1. A quick refresher: MOFs, ZIFs and ZIF-L

1.1. What are MOFs?

Metal–organic frameworks (MOFs) are crystalline materials built from:

• Metal ions or clusters (like Zn²⁺, Co²⁺, etc.)

• Organic linkers (often multi-dentate ligands with nitrogen or oxygen atoms)

These components assemble into ordered, porous networks. MOFs are famous for:

• Very high surface areas

• Tunable pore sizes and shapes

• Chemical and thermal stability (depending on the system)

• Flexible chemistry – you can design them by choosing different metals and linkers

Because of this, MOFs are used or explored in:

• Gas storage and separation

• Heterogeneous catalysis

• Sensors

• Drug delivery and other biomedical uses

Their performance depends not just on the chemical composition, but also on morphology (2D vs 3D, particle size, crystal shape) and the accessibility of active sites.

1.2. ZIFs: Zeolitic imidazolate frameworks

Within the MOF family, there is a sub-class called zeolitic imidazolate frameworks (ZIFs). These are built from:

• Tetrahedral metal ions, typically Zn²⁺ or Co²⁺

• Imidazolate-based ligands (derivatives of imidazole)

ZIFs combine features of both MOFs and traditional zeolites:

• High porosity and surface area

• Good chemical and thermal stability

• Pore structures and topologies somewhat analogous to zeolites

• Tunable acidic and basic sites (from metal centers and nitrogen-containing ligands)

Because of these properties, ZIFs have been studied for:

• Gas storage and CO₂ capture

• Catalysis

• Energy applications

• Membranes and separation

• Biomedical usages

The most famous ZIF is probably ZIF-8, but it’s not the only one.

1.3. What is ZIF-L?

ZIF-L is a special member of the ZIF family. It is:

• Two-dimensional (2D) in structure

• Leaf-like in morphology – thin plate-like crystals

• Built from Zn²⁺ ions and 2-methylimidazole (2-mIm)

• Synthesized in water at room temperature, which is a “green” and mild approach

Structurally, ZIF-L has:

• Two types of Zn²⁺ environments

• Both coordinated 2-mIm ligands and one “free” 2-mIm molecule in the structure

• Neighboring layers connected not by deprotonated linkers but by hydrogen bonding

This is a key difference from ZIF-8. In ZIF-8, the 3D framework is more rigidly connected, while in ZIF-L the 2D layers and H-bonding make some metal sites more readily accessible. As a result:

• Lewis acid (LA) sites and Lewis base (LB) sites on ZIF-L can be more exposed

• This opens the door to using ZIF-L as a solid Lewis pair catalyst

Given that ZIF-8 has already shown catalytic activity in some polymerizations, it is natural to ask whether ZIF-L, with its different structure and accessible sites, might perform even better.

2. Why focus on ultrahigh molecular weight PMMA?

PMMA (poly(methyl methacrylate)) is one of the most widely used vinyl polymers. It’s valued for:

• Optical clarity

• Good mechanical properties

• Weather and UV resistance

• Biocompatibility in many settings

In advanced applications, especially:

• Lithium polymer batteries (as a matrix in gel polymer electrolytes)

• Biomedical devices such as bone cements and dental materials

• Actuators, optics and sensors

the molecular weight of PMMA becomes crucial.

2.1. What does “ultrahigh molecular weight” mean?

Typical PMMA might have number-average molecular weights (Mn) in the tens or hundreds of kg/mol. In this work:

• The target is ultrahigh molecular weight (UHMW) PMMA, with Mn reaching almost 1390 kg/mol.

Such long chains:

• Improve mechanical strength and toughness

• Help form robust, continuous films

• Enhance performance in gel polymer electrolytes (ionic conduction + mechanical integrity)

However, conventional routes to UHMW PMMA often require:

• Very high pressures

• Long reaction times

• Special solvents and co-catalysts

That makes the process less attractive for greener, simpler, or industrially friendly setups.

3. ZIF-L as a Lewis pair catalyst for MMA polymerization

3.1. Lewis pair polymerization (LPP) in simple terms

In Lewis pair polymerization (LPP), two components work together:

• A Lewis acid (LA) – an electron pair acceptor

• A Lewis base (LB) – an electron pair donor

Together, they activate the monomer and create a zwitterionic active species (a species bearing both positive and negative charges on different parts). This active species then adds more monomer units and grows into a polymer chain.

Key ideas:

• The LA often coordinates or “activates” the double bond of a vinyl monomer (like MMA).

• The LB attacks this activated monomer, generating a charged growing chain.

• The balance between LA and LB, and their ratio, heavily affects the reaction rate, molecular weight, and control over the polymerization.

ZIF-L contains both types of sites:

• Metal centers (Zn²⁺) acting as Lewis acid sites

• Nitrogen-containing imidazole ligands providing basic (Lewis base) character

So the material itself is like a built-in Lewis pair, but embedded in a solid, recoverable framework.

