Michael-type addition reactions in NIPAAm-cysteamine copolymers follow second order rate laws with steric hindrance

This work investigates the differential reaction rates seen among several Michael-Type acceptors when reacted with poly(NIPAAm-co-cysteamine). This work differs from many of the previous studies upon mercaptans in that it examines systems used for network and gel formation. We find that the reaction rates of poly(NIPAAm-co-cysteamine) cross-linked with Michael type acceptors follow traditional second order rate laws. In addition, we further confirm that these reactions are pH sensitive, reliant upon the pK (a) of the conjugated thiols, and on local chain chemistry. Additionally, this work determines that the reaction of difunctional acrylates with the macromolecular NIPAAm molecules leads to an apparent, but not significant, increase in the rate of reaction. The low magnitude of this increase is likely indicative of increased steric hindrance arising from network formation, or reduced diffusion in the NIPAAm polymer chains. Statistical analysis shows pH and ratio of thiol to acrylates significantly affect reaction rates (p < 0.05). The type of acrylate (PEGDA, PEGMA, or HEA) does not return as significant globally or within a pH range. Since localizing charge on a chain raises the effective pK (a) of nearby acids, gains in reaction rate from increasing chain functionality are shown to increase much less than would be expected from the increased concentration.