March 10, 2025 | UR Gate

Polymer Precipitation for Molecular Weight Determination

Theoretical Principle

Commercial polymers consist of a mixture of polymer chains of different lengths, meaning they have varying values of n (the number of repeating monomer units in the polymer chain). However, they share similar chemical properties. These chains differ in ̅Dp or ̅Xn, which represents the degree of polymerization, meaning they have different molecular weights. As a result, there are chains with high molecular weights, others with medium molecular weights, and some with low molecular weights. Polymers with chains of varying molecular weights are referred to as polydispersed polymers, meaning they have a wide distribution of molecular weight. On the other hand, polymers with chains of similar lengths, and thus similar molecular weights, are called monodispersed polymers, meaning they have a narrow distribution of molecular weight.

Separating a polymer mixture (polymer chains of different molecular weights but similar chemical composition) is not possible using conventional separation techniques such as distillation, crystallization, or sublimation. Instead, separation is achieved using methods that rely on the differences in the solubility of polymer chains based on their molecular weights, as solubility decreases with increasing molecular weight. To obtain polymer fractions with close molecular weight ranges and subsequently determine their molecular weights individually, the following methods can be used:

Precipitation Fractionation
Fractionation Using Solvents
Fractionation Using Gel Permeation Chromatography (GPC)

The simplest and most cost-effective method is precipitation fractionation (sequential precipitation). This involves adding a precipitating agent to a solution of the commercial polymer. Due to the differences in polymer chain lengths, they will precipitate sequentially according to their molecular weights. The longer chains will precipitate first, followed by shorter chains.

Practical Principle

Polymer precipitation is carried out by adding a specific precipitating agent to the polymer solution. This agent reduces the solubility of the polymer chains in the solvent, leading to precipitation.

Accurately weigh 2.5 g of the polymer sample (commercial polystyrene foam) and dissolve it in a beaker using a suitable solvent, such as ethyl acetate or butyl acetate, ensuring the beaker is dry. The appropriate solvent volume should be used.
Fill a burette with the precipitating agent, which can be ethanol or methanol.
Gradually add ethanol or methanol from the burette while stirring the solution until it becomes turbid and the first white precipitate forms. Stop adding the precipitating agent at this point and record the volume dispensed from the burette.
Set the solution aside to allow the precipitate to separate from the liquid. The precipitate is separated using decantation, while the clear supernatant is retained for further processing.
Add a few additional drops of the precipitating agent to the white precipitate to enhance its aggregation.
Repeat the addition of the precipitating agent to the clear polymer solution until it turns turbid again, forming a second precipitate. Separate it using the same method, recording the dispensed volume.

The isolated polymer fractions are placed in special weighing dishes or watch glasses and left exposed in a clean area until the next week. Once dry, their weights are recorded, and they become ready for molecular weight determination using one of the standard methods, such as the viscosity method.

Discussion

What are mono-dispersed and poly-dispersed polymers? How can they be practically distinguished?

Mono-dispersed polymers: These have nearly equal molecular weights for all molecules. They can be distinguished practically by observing that their chemical reactions and physical properties will be very similar.
Poly-dispersed polymers: These have a wide range of molecular weights. Practically, they can be distinguished using methods such as sequential precipitation or viscosity measurements, where poly-dispersed polymers show more complex behaviors due to the variation in molecular weights.

What is meant by the molecular weight distribution of a polymer?

It refers to the distribution of molecular weights of the repeating monomers in the polymer. This distribution determines the diversity of molecular sizes in the polymer, which affects properties such as viscosity, rigidity, and elasticity.

What are the types of molecular weights in polymers, and what is the relationship between them and the types of distribution?

Types of molecular weights include the weight-average molecular weight (Mw), the number-average molecular weight (Mn), and the polydispersity index (PDI).
Polymers with a broad molecular weight distribution have a higher PDI, while mono-dispersed polymers have a PDI close to 1.

Which isolated fractions are easier to generate?

Fractions with lower molecular weights are easier to generate and handle compared to those with higher molecular weights.

Which isolated fractions have the highest molecular weight?

Fractions with higher molecular weights precipitate first when a precipitating agent is added to the solution, as they have stronger intermolecular forces.

Do the uses of a single polymer vary with its molecular weight? Explain.

Yes, the uses of a polymer depend on its molecular weight. Polymers with higher molecular weights are more rigid and less flexible, while those with lower molecular weights are more flexible and less rigid.

How is the molecular weight of a polymer determined?

The molecular weight of a polymer can be determined using techniques such as sedimentation, viscosity measurements, or advanced spectroscopy methods like MALS (Multiple Angle Light Scattering).

What is the fundamental principle of fractional precipitation of polymers?

The fundamental principle of fractional precipitation is that polymers with different molecular weights precipitate at different rates when a precipitating agent is added to the solution. Polymers with higher molecular weights precipitate first, while those with lower molecular weights precipitate later.