March 08, 2025 | UR Gate

Biphasic Precipitation of Polymers with Experimental Procedure

Biphasic Precipitation of Polymers – Industrial Chemistry Lab

Theoretical Principle

Synthetic polymers are composed of polymer chains with varying molecular weights, and this variation arises from the polymerization methods used, such as free radical polymerization or step-growth polymerization. The degree of this variation is expressed by the dispersity (Đ), which is the ratio of the weight-average molecular weight to the number-average molecular weight (Mw/Mn).

Polydispersed Polymer: A polymer consisting of chains with different molecular weights, resulting in a wide distribution of chain lengths. Most synthetic polymers are polydispersed, as it is difficult to control the growth of all chains during polymerization.

Monodispersed Polymer: A polymer consisting of identical chains with the same molecular weight. Monodispersed polymers are rarely achieved in practice, except in biopolymers (like proteins) or when using special polymerization techniques such as living polymerization.

Monodispersed and Polydispersed Polymers in Industrial Chemistry

Note:
Polymers with different molecular weights cannot be separated by traditional methods like distillation, crystallization, or sublimation; instead, separation depends on the solubility of the polymer chains based on their molecular weights.

Methods for Fractionating and Separating Polymers by Molecular Weight

Precipitation Fractionation: This method relies on using a precipitant that decreases the solubility of the polymer, causing larger molecules to precipitate first, followed by the smaller ones.
Solvent Fractionation: This method involves using different solvents that have varying abilities to dissolve specific parts of the polymer, allowing separation based on their solubility.
Thermal Fractionation: This method involves heating the polymer to different temperatures, where the lower molecular weight components dissolve first, enabling gradual separation based on the melting point.
Size Exclusion Chromatography (SEC): This method involves passing the polymer through a porous medium, where small molecules enter the pores while large molecules pass through quickly, separating them based on size.

Note:
In this experiment, precipitation fractionation will be used, where a precipitant is added to a polymer solution, causing larger polymer molecules to precipitate first, followed by the smaller ones.

Experimental Principle

1. Dissolving the Polymer:

The polymer is dissolved in an appropriate solvent, such as tetrahydrofuran (THF) or acetone, to create a homogeneous and clear solution.
A suitable precipitant is chosen that is poorly solvable for the polymer but miscible with the original solvent, such as ethanol or methanol.

2. Gradual Addition of the Precipitant:

The burette is filled with the precipitant and added to the polymer solution slowly with continuous stirring.
The addition continues until turbidity appears, indicating the beginning of precipitation.
Stirring continues until a white precipitate forms, signaling that the higher molecular weight polymer molecules are precipitating.

3. Separation of the Initial Precipitate:

The addition of the precipitant is stopped, and the precipitate is separated from the clear solution using decantation or centrifugation for improved separation efficiency.
The precipitate is washed by adding small amounts of the precipitant to enhance aggregation and reduce the loss of particles.

4. Repeating the Process to Obtain Different Fractions:

The same procedure is repeated on the remaining clear solution, gradually adding more precipitant.
Four different polymer fractions are obtained, each with different molecular weights.
Each separated fraction is placed in a watch glass and left to dry for one week.