Poly(methyl methacrylate), or PMMA, is one of those “quietly essential” polymers you see everywhere without noticing. It appears as transparent panels instead of glass, as bone cement in orthopedics, as a component in dental fillings, in optical devices, and even as a matrix in gel polymer electrolytes for lithium batteries.

A key point that often gets overlooked: PMMA’s performance is strongly tied to its molecular weight. High molecular weight PMMA tends to be tougher, more mechanically stable, and better suited for demanding applications such as high-performance electrolytes or durable medical materials. Pushing that further, ultrahigh molecular weight (UHMW) PMMA opens even more possibilities—but it is not easy to make.

Traditionally, reaching very high molecular weights requires harsh or impractical conditions: high pressures, long reaction times, special solvents, or complex catalyst systems. That’s where a special metal–organic framework enters the story: ZIF-L, a two-dimensional zeolitic imidazolate framework.

In this blog, we’ll walk through how a 2D MOF with a leaf-like morphology, ZIF-L, can act as a heterogeneous Lewis pair catalyst for polymerizing methyl methacrylate (MMA) into UHMW PMMA—without solvent and without any co-catalyst. We’ll keep the explanation simple, but still faithful to the science.

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1. From MOFs to ZIFs to ZIF-L: Setting the Stage

What are MOFs?

Metal–organic frameworks (MOFs) are crystalline materials built from metal ions (or clusters) connected by organic linkers. Imagine a 3D scaffold made of metal nodes and organic rods. Because of their design, MOFs:

• Have huge internal surface area

• Offer tunable pore sizes and shapes

• Often show good thermal and chemical stability

These properties make them attractive in gas storage, separation, catalysis, sensing, and biomedical applications.

What are ZIFs?

A specific family of MOFs is called zeolitic imidazolate frameworks (ZIFs). Here:

• The metal is usually Zn²⁺ or Co²⁺

• The organic linker is an imidazolate (a deprotonated imidazole-based molecule)

Structurally, ZIFs resemble zeolites but with organic bridges instead of simple oxygen atoms. They combine:

• The robustness of inorganic zeolites

• The tunability and functionality of MOFs

Because of their high surface area, porosity, and stability, ZIFs are widely explored in gas storage, separations, catalysis, and energy-related fields.

Enter ZIF-L: A 2D, Leaf-Like Framework

Most people in the MOF world know ZIF-8, a classic 3D ZIF with a sodalite-type structure. ZIF-L, on the other hand, is a 2D form made from:

• Zinc nitrate hexahydrate

• 2-methylimidazole (2-mIm)

• In water, at room temperature

ZIF-L has:

• A leaf-like morphology (hence the “L”)

• Plate-like crystals with micrometer-scale length and width, but nanometer-scale thickness

• A porous internal structure similar in some respects to ZIF-8, but with different topology and connectivity

Structurally, ZIF-L is interesting because:

• It contains two types of Zn²⁺ centers

• Each Zn is coordinated by nitrogen atoms from 2-mIm linkers

• There is also a “free” 2-mIm molecule in the structure

• Neighboring 2D layers are held together by hydrogen bonds, not by deprotonated linkers as in ZIF-8

This last point matters: because layers are connected only by hydrogen bonds, the metal sites in ZIF-L are more accessible than in ZIF-8. That means ZIF-L can present Lewis acidic metal centers and Lewis basic nitrogen sites in a way that is quite attractive for catalysis.

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2. Why PMMA and Why Molecular Weight Matters

Methyl methacrylate (MMA) is the monomer used to make PMMA. Polymerizing MMA can be done with various techniques (radical, anionic, controlled/living methods), but making ultrahigh molecular weight PMMA is challenging.

PMMA’s applications depend heavily on its molecular weight:

• In gel polymer electrolytes (GPEs) for batteries, high molecular weight PMMA offers:

  • Better mechanical strength

  • Improved film-forming ability

  • Good dimensional stability

• In biomedical uses (bone cement, dental fillings), higher molecular weight often translates into improved toughness and durability.

