We report the cocrystallization of the regio- and stereoregular chiral copolymer poly(limonene carbonate). To the best of our knowledge, this marks the first example of an amorphous, enantiomerically pure polymer that becomes crystalline upon stereocomplexation with its complementary enantiomer. By analyzing X-ray powder diffraction data, we propose a packing model in which sheets of enantiopure chains interdigitate with layers of the opposite enantiomer, forming a "steric zipper".

The crystal structure of the trigonal form of isotactic polypropylene in propylene–pentene random copolymers is presented. These copolymers crystallize in the α form of iPP for concentrations of pentene lower than 7–8 mol % and in the trigonal form for higher pentene concentration. The trigonal form does not crystallize by cooling from the melt but crystallizes from the amorphous by cold-crystallization or for samples with high pentene concentration by aging the amorphous samples at room temperature. The pentene units are included in the crystals of trigonal form and, at low concentration, also in the crystals of α form, producing increase of the unit cell dimension. The change of crystallization habit from monoclinic into trigonal occurring for concentrations higher than 10 mol % allows incorporation of higher amounts of pentene units in the crystals of the trigonal form than in the α form. The crystal structure of the trigonal form has been studied by analysis of the X-ray fiber and powder diffraction patterns and electron diffraction of single crystals of propylene–pentene copolymers having concentrations of pentene units in the range 20–54 mol %. Chains of propylene–pentene copolymers in the 3-fold helical chain conformation are packed in a trigonal unit cell according to the space group R3c or R3̅c. The dimension of the unit cell axes depends on the amounts of pentene units included in the crystal lattice, and when pentene concentration approaches 50 mol %, the value of a = b axes becomes practically equal to that of the unit cell of the stable form I of isotactic polybutene.

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.

Abstract:We report the cocrystallization of the regio-and stereoregular chiral copolymer poly(limonene carbonate). To the best of our knowledge, this marks the first example of an amorphous, enantiomerically pure polymer that becomes crystalline upon stereocomplexation with its complementary enantiomer. By analyzing X-ray powder diffraction data, we propose a packing model in which sheets of enantiopure chains interdigitate with layers of the opposite enantiomer, forming a "steric zipper".

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