X-ray analysis and molecular modeling of poly(vinyl alcohol)s with different stereoregularities

Wide-angle X-ray methods have been used to compare the structures of poly(v inyl alcohol) (PVA) with syndiotactic diad (S-diad) contents in the range 5 1-63%. The fiber diagram of a PVA with 51.2% S-diad content (essentially at actic) is indexed by a monoclinic unit cell with dimensions a = 7.82 +/- 0. 03 Angstrom, b = 2.53 +/- 0.01 Angstrom (chain axis), c = 5.52 +/- 0.01 Ang strom, and beta = 91.5 +/- 0.2 degrees. These dimensions are very similar t o those proposed by Bunn (Nature 1948, 161, 929) and Sakurada (Bull. Inst. Chant. Res., Kyoto Univ. 1950, 23, 78). As the S-diad content is increased, the crystallites become larger and less distorted and there is lateral con traction of the crystal structure, suggesting that the chains can pack more efficiently. For a syndiotacticity-rich PVA with a S-diad content of 63.1% , the unit cell dimensions are as follows: a = 7.63 +/- 0.02 Angstrom, b = 2.54 +/- 0.01 Angstrom, c = 5.41 +/- 0.01 Angstrom, and beta = 91.2 +/- 0.1 degrees. The latter structure has a theoretical density of 1.40 g/mL, whic h is similar to 5% higher than the value of 1.34 g/mL predicted for the ata ctic polymer. The efficiency of chain packing in the two unit cells has bee n compared by molecular dynamics modeling of arrays of chain segments of di fferent random sequences, with 50% and 65% S-diad contents. For both struct ures, the results favor hydrogen bonding similar to that proposed by Bunn r ather than that due to Sakurada. NVT calculations show that the larger unit cell is favored for the atactic polymer, while the smaller unit cell is fa vored for the 65% syndiotactic polymer. A NPT assembly for the atactic mode l starting in the contracted unit cell expanded and equilibrated close to t he observed, lower density structure. Likewise, a starting model for the 65 % syndiotactic polymer packed with the dimensions for the atactic polymer w as found to adjust to the observed contracted structure. The changes are to be understood in terms of the balance between hydrogen bonds (electrostati c) and van der Waals forces, which vary depending on the tacticity.