Elucidation of network structure in cationic photopolymerization of cyclic ether comonomers

Abstract

Although cationic photopolymerizations are not inhibited by oxygen, polymerization rates and conversions of commonly used cycloaliphatic epoxides are typically much lower than industrially prevalent acrylates undergoing free-radical photopolymerization. The addition of oxetanes can dramatically improve epoxide kinetic outcomes; however, a clear understanding of how this addition impacts polymer structure has been lacking. Herein, copolymerization of epoxide and oxetane monomers is established via Raman spectroscopy paired with Soxhlet extraction and then confirmed using dynamic mechanical analysis. The microstructure of the resulting copolymer network is determined by calculating reactivity ratios for an industrially relevant cycloaliphatic epoxide paired with four different oxetanes using Raman spectroscopy to resolve individual conversions for each monomer. Reactivity ratios calculated using the Mayo–Lewis, Fineman–Ross, and Kelen–Tüdös methods are all in agreement for these epoxide/oxetane systems, and the program developed using the Kelen–Tüdös method is made available for future studies. Some monomer pairings result in random copolymers, while others result in blocky copolymers. The reactivity of each oxetane with the cycloaliphatic epoxide is influenced by monomer structure and purity; however, care must be taken to ensure that other reactions, such as the Activated Monomer mechanism due to interaction between epoxide and hydroxyl moeities, do not confound the reactivity ratio results.