Molecular and kinetic design for the expanded control of molecular weights in the ring-opening metathesis polymerization of norbornene-substituted polyhedral oligomeric silsesquioxanes

Abstract

Cube-like polyhedral oligomeric silsesquioxane (POSS) is a promising candidate for isotropically bulky pendants to expand the dimensional limit of polymer main chains. This paper presents molecular and kinetic insights into the controlled synthesis of rod-like POSS-containing polynorbornenes. Ring-opening metathesis polymerization (ROMP) was performed on three norbornene-substituted POSS monomers with different spacers. For monomers possessing non- and amide functionalities at the spacers, ROMP at the maximum concentration ([M]₀ = 0.4 M) led to 100% conversion, predictable molecular weights (M_n ≤ 1236 kDa) and low dispersities (Đ ≤ 1.20) in homopolymers. Scaling analysis for POSS-containing polynorbornenes revealed an unusual finding, namely, that the periodic clustering of POSS pendants favored by long flexible spacers (16-atom chains) enhanced the rigidity of polynorbornene main chains, leading to their rod-like conformation. Kinetically optimized ROMP allowed the subsequent addition of a macromonomer to create POSS-bottlebrush copolymers (POSSBBCPs). These POSSBBCPs self-assembled into thin films to form ordered nanostructures with diverse morphologies and periodicities greater than 100 nm.