Determination of the Crystal Structure of Syndiotactic 3,4-Poly(2-methyl-1,3-butadiene) by Molecular Mechanics and X-ray Diffraction

The crystal structure of syndiotactic 3,4-poly-(2-methyl-1,3-butadiene) has been determined by the joint use of molecular mechanics and X-ray diffraction. Molecular mechanics calculations have been performed both on the isolated chain and on the crystal by the use of various force fields. The energy minimizations predict a model of the crystal structure. The calculated X-ray powder diffraction pattern is in good agreement with the experimental one. This model has been refined in order to obtain the best agreement with all the experimental data. The space group is Pbcm and the parameters of the unit cell are a=0.66 nm, b=1.32 nm, c=0.527 nm. The presence of defects in the crystal has been taken into account. Analogies and differences with the crystal structure of syndiotactic 1,2-poly(1,3-butadiene) are discussed.