A Study on the Effect of Catalyst Types on Microstructure of Ethylene/1-Hexene Copolymer by Thermal Fractionation

Olefinic copolymers of ethylene/1-hexene were synthesized by three main types of supported catalysts, including Philips, Ziegler-Natta and metallocene catalysts, and the specific microstructural features delivered by each class of the catalysts were studied. The heterogeneity of comonomer distribution was studied by thermal fractionation using successive self nucleation annealing (SSA) as a novel method. It was observed that the comonomer incorporation ability decreased in the order of metallocene > Ziegler-Natta > Philips. The chemical composition distribution (CCD) of the produced copolymers was investigated by differential scanning calorimetry (DSC). Interestingly, we found that the copolymers produced by the directly supported metallocene catalyst possessed heterogeneous comonomer distributions, similar to the samples produced by Ziegler–Nata and Philips catalysts. The lamellar thickness distributions of the copolymers were calculated by the DSC curve deconvolution into a number of standard distribution functions. It was shown that the type of the supported catalyst did not affect the lamella thickness distribution to a discernible extent. The range of lamellar thickness was about 2-29 Å for the polymers produced using directly supported metallocene, Zigler-Natta and Philips catalysts, while the corresponding value for copolymers made by homogeneous metallocene catalyst was in the range of 2-7 Å. It could be concluded that in-situ supporting of metallocene catalyst increased the comonomer incorporation in a more homogeneous fashion. In addition, comonomer units were distributed more homogeneously at higher 1-hexene concentrations as could be realized from the approach of DSC index (DSCI) which showed that the DSC index moved towards unity at higher comonomer levels.