| Abstract
| - The crystal structure of form II of isotactic poly(4-methyl-1-pentene) is presented. Samplesof isotactic poly(4-methyl-1-pentene) in the pure form II have been obtained by crystallization fromtetramethyltin solutions. The crystal structure has been determined by using X-ray powder diffractiondata and packing energy calculations. Chains in 41 helical conformation are packed in a monoclinic unitcell with axes a = 10.49 Å, b = 18.89 Å, c = 7.13 Å, and γ = 113.7°, according to the space group P21/b.Four chains are included in the unit cell with a calculated density of 0.862 g/cm3, according to theexperimental density of 0.850 g/cm3. Statistical disorder in the position of up and down chains in eachsite of the lattice is present in the structure. This structure provides an example of symmetry breakingin polymer crystals since the local 41 helical symmetry of the chains is not maintained in the lattice ascrystallographic symmetry. As a consequence, the chains do not pack in a tetragonal lattice, as generallyoccurs for chains in 41 helical conformation but are packed in a monoclinic lattice having lower symmetry.
|
