Thermally Rearranged Polymers Research Articles

Enhanced mechanical robustness and separation performance in triptycene modulated thermally rearranged copolyimide membranes

Thermally rearranged (TR) polymers have seen a growing role in gas separation membranes. Further exploitation of TR membranes with enhanced mechanical properties and gas separation performance are urgently demanded. Herein, a new series of TR polymers incorporating rigid triptycene units were prepared containing amino-functionalized neighbored benzene propellers and thermally rearrangeable o-hydroxy groups. The resultant TR membranes exhibit increased chain d-spacing and gas permeability with the increase of thermal arrangement temperatures. Moreover, the presence of triptycene enhances the mechanical strength of TR polymers due to the π-π stacking effect of triptycene moieties. The 400 °C TR polymer 6FDA-6FAP:DAT(2:1) displays a H2 permeability of 345.3 Barrer with a H2/CH4 selectivity of 83.6, 440% and 118% higher than the precursor, respectively, exceeding the latest Robeson upper bound. The design principle of TR membranes in this work provides an alternative route for the development of mechanically robust and high performance membranes with potentials for energy efficient gas separations.

Transport, Spectroscopic, and Electrical Properties of Thermally Rearranged Nanocomposite Membranes

AbstractNanocomposite membranes are a class of innovative filtering materials consisting of nanofillers embedded in a polymeric or inorganic oxide matrix that functionalizes the membrane. Thermally rearranged (TR) polymers have a good blend of selectivity and permeability. Nanocomposite membranes were tested after applying a TR process. The selectivity decreases with increasing permeability, showing a trade‐off relationship between permeability and selectivity. TR polymer nanocomposite exhibits higher gas permeability than silica‐doped and pure polymer. The selectivity for H2/N2 and H2/CO2 gas pairs exceeds the Robeson upper‐bound limit, and in the case of the H2/CH4 gas pair. The selectivity for H2/N2 , H2/CH4 and H2/CO2 gas pairs exceeds the Robeson upper‐bound limit.

Thermally rearranged nanofibrous composite membranes for carbon dioxide absorption and stripping

We develop thermally rearranged (TR) nanofibre membranes for the capture of CO2 using membrane gas absorption. The porous nature of the electrospun nanofibre membranes and the hydrophobicity of the TR polymers provide ideal properties for both CO2 absorption and stripping. The use of thermal rearrangement after fabricating the nanofibre composite structures (TR-NFM) also interconnects the nanofibres and greatly improves mechanical strength. Surface hydrophobicity is further increased by electrospraying of nanoparticles onto the nanofibre membrane surface to increase roughness (TR-NCM), leading to higher breakthrough pressures and stable CO2 stripping performance for up to 270 h at 100 °C without membrane wetting. The TR-NFM and TR-NCM membranes both show an order of magnitude increase in CO2 absorption flux compared to a standard polytetrafluoroethylene (PTFE) membrane, offering strong potential for commercial application.

Polymers of intrinsic microporosity and thermally rearranged polymer membranes for highly efficient gas separation

• PIMs and TR polymers are referred to polymers with a highly microporous structure. • The major challenge of PIMs and TR polymers is achieving a high permselectivity. • We studied the monomer molecular design of energy-efficient PIM/TR based membranes. • Different methods for synthesizing PIMs and TR polymers are described. • The presence of functional/pendant groups such as are investigated. • Different strategies for preparation and tailoring of their properties are discussed. In the last decades, many novel polymeric materials have been considered to prepare highly energy-efficient gas separation membranes. There is a continuous search to overcome the trade-off relationship between the gas permeability and selectivity to improve the membrane separation performance during long-term operation. One of the most important research fields of molecular design and polymer chemistry is the synthesis of new polymers with a highly microporous structure, referred to as polymers of intrinsic microporosity (PIMs) and thermally rearranged (TR) polymers. The major challenge of PIMs and TR polymers, required for their application in membrane gas separation, is achieving a high permeability and selectivity, which remain stable over prolonged periods and under all possible operating conditions. Limits of their use can be low mechanical stability, sophisticated and challanging synthesis methods, physical aging of PIMs, etc. For TR polymers, identifying the optimum temperature in the thermal treatment process is challenging, and the preparation of thin-film TR membranes from PBO precursor via conventional methods is difficult. Moreover, the productivity of the membrane modules and the adhesion between the support/polymer and nanomaterials/polymer need to be improved in mixed matrix membranes (MMMs). Also, the formation of defects in the membrane structure is the main challenge to overcome for the fabrication of thin-film composites (TFCs). In this manuscript, we carefully review the molecular design and the properties and performance of PIMs and TR materials as energy-efficient membranes for various gas separations, emphasizing CO 2 /CH 4 and CO 2 /N 2 . Different methods for synthesizing PIMs and TR polymers are described, and the resulting polymers are evaluated for gas separation. Structure designing for the synthesis of PIMs and TR membranes and the influence of different chemical structures of monomers for creating chain rigidity by non-planar groups, and tuning the angle of contorted centres, are investigated. Moreover, the presence of functional groups and other substituents, such as fluorinated, sulfonated, carboxylic, tetrazole, and other groups, are investigated. Different strategies for membrane preparation and tailoring of their properties, such as cross-linking, co-polymerization, blending with other polymeric or organic and inorganic materials, are discussed as well.

