In Situ Promotion of Long-Chain Branching in Polyethylene from Ziegler-Natta Catalysts

Polyethylene (PE) synthesized by Ziegler-Natta catalysts (MgCl2/TiCl4 catalysts) lack long-chain branching (LCB) structure, which to a great extent limits their end uses. In this paper, we report an in situ methodology of incorporating LCB structure into PE of Ziegler-Natta catalysts. The chemistry centers on a bifunctional reagent, omega-alkenylmethyldichlorosilane, where the reagent is first copolymerized with ethylene to anchor on some polymer chains pendant alkylmethyldichlorosilane groups, then upon treating the polymer with water adjacent polymer chain-borne alkylmethyldichlorosilane groups undergo one-on-one interchain hydrolytic condensation forming a dimeric alkylme-thylsiloxane bridge interconnecting two different polymer chains as H-shape LCB structure. Control of omega-alkenylmethyldichlorosilane concentration during copolymerization is key to realize the chemistry. Gel-free LCB-PEs containing controlled amounts of LCB structure were synthesized by accompanying the copolymerization with hydrogen (H-2) as chain transfer agent. The effect of alkenyl chain length of omega-alkenylmethyldichlorosilane on the formation efficiency of LCB structure was studied. The chemistry was generally applicable to different Ziegler-Natta catalysts. LCB-PEs containing increasing amounts of LCB structure exhibited proportionally increased extensional flow properties. Moreover, compared to linear plain PE, LCB-PEs were overall found to increase crystallization rates and thus increase crystallinities.

Automated summary

This paper introduced a in situ method for incorporating LCB structure into PE synthesized by Ziegler-Natta catalysts. By controlling the concentration of the bifunctional reagent during copolymerization, gel-free LCB-PEs with controlled amounts of LCB structure were successfully synthesized, showing improved extensional flow properties. Moreover, LCB-PEs demonstrated increased crystallization rates and crystallinities when compared to linear plain PE.