reference. Th i s point deserves further ex- planation: It is well known (2) that naturally occurring carbon contains about 1.1% isotope C l 3 which h a s nuclear spin number equal to 1/2. A s a re- su l t of its associated nuclear magnetic moment, t h e absorption peak due to the hydrogen attached to the carbon will be spli t into two peaks with the separation between them approx. 120 c p s (2 ppm on 60 Mc. chart scale). Since only 1.1% of naturally occurring carbon is C13, and the res t is C ' which does not p o s s e s s a nuclear magnetic moment, the C l 3

satellite peaks are rather weak and show up only when the main peak is intense; the intensity ratio is approx. 200 to 1. Bes ides the satellite peaks from C ' coupling, the mechanical spinning of the sample used in averaging the magnetic field inhomogeneity a l so generates sideband peaks. The separation of sideband peak from the main peak i s variable and equal to the spinning frequency or multiples of it. Both effects are demonstrated in a spectrum of pure tetramethyl ethylene shown in Figure

POLYMER LETTERS

T M!

-CH,

I1 1 I

?-

1009

c I r

6 5 4 3 2 1 0 P P M

Fig. 2. Reproduction of NMR spectrum of Ziegler-type high cis-poly- isoprene from Binder’s article. The assignments on weak peaks are self-explanatory. Peak pairs with separation approx. 2 ppm are from C band.

coupling and those with separation approx. 0.7 ppm are spin side-

1. In Figure 2, a reproduced spectrum of Ziegler-type high cis-polyiso- prene from Binder’s article is given. The peaks which Binder assigned to cyclic structures are shown to originate either from C13 coupling or from spin sidebands. The reassignment of these peaks near 1 ppm makes it very doubtful that the particular sample of Ziegler-type high cis-poly- isoprene used for the spectrum contains 13% cyclic structures a s exti- mated by IR.

In view of the fact that polymers from butadiene a s well a s from iso- prene can be readily cyclized, it is plausible that, depending on their history, such polymers may contain cyclic structures. It seems prema- ture, however, to infer t h e presence of cyclic structures in polyisoprenes and polybutadienes in general.

1010

References

POLYMER LETTERS

(1) J. L. Binder, J . Polymer Sci. B, 4 19 (1966). ( 2 ) Most texts in high resolution NMR, such a s R. H. Bible, Jr.,

Interpretation of NMR Spectra, Plenum Press, New York, 1965, pp. 63-65.

Hung Yu Chen

The Goodyear Tire & Rubber Company Research Division Akron, Ohio 44316

Received June 10, 1966 Revised September 6, 1966