Molecular structure and rubber-like elasticity I. The crystal structures ofβgutta-percha, rubber and polychloroprene

Abstract

The crystal structures ofβgutta-percha, rubber and polychloroprene have been determined by interpretation of X-ray diffraction photographs.βGutta-percha (—CH2—C(CH3)=CH—CH2—)n is orthorhombic, with axial lengths a0= 7.78 A,b0= 11.78 A,c0= 4.72 A. Four long-chain molecules pass through this cell parallel to thecaxis. The space group isP212121, and the co-ordinates of the five carbon atoms of one structural unit are: (CH2) 0.926 (C) 0.000 (CH) 0.000 (CH2) 0.074 (CH3) 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 thetransconfiguration, but the connecting links (CH2—CH2) 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 thetransdouble-bond configuration, and differ fromβmolecules only in the positions of the CH2—CH2bonds; the chain form is thus different from that ofβmolecules. Crystalline rubber (also (—CH2— C(CH3)=CH — CH2— )n) is monoclinic, and hasa0= 12.46 A,b0= 8.89 A,c0= 8.10 A,β= 92°. Four long-chain molecules pass through this cell parallel to thecaxis. The space group isP21/aand the co-ordinates of the ten carbon atoms of one structural chain unit are (CH2) 0.753 (C) 0.854 (CH) 0.845 (CH2) 0.745 (CH3) 0.968 0.899 0.865 0.905 0.959 0.876 0.802 0.703 0.542 0.457 0.773 (CH2) 0.744 (C) 0.644 (CH) 0.659 (CH2) 0.757 (CH3) 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 thecisconfiguration. 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 (—CH2—CCl=CH—CH2— )nis completely analogous to that of (i gutta-percha. The cell is orthorhombic, witha0= 8.84 A,b0= 10.24 A,c0= 4.79 A. The co-ordinates of carbon and chlorine atoms are (CH2) 0.077 (C) 0.033 (CH) 0.008 (CH2) 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