(373p) Effect of Grafting Density and Sidechain Length on Mechanical and Gas Transport Properties of Poly(ladder) Romps

Chemical purification methods like distillation, drying, and evaporation have been widely used for centuries. However, these separation processes account for approximately 16 quadrillion BTUs (16 Quads) of energy in the United States annually, with distillation responsible for 53% of this energy use and roughly 16% of all CO₂ emissions. Replacing these processes with more energy efficient alternatives could save 100 million tonnes of CO₂ emissions and $4 billion in energy costs annually. Membranes are an attractive alternative to many separation processes since they do not require phase changes, moving parts, or thermal regeneration to maintain separation performance. Incorporating membrane-based technologies could lead to a 90% reduction in energy costs compared to distillation.

Membranes made from polymers like polysulfones, polyimides, polyphenylene oxides, cellulose acetates, and silicon rubber are already used in industry for hydrogen recovery, nitrogen production, vapor recovery, and natural gas treatment. Despite being in use for several decades, these membranes are limited in performance for other separation processes like CO₂ capture from flue gas. Additionally, many of these aforementioned polymers lack plasticization resistance (thereby reducing size-sieving ability) and suffer from physical aging (leading to decreased permeability over time). Polymers of Intrinsic Microporosity (PIMs), in particular PIM-1 have a ladder-like structure that limits dense packing and therefore has high free volume. PIM materials have shown high selectivity and permeabilities for CO₂ and other mixed and pure gases due to their backbone structure. However, like many other membranes, PIMs experience physical aging and are prone to plasticization.

To address these limitations, microporous bottlebrush polymers featuring rigid, pore-generating side chains on flexible backbones were synthesized through a reaction based on Ring Opening Metathesis Polymerization (ROMP). Macromonomers were produced from Diels-Alder reactions before converting into high molecular weight polymer with low polydispersity by ROMP. The first-generation poly(ladder) ROMPs showed tremendous potential for membrane-based gas separations. OMe-ROMP and CF₃-ROMP displayed permeabilities and selectivities similar to PIMs, and demonstrated stability against pure-gas CO2 plasticization with no discernable plasticization pressure point up to 51 bar. CF₃-ROMP also exhibited record-breaking CO₂ permeability, ideal for carbon capture applications.

One limitation of these poly(ladder) ROMPs is their tendency to form brittle films. Previous studies suggests that bottlebrush melts and lightly crosslinked gels can achieve higher strain at breaks by increasing the degree of polymerization of the side chains (n_sc) and by decreasing grafting density, resulting in super elastic materials that are 1,000 times softer than conventional elastomers.

In this study, OMe-ROMP macromonomers were either separated by molecular weight using silica gel chromatography and underwent ROMP to produce poly(ladder) ROMPs with monodispersed sidechains or copolymerized with simple norbornenes to probe the effects of increasing n_sc and grafting density on the elasticity and gas separation performance, respectively. Using copolymerization to alter the grafting density of poly(ladder) ROMPs resulted in more elastic films with greater selectivity, but decreased permeability and plasticization resistance. As the sidechain length increased, more elastic films were formed with improvements to gas separation performance and increased CO₂/CH₄ mixed-gas plasticization resistance. Altering grafting density and length of the macromonomers provided greater insight into the design of poly(ladder) ROMPs that gives rise to greater mechanical elasticity, unprecedented plasticization resistance, and remarkable gas separation performance.