3.2. Why ZIF-L and not just any MOF or salt?

To test whether ZIF-L is truly special, the researchers compared it with:

• Various zinc salts

• The free 2-methylimidazole ligand

• Other known materials

Under the same polymerization conditions, these alternatives did not reach the same performance as ZIF-L. The ZIF-L catalyst gave:

• High conversion of methyl methacrylate (MMA)

• Ultrahigh molecular weight PMMA

To better understand why, they examined the acid–base properties using temperature-programmed desorption (TPD) with:

• NH₃ (to probe acidic sites)

• CO₂ (to probe basic sites)

The results showed:

• ZIF-L has more acidic sites than basic sites overall.

• The Lewis acid/base ratio is favorable for MMA polymerization. In similar systems, having at least a 2:1 ratio of acidic to basic sites tends to promote efficient LPP.

Compared to ZIF-8, ZIF-L has:

• A more accessible metallic part

• A different structural environment

• Hence, potentially higher catalytic performance for MMA polymerization.

4. How the polymerization is carried out

4.1. Greenish and simple conditions

The polymerization setup is intentionally straightforward:

• Bulk polymerization of MMA

• No solvent

• No co-catalyst

• Using only ZIF-L as the catalyst

• Under a dry, inert atmosphere (argon)

• At controlled temperatures (e.g., around 140 °C in optimal conditions)

Before use, ZIF-L is:

• Activated by heating under vacuum to remove residual water and guest molecules.

Then:

1. Activated ZIF-L and MMA are placed in a Schlenk flask.

2. The system is purged with argon and sealed.

3. The mixture is heated in an oil bath at the chosen temperature for a set time.

4. The reaction is quenched by cooling and exposing to air.

5. Aliquots are analyzed by ¹H NMR to determine monomer conversion.

6. The polymer is separated from the solid catalyst, precipitated, washed, and dried.

4.2. Tuning molecular weight and conversion

By systematically varying:

• Temperature

• Polymerization time

• Monomer-to-catalyst ratio

they observed clear trends:

• Higher temperature → faster reaction and higher molecular weight (up to a limit).

• Longer reaction time → higher conversion and longer chains.

• Higher MMA/ZIF-L ratio → usually lower molecular weight (less catalyst per monomer).

Under optimal conditions (e.g., 140 °C and long enough reaction time), they obtained UHMW PMMA with Mn up to ~1390 kg/mol.

5. Evidence for “living” character and the role of ZIF-L

5.1. Kinetics and “living” features

The polymerization kinetics show:

• At the beginning, the polymerization rate increases with conversion, resembling first-order kinetics with respect to monomer.

• Later, the rate slows down and deviates from ideal first-order behavior.

Why does the rate slow down?

• As UHMW PMMA forms, the viscosity of the reaction mixture increases dramatically.

• High viscosity restricts the mobility of chains and monomers, making further reaction more difficult.

• At some point, the system becomes so viscous that polymerization is strongly hindered, even if monomer is still present.

Despite this, the system shows several hallmarks of a “living-like” polymerization:

• Linear increase of molecular weight with monomer conversion, especially in the earlier and mid stages.

• The possibility of chain extension: preformed PMMA chains can be re-exposed to MMA (under inert conditions) and continue growing.

This suggests:

• The active species are not quickly destroyed.

• Chain termination is relatively limited during the main growth phase.

• ZIF-L can reinitiate or maintain active sites that propagate chain growth.

5.2. Structural analysis of the polymer

To understand the polymer structure and chain ends, several techniques were used:

• ¹H NMR:

  • Shows characteristic PMMA peaks (backbone CH₂, side-group CH₃ and COOCH₃).

  • A specific signal around 1.64 ppm indicates the presence of a six-membered lactone ring at the chain end.

  • Signals around 2.90–2.98 ppm correspond to –OH groups in the polymer structure.

• FT-IR:

  • The absence of the C=C stretching band confirms that the double bonds of MMA have been consumed and converted into single bonds in PMMA.

  • Shifts in certain bands reflect the polymer structure and lack of residual vinyl groups.

• MALDI-TOF mass spectrometry (on lower molecular weight PMMA):

  • The mass differences between peaks match the molecular weight of one MMA unit, proving that the chains are built from repeating MMA units.

  • A plot of m/z versus the number of repeat units gives a straight line with a slope equal to the MMA unit mass.

  • The intercept corresponds to a chain end containing a six-membered lactone ring.

All of this supports the idea that:

• PMMA chains are linear, built unit-by-unit from MMA.

• The chain ends are associated with a lactone ring formed by intramolecular backbiting during the final stages or upon exposure to air.

• Under inert conditions, those chain ends can still be extended, but under air they become effectively terminated.

5.3. Proposed mechanism: Zwitterionic Lewis pair polymerization

Putting the pieces together, the researchers propose that the mechanism proceeds through zwitterionic Lewis pair polymerization:

1. Activation of monomer

   • Lewis acid sites on ZIF-L (Zn²⁺ centers) coordinate to the C=C of MMA, forming an “activated” monomer.

