Tacticity distribution of isotactic polypropylene prepared with heterogeneous Ziegler-Natta catalyst. 2. Application and analysis of SSA data for polypropylene

Accurate characterisation of the tacticity distribution in isotactic polypropylene is important for obtaining better structure property correlations and for the indirect investigation of the active sites in heterogeneous Ziegler-Natta catalysts. In this work, successive self-nucleation and annealing (SSA) DSC measurement is applied for the study of tacticity distribution in polypropylene fractions with varying isotacticity. The results are compared with ¹³C NMR and temperature rising elution fractionation (TREF). Analysis of the lamellar thicknesses and average meso sequence lengths (MSL) revealed that the fractions could be classified into three main groups according to the lamellar structure generated in SSA. The fractions with relatively low isotacticity crystallise mainly by the longest crystallisable sequence of the chain. In more isotactic fractions, the lamellar thicknesses are close to the average crystallisable sequence lengths of the chains. For the highest isotacticity fractions correlation was not found. Because of these differences the correlation between the melting temperature and isotacticity (meso diad-%) was non-linear. Good correspondence between the SSA melting curves and the TREF fractograms was observed.