Synthesis and structure of deuterated ultra-low cross-linked poly(N-isopropylacrylamide) microgels

Ultra-low cross-linked (ULC) microgels are extremely soft polymers with advanced material properties exhibiting a colloid-to-polymer transition. Poly(N-isopropylacrylamide) (pNIPAM) microgels are commonly synthesized by radical precipitation polymerization of the fully protonated monomer N-isopropylacrylamide (NIPAM). Analogous deuterated monomers arouse interest regarding their varying scattering length densities in small-angle neutron scattering (SANS). An isotope substitution in the main monomer of a microgel changes its scattering length density, which enables the exploitation of the technique of contrast variation in SANS. Here, we demonstrate that the synthesis of the deuterated pNIPAM ULC microgels encounters challenges related to the self-cross-linking mechanism of the polymer chains. The location of the deuterium isotopes is crucial for the generation of deuterated pNIPAM ULC microgels: when the isopropyl group of NIPAM is deuterated, the cross-linking is strongly restrained and the formation of microgels is precluded. However, the deuteration of the vinyl group of NIPAM, ending as backbone in the pNIPAM chains, allows the self-cross-linking. Ab initio calculations of the bond dissociation enthalpies endorse that the cross-linking of the pNIPAM chains occurs via hydrogen atom abstraction at the tertiary carbon atom of the isopropyl group. Additionally, we show that the deuteration of the vinyl group barely shifts the transition temperature of the polymer compared to protonated pNIPAM, whereas deuteration of the isopropyl group results in a significant shift of the transition temperature toward higher temperatures. Finally, the deuterated ULC micro-and nanogels reveal a stronger cross-linked network with lower swelling ability compared to the protonated microgels.