Direct Synthesis of High-Melt-Strength Polypropylene Using the Fourth Generation Heterogeneous Ziegler-Natta Catalyst and Commercial Production Process

The most of commercial isotactic polypropylene (PP) polymers exhibit low melt strength due to lack of a long-chain-branched (LCB) structure, which significantly limits PP processing operations. There are several chemical methods available today to prepare high-melt-strength PP (HMS-PP) products containing a small amount of LCB structures. However, the processes require multiple reaction steps, forming a complicated mixture of branch structures (i.e., branch length, branch density, and impurity, etc.). This work discusses an efficient and economic method following a commercial PP production process (i.e., polymerization and then extrusion). The direct-polymerization step is centered on an asymmetric diene co-monomer p-(3-butenyl)styrene (BSt) that can be randomly incorporated in the PP chain with a mono-enchainment fashion by the fourth generation MgCl2-supported TiCl4 catalyst in the slurry polymerization process. The predominate coordination/1,2-insertion of the alpha-olefin moiety in the BSt co-monomer forms linear PP-co-BSt copolymer (granules), with few pendant styrene moieties randomly distributed along the polymer chain. Following the common PP extruding and pelletizing procedure at 200 degrees C, the molten PP-co-BSt copolymer engages the in situ coupling reaction between pendant styrene units in neighboring polymer chains to spontaneously form PP branches with long branch lengths. The branch densities in the resulting LCB-PP pellets are controlled by the BSt content in the copolymer and the degree of the thermally induced coupling reaction. The resulting LCB-PP polymer, with an average of one to four LCB structures per chain, is completely melt-processable, exhibiting high melt strength under elongation flows. The same strain-hardening phenomenon was also observed in the polymer blends between these LCB-PP polymers and linear PP homopolymer with the composition up to 1/1 weight ratio.