Synthesis of imidazolium-mediated Poly(benzoxazole) Ionene and composites with ionic liquids as advanced gas separation membranes

Thermally rearranged (TR) polymers and ionic polymers are two material classes which have been employed in leading gas separation membranes. This work introduces a novel approach of combining the benzoxazole functionality associated with TR polymers with tailorable cationic groups, yielding a new type of imidazolium-mediated poly(benzoxazole) ionene polymer, “Im-PBO-Ionene” with the aim of enhanced CO2 separation performance. The structural changes exhibited from the Coulombic interactions between the ionene backbone and the “free” ionic liquid (IL) resulted in enhanced gas separation performance, shown in fundamental characterizations, observed through increased diffusivities and more notably, retained high selectivities and 3x or 5x respective increases in CO2 permeability upon the addition of 1 or 2 equivalents of IL per polymer repeat unit. These new high-performance ionenes demonstrate the versatility of ionene design and potential of the ionene + IL material platform for gas separation membranes with versatile incorporation of sophisticated functional and structural features.

Recent progress in microporous polymers from thermally rearranged polymers and polymers of intrinsic microporosity for membrane gas separation: Pushing performance limits and revisiting trade‐off lines

AbstractPolymers are promising materials for gas separation membranes. However, the trade‐off relationship between gas permeability and selectivity remains an obstacle for achieving polymer membranes that exhibit high gas permeation with desirable separation efficiency. Improving polymer microporosity is of interest in gas separation membranes to enhance gas transport behavior. Polymer modifications by (a) incorporating intrinsically microporous units and/or (b) increasing chain rigidity can enhance microporosity in conventional polymer membrane materials such as polyimides. These strategies are adopted for new classes of microporous polymers, thermally rearranged (TR) polymers, and polymers with intrinsic microporosity (PIMs), to maximize gas transport properties. Their outstanding gas separation performances have redefined the traditional trade‐off lines. This review aims to explore the advances in microporous polymers for gas separation applications. The approaches on TR polymers and PIMs to enhance their microporosity are listed, and their developments are evaluated in the context of revisiting performance limits for industrially relevant gas separation applications.

Thermally rearranged polymer membranes containing highly rigid biphenyl ortho-hydroxyl diamine for hydrogen separation

A novel bulky and rigid ortho-hydroxyl diamine, 3,3′-diamino-5,5′,6,6′-tetramethyl-[1,1′-biphenyl]-2,2′-diol (TMBDA), was synthesized for gas separation membranes in this study. The chemical structure of TMBDA with four methyl groups and a fixed twist biphenyl center was rationally designed, and consequently presented a superior energy barrier of rotation, which efficiently suppressed chain motion and enhanced polymer rigidity. Four types of TMBDA-based polyimides (PIs) were prepared with two commercially available dianhydrides, i.e., 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and pyromellitic dianhydride (PMDA), from two different synthetic routes, azeotropic and chemical imidization. Thermally rearranged (TR) polymers were obtained from TMBDA-based PIs by thermal treatment at various temperatures. Thermal properties and physicochemical characteristics of TMBDA-based PIs and TR polymers were investigated. The resulting polymers exhibited high glass transition temperatures (Tg) and contorted backbone structures with three discrete d-spacing values, where the smallest interchain d-spacing was located between the kinetic diameters of H2 and CH4 molecules. Moreover, TMBDA-based TR polymers showed outstanding performances in hydrogen separation with comparable H2 permeability and higher selectivity than reported TRs. For instance, 6F-TM-Ac-425 exhibited a H2 permeability of 325 Barrer with H2/CH4 and H2/N2 selectivities of 50 and 39, respectively, approaching to the 2008 Robeson upper bound.

