Structural Evolution of RAFT-Modified Unsaturated Polyester Copolymers: Effects of CPDT Concentration, Acidic Comonomer Structure, and Polyester Matrix Architecture

Unsaturated polyester resins (UPRs) represent challenging systems for reversible-deactivation radical polymerization (RDRP) because chain propagation, branching, and localized gelation may occur concurrently. This study systematically investigates the influence of the concentration of the RAFT agent 2-cyano-2-propyl dodecyl trithiocarbonate (CPDT), the chemical structure of the polyester prepolymer, and the nature of the acidic comonomer on the structural evolution of RAFT-modified unsaturated polyester copolymers. Three copolymer series synthesized at different CPDT concentrations were investigated: p-EGM:AA:[CPDT], p-EGM:MAA:[CPDT], and p-PGM:MAA:[CPDT]. Structural changes were characterized using H NMR, H–H COSY, UV–Vis spectroscopy, and gel permeation chromatography (GPC). Semi-quantitative analysis of normalized H NMR integral intensities was performed using Relative Vinyl Intensity (RVI), CPDT-associated methyl intensity (MI*), and normalized aliphatic intensity (AI*) to compare changes in selected proton environments among the investigated copolymer series. Increasing CPDT concentration was accompanied by a decrease in the normalized residual maleate vinyl signal, although the magnitude of this change depended strongly on copolymer composition. The most pronounced decrease in RVI was observed for the p-EGM:AA:[CPDT] series, from 0.6291 to 0.0528, whereas substantially smaller changes were observed for the p-EGM:MAA:[CPDT] series. The MI* and AI* profiles exhibited composition-dependent variations, reflecting changes in the relative contributions of CPDT-associated methyl and overlapping aliphatic proton environments, respectively. Because the aliphatic region used for AI* contains overlapping polymer- and CPDT-derived contributions, AI* is not interpreted as a quantitative measure of polymer-backbone branching. Overall, the combined NMR and GPC/SEC results reveal composition-dependent structural changes accompanying RAFT copolymerization and demonstrate that both the polyester matrix and the acidic comonomer influence the response of these heterogeneous unsaturated polyester systems to variations in CPDT concentration.