Reaction Kinetics in Acid-Catalyzed Deprotection of Polymer Films

A quantitative description of kinetics in acid-catalyzed polymer deprotection reactions requires proper identification of the controlling mechanisms. We examined the acid-catalyzed deprotection of a glassy poly(4-hydroxystyrene- co - tert -butyl acrylate) resin using infrared absorbance spectroscopy and stochastic simulations. We interpret experimental data with a model that explicitly accounts for acid transport, where heterogeneities at local length scales are introduced through a nonexponential distribution of waiting times between successive hopping events. A subdiffusive behavior with long-tail kinetics predicts key attributes of the observed deprotection rates, such as a fast initial deprotection, slow conversion at long times, and a nonlinear dependence on acid loading. Most importantly, only two parameters are introduced to offer a near-quantitative description of deprotection levels at low acid loadings and short times. The model is extended to high acid loadings and long times by incorporating a simple acid depletion model based on mutual encounters. Our study suggests that macroscopic deprotection rates are controlled by acid transport in the glassy deprotected polymer, which presents with a strongly non-Fickian behavior.