Grant

The Infra-red Spectrum of Syndiotactic Polypropylene I. J. GRANT and I. M. WARD The infra-red spectrum of syndiotactic polypropylene is reported, and comparison made with that of the isotactic polymer. It is concluded that several o f the absorption bands in the range 1 500 c m - ~ to 200 c m - 1 are characteristic of the portions o f chains in the syndiotactic helical configuration, and therefore give an empirical measure of the stereoregularity.

IN A previous publication 1 the infra-red (i.r.) spectra of isotactic and atactic polypropylene were described. Assignments for the principal vibrations were attempted following the preliminary analysis of the isotactic polypropylene spectrum proposed by PeraldoL Similar attempts have also been made by Krimm ~, Tobin 4, and Liang, Lytton and Boone 5. With the exception of Tobin it is assumed that the multiplicity of absorptions arises from the interactions between modes of vibration of different units within a chain molecule rather than between units belonging to different chain molecules. The present communication describes the measurement of the i.r. spectrum of syndiotactic polypropylene and considers the general assignment of absorptions in both the isotactic and syndiotactic polymers. EXPERIMENTAL

Preparation of samples

The spectrum of isotactic polypropylene was obtained' from a standard commercial grade moulding powder and the atactic polymer spectrum was obtained from a polymer prepared with a non-stereospecific catalyst. The syndiotactic spectrum was obtained from an experimental sample of polymer prepared by a method similar to that described by Natta et al?. To confirm that this polymer was primarily syndiotactic high resolution proton magnetic resonance (p.m.r,) spectra and X-ray diffraction spectra were obtained. Figure 1 is a high resolution p.m.r, spectrum of the syndiotactic polypropylene used for the i.r. spectra. This p.m.r, spectrum was obtained from a 15% w / v solution of the polymer in orthodichlorobenzene at 140°C using a Varian Associates A60 spectrometer. This spectrum shows the presence of equivalent methylene protons only, as would be expected for syndiotactic polymer 7, the more complicated ABC2 type spectrum for isotactic polymer being absent within the sensitivity of the spectrometer. In a similar manner the X-ray diffraction photographs showed reflections identical to those described by Natta et al. 8 for syndiotactic polymer and complete absence of reflections expected for isotactic polymer. Measurement of i.r. spectra

The i.r. spectrum of isotactic polypropylene was recorded using films of thickness 0.15mm to 1.0mm. The i.r. spectra of the syndiotactic and 223

I. J. G R A N T and I. M. WARD

Figure 1---60 Mc/s proton magnetic resonance spectrum of a 15 per cent w/v solution of syndiotactic polypropylene in or tho-dichlorobenzene at 140"C

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Figure 2--Infra-red spectra of the 3 I00 cm - t to 2700 cm -1 region of: (a) Syndiotactic polypropylene, 25"C; (b) Isotactic polypropylene, 25"C; (c) Molten syndiotaetic polyproylene; (d) Molten isotactic polypropylene; (e) Atactic polypropylene, 25" C

THE INFRA-RED SPECTRUM OF SYNDIOTACTIC POLYPROPYLENE I00

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Figure 3--Infra-red spectra of the 2000 crn-1 to 666 cm-1 region of: (a) Syndiotactic polypropylene, 25°C; (b) Isotactic polypropylene, 25°C; (c) Molten syndiotactic polypropylene; (d) Molten isotactic polypropylene; (e) Atactic polypropylene, 25"C

atactic polymer samples were recorded using films of thickness 0.15 mm to 0-3 mm supported between rocksalt plates of 4 m m thickness in the 4 000 cm -1 to 666 cm -t spectral region, In the 666 cm -1 to 222 cm -1 region the syndiotactic spectrum was recorded using unsupported films of 0"5 mm to 1-0 mm thickness. The molten spectra of syndiotactic and isotactic polypropylene were obtained from films supported between rocksalt plates 225

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Figure 4--Infra-red spectra of the 666 cm -~ to 222 crn -z region of: (a) Syndiotactic polypropylene, 25°C; (b) Isotactic polypropylene, 25°C

placed in a thermostatically controlled oven maintained at a temperature 20 ° above the melting point of the polymer. The i.r. spectra were measured using a Grubb Parsons single beam spectrometer Type $3 equipped with a rocksalt prism for the region 1 500cm -1 to 6 6 6 c m -1 and a lithium fluoride prism for the region 4 000 cm -1 to 1 500 cm-L In addition spectra were obtained for the range 666 cm -1 to 222 cm -1 using a Grubb Parsons DM4 Spectrometer (with the cooperation of Dr A. E. Martin). In the region 4 000 cm -x to 666 cm -~ the spectrometer was calibrated against gas samples contained in a 10 cm gas cell and in the region 666 cm -1 to 222 cm -1 atmospheric water vapour was used to calibrate the instrument. RESULTS

