A quantum mechanical study of methacrylate free-radical polymerizations

Area of Science:

• Polymer Chemistry
• Computational Chemistry
• Physical Chemistry

Background:

• Pulsed-laser initiated polymerization (PLP) coupled with size-exclusion chromatography (SEC) measures polymerization kinetics.
• Understanding the factors influencing propagation rate coefficients (kp) is crucial for polymer design.

Purpose of the Study:

• To develop accurate quantitative structure-property relationship (QSAR) models for predicting methacrylate polymerization kinetics.
• To compare the predictive capabilities of semiempirical and ab initio theoretical methods for activation energies and rate coefficients.
• To elucidate the reasons behind experimentally observed high kp values for larger methacrylate monomers.

Main Methods:

• Theoretical calculations using semiempirical (AM1-CI) and ab initio (HF, DFT) methods.
• Calculation of Gibb's free energies (ΔG‡) and activation energies (Ea).
• Development of QSAR models using charge and quantum chemical descriptors.

Main Results:

• Semiempirical AM1-CI successfully predicted relative activation energies (R² = 0.89).
• DFT methods provided more accurate absolute activation energies but less reliable relative values.
• Accurate QSAR models for kp were developed: semiempirical (R² = 0.959) and DFT (R² = 0.979).
• A second DFT model for hydrocarbon methacrylates showed high accuracy (R² = 0.979).

Conclusions:

• Both semiempirical and DFT methods can be used to model methacrylate polymerization kinetics.
• QSAR models based on quantum chemical descriptors offer a powerful tool for predicting kp.
• Theoretical analysis provides explanations for experimentally observed kinetic behaviors in methacrylate polymerization.