• For optical and sensor applications, properties such as transparency, thermal behavior, and mechanical stability are affected by chain length.

Past attempts to obtain UHMW PMMA often needed:

• High-pressure conditions

• Long reaction times

• Special polar solvents like DMSO

• Added organic co-catalysts (e.g., particular amines)

The work we’re discussing here asks a simple but powerful question:

Can we make UHMW PMMA under simpler, greener conditions using a solid catalyst?

The answer, surprisingly, is yes—and ZIF-L is the key.

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3. Building and Characterizing ZIF-L

How ZIF-L is Prepared

ZIF-L is synthesized in water at room temperature, using:

• Zn(NO₃)₂·6H₂O

• 2-methylimidazole

• A specific Zn²⁺/2-mIm ratio (1:8 in the described synthesis)

The solutions are mixed, stirred for a few hours, and the resulting solid is collected, washed, and dried. No harsh conditions or exotic solvents are required.

What Confirms the Structure?

Several standard characterization techniques are used (without going into image-specific details):

• Scanning Electron Microscopy (SEM)
  Confirms the leaf-like morphology, with micrometer-sized particles that are thin and plate-like.

• Powder X-ray Diffraction (PXRD)
  Shows that the material is highly crystalline and matches known ZIF-L patterns. The peaks also show that it’s structurally similar to ZIF-8 in some ways, but clearly distinct.

• Fourier Transform Infrared Spectroscopy (FT-IR)
  Identifies the vibrations associated with:

  • Zn–N bonds (evidence of metal–linker coordination)

  • The imidazole ring (bending and stretching modes)

  • Hydrogen bonding between N–H groups and nitrogen atoms

• Thermogravimetric Analysis (TGA)
  Shows how the material behaves when heated:

  • Minor mass loss at lower temperatures is associated with desorption of water and weakly bound molecules.

  • A plateau at intermediate temperatures indicates good thermal stability of the framework once volatile species are gone.

• Nitrogen Sorption (BET Analysis)
  Reveals a microporous structure, with a type-I isotherm and modest but clear surface area and pore volume.

Together, these measurements paint a consistent picture: ZIF-L is a stable, crystalline, microporous 2D MOF with accessible metal and nitrogen sites—promising for catalytic applications.

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4. ZIF-L as a Catalyst: Bulk Polymerization of MMA

Once ZIF-L is in hand, the next step is to test whether it can actually catalyze the polymerization of MMA.

Key Features of the Polymerization Setup

• Heterogeneous catalyst: ZIF-L is a solid.

• Solvent-free: MMA is polymerized in bulk; no additional liquid medium is used.

• No co-catalyst: Unlike many Lewis pair polymerizations, no extra organic Lewis base or acid is added. ZIF-L alone provides the required sites.

• Inert atmosphere: Reactions are carried out under an inert gas (e.g., argon) using standard Schlenk techniques.

• Temperature and time: The study explores a range of temperatures and reaction times.

Before the reaction, ZIF-L is usually “activated” by gentle heating under vacuum to remove residual adsorbed species and open up its pores and active sites.

ZIF-L, MMA, and no solvent: that’s the basic recipe.

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5. How Well Does It Work? Molecular Weight, Conversion, and “Living” Behavior

The performance of ZIF-L as a catalyst for MMA polymerization is evaluated by looking at:

• Monomer conversion (how much MMA turns into PMMA)

• Number-average molecular weight (Mn) of the polymer

• Dispersity (Ð), which indicates how narrow or broad the molecular weight distribution is

Effects of Temperature and Time

When the polymerization conditions are varied:

• Increasing the temperature (for the same reaction time and catalyst loading) generally:

  • Increases the conversion

  • Increases the molecular weight (up to a point)

• Increasing the reaction time at a fixed temperature:

