Polynorbornene with silicon cluster pendant groups

Abstract

Multicyclic oligosilane clusters like permethylated bicyclo[2.2.2]octasilane (Si[2.2.2]) represent a new class of electronic insulators that harness quantum effects (i.e., destructive σ-quantum interference (σ-DQI)) to suppress charge transport. Polymeric insulators that operate from σ-DQI principles are highly promising for applications in ultrathin dielectrics. To realize such applications, however, it is crucial to devise strategies for incorporating molecular silicon clusters into polymeric materials. Here we describe the design, synthesis, and characterization of the first polymers with oligosilane clusters as pendant groups. We first describe our process to find a successful synthetic route to install alkyl-spaced norbornene termini onto Si[2.2.2] clusters. Next, we show these clusters remain structurally intact under ring-opening metathesis polymerization (ROMP) conditions to yield high molecular weight polymers with relatively narrow polydispersity (M_n = 902–1289 kDa, Đ = 1.54–1.68). UV-vis absorbance studies show that the cluster-polymer hybrids retain the same optical absorbance features that are specific to the discrete monomeric cluster. Crucially, we find that installing the Si[2.2.2] cluster pendants to polynorbornene backbones results in emergent improvements in thermal stability, where the decomposition temperature of the cluster-polymer hybrid (T_d ∼ 340 °C) increases by 80 °C compared to the free monomer. These results suggest cluster-polymer hybridization is a general strategy to improve the thermal stability of silicon cluster materials. We anticipate that the synthesis and characterization of the silicon cluster-polymer hybrids herein will be a key stepstone toward evaluating silicon clusters as σ-quantum interference-enabled dielectric materials.