Molecular Structure and Rubber-like Elasticity. I. The Crystal Structures of β-Gutta-Percha, Rubber and Polychloroprene

Abstract

The crystal structures of β gutta-percha, rubber and polychloroprene have been determined by interpretation of X-ray diffraction photographs. β Gutta-percha (—CH 2 —C(CH 3 )=CH—CH 2 —)n is orthorhombic, with axial lengths a 0 = 7.78 A, b 0 = 11.78 A, c 0 = 4.72 A. Four long-chain molecules pass through this cell parallel to the c axis. The space group is P 2 1 2 1 2 1 , and the co-ordinates of the five carbon atoms of one structural unit are: (CH 2 ) 0.926 (C) 0.000 (CH) 0.000 (CH 2 ) 0.074 (CH 3 ) 0.970 0.110 0.146 0.074 0.110 0.277 0.676 0.960 0.177 0.462 0.980 The molecules are asymmetric; all the molecules passing through any one crystal are identical—either left-handed or right-handed, not mixed. The carbon chain is a non-planar zigzag; each chain unit C—C=C—C is planar, and has the trans configuration, but the connecting links (CH 2 —CH 2 ) lie in a different plane; the plane —C—C—C = makes an angle of 115° with plane —C—C=C—. a Gutta-percha molecules probably also have the trans double-bond configuration, and differ from β molecules only in the positions of the CH 2 —CH 2 bonds; the chain form is thus different from that of β molecules. Crystalline rubber (also (—CH 2 — C(CH 3 )=CH — CH 2 — ) n ) is monoclinic, and has a 0 = 12.46 A, b 0 = 8.89 A, c 0 = 8.10 A, β = 92°. Four long-chain molecules pass through this cell parallel to the c axis. The space group is P 2 1 / a and the co-ordinates of the ten carbon atoms of one structural chain unit are (CH 2 ) 0.753 (C) 0.854 (CH) 0.845 (CH 2 ) 0.745 (CH 3 ) 0.968 0.899 0.865 0.905 0.959 0.876 0.802 0.703 0.542 0.457 0.773 (CH 2 ) 0.744 (C) 0.644 (CH) 0.659 (CH 2 ) 0.757 (CH 3 ) 0.532 0.834 0.874 0.905 0.834 0.828 0.326 0.215 0.052 0.975 0.268 The molecules are asymmetric; two of the molecules passing through the unit cell are left-handed and two right-handed. The chain carbon atoms of any one molecule form a non-planar zigzag, in which the double-bond units C— C=C— C have the cis configuration. The two isoprene units C —C—C=C —C— which make up the structural unit are not identical in configuration; the distortion appears to be due to intermolecular forces. The structure of crystalline polychloroprene (—CH 2 —CCl=CH—CH 2 — ) n is completely analogous to that of (i gutta-percha. The cell is orthorhombic, with a 0 = 8.84 A, b 0 = 10.24 A, c 0 = 4.79 A. The co-ordinates of carbon and chlorine atoms are (CH 2 ) 0.077 (C) 0.033 (CH) 0.008 (CH 2 ) 0.970 Cl 0.167 0.054 0.115 0.034 0.099 0.243 0.278 0.000 0.787 0.509 0.000