Uncovering three-body competition of chain growth, degradation and re-aggregation for polyphenylacetylenes during solution polymerization†

By employing phenylacetylene as a model system to understand the structural evolution of conjugated polymers during solution polymerization, we reveal for the first time the three-body competition mechanism involving chain growth, degradation, and re-aggregation processes in poly(phenylacetylene) (PPA) synthesis. Key findings include: (1) identification of a universal two-stage degradation phenomenon (5.0 min < time < 200 h) independent of solvent or atmosphere, comprising a slow polymerization-dominated degradation of large fragments followed by rapid degradation-dominated breakdown of smaller fragments; (2) demonstrated solvent- and atmosphere-dependent relationships for maximum conversion, apparent molar mass (M_w,app), and characteristic transition time (t_transit), where inert atmosphere and polar solvents prolong t_transit, indicating distinct activation energies for thermal-versus oxidative degradation pathways; (3) quantification through light scattering of absolute-to-apparent molar mass ratios (M_w,abs/M_w,app = 2.3–3.3) across solvents, establishing a critical conversion factor for molecular weight comparisons in conjugated polymer studies. Simplified component analysis combining M_w,abs, average hydrodynamic radius (〈R_h〉), and size distribution (f(R_h)) further unveils an unexpected competition between degradation (1–10 nm fragments) and re-aggregation (40–400 nm clusters) in dilute solutions. Our results suggest SEC flow fields may disrupt weakly-bound aggregates. This work provides fundamental insights into dynamic competition mechanisms governing conjugated polymer synthesis, with implications for controlled fabrication of polymeric architectures.