Catalyst Type Effects on Structure/Property Relations of Polypropylene Random Copolymers

Polypropylene (PP) random copolymers are an important class of commercial materials that can be tuned for a wide range of different applications. The structure-property relations are mainly discussed in this article focusing on two major influencing factors, the catalyst system and the type and amount of comonomer. Ziegler-Natta (ZN) catalyst systems still dominate the production of polypropylene and todays sixth-generation ZN catalysts are based on postphthalate donor systems. Metallocene (MC) catalysts offer possibilities to exactly control the microstructure of the polymer even better. Mostly, ethylene is used as comonomer being randomly distributed for up to 7.3 mol% when using ZN catalysts. For single site catalysts, homogenous (single phase) materials can be achieved up to ethylene contents of 20.8 mol%. The incorporated comonomer reduces the crystallization speed and the lamellar thickness of the polymer thus affording soft and highly transparent materials. Random copolymers and terpolymers with higher alpha-olefins only play a minor role but offer an additional possibility for property adjustments. PP random copolymers can also be varied in their performance through multistage polymerizations, altering the comonomer content and or the molecular weight in the different polymerization reactors.