Deciphering the function of Zn(Et)2 in tailoring comonomer insertion sequence during coordinative chain transfer ethylene/propylene copolymerization

Abstract

Conventionally, the molecular weight of polyolefin-based resins is regulated by H₂ during polymerization. In pursuit of finding an alternative for the substitution of H_2, coordinative chain transfer copolymerization (CCTcoP), comprising of (EBI)ZrCl₂, MMAO-12, TIBA, and Zn(Et)₂ (DEZ), was undertaken. Chemical composition determination by ¹³C NMR analysis revealed that zinc-assisted copolymerization (ZAC) altered comonomer sequence distribution and its content in the resulting copolymers. DEZ induced different propagating sites exhibiting an exclusive tendency for ethylene insertion ($r_E$: 11.58–18.17). This preference steered the active centers toward the production of crystallizable segmented copolymers, primarily long ethylene-rich segments possessing more isolated propylene units, a higher EEE triad (45.1–55.9 %), a larger average ethylene sequence $\overline{n}$_E (4.1–5.8), a reduced random index (0.37–0.50), and an appreciably higher melting temperature (DSC). Conversely, zinc-unassisted copolymerization (ZUC) afforded random copolymers with noticeably higher random index (0.65–0.82), lower melting points, $r_E$: 3.53–5.87, EEE triads (12.9–17.9 %), and $\overline{n}$_E (1.75–2.54).