Unperturbed dimensions and local stiffness of poly(vinyl chloride) with stereochemical sequences composed of repeating units

The unperturbed dimensions, preferred local conformations, and conformational entropies have been examined in rotational isomeric state models of poly(vinyl chloride) with stereochemical sequences defined by 46 repeating patterns. All possible repeats of 1-7 diads were considered, as well as selected repeats of 8-11 diads. Most of these chains have unperturbed dimensions close to the averages in a typical atactic poly(vinyl chloride). One exception is the chain with the repeating stereochemical sequence mmr, where m and r denote meso and raceme diads, respectively. This repeating stereochemical sequence yields the smallest unperturbed dimensions obtainable with poly(vinyl chloride), due to three competing preferred conformations within the mm unit and the inability of this short unit to complete a single turn of the 3(1) helix typical of an isotactic vinyl polymer. The series mr(x), x = 1, 2,..., exhibits unperturbed dimensions with a strong odd-even effect superimposed on a general tendency for an increase toward the asymptotic limit defined by the syndiotactic chain. The larger unperturbed dimensions are obtained when x is odd. Given the strong preference for the tt conformation in an r diad, the preferred conformations of mr(x)m have the appearance of a bend when x is even, with both m diads tending to depart from the same side of the r(x) rod. When x is odd, these m diads have a greater probability of departing the r(x) rod in opposite directions, producing a greater extension of the chain. In contrast with the odd-even effect observed in the unperturbed dimensions, the average conformational entropy per bond exhibits a monotonic decrease as x increases through the series mr(x). Similar behavior is seen in the mr, series in a rotational isomeric state model for polystyrene. The screening of the 46 repeating stereochemical sequences of poly(vinyl chloride) identifies mmr and mr(x) as being worthy of further study with multichain simulations at bulk density.