Highly permeable Thermally Rearranged Mixed Matrix Membranes (TR-MMM)

Thermally Rearranged (TR) polymers and Mixed Matrix Membranes (MMMs) are two effective approaches used to advance the performance of gas separation membranes. Conversion of thermally activated groups in TR membranes result in hourglass-shaped cavities and unusually high permselectivity, whereas the inclusion of nanoparticles with engineered pore volume, window size and/or surface chemistry in MMMs can add fast and selective pathways for gas transport. In this study, we explored the effects of combining these two approaches by adding ultra-porous and highly thermostable PAF-1 nanoparticles into the TR-able polymer, 6FDA-HAB5DAM5 (DAM). Gas separation performances of TR-MMMs were evaluated by comparison with the pure polymer and another TR-MMM bearing an already thermally-treated PAF-1 additive (cPAF). While both additives enhanced gas transport, only PAF-1 stabilized the TR conversion reaction and improved the TR-MMM's material properties. Minor variations in cPAF surface changed the TR-MMM interfacial interactions as well as other properties crucial to the application of advanced membrane materials, namely aging, plasticization, and mechanical stability. By combining thermal rearrangement process with PAF-1, TR-MMM demonstrated a 55-fold increase in CO2 gas permeability (37-fold for H2) at similar gas selectivities, but without the handling issues observed for the pure TR polymer membrane.

Mixed matrix membranes with a thermally rearranged polymer and ZIF-8 for hydrogen separation

Mixed matrix membranes (MMMs) with a thermally rearranged (TR) polymer and zeolitic imidazolate framework-8 (ZIF-8) were prepared in this study. ZIF-8 contributed to not only enhancing gas permeability, but also promoting thermal conversion of TR polymer membranes. As a result, TR ZIF-8 MMMs can be prepared at lower temperatures compared to the original TR polymers. The thermal rearrangement process also increased gas separation properties as it contributes to a tunable cavity size and hour-glass shape cavity distributions. Moreover, interfacial voids between the polymer phase and ZIF crystals which usually occur in most MMMs disappeared during thermal rearrangement of the TR polymer. As a result, the TR ZIF-8 MMMs showed excellent hydrogen separation properties with a H2 permeability of 1,206 Barrer, H2/N2 selectivity of 22.3, and H2/CH4 selectivity of 25.7.

An atomistic insight on CO2 plasticization resistance of thermally rearranged 6FDA-bisAPAF

An emerging class of thermally rearranged (TR) polymer has been of great interest for its extraordinary transport properties. More importantly, harnessing the full potential of TR polymer as gas separation membrane for CO2/CH4 separation is through its ability to resist plasticization at high pressures. Accordingly, we report here on the effect of CO2-induced plasticization on polyimide precursor (6FDA-bis-APAF: 4,4-hexafluoro isopropylidene- diphthalic anhydride – 2,2′-bis(3-amino-4-hydroxyphenyl) – hexafluoropropane) and on the resulting thermally rearranged polybenzoxazole (TR-PBO) polymer membranes as investigated through the radial distribution function and accessible free volume analyses. Using molecular simulation techniques, structural properties such as d-spacing, glass transition temperature, fractional free volume, etc. were estimated in agreement with wide range of experimental observations, which are published within the last decade. Results showed that, TR polymer displayed restricted % FFV increase up to 40 bar due to its limited chain mobility as indicated by the dihedral distribution, and sorption sites on its backbone with lower affinity to CO2 as shown by the RDF analyses. Additionally, analysis of free volume elements suggests that the ability of TR polymers to maintain their interconnected microstructure and resistance to CO2-induced plasticization at high pressures leads also to higher diffusion and hence permeation performances and as a result, make them promising materials in gas separation applications.

