Simulations of crystallization in syndiotactic polypropylene

Abstract

Crystallization in syndiotactic polypropylene was investigated using Monte Carlo methods to generate polymer chains having sequences with representative stereochemistries. The chains were then slid by one another in parallel arrangements to look for matches that could lead to crystallite formation. The results were used to estimate degrees of crystallinity, melting points and some other related physical properties as a function of stereochemical structure. The alternating (racemic) stereochemical configurations in this form of polypropylene appeared to be more favourable for such matches than the replicating (meso) ones in the isotactic modification. This suggests increased degrees of crystallinity in the syndiotactic polypropylene at the same level of stereoregularity as the isotactic polymer, as was also predicted by Natta. The simulations are consistent with syndiotactic polypropylene having a lower enthalpy of fusion and with the negative surface free energies predicted for samples with lower degrees of syndiotacticity. The predicted melting points are such that for syndiotactic polypropylene to exhibit room temperature crystallinity it would have to have higher degrees of crystallinity than would be acceptable for a thermoplastic elastomer. The simulations also indicate that syndiotactic crystallization would not coexist with isotactic crystallization in the same sample of polypropylene unless there was an extremely wide range of replication probabilities.