Long chain branched and cross-linkable polypropylene; synthesis and applications

This paper discusses synthesis of two new polypropylene structures, including long chain branched polypropylene (LCBPP) and cross-linked polypropylene (x-PP) that show unique physical properties and applications. Both PP structures are prepared by using the same bi-functional T-reagent, i.e. p-(3-butenyl)styrene (BSt), and suitable metallocene catalyst systems. In the presence of rac-Me₂Si(2-Me-4-Ph-Ind)ZrCl₂/MAO and H ₂, the BSt simultaneously serves as a comonomer and chain transfer agent resulting in a LCB-PP with high molecular weight and desirable branch microstructure. Evidentially, the LCB structure is very sensitive to the melt rheology, including strain hardening under extensional flow conditions. The dramatic increase of melt strength is essential in film processing and thermo-forming for preparing PP thin films and forms that show good strength and low density. On the other hand, rac-CH₂(3-t-Butyl-Ind) ₂ZrCl₂/MAO catalyst incorporates BSt units with a preferred α-olefin insertion over styrene moiety. The feature permits the preparation of linear PP-BSt copolymers with high molecular weight and good catalyst activity. The resulting PP-BSt copolymers having some flexible pendent styrene moieties are completely soluble in xylene at elevated temperatures, and the solution case films show active cross-linking activity at temperatures >160°C by engaging in a Diels-Alder [2+4] inter-chain cycloaddition reaction between the pendent styrene units. Evidently, the flexibility of styrene units is important, which enhances the inter-chain cyclization to form a complete 3-D network, even with a very small amount of BSt units. The cross-linking feature provides PP thin film with good electric and thermal stability, which result in high breakdown strength under high applied electric field and elevated temperature conditions and increased energy storage capacity in capacitors.