Sorption, diffusion, and permeability of humid gases and aging of thermally rearranged (TR) polymer membranes from a novel ortho-hydroxypolyimide

We studied in this work the properties of a new membrane (TR-PBO) obtained by solid state thermal rearrangement at 450°C of a recently developed polyimide precursor, (mHAB-6FDA), which was synthesized by reaction of (3,3-diamino-4,4-dihydroxybiphenyl, mHAB) with 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA). The mHAB monomer is an isomer of the commercial 3,3′-dihydroxybenzidine (pHAB), used to form the more popular polyimide precursor pHAB-6FDA. TR-PBO membranes obtained from mHAB-6FDA showed excellent CO2 permeability (720 Barrer) and good CO2/CH4 ideal selectivity of 23. We found out that the thermal rearrangement enhances the solubility and diffusion coefficients of CO2 at 10bar by factors as high as 1.3 and 5, respectively. Larger enhancements, however, were observed in the case of CH4, causing the diffusivity selectivity to decrease by a factor of 2.6 and the solubility selectivity to decrease by a factor of ~1.5 upon rearrangement. The pure gas solubility was modeled with the Dual Sorption Mode model and the NELF model. The two models were then used to predict the mixed gas behavior in terms of solubility-selectivity, highlighting the effects of competition that are consistent with those observed in other glassy polymers.We also performed moisture sorption tests and gas permeability measurements in the presence of humidity. It was observed that the thermal rearrangement increases the membrane hydrophobicity and, consistently, the CO2 and CH4 permeability of mTR-PBO membranes is much more stable in the presence of humidity than that of the precursor polyimide membranes. Finally, the effect of aging on the membrane performance was analyzed. A 30% decrease in the CO2 permeability of TR polymer membranes (around 50µm thick) was observed after 6 months, while the selectivity increased by 20%. These results indicate that, even after 6 months, the performance of the TR polymer membrane was outstanding and close to Robeson's upper bound.

Influence of temperature on gas solubility in thermally rearranged (TR) polymers

Thermally rearranged (TR) polymers have been the subject of many fundamental studies, but the effect of TR conversion on temperature-dependent transport properties is largely unexplored. Sorption isotherms for N2, CH4, and CO2 in HAB-6FDA polyimide and its TR analogs were measured at temperatures ranging from −10°C to 50°C and pressures up to 27atm. Solubilities increase with decreasing temperature for each gas and sample tested. At low TR conversions, the sorption process initially becomes less exothermic. However, enthalpies of sorption do not significantly change with TR conversion after the initial stages of rearrangement. Enthalpies of sorption in TR polymers are qualitatively similar to those of other high free volume materials. Solubility selectivity for CO2/CH4 at 10atm did not change with temperature due to similar enthalpies of sorption for CO2 and CH4. Sorption data were fit to the dual mode model at different temperatures, and model parameters were correlated with polymer and penetrant properties.

Analysis of the transport properties of thermally rearranged (TR) polymers and polymers of intrinsic microporosity (PIM) relative to upper bound performance

A critical analysis comparing the diffusion selectivity, solubility selectivity, diffusivity, and solubility coefficients of thermally rearranged (TR) polymers and polymers of intrinsic microporosity (PIM), two types of polymers which consistently perform at or beyond the polymer upper bound for certain gas pairs (O2/N2 and CO2/CH4), is reported here. Although most polymers in these two classes exhibit transport properties in the range of typical glassy polymers, several variants offer exceptional performance. The diffusivity selectivity for O2/N2 and CO2/CH4 for TR polymers and PIM lies outside the range of the glassy polymer database. The solubility coefficients for O2 are at the upper end of the range of both TR and PIM polymers, as is also the case for CO2 solubility coefficients for PIM polymers. The O2/N2 and CO2/CH4 solubility selectivities for both PIM and TR polymers are in the range of typical glassy polymers. Thus, unique separation values for these polymers are a manifestation of maximizing diffusivity selectivity combined with very high gas solubility. Additionally, the previously determined diffusion gas kinetic diameters, dg, were found to correlate best with diffusion coefficients of PIM and TR polymers and serve to better analyze transport properties of upper bound materials.

Spirobisindane-based polyimide as efficient precursor of thermally-rearranged and carbon molecular sieve membranes for enhanced propylene/propane separation

High performance thermally-rearranged (TR) and carbon molecular sieve (CMS) membranes made from an intrinsically microporous polymer precursor PIM-6FDA-OH are reported for the separation of propylene from propane. Thermal rearrangement of PIM-6FDA-OH to the corresponding polybenzoxazole (PBO) membrane resulted in a pure-gas C3H6/C3H8 selectivity of 15 and C3H6 permeability of 14 Barrer, positioning it above the polymeric C3H6/C3H8 upper bound. For the first time, the C3H6/C3H8 mixed-gas properties of a TR polymer were investigated and showed a C3H6 permeability of 11 Barrer and C3H6/C3H8 selectivity of ~11, essentially independent of feed pressure up to 5bar. The CMS membrane made by treatment at 600°C showed further improvement in performance as demonstrated with a pure-gas C3H6/C3H8 selectivity of 33 and a C3H6 permeability of 45 Barrer. The mixed-gas C3H6/C3H8 selectivity dropped from 24 to 17 from 2 to 5bar feed pressure due to a decrease in C3H6 permeability most likely caused by competitive sorption without any evidence of plasticization.

