Reversible addition fragmentation chain transfer (RAFT) copolymerizations of methacrylic acid N-hydroxysuccinimide ester and cyclic N-vinylamide derivatives (N-vinylpyrrolidone, N-vinylpiperidone, and N-vinylcaprolactam) were successfully performed with methyl 2-(ethoxycarbonothioylthio)propanoate as chain transfer agent (CTA). Effects of different reaction parameters, such as solvent type, temperature, and CTA-to-initiator (C/I) ratio, were studied to optimize the polymerization conditions in order to obtain copolymers with variable chemical composition, controlled molecular weight, and narrow polydispersity index (PDI). The solvent type has a high impact on the polymerization reaction, and a high C/I ratio decreases polydispersity as well as conversion. Increased steric hindrance through an enlarged lactam ring offsets the monomer reactivity. The controlled character of RAFT polymerization was evidenced by the low PDI of the copolymers and a linear relationship between conversion and molecular weight. Biohybrid nanogels were synthesized by direct coupling between reactive copolymers and enhanced green fluorescent protein (EGFP) or cellulase (CelA2_M2) at room temperature in a water-in-oil emulsion. The EGFP-conjugated nanogels were fluorescent, while the CelA2_M2 encapsulated in nanogels retained its catalytic activity, as demonstrated by the hydrolysis of 4-methylumbelliferyl-beta-D-cellobioside.