Configurational-conformational statistics of poly(ethylene-propylene)s

Using the periodic condition method of rotational isomeric state model, the mean-square end-to-end distance and the mean-square radius of gyration, as a function of composition and stereochemical tacticity, of Bernoullian pol y(ethylene-propylene) chains were extensively calculated. When both the fra ction of ethylene monomers and the fraction of meso dyad (f(m)) in the prop ylene sequence are 0.5, the characteristic ratio calculated for the mean-sq uare end-to-end distance of PEP first decreases monotonously before reachin g its minimum at N = 3, where N is the length of the periodic microstructur e sequence. Then it increases and passes through a shallow extreme value at N = 5, and finally goes gradually to its asymptotic value. A substantial i ndependence of this conformational characteristic from the period length is achieved when it is greater than 6. Theoretical data are comparable with t he experimental results for alternating PEP The stereochemical structure of the propylene sequence has a strong influence on the characteristic ratio of the chain when the occurrence probability of ethylene units is low. Ther e is a variation of only 7.2% for <S-2> /nl(2) for f(m) from 0.90 to 1.0.