Simultaneous Cross-Linking Olefin Polymerization Technology for Heterophasic Copolymerization of Propylene toward High-Rubber-Content TPO: Impact of New Nonconjugated α,ω-Diene with Increased Conformational Flexibility as Cross-Linker

Abstract

To address particle agglomeration caused by ethylene-propylene rubber (EPR) phase migration during the industrial production of high-rubber-content thermoplastic polyolefin elastomers (TPO) and to overcome the low secondary insertion efficiency of 1,9-decadiene (DD), this study proposes a conformational-flexibility modulation strategy grounded in our previously established triconcentration framework (Ccritical–Cinduction–Cgel) for simultaneous cross-linking olefin polymerization (SCOP). A novel nonconjugated α,ω-diene cross-linker, bis(7-octenyl)dimethylsilane (D8M2), featuring a flexible –Si(CH3)2– spacer, was designed and synthesized. The low rotational energy barrier and elongated bond length of Si–C bonds in D8M2 significantly mitigate steric hindrance at the active center, boosting the fraction of effective long-chain branching (LCB) formation from ∼8% (for DD) to 94.2%. Systematic investigation of D8M2 concentration effects on polymerization kinetics, chain topology, and phase morphology confirms the general applicability of the triconcentration framework to organosilicon cross-linkers. Mechanistic studies reveal that the conformational-entropy gain of the –Si(CH3)2– group accelerates arm-retraction relaxation despite promoting efficient cross-linking, creating an intrinsic trade-off between high insertion efficiency and weakened topological constraints. Consequently, a relatively high concentration (∼0.05 mol/L) is required to achieve discrete, nonagglomerated particles. This work elucidates the quantitative relationship between conformational flexibility and topological constraint strength, demonstrating that flexible spacers can broaden the effective operating window while lowering the characteristic concentrations, thereby providing crucial guidance for the rational molecular design of next-generation SCOP cross-linkers.