The Role of Intimate Dipole Carbonyl-Carbonyl and Hydrogen-Carbonyl Interactions in the Stereocomplexation and Crystallization: The Case of Poly(Cyclohexene Carbonate).

Enantiopure isotactic poly(cyclohexene carbonate) (PCHC) has been synthesized with chiral Zn-β-diiminate catalyst. PCHC crystallizes both as enantiopure polymer (R)-PCHC and (S)-PCHC and upon stereocomplexation of the two enantiomers. We report the crystal structures of the enantiopure polymer and of the stereocomplex (R/S)-PCHC and explain their crystallization based on the establishment of multiple attractive H---O═C interactions between oxygen atoms of carbonyl groups and the hydrogen atoms of the cyclohexyl rings and C═O---C═O intimate dipole interactions between carbonyl groups of chains of opposite chirality in the stereocomplex. The crystal structure of the enantiopure polymer is characterized by chains in 2/1 helical conformation packed in the orthorhombic unit cell with axes a = 11.55 Å, b = 9.42 Å, and c = 7.36 Å, according to the space group P212121, with steric interdigitation between chains of similar chirality favored by multiple attractive H---O═C interactions. The stereocomplex crystallizes in an orthorhombic unit cell with axes a = 10.40 Å, b = 8.41 Å, and c = 7.36 Å, according to the space group Pbc21, driven by establishment of additional C═O---C═O dipole interactions between carbonyl groups of chains of opposite chirality, besides of the multiple attractive H---O═C interactions.