Claisen thermally rearranged (CTR) polymers.

Thermally rearranged (TR) polymers, which are considered the next-generation of membrane materials because of their excellent transport properties and high thermal and chemical stability, are proven to have significant drawbacks because of the high temperature required for the rearrangement and low degree of conversion during this process. We demonstrate that using a [3,3]-sigmatropic rearrangement, the temperature required for the rearrangement of a solid glassy polymer was reduced by 200°C. Conversions of functionalized polyimide to polybenzoxazole of more than 97% were achieved. These highly mechanically stable polymers were almost five times more permeable and had more than two times higher degrees of conversion than the reference polymer treated under the same conditions. Properties of these second-generation TR polymers provide the possibility of preparing efficient polymer membranes in a form of, for example, thin-film composite membranes for various gas and liquid membrane separation applications.

Gas permeation properties of thermally rearranged (TR) isomers and their aromatic polyimide precursors

This study explores the influence of meta/para and ortho-position functional group structures on gas transport properties of two isomeric polyimides and their thermally rearranged (TR) polymers. The diamine isomers, 3,3′-dihydroxy-4,4′-diamino-biphenyl (p-HAB) and 4,4′-dihydroxybiphenyl-3,3′-diamino-biphenyl (m-HAB), were polymerized with 2,2′-bis-(3,4-dicarboxy-phenyl) hexafluoropropane dianhydride (6FDA) to form two aromatic polyimide isomers. These polyimides were also prepared with either hydroxyl or acetate groups at the ortho-positions to the imide ring, and the polyimides were partially converted to the corresponding polybenzoxazoles by thermal treatment at various temperatures and times. Single gas permeabilities of CH4, H2, O2, N2 and CO2 were measured at 35°C and upstream pressures ranging from 3atm to 17atm. Polyacetylimides and their corresponding TR polymers showed higher gas permeability and lower selectivity than polyhydroxyimides and their TR analogs, respectively. The effect of isomerism on gas transport properties was also investigated. Para-connected polyhydroxyimide had higher fractional free volume, and thus, gas permeability coefficients than meta-connected polyhydroxyimide. Interestingly, in contrast to the meta/para effects previously reported in the literature for linear aromatic polymers, the meta-linked polyacetylimide had higher permeabilities and lower selectivities than its para-linked analog, presumably due to steric hindrance from the acetate groups that may inhibit the rotational mobility of the phenylene units in the para-linked isomer. For TR polymers derived from either polyhydroxyimides or polyacetylimides, para-configured polymers had higher gas permeabilities than meta-configured analogs. Finally, polyimide precursors for TR polymers can be sensitive to synthesis and casting conditions, and this study demonstrates how small variations in synthesis and casting procedure can have a significant effect on physical and gas permeation properties.

Ternary mixed-gas separation for flue gas CO2 capture using high performance thermally rearranged (TR) hollow fiber membranes

Abstract A comprehensive study evaluated the mixed-gas (CO2/N2/O2) separation performance of thermally rearranged polybenzoxazole-co-imide (TR-PBOI-AD5) hollow fiber membranes fabricated in-house (pure-gas CO2 permeance of 481 GPU and ideal CO2/N2 selectivity of 17.7). The criteria for the determining the two major operating conditions (pressure ratio and feed flow rate) were identified in order to deliver optimal separation performance with low process energy consumption. By varying the feed CO2 concentrations, it was found that a single-stage operation using TR-PBOI-AD5 (produced by bis-APAF and DAM as the TR-able and non-TR-able diamines, respectively, based on 6FDA) hollow fiber membranes was not sufficient to meet the required 90% permeate CO2 purity target, while a two-stage operation using the same membranes led to a permeate CO2 purity (81%) closer to the target value. The comparison study among the TR polymer membranes and three other polymer membranes highlighted the significance of an appropriate choice of a mixed-gas separation operating scheme to fully utilize the exceptional permeation properties of the recently developed high performance membranes such as TR polymeric membranes.

