Crystallization of Alternating Limonene Oxide/Carbon Dioxide Copolymers: Determination of the Crystal Structure of Stereocomplex Poly(limonene carbonate)

The crystal structure of the alternating copolymer of limonene epoxide and carbon dioxide has been studied. Highly stereoregular and regioregular alternating copolymers have been prepared starting from cis/trans mixtures of both enantiopure (R) and (S) isomers of 1-methyl-4-(1-methylvinyl)-7-oxabicyclo[4.1.0] heptane (limonene oxide) using a β-diiminate zinc complex under mild conditions of temperature and pressure, to yield enantiopure poly(1S,2S,4R-limonene carbonate) (PRLC) and poly(1R,2R,4S-limonene carbonate) (PSLC) products, respectively. Attempts to crystallize the pure enatiomers failed, whereas racemic mixtures readily crystallize from solution, forming racemic crystals. In the crystals, enantiomorphic chains are packed in an orthorhombic unit cell with axes a = 9.71 ± 0.05 Å, b = 10.68 ± 0.05 Å, and c = 11.31 ± 0.05 Å (chain axis) according to the space group Pbc21 and 2 chains (4 monomeric units)/unit cell. The chain periodicity c of 11.31 Å can be accounted for by a s(2/1) helical conformation with nearly all-trans bonds in the backbone. Isoclined chains of opposite chirality are packed in the unit cell forming well interdigitated bc layers piled along a via zipper interactions of the enantiomorphic side groups belonging to adjacent chains. The structure is characterized by the presence of different types and degrees of disorder that arises from the nearly random rotation of isopropenyl groups around the connection bond to the cyclohexane rings (conformational disorder), twisting of cyclohexane cycles, and up/down positional disorder of isochiral chains in the lattice positions (substitution type disorder). It is argued that the formation of racemic crystals instead of a mixture of enantiopure crystals is stabilized by the favorable interactions of the polar carboxyl groups and the zipper interactions of the lateral chiral groups of first neighboring chains along b. It is also inferred that the crystallization of enantiopure crystals is prevented by the slow crystallization kinetics and the less favorable interactions between chains in an isochiral packing.