A novel methacrylic-based R-transmer, named 2-(methacryloyloxy)ethyl 4-cyano-4-(((propylthio)carbonothioyl)thio)pentanoate, was synthesized, and its reactivity (propagation versus reversible chain transfer) in reversible addition–fragmentation chain transfer self-condensing vinyl polymerization (RAFT-SCVP) with methyl methacrylate (MMA) was studied to apprehend its impact on hyperbranched polymers (HBP) structure. The resulting HBP were characterized by nuclear magnetic resonance (NMR) spectroscopy, size exclusion chromatography (SEC) and differential scanning calorimetry (DSC) to determine, respectively, their composition and degree of branching (DB), number average molecular weight (Mn) and dispersity (Đ), and glass transition temperature (Tg). Homopolymers of methacrylic-based R-transmer and copolymers with MMA were considered to study the reactivity of the methacrylic-based R-transmer with different initial molar ratios [MMA]0/[R-transmer]0 through kinetic studies and its consequences on HBP macromolecular characteristics and thermal properties. By varying the [MMA]0/[R-transmer]0 ratio from 1:1 to 100:1, HBP with increasing Mn (from 6,400 to 14,900 g mol–1) and Tg values (from 36.1 to 87.9 °C), decreasing Đ values (from 4.07 to 1.71) and DB values (from 0.30 to 0.03) were obtained.