Synthesis and characterization of thermally rearranged (TR) polybenzoxazoles: Influence of isomeric structure on gas transport properties

Isomeric polyhydroxyimides based on 3,3′-diamino-4,4′-dihydroxybiphenyl (m-HAB) or 3,3′-dihydroxy-4,4′-diaminobiphenyl (p-HAB) with 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) were prepared via an ester-acid monomer. The polyhydroxyimides were then acetylated using acetic anhydride to change the ortho-functional groups on the polymer chains. These ortho-functional polyimides were used as precursors for thermal rearrangement (TR) to polybenzoxazoles for gas separation membranes. The permeability coefficients of TR polymers significantly improved as the ortho-functional polyimides converted to polybenzoxazoles. The influence of meta and para isomeric monomers on the gas transport properties of the resulting TR polybenzoxazoles were studied. In addition, gas permeation properties show a dependency on the size of the ortho-functionality of the polyimide precursors.

Effect of polymer structure on gas transport properties of selected aromatic polyimides, polyamides and TR polymers

Thermally rearranged (TR) polymers are formed through a thermally induced solid-state reaction of polyimides or polyamides that contain nucleophilic reactive groups ortho-positioned to their diamine. Naturally, the transport properties of TR polymers are intimately related to the chemical structure and reactivity of their precursors. Herein, we report characterization and transport properties for three poly(hydroxyimide) precursors prepared via thermal imidization in solution and for their corresponding TR polymers. Structural modifications to the polymer backbone can be used to control thermal rearrangement reaction kinetics. In regards to TR polymer formation, samples prepared from diamines with biphenyl functionality reacted more efficiently than those prepared from diamines with hexafluoroisopropylidene-linked aromatic units. However, hexafluoroisopropylidene functional units provided the highest combinations of permeability and selectivity for separations involving H2, N2, O2, CH4, and CO2. Differences in permeability between samples correlated well with changes in free volume, and 3 poly(hydroxyimide)s showed unusually high selectivities for their given free volume. The effect of synthesis route was also investigated for a specific TR polymer derived from 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA). Poly(hydroxyimide) precursors prepared via thermal imidization in solution and thermal imidization in the solid-state showed nearly identical permeabilities and selectivities regardless of synthesis route. However, after thermal rearrangement, the TR polymers prepared from polyimides synthesized via solid-state imidization have higher gas permeabilities than their solution-imidized analogs. In addition to light gas permeabilities, plasticization effects were investigated with CO2 hysteresis loops for all samples, and pure-gas olefin/paraffin permeabilities were determined for a TR polymer derived from 2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane (APAF) and 6FDA. With the exception of HAB–6FDA polyimides, pure-gas CO2 feed pressures up to approximately 50bar do not reveal a plasticization pressure point, but conditioning effects are observed for most samples. APAF–6FDA TR polymers have pure-gas permeabilities and selectivities beyond the propylene/propane upper bound.

Gas Membranes for CO2/CH4 (Biogas) Separation: A Review

Gas separation by membrane carries many advantages over other methods of gas separation, including the amenability of the gas separation process to simplification and scaling up, and the elimination of the need for phase change of the gas. Material composition is, by far, the most important factor in determining the gas separation effectiveness of a membrane. This article is a review of the existing research on polymers as used in the separation of carbon dioxide/methane (CO2/CH4), generally focusing on polyimide and polysulfone compounds. Data on thermally rearranged (TR) polymer membranes and mixed matrix membranes were also included. In this study, the membrane performance of different polymeric materials was examined by comparing the data on CO2/CH4 selectivity obtained through a literature review and the upper bound proposed by Robeson in 2008. Results showed that polyimide and polysulfone membranes are highly applicable to the separation of CO2/CH4 gas mixtures: polyimide showed particular promise for commercial implementation. Polysulfone membranes exhibited high selectivity but low CO2 permeability that failed to exceed the upper bound. However, case studies suggested that polysulfones in the hollow fiber membrane form have a wide variety of applications in an industrial setting. On the other hand, the TR polymer membranes, which are currently used for research, had a CO2 permeability of 5–5,903 Barrer and CO2/CH4 selectivity of 3.8–124, which exceeded the upper bound proposed by Robeson. We, thus, came to the conclusion that future research on CO2/CH4 gas separation should focus on polysulfone, polyimide, and TR membranes as materials for economically efficient and highly effective gas separation membranes.