The i.r, spectra of syndiotactic, isotactic and atactic polypropylene along with the spectra of molten syndiotactic and molten isotactic polypropylene are shown in Figure 2 for the 3 100 cm -1 to 2 700 cm -1 region and in Figure 3 for the 2 000 cm -1 to .666 cm -1 region. Figure 4 shows the spectra of syndiotactic and isotactic polypropylene for the 666 cm -1 to 222 cm -1 spectral region. The frequencies of the absorption bands together with indications of their intensity and tentative assignment are listed in Tables 1 and 2 for syndiotactic and isotactic polypropylene respectively. Table 1 also records the frequencies observed in the molten spectrum together with indications of their intensities. DISCUSSION

(i)

Comparison of solid phase spectra

It can be seen that although the principal internal carbon-hydrogen stretching and bending vibration absorptions of the isotaetic and syndiotactic 226

THE INFRA-RED SPECTRUM OF SYNDIOTACTIC POLYPROPYLENE T a b l e 1. Characteristics of syndiotactic polypropylene absorption bands Frequency cm - ~

Relative strength

Frequency above m.pt

Relative strength

2955 2923 2882 2869 2841 2809

S S Sh. Sh. S W

2955 2920

S

2869 2839 2809

Sh. S W

1461 1437 1376 1369 1347 1332 1311 1290 1262 1242 1229 1200 1162 1154 1128 1083 1061 1034 1002 976 963 935 905 868 839 811 775 750

S Sh. S Sh. Sh. W W W W W W W M M W W W W M S M W W

1457 1437 1378 1369

S Sh. S Sh.

1229 1205

W W

1155

M

972 961

M

899 868 835 816

W vW W W

567 537 484 469 431 344

W W M M W W

M

W M W W

S

Tentative assignment

Asymm.--CH 3 stretching Asymm. ---CH 2 stretching 2X---CH2 bending Symm. ----CHa stretching Symm. - - C H 2 stretching - 4 2 H stretching Asymm. ---CH3 bending ---CH2 bending Symm. - - C H 3 bending

- - C H 3 wagging (amorphous)

M

Water vapour absorption

(S--strong, M--medium, W--weak, Sh.--shoulder, *--not determined in this work.)

p o l y m e r s are v e r y similar, there are considerable differences in the range 1 500 c m -x to 220 c m -1. It is p a r t i c u l a r l y noticeable that the ab so r p t i o n s at 1 1 6 7 c m -1, 9 9 7 c m -x and 841 c m -1 w h i c h are considered b y some w o r k e r s ~,9 to be associated with the threefold helix of isotactic polyp r o p y l e n e are absent f r o m the spectrum of the syndiotactic polymer. T h e spectrum of t h e syndiotactic p o l y m e r does, however, contain a considerable 227

I. J. GRANT and I. M. WARD

Table 2. Characteristics of isotactic polypropylene absorption bands Frequency cm-1

Relative strength

Frequency above m.pt

2958

S

2956

2923 2881 2869 2839 2810

S S S S W

2915 2871 2844 2810 1451

1458 1440 1377 1360 1329 1303 1297 1255 1219 1168 1153 1103 1045 997 972 940 899 841 809

S M W W W W W S Sh. W W S S W W S M

567 528 458 398 322 247

W M S M W W

Sho

1373 1353 1317 1248 1150 1100

Assignment (Re[. 1) Asymm. - - C H a stretching (A and E modes) Symm. - - ~ H 3 stretching 2X--CHz bending Symm. - - ~ H a stretching Symm. ---CH 2 stretching ---CH stretching Asymm. - - C H 3 bending (A and E modes) - - C H 2 bending (E) Symm. ----CH3 bending (E) ---CH bending (E) ----CH~ wagging (E) ---CH bending (A) ----CH2 twisting (E) ---CH wagging (A) - - C H wagging (E) - - C H 3 wagging (amorphous)

996 971 897 830 810 Water vapour absorption Assignments of these bands not attempted by McDonald and Ward 1

(S---strons, M - - m e d i u m . W - - w e a k , Sh.----shouldcr. *--not determined in this work.)

n u m b e r of a b s o r p t i o n s which are n o t present in the m o l t e n spectrum. T h e s e occur at 1 332, 1 311, 1 290, 1 262, 1 242, 1 162, 1 128, 1 083, 1 034, 1 002, 935, 867"5 a n d 811 c m -1 a n d we consider that they are characteristic of the syndiotactic helix. T h e n u m b e r of f u n d a m e n t a l v i b r a t i o n s for a r e p e a t unit of a p o l y p r o p y l e n e helix is ( 3 N - 4 ) where N is the n u m b e r of a t o m s in one c o m p l e t e t u r n of the helix. F o r isotactic p o l y p r o p y l e n e , with three m o n o m e r units in o n e c o m p l e t e t u r n of t h e helix ~°, ( 3 N - 4 ) = ( 3 x 2 7 - 4 ) = 7 7 modes of v i b r a t i o n a n d for syndiotactic p o l y p r o p y l e n e with a b i n a r y helix 7 consisting of four m o n o m e r units in a c o m p l e t e turn this analysis yields (3 x 36 - 4) = 104 m o d e s of vibration. C o m p a r i s o n of the u n p o l a r i z e d spectra of isotactic a n d syndiotactic p o l y p r o p y l e n e , shown in Figures 2 a n d 3, clearly shows that there are m a n y m o r e e x t e r n a l v i b r a t i o n s in the syndio228