2. Formation of zwitterionic active species

   • Lewis base sites (nitrogen donors) attack the activated monomer.

   • This generates a positively charged unit near the metal and a negatively charged site on the growing chain – a zwitterionic active species.

3. Propagation

   • The zwitterionic species reacts with more Lewis acid-activated monomer units.

   • Each step adds one MMA unit to the growing chain, extending PMMA.

4. Backbiting and lactone formation

   • At later stages, an intramolecular cyclization (“backbiting”) can occur.

   • This forms a six-membered lactone ring at the chain end and effectively stops further growth, especially upon exposure to air.

5. Chain extension under inert conditions

   • If chains with suitable ends are re-exposed to MMA under inert conditions, chain extension can resume.

   • This is consistent with a “living-like” behavior.

An important detail is that the Lewis pair is embedded in the solid ZIF-L framework. That makes the system heterogeneous: the catalyst is a solid powder, and the monomer/polymer is in the liquid phase (before gel-like viscosity develops).

6. Stability, recoverability and reusability of ZIF-L

For any heterogeneous catalyst, especially a MOF, two practical questions are critical:

1. Does it survive the reaction conditions?

2. Can we recover and reuse it without losing performance?

6.1. Structural stability

After multiple polymerization cycles, the recovered ZIF-L was characterized again using:

• PXRD (powder X-ray diffraction) – to check crystal structure

• SEM (scanning electron microscopy) – to look at morphology

• BET surface area measurements – to examine porosity

The results showed:

• The basic leaf-like morphology and framework structure were preserved.

• The material remained highly crystalline.

• The porosity and surface characteristics stayed within an acceptable range.

This indicates that ZIF-L is structurally robust under the polymerization conditions (elevated temperature, bulk monomer, long reaction times).

6.2. Metal leaching and contamination

Using ICP (inductively coupled plasma) analysis, the amount of zinc leached into:

• The reaction mixture

• The final PMMA product

was measured.

The detected Zn content in PMMA was extremely low (on the order of 0.0005%), which is:

• Negligible from a performance standpoint

• Very attractive for applications where metal contamination must be minimized (e.g., electronic or biomedical uses)

This is a big advantage over many homogeneous catalysts, where removing metal residues from polymer products can be difficult.

6.3. Reuse of ZIF-L as a catalyst

The catalyst was:

1. Recovered by filtration

2. Carefully washed and dried

3. Reused under the same polymerization conditions

Even after at least three cycles, the catalyst:

• Still produced high conversions of MMA

• Still yielded high molecular weight PMMA (with only a slight decrease in Mn)

The small reduction in Mn suggests only minor structural or surface changes over time, which were not critical within the tested cycles.

Overall, ZIF-L behaves as a recoverable and reusable heterogeneous catalyst with:

• High activity

• High stability

• Very limited leaching

7. Why this matters: beyond one polymer and one catalyst

Let’s zoom out and look at the broader implications.

This work demonstrates that:

• A 2D ZIF (ZIF-L) can act as an efficient Lewis pair catalyst for vinyl monomers like MMA.

• You can obtain ultrahigh molecular weight PMMA (Mn ≈ 1390 kg/mol) in a solvent-free, co-catalyst-free medium.

• The polymerization process shows living-like features, allowing chain extension and controlled growth.

• The catalyst is heterogeneous, stable, recoverable and reusable, with very low metal contamination in the final polymer.

From an application point of view, such UHMW PMMA can be very attractive for:

• Gel polymer electrolytes in lithium batteries, where high molecular weight helps form robust, stable films

• Biomedical materials, where cleaner polymers with minimal metal content are preferred

• Optical and sensor technologies, which require high-purity, mechanically stable polymers

From a materials science point of view, this opens doors to:

• Using other 2D MOFs or ZIFs as platforms for Lewis pair polymerization of different monomers

• Designing tailored catalysts where acid–base balance, porosity and morphology are tuned for specific polymer targets

• Combining the advantages of MOFs (modularity, porosity, ease of functionalization) with controlled polymer chemistry

8. Closing thoughts

To sum it up in everyday language:

• ZIF-L is a special, leaf-like 2D porous material built from zinc and an imidazole derivative.

• It naturally carries both acidic and basic sites, so it behaves like a built-in solid “Lewis pair”.

• Under mild but well-chosen conditions, it can catalyze the polymerization of methyl methacrylate (MMA) into extremely long PMMA chains without the need for a liquid solvent or extra co-catalyst.

• The process shows many features of a living polymerization, which is valuable when you want control over molecular weight and structure.

• The catalyst can be filtered off, washed, dried, and reused multiple times, keeping its activity and structure.

Because it combines:

• Green synthesis of the catalyst itself (made in water at room temperature)

• Solvent-free polymerization

• High-performance UHMW PMMA

• Reusability and low contamination

ZIF-L stands out as a promising platform for future polymerization catalysis, not just for PMMA but potentially for a broader range of vinyl monomers.