Effects of neighboring sulfides and pH on ester hydrolysis in thiol–acrylate photopolymers

Networks synthesized through thiol–acrylate photopolymerization or Michael-type addition step growth reactions contain esters with neighboring sulfide groups. Previous work has demonstrated that these esters are readily hydrolyzable at physiological pH. Here, the influence of the distance between the sulfide and ester, as well as the water concentration, on ester hydrolysis was characterized. These preliminary results indicate that reducing the number of carbons between the sulfide and the ester from 2 to 1 increased the rate of ester hydrolysis from 0.022 ± 0.001 to 0.08 ± 0.015 days−1. Increases in ester hydrolysis rates were also observed as hydrophilicity increased for oligomers prepared from a trithiol, tetrathiol and dithiol monomer (0.012 ± 0.003, 0.032 ± 0.004, and 0.091 ± 0.003 days−1, respectively). Additionally, in bulk-eroding polymeric biomaterials, variations in pH impacted the ester hydrolysis rate. This work confirms that small variations in buffer pH predictably alter the mass loss profile of a thiol–acrylate photopolymer. More specifically, as buffer pH was changed from 7.4 to 8.0, the rate of ester hydrolysis increased from 0.074 ± 0.003 to 0.28 ± 0.005 days−1. The magnitude of this observed change in ester hydrolysis rate was correlated to the increase in hydroxide ion concentration that accompanied this pH change.