THE INFRA-RED SPECTRUM OF SYNDIOTACTIC POLYPROPYLENE tactic spectrum compared with the isotactic spectrum. The ( 3 N - 4 ) analysis for the number of fundamental modes of vibration indicates that the ratio of the number of isotactic modes to syndiotactic modes is 77/104=0"74. In the spectral range 4 0 0 0 c m -~ to 220cm -1, for unpolarized spectra, there are 30 isotactic absorption bands and 39 syndiotactic absorption bands which yield an isotactic to syndiotactic ratio of 0.77. The close agreement of calculated and observed ratios suggests that this approach to assigning the modes of vibration in a syndiotactic polypropylene helix is substantially correct. (ii) Comparison of molten and atactic spectra Comparison of molten isotactic, molten syndiotactic and atactic spectra shows that, in the first place, the three are approximately similar. Several absorption bands which are characteristic either of the isotactic or syndiotactic polymer disappear or are reduced in intensity on melting but return to their original intensity upon cooling. One common feature of these spectra is the bands at 1 155 cm -1 and 973 cm -~ which retain their original intensity on melting. These two bands are found in the i.r. spectra of all steric forms of polypropylene whether in a solid or in a molten state suggesting that these bands derive from the chemical rather than the structural nature of polypropylene. Subsidiary differences are observed between the molten spectra and that of the atactic polymer. It is interesting to note that the molten syndiotactic and atactic spectra are very closely similar and differ significantly from the molten isotactic spectrum. For example in both the molten syndiotactic and atactic spectra the absorption ca. 973 cm -~ appears as a doublet s with peaks at 976 and 963 cm -1 and bands in the 1 200cm -~ region appear to be very similar. This suggests first that the pattern of stereoregularity in the atactic polymer, although not sufficiently regular to allow .crystallization is nearer to that in a syndiotactic rather than an isotactic polymer; and secondly that certain absorptions are observed in the molten state which are characteristic of the chain configurations of syndiotactic and isotactic polymers, even though the absorptions characteristic of the different types of helix are no longer present. This interpretation was confirmed by the similarity of the p.m.r, spectra of syndiotactic and atactic polymers. (iii) Conclusion In the previous publication from this laboratory 1 McDonald and Ward following Perald& and Krimm 3 assumed that both the internal and external modes of vibration would be affected by the intramolecular interactions. On the other hand, Liang, Lytton and Boone 5 assumed that only the external modes of vibration would be affected. These unpolarized spectra of syndiotactic polypropylene do not throw any further light on this particular controversy. The present investigation does, however, suggest that the approach of considering several of the absorptions in the 1 500 to 220 cm -1 region as arising from interactions within the helix to be a correct one. Furthermore, in syndiotactic polymer, as 229

I. J. GRANT and I. M. WARD in isotactic polymer, certain absorptions characterize the portions of chains existing in the helical configuration and therefore give an empirical measure of the stereoregularity. This measure will depend on the presence of at least four m o n o m e r units being alternately d and 1 substituted, as against three being identically substituted in the isotactic case. (Received A u g u s t 1964) Research Department, I C I Fibres Ltd, H o o k s t o n e R o a d , Harrogate REFERENCES 1 McDON^LO, M. P. and WARD,I. M. Polymer, Lond. 1961, 2, 341 2 PERALDO, M. Gazz. chim. ital. 1959, 89, 798 3 KmMM, S. Advanc. Polym. Sci. 1960, 2, 51 4 TOBIN,M. C. J. phys. Chem. 1960, 64, 216 LI.~O, C. Y., LYTTON,M. R. and BOONE,C. J. 1. Polym. Sci. 1961, 54, 523 N^TrA, G., PASQUON,I. and ZAMBF-LLI,A. J. Polym. Sci., Part C, Polymer Symposia No. 4, p 411 r TINCnER, W. C., American Chemical Society Division of Polymer Chemistry, Papers presented at Atlantic City Meeting Vol. III, p 142, 1962 s NArr^, G., PASQUtm, I., C o ~ I ~ , P., PERJa_DO,M., I~OORARO,M. and Z~B~LLI, A. Atti. Accad. Lincei, 1960, 28, 539 9 BRADER, J. J. J. appl. Polym. Sci. 1960, 3, 370 lo Nm'rA, G., CORRADINI,P. and CESAm, M. Atti Accad. Lincei, 1'956,21, 365

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