  • Allows the polymer chains to grow longer, leading to higher molecular weights

At a suitable combination of 140 °C and 24 hours, with a certain monomer-to-catalyst ratio, the system produces PMMA with:

• Ultrahigh molecular weight, Mn up to around 1390 kg/mol

• Still relatively controlled dispersity (not extremely broad)

This is a very high molecular weight for PMMA, especially considering:

• No co-catalyst is used

• The medium is solvent-free

• The catalyst is heterogeneous and recyclable

Effect of Monomer/Catalyst Ratio

If you increase the MMA/ZIF-L ratio (i.e., use more monomer per catalyst site):

• The molecular weight tends to decrease

• The conversion also decreases somewhat, especially at higher ratios

This makes sense: with fewer catalytic sites available per amount of monomer, each chain has fewer chances to grow to extreme lengths before the system becomes too viscous or diffusion-limited.

Signs of “Living” Polymerization

The study examines how molecular weight grows as monomer conversion increases. Key observations:

• Mn increases roughly linearly with conversion, a classic signature of a “living” or controlled polymerization.

• The system can undergo chain extension: previously made PMMA can help propagate further when more monomer is added under suitable conditions.

In a living or living-like polymerization:

• Active chain ends don’t terminate immediately.

• The number of active chains stays relatively constant.

• Chains grow in a more uniform fashion, making higher molecular weights easier to achieve under control.

In reality, the system is “living-like” rather than perfectly ideal—some chain termination does occur, especially when exposed to air—but the trend is clearly closer to controlled polymerization than to a simple uncontrolled radical process.

Kinetics and Viscosity Effects

Kinetic analysis shows:

• Initially, the rate of polymerization behaves as if it follows first-order kinetics with respect to monomer.

• As time passes and the molecular weight increases, the reaction mixture becomes more viscous.

• Higher viscosity restricts the mobility of monomers and growing chains, so the rate slows and deviates from perfect first-order behavior.

In other words, the chemistry wants to keep going, but the physical properties of the mixture start getting in the way.

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6. What Does the Polymer Look Like? Structural Insights into PMMA

To confirm that PMMA is really being formed—and to understand its end groups and chain structure—several techniques are used.

1H NMR

Proton NMR provides:

• Signals corresponding to the backbone methylene and methine protons of PMMA

• A clear methoxy group signal (from the –COOCH₃ side groups)

• Additional signals that point to the presence of hydroxyl (–OH) groups and a lactone ring at the chain end

The lactone ring is associated with an intramolecular “backbiting” process during chain termination: the end of the growing chain folds back and forms a cyclic structure. This is consistent with what’s known for some Lewis pair polymerizations when acidic sites are involved.

MALDI-TOF Mass Spectrometry

Mass spectrometry of low-molecular-weight PMMA provides:

• A series of peaks, with spacing equal to the mass of one MMA unit

• A linear relationship between m/z and the number of repeat units

The intercept indicates a lactone-containing chain end, supporting the idea that many chains terminate in that way.

FT-IR and DSC

Infrared spectroscopy:

• Shows absorption bands consistent with PMMA

• The absence of a C=C double-bond feature confirms that the monomer double bonds have been consumed.

Differential scanning calorimetry (DSC):

• Gives a glass transition temperature (Tg) for the PMMA around 125 °C, in line with expectations for high-molecular-weight PMMA.

All of this builds a coherent picture: ZIF-L catalyzes the formation of linear PMMA chains, many ending in a lactone ring, with very high molecular weights and properties consistent with high-performance PMMA.

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7. How Does ZIF-L Actually Catalyze the Reaction? The Lewis Pair Mechanism

ZIF-L contains both:

• Lewis acidic sites (Zn²⁺ centers)

• Lewis basic sites (nitrogen atoms in imidazolate linkers)

To probe this, the study uses temperature-programmed desorption (TPD) with ammonia (for acidity) and carbon dioxide (for basicity). The results show:

• ZIF-L has both acid and base sites, but acidic sites dominate.

• The acid/base ratio is in a range where Lewis pair polymerization of MMA is expected to be effective.

Zwitterionic Lewis Pair Polymerization (LPP)

The proposed mechanism is a zwitterionic LPP:

1. Activation of monomer by Lewis acid (LA):
   MMA coordinates to an acidic Zn site, making its double bond more electrophilic.

2. Attack by Lewis base (LB):
   A basic site (e.g., a nitrogen atom in the framework) attacks the activated monomer, creating a zwitterionic active species—a chain end with both positive and negative character.

3. Propagation:
   The zwitterionic species adds more LA-activated MMA units. The growing chain carries the LB end, and the LA site activates the next monomer. This repeating sequence leads to chain growth.

4. Termination (often in air):
   Under air exposure, backbiting cyclization can lead to formation of a six-membered lactone ring at the chain end. Once this lactone end forms, the chain can no longer propagate new monomers in air. Under inert conditions, however, chains may continue growth via extension.

This mechanism explains:

• The living-like nature of the polymerization (controlled growth)

• The presence of lactone chain ends in the final polymer

• The dependence of activity on the balance of Lewis acid and base sites in ZIF-L

It also highlights why ZIF-L, with its accessible metal centers and hydrogen-bonded layers, can outperform some other MOFs in this type of catalysis.

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8. Can the Catalyst Be Reused? Stability and Recyclability of ZIF-L

One of the big advantages of heterogeneous catalysts is that they can often be:

• Recovered

• Reused

• Evaluated for metal leaching

In this system:

• After polymerization, ZIF-L can be filtered off, washed, and dried.

• Structural studies (PXRD, SEM, surface area measurements) after multiple cycles show that ZIF-L retains its crystalline structure and morphology.

• Inductively coupled plasma (ICP) analysis of zinc in the reaction mixture shows only very small amounts of Zn leaching, which is important for:

  • Product purity (especially for biomedical or electronic applications)

  • Catalyst durability

When reused in further MMA polymerizations under the same conditions, recovered ZIF-L:

• Maintains high catalytic activity

• Produces PMMA with only slightly lower molecular weight after several cycles

This indicates that the framework is robust under the reaction conditions and that structural changes during long reaction times are minor.

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9. Why This Matters: Green Polymerization and Future Directions

Putting everything together, this work shows that:

• A 2D MOF (ZIF-L) can be used as a solid Lewis pair catalyst.

• The system operates under solvent-free conditions and without co-catalysts, which moves it closer to green chemistry principles.

• It can produce ultrahigh molecular weight PMMA with Mn around 1.39 million g/mol, which is highly relevant for:

  • Gel polymer electrolytes

  • High-performance structural and biomedical applications

  • Advanced optical and sensor materials

Some key takeaways:

1. Structure–function link:
   The 2D, hydrogen-bonded architecture of ZIF-L makes metal sites more accessible than in many 3D MOFs. This structural feature directly influences its catalytic behavior.

2. Dual-site catalysis:
   Having both Lewis acidic and basic sites in one solid enables internal Lewis pairs, avoiding the need to add separate molecular acids and bases.

3. Controlled, living-like polymer growth:
   The polymerization behaves in a way that resembles living polymerization, allowing systematic tuning of molecular weight via reaction conditions.

4. Recyclability and low leaching:
   ZIF-L remains structurally stable and can be reused several times with minimal Zn contamination in the product.

Going forward, concepts from this system could be extended to:

• Other vinyl monomers, not just MMA

• Designing new MOFs or 2D frameworks specifically engineered for Lewis pair polymerization

• Tailoring catalysts for specific target molecular weights and architectures (block copolymers, gradient copolymers, etc.)

In short, ZIF-L is more than just an interesting 2D MOF. It is a practical and promising platform for greener, more controlled synthesis of ultrahigh molecular weight polymers—especially PMMA, which continues to play a central role in advanced materials and energy technologies.