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

Abstract

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, ω-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 alkylmethylsiloxane bridge interconnecting two different polymer chains as H-shape LCB structure. Control of ω-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 (H2) as chain transfer agent. The effect of alkenyl chain length of ω-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.