LETTERS TO THE EDITORS 307

Chain Conjiguration in Crystalline Vinyl Polymers

G. Natta et U Z . ' * ~ have reported the preparation of crystalline poly- propylene, poly-a-butene, and polystyrene, and have suggested, on the basis of x-ray diffraction evidence, that the molecules in these polymers and in polyvinyl isobutyl ether3 are "isotactic," that is, in any one molecule the substituent groups are identically placed, either all in left-hand or all in right-hand positions, at any rate within the length of chain in a crystal- line region. (Ordinary polystyrene, and in fact most vinyl polymers pre- pared in the ordinary way, do not crystallize, presumably because their substituent groups are indiscriminately placed in left and right positions.) They do not, however, discuss the chain configuration. We wish to point out that the probable chain configuration may be inferred from the mag- nitude of the chain repeat distance, and that this configuration gives strong support to their conclusion that the molecules are isotactic.

Singly-linked carbon chains are likely to assume configurations in which the bonds of linked carbon atoms are staggered (unless all staggered con- figurations give rise to severe overcrowding of substituent groups, in which case there may be deviations from the ideal staggered positions). The plane zigzag which occurs in unsubstituted CHz chains is the simplest stag- gered configuration; in this chain any three successive chain-bonds form a trans configuration. When substituent groups are present in such posi- tions that the all-trans chain would lead to overcrowding, gauche chain- bond configurations are likely. A convenient nomenclature whereby all the possible staggered chain configurations can be considered systematically and referred to unambiguously has been suggested in an earlier paper by one of US;^ in this nomenclature, the position of each successive chain-bond with respect to the two preceding bonds is referred to by the letters A , B, and C, in which A refers to the trans and B and C to the left and right gauche positions, respectively. The geometrical repeating unit, when it comprises more than one bond-sequence, is referred to by a numerical subscript: thus, A2, (AB), , ABAC, A3BA3C, (ABC)2.

Each type of chain has its own characteristic repeat distance: 2.5 A. for Az , 4.4 A. for ABAC, 6.2 A. for (AB)3 or (AC), , 8.8 A. for A3BAgC, and so on. These figures are based on the C-C bond length of 1.53 A. and an angle of 109l/," between chain-bonds; if the angle is opened a little, as is likely especially in polymers with large substituent groups, the repeat distances are increased a little. The chain repeat distances for the crystal- line vinyl polymers are 6.2-6.7 A., and it appears therefore that the chain form is or (AC)3.

Now this is precisely the type of chain which on stereochemical grounds would be expected in an isotactic vinyl polymer molecule. In a chain in which every alternate carbon atom holds a single substituent group such as CH, or CsH5, and always in the same space-position, the plane zigzag chain would lead to gross overcrowding, bringing successive substituent groups within 2.5-2.6 A. of each other; methyl groups or benzene rings require

308 LETTERS TO THb; EDITORS

3.5 A. for comfort. Consequently, alternate bonds are likely to take up gauche positions, because in this way the substitueut groups can be well separated; moreover, they will all take up identical gauche positions, all B’s or all C’s; in other words the chain configuration has an ABAB . . . (or the enantiomorphous ACAC . . .) sequence. Such a chain is found to be a helix with three chemical units in the geometrical repeating unit, i.e., it is (AB)3 or (AC)3.

This type of chain, one of the fundamental types mentioned in reference 4, is illustrated in Figure 1 with phenyl substituent groups in the most probable positions. The configuration is arrived at in the following way.

Fig. 1. Left: The chain (A& Center and right: Side and end views of iso- tactic polystyrene molecule.

Fig. 2. Method of arriving at configuration of Figure 1.

In Figure 2, the chain is initially in the lorin ol' a plauc zigzag, which puts R 4 much too close to Kz; if C1CzC3 is kept stationary, and rotation of C3C4 about CzC3 as axis occurs, Cd inoves along the broken circle, either fornards through 120" to C: or backkards through 120" to C:. If it moves for- wards to C: it finds itself contacting both C, arid the first carbon atom of Hz a t rather short distances of 2.9 h.; if it moves backwards to C: the contacts are with C1 and €I2. The latter is obviously preferable because there is less overcrowding (Iiydrogeii being very much smaller than carbon j. In addition, the for\\ard position nould bring the first carbon of R: much too close to HI (1.7A.j, whereas the bacbw ard position puts the first carbon of R: at the satisfactory distance of 2.8 A. from HZ. On both counts, the backward position is the better. This move, repeated at alternate bonds along the chain, gives the left-hand helix of Figure 1. If rotation had been performed about C3C4 as axis instead of CzC3, a right-hand helix, the mirror image of Figure 1, would be obtained. (Note that the molecule, if regarded as infinitely long, is not intrinsically asymmetric, for in the plane zigzag configuration there are planes of syininrtry through the chain atonis a t right angles to the chain axis.)

The benzene rings in Figure 1 are tilted a t the angle wliich in a niodel was found to give the best clearance between hydrogen atoms, but to avoid confusion the hydrogen atoms are not shown in the drawings.

I t may be noted that the unit cells proposed by Natta for polystyrene and poly-a-butene are of hexagonal type; molecules of configuration ( A B ) 3 or ( A C ) , have thrccfold screw axes which nould fit appropriately in arrangements of trigonal or rhombohedra1 symmetry having unit cells of this type. Moreover, we note that iir thc x-ray diffraction photographs of polystyrene and polypropylene there are very strong reflections on the first layer line from planes inclined a t some 40 " to the equat m ; this is con- sistent with the configuration we propose, the molecular transform of which would have a strong maximum in this region.

It is an interesting question whether in any one crystalline region there are equal numbers of left- and right-hand helices, or on the contrary all left or all right helices. Since the molecule can assume either form, this will depend on the relative packing capabilities of these two possible ar- rangements.

The chain repeat distance in polyvinyl isobutyl ether prepared a t a low temperature is given by Schildknecht et aL3 as 6.20 A., but our own x-ray diffraction photograph (taken in 1942) of a stretched sample of Oppanol C (a polymer of the same substance but prepared a t a rather higher tempera- ture) gives 6.60 A., which is very similar to the figures given by Natta for polypropylene (6.50 A.), polystyrene (6.65 A.) and poly-a-butylene (6.7 A.). A chain repeat distance of 6.20 A. would mean that the bond angles are near the tetrahedral figure of 1091/20. The larger repeat distances indi- cate larger bond angles. Assuming that all the bond angles are opened equally, they would be 1 141/2 " in polypropylene, 116 " in Oppanol C, 11 6'/2 " in polystyrene, and 117" in poly-a-butylene. These values seem not un-

310 LETTERS TO THE EDITORS

reasonable; they are probably due to repulsion between chain atoms (C, and C; in Fig. Z), and repulsion between chain atoms and the first atoms of substituent groups (C, and Ri).

It is worth remembering that there is one other vinyl polymer (poly- vinyl chloride) which, though poorly crystalline, has enough regularity to give a diffuse x-ray fiber pattern, and the chain repeat distance obtained from this5 is 5.1 A. This length, being equal to that of two zigzag spans, suggests strongly that the chain is a plane zigzag but that the chlorine atoms are alternately in left and right positions. (The chain, in the nomenclature used here,4 would be called A4.) When large substituents are alternately in left and right positions, there is no need to depart from the plane zigzag (all-trans) configuration which is preferred by the chain carbon atoms. From the very diffuse character of the x-ray pattern, i t is evident that the regular alternation of the chlorine atoms in ordinary polyvinyl chloride is maintained only over short lengths of chain-some tens of atoms rather than the hundreds in most crystalline polymers.

Note added in proof: Since this paper was submitted for publication, Professor Natta has drawn our attention to the following recent papers in which the same chain configu- ration is suggested: G. Natta and P. Corradini, Atti accad. nazl. Lincei, 352,73 (1955) (polypropylene); Rend. accad. nazl. Lincei, viii, 18, No. 1, p. 19 (1955) (polystyrene). The arrangement of the molecules in crystals of isotactic polystyrene is also considered in a paper by the same authors to appear soon in “Die Macromolekulare Chemie.” Our own paper deals with certain aspects of the stereochemistry (van der Waals repul- sions within the molecule) in rather more detail, and suggests a different orientation of the benzene rings (obtained by rotation around the bonds linking them to the chains) which we consider more probable than that suggested by Natta and Corradini. But we are in complete agreement about the chain configuration.

References

1. G. Natta el al., J. Am. Chem. Soc., 1955, 77, 1708. 2. G. Natta, J . Polymer Sci., 1955, 16, 143. 3. C. E. Schildknecht et al., Ind. Eng. Chem., 1948,40,2104. 4. C. W. Bunn, Proc. Roy. Soc., 1942, A180, 67. 5. C. S. Fuller, Chem. Reviews, 1940, 26, 143.

C. W. BUNN E. R . HOWELLS

Imperial Chemical Industries Ltd. Welwyn Garden City, Herts, England

Received June 20, 1955

A Rebuttal t o “The Penultimate Unit E’ect in Free Radical Copolymerization9’

A recent letter by R. M. Joshil contains a wholly fallacious criticism of the paper2 in which I originally drew attention to the evidence for the exist- ence of the penultimate unit effect in free radical copollmerization.