Question

How might you determine experimentally whether a particular polymerization is propagating by a step-growth or a chain-growth mechanism?

Short answer

Answer: To experimentally determine if a polymerization is propagating through a step-growth or chain-growth mechanism, you can analyze the molecular weight distribution and the dependency of molecular weight on conversion. By performing techniques such as gel permeation chromatography (GPC) or size-exclusion chromatography (SEC), you can measure the molecular weight distribution of the polymer. A wide distribution indicates step-growth polymerization, while a narrow distribution suggests chain-growth polymerization. Additionally, by measuring molecular weight as a function of conversion, you can observe the relationship between these variables. If the molecular weight increases gradually with increasing conversion, it is a step-growth polymerization. If the molecular weight stays relatively constant throughout the reaction, chain-growth polymerization is likely occurring.

Step-by-step solution

Understanding step-growth and chain-growth polymerization

Step-growth polymerization involves the formation of polymer chains via the reaction between monomers with two reactive groups, leading to a distribution of molecular weights and dependence on conversion. Chain-growth polymerization, on the other hand, involves the propagation of a single chain through the sequential addition of monomers, resulting in a more narrow distribution of molecular weights and no significant dependence on conversion.

Measure molecular weight distribution

To determine the molecular weight distribution of the polymer, perform gel permeation chromatography (GPC) or size-exclusion chromatography (SEC). These techniques will help to measure the molecular weight distribution of the polymer sample and provide a visual representation of the dispersity, allowing you to determine if the reaction followed a step-growth or chain-growth mechanism. A wide distribution indicates step-growth polymerization, while a narrow distribution suggests chain-growth polymerization.

Measure molecular weight as a function of conversion

Analyze the dependency of the molecular weight on the conversion by taking samples of the reaction at different time intervals. For each sample, measure the molecular weight using techniques such as GPC or SEC, and calculate the conversion using methods like proton nuclear magnetic resonance (1H NMR) or Fourier-transform infrared spectroscopy (FTIR). Plot the molecular weight against conversion, allowing you to observe the relationship between these variables. If the molecular weight increases gradually with increasing conversion and reaches the theoretical value after reaching high conversion, it is a step-growth polymerization. If the molecular weight stays relatively constant throughout the reaction, chain-growth polymerization is likely occurring. By analyzing and comparing the molecular weight distribution and the dependency of molecular weight on conversion, you can determine if the polymerization is propagating through a step-growth or